SNOW
ON
ANTARCTIC
SEA ICE
Robert
A. Massom,
• HajoEicken,
2 Christian
Haas,
3 MartinO. Jeffries,
2
MarkR. Drinkwater,
4'5Matthew
Sturm,
6 Anthony
P.Worby,
•'7Xingren
Wu,•'7
VictoriaI. Lytle,
•'7ShukiUshio,
8 KimMorris,
2 PhillipA. Reid,
•'9
Stephen
G. Warren,
to andlanAllison
•'7
Abstract. Snowon Antarcticsea ice playsa complex
and highlyvariable role in air-sea-iceinteractionprocessesand the Earth's climate system.Using data collectedmostlyduringthe past10 years,thispaperreviews
the followingtopics:snowthicknessand snowtype and
their geographicaland seasonalvariations;snow grain
size, density,and salinity; frequencyof occurrenceof
slush;thermal conductivity,snowsurfacetemperature,
and temperaturegradientswithin snow;and the effect of
snowthicknesson albedo.Major findingsincludelarge
regionalandseasonaldifferencesin snowpropertiesand
1.
INTRODUCTION
thicknesses;
the consequences
of thicker snowand thinner ice in the Antarcticrelativeto the Arctic (e.g., the
importanceof flooding and snow-iceformation); the
potential impact of increasingsnowfallresultingfrom
global climate change;lower observedvalues of snow
thermalconductivitythan thosetypicallyusedin models;
periodic large-scalemelt in winter; and the contrastin
summer melt processesbetween the Arctic and the
Antarctic. Both climate modelingand remote sensing
would benefit by taking accountof the differencesbetween the two polar regions.
constantlyevolveasa resultof a rangeof metamorphism
processes.
At maximumextenteachyear (September-October),
Becauseof its low thermal conductivity(about an
seaice coversa vast area of the SouthernOcean (---19 order of magnitudelower than that of sea ice), snow
millionkm2),attaining
latitudes
asfar northas---55øSgreatlymodifiesthe seaice thermodynamically,
affecting
[Gloersenet al., 1992]. In so doing,it profoundlyalters seasonalaccretion[Maykutand Untersteiner,
1971] and
the exchangeof energy and mass between ocean and ablation rates. The latter in turn affect the structure and
atmosphereand forms an integral part of the global dynamicsof the underlyingoceanvia the input of brine
climatesystem.These effectsare significantlyamplified [Gordonand Huber, 1990] and freshwater[Fahrbachet
by the presenceof an insulativesnowcoverwhichis itself al., 1991],respectively.
Becauseof its high albedocomhighlyvariable in thicknessand properties.Persistently pared with that of sea ice, snow dominatesthe surface
strongwinds redistributethe snow, and its properties shortwaveenergyexchange[Maykut,1986].By smoothing the ice surface,snowgreatlymodifiesthe ice-airdrag
coefficient[Andreaset al., 1993] and the bulk transfer
coefficients
for latent and sensibleheat [Andreas,1987].
• AntarcticCooperative
Research
Centre,c/oUniversity
of
It
also
has
a
first-ordereffecton the microwaveproperTasmania, Hobart, Tasmania, Australia.
2 Geophysical
Institute,Universityof AlaskaFairbanks, ties of the surface,leadingto ambiguityin retrievalsof
sea-icetype and concentrationfrom satellitedata [e.g.,
USA.
Comiso,1983;Drinkwateret al., 1995;Lohanick,1990].
3Alfred-Wegener-Institut
fiir Polar-und
Meeresforschung,
Snow also reflects photosyntheticallyactive radiation
Bremerhaven,Germany.
4 Oceans/Sea-Ice
Unit, EuropeanSpaceAgency,Noord- (PAR) and so reducesthe amount availableto sea-ice
wijk, Netherlands.
algae [Sullivanet al., 1985].
sFormerlyat NASAJetPropulsion
Laboratory,
California
Modeling studieshave shownthat the Antarctic seaInstituteof Technology,Pasadena,California,USA.
ice zone is highly sensitiveto global climate change
6U.S.ArmyColdRegions
Research
andEngineering
Lab- [Mitchellet al., 1990;Wu et al., 1999],with snowon sea
oratory-Alaska,Fort Wainwright,Alaska, USA.
ice enhancingits sensitivity[Eickenet al., 1994].This is
7Alsoat Australian
AntarcticDivision,Kingston,
Tasmaan important factor given that some models predict
nia, Australia.
enhanced
net precipitationasa resultof globalwarming
8NationalInstituteof PolarResearch,
Tokyo,Japan.
9NOWatClimatic
Research
Unit,University
of EastAnglia, [e.g.,Manabeet al., 1991].Enhancedsnowaccumulation
on sea ice may result in an overall coolingeffect on
Norwich, England.
mDepartmentof AtmosphericSciences,
Universityof globalclimate[Ledley,1991].However,while the importance of sea ice is well established,detailed field studies
Washington,Seattle,Washington,USA.
Copyright2001 by the AmericanGeophysical
Union.
Reviewsof Geophysics,
39, 3 / August2001
pages413-445
8755-12 09/01/2 000 RG 000085 $15. O0
Papernumber2000RG000085
ß 413
ß
414 ß Massom et al.: SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWS OF GEOPHYSICS
OøW
Atlantic Ocean
X
_70øS '%,
/,
Weddell
••_
•
IndianOcean
Sea
Belli.
n'
"n•
t•••
gsha_usen
,• /•-•West
90øWI
•ea
_
•
J
'•
\Amundsen •&
•k
Sea
x
......
East
'-%.' ,
Antarctica
• 1
I_
I •'• !
I
90øE
Antarctica[
8oøs7oøs
•;;;•
••
/•
I
•
Pacific Ocean
P
I - )•
ß'....
i
-•
/
/
Western
....
,
180øE
Figure 1. Map of Antarctica,showingthe locationsof the different seasand oceansdiscussed
in the text.
on snow distribution and properties have only been
conductedin the past 5-10 years. These studies are
beginningto establishthe full significanceof snow on
Antarctic sea ice as a key component of the global
climate system.
In this paper we review the major findings.Section2
is a summary of snow data from five Antarctic sectors
(designatedby Gloersenet al. [1992]),namely,the Weddell Sea (20øE-60øW),the Indian Ocean (20øE-90øE),
the westernPacificOcean (90øE-160øE),the RossSea
(160øE-140øW),and the Bellingshausen
and Amundsen
Seas(140øW-60øW),as shownin Figure 1. The Indian
and western Pacific Ocean sectorsare collectivelyreferred
to as the East Antarctic
sector. Section 3 assesses
the significanceof snow in the air-sea-iceinteraction
system.New findingshave significantimplicationsfor
modeling (both physicaland biological) and remotesensingstudiesof Antarctic seaice. Gapsin our current
knowledgeare identified.Finally, the possibleenhanced
role of snowunder global warming conditionsis examined. Throughout,snowis describedusingthe combined
morphological and process-orientedclassificationof
snow types of Colbecket al. [1990], and sea ice is described using the World Meteorological Organization
(WMO) nomenclature[WMO, 1971].
2.
2.1.
PHYSICAL
PROPERTIES
Snow Thickness Distribution
Snowthickness(Zs) is typicallymeasureddirectlyin
situ or by routine hourly visual estimatesfrom shipsin
transit. At present, no remote-sensingtechnique can
provide reliable measurementsfrom space, although
promisingmethodsare under development[e.g.,Arrigo
et al., 1996;Markus and Cavalieri,1998].
2.1.1. Snow thicknessdistributionon pack ice.
Becauseof persistentlystrongwinds over Antarctic sea
ice, typicallyimmediatelyfollowingprecipitationevents
[Massomet al., 1997, 1998],windblownredistributionis
a key factor affecting the snow thicknessdistribution.
Redistribution of dry, unconsolidatedsnow occurs at
windspeeds
of >6-8 m s-• [Andreas
andClaffey,
1995].
As a result, thicknessmay not be directly related to
either the frequencyor duration of snowfall.
Mean snowthicknessesare givenin Table 1 and vary
widely(from 0.02 to 0.49 m) both seasonally
andregionally due to differencesin precipitationregimes (see
section3.5) and the age of the underlyingice. Similar
regionaland seasonaldifferencesare noted in frequency
distributionsfrom different sectors,as shownin Figure
2. Snow thicker than ---0.25 m representsdrifted snow
or, in the case of snow thicker than •--1.0 m, multiyear
accumulationon perennialfloes3-10 m thick [Jeffrieset
al., 1994a; Massom et al., 1997, 1998; Wadhams et al.,
1987].For comparison,snowthicknesson Arctic multiyear ice attainsa thicknessof 0.26-0.42 m at the end of
the accumulationseasonin May [Warrenet al., 1999].
The winter distributionsin Figures2a, 2c, and 2e are
similar,with few data from the thin class(0-0.05 m), as
only a small proportion of the pack comprisedthin ice.
That the winter Weddell Seadistribution(in Figure2b)
resemblesthe RossSeaautumnaldistribution(in Figure
2d) isbecausethe newpancakeice formationdominated
39, 3 / REVIEWSOF GEOPHYSICS
Massomet al.: SNOW ON ANTARCTIC SEAICE ß 415
TABLE 1. Mean Snow Thicknesses Measured Over Antarctic Sea Ice, From Drill-Hole Measurements
(Unless Otherwise Stated)a
Region
Season,
Yearb
Meanandlrr, rn
Range,
cm
n
Reference
Indian and WesternPacific Oceans
62ø-110øE,
ß..
0.05-0.15
_+ 0.13
+_ 0.16
_+ 0.20
+_ 0.12
0.01-0.45
0.01-0.76
0.05-1.40
0.00-1.50
47
67
113
186
Worbyand Massom[1995]
Worbyand Massom[1995]
Worbyand Massom[1995]
Worbyand Massom[1995]
0.13 + 0.09
0.02-2.01
794
0.13 +_0.12 (SO)
0.01-0.90
165
Massomet al. [1998]
Worbyet al. [1998]
spring1988
.-.
Allison et al. [1993]
58ø-66øS
0.10
0.12
0.15
0.06
64ø-106øE
62ø-102øE
75ø-150øE
spring1991
spring1992
spring1994
139ø-141øE,
autumn 1993
144ø-150øE
138ø-142øE
138ø-142øE
winter
winter
161ø-168øE
summer
0.02 +_ 0.05
0.01-0.23
164.5øE,
summer 1991
0.16 +_0.03(OM)
0.10-0.27
167øE, 78øS,
102øW, 75øS
summer 1992
0.18 +_0.03 (FI)
0.27 +_0.04 (FI)
165øE
summer
0.125-0.34
0.21-0.36
0.02-0.60
1995
1995
Bellingshausen,
Amundson,and RossSeas
1990-1991
100
56 (AA)
Jeffriesand Weeks[1993]
Danielsonand Jeff-hes
[1992]
75.5øS
1965
50
31
.-'
Veazeyet al. [1994]
Paigeand Lee [1967]
(FI)
75øW
85øW
105øW
120øW
70ø-160øW
late summer 1994
late summer 1994
late summer 1994
late summer 1994
summer/autumn
70ø-160øW
summer/autumn
+_ 0.16
+_ 0.24
_+ 0.30
_+ 0.26
+_ 0.15
0.12-0.69
0.05-0.98
0.05-0.78
0.05-1.01
0.16-0.70
35
75
19
41
100
Haas et al. [1996]
Haas et al. [1996]
Haas et al. [1996]
Haas et al. [1996]
Jeffiieset al. [1994a]
0.34 _+0.15
0.17-1.98
16
Jeffrieset al. [1994b]
31
Veazeyet al. [1994]
Jeffriesand Adolphs[1997]
0.45
0.29
0.46
0.49
0.34
1992
1992
160øE-100øW
autumn1992
0.27 _+0.04 (FI)
0.21-0.36
165ø-180øW
autumn/winter 1995
0.15 +_ 0.11
165ø-180øW
109ø-171øW
autumn/winter1995
winter/spring1994
0.11 +_0.10 (SO)
0.13 +_0.07
0.00-0.83
0.00-1.50
0.05-0.32
68ø-80øW,
autumn/winter 1999
0.03 + 0.03
62ø-69øS
109ø-171øW
109ø-171øW
109ø-171øW
75ø-110øW
75ø-110øW
80ø-110ø and
winter/spring1994
winter/spring1994
winter/spring1994
winter/spring1993
winter/spring1993
winter/spring1995
0.04 + 0.03 (SO)
0.29 _+0.09
0.27 +_0.09
0.17 +_0.10 (SO)
0.23 + 0.16 (SO)
0.19 ___
0.12 (SO)
0.22 _+0.09
ß ..
3,670
11,114
2,413
404
Sturmet al. [1998]
S. Stammerjohn
(personalcommunication,
2000)
109
0.00-1.75
0.11-0.79
0.00-0.75
0.03-0.82
0.00-1.00
0.07-0.33
7,583
2,227
9,125
1,113
4,071
2,882
Sturmet al. [1998]
Jeffrieset al. [1998b]
Jeffrieset al. [1998b]
Jeffrieset al. [1994b]
Worbyet al. [1996b]
Sturmet al. [1995, 1998]
155ø-180øW
Weddell Sea
0ø, 58ø-60øS
46ø-54øW,59ø-
spring1981
spring1988
0.30
0.31
"'
0.08-0.50
26
1,553
64øS
0ø-55øW
36ø-40øW
spring1989
spring1989
spring/summer
1983
40øW, 64ø-
summer/autumn
74øS
0.26 +_0.23
0.18
0.41 (SO)
...
0.01-1.29
0.01-0.80
"'
0.02-0.56
5,339
2,650
77
ClarkeandAckley [1982]
Eickenet al. [1994],Langeand
Eicken [1991]
Eickenet al. [1994]
Meeseet al. [1990]
Comisoand Sullivan[1986]
Gow et al. [1987]
1980
5øE-55øW
autumn/winter
0.28 _+ 0.15
0.08-0.62
5øE-55øW
autumn/winter1992
1992
0.32 _+0.31 (SO)
0.01-1.00
5øE-20øW
autumn/winter 1994
0.12 +_ 0.06
0.01-0.63
5øE-20øW
autumn/winter1994
0.10 _+0.06 (SO)
0.01-1.00
5øW-7øE, 56ø-
winter 1986
0.11
-..
70øS
0ø-50øW
0ø-50øW
20ø-40øW
45ø-57øW
winter 1992
winter 1992
late summer 1997
late summer 1997
0.14
0.12
0.22
0.11
+ 0.17
+ 0.14
_+ 0.15
+ 0.14
0.01-0.95
...
0.01-1.20
0.01-0.93
4,000
173
Fisherand Lytle [1998]
V. Lytle (unpublisheddata, 2001)
1,830
360
4,238
582
386
716
783
V. Lytle (unpublisheddata, 2001)
Wadhamset al. [1987]
Massomet al. [1997]
Eicken et al. [1994]
Haas et al. [1998]
Haas et al. [1998]
ammdenotesa rangeof areaaverages;
SO denotesshipobservations;
FI denotesfastice;andOM denotesmeasurements
from onefloeonly.
bSpring
meansSeptember-November,
summermeansDecember-February,
autumnmeansMarch-May,andwintermeansJune-August.
CMinimum does not include snow on nilas, which would have zero thickness.
416 ß Massom et al.' SNOW ON ANTARCTIC SEA ICE
.4
39, 3 / REVIEWSOF GEOPHYSICS
, i i i ! ! ! i , I , i , i , i i I i
t c)Ross/Amundsen
Seas
0.2
1
Mean
=0.29
m
a) Indian Ocean
0.3
O.2
0.1
0.1]•
-7 4
n:7583
_ ear•
-0.13,m
I
0.3--
I
d) Ross Sea
0.0
......
-
, ß
-
Mean - 0.14 m
-
n = 3018
0.1-
b) Weddell Sea
0.3]__[
[
--
.
0.0
Mean
=0.14
m
--
•......
I
I
0.3tI ' _
e) Bellingshausen/
0.2
0.2- 586
•
ea•=0.22
m
= 2882
0.1
0.0
..................
0.2
0.0
0.4
0.6
0.8
.
0.0 ,
•
•
0.0
0.5
1.0
1.0
_
Amundsen
Seas _
1.5
Snow Thickness, m
Snow Thickness, m
Figure 2. Frequencydistributionsof snowthicknessfrom differentAntarcticsectors:(a) Indian Ocean,
winter 1995 [after Massomet al., 1998]; (b) Weddell Sea, winter 1992 [after Massomet al., 1997]; (c)
Amundsenand RossSeas,winter-spring1994;(d) RossSea,autumn1995;and (e) Bellingshausen
and Ross
Seas,winter 1995. Figures2c-2e are after Sturmet al. [1998].
the early part of the former cruise.In autumn,the snow
(and ice) had yet to attain its maximumthickness.Seasonal and regional variability is further discussedin
section 3.1. Relatively few snow thicknessdata have
downoff the Antarcticcontinent),significantaccumulation can occurin other regions[Kawamuraet al., 1995;
Ushioand Takizawa,1993]. Data from one transectare
shownin Table 2. While relativelylow accumulations
are
been collected in summer.
measuredat sitesnearestthe coast(transectL0), snow
2.1.2.
Snow thickness distribution
on fast ice.
thicknessincreasessignificantly
with increasingdistance
Fast ice is sea ice that forms at, and remains attached offshore(attainingthicknesses
up to 1.65m). Fartherto
("fast") to, the coastor groundedicebergs.At its maxi- the east,the averagesnowthicknessrangesfrom 0.6 to
mum extent(in November),Antarcticfastice coversan 1.0 m on the "seaward"fast-icezone (6-15 km offestimated
550,000km2 [Fedotov
etal., 1998].Whilevir- shore),where the wind weakensand accumulationoctually no snow accumulatesin fast-ice regionsaffected curs in the form of snow blown off the continent as well
by katabaticwinds(strong,persistentwindsthat cascade as precipitation from cyclones[Fedotovet al., 1998].
TABLE 2.
SnowThickness(Zs)and Sea-IceThickness(Zi) Data From Stations on Fast Ice in Lfitzow-Holm Bay, East
Antarctica [After Kawamura et al., 1995] a
LO
Date
Zs
May 1990
Aug. 1990
Oct. 1990
L1
Zi
L2
L3
L4
L5
Zs
Zi
Zs
Zi
Zs
Zi
Zs
Zi
Zs
Zt
......
......
0.11
0.15
1.44
1.70
0.39
0.70
2.20
2.50
0.63
0.97
2.15
2.13
0.72
1.20
2.20
2.13
......
1.40
2.13
......
0.29
1.98
1.05
2.22
1.39
2.24
1.59
2.37
1.65
2.12
0.05
0.22
1.45
1.74
0.10
0.36
2.10
2.15
0.35
0.86
3.03
3.25
0.58
0.92
3.16
2.81
0.47
1.12
2.88
2.97
0.20
2.04
0.53
2.36
0.98
0.53
3.25
3.38
1.13
............
2.86
1.42
2.80
April 1991
Aug. 1991
0.05
0.20
0.72
1.34
Oct. 1991
Jan. 1992
0.15
1.60
..................
aTransect
L0 to L5 runsoffshorefromeastto west,from -69ø16'S,39ø30'Eto -69ø17'S,38ø30'E.
39, 3 / REVIEWS OF GEOPHYSICS
Massom et al.: SNOW ON ANTARCTIC SEA ICE. 417
--
Periods of Snowfall
...." " , . - -
lSiJj
_
...
-
.....
Periods of Surface Hoar-Frost Deposition
5.
a)
I
0
.P
<p -5
....
::l
(is
.... -10
Q)
0.
E -15
~
.:: -20
<
-25
-30
30.+-~~~----~----~----~----~--~~1400
b)
1"
E
25
I, '"
300
20
.§~
"0
~ 15
200
~
i5
Cf)
-g
J-
"0
10
c:
100 ~
~
5
o I"
July 29
I
I
Aug. 3
Aug. 8
I
i
I
10
Aug. 13 Aug. 18 Aug. 23 Aug. 28
Day, 1995
Figure 3. Time series of (a) near-surface air temperature and (b) wind speed and direction measured from
the R/V Aurora Australis while in the pack ice, August' 1995, off East Antarctica. The horizontal dashed line
denotes the approximate wind speed above which aeolian transport of unconsolidated snow occurs. Periods
of snowfall and surface hoar-frost deposition are also marked. After Massom et al. [1998].
Petrov [1967] found the maximum snow depth 13 kIn
offshore of Mirny.
2.2.
Snow Properties
2.2.1. Snow stratigraphic and textural characteristics. The temporal evolution of an accumulating snow
cover is highly complex. At the microscopic level, processes of crystalline modification, collectively referred to
as snow metamorphism, occur immediately after deposition and significantly modify the snow texture. Drysnow metamorphIsm, which is driven by water vapor
movement in pores, is subdivided into (1) equitemperature (ET) and (2) temperature-gradient (TG) processes
[Colbeck, 1982; Granberg, 1998; Sommerfeld and
LaChapelle, 1970]. These processes are also known as
"kinetic" and "equilibrium" growth, respectively [Colbeck, 1982]. In ET metamorphism, snowflakes rapidly
transform into an aggregate of rounded smooth grains in
a snowpack that is roughly isothermal; it is also termed
destructive metamorphism, as it effectively destroys the
crystal structure. This process is most important during
the early stages of snow deposition, when wind also
mechanically fragments and compacts the snow to create
a medium- to high-density surface layer known as "wind
slab." In addition to crystal rounding, the overall effect
of ET metamorphism is to reduce the snow surface to
volume ratio, to increase the density (by filling in pore
spaces), to round the grains, and to increase the snow
strength (by building bonds between crystals). Temperature-gradient metamorphism occurs where a large temperature gradient (generally less than about -lOoe
m- 1 ), set up by the insulative effect of the snow cover,
promotes the rapid upward migration of water vapor
through the snowpack. Significant crystal enlargement
occurs by this kinetic growth (hence the alternative term
constructive metamorphism), and the formation of more
complex (angular and faceted) crystals takes place. This
418 ß Massom et al.: SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWSOF GEOPHYSICS
Figure 4. Photographof a typicalwinter snowpit (thickness0.25 m) from the AmundsenSea, September
1994, showingdistinctlayering.
producesa lower-densitysnow,with poor intercrystalline bonding.Wet-snowmetamorphismoccurswhenliquid water is present(due to melt and/orthe incursionof
seawater).Under these conditions,rates of metamorphism tend to accelerate,with small grainsbeing preferentiallydestroyedand large grainsbecomingrounded.
Subsequentrefreezing results in a coarse-grained,icy
snowcharacterizedby large, bonded grain clusters.In
are presentedin Table 3. Typical mean grain size and
densityas a functionof snowtype in winter are givenin
Table 4. Grain size is measuredfrom vertical profiles
[e.g., Massom et al., 1998] and by sievingbulk snow
samples[Sturm,1991].The "grainsize"we report here is
for the longestdimension;for a roundedcrystal,it is the
diameter. For new and recent snow, densityand grain
sizedependupon the conditionsduringdeposition[Colthe caseof snow on Antarctic sea ice, the introduction of beck et al., 1990]. Sturm et al. [1998] suggestthat the
brine into the snowfurther complicatestheseprocesses relatively high air temperatures and persistentwinds
that often accompanysnowfall[Drinkwater,1995;Mas(seesection2.2.4).
Snow-crystalmetamorphism,in combinationwith ad- soreet al., 1998] accountfor the relative abundanceof
ditionalsnowaccumulation,intermittentmeltwaterper- softto moderateslabswith a fine-grainedtexture(Table
colation,and upward migrationof brine, resultsin the 3). They note a near-absenceof hard slabslike those
formationof a complexsnowpack.Another key factor is commonlyobservedin Arctic Alaska under similarwind
the interplayof environmentalvariables(local topogra- conditionsbut significantlylower temperatures[Benson
phy, temperature,wetness,solar radiation,wind) both and Sturm, 1993].
during and after deposition.Weather conditionsin the
While recent snowcoversare characterizedby anguAntarctic sea-icezone are highly dynamicand variable, lar (mechanically
fragmented)or roundedcrystalforms,
with periodsof high temperatures(to -0øC) in winter significantlylarger crystalsin the form of faceted snow
and strongwindsoften accompanying
snowfall[Massom and depth hoar dominate in older and colder snow
et al., 1997],asis shownin Figure3. As a result,snowon covers[Massomet al., 1997, 1998],as is shownin Tables
Antarctic sea ice by no means forms a uniform slab. 3 and 4. Depth-hoar formation occursat temperature
Rather, it comprisesnumerous layers with different gradients
of about-10 ø to -25øCm-1 [Akitaya,
1974;
physicalcharacteristics
(Figure 4) [Massomet al., 1997; Armstrong,1980;Colbeck,1982].Bulk temperaturegraSturm et al., 1998]. Particularlycomplexstratigraphies dient (VT) is assumedto be vertical and linear and is
are observedin older snowcoverson multiyear or fast definedby (Ts - Ti)/Zs, where Ts is the snow-surface
ice [Massomet al., 1998].
temperature and Ti is the snow/iceinterface temperaThe sixdominantsnowclassesin winter are (1) new ture. New, thin snow covers soon exhibit textural comand recent snow,(2) soft and moderateslab, (3) hard plexity as a result of rapid depth-hoarformation due to
slab,(4) facetedsnowanddepthhoar,(5) icylayers,and steeptemperaturegradients[Massomet al., 1998]. The
(6) salineslush[Massomet al., 1998;Sturmet al., 1998]. greatest proportions of depth hoar occur in autumn
Photomicrographsof four typesare shownin Figure 5. (Table 3) and are probablylinked to the larger (negaPercentagesof different snowtypesobservedfrom the tive) VT acrossa thinner snowpack(see section3.4.1.).
Indian Ocean [Massomet al., 1998] and the BellingsAnother important feature is the prevalence of icy
hausen,Amundsen,and RossSeas[Sturmet al., 1998] layers,whichare presentin all seasons
(Table 3). On the
39
3 / REVIEWS OF GEOPHYSICS
Massom et al.' SNOW
ON ANTARCTIC
SEA ICE ß 419
Figure 5. Photomicrographsof the four primary snow types encountered in late winter 1994 in the
Bellingshausen
and RossSeas.From Sturmet al. [1998].
snowsurface,wind and polycrystallinecrustsform rap- they cover large areas and play a significant role in
idly due to synopticvariabilityin Tair and relative hu- "locking up" the snow and reducing aeolian erosion,
midity, the frequencyof high winds,and even sleet/rain redistribution, and snow sublimation [Eicken et al.,
freezingon hittingthe surfaceat lowerlatitudes(evenin 1994; Massom et al., 1997]. They probably also limit
winter) [Massomet al., 1997;Sturmet al., 1998].Surface wind pumping [Waddingtonet al., 1996]. When buried,
wind-glazingoccursby the formation of regelationice crusts intercept meltwater intermittently percolating
filmson the snowsurfacefollowingthe kineticheatingof downward during occasionalwarm periods [Colbeck,
wind-drivensaltatingdrift snow[Goodwin,1990].Other 1991], even in winter [Jeffrieset al., 1997b;Massomet
icy snowsurfacefeatures,includingglaze, occurby ra- al., 1998; Sturm et al., 1998]. This meltwater spreads
diational effects [Colbeck,1991]. By these processes, out laterally and subsequentlyrefreezes, and the resnow microrelief
features such as hoarfrost
flowers
sultant icy layers can attain thicknessesof 0.05 m, but
transforminto icy rippled and "nubbly"layersconsisting are commonly less than 0.01 m, and extend horizonof irregular lumpsof ice, 0.01-0.02 m high and spaced tally for distancesof meters, having a significanteffect
every few centimeters.Upwind faces of sastrugi are on polarization in passivemicrowave remote sensing
particularlysusceptibleto crust formation [Massoreet [Garrity, 1992]. Horizontal icy layers also form largely
impermeable barriers to the upward flux of water
al., 1997].
Although surface crusts constitute only a small vapor in colder snow,leading to enhanceddepth-hoar
fraction of the total snowvolume (e.g., <3% in Au- formation underneath [Colbeck, 1991]. Where meltgust 1995 in East Antarctica [Massomet al., 1998]), water percolatesdownwardthrough the snowpackand
TABLE 3.
The Percentageof Different Snow Types Sampled on Seven Different Antarctic Sea-Ice Cruises.
Autumn/
Time
Regiona
Number of snowpits
% Icy (6a, 8a to 8c, 9d, 9c)
% New and recent(la-le, 2a, 2b)
% Soft and moderateslab(3b, 3c)
% Hard slab(3a)
% Faceted/Depthhoar (4a-4c,
5a-5c)
% Slush(6c)
Mean depth, m
Mean VT, øCm-•
Spdng'94
EastAnt.
10
16
11
7
9
54
3
0.15
....
Spdng'94
Am./Ross
164
20
14
23
6
31
6
0.28
23
a Classifications
(6a, etc.) are accordingto Colbecket al. [1990].
Winter/
Autumn '93
EastAnt.
Winter '95
Ross
16
18
12
7
11
48
73
26
8
10
0
47
4
0.17
-66
9
0.14
-82
Winter '95
EastAnt.
46
18
23
8
9
39
3
0.13
-47 (n = 751)
Spring'94
BelL/Am.
58
9
30
55
6
0.20
-41
Winter/
Spdng '95
BelL/Ross
45
46
4
13
0
29
8
0.22
-44
420 ß Massom et al' SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWS OF GEOPHYSICS
TABLE 4. Mean Grain Size and Density as a Function of
the Major Snow Types, From Sieving of Snow in Winter in
the Amundsen and Ross Seas, With let [After Sturrn et al.,
19981a
Snow '
Grain Size,
Type
mm
Snow Density,
na
kgm-3
New/recent snow
1.1 _+ 0.6
2
310
Soft-moderate
Hard slabs
1.1 _+ 0.4
0.9 _+ 0.3
21
4
340
410
2.0 _+0.9
1.3 +_0.6
29
8
290
350
slabs
Depth hoar
Melt-grain clusters
aNumber of snowpits.
refreezes on the sea ice surface, "superimposed"ice
forms (see section3.2.2).
Snow texture is also intermittently altered by a wetting or dampeningof the snowby melting,floodingwith
seawater,or the incorporationof brine from the underlying sea-ice surface. Flooding creates large, poorly
bondedroundedcrystalsin the form of slush[Colbecket
al., 1990].Above the slushlayer, capillary"wicking"of
seawaterupwardinto the snowmasssignificantlyalters
the snowtextureto a heightof 0.1-0.2 m [Massomet al.,
1997,1998](Figure6). The overallimportanceof flooding is discussedin section3.2.
Where the snow is damp/wetwith a low water content, which is often the case close to the snow/ice inter-
face, clustersof roundedcrystalsform, held togetherby
largeice-to-icebonds[Colbeck,1986].Subsequent
freezing leads to rounded melt-freeze particles,wherebyindividual crystalsare frozen into a solid polycrystalline
grain;this producesa coarse-grainedsnow.In summer,
snowis characterizedby extensiveicinessin the form of
icy grain clusters,ice lenses,ice layers, and percolation
columnsas a result of melt-refreeze cycling,with grain
clustersup to 1 cm across[Haaset al., 2001;Morrisand
Jeffries,2001]. These large clustersmay have a strong
signature at microwavelengths.In winter, dampened
grain clustersare more prevalenttoward the snowpack
base.
Estimatesof meangrainsizeare presentedin Table 5.
They range from 1.5 to 2.9 mm and exhibit a considerable spatialand temporalsimilarity.Figure 7 presentsa
typicalfrequencydistributionof grainsizefrom snowpit
measurements in winter. The mode is 2.0-2.5 mm, and
grain size rangesfrom 0.05 mm for mechanicallyfragmented spiculesof new snowto 10 mm for well-developeddepth-hoarcupsin older,coldsnow.Skeletaldepth
hoar comprisesloose accumulationsof cuppedand columnar crystalsseparatedby 1-2 mm voids [Akitaya,
1974].Sturmet al. [1998] observedthat if a givenlayer
Snow.:
Su.i:.'rf:::•ce'•:::::,
:..........................................
.=•
.................................
......................
IcyLayers
Figure 6. A photographof dyed seawaterpercolating,by capillarysuction,upwardthrougha block of
naturalAntarcticsnow,removedfromwinterseaice (in the AmundsenSea,September1994)andtransported
intactto a coldroom.In thisexperiment,2 cm of seawaterwaspouredaroundthe snowbase.The photograph
was taken 30 min later. The whole snowcolumn(14 cm thick) was affected,with the lower 7.5 cm being
saturated.Note the lateral spreadingout of seawateracrossicy lensesin the upperpart of the snow,whichis
typicallycolder.
Massom et al' SNOW
39, 3 / REVIEWS OF GEOPHYSICS
TABLE 5.
ON ANTARCTIC
SEA ICE ß 421
Mean Snow Grain Size Measured in Snow Pits, With l•r
Grain
Region
75ø-150øE
Season,Year
Size,mm
Range,mm
Indian and WesternPacific Oceans
1.7 _+0.8
0.05-4.00
spring1994
139ø-141øE,144ø-150øE
autumn 1993
1.5 _+1.5
0.05-10.0
138ø-142øE
winter 1995
1.6 +_ 1.2
0.01-10.0
na
Reference
17
54
Worbyand Massom[1995]
Worbyand Massom[1995]
Massomet al. [1998]
167
Bellingshausen,
Amundsen,and RossSeas
70ø-130øW
109ø-171øW
20ø-55øW
0ø-50øW
late summer 1994
2.5 __+1.0
1.00-4.00
winter/spring1994
1.6 _+0.7
0.80-3.60
11
49
late summer 1997
winter 1992
Weddell Sea
2.9 _+ 1.2
2.7 _+ 3.1
0.5-10.0
0.2-10.0
129
90
Haas et al. [2001]
Sturmet al. [1998]
Haas et al. [2001]
Massomet al. [1997]
aNumberof snowsamples.
had been subjectedto wetting,its grain-sizedistribution
showeda distinctbimodality.
By virtue of its effect on the ratio of absorptionto
scattering,grain-sizegrowthreducessnowalbedo,particularly at near-infraredwavelengths[Grenfellet al.,
1994;Warren,1982;Wiscombe
and Warren,1980].Moreover, grain enlargementcan increasethe intensity of
shortwaveradiationpenetrationby decreasingthe spectral extinction coefficient [Maykut, 1986]. Similarly,
grain-sizevariabilityaffectsthe surfaceelectromagnetic
signaturemeasuredby satellitemicrowavesensors[Hallikainenand Winebrenner,1992], particularlyat higher
frequencies[Comiso,1983].
However, more than just a singlemeasureof grain
sizeis neededto understandtheseeffects.For spherical
grains,there is no ambiguity.However,for nonspherical
grains,the sizesreportedhere in Figure 7 and Tables3
and 4 are the longest dimension.This is probably a
usefulmeasurefor explainingthe microwaveemissivity,
but for snowpropertiesin the solar spectrum,the most
relevantdimensionis the shortestone (e.g.,the width of
the column,the thicknessof a plate, or the thicknessof
one wall of a faceted cup. Specifically,the optically
effectivegrain radiusis 3 V/S, where V/S is the volumeto-surfaceratio [Grenfelland Warren, 1999]). This is
becausethe ratio of absorptionto scatteringis determined by the average distance that photons travel
throughice betweenopportunitiesfor scatteringat airice interfaces.For example,the depth hoar in Figure 5
mayhavea grainsizeaslargeast0 mm asdeterminedby
sieving,but the optically effectivegrain diameter appearsto be about0.5 mm. Albedo decreases
asgrainsize
increases.This normally resultsin a decreaseof albedo
with snowage as destructivemetamorphismproceeds.
However, growth of depth hoar by constructivemetamorphism,althoughit increasescrystallength,doesnot
necessarily
increasethe averagecrystalthickness.
2.2.2. Snowdensity. Snowdensityaffectsthe effectivethermalconductivity(see section2.2.5) and air
permeabilityand determinesthe snow-waterequivalent
and dielectricpropertiesof the snowmass[Hallikainen
and Winebrenner,1992]. Large-scalemeans vary from
---290to 390kgm-3 (Table6), similartovalues
foundin
the Arctic [Warrenet al., 1999, Figure tt]. Density as a
functionof snowtyperanges
from108kgm-3 for new
snowto >760kgm-3 for icylayers[Sturm
etal., 1998].
Again, greatvariabilityoccurson smallspatialscales.In
East Antarctica in August 1995, for example,average
0.3
density
rangedfrom240to 600kg m-3 across
a single
floe
Mean
= 1.6 mm
n=
167
as determined
from
47 measurements
at meter
intervals[Massomet al., 1998].
Typicalwintertimefrequencydensitydistributionsare
shown in Figure 8. The distributionfrom the Indian
Ocean(Figure8a) shows
a modeat 300-350kg m-3,
witha minorpeakat ---700kgm-3 comprising
icylayers.
Lowermodalvalues(225-250kg m-3) havebeenreported from the Bellingshausen
and AmundsenSeasin
the late-winter to early spring [Jeffrieset al., t997b],
0.0
,
,
,
,
ITM ,
reflectingthe higherproportionof depthhoar presentin
10
12
0
2
4
6
8
the older,coldersnowcover(seesection2.2.1).The few
Grain Size, mm
datacollectedin summerreveala highmeandensitydue
[Haaset al., 2001;Morrisand
Figure 7. A typicalfrequencydistributionof snowgrain size to melt-refreezeprocesses
in winter, from East Antarctica.After Massomet al. [1998].
Jeffries,2001]. The densityof icy samplestends to be
422 ß Massom et al.' SNOW ON ANTARCTIC SEA ICE
TABLE 6.
39, 3 / REVIEWS OF GEOPHYSICS
Snow Densities (Mean and Standard Deviation) Measured Over Antarctic Sea Ice
Range,kg
Region
Season,
Year
Ps,kgm-3
m-•
na
Reference
WesternPacific Ocean
autumn 1993
winter 1995
139ø-141øE, 144ø-150øE
138ø-142øE
390 _+ 170
360 _+ 110
120-660
120-760
55
170
Worbyand Massom[1995]
Massomet al. [1998]
414 b
Bellingshausen,
Amundsen,and RossSeas
95ø-165øW
autumn
1992
165ø-180øE
autumn/winter
340 _+ 80
99-543
134
350 _+ 80
240-817
73
75ø-110øW
109ø-171øW
80ø-110øW,155ø-180øW
winter/spring1993
winter/spring1994
winter/spring1995
247 _+210
360 _+40
380 _+80
108-467
300-440
290-480
70ø-130øW
late summer
391 _+ 71
212-518
5øE-55øW
autumn/winter
0ø-50øW
winter
1995
1994
Jeffrieset al. [1994a]
Sturmet al. [1998]
Jeffrieset al. [1997a]
Sturmet al. [1998]
Sturmet al. [1995, 1998]
Haas et al. [1996]
210
255
45
37
Weddell Sea
1992
290
200-600
...
320
180-670
132
290 _+70
110-550
206
349 _+ 72
130-496
130
1992
0ø-55øW
spring1989
20ø-57øW
late summer
1997
V. Lytle (unpublisheddata, 2001)
Massomet al. [1997]
Eickenet al. [1994]
Haas et al. [1998]
aNumber of snowpits.
bDensity
whenadjusted
to account
for thetotalproportion
of icylayerspresent.
underestimateddue to the difficultyof samplecollection
for thesethin layers.
Generally, snow densitycan be estimatedfrom the
prevailingand recent weather conditions.For dry cold
conditions,
low-density
snow(200-300kgm-3) predominates; for warm, windy conditions,medium- to high-
density
snowpredominates
(350-500kgm-3); andfor
melt-refreeze conditions,high-densitysnow predomi-
nates(400-700kgm-3).Snowoverburden
issolowthat
0.4
,
I
•
vertical compactionand densificationof the seasonal
snowcover is likely to be very limited.
I
a) Indian Ocean
2.2.3.
0.3
Mean
=360
kg
m
'3
0,2-"
Snow wetness.
contentis typicallymeasuredusingcalorimetry,dilution,
and dielectrictechniques.Colbecket al. [1990] give a
general classificationof liquid water content: Snow is
saidto be drywhenthepercentage.of
liquidwateris0%,
moist at <3%, wet at 3-8%, very wet at 8-15%, and
slush(saturated)at > 15%. While few quantitativesnow-
0,1--
wetness measurements
0.0
'
r::r 0.4
,
'--
i
,
I
'
I
'
i
b) Weddell Sea
0.3
320
kg
m
'3
132
0.2-
0.1-
0.0
Snow wetness or free-water
0
,
i
200 400 600 8•)0
Snow Density,kg m-3
i
lOOO
have been made over Antarctic
sea ice, observationssuggestthat snowwetting due to
melt, flooding, and even rainfall (at times) is a key
factor, greatly modifyingthe snow texture, properties,
salinity, and optical and microwave characteristics
[Drinkwater,1995; Hallikainen and Winebrenner,1992;
Warren, 1982]. Although melt is most intense in late
springand summer,it occasionally
occursat other times,
includingwinter (section3.3). Floodingby seawaterincursionoccursthroughoutthe year [Ackleyet al., 1990;
Eickenet al., 1995;Jeffrieset al., 1998a;Takizawa,1984;
Wadhamset al., 1987; Worby and Massom, 1995]. In
places,the massof the relativelythick snowcover(compared with the seaice) depresses
the ice surfacebelow
sea level. Seawater infiltration
then occurs where cracks
or brine-drainagechannelsinterconnectthe ocean and
ice surface, saturating the basal snow. The resultant
Figure 8. Typical frequency distributionsof snow density slushbecomesmeteoric ice or "snow ice" on freezing
measuredin snowpitsfrom twowintercruisesin (a) the Indian (seesection3.2). Floodingalsoseedsthe snow/iceinterOcean [after Massomet al., 1998] and (b) the Weddell Sea face with nutrients and algae, which form productive
"infiltration" algal communities[Ackleyand Sullivan,
[after Massomet al., 1997].
Massom et al'
39, 3 /REVIEWS OF GEOPHYSICS
SNOW
ON ANTARCTIC
SEA ICE ß 423
TABLE 7. Percentage
of SnowPits and Drilled HolesContainingSlushand NegativeFreeboards(zs) and the Proportion
of Snow Ice Measured in Sea-Ice Core Analysesa
% Pits With
Region
136ø-138øE,
Season,
Year
early spring 1991
Slush(n)
% zf -< 0 m (n)
MeanZs/Z
i
% SnowIce
Indian and WesternPacific Oceans
......
0.37 (7)
Reference
30
Worbyand Massom[1995]
0.10(39)
18
Worbyand Massom[1995]
23
Worbyand Massom[1995]
63ø-65øS
64ø-106øE,
late spring1991
59ø-69øS
spring1992
ß..
39 (23)
0.07 (67)
spring1994
40 (10)
35 (54)
0.18 (106)
139ø-150øE,
autumn 1993
53 (17)
11 (134)
0.09 (186)
64 ø-66 øS
138ø-142øE
winter
29 (46)
18 (577)
0.19 (584)
165ø-180øW
autumn/winter
62ø-102øE,
60ø-69øS
75ø-150øE,
Worbyand Massom[1995]
59ø-69øS
1995
9
Worbyand Massom[1995]
Massomet al. [1998]
winter/spring1995
Bellingshausen,
Amundsen,and RossSeas
36 (73)
29 (3671)
0.21/0.31
25(15/38)
b Jeffries
andAdolphs[1997];
Sturmet al. [1998];
Jeffrieset al. [1998b]
0 (43)
17 (3253)
0.18 (3253) 12(7/25)b Jeffries
et al. [2001];
Morris and Jeffries[2001]
?
Sturmet al. [1998]
51 (45)
34 (4025)
0.32 (4176)
75ø-110øW
109ø-171øW
winter/spring1993
winter/spring1994
35
46 (165)
18
51 (2227)
70ø-130øW
late summer1994
12 (170)
17 (170)
0ø-55øW
winter/spring1989
7øE-5øW,
winter 1986
56ø-70øS
0ø-50øW
winter
46ø-54øW,
spring1988
59ø-64øS
45ø-55øW
summer
1995
175øE-175øW autumn/winter1998
80ø-110øW,
155ø-180øW
0.29
0.35
24
38
Jeffrieset al. [1997b]
Sturmet al. [1998];
Jeffrieset al. [1998a]
Haas et al. [1996]
Weddell Sea
1992
1992
50
ß..
ß..
40
0.27 (5339)
17
0.18 (4238)
...
Ackleyet al. [1990];
Lange et al. [1990]
Wadhamset al. [1987]
0.14 (386)
0.23 (1553)
...
..-
Massomet al. [1997]
Eickenet al. [1994]
11.5(468)
15
30-50
8
Drinkwaterand Lytle [1997]
(-100 holes)
aEickenet al. [1994] estimatedthat snowcontributed-8% to the total sea-icemassin the Weddell Sea,basedon resultsfrom severalfield
experiments.Numbersof measurements
(pits and/or holes)are in parentheses.
The ratio of snowto ice thicknessis alsogiven.
bThefirstfigureisfor theinnerpack,andthesecond
is for theouterpack(seetext).
1994]. The effect of this snow overburdenis a fundamental differencebetweenthe two polar regions;such
flooding is common in the Antarctic but rare in the
Arctic (althoughit mayplaya significantrole in marginal
seassuchas the GreenlandSea).
Table 7 showshow frequentlynegativeice freeboard
(zf) wasobserved
on differentcruises;
thisvariesfrom
11 to 51%, and is paralleledin the percentageof slush
occurrence.Freeboard is typicallycloseto zero [Jeffries
et al., 1998b],possiblydue to cyclesof snow-iceformation, implyingthat little additional accumulationis necessaryto submergethe snow/iceinterface and cause
flooding[Eickenet al., 1994].Sturmet al. [1998] suggest
that there is a "self-balancing"mechanismin snow-ice
formation,wherebyif a negativefreeboardis observed,
it has to be a short-livedphenomenonin most cases,as
snow-icewill form shortly thereafter, reassertingthe
balanceand producinga zero freeboard.
Other processesare responsiblefor flooding, as depicted in Figure 9. In the outer pack, or marginal ice
zone, wave-ice interaction can fragment existingfloes
and can tilt smaller
floes to introduce
seawater
onto
their surfaces,removingmuch of the snowcover.Saline
ponds,causedby the fracturingof, and rubble buildup
on, small floes and subsequentwave overwashingor
flooding,are a commonfeature of the marginalice zone
at all timesof year [Ackleyand Sullivan,1994;Massomet
al., 1997, 1998; Wadhamset al., 1987].These "deformation ponds"(Figure 9b), which are not to be confused
with freshwater melt ponds, cover up to 20-30% of
the surface in places and occur up to 200 km in from
the ice edge [Massom et al., 1998]. In lowering the
surfacealbedo, these processesmay play a significant
role in enhancingthe rapid meltback of the Antarctic
seasonalsea-ice zone during the austral spring-summer period.
424 ß Massom et al'
SNOW
ON ANTARCTIC
SEA ICE
39, 3 / REVIEWS OF GEOPHYSICS
Figure 9. Examplesof processesresponsiblefor introducingseawater/highsalinityinto the snowcovering
Antarcticseaice: (a) frostflowers;(b) deformationponding;(c) wave-inducedice crackingand squirting•and
(d) ice depressionadjacentto pressureridges.Letter F denotesregionsof flooding,and P denotesponding.
Ridgedfloesmay at timeshavelessflooding(and lesssnow-iceformation),due to the increasedbuoyancyof
the ice masscontainedin the ridges[Jeffrieset al., 1998a].
Periodically,long-periodswellpenetratesdeeplyinto
the pack and floodsthe surfaceof the interior sea-ice
zone [Moralset al., 1998;Squireet al., 1986]. Worbyet al.
[1996a] observedthe breakup of ice by swell 300 km
south of the East Antarctic ice edge in winter. As the
swell propagated through the pack, linear fractures
openedon the crestbefore closingon the trougha few
secondslater, leadingto the "squirting"of seawateronto
the floe surface, as shown in Figure 9c. This process
reportedthat for air temperaturesbetween-5 øand 0øC,
significantfree water is often presentin the snowcover,
to a maximumof 2% just abovethe snow/iceinterface.
Miitzler et al. [1982] showedthat snowwetnessgreater
than 1% has a significanteffect on the microwaveemissivity.
flooded
Mean snow salinities, measured from the electrical con-
an estimated
20% of the surface over a merid-
ional band ---75 km wide [Massoreet al., 1999] and
causeda distinct greyingof the surface.Although the
frequencyof sucheventsis unknown,it is likely that they
periodicallyadd salt and water to the snow.Suchprocessessignificantlyalter both passive[Massoreet al.,
1999] and active[Remundet al., 2000] microwavesignatures from satellite remote sensing.
Even in the absenceof flooding,the basalsnowlayer
tends to be moist or wet [Massoreet al., 1998], due to
wickingof brine upwardthroughthe snowcover.Steffen
and DeMa•a [1996]observedthat brine in snowremains
liquid to temperaturesaslow as -15øC. Brine expulsion
onto the ice surfaceoccursduringthe initial stagesof ice
formation [Perovichand Richter-Menge,1994] and also
occasionallythroughoutthe lifetime of a floe aswarming
intermittentlyincreasesthe ice porosity [Tuckeret al.,
1992]. Garrity [1992] measured a wetnessof 2% just
abovethe slushlayer in the Weddell Sea in winter and
2.2.4. Snowsalinity. As a resultof the aboveprocesses,snow on Antarctic sea ice outside the summer
melt periodis typicallysaline[Massoreet al., 1997,1998].
ductivityof meltedsnowsamples,are presentedin Table
8. While rangesare large, bulk valuesfrom three East
Antarctic cruisesare strikinglysimilar to thoseobtained
from the Weddell, Bellingshausen,
Amundsen,and Ross
Seasin winter. In all cases,salinitiescover a wide range
of values(0.1-91.8 practicalsalinityunits (psu)). A typical frequencydistributionis presentedin Figure 10. On
many of the voyages,particularly in the Indian and
western Pacific Ocean regions, no salt-free snow was
found.
As a resultof capillarysuctionof brine, high salinities
(> 10 psu) occurup to about0.1 m in the snowcolumn,
but mainlyin the 0.00-0.05 m layer [Massoreet al., 1997,
1998],asshownin Figure 11. Mean salinitiesof the basal
snowlayer (0.00-0.03 m abovethe sea-icesurface)are
presentedin Table 9 and are consistentlyhigh in autumn-spring,rangingfrom 15 to 24 psu.However,there
is again a high degree of horizontalvariability on small
39, 3 / REVIEWSOF GEOPHYSICS
Massomet al.: SNOW ON ANTARCTICSEAICE ß 425
TABLE 8. Volume-IntegratedAverageSnowSalinities From Various Antarctic Sea-IceExperiments
Region
Season,Year
SnowSalinity,psu
Range,psu
n
Reference
Worbyand Massom[1995]
Worbyand Massom[1995]
Indian and WestPacific Oceans
75ø-150øE
spring,1994
139ø-141øE,
autumn, 1993
8.8 _+ 13.3
8.4 _+ 13.5
0.1-66.4
0.1-47.1
41
51
138ø-142øE
winter, 1995
8.5 _+ 10.6
0.1-54.5
202
Massomet al. [1998]
75ø-110øW
165ø-180øW
175øE-175øW
109ø-171øW
autumn/winter, 1993
autumn/winter, 1995
autumn/winter, 1998
Jeffrieset al. [1997a]
Jeffriesand Morris [2001]
Jeffriesand Morris [2001]
Jeffrieset al. [1997b];
Sturmet al. [1998]
Haas et al. [1996]
144ø-150øE
Bellingshausen,
Amundsen,and RossSeas
winter/spring,1994
8.7
8.7
13.1
2.6
_+ 10.6
+_ 10.2
_+ 18.5
_+ 6.5
0.02-36.3
0.02-39.8
0.01-91.8
0.01-40.5
84
193
179
333
70øN-130øW
late summer, 1994
0.02 +_ 0.08
0.00-0.36
37
0ø-55øW
spring,1989
20ø-55øW
5øE-55øW
0ø-50øW
late summer, 1997
autumn/winter, 1992
winter, 1992
4.0
0.15 _+0.8
2.6
8.7 _+10.7
Weddell Sea
scales,withMassomet al. [1998]reportingvaluesranging
from 4.7 to 34.8 psualongone transectacrossa smooth,
unridgedfirst-yearfloe.
Particularlyhighbasalsalinities(>34 psu) can occur
due to the incorporationof "frostflowers"(Figure 9a)
into the accumulating
snowmass[Massomet al., 1997].
Frost flowers form dense mats, a few centimeters thick,
on snow-freenew ice (nilas)by ice depositeddirectlyon
the surfacefrom the vapor phase[Tuckeret al., 1992]
and attain salinitiesas high as 110 psu [Drinkwaterand
Crocker,1988] as they absorbconcentratedbrine from
the ice surfaceby capillarysuction.They are a strong
sourceof radar backscatter,becausethey form a highly
saline,roughdielectricinterfacecomparedwith smooth
seaice [Drinkwaterand Crocker,1988].Further concen-
0.0-41.9
0.0-8.6
......
0.0-48.8
87
130
189
Eickenet al. [1994]
Haas et al. [1998]
Lytle and Ackley [1992]
Massomet al. [1997]
tration of brine may occurin the snowmassby sublimation, and with colder temperaturesas more solid ice
forms (and alsocrystallizationof the differentconstituent saltsat lower temperatures)[Anderson,1960].Such
processes
raisethe snowmeltingpoint,particularlyin its
lower layers.
The low "background"salinity of <1 psu above a
heightof about0.3 rn in Figure 11 is due to the input of
aerosolsaltsor seasprayfrom above,or the incorporation of snowthat has acquiredsaltwhile blowingacross
the surface of bare ice. Blowing snow collectedjust
abovethe surfacehas salinitiesrangingfrom 0.1 to 2.0
psu[Massomet al., 1998].The few datacollectedduring
summer [Haas et al., 2001; Moralsand Jeffries,2001]
indicatelow snowsalinitiesof <1.0 psu(Table 8), probably as a result of desalinationby percolatingsnow
meltwater.
I
0.5
I
I
I
I
I
0.4-
i
I
•
I
= 8.5 psu
'l
= 202
0.20.'1-
0
10
20
30
50
60
Salinity,psu
Figure 10. A frequencydistributionof snowsalinityfrom the
Indian Ocean, August 1995. Typical frequencydistributions
(outsidesummer)are weaklybimodal,with a dominantlowsalinitypeak tailing off to secondarypeaksat 20.0-30.0 psu,
corresponding
to slushand dampenedbasalsnow.After Massom et al. [1998].
2.2.5. Effective thermal conductivity of snow.
Calculationsof conductiveheat flux through the snow,
which are essentialin the modeling of sea-ice growth
and decay, depend not only on V T but also on the
chosen value of the snow thermal conductivity.The
latter is definedas the proportionalit3,.
constantbetween
V T and the total heat transportacrossthe snowand is
generallyexpressedas "effective"thermal conductivity
to accountfor all of the processesin the snowpack,
includingconductionand vapor diffusion,but without
airflow[Sturmand Johnson,1992;Yen, 1981].
On small scales,the effective thermal conductivity
acrossthe layeredsnowmediumis determinednot only
by the snowtemperaturebut alsoby its texturalcharacteristics. Various
derivations
of snow effective
thermal
conductivityfor individual layers containingpredominantly one snow type, and defined here as keff, are
reviewedby Sturmet al. [1997].Sturmet al. [1998] took
direct measurementsof keff for dry snowsamplesin the
Amundsenand RossSeasusinga needleprobe appara-
426 ß Massom et al.: SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWS OF GEOPHYSICS
6O
a) Indian Ocean
50.
Winter
ß
b) Weddell Sea
1995
Winter
1992
[]
• 3O
•020
[]
lO
[]
[]
I;'l..="!
o
0.0
0,1
i , 0.4
i , 0.15
0'.2 , 0.3
i
'
i
0.60.0
.
ß I. I-.'
I
'
0.1
i
0.2
. I'.
'
I
ii
ß
0.3
i
0.4
ß
l_ ß
'
ß
i
i
,
0.5
0.6
Height Above Sea-Ice Surface, m
Figure 11. Snow salinityas a functionof height in the snowcolumnabovethe ice surface,for (a) East
Antarctica,August1995 [afterMassomet al., 1998],and (b) the Weddell Sea,June-July1992[afterMassom
et al., 1997].
tus [Sturm and Johnson,1992], with tests designedto
precludethermally driven convectionand radiation effects. Analysis of these samplesshowsthat snow on
Antarctic sea ice exhibitsa wide range of valuesfor keff
(Table 10), dependingnot onlyon density(Ps) but also
on snow microstructure,i.e., grain size, porosity,grain
type, and bonding.Sturm et al. [1997] concludedthat
relationshipsbetweenthermal conductivityand density
only occur because(complex)relationshipsexist between densityand microstructure.Althoughkeff showsa
strong density-dependencefor snow composed of
rounded grains or windblownfragments,there is little
density-dependence
for snowcomposedof depthhoar or
faceted grains.
Recentwork by Massomet al. [1998]and Sturmet al.
[1998] has usedthis additionaldetailedinformationon
keff as a functionof snowtype (texture) to derivenew
valuesfor the bulk effectivethermalconductivity
(kbulk)
of snow on Antarctic sea ice. Whereas keff refers to
individuallayersof snow,kbulkrefers to the sum of the
measurementsfor all of these layers,usingthe proporTABLE 9.
tions of different snow types observedduring entire
cruises(e.g.,asgivenin Table 3). Resultantvaluesusing
datafrom four field experimentsto varioussectorsof the
Antarctic pack are 0.112 (autumn-winter), 0.124
(spring),0.138(winter-spring),0.141(winter),and0.164
W m-1 K-1 (winter).Thesevaluesare significantly
lower than thosetypicallyassumedin models,for exam-
ple, -0.31 W m-1 K-1 givenby Hibler [1979]and
Maykut [1986]. Other workers [e.g.,Jordan,1991; Yen,
1981] calculatesnowkbulkas a function of density.For
comparison,Brandt and Warren [1997] measuredkeff
values of -0.18-0.35
W m -• K -• for continental Ant-
arctic snow,with kbulkincreasingwith depth. They suggest that the lower values are associatedwith a newer
snowcoverthat is significantly
lesssintered(i.e., hasless
intergrainbonding)than the compactedlayersat depth.
The discrepancybetweenmeasuredvaluesand those
typicallyusedin modelsalso highlightsthe great difference in the scalesinvolved, i.e., the small scale of mea-
surements(basicallyhand-specimensize) versusthe
much larger scaleof the model domain (tens to hun-
Mean Salinities of the Basal Snow Layer (0.00-0.03 m Above the Sea-Ice Surface)
Region
Season,Year
Salinity,psu
Range,psu
n
Reference
WesternPacific Ocean
Worbyand Massom[1995]
Massomet al. [1998]
139ø-141øE, 144ø-150øE
autumn 1993
23.7
1.8-46.5
11
138ø-142øE
winter
17.0
0.1-54.4
76
165ø-180øW
165ø-180øW
175øE-175øW
autumn/winter 1995
autumn/winter 1995
autumn/winter 1998
20.7 _+ 10.8
15.8 _+ 14.3 a
22.4 -+-20.8 a
0.1-49.0
0.1-39.1
0.7-91.8
61
80
93
109ø-171øW
80ø-110øW,155ø-180øW
75ø-110øW
winter/spring1994
winter/spring1995
winter/spring1993
13.3 _+8.0
16.6 _+10.9
15.7
1.0-41.0
0.0-47.0
2.0-29.4
18
47
34
Sturmet al. [1998]
Jeffriesand Morris [2001]
Jeffriesand Morris [2001]
Sturmet al. [1998]
Sturmet al. [1998]
Jeffrieset al. [1994a]
0ø-50øW
winter
0.5-47.8
55
Massomet al. [1997]
1995
Bellingshausen,
Amundsen,and Ross Seas
aMeasurements
taken 0-0.05
1992
rn above the sea-ice surface.
Weddell Sea
16.2
39, 3 / REVIEWSOF GEOPHYSICS
Massom et al.: SNOW ON ANTARCTIC SEA ICE ß 427
TABLE 10. Average Effective Thermal Conductivities of Snow Types, Including 173 Antarctic Measurements Made by
Sturm et al. [1998] a
Snow
Type
ICSGClass
b
n
Snow
Density,
kgm-3
kef•Wm-• K-•
Range
inkef
• Wm-• K- •
New snow
la-le
10
133 +_ 53
0.070
Recent snow
2a, 2b
21
254 _+68
0.128
0.056-0.200
3a
3b
51
9
320 +_81
345 _+36
0.169
0.163
0.051-0.632
0.083-0.278
Small roundedgrains
Larger roundedgrains
Mixed forms
0.039-0.117
3c, 4c
26
321 ___
29
0.153
0.099-0.218
5b
171
225 _+55
0.072
0.021-0.142
Depth hoar, indurated
Clusteredroundedgrains
Refrozen wet polygrains
5a-5c
6a
6b
9
1
20
345 _+73
290
422 +_58
0.183
0.188
0.250
0.062-0.330
...
0.095-0.445
Soft to moderate wind slab
Hard wind slab
Hard drift snow
3a, 9d
3a, 9d
3a, 9d
50
16
77
348 ___
33
379 _+45
440 _+34
0.167
0.237
0.355
0.061-0.281
0.141-0.316
0.150-0.541
Very hard wind slab
3a, 9d
22
488 _+50
0.452
0.240-0.654
Depth hoar in chains,
weak
aRanges
andaverage
snowdensities
arealsogiven.SeealsoSturmetal. [1997].The thermalconductivity
of seaiceapproximates
2.20W m-•
K-• [Untersteiner,
1961].After Sturmet al. [1998].
bColbeck
etal. [1990].
dreds of kilometers).The term keff when used on the
differentscaleshasa complexandwide rangeof physical
meanings,whichare beyondthe scopeof thispaper,but
is likely to be responsiblefor some if not all of the
mismatch
the estimatesgivenin the 1993 table. The dramaticrise
in albedo causedby addinga thin snowlayer, of only a
few millimeters, to thick sea ice is discussed and ex-
plainedby Warrenet al. [1997].
in values.
2.2.6. Albedo of Antarctic sea ice. Sunlight is
scatteredby bubblesand brine pocketsin seaice [Grenfell, 1983]. The albedo of bare sea ice is determined
mainly by its thickness,with albedo increasingwith
thickness[Perovich,1998]. However, in the Antarctic,
the sea ice usuallyacquiresa snowcover within a few
daysafter it forms. Thereafter its albedo is determined
almost entirely by the snow properties, primarily the
snowdepthbut alsothe grain sizeandwetness[Brandtet
al., 1999]. In the Ross Sea, for example,Zhou et al.
[2001]foundthat the total albedoof the snowon seaice
decreasedbetweenthe continentand the ice edge,i.e.,
3.
THE IMPORTANCE
OF SNOW
ON
ANTARCTIC
SEA ICE
3.1.
Snow Thickness Distribution
3.1.1. Small-scale variability. On decameter
scales,significantthickeningoccurson rough (ridged)
ice through the formation of sastrugi[Massomet al.,
-1
1997, 1998].Formedwhenwind speedsexceed-7 rn s
[Gow, 1965], sastrugi(Figure 12a) contain snow 2-5
timesthickerthan on adjacent,wind-scouredice [Eicken
from south to north, due to an increase in snow wetness
and grain size.
Measurementswere made on two East Antarcticvoy- TABLE 11. Broadband Solar Albedos for Cold Antarctic
ages (in October-November,1988 and 1996) of the Sea-Ice Types, From Brandt et al. [1999] a
spectralandbroadbandsolarirradianceincidenton, and
Thin Snow,
Thick
reflected by, the various types of ice encountered in
Continuous,
Snow,
springtime,using a spectral radiometer covering the
<3 cm
>3 cm
Ice Type
Snow-Free
wavelengthrange 320-1060 nm with 11 filters and a
broadbandpyranometer(0.3-2.8 Ixm). Most measure- Open water
0.07
......
ments were made within
2 hours of solar noon in order
Grease ice
to experiencethe maximumsolaraltitude.Surfacetypes Nilas,<0.1 m
Young greyice, 0.1-0.15 m
sampledwere water, greaseice, pancakes,nilas, snowYounggrey-whiteice, 0.15coverednilas,snow-freefirst-yearice, and snow-covered
0.30 m
first-yearice.
First-yearice, 0.3-0.7 m
0.09
......
0.16b
0.25
(0.35)
0.42
(0.52)
(0.62)
...
(0.70)
(0.74)
(0.42)
(0.72)
0.77
0.49
0.81
0.85
The resultsof broadbandsolar albedos,coveringthe First-yearice, >0.7 m
wavelengthrange0.3-2.8 Ixm,are summarizedin Table
aValuesin parenthesesare interpolatedor extrapolated,not mea11. This is an updatedversionof the table publishedby sured.Ice with patchysnowcoverhasan albedointermediatebetween
Allison et al. [1993],which had been basedon the 1988 snow-freeice and ice with thin snowcover,weightedby the fractional
area of snowpatches.
experimentonly. The most important differenceis that
bNilasalbedovarieswithicethickness
from0.07to 0.25;0.16is the
the albedosin the "thin snow"categoryare higherthan middle of the range.
428 ß Massom et al.: SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWSOF GEOPHYSICS
Figure 12. Significantlocalizedsnowthickeningoccursin the form of (a) sastrugiforming in the lee of
surfaceroughness
featuressuchaspressureridges,and (b) barchandunes,Indian Oceanin August1995.Note
the crossbeddingin Figure 12a, due to floe rotation and changesin prevailingwind direction.
et al., 1995;Massomet al., 1997] and can affect more
than 50% of the surfacearea of ridgedfloes[Massomet
al., 1998].On largeflatter floeswhere the areal extentof
ridging is less than 5%, low barchan dunes dominate
(Figure 12b).Typicallycovering40-50% of the surface,
thesedunesincreasesnowthickness(%) by a factor of
1.5-3 [Massomet al., 1997, 1998]. Such thickeningcan
significantlyaffect the isostasyof newlyrefrozenlead ice
[Massomet al., 1998].Decimeter-scale
point aerodynamic
roughnessfeatures,suchas weatheredclumpsof surface
hoarfrostcrystals(initiallyformedunder clear,cold,and
calmconditions
[Langetal., 1984])or frostflowers,mayact
asinitialfocifor low duneformation[Massometal., 1997].
Initially migratory,dunes become semipermanentwith
time as icy crustsform on their surfaces.
As a resultof theseprocesses,
zscanvaryasmuchacross
a singlefloe asit doesbetweenfloesanddifferenticetypes.
On the meter to decameterscale,good correlationbe-
tweensnowandice thickness
(zi) occursonlyin regionsof
new andyoungice,asnotedin the easternWeddellSeaby
Massometal. [1997]earlyin the growthseason.In general,
theyfoundthat the correlationbetweenz• andzi decreased
with increasing
zi. The degreeof scatterin the relationship
betweenz• andzi increases
with the amountof ridging(ice
deformation)[Eickenet al., 1994].
3.1.2. Snow-surface
roughness. Usingsemivariogram analysis,Sturm et al. [1998] reported lengthsof
snow-surface
featuresin the Bellingshausen,
Amundsen,
and Ross Seas ranging from 3 to 70 m, with average
values of 12.7 m in August-September1995, 22 m in
September-October 1994, and 23.3 m in May-June
1995. This comparedwith a range of structure lengths
for the ice surfaceof ---4-6 m. The averagesnowstructural amplitude was, at <0.1 m, small, but larger than
that of the underlyingice (--•0.05m). Thus the addition
of snow increasesthe topographicrelief but tends to
39, 3 / REVIEWSOF GEOPHYSICS
Massom et al.: SNOW ON ANTARCTIC SEA ICE ß 429
"smoothout" the higher-frequency(i.e., rougher) ice
surface.Sturmet al. [1998]further concludedthat anyice
surfaceroughnessfeature with a relief >0.2 m "nucleated" a significantsnow surfacefeature. Similar work
was carried out in the Weddell Sea by Andreas et al.
[1993]. In the Ross Sea, Tin and Jeffries[2001] found
significantcorrelationsbetween mean snow elevation
and meanzi and betweensnow-surface
roughnessandzi
U.,IP_ '.
'.
:
:
:
:
:
:
:
." "•
...>/,::"
'".....
'...;
..D'.•'
i'•....
•i ii W;•g',i?
....
...::../"/'").....
';.. "". ":'..'"
i Sba..';..'"?'".. ..'"'
.'/i/....
...................
in the Ross Sea.
The aerodynamicsmoothingeffect of snowtends to
reduce the surface drag coefficient of deformed ice
[Andreaset al., 1993];for a givenwind speed,the drag
coefficientis used to determine the atmosphericstress
on the ice surface in order to compute ice motion.
Andreasand Claffey[1995] andAndreas[1995] showthe
surfacedragcoefficientto be criticallydependenton the
alignmentof the dominantsnowdriftpattern relative to
the mean wind direction.
Where
wind direction
:
';!
1:3
WhiteIce
i:./i
"..'
. . .iii
..'"'
i :..' ..'.."..".."
....
I/.•'::..•
0 Medium
First-Year
Ice'..
'..".'. '.j:.:--:.
•-:-:..:.:
..'.' ..'..'
". ",}'-;'"
'"'," ." ."
ß FastIceFloes
"..'
'""'
II ',.'
"'•"..'•J,
'-, '--,'•-•.'
I '"'" "'
0 Second-Year
Ice
1.5
1.0
is rela-
tivelyconstant,they estimatethat the formationof snowdrifts aerodynamicallystreamlinesthe surface and can
lead to a 30% decreasein drag coefficientin 12 hours.
Conversely,a significantshift in wind direction or floe
rotationcanlead to drift erosion(sastrugidevelopment)
and a rapid increase in the drag coefficient before
streamliningonce againoccurs.Sastrugican also significantly alter the bidirectionalreflectancepattern of the
surfacebut causeonly a slightreductionin albedo[Warren et al., 1998].
3.1.3. Patternsof large-scalevariability. In this
sectionwe examineregional and seasonalvariability in
snow thickness distribution.
0.5
..
0.0
• 0.5
•
1.0
•'• 1.5
2.0
2.5
- Northwest
3.0
3,900
....
I
....
3,400
Southeast
I
.
2,900
.
.
,
I
,
,
2,400
,
,
I
,
1,900
,
,
,
1,500
Distance Along Track, km
Figure 13. (a) Map showingthe cruise track of R/V Po-
3.1.3.1. Regional variability. Snow thicknesseslarstern,June-August1992.(b) Mean ice andsnowthicknesses
appearto be greatestin the Bellingshausen,
Amundsen,
and RossSea sectors(Table 1), due to higherprecipitationratesin theseregions(seesection3.5). Moreover,
a large contrastin Zs existsbetween the SE and NW
Weddell Seain winter [Langeand Eicken, 1991;Massom
et al., 1997] (Figure 13). Thick snow to the west of
--•35øW and north of--•65øS occurs in the following
western limb of the Weddell Gyre [Massom, 1992],
which containsa significantproportion of second-year
and multiyearice [Drinkwateret al., 1993].In the eastern
Ross Sea in summer(January-February),Zs increases
from west to east [Morrisand Jeffries,2001].
A significantthickeningof snow (and ice) inward
from the ice edge has been noted in East Antarctica
[Allisonand Worby,1994]and in the Bellingshausen
and
AmundsenSeasin late winter [Worbyet al., 1996b].The
thinner snowcoverin the outer packmaybe relatednot
so muchto lower precipitationratesbut rather to other
from samplingsitesalong the cruisetrack, with locationindicatorsA-D as shownin Figure 13a. The curvesare interpolated throughthe mean data points.After DrinkwaterandHaas
[1994].
ice) occurredin a 200-km-widecoastalzone, with the
thickestoccurringin a continentalshelf zone 200-600
km from the coast.In both cases,observedpatternsmay
also result from snow-ice formation, as discussedbelow
(section3.2).
3.1.3.2. Seasonal variability. Monthly snowthicknessdistributionsfor East Antarctica were presented by Worbyet al. [1998], based on 10 years of
ship-basedobservationsfor the months March-April
and August-December(Figure 14). The mean snow
thickness varies between 0.11 m in March
and 0.17 m in
both Septemberand December.Thesedata showthat in
factors such as the removal of the snow cover from older
March in East Antarctica, less than 10% of the snow
ice by wave and swellaction.Moreover, the snowin the coveris thicker than 0.1 m, with approximately20% of
outerpackis thinner becauseit hasnot had asmuchtime the pack comprisinglargely snow-freeice. This is conto accumulatecomparedwith the inner pack;that is, the sistentwith rapid new ice growthat thistime of year. By
April, approximately30% of the snowcover is thicker
ice in the outer pack is younger.
Work in the western Ross Sea in May-June 1995 than 0.1 m, althoughmostof this is in the range 0.1-0.2
[Jeffries
andAdolphs,1997] and May-June 1998 [Morris m. There is a lower percentageof snow-freeice than in
and Jeffries,2001] showedthat the thinnestsnow(and March as the ice cover becomes more consolidated and
430 ß Massom et al' SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWSOF GEOPHYSICS
(a)
(b)
0.4
March
March i
0.4
0.2
0.2
.
0.0.
0.6
0.6
0.4
.
April
0.2
0.0
.
........................
...........
0.2
..........
0.0 .......
0.4
August -:
0.4
,
0.2
0.0
0.4
,-
.....
'........
,
:
,
,
....
i
....
]-..........................
..............
0.4
0.2
0.2
•
0.0 ..........
0.6
............................................................................................
October
October
0.4
0.4
..........0.2
0.2
0.0
........
0.0
0.6
0.6
November
• 0.4
0.4
>
o
.......
'........
,
.............................................................................................
.:
Nøvember
i
:
:
0.2
0.2
..
:
0.0 ..........
............................................................................................
0.6
December
•
...........................
......
0.4
0.4......
_
............
'
0.2
0.2
0.0•
0.0 .........
6&
4•
•
o
•
•
i
0
i
i
i
OJ •:• q:) •
0
OJ
Figure 14. Monthly thicknessdistributions(in meters)from ship-based
observations
in East Antarcticafor
(a) seaice and(b) snow.The majorityof the datahavebeencollectedduringspring.From Worbyet al. [1998].
beginsto accumulatea snowcover.By winter (August),
there is a strongpeak in the 0.1-0.2 m snowthickness
category.The fractionof snow-freeice is about 10% and
remainsfairly constantthroughoutspringas leadsopen
within the pack and new ice is formed. The changing
snowthicknessdistributionfrom Augustto Decemberis
characterizedby a shift in the modal thicknesstoward
thinner snowcategoriesand a decreasein the coverage
of thicker snow.This is partly due to the increasein the
fractionof openwater within the pack,whichis why the
monthly-mean snow thicknessvalues are not significantlydifferent throughoutthe spring.The long tails in
39, 3 / REVIEWSOF GEOPHYSICS
Massom et al.: SNOW ON ANTARCTIC SEA ICE ß 431
the thickness distribution
curves reflect the snow cover
high net upwardheat fluxesand bringingnutrientsand
on very thick floes, not snowdriftsaround ridges, as biota to the snow/iceinterface to support an ice algal
thesewere not includedin the ship-basedobservations. bloom [Fritsenet al., 1994]. Moreover, Lytle and Ackley
In contrast to the winter, thicker snow is encountered [2001] suggestthat in the Weddell Sea, extensiveice
in summerin the SE comparedwith the NW Weddell basal melt due to the ocean heat flux [McPheeet al.,
Sea(at leastin 1997),asshownby Haas et al. [2001]and 1996] is largelybalancedby continualsurfacesnow-ice
reflectedin Table 1. This is likely the result of enhanced growth. Such a mechanismhas significantimplications
meltingin the high-latitudenorthwestrather than of less for the ocean freshwaterbudget, as it transportssnow
precipitation.Haas et al. [1996] reported snow thick- vertically downwardthrough the sea ice to melt at the
nessesto be lower in the centralBellingshausen
Seathan base.Similarprocesses
havebeen observedin the Pacific
farther south. It is not known whether this results from
Ocean sectorby Jeffrieset al. [2001].
enhancedmelting or whether floes to the north were
3.2.1. Spatialand seasonalvariabilityin flooding
simply younger (multiyear ice was observedin the and snow-ice occurrence. Considerablevariability
AmundsenSea at the time (February,1994)). One as- has been noted in the seasonaloccurrenceof flooding
pect of melting snowin theseregionsis that it contrib- and concomitant snow-ice formation. Results shown in
utesto the formationof "superimposed
ice" rather than Table 7 suggestthat snow-iceformation is seasonally
simplybeing removedas meltwater(see section3.2.2). dependent,with a lower percentageobservedearly in
Another exampleof relativelythick snowoccurrencein the growth seasoncomparedwith spring [Worbyand
summeris describedin the easternRossSea [Morrisand Massom,1995].Sturmet al. [1998] alsonote an increase
in snow-ice thickness from autumn to winter, correJeffries,2001].
spondingto the increasein snowaccumulation;however,
3.2.
The Contribution
of Snow to Sea-Ice Formation
A key role of snowin Antarcticseaice, and onewhich
setsit apart from snow-seaice interactionsin the Arctic
[Jeffheset al., 1998a],is the widespreadcontributionit
makesto seawaterfloodingand the subsequentthermodynamicthickeningof the ice from aboveby snow-ice
formation. Snowice is distinguishable
from other forms
of granular ice (e.g., frazil ice) by its oxygenisotope
signature
(•80) andto someextent(although
notdefinitively)by its fine-grainedstructure[GowandEpstein,
1972;Eicken, 1998;Jeffrieset al., 1998b].
While floodingis a widespreadphenomenon(section
2.2.4), snowis alsoincorporatedinto sea-iceformation
by other processes.These includethe collapseand refreezing of unconsolidatedridges,rafting of snow-covered floes[LangeandHubberten,1992],and the freezing
of brashice (sea ice pulverizedby wave action in the
outer pack). By such mechanisms,it is possiblefor
snow-icelayersto form at any depth within a floe.
Mean regional proportionsof snow ice calculated
from ice coresvary from 8 to 38% (Table 7). A direct
comparisonof theseresultsis not possibledue to ambiguities in the values presentedby different authors,
which represent either an averagepercentageof the
compositionof individualcoresor a percentageof the
total length of core sampled[Jeffries,1997]. Most authors are not specificabout which value they are presenting;in spiteof this,however,it is clearthat snow-ice
formation is an important processin the formation of
Antarctic sea ice and a significantcontributor to the
total massbudget of the sea ice.
Other interestingfindingshave emergedfrom these
studies.For example,it hasbeen observedthat refreezing of slushand "gap layers"and the formationof snow
ice in autumn are responsiblefor strong convection
eventswithin the ice [LytleandAckley,1996].This transportswarm seawaterinto the upperice layers,producing
the ratio
of snow to snow ice was observed
to remain
nearly constant,due to snow-iceformation. The spatial
variability of snow-iceoccurrencein the Ross Sea in
autumn, when there is a small amount of snow ice in the
inner pack ice and 2-4 times as much in the outer pack
ice, is similar to the seasonalvariability observedelsewhere in the Antarctic pack [Jeffheset al., 2001].
Increasesin surfacefloodingobservedin summerby
Drinkwaterand Lytle [1997] and Jeffrieset al. [1998b,
2001] are probablydue to basalice melt by high oceanic
heat fluxes.Working in the easternRossSea in summer,
Morris and Jeffries[2001] further observeda relatively
thick snow cover (comparedwith early winter). Both
allowthe snowload to depressthe ice, and coincidewith
an increasein ice porosity.Sea-iceporosityand permeability are determinedby its liquid-brinevolume,which
is in turn a functionof temperatureand salinity[Coxand
Weeks,1975]. In the absenceof cracking,Crockerand
Wadhams[1989] suggestthat brine drainage channels
interconnect
between
the ocean
and ice surface when
the ice temperatureexceeds-8øC. Goldenet al. [1998]
further suggestthat seaice becomespermeablewhen its
temperatureexceedsabout -5øC for a salinityof 5 psu,
allowing brine carrying heat and nutrients to move
throughthe ice. It can be seenfrom Table 12 that these
temperature thresholdsare often exceeded.Numerical
simulationsof floodingand snow-iceformation indicate
that upwellingthrough fracturesis more effectivethan
percolationupward throughan interconnectednetwork
of brine channelsin introducingbrine/seawaterto the
snow/iceinterface[Maksymand Jeffries,2000].
Flooding does not alwaysaccompanynegativefreeboardsin spring[Andreaset al., 1993] and winter [Massom et al., 1997, 1998]. When the ice is cold, flooding
may be due more to deformationprocesses(cracking
and depressionadjacentto ridges,and rafting) than to
percolationconduits[Eickenet al., 1994].However, cy-
432 ß Massom et al.: SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWS OF GEOPHYSICS
clical warming eventsin winter (see Figure 3) cause hoar would decreasethe conductiveheat flux by a factor
periodicwarming of the ice that is sufficientto induce of 2 for 0.15-m-thick snow under a similar VT. On the
brine percolation. High-temperature events in winter other hand, floodingreduceszs and thereforedecreases
are discussed in section 3.3.2. Further
discussion of brine
exchangeis givenby Maksymand Jeffries[2000].
The onsetof floodingis characterizedby a decreasein
the radar scatteringsignatureat high incidenceangles
during autumn [Drinkwateret al., 1998;Drinkwaterand
Lytle, 1997; Haas, 2001]. Summer melt and refreeze
(superimposed
ice formation)resultsin an increasein
area-averagedradar backscatterof 2-5 dB compared
with winter values[Drinkwaterand Lytle, 1997;Haas, in
press;Morriset al., 1998].Thesefindingsare contraryto
the snow insulation.
g.2.2. Superimposedice formation. Snow contributes directly to Antarctic sea-ice thermodynamic
growth not only by snow-iceformation but also to a
lesserextent by the formation of superimposedice. By
this process,meltwater percolatingdown through the
snow refreezes
on contact with the colder ice surface or
Sea. Similar findingshave come from East Antarctica
[Worbyet al., 1998], althoughhigh percentagesof snow
ice have been reported at times. In contrast,Jeffriesand
/ldolphs [1997] and Jeffrieset al. [1997a, 1998a, 2001]
found that snowice playsa greater role in the thermodynamicthickeningof the ice cover in the easternand
westernPacificsectorsthan doeseither frazil or congelation ice. This is attributedto higherprecipitationrates
in the easternPacificsectorcomparedwith the Weddell
Sea and East Antarctica (as discussed
in section3.5).
Latitudinal variability has also been observed in the
westernRoss Sea [Jeffn'es
and/ldolphs, 1997;Jeffn'eset
al., 2001], with the percentageof snowice encountered
in the outer pack ice regionbeingmore than doublethat
in the inner pack ice (Table 7).
Becauseof the widespreadoccurrenceof snow-ice
slushlayer to form a layer of ice with a polygonalcrystal
structure[Eicken,1998;Haas et al., in press;Jeffrieset al.,
1994b, 1997a;Morris and Jeffn'es,2001]. Superimposed
ice appearsto be a widespreadfeature of the summer
snowcoveron perennialice [Haaset al., 2001;Jeffn'es
et
al., 1994b, 1997a;Jeffriesand Morris, 2001] and is also
associated
with fast-iceregions[Kawamuraet al., 1997].
In the Bellingshausen
and AmundsenSeas,Jeffrieset al.
[1997a] reported that superimposedice comprised5%
of the ice sampledin the region.
Superimposedice formation has also been observed
in winter [Jeffries
et al., 1997b;Massomet al., 1998;Sturm
et al., 1998;WorbyandMassom,1995],onceagainshowing that significantsurfacemelt eventsoccurthroughout
the year. Although its spatialvariabilityis unknown,it
may be more prevalentcloserto the marginalice zone
where more melt-freeze cyclesoccur due to generally
warmer air temperatures.
Being rough on the centimeterscale [Massomet al.,
1998], and owing to its relatively low salinityand the
large number of gasbubblespresent[Haas et al., 2001],
superimposed
ice potentiallycontributesto a significant
increasein microwavescattering[Holt and Digby, 1985;
Ohstortand Gogineni,1985]. Haas [2001] observeda
pronouncedseasonalcyclein satelliteradar backscatter
from Antarctic sea ice, with higher values in summer
than in winter due to extensivesuperimposed-ice
formation on perennial ice in summer.It may also affect the
ice optical and mechanicalproperties and inhibit the
formation, the snow thicknessdata described in section
influx of brine from below, as it forms a consolidated,
the decrease in radar backscatter
observed in the Arctic
summer[Winebrenner
et al., 1998].Floodingmay alsobe
responsiblefor maskingthe passive-microwave
signature
of Antarctic multiyear ice [Comisoet al., 1992].
Significantregionalvariabilityhasalsobeen observed
in the
contribution
of snow
ice to the
sea-ice
mass
balance[Jeffries
et al., 1998a].Core analysesby/lckley et
al. [1990], Eicken et al. [1994], and Lange et al. [1990]
suggestthat snow-iceformation makesonly a moderate
contribution
2.1 must
to the sea-ice mass balance in the Weddell
underestimate
the
total
snow
accumulation
impermeable low-porositylayer on top of otherwise
prior to the observationtime. In the Ross,Amundsen, "rotten" sea ice. Superimposedice formation may also
and BellingshausenSeas,for example,the snowthick- contribute to the "preservation"of the ice cover if it
ness is often less than the thickness of the snow-ice
exceedsthe rate of bottom melting.That superimposed
layers because 45-60% of the snow cover has been ice is not associated with the seasonal ice zone in sumentrained in the floes as snow ice [Sturm et al., 1998; mer may be becauseit may be flooded soon after forJeffrieset al., 1998b, 2001]. The strongcorrelationbe- mation due to bottom melting.
tween snowdepth and snow-icelayer thicknesssuggests
a meansto estimatetotal snowaccumulationfrom ship- 3.3. Snowmelt Processes
basedestimatesof snowdepth [Sturmet al., 1998;Jeffries
et al., 2001].
3.3.1. Summerconditions. Melt ponds,whichare
While snow-ice formation effectively reduces the so characteristicof the Arctic summer,are largelylackthicknessof the snow, it also enhancesthe snow'sinsu- ing in the south.Andreasand Ackley [1982] attributed
lative effect by increasingdepth-hoar formation as a this to the low relative humidity associatedwith dry
result of raisingthe snow/iceinterfacetemperatureand winds draining off the continent, persistentlystrong
steepeningthe temperaturegradientin winter [Sturmet winds causingincreasedlatent heat lossesfrom ice to
al., 1998]. Sturm et al. [1998] suggestthat a typical atmosphere,and generallybelow-freezingair temperametamorphic change from a moderate slab to depth tures, which together prevent the elevation of snow
39, 3 / REVIEWS OF GEOPHYSICS
Massom et al.: SNOW ON ANTARCTIC SEA ICE ß 433
wetnessto high levelsaroundAntarctica.As a result,the nodules.Becauseof prolongedhigh temperatures(over
snow cover remains largely intact, and the surface al- a 2- to 3-dayperiod) and sleetand evenrain, all shallow
bedo remainshigh, in regionsretaining ice during sum- areas of unconsolidatedlow-densitysnow (originally
mer, i.e., --4 x 106 km2 in Februaryin the western 0.10-0.15 m thick) melted away,leavinga layer of slush
Weddell, Amundsen,Bellingshausen,and eastern Ross on rotting sea ice with a pitted, porous grey surface
Seas[Gloersenet al., 1992].The snowcoveritself delays [Haaset al., 1992;Massomet al., 1997]but no melt ponds
or retardssummerablationfrom aboveby restrictingthe (due to drainagethroughenlargedbrine drainagechaninput of solarradiationto melt the ice [Gow and Tucker, nels). The effect on radar backscatteris discussedby
1990].
Drinkwater et al. [1995]. The only regions of snow to
Melt pondswere observedin the NW Weddell Sea survivewere hardenedbarchandunes(Figure 15b).The
and the Amundsenand BellingshausenSeasin summer effect sucheventshave on the salinity and structureof
(February) 1995; these had a significanteffect on the the underlyingseaice and oceanis unknown,althoughit
radar backscattersignatureof the ice cover [Drinkwater is likely that the downwardflux of freshwateris considet al., 1998; Drinkwaterand Liu, 2000]. These authors erable.
attribute the melt-pond formation to anomalouslyintensewarming eventsin 1994-1995. Pondswith salinitiesbelowthoseof seawaterwere alsoreportedfrom the 3.4. SnowThermalPropertiesand Thermal
NW Weddell Sea in January-February1997 by Haas et Environment
al. [2001]. Some containeddensealgal populations.
Antarctic sea-icemelt occursprimarily from below
3.4.1. Snow temperature. Becauseof its insula[AndreasandAckley,1982;Gordon,1981],assummerair tive properties,snow greatly influencesthe magnitude
temperatureslargely remain below freezing in those and directionof heat fluxesand the ice-surfacetemperareas retaining ice. Surface melt in the Weddell Sea ature. Snowtemperaturein turn regulatesthe composiduring the onset of summer occursin patchesand is tion and salinityof the liquid phase [Weeksand Ackley,
neither spatiallynor temporallycontiguousat any given 1986] and the connectivityof the pore/brine channel
time. Ephemeralmelt southof the seasonalmelt front is pathwaysin seaice [Coxand Weeks,1975]. It alsoplays
due to periodic incursionsof warm maritime air, with a central role in the structuringof sea-ice microbial
subsequent
returnsto coldweatherleadingto refreezing communities[Eicken,1992] and the passivemicrowave
[Drinkwateret al., 1998;Drinkwaterand Liu, 2000;Morris signatureof the surface[Comisoet al., 1992].
et al., 1998].The resultantextensiveicinessin the snow
In winter, the snow/iceinterfacetemperature(T i) is
cover may contributeto the observedincreasein radar typicallysignificantlywarmer than that of the snowsurbackscatterin summer[Haas,2001]. In thesestudiesan face (Ts), with the snowlargelyprotectingthe ice from
important distinction is made between seasonaland large variabilityin Tair (Table 12). While Ts is strongly
perennial ice. Covered in thick snow,the latter reacts coupledto, and approximates,Tair [Guestand Davidson,
slowestduringbrief periodsof warming.Drinkwaterand 1994], Ti is highly dependenton zs and is poorly correLiu [2000] concludethat Antarctic sea-icesurfacemelt- lated with Ts.
ing is sparseand relativelyshortlived comparedwith the
Seasonaland interannual differencesin Ts and Ti
protractedsummermelt seasonin the Arctic.
translateinto significantvariabilityin V T [Sturmet al.,
3.3.2. Extensivesurfacemelt in winter. Substan- 1998], which affectssnowtexture through its effect on
tial surfacemelt periodicallyoccursat other times of metamorphism[Colbeck,1982]. Average temperature
year, even in winter [Massomet al., 1997, 1998], as gradientsin winter (shownin Table 12) are typically
rapidly movingstormstransportwarm, moist maritime lower than the critical threshold of about -25øC m- • for
air acrossthe pack [Andreas,1985]. The snowcover, if depth-hoargrowth[Akitaya,1974;Armstrong,1980;Marthick enough,largely protectsthe underlyingice from bouty, 1980]. Sturm et al. [1998] showed that in the
melt before freezing conditionsreturn, but undergoes Amundsen,Bellingshausen,and Ross Seas,depth hoar
considerablemetamorphismresultingin an icier, coars- comprisednearly twice as much of the snowpackin
er-grainedtexture. This sequencerecurs a number of autumn than in winter, due to the higher V T acrossa
times throughouta season[Massomet al., 1997, 1998]. thinner autumn snow cover.
Lessfrequently,andwhere hightemperaturespersist,
3.4.2. Conductiveheat flux through the snow.
the snowcovermay be removedaltogetherby melt. In Usingthe newlowerkbulk
valueof 0.164W m-• K-•
the northwesternWeddell Sea in July 1992, an increase givenin section2.2.5,Massomet al. [1998]examinedthe
in Tair from -22øC to 0øCovera 12-hourperiod(Figure temporalvariabilityof the conductiveheat flux (Fa) by
15a) was accompaniedby significantincreasesin wind measuringVT over a 16-day period in August 1995
speedand relative humidity,with a low, longwaveradia- (Figure16). Rapid andlargechanges
of rair while Ti was
tion-trapping cloud base (also observed by Oelke much steadier resulted in large heat flux changes
[1997]). This led to a reversalof the sensibleheat flux, througha snowcoverthat varied in thicknessfrom 0.22
melt, and condensationat the surface[Drinkwater,1995], to 0.11 m (due to aeolianerosion).Suchtemporalvariwhich subsequently
refroze to form centimeter-scale
icy ations are similar to near-contemporaryspatial varia-
434 ß Massom et al.' SNOW ON ANTARCTIC SEA ICE
39, 3 / REVIEWS OF GEOPHYSICS
Snowfall
5
a) o
ø
'
' [..I i •..'•
-õ
E
-30
....... ,........................•
375
25Directionf•,•
•
325 •
0
20
275
"'"
15
225 •
10
175
5
c:
125
12
0
1020
10
.c: 1010-- !
•
:3
w
\Pressure
"
',
!lB
1000
Q. 990
Cloud
c),.,,,,,
<• 980 .,
-,
0
21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0
Time, Days in July,1992
Figure 15. (a) Near-surfacemeteorological
variablesmeasuredfrom R/V Polarstemwhile driftingwith the
pack ice in the NW Weddell Sea (at -61øS, 43øW), July 21-25, 1992. After Massomet al. [1997]. (b) A
photograph,taken on July 25, 1992, showingthe effectsof the catastrophicsnowmeltassociatedwith the
significantsynoptic-scale
temperatureincreasedue to warm air advection,shownin Figure 15a.The surviving
barchandune in the foregroundis -5 m across.
39, 3 / REVIEWS OF GEOPHYSICS
Massom et al.' SNOW ON ANTARCTIC SEA ICE ß 435
tions observedacrosssinglefloes as a result of variable
zs [Massomet al., 1998].
Sturmet al. [1998] observedlocal variationsin F a to
be greater in autumn than in winter and concludedthat
suchvariabilitycannotbe predictedasa simplefunction
of Zs, as snowstratigraphydoesnot scalewith Zs. On a
regional scale they derived mean values for entire
cruises
ranging
from-2.9 _+5.0and-6.1 _+5.3W m-2
inwinterto -9.2 _+9.9W m-2 in autumn.
Theyattribute
these differences
to the combined
effect
of seasonal
differencesin mean valuesof Ts, kbulk
, •7T, and Zs, as
+++
++
++
o +
0 0 0
0 0
0 0
•
II
II
II
+1• +1
+1 +1
+1+1
ß
+1 +1
I
noted
earlier.
Recent large-scalemodeling studies by Wu et al.
[1999] imply that the use of correctkbulkvaluesis as
crucialto the model performanceas the presenceof a
snow layer and the value of the snowfall rate. They
investigatedthe effects of variationsin snow thickness
and the magnitudeof the thermal conductivityon accretion/ablationin a coupledatmosphere-seaice model.
Usingthelowervalueof 0.164W m-• K-• [afterMassometal., 1998]instead
of 0.31W m-• K-• changed
the
modeledsea-icethicknessdistributionsignificantly,
with
the ice becomingthinner on averageby about 0.2 rn in
the Antarctic(and about0.4 rn in the Arctic). A similar
experimentconductedby Fichefetet al. [2000] with a
global, coupled ocean-sea ice model suggeststhat
changesin the oceanic heat flux at the ice base could
reducethis thinning to 0.10 rn in the Antarctic.
3.5. Precipitation
Precipitationand accumulationare difficult to measure in situ on sea ice becauseof strongwinds and the
formation
t'•
o• +1
oo
+1 +1
of snow ice. Variables
obtained
from
atmo-
sphericanalysismodelsmay be used to calculatemoisture fluxes,with the residualprovidingan estimateof the
net surfacemoisturebalance,the equivalentof precipitation minus evaporation(P-E) or accumulation.This
procedureprovidesa larger spatialand temporalcoverage than direct measurementsand hasbeen successfully
appliedto the calculationof snowaccumulationoverthe
Antarctic continent [Budd et al., 1995; Cullatheret al.,
1998]. Owing to the relative coarsenessof the data,
resultsobtainedfrom atmosphericparametersare difficult to comparewith directmeasurements,
as the latter
are spatially and temporally limited. Moreover, those
measurements
that do existoften do not representthe
total snow accumulation
due to snow "loss" when snow
iceforms(seesection3.2.1).The magnitudeof snowloss
directlyinto the oceanvia leads[Eickenet al., 1994] and
windblownsublimation[D•ry et al., 1998;Pomeroyand
Gray, 1994] is alsolargelyunknownover sea ice.
A typicalaccumulationdistributionis shownin Figure
17, where 39 months of Global Assimilation and Predic-
tion (GASP) data were used to calculatehorizontal
moisturefluxes[Buddet al., 1995;ReidandBudd, 1995].
Standingwavesof cyclonicactivitycan be seen off the
coastof the continent,particularlyin the easternlongitudes. Here areas of high accumulationare associated
436 ß Massom et al.: SNOW ON ANTARCTIC SEA ICE
0
I
39, 3 / REVIEWSOF GEOPHYSICS
I
-10
i
i
i
i
I
"' "'""\
-
-15
-25
-30
-35
5
'
o
-lO
-2o
-25
4
8
12
16
20
Time, Days in August, 1995
Figure 16. (a) The relationshipbetweenTair and Ti derivedfrom hourly thermistormeasurements
on a
singlefirst-yearfloe. (b) Vertical conductive
heat flux in the snow.Negativeimpliesa net conductiveheatloss,
and positiveimpliesa net gain.After Massomet al. [1998].
with air massesflowing south toward the coast, with becomes thicker in winter over most of the Antarctic
outflowproducingdrier conditions.Figure 18 showsthe sea-icezone(Figure19). For "no-snow,"the decreasein
annualcycleof zonal mean net accumulationfor several thermal insulation causesincreasedice growth at the
latitudes north of the Antarctic
coast. A well-defined
baseof the ice. A doublingof the snowfallresultsin an
annualcycleis shown,with maximumvaluesof accumu- increasein ice thicknessby increasedsnow-iceformalation occurringin July through Septemberand a dis- tion, which counterbalances the increase in insulation
tinct minimum occurringduring December. While the affordedby the thicker snow.By contrast,both no-snow
maximum snow accumulation around the zone is shown to
and double-snowconditionsresult in an apparentthinbe about44 mm peryear(duringJuly),accumulation
rates ning of ice in the Arctic. These differences are due
primarily to the relative differencesin zi and Zsbetween
varyquiteconsiderably
withinthe zone(Figure17).
the two polar regions.
Eicken et al. [1995] further suggestedthat the de3.6. Implications
of GlobalWarming
Global climatemodels[e.g.,Hunt et al., 1995]predict crease in conductive heat flux under a thicker snow cover
increasedprecipitationover the Antarctic sea-icezone. would enhancethe sensitivityof basal thermodynamic
The implications,and feedbacksinvolved,are complex. ice growthto oceanicheat flux while decreasingthe ice
Using a global coupled atmosphere-oceansea-ice surfacesensitivity.Of course,dynamicprocesses
suchas
model, Wu et al. [1999] showedthat changesin snowfall ridging and rafting would continue to thicken sea ice
related to globalclimatechangehaveoppositeeffectsin [Worbyet al., 1998].
the Arctic and Antarctic sea-iceregions.When either
Figure 20 is a schematicof possibleeffectson Antsnow is removed or the snowfall rate is doubled, ice arcticair-sea-iceinteractionsand biota of increasedpre-
39
3 / REVIEWS OF GEOPHYSICS
Massom et al' SNOW
ON ANTARCTIC
SEA ICE ß 437
MeanNetAccumulation
Rate,mmmonth
'•
2
5
10
20
40
60
80
..
Minimum Vector
0.2 rn s "•
lOO
.
Maximum
Vector
15.6 m s •
Figure17. Mapof themeannetaccumulation
rate(mmmonth-•) fromGlobalAssimilation
andPrediction
(GASP) data (for the period September1989to November1992),with the reduced-gridyearlymean surface
windfield in m s-• for the 0.81 sigmalevelsuperimposed.
The verticalcoordinate
sigmais the terrain
followingverticalheightrepresentedaspressureoverthe surfacepressure.This figureusesdata obtainedfrom
the studywhich contributedto Reid and Budd [1995] and Budd et al. [1995].
cipitationat high latitudesas a resultof globalwarming.
An increasein freshwaterfluxinto the openoceanin the
form of precipitationwould increasethe stabilityof the
surfaceoceanlayers,therebyenhancingsea-iceproduction [Fichefetand MoralesMaqueda, 1999] and increasing the ice thicknessby inhibitingdeep-oceanconvection
and the upwellingof warmerdeepwaters[Martinsonand
Iannuzzi, 1998]. The increasedcontributionof snow-ice
formationmay alsolead to a significantlymodifiedbrine
rejection rate. Under current conditions,a factor contributing to snow-iceformation appearsto be the effect
of highoceanicheat fluxesin certainregions(e.g.,Maud
Rise, Weddell Sea, and RossSea), leadingto basalice
melt or a cessationof thermodynamic basal sea-ice
growth [Ackleyet al., 1995;Jeffrieset al., 2001.].Work by
Lytle and Ackley [2001] further suggeststhat snow-ice
basalmelt may lead to the continualvertical downward
transport of snow ice to melt at the ice base. Such a
freshwaterflux would, they argue,be sufficientto inhibit
deep-oceanconvectionunder certain circumstances.
Eickenet al. [1995] further suggested
that an increase
in snow flooding would be further enhanced by the
increasedporosityof the ice due to an increasein its
temperature.They suggestthat snowloadingand flooding would have a particularly large effect within the
advancingice edgezone or early in the growthseasonby
both decreasingice growth rates and causingenhanced
snow-iceformationdue to loadingon comparativelythin
ice. Predictedincreasesin precipitationin spring-summer may have a significanteffect on the amount of ice
that survives the austral summer by affecting snow
shieldingand snow-iceformation [Eickenet al., 1995].
This may in turn affect sea-iceextent in the subsequent
winter [Comisoand Gordon, 1998].
Another significanteffectwould be on sea-icemicrobial communities.
A thicker
snow cover would
reduce
the intensity and modify the spectral compositionof
photosynthetically
activeradiation [Sullivanet al., 1985].
On the other hand, floodingmay bring nutrientsup to
the relativelylight ice surface,allowinga communityto
developthat is to somedegreeprotectedfrom grazingby
krill and other larger organisms.A thicker snow layer
may afford greater protectionfrom ultravioletradiation
for ice-lovingorganisms[Perovich,1995].
438 ß Massom et al.- SNOW ON ANTARCTIC SEA ICE
5O
I
39, 3 / REVIEWS OF GEOPHYSICS
I
t
...........
....
68.2
66.0
63.7
61.5
........
4O
I
I
I
I
øS
øS
øS
øS
/,
/.
,/
3O
\,
...,
,.
,
2O
J
F
M
A
M
J
J
a
I
i
S
0
I
N
D
J
Time, Month
Figure18. Seasonal
cycleof thezonalmeansnowaccumulation
rate(mmmonth-•)forseveral
latitudes
off
the Antarcticcoastfor the periodSeptember1989to November1992.This figureusesdata obtainedfrom the
studywhich contributedto Reid and Budd [1995] and Budd et al. [1995].
4.
CONCLUSIONS
and the fact that significantsnowmelteventscan affect
largeareas,evenin winter. It is likely that snowwill play
an increasinglyimportantrole if globalchangeincreases
high-latitudesnowfall.
Sturmet al. [1998] observedthat the cyclicalpassage
of warm and cold fronts,while producingheterogeneity
in winter at scalesof <100 km, alsoproduceslarge-scale
homogeneity.Note, for example,how similar the pro-
Recent field programshave greatly improved our
understandingof the complexroles of snowin interactions of Antarctic sea ice with the atmosphere,ocean,
and biota. Important findingshave emerged,including
the role of snow in snow-ice formation, lower effective
thermalconductivities
than are typicallyusedin models,
.6
i
i
i
I
I
i
1.4
1.2'"'--.
,,•ø
% •'• ,•.
--'
,•..• ---, '"'- ""
---
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-----
i
'.......
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•x••-•.
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I
Sl ....
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.--.---'"'"
-. ....
---" .......
--
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/
/
0.6 "'"
0.4
'J
F
M
A
M
J
J
A
S
O
N
D
Time, Month
Figure 19. The seasonalcycleof the area-weightedaverageof Antarcticsea-icethickness,asmodeledby Wu
et al. [1999].Curvesare labeledasfollows:C, controlrun; SI, ice; OS,no snow;0.5S,half asmuchsnowfallon
the surfaceas providedby the originalmodel;2S, twice as much snowfallas in the originalmodel;and LK,
kbulk-- 0.164W m-• K-• (ratherthan0.31W m-• K-•, asusedin curveC). Reprinted
fromWu et al. [1999]
with permissionfrom Springer-Verlag.
39, 3 / REVIEWS
OF GEOPHYSICS
INCREASED
Massom
et al.: SNOWON ANTARCTIC
SEAICEß 439
PRECIPITATION
Possible increase in
Cooling
1
+
summer
sea-ice
extent
Delayin summersurfacemelt
ii?i
Increase in sea-ice albedo
':•---•::"::!i!•
':•:
I.ncmased;snow
loading
and insulation
......
................
........
Increase in
"infiltration"
community
--•
t
Modified
PAR+ Decrease
inUVpenetration
::::'
'•'•':
":':'•::"
'':*":'
''
.
..
'•lncreased
snow
ice
formation
• • G,•,•,
...........
,•...,•
....,• biological
Modified
sea-ice
,•,______
___
__________
,•,.,.,,•,u,u,,
(algal
etc.)
-'......
.•••-..'••-'
...... -:.-:• snow
ice
to
seaice
mass
communities
Modified bdne rejection rate
Moreflooding
More
•
.._.____:.___..t
.h.e.r.m_o
•dyn_a_rnjc
-g•_wt_h.;,
Effect on water mass formation and modification,
permeable
oceancirculation,and ocean aigal ecology
Warmer
ice
Melt from below?
Figure20. Schematic
of theeffectonthesnow-ice-ocean-atmosphere-biota
system
of enhanced
precipitation due to predictedglobalwarming.
For example,improvedaccumulation
portionsof differentsnowtypesare from differentEast snowprocesses.
Antarcticcruisesin Table 3. While point predictionof data are requiredto validateglobalcirculationmodels
snowpropertiesis impossible,
predictablestratigraphic andto resolvespatialandtemporalvariability.Thisis a
and texturalsequences
do developeachwinter at the difficulttask in the windy Antarcticenvironment,and
regionalscale,with the proportions
of depthhoar and much reliancewill continueto be placed on improved
dataas
icy snowincreasing
and decreasing,
respectively,
with modelingusingthebestavailablemeteorological
increasing
latitude(Figure21). Sturmet al. [1998]sur- input.Furthermodelingis requiredto determinethe
misedthat periodicfloodingand subsequent
snow-ice role of aeolian redistributionand loss by sublimation
formationeffectivelydiminishthe degreeto whichsnow and the role of crustsin lockingsnowin place.Sublimametamorphism
processes
cancreatelarge-scale
inhomo- tion of blowingsnowcan lead to significantepisodic
vertical and horizontalfluxesof water and energy[D•ry
geneityin the snowpackas the seasonprogresses.
Significant
contrasts
existbetweenthe snowcoverson et al., 1998; Pomeroyand Gray, 1994], which are not
seaice in the two polar regions,suggesting
that models presentlyincorporatedin models.
Snowis currentlyrepresented
in a simplisticfashion
needto parameterizesnowand ice interactions
differently. In the Arctic, snow-iceformationis relatively in numerical sea ice-climate models,in terms of both its
uncommon
(althoughit mayplay a significant
role in thicknessdistributionand properties.Given the rapid
thatoccursin snowon Antarcticseaice,
seasonalfirst-yearice in marginalseassuch as the metamorphism
GreenlandSea), and hard wind slabsand depthhoar and the effect this has on the effective thermal conducmakeup the bulk of the snowcover.Moreover,melt tivity, time-dependentchangesin snow parameters
features,while commonin the Antarcticevenin winter, shouldideally be includedin models,in additionto
in snowthickness.
Similarly,oceanswellpeneare rare in the highArcticuntilspring.However,Arctic changes
melt commences
veryrapidlyby the end of June,sothat tration playsa significantrole in modifyingand even
the snowand eventhe top ice layersdiminishwithin a removingthe snowcoverin the outerpackice. Again,
are not includedin presentmodels,due
coupleof weeksandcoverup to 30% of the ice surface suchprocesses
[Tuckeretal., 1992].In contrast,
summersurfacemeltis in part to their complexity.Given their importance,
formationprocesses
shouldalsobe includedin
only very moderatein the Antarctic.More data are snow-ice
modelingof Antarcticseaice, with models
requiredto better characterize
the distributionand large-scale
(i.e.,strictconproperties
of snowin summer.Muchof the large-scale'takinginto accountenergeticconstraints
information on melt extent in the seasonal sea-ice zone
servationof massand energy)for freezingof a flooded
has been inferred from satellite radar remote sensing snow layer.
[Drinkwaterand Liu, 2000].
Othersignificant
gapsremainin ourunderstanding
of
Measured values of the bulk effective thermal con-
ductivityof snoware lower than thosecurrentlypre-
440 ß Massom et al' SNOW ON ANTARCTIC SEA ICE
I
80-
I
a) Icy and Melt-Grain
Clusters
60-
A
'(D
40-
0
o
o
c-
20A
0-
o
80-
(•
60-
n
40-
]
'
I
'
I
'
I
,
I
,
I
,
b) Depth Hoar
39, 3 / REVIEWS OF GEOPHYSICS
ecosystems
and the structureof the oceanicmixed layer
is unknown.Thermodynamiceffectsof a snowcover on
ice connectivity,brine channelformation,brine volume,
and ice porosity,and their variability, are complexand
poorly understood, although progressis being made
[e.g.,Goldenet al., 1998;Maksymand Jeffries,2000].
Regardingsnowthickness,an effort to systematically
and regularly record snow (and ice) thicknessalong
regularly spacedmeridional transectsby ship has recentlybeen initiated within the ScientificCommittee on
AntarcticResearch(SCAR) AntarcticSea-IceProcesses
and Climate (ASPeCt) program [Worbyand Ackley,
2000]. While promisingeffortshave recentlybeen made
to derive circumpolarsnow thicknessdistributionsremotelyfrom satellitepassivemicrowavedata [e.g.,Ardgo
et al., 1996;Markus and Cavalieri,1998], more independent data are required for validation under a range of
seasonsand conditions. Once validated, such data will
20-
improve our understandingof possiblelinks between
spatialpatternsof snow-iceformationand regionalpatA
terns in snowdepth.
0-1•
-ß
I
'
I
'
I
In addition to collectingdata along specifiedcruise
72
70
68
66
transectswhere possible,future experimentsshouldideally be designedto resolvetemporalvariabilityin snow
Latitude,øS
coverproperties,includingmicrowavesignature.In parFigure 21. Differencesin the proportionsof (a) icysnowand ticular, more work is required to estimatethe regional
(b) depthhoar with latitude.Thesedata are from snowpits in
the Amundsen
and Ross Seas in the winter
and seasonal contribution
of snow ice to the overall mass
of 1994. After
balanceof the Antarctic pack. As Jeffrieset al. [1998b]
point out, the occurrenceof floodingand snow-iceformation is a complexfunction of the interplay between
snowaccumulationand redistribution,basalice growth
scribedin models.In a recentArctic experiment,Sturm and melt, and the temperature-dependentice permeet al. [2001] deriveda similarlylow value.However,after ability and porosity.Suchexperimentsmay bestbe carcomparingthe heat flux computedfrom ice growthrates ried out from drifting ice camps.
with that computedfrom temperaturegradientsin the
Finally, observationssuggestthat the accurate resnow,they suggestthat the bulk value is too low if the trieval of sea-icegeophysical
parameters(e.g., ice consnowcoveris treated like a homogeneouslayer to which centration) from satellite microwavedata requires a
a one-dimensionalconductiveheat flow model is ap- better understandingof the role of snowand the signifplied. Sturm et al. [2001] argue that lateral as well as icant differences in conditions between the Antarctic
vertical heat flow is common,resultingin the develop- and Arctic. A wet or aging and metamorphosingsnow
Sturmet al. [1998].
ment of areas of concentrated
heat loss at the snow and
cover makes a significantcontributionto the observed
ice surface,and increasingthe effectiveconductivityof variancein both the radiometricbrightnesstemperature
the snowby a factor of 1.4 when aggregatedover large [Comisoet al., 1992] and radar backscatterof sea ice
areas.This accountsfor someof the discrepancyin heat [Hallikainenand Winebrenner,
1992].
fluxes, while nonconductive heat-transfer mechanisms,
like natural and forcedair convection,may alsoincrease
ACKNOWLEDGMENTS. The authorsareverygratefulto
kbulk.Further work is necessary,as lower values have
importantimplicationsin the computationof thermody- the captains,crews,and helicopterpilotson the R/Vs Aurora
namic ice growth and salt rejectionrates. Difficulty re- Australis,Nathaniel B. Palmer, Polarstem, and Icebird and to a
on the variouscruises,who are
mains in reconcilingthe differencein scalesinvolved, greatnumberof field assistants
i.e., between direct measurementsof thermal conductiv-
ity and thoseheat-transferprocessesaddressedby models,which operate at a significantlylarger scale.
Presentcomputationsof salt flux into the underlying
ocean take no account of salt rejected upward and
storedin the "blotting pad" of snowand the effect this
has on the ocean stability.Moreover, the effect of the
release of this salt at summer melt on high-latitude
unfortunatelyfar too numerousto list here. R.M. thanksJoey
Comisoof NASA GoddardSpaceFlight Centerfor hissupport
duringWWGS '92 and the Alfred-Wegener-InstitutfQr Polarund Meeresforschung,the U.S. National Research Council,
and the National
Science Foundation.
The
contributions
of
Jeffries,Morris, and Sturm were made possibleby NSF-OPP
grants 9117211, 9316767, and 9614844, Edison Chouest Offshore, and Antarctic Support Associatespersonnel.M.R.D.
completedthiswork at the Jet PropulsionLaboratory,Califor-
39, 3 / REVIEWSOF GEOPHYSICS
Massom et al.: SNOW ON ANTARCTIC SEA ICE ß 441
nia Institute of Technology,under contractto NASA code YS Brandt, R. E., C. S. Roesler, and S. G. Warren, Spectral
albedo,absorptance,and transmittanceof Antarctic seaice,
with grant665.21.02.S.W. acknowledges
supportby NSF grant
in Proceedings
of the Fifth Conferenceon Polar Meteorology
OPP-98-15156.We thank Peter Cargill and two anonymous
and
Oceanography,
pp. 456-459, Am. Meteorol. Soc.,Bosreviewersfor their excellent suggestions,
the editor James
ton, Mass., 1999.
Smith, and Bill Budd of the Antarctic CRC. This paper is
dedicated to the memory of our friend and colleague Al Budd, W. F., P. A. Reid, and L. J. Minty, Antarctic moisture
flux and net accumulationfrom global atmosphericanalyLohanick.
ses,Ann. Glaciol., 21, 149-156, 1995.
ThomasTorgersenwas the Editor responsiblefor this paClarke, D. B., and S. F. Ackley,Physical,chemicaland biologper. He thanksAndreasEdgar and an anonymousreviewerfor
ical propertiesof winter seaice in the Weddell Sea,Antarct.
technicalreviewsand Peter Cargill for the cross-disciplinary J. U.S., 17(5), 107-109, 1982.
review.
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