GEOPHYSICAL RESEARCH LETFERS, VOL. 28, NO. 17, PAGES 3301-3304, SEPTEMBER 1, 2001
Coupled oscillations in Antarctic sea ice and atmosphere in the
South
Pacific
sector
Silvia A. Venegas Danish Center for Earth System Science, Niels Bohr Institute for Astronomy, Physics and Geophysics, University of Copenhagen, Denmark
Mark
R. Drinkwater
Oceans/IceUnit (EEM-FSO), EuropeanSpaceAgency- ESTEC, Noordwijk, The Netherlands
Gary Shaffer Danish Center for Earth System Science, Niels Bohr Institute for Astronomy, Physics and Geophysics, University of Copenhagen, Denmark
and Methods Abstract. Interannual oscillations with period of about 3-6 Data years have dominated the winter atmospheric and ice variThe data consistof 20 years(1979-1998)of winter anomaability in the Ross, Amundsen, and Bellingshausen Seas, Antarctica, during 1979-1998. Anomalies in sea ice concen- lies (averageJune to September)of sea ice concentration tration and drift propagate eastward in the winter ice pack, (SIC), ice drift (ID), and sea level pressure(SLP) in the coupledto sea level pressureanomalies. This signal accounts Ross, Amundsen and BellingshausenSeas,from 55øS to the for the ice and atmosphericcomponentsof the Antarctic Circumpolar Wave during its eastward passagethrough the Pacific
sector of Antarctica.
Sea ice concentration
anoma-
lies at the ice margin seem to result from air-ice-sea interactions which involve both the dynamic effect of the anomalous, wind-driven ice motion, and the thermodynamic ice growth and melting due to surface air and ocean tempera-
SouthPole. SIC data are derivedfrom SMMR and SSM/I satellite passivemicrowaveobservations[Cavalieri et al., 1997]and rangefrom 0 (ice-freegrid) to 1 (ice-covered grid).
ID data are obtained by computer-tracking of features in the satellite images in combination with buoy data from the
ture anomalies.
InternationalProgrammefor Antarctic Buoys[Drinkwater et al., 1999].SLP data comefromthe NCEP-NCAR ReanalysisProject[Kalnayet al., 1996].The erroneous inclusionof
Introduction
south of 40øS. However, this error rapidly decreasesfrom
the Australian
Climate variability in the Southern Ocean has been a subject of increasing interest during the last few years. Recent investigationsrevealed the existenceof concurrent anomalies in sea ice extent, sea surfacetemperature, sea level pressure, meridional wind and sea level height in a zonal belt around
PAOBS
SLP data
has affected
this dataset
synopticto monthly and longertimescales[Kistler et al., 2001] and should be negligiblefor our application. All datasets cover a spatial grid of 1ø latitude x 2.5ø longitude
and are describedin Venegasand Drinkwater[2001]. The Multi Taper Method-
Singular Value Decomposi-
tion (MTM-SVD) technique[Mann and Park, 1999]is ap-
56øS[White andPeterson,1996;JacobsandMitchell,1996]. plied to the joint SIC, ID and SLP anomalies to isolate staThis Antarctic CircumpolarWave (ACW) propagateseast- tisticany significant narrowband oscillations that are correward with a period of about 4-5 years, taking about 8-10 lated among the three fields. Each grid point time series years to encircle the Antarctic continent. Model simulations is first Fourier transformed using the MTM approach with of the ACW demonstrate that there must be a coupling be- k = 1,..., 3 orthogonal tapers. The three independent spectween the ocean and the atmosphere for this phenomenon tralestimates Y• (f) computed foreachofthem = 1,..., M
to exist [White et al., 1998].Seasurfacetemperature(SST) anomalies are negatively correlated with sea ice extent and likely help reestablishthe ACW from one winter to the next
timeseries areorganized inaM x3matrix A(f) = [Y•(f)]
for which a singular value decomposition is performed at each frequencyf. The left and right singular vectors so ob[Gloersenand White,2001]. Here we investigatethe effects tained are used to reconstruct the spatial and temporal patof both thermodynamics and dynamics upon the nature of terns of the signal associatedwith a given frequency. For method see Mann and the ice pack in the Pacific sector of Antarctica and illustrate further details on the MTM-SVD the importance of the basin-scaleice drift in driving and Park [1999]and Venegas and Drinkwater[2001]. sustaining ice edge anomalieson interannual timescales.
The Interannual
Signal
Figure 1 showsthe Local Fractional Variance spectrum of the joint SIC, ID, and SLP anomalies. The statistically sig-
nificantpeakat periodsof 3-6years(0.16-0.32 cycles yr-x) Copyright2001by theAmericanGeophysical Union. Papernumber2001GL012991. 0094-8276/01/2001GL012991
$05.00
reveals a quasi-periodic signal in the concurrent winter SIC, ID and SLP anomalies. The broadnessof the peak suggests frequency modulation during the 20 years. From a characteristic oscillation period near four years and for consistency 3301
3302
VENEGAS
ET AL.: ANTARCTIC
ICE AND ATMOSPHERE
period (years) 10
5
3.3
2.5
site in a high pressurecase). This suggeststhe generationof
2
o.8 r___ ......
o.75F......... ."..-.. :':.•................... !.................... !................... '-' ..................
.......
07
JJj
•
'
.
................. .
'- o.65•.................. •',,•....... •
.....
•!
--
- .......... !....... "'"';"•;'"'4'
> o• 0.6 ............... I-'"' e-
i•
.o
o.ss o.5
......
99% 95%
• .......
....
VARIABILITY
90%
air temperature anomalies in phase with the SST anomalies that further favor ice melting and growth, thereby adding to the thermodynamic effect of the SST on the ice extent. As such, ice edge anomalies seem to result from a combination of the dynamic effect of the wind-driven ice motion and the thermodynamic effect of surface air and ocean temperature anomalies. Since the ice drift responds within 12 hours to changesin SLP, dynamics initially dominates over
........ i..................
....i..........
.--..' ........
so%
O.45
0 ø (year 1)
AS
.............V,.... V'
0 35
0
.................. ,i................... ,i.................. ............... 1 0.1
0.2
0.3
0.4
0.5
frequency(cycles/year)
Figure 1. Local fractional variancespectrumof the joint seaice concentration(SIC), ice drift (ID) and sea level pressure(SLP) winter anomalies for the period 1979-1998. The spectrum is formed from the first singular value at each frequency, proportional to the variance explained by the most significant signal de-
tected in the data at that frequency. Significancelevels(dashed lines) are obtained by bootstrap resampling.
with
similar-scale
oscillations
observed
in the Weddell
90 ø (year 2)
Sea
[Venegasand Drinkwater,2001],we will refer to this signal as the Quasi-Quadrennial(QQ) oscillation.The evolutionof the spatial patterns during a QQ oscillation is shownin Figure 2. A positive SIC anomaly forming in the western Ross Sea in years 1-2 is seen to progress eastward to the RossAmundsen Sea in year 3, to the Amundsen-Bellingshausen Sea in year 4 and finally to the west of the Antarctic Peninsula in year 1. A negative SIC anomaly can be equally traced from its origin in the western Ross Sea in years 3-4. Such an eastward propagation of ice margin anomalies is indeed
180 ø (year 3)
observedin year-rounddata [Gloersenand Huang,1999].
/
SLP and ID anomalies propagate eastward in tandem with
the SIC anomalies.
Anomalous
meridional
.
,/,•.
ice flow in
the Ross-Amundsen Sea is aligned with the isobars of the
SLP anomalies(years1 and 3), indicatingthat this flow responds to changesin meridional geostrophic wind through
changesin air-icedrag. Poleward(equatorward)ID anomalies reduce (enhance)the ice flux toward the ice margin, therebycontributingto below (above)normal ice extent by compaction(dilation) of the ice pack. The resultingpat-
270ø (year 4)
tern is a series of eastward-propagating troughs and crests
in northward ice extent (negative and positive ice margin SIC anomalies)on the leadingand trailing sidesof the propagating low pressureanomalies,respectively. These coupled o•
air-iceanomalies moveat a speedof -•45øyr-1 (-•8 cms-1)
•
and account for the atmospheric and ice components of the
o•
ACW in the Pacificsector[White and Peterson,1996]. The oceanic component of the ACW is provided by the eastward advection of SST anomalies by the Antarctic Cir-
Figure
cumpolarCurrent (ACC). Warm (cold) SST anomaliespro-
SLP winter anomalies at four consecutive phases of a 4-yr period
vide a thermodynamic forcing of the ice edge through en-
oscillation(frequency0.25 cyclesyr-1) in the RossSea (RS),
hancedice melting (growth), thus being out of phasewith the ice edgeanomalies[Gloersenand White, 2001]. In ad-
AmundsenSea (AS) and Bellingshausen Sea (BS). SLP anomalies are in green (solid: positive, dashed: negative). SIC anomalies are in blue (positive) and red (negative). ID anomaliesare black
dition to the SST forcing, meridional winds on the leading
2. Spatial patterns of the reconstructedSIC, ID and
arrows. Similar structure at these phases is found for all bands
(trailing) edge of the propagatinglow pressureanomalies from 3 to 6 years. Contour intervals for SLP and SIC are 1.5 hPa lead to cold (warm) air advectiononto the ice pack (oppo- and 0.08, respectively.
VENEGAS
ET AL.: ANTARCTIC
ICE AND ATMOSPHERE enhanced
(a) climatology
VARIABILITY
ice formation
3303
could contribute
to increased
dense
water formation, especially in coastal polynyas over broad
regionsof the continentalshelf[Drinkwater,1996]. The interannual variability in coupled air-ice-sea interactions described here could lead to modulation in High Salinity Shelf Water production on these timescales. The contrasting situations in years I and 3 result from the propagation of the coupled air-ice anomalies through the Ross, Amundsen and BellingshausenSeas. Such propagation givesrise to an out-of-phasebehavior of the winter ice extent in the Ross-AmundsenSeas and the Bellingshausen Sea, as illustrated in Figure 4. The figure also shows how anomalous ice extent in the Ross-AmundsenSeas is negatively correlated with anomalous SLP over the Amundsen
(b) year 1
Sea.
The
dominance
of the interannual
timescale
in these
winter time series confirms that the QQ signal accounts for a large fraction of the total ice and atmospheric variance. According to Figure 4, the extreme situation of year I oc-
curredin the wintersof 1980, 1982 (weak), 1987, 1991-92 and 1995 and the extreme situation of year 3 occurred in
the wintersof 1979, 1985, 1989-90,1993 (weak) and 1996. The QQ cycles are best defined from 1984 to 1993. They are disturbed around 1982-83 and 1993-94, likely suggesting the interference of other, longer-period oscillations.
(c) year 3
Conclusions
Figure 3. The winter SLP field (thin lines) and ID field (arrows), and the 0.15 SIC isoline(representingthe winter ice margin, thick line) for (a) winter climatology1979-1998,(b) year 1, and (c) year 3. The patterns in (b) and (c) are obtained as the reconstructed winter anomalies shown in years I and 3 of Fig-
ure 2, respectively,addedto the winter climatologyshownin (a). Contour
interval
for SLP
is 3 hPa.
thermodynamics in reconfiguring the ice edge.
Coupled ice and atmosphere oscillations with period of about 3-6 years have dominated the winter variability in the Pacific sector of Antarctica during 1979-1998. Areas of anomalous ice concentration and drift propagate eastward in the ice pack coupled to SLP anomalies. The propagation of the atmospheric anomalies leads to a zonal shift in the location of the low pressurecenter from the Ross to the Amundsen Sea during an oscillation. The wind-driven ice circulation adjusts to the atmospheric changes, displaying either a well-defined Ross Sea gyre or a strong northeastward ice drift over the region. This signal accounts for the ACW ice and atmosphericcomponentsin the Ross, Amundsen,and BellingshausenSeas. Accompanyingsurfaceair and ocean temperature anomalies are out of phase with the ice extent anomalies. Sea ice anomalies at the ice margin seem to result from air-ice-sea
interactions
which involve
both the
dynamic effect of the wind-driven anomalous ice motion and ID, and SLP winter fields(a) with the completewinter fields the thermodynamic effect of surface ocean and air temperin the two extreme phasesof a characteristicQQ cycle (b atures leading to ice growth and melting.
Figure3 shows a comparison of the climatological SIC,
and c). Climatologicalgeostrophicwinds and ice drift are predominantly zonal in the entire region except for outflow
fromthe westernRossSea. The situationin year I (b) shows a well-defined Ross Sea gyre with a northward branch near 180ø and a southward branch near 140øW, associatedwith a low-pressure center over the Ross Sea. The southward ID component is responsiblefor reduced ice extent in the Amundsen Sea and for coastal ice convergenceand ridging. Also, warm air advection by the northerly wind component favors ice edge retreat by enhancedmelting. In the situation
of year3 (c), the low pressurecenteris shiftedeastwardover the Amundsen-BellingshausenSea. The strong equatorward component of the wind-driven ID leads to anomalousnorthward ice extent in the Amundsen Sea. Cold air advection by the southerly wind component contributes to enhanced ice growth rates in the divergent regions of the ice pack and at the ice margin. Anomalous southerly winds and ice drift and
o
.........
....
...... ..... ß• 0 •
O. 2........ O. 4 1978
' 1980
' 1982
' 1984
• 1986
' 1988
' 1990
' 1992
' 1994
' 1996
'-'•6 12 1998
years
Figure 4. Time series of winter SIC anomalies in the Ross-AmundsenSeas (RAS), (65øS-140øW, thick solid line); winter SIC anomalies in the BellingshausenSea (BS) (65øS70øW, thin solid line); winter SLP over the Amundsen Sea (AS) (65øS-105øW, dashed line). Correlation coefficients: r(SICRAS, SICBS)---0.67, r(SLPAS, SICRAS)=-0.78, r(SLPAS, SICBS)=0.84, all significantat the 99% level.
3304
VENEGAS
ET AL.: ANTARCTIC
ICE AND ATMOSPHERE
Since the winter sea ice cover is largely reduced during summer, the memory of the ACW from one winter to the next seems to reside, at least partly, in its oceanic compo-
nent throughanomaliesof SST [Gloersenand White,2001], and perhaps also sea surface salinity, advected eastward by the ACC. However, the results presentedhere suggestthat
the dynamiceffectof the meridionalice motion (associated with the anomalousatmosphericcirculation)is alsoimportant for reestablishing the winter ice edge anomalies after
the summer melt. Furthermore, the ice itself has a memory in terms of ice thickness anomalies in the perennial ice
coverwhichsurvivesthe summer(due to prior convergence and deformation)[Drinkwater et al., 2001]. When sucha thickness anomaly is transported to the ice margin the following winter, it becomes a freshwater anomaly via melt. This freshwater anomaly takes time to dissipate and produces stratification of the upper ocean near the ice edge. The resulting reduced ocean heat flux potentially allows ice edge anomalies to persist for longer periods. Similar coupled mechanismsare observedin the Atlantic
sectorof Antarctica [Venegasand Drinkwater,2001],which indicates that the passageof the ACW around Antarctica is likely due to local air-ice-sea interactions that are phaseshifted among the different basins. Several investigations have related the anomalous SLP pattern of year I to the E1
Nifio-SouthernOscillation(ENSO) phenomenon[Garreaud and Battisti, 1999;Harangozo,2000]. SLP anomaliesin Figure 2 seem to enter the subpolar region from the northwest, further suggesting their origin to be in the low latitude,
westernSouthPacific[Petersonand White,1998]. The link between ENSO and the ACW-related QQ oscillations described here warrants further study.
Acknowledgments.
The SMMR and SSM/I SIC data
are provided by the National Snow and Ice Data Center, Colorado. The ID data were produced by the Polar Remote Sensing Group at the JPL, California. The SLP data were obtained from
the NCAR/NCEP ReanalysisProject. S.A.V. and G.S. are grateful to the Danish National Research Foundation for its support of this work.
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(ReceivedFebruary 8, 2001; revisedJune 20, 2001; acceptedJuly 5, 2001.)
