How to form halocline water?
Atlantic water
- cannot form Halocline
water simply by mixing
(Aagaard, 1981)
Surface Water
Adapted from Steele and Boyd, 1998
ADVECTIVE HC
Temp
Fresh
Aagaard et al, 1981
Salty
~ 50m
ADD
COLD, SALTY
~ 100m
~ 50m
Salty
Convect
Temp
CONVECTIVE HC
Rudels et al, 1996
ADD
COLD, FRESH
~ 100m
Steele and Boyd, 1998 – source of halocline water differs (advective or convective)
Woodgate et al, 2001 – temperature of halocline water differs
(convective must be at freezing temperature, advective may or may not be at freezing)
Halocline formation
Woodgate etal, 2001
Retreat of the Cold
Halocline
(Steele and Boyd, 1998)
In 1995, only Makarov
has a cold halocline
Use salinity in 40-60m
band as an indicator
Retreat of the Cold Halocline
(Steele and Boyd, 1998)
Injection point of freshwater
(Russian Rivers) has changed
Backed up by chemical data,
Ekwurzel et al., 2001
PREVIOUS – RW into Eurasian Basin – CHL in Eurasian Basin
1995
– RW along shelf instead – no CHL in Eurasian Basin
Decadal averages of Russian Data
1950s to 1980s
http://nsidc.org/data/g01961.html
(see also Swift et al, 2005 annual averages in boxes 1948-1993
- ftp:://odf.ucsd.edu/pub/jswift/arctic_aari_method_B
“Levitus” for the Arctic
http://psc.apl,washington.edu/Climatology.html
The (partial) return of
the Cold Halocline
(Boyd et al, 2002)
Consider (upper 80m) S over Lomo Ridge
- 1995 ~ 34 psu – no CHL
- 1997 ~ 33.55 psu
- 1999 ~ 33 psu
- 2000 ~ 33.3 psu – CHL returning
What could be causing this??
So far, this is EASTERN Arctic story,
what about the Western Arctic
Western versus Eastern Arctic Halocline
WESTERN ARCTIC
(PACIFIC) HALOCLINE
- greater salinity range
- fresher at surface
- general Tmax above Tmin
- very varied
- (rich in nutrients)
Adapted from Steele and Boyd 1998
EASTERN ARCTIC (ATLANTIC) HALOCLINE
- less salinity range
- saltier at surface
- sharper bend in TS space
Shift of Pacific/Atlantic Front
JGR, 1996
- use TS and chemistry to show Pacific
Atlantic Front retreated from Lomo Ridge
to Mendeleev Ridge by 1993
JGR, 2005
Historic Russian Data
- silicate profiles in central Makarov
- Si max disappears in late 1980s
Bering Strait and the Chukchi Sea
Nutrient-rich
Anadyr waters
Bering Shelf
waters
Alaskan
Coastal Current
(warm, fresh,
seasonal)
Siberian
Coastal Current
(cold, fresh,
seasonal)
Stagnation
Zones over
Herald and
Hanna Shoals
COLDER
SALTIER
RICHER IN NUTRIENTS
WARMER
FRESHER
LOWER IN NUTRIENTS
To first order,
except for
- cooling
- input from
coastal
polynyas,
Chukchi
dominated by
input through
Bering Strait
Export to Arctic
~
Input through
Bering Strait
Woodgate et al, DSR, 2005, http://psc.apl.washington.edu/Chukchi.html
JGR, 2004
ACW=Alaskan Coastal Water
sBSW = summer Bering Sea Water
Generic Pacific Water
circulation
Steele et al, 2004
- change in pathway with change in Atmospheric state
- shift of Pacific/Atlantic boundary from Lomonosov Ridge
BUT
- doesn’t always match Fram Strait outflow – is there a better tracer
- how get the Pacific Water off from the Chukchi
Chukchi Sea Outflow
Long
Strait
= TOPOGRAPHIC CONSTRAINTS
(Potential Vorticity Conservation)
- Taylor columns in Chukchi
- flow ~ along isobaths eastward
BUT WE SEE PW GETS AWAY
FROM TOPOGRAPHY
= FRICTION (TOP or BOTTOM)
= DENSITY DIFFERENCES
- dense water outflows
=Four main outflows
1. Barrow Canyon 2. Central Gap
3. Herald Canyon 4. Long Strait??
= Most nutrients in West
= Outflows move east & north
= Seasonal & interannual variability
in TS (thus density and equilibrium
depth) and also in volume
= WIND EFFECTS
- upwelling and downwelling
- undercurrents
= EDDIES
= INERTIAL and TIDAL
OSCILLATIONS AND MIXING
Chukchi slope velocity 2002-2003
red=73 20N blue=73 37N
Chukchi Slope 2002-2003
TS-properties
Temperature
Maximum is
December – March,
i.e. advective from the
south
Intrusions of Atlantic
Water in Autumn
73 20N
- red 60m/70m water
73 37N
- cyan 60m/110m water
- navy 100m/110m water
Dense Water Outflow
– e.g. from coastal polynyas
(e.g. Martin et al, 2004)
X
Wind
Ice
S flux as new ice
ICE
Temperature
ICE
Thickness
Dense water on shelf
Dense water flows
down shelf as a
descending plume,
entraining water.
(i.e. down, but not OUT)
2002, JGR
- can get the salinities,
but volume is small
Wind effects
X
Wind
X
Wind
isopycnal
Upwelling of deeper water
- can come up canyon
onto the shelf
- cf Chukchi slope
canyons, Barrow Canyon,
and many others
If initial stratification enough,
can get undercurrent
opposite to the wind
(Yoshida Undercurrent)
- cf Beaufort slope
Results of a strong westward wind
3rd October
5th October
Ship’s ADCP of the Beaufort
slope current system
(red= towards you)
(Andreas Muncheow, UDel)
Upwelling versus polynyas??
Use Silicate to track
Pacific Water in the
Chukchi Borderland
Can we get this
TS from Bering
Strait??
NO ... salinities are only near 34 psu in extreme winters,
and then the waters are at freezing, not warmer
Bering Strait
TS 1990-1991
Woodgate et al, 2005
Influence of shelf waters??
Along the Chukchi
Shelf, upwelling and
diapycnal mixing of
lower halocline waters
and Pacific waters
(Note ventilation by
polynya waters
couldn’t give this T-S
structure)
Do the volumes work out?
Volume PW at 33.1 psu per year
~ 6x 10^12 m3/yr (0.6 Sv for 4 months)
In Arctic ~ half pure, half mixed with AW (1:1)
Thus need 3 x 10^12 m3/yr AW to be raised onto shelf
Observed upwelling events in Barrow Canyon
= 2-3 x 10^11 m3 per event
Therefore need 10 events
and there are multiple wind events, and multiple canyons
So – plausible
Also, this ventilation rate is an order of magnitude
higher than estimates of polynya water formation
Woodgate et al, 2005, GRL
The Eddy Band-wagon
Eddies in the Beaufort Sea
e.g.,Hunkins and Manley, Plueddemann and MANY others
http://www.whoi.edu/science/PO/arcticgroup/projects/eddies.html
halocline depth
Arctic Rossby Radius
Predominantly Anticyclonic
Eddies in the
non-Beaufort
Arctic
LM2
LM3
LM1
TWO EDDY TYPES
Cold (Tf), Fresh, near surface, AC,
- likely from shelf polynyas
Warm, Salty, ~ 1000m deep, AC
- instabilities on upstream front
(e.g. St Anna)
40 cm/s; ~ 10km radius, but volume flux ~ 0.1 Sv or less
Upper Arctic Ocean Circulation and Ventilation
HALOCLINE BASICS
Formation possibilities
Western versus Eastern Arctic
Different branches of PW
Rudels et al,1996
CIRCULATION CHANGES
Shift in rivers outflow
Retreat and recovery of CHL
Shift of Pacific/Atlantic Front
MECHANISMS OF SHELF-BASIN EXCHANGE
Winddriven upwelling and mixing of AW with PW
Potential vorticity conservation, following topography
Dense water outflow from polynyas
Eddies (AC, ~ scale of Rossby radius)
Winddriven undercurrents (Yoshida jet)
Tides and inertial oscillations
Photo PSC/UW