JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL.
88, NO. C14, PAGES 9717-9722, NOVEMBER
20, 1983
Satellite ObservedBehavior of the Terra Nova Bay Polynya
DENNIS D. KURTZ
AND DAVID
H. BROMWICH
Institute of Polar Studies,Ohio State University
Infraredsatelliteimagesindicatethat a polynya,surroundedby a zone of loosepack ice,persistedin
Terra Nova Bay, Antarcticathroughthe winter of 1979;the featuremay recureachyear.The affected
region(polynyaandloosepack)occupied
roughly25000km2 of the westernRossSea.Throughout
the
winterthe areaof openwaterfluctuatedquasi-periodically
with a periodof 15-20 days.Averagepolynya
areawas1000km2;maximumareawasapproximately
5000km2. Fluctuations
wereassociated
with the
magnitudeof the zonal geostrophicwind, with a closingpolynyabeing relatedto strong,persistent
easterlies.
An open,or opening,polynyawaslinkedwith persistent
westerlyor weakeasterlywindsand
the probabledescentof adiabaticallywarming,drift-bearingair from the plateau.This air entersTerra
Nova Bay throughthe ReevesGlacier valley,probablyas katabaticsurfacewinds.Polynyaformation
wasprobablydueto the actionof thesewindson the coastalpackice,whilearealfluctuations
reflected
the interplaybetweenthe katabaticwindsand synopticscalemotionsadvectingice toward and away
from the bay.
must frequently cross Victoria Land farther south. Cyclones
sometimes behave in this fashion when cyclogenesistakes
place in the Victoria Land Trough [U.S. Navy Weather Service, 1970]; however, this does not explain why winter open
water persistsonly in Terra Nova Bay. The presentcomparispatialscales.
The WeddellPolynyais a roughly3 x l0 s km2 son of this polynya'sbehavior with synopticscaleconditions,
area of open water which irregularlyforms during winter and through one winter, constitutes a needed test of Knapp's
spring[Carsey,1980; Martinsonet al., 1981]; this featuremay model for the geographicsettingof the westernRossSea.
Wintertime open water has been known to exist in Terra
be a result of extensiveopen-oceanconvection[Martinson et
al., 1981]. Much smallerpolynyascan persistin coastalareas Nova Bay since1912 when six men of Scott'sNorthern Party
which are sheltered from drifting sea ice and can undergo were stranded on InexpressibleIsland [Priestley, 1914; Bromsubstantial oscillations in size [Knapp, 1967, 1972; Streten, wich and Kurtz, 1982]. In the mid-1960's, Knapp [1972] noted
1973]. Theseare distinctfrom shoreleadswhich open tempor- this polynya in a generalsurveyof Antarctic coastalpolynyas,
arily due to seawardadvectionof seaice by strongwinds and using experimentalNIMBUS 1 and 2 infrared photographs,
and gave a sequenceof imagesillustratingits temporal variawhichform partly by chance.
Knowledgeof Antarctic coastalpolynyasis limited because bility (his Figure 6). $zekielda [1974] probably examined it
of the difficulty of observing small areas during the polar more recently using early operational infrared satellite imanight. However, their extent can be approximatelyinferred gery. His geographiccoordinatesare incorrect,but the profile
from the ~ 30 km resolution microwave brightnesstemper- of the Drygalski Ice Tongue is apparent in his figure 3. Szeature distributions monitored since 1973 [Carsey, 1980; kielda noted that the polynya persistedthroughoutthe winter
Weller, 1980]. Relatively high resolution (~3 km) infrared and attributed its presenceto marine upwellingor submarine
satelliteimagerypermitsdaily observationof localizedsea ice volcanism.The nature and frequencyof polynya fluctuations
behaviorthroughoutthe winter [e.g.,Dey, 1981]. The present and their relationshipsto synoptic scale weather were not
discussed.
studyusesthe latter to examineseaice conditionsin the westInformation from sunlit periodsalso indicatesunusualconern RossSea where a prominent featureis a polynya in Terra
ditions
in Terra Nova Bay. Sea ice chartsfrom 1973 to present
Nova Bay (Figure 1).
[Fleet
Weather
Facility, 1975, 1977, 1979; Naval Polar OceanKnapp [1972], drawing on earlier work by Zubov, hypothesizedthat Antarctic coastalpolynyasform when strongwinds ographyCenter, 1981, 1983] consistentlyshow open water or
associatedwith traveling cyclonescausenet sea ice displace- reducedice concentrationsfrom October to early March. The
ments away from stationary ice boundaries.Thus, zonally anomaly persistsin spring, surroundedby concentratedpack
propagating storms north of an irregular east-westcoastline ice in the Ross Sea, during the summer when ice tends to
(East Antarctica for instance)shouldgeneratepolynyasto the remain along the Victoria Land coast, and during fall when
westof capelikeprojectionsand along the east cost of protec- pack ice is rapidly forming in the open sea.That the Antarctic
sea ice edge retreats to its most southerlypoint in the central
ted bays.This situation was verifiedby Knapp's analysisfor 2
and western Ross Sea [Streten and Pike, 1980] suggestsan
winter months of variations in the area of open water west of
the fast ice anchoredby Fram Bank (longitude ~ 70øE),about atypical characterfor this whole region. Prior to 1980, sea ice
70 km north of Cape Darnley. Knapp's model requires that chartslabel Terra Nova Bay as being filled with concentrated
for a polynya to form in Terra Nova Bay depressioncenters pack from mid-March to early October. These charts were
INTRODUCTION
Many areasof wintertime open water are now known to be
presentwithin the Antarctic northernice limit. Theseplay an
important role in the surfaceenergybalanceof the pack ice
belt [Weller, 1980] and exist on a variety of temporal and
•1••.• 1111,•11• •.
Copyright 1983by the AmericanGeophysicalUnion.
Paper number 3C1039.
0148-0227/83/003C- 1039505.00
•,•11
•11
•,• 1,•• 1 • •.1•.• 11•1
o ffl.•o1o
• 11•.•. o • ffl.• 1•1•
• •,.•11•.•.1 •.l •,.•l 1o
Ill
that area during winter were not of navigational interest.
Newer charts are more detailed.For the period betweenmid
March and late Septemberduring the years 1980-1982, 41%
of the analysesshowreducedice concentrations
or new ice.in
9717
9718
KURTZ AND BROMWlCH' TERRA NOVA BAY POLYNYA
-74•S
170øE
•ooE
study
area
.ova
ROSS
SEA
Cape
Washington
Marble Pt.
ROSS
ICE
SHELF
170øE
.- ....
:::
-
TERRA
..........
75øS
•/ooøii
Inexpressible
Island
NOVA
:D
Glaciers
\ ..-"'""-') i........._............._...:.:•:•.••..:.....•:
•
•..... a Fast
Ice
t
•/•
•!•
01
.;.•
krn
5•0
CONTOUR
INTERVAL
1000m
Fig. 1. Location map showingTerra Nova Bay and surroundingfeatures.
Terra Nova Bay. These findingsindicate that ice charts alone
cannot be usedto derive ice concentrationsfor the inner pack
during the polar night.
Western Ross Sea wind and ice conditionswere compared
for March through October 1979 to elucidatemechanismsof
polynya formation and maintenancein Terra Nova Bay. Sea
ice behaviorwas monitored by usingthermal infrared imagery
from the two near polar-orbiting satellitesadministeredby the
U.S. Air Force Defense Meteorological Satellite Program
(DMSP). Images were selectedfrom 1979, the FGGE (First
GARP Global Experiment)year, and were used in conjunction with twice-daily (0000 and 1200 GMT) synoptic analyses
for that year. The latter constitutethe best available record of
the surfacepressurefield over the Southern Ocean [Guyruer
and Le Marshall, 1981] and enabled derivation of the western
Ross Sea wind field. It is important to note, however, that the
southernhemispherecirculation was highly anomalousduring
FGGE [van Loon and Rogers, 1981; Trenberth and van Loon,
1981].
SATELLITE IMAGERY ANALYSIS
Infrared positive transparencies were analyzed by using
straightforward photo-interpretive techniques.Original digital
information was not available. Although 64 gray shadesare
present, the human eye can normally distinguish between
about 14 shades[Dickinson et al., 1974] (i.e., objects of sufficient size with an equivalent temperature contrast of at least
7 K can be observed).The Drygalski Ice Tongue is a readily
identifiablegeographicfeatureon satelliteimages.Its profile is
evident even when no open water is present.In addition, Cape
Washington (Figure 1) and the outline of Terra Nova Bay
us,ually
exhibitsufficient
radiativecontrastwith openwaterto
be discernable.Thermal featuresvisible on cloud-freeimages
accurately reflect relative temperature distributions on the
land, sea, or ice surface. Intervening cloud alters observed
temperature,thereby modifying the appearanceof the surface.
But, clouds and moving features in the pack ice could be
tracked by superimposingsuccessive
images,eliminatingmuch
KURTZ AND BROMWICH' TERRA NOVA BAY POLYNYA
9719
TABLE 1. Estimatesof Monthly Mean Extent of Open Water in the Terra Nova Bay Polynya
Mean
Area of
Number
Month,
Open Water,
1979
km2
S
S•
2700
1100
March
3300
April
May
900
1000
June
1100
1000
July
August
September
1100
1400
1500
600
1100
800
October
Total
1400
1300
900
1100
210
90
900
800
Maximum
Area of
of
Open Water,
Days
km2
6
220
160
17
23
3000
3000
230
19
4500
120
230
200
25
23
16
3000
5000
3400
18
147
4500
5000
Values used in calculatingthe means are based upon minimum estimatesof open water extent on any
givenday.
of the uncertainty associatedwith cloudy images.Interpretations are biased becausethe polynya could only be observed
with cloud-free or partly cloudy conditions; however, acceptable imageswere available for more than half the days from
March through October, 1979 (Table 1).
Discrimination between open water, consolidated sea ice,
and cloud was necessaryfor the purposesof this study. These
distinctions were possible on most images. Open water is
much warmer than adjacent sea ice and thus appearsdarker.
Sea ice, particularly old and/or thick ice, has a low surface
temperatureand appearslight gray. It is also characterizedby
a network
of leads and fractures
wherein
individual
leads and
crack patterns persist for 2-5 days (compare Ackley and
Hibler [1977]). Sea ice boundaries are typically abrupt and
roughly parallel to nearby fractures.Although clouds often
have temperatures close to those of the underlying sea ice,
they display curved, frequently diffuse boundaries, have an
uneven texture, and are not fractured.
,..
;•.:.
The
most
difficult
distinction
to make
was that
ature, and the ice cover is neither of the scale nor the thickness
for a network of fractures to be visible. Thus, even when obvi-
ous open water was present,it was sometimesdifficult to determine accurately its boundaries.Areas of open water were
determined by projecting images onto a grid and by comparing the desiredarea to a known standard. Low level cloud
or fog may have a similar thermal signatureto that of patchy
ice. However, persistenceof areasdesignatedas patchy ice for
several days leads us to infer that sea surfaceobscuration by
low level hydrometeorswas not a problem.
POLYNYA
DESCRIPTION
Terra Nova Bay is the outlet for three large glaciers(Figure
1). The floating extensionof the David Glacier, the Drygalski
Ice Tongue, is the southernboundary of the bay. North of this
---•.
.:
..
LANDO./•
.•
--•
.
• ::•
...........
"'??'?•
•:'"::'•:'•:•:•:•" •: ?"•-•
0
:ClOUd
200
between
open and partly open water which presumably contained
patchesof thin ice. The two situationsdiffer little in temper-
400 km
Fig. 2.(a) Infrared satelliteimage of western Ross Sea, September 13, 1979. Very little open water is presentin Terra
Nova Bay (T). Light tones indicate cold temperatures,dark areas are warm. Symbols are identical throughout: D,
Drygalski Ice Tongue; E, thermal signatureof possibledescending,drift-bearing air (seetext); L, region of loose pack ice
surroundingthe Terra Nova Bay polynya (P); R, pack ice in the RossSea.(b) Infrared satelliteimage of westernRossSea,
September16, 1979.The polynya occupiesall of Terra Nova Bay. (c) Infrared satelliteimage of westernRossSea, May 24,
1979. Some open water is present in Terra Nova Bay; the thermal signature of what may be descending,drift-bearing
warm air is visiblewestof the bay. This air entersthe bay through the ReevesGlacier.
9720
KURTZ AND BROMWICH: TERRA NOVA BAY POLYNYA
Area of open water - km 2
Geostrophic zonal wind speed - ms -•
ly parallel ice surfacetopography, implying a physiographic
influence.The signaturemay reflect the presenceof descendMay 15
15
ing, adiabaticallywarming air which is transportingdrift. Infrared satelliteimageryhas been usedto observestrongvalley
winds in Alaska [Maryill and Jayaweera, 1975]. This feature
2O
2O
appears to enter into Terra Nova Bay through the Reeves
Glacier valley and is commonly associatedwith an open, or
25
25
opening,polynya. Moderate to strong persistentwinds were
observedto blow from this valley by the men of Scott'sNorthern Party in 1912 [Bromwichand Kurtz, 1982]. The high frequency of intermediate speeds and the unidirectional
June I
1
characterof that wind suggesta katabatic origin. Strong,persistent, jetlike katabatic outflow is suggestedby Parish's
5
5
[1982] simulation which shows a pronounced confluence
(from ~ 3% of East Antarctica) of winter surfacewinds on the
10
10
ice sheetinland from the bay. Typically, katabatic winds die
out within a few kilometers beyond the foot of the terrain
slope [Schwerdtfeger,1970]. Here, by contrast, the strong
15
15
winds extend at least to InexpressibleIsland, about 25 km
Fig. 3. Comparisonof polynya area and zonal geostrophicwind, eastwardof the slopebreak, and probably well beyondthat.
May 15 to June 15, 1979,representativeof the entire winter.
The dynamicalbasisfor this situation will be discussedin a
subsequentpaper. The wind was assumedby the Northern
Party to be responsiblefor the failure of seaice to consolidate
in the immediate vicinity of InexpressibleIsland, and this is
the Reevesand Priestley glaciersconvergeas the Nansen Ice
supported by our analysis of their descriptiveobservations
Sheet, whose calving wall forms much of the bay's western
[Brornwichand Kurtz, 1982]. The present analysisattempts to
border.
determinethe relative influenceof synopticscaleweather and
The unusual conditions which influence the Terra Nova
katabatic outflow on polynya behavior.
Bay polynya apparently affect a large region in the western
2000
4000
6000
-2O
-10
0
10
Ross Sea. This affected area is divisible into two zones. The
polynya itself is the area of open water closerto the coast.A
larger region of patchy or thin ice surroundsthe polynya on
its seawardsides,being bordered on the south by the DrygalskiIce Tongue.Seaice thereappearsfeatureless
on infrared
images,in contrastto the fracturedRossSea pack. This featurelesszone is lighter in tone (colder) at greater distances
from the open water. A concentricarrangementof seaice and
openwater was alsodescribedby Szekielda[1974]. The northern boundary of the affected zone is diffuse, but the eastern
edgeis marked by a band of cold, fracturedseaice. Fractures
are organizedin a radial patterncenteringon the polynyaand
extendinginto the consolidatedRossSeapack ice.
The polynya fluctuates in size from a minimum, when
almost no open water is visible (Figure 2a), to conditions
where all of Terra Nova Bay is open (Figure 2b). Maximum
wintertimepolynyaarea is approximately5000 km2. The
westernportion of the bay, immediatelyeast of the outlet of
the ReevesGlacier (Figure 1), is normally open. Maximum
polynya width appearsto be controlledby the length of the
Drygalski Ice Tongue. Wintertime open water never extends
very far east of it. The region of patchy and thin ice surrounding the polynya extendsnorth and east of Terra Nova
Bay (Figure 2c) and occupieda maximum area of approxi-
mately25000km2. Its shapeprobablyreflectsthe distribution
of conditions which, while not able to maintain open water,
neverthelessprevent sea ice from consolidating.This zone of
partial ice cover undergoesareal fluctuationsin phase with
thoseof the polynya,but doesso to a lesserdegree.
Satelliteimagesalso record thermal featureson land which
may be related to polynya conditions.The relatively warm
region visible west of Terra Nova Bay in Figure 2c occurs
only along this portion of the westernRoss Sea coast. This
phenomenonis not always present,suggestingsomemeteorologicalcontrol; its shapeand internal thermal structureclose-
POLYNYA FLUCTUATIONS
Areal variations of the Terra Nova Bay polynya were compared with the zonal componentof the estimatedsurfacegeostrophic wind to examine the relationship betweensynoptic
scaleforcingand polynya behavior.The geostrophicwind was
taken to indicatedirectionof seaice movement;the following
analysessupport this approach. Brown [1981] noted that the
(frictionally retarded and deflected)surfacewind blows over
the wintertime pack ice of the Beaufort Sea at an average
angle of 30ø to the left (looking downwind) of the isobars.
According to Knauss [1978], Nansen, while frozen in the
Arctic pack, observedthat when the wind blew the ice appeared to move at an angle of 200-40ø to the right of the
wind. The geostrophicwind is a good approximationto the
free atmosphericflow when the pressurefield is slowlyvarying
and fairly uniform and where the fetch is relatively large.
Then, if the wind-inducedice drift is much larger than the
current drift, sea ice moves in the direction of the surface
geostrophicwind. Further support comesfrom the recentempirical investigationof Arctic sea ice motions by Thorndike
and Colony [1982]. They found that, within 400 km of the
coast,the complexnumber equation
u - t7= 0.008e-•øG
(1)
explainsroughly 50% of the variancein winter and springsea
ice motions on time scalesof from 1 to 30 days. The expressionon the left is the wind-inducedseaice velocity,overall
ice movement(u) minus the mean oceancurrent e. The angular differencebetween this movement and the geostrophic
wind (G) is 5ø to the right, and ice movesat 0.8% of the speed
of G. In the absenceof any in situ measurements,a similar
momentumbalanceis assumedfor the RossSea pack in spite
of the likely differences
in atmosphericstability,surfaceroughness,and pack ice characteristics.However, the directionsof
KURTZ AND BROMWICH'TERRANOVA BAY POLYNYA
Coriolis force, frictional deflection of the geostrophicwind,
and sea ice movement relative to the geostrophicwind are
oppositeto thosein the northernhemisphere.
The geostrophicwind vector was estimatedby measuring
averageorientation and spacingof isobarsover the RossSea
within a 100 to 200-kin radius of Terra Nova Bay on the twice
daily mean sea level pressureanalysesproducedby the Australian Bureauof Meteorology,Melbourne.Geostrophicwind
9721
anddirectionof thezonalwind.Durationof •hangesin the
wind field may also influence polynya response.Marked
changesof shortdurationappearto havelittle effect.Changes
in the wind field eliciting major changesin polynya ice conditions are of fairly long (> 36 hours) duration. Also, maximum polynya area is also controlled by the length of the
Drygalski Ice Tongue which determinesmaximum polynya
width.
The polynya was seldomif ever eliminatedby synoptically
driven sea ice advection,indicatingthat other phenomenaare
V•= (o•/f?s){Ap/AN)
(2) responsiblefor its presence.Table 1 shows monthly mean
wheref75 is the Coriolisparameterevaluatedat 75øS,Ap is the polynyaareasfor March-October 1979.Averagearea of open
pressuredifferenceacrosswhich the normal isobar spacingAN water in March is quite large,but, owing to the initial formawas measured,and • = (RT/p) where R is the gasconstantfor tion of a definite polynya during that month, it does not redry air and p and T are the climatologicalmonthly average flect the magnitude of the physicalprocessesinvolved. The
surfacepressureand temperature at 75øS, 165øE [Taljaard et polynyawas well definedby April, with polynya area remaining constant from April through July. A polynya area of
al., 1969].
speedV•wascalculated
fromtheformula
Frictional
deflection and retardation
of the winter surface
geostrophicwind for the Beaufort Sea [Brown, 1981] were
used to estimate
the surface wind over the Ross Sea from the
geostrophicvalues.The vector resultant of theseestimatesfor
the period from March through October 1979 was from 176ø
roughly1000km2 probablyreflectsthe magnitudeof wintertime processesthat influenceits formation. Increasing area
coincideswith increasingsolar insolation; sunriseat 75øSis
on August 10.
CONCLUSIONS
at 4.2 m s-x with a directionalconstancy
of 0.85.About90%
Polynya fluctuations in Terra Nova Bay were associated
with a variety of synopticsituations.These included western
in a direction roughly parallel to the coastal mountains west Ross Sea lows traveling from south to north and north to
of Terra Nova Bay. Automatic weather station measurements south; cyclonesoriginatingat variouspoints along the Victoof the March-October surfacewind at Marble Point (Figure ria Land coast; and, most frequently,troughsconnectedwith
1) during 1980 and 1982 provide comparativedata. The wind depressionsmoving eastward acrossthe Ross Sea entrance.
regime there, about 300 km due south of InexpressibleIsland, Thus, though Knapp's model provides a convincingexplais dominated by large-scaleeffects.The 1980 (1982) resultant nation for synopticallyforcedpolynyasalong the East Antarcwindvectorwasfrom192(198)degrees
at 2.9(2.3)m s- a with tic coast[Knapp, 1972] and in the westernWeddell Sea [Kyle
a directional constancyof 0.70 (0.69); 69% of the observed and Schwerdtfeger,1974], more complex and varied synoptic
occurin the westernRossSea.Theseconsiderations
wind directionsfell in the quadrant between southeastand sequences
southwest.Here again,most of the winds blew roughly paral- and the low, but significant,correlationbetweenpolynya fluclel to the Transantarctic
Mountains.
It is contended that the
tation and zonal geostrophicwind indicatethat synopticscale
similarity betweenthe observedand derivedwind records,de- motions, by their associatedsea ice advection, only modify
spite differing observationperiods and spatial separationof polynya extent.
The principal influence on polynya formation and mainthe sites,lendscredibilityto the pressureanalysesfrom which
the 1979 surface winds were estimated. This situation is also
tenance is probably persistentkatabatic winds that remain
consistentwith the possiblepresenceof frequent, southerly, stablefor tens of kilometersbeyond the coastalslope break.
is reflected
by the 1000km2 averagepolynya
wintertimebarrier windsblowing parallel to the Transantarc- Their magnitude
tic Mountains [Schwerdtfeger,1979] over the western Ross size during the polar night; synopticallydriven areal fluctu-
of the wind directionslay in the quadrant betweensoutheast
and southwest.Thus, most of the estimated surface winds blew
Sea.
ationsat timesexceed2500 km2. The nucleusof openwater
Ninety percent of the estimated geostrophicwinds were
from directionsbetweeneastand south.Strongwesterlieswere
infrequent. Polynya areal fluctuations occurred quasiperiodically, with a period of 15-20 days (Figure 3), and
appear to be related to changesin the geostrophicwind field.
An opening or extensivepolynya was concurrentwith or immediately precededby winds with marked westerlyor weak
easterly components.Rapid decreasesin polynya area were
usuallyassociatedwith relativelystrongeasterlywinds(zonal
near the Reeves Glacier, wind Observationsin 1912, and
Parish's [1982] simulation support this hypothesis. Furthermore,rapid seasurfacefreezingwas observedduring calm
intervals by the Northern Party [Bromwichand Kurtz, 1982],
indicatingthat bay waterswill freezein the absenceof katabatic winds regardlessof the geostrophicwindfield.The combination of katabatic persistenceand stability is anomalous
and fundamentalfor annual formation of the Terra Nova Bay
polynya. Typical katabatic winds, experiencing katabatic
jumps [Ball, 1957; Parish, 1981], would dissipate much of
their energynear the coast and would not affect sea ice conditionsgreat distancesoffshore.
component> 6 m s-•). Somecorrelationbetweenthe areaof
open water and the zonal wind component is expectedbecausepolynya area varies principally by the east-westmigration of its easternboundary.However, correlation betweenthe
two is low (rz - 0.30) thoughsignificantlygreaterthan zero (at
the 0.1% confidencelevel). Best correlation is exhibited with
zero lag. No correlation betweenpolynya area and the meridional geostrophicwind is expected or present at the 5%
confidencelevel (rm= 0.15). The low rz value suggeststhat
Acknowledqments.We thank Terri Gregory of the DMSP Library,
Space Science and Engineering Center, University of WisconsinMadison
for
valuable
assistance.
'-' ...........
L{.)11{•,[,l{•}11
Ol
the
1¾1•1,1 IJl•;
FUllIt
meteorological observations was supported by NSF grant DPP7925040 to Charles R. Stearns.This work was supportedin part by
NSF grant DPP-8100142 and is contribution 470 of the Institute of
polynya area variesonly partly in responseto the strength Polar Studies,The Ohio StateUniversity.
9722
KURTZ AND BROMWICH:TERRANOVA BAY POLYNYA
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(ReceivedJanuary 14, 1983;
revised March 22, 1983;
acceptedJune 16, 1983.)