Simulations of ice-ocean dynamics in the Weddell Sea 2...

Simulations of ice-ocean dynamics in the Weddell Sea

2. Interannual variability 1985–1993

R. Timmermann, H. H. Hellmer, and A. BeckmannAlfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Received 30 November 2000; revised 26 June 2001; accepted 16 July 2001; published 28 March 2002.

[1] Investigations of sea ice-ocean interaction on the continental shelf in the southwestern WeddellSea reveal a strong correlation between fluctuations of atmospheric forcing and sea ice formation.Anomalies of meridional wind stress in the inner Weddell Sea are consistent with the phase of theAntarctic Circumpolar Wave. Positive anomalies of northward wind stress cause an increase of seaice export in the same year and of sea ice formation in the following year, leading to an increasedproduction of High-Salinity Shelf Water. Driven by a varying zonal density distribution over thecontinental shelf, the circulation in the Filchner-Ronne Ice Shelf cavity fluctuates between twomodes, each of which features a characteristic distribution of basal freezing and melting regions.Thus signals of interannual atmospheric variability propagate into the deep ocean and the sub-iceshelf cavities. INDEX TERMS: 4255 Oceanography: General: Numerical modeling; 4207Oceanography: General: Arctic and Antarctic oceanography; 4215 Oceanography: General:Climate and interannual variability (3309); 4219 Oceanography: General: Continental shelfprocesses; KEYWORDS: Antarctic Circumpolar Wave, Weddell Sea water mass formation, Filchner-Ronne Ice Shelf, sub—ice shelf circulation, interannual variability, BRIOS

1. Introduction

[2] Water mass transformation in the Weddell Sea is stronglyinfluenced by ice-ocean interaction. Intense cooling and brinerelease during sea ice formation on the southwestern continentalshelf lead to an increase in surface water density and deepconvection and thus to the formation of High-Salinity Shelf Water(HSSW), one ingredient for the formation of Weddell Sea BottomWater (WSBW) [Foster and Carmack, 1976]. An alternative wayof WSBW formation is provided by mixing of Warm Deep Water(WDW) with Ice Shelf Water (ISW), which pours out of theFilchner-Ronne Ice Shelf cavity and sinks down on the continentalslope [Foldvik et al., 1985]. This paper aims at an investigation ofthe seasonal and interannual variability of the coupled ice-oceansystem and its response to atmospheric anomalies.[3] One of the prominent signals of variability in the Southern

Ocean is the Antarctic Circumpolar Wave (ACW) [White andPeterson, 1996]. On the basis of an analysis of European Centrefor Medium-Range Weather Forecasts (ECMWF) reanalysis dataalong 56�S and of remote sensing sea ice concentration data, theACW has been described as a set of closely connected anomaliesof sea surface pressure and temperature, meridional wind stress,and sea ice extent. Positive anomalies of meridional wind stresscorrelate with an increased ice extent and vice versa. These patternsare described as propagating around Antarctica with a predom-inantly 4 year period, typically featuring a quadrupole structure ofpositive and negative anomalies.[4] Further examination of the ECMWF reanalysis data set,

however, reveals an ACW-related oscillation also in the innerWeddell Sea, i.e. the region south of the line Kapp Norvegia-Joinville Island. Time series of 3 monthly running means andannual means of meridional wind speed (Figure 1) reveal periodicfluctuations that are in phase with the ACW. Maxima of northwardwind stress are found in 1988 and 1992, and minima are found in1986 and 1990; 1990 is the only year in which the annual meanwind stress in the inner Weddell Sea is southward. This oscillation

can also be partly seen in the area-averaged 2 m air temperature(Figure 2); the southward anomaly of meridional wind stress in1990 is linked to a warm anomaliy of near-surface temperature,while the strong northward wind of 1992 advects anomalously coldair. The minimum of northward wind stress in 1986 and thesubsequent maximum in 1988, however, are not reflected byanomalies of 2 m temperature corresponding to the analyses ofWhite and Peterson [1996] in which anomalies of 1986/1988 areless pronounced than in the years from 1990 onward. Thus theACW is not an ideally periodic system; additional fluctuationssuperimpose the 4 year period oscillation. In this paper we willinvestigate effects of ACW-related atmospheric variability on seaice formation and water mass production in the inner Weddell Sea,investigating processes on the continental shelf (section 2) and inthe sub-ice shelf cavities (section 3) followed by a quantification ofthe freshwater budget in the Weddell Sea (section 4).[5] To achieve a self-consistent representation of both ice and

ocean dynamics (including the ice shelves), we use the coupled ice-ocean model BRIOS-2 described by Timmermann et al. [2002].With a grid focused on theWeddell sector of the Southern Ocean thismodel provides a reasonable horizontal resolution (20–50 km) inthe area of our main interest. The s coordinate ensures a high verticalresolution on the continental shelf, and consideration of the majorice shelves provides an adequate representation of sub-ice shelfprocesses and their impact on the hydrography of the Weddell Sea.

2. Sea Ice-Ocean Interaction in the SouthernWeddell Sea

2.1. Sea Ice Formation and Export

[6] Time series of the freshwater flux caused by sea iceformation in the inner Weddell Sea reveal the seasonal cycle offreezing and melting (Figure 3). Typical maxima of monthlymean freshwater extract or input are 200–300 mSv (1 mSv = 103

m3 s�1). The annual averages range from 18 to 59 mSv, with a 9year mean of 33.7 mSv [Timmermann et al., 2001]. Thus roughly1012 m3 yr�1 of fresh water are extracted from the inner Weddell

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C3, 3025, 10.1029/2000JC000742, 2002

Copyright 2002 by the American Geophysical Union.0148-0227/02/2000JC000742$09.00

11 - 1

Sea and exported across the line Kapp Norvegia-Joinville Island.Time series of monthly mean freshwater export (Figure 4) show aseasonal cycle that is quite similar to the seasonality of sea iceproduction (Figure 3). Compared to sea ice production, the iceexport maxima are delayed by a few weeks. As sea ice canacumulate because of dynamic effects, ice export occurs inter-mittently, driven by synoptic wind variability.[7] Unlike the time series of monthly means, the correlation

between annual means of ice production and export is not obvious;specifically, effects of ice production anomalies on ice exportanomalies in the same or the following year are not discernible.Apparently, the interannual variability of ice export is not (only)due to variations in sea ice production but is dominated by

temporary accumulation and the varying amount of sea ice thatsurvives the summer melting period.[8] On the other hand, a relation between ice export and the sea

ice production in the following year can easily be found. After thelarge ice export in 1992 a positive anomaly of sea ice formation in1993 is notable: an anomalous low ice coverage on the southwest-ern continental shelf during February/March 1993 is found both inthe model and in the observations, allowing the formation of largeamounts of sea ice with the beginning of autumn. This event isconsistent with the phase of the ACW: Winter 1992 features apositive anomaly of meridional (northward) wind in the innerWeddell Sea (Figure 1). Similar to that, albeit not as pronounced,is the correlation between the positive ice export anomaly in 1987

Jan.85 Jan.86 Jan.87 Jan.88 Jan.89 Jan.90 Jan.91 Jan.92 Jan.93 Jan.94-2

-1

0

1

2m

/s

1985 1986 1987 1988 1989 1990 1991 1992 1993-0.5

0.0

0.5

1.0

1.5

m/s

Figure 1. (top) Three month running means and (bottom) annual means of meridional (northward) 10 m wind speedfrom the ECMWF reanalyses of 1985–1993 averaged over the inner Weddell Sea.


-20

-15

-10

-5

0

C

1985 1986 1987 1988 1989 1990 1991 1992 1993-13.0

-12.5

-12.0

-11.5

-11.0

-10.5

Coo

Figure 2. (top) Three month running means and (bottom) annual means of 2 m temperature from the ECMWFreanalyses of 1985–1993 averaged over the inner Weddell Sea.

11 - 2 TIMMERMANN ET AL.: WEDDELL SEA INTERANNUAL VARIABILITY

and sea ice formation in 1988; however, these events appear not tobe linked to the phase of the ACW.[9] Comiso and Gordon [1998] demonstrated that at least for

the period 1979–1995, positive anomalies of winter ice extent inthe Atlantic sector of the Southern Ocean were succeeded bynegative anomalies of summer sea ice extent and vice versa. Thissupports the hypothesis that maxima of ice extent are not onlyproduced by a higher ice volume but also by an increased sea iceadvection out of the inner Weddell Sea. This would cause areduced ice volume in the following summer, which in turn leadsto an enhanced sea ice production (not necessarily volume) in thesubsequent autumn and winter.[10] The sea ice export minimum in the simulation of 1990 is

consistent with an annual mean southward wind stress over the

inner Weddell Sea (Figure 1). Sea ice formation in this regionappears not be affected; however, we will demonstrate below howthis event affects the freshwater flux and water mass formationover the continental shelf.[11] Time series of the sea ice volume flux across the line Kapp

Norvegia-Joinville Island have been derived from upward lookingsonars measurements by Harms et al. [2001]. Analyzing thesimulated volume flux of sea ice (not shown) instead of the seaice-related freshwater export reveals a time series quite similar toFigure 4. The 9 year mean simulated ice volume transport amountsto (42 ± 26) � 103 m3 s�1, which is close to the estimated (46 ± 8)� 103 m3 s�1 of Harms et al. [2001]. So, model results andobservations are very close in the long-term mean. Uncertainties inthe assumptions of both our model and the observations, however,


-400

-200

0

200

400

mSv

33.7

1985 1986 1987 1988 1989 1990 1991 1992 19930

10

20

30

40

50

60

mSv 33.7

Figure 3. (top) Monthly and (bottom) annual means of the simulated sea ice-related freshwater flux extracted fromthe inner Weddell Sea. Positive values indicate sea ice formation, i.e., loss of fresh water; 1 mSv = 103 m3 s�1.


-500

50100150200250

mSv

33.7

1985 1986 1987 1988 1989 1990 1991 1992 1993-20

0

20

40

60

80

mSv 33.7

Figure 4. (top) Monthly and (bottom) annual means of the freshwater export due to the drift of sea ice and snow outof the inner Weddell Sea.

TIMMERMANN ET AL.: WEDDELL SEA INTERANNUAL VARIABILITY 11 - 3

exclude a good agreement between the individual annual means ofboth time series.

2.2. Water Mass Formation

[12] Timmermann et al. [2001] discussed the simulated meanwater mass structure of the Weddell Sea. Comparing the clima-tological monthly means of March and September (Figures 5aand 5b), one notices only a small variability. A pronouncedseasonal cycle can be seen in the temperature of the light, near-surface water masses (S < 34.4) in the central Weddell Sea.However, variations in properties of the denser, saline water onthe continental shelf are small. In this region the water column iscovered by sea ice for most of the year; the ice-free period insummer is too short to provide much of a warming. Relativelystable stratification in summer reduces vertical mixing and theinput of ice shelf water (ISW) from the sub-ice shelf cavitiesprovides an additional cooling.[13] HSSW with S > 34.75 is poorly represented in the 9 year

mean and even in the climatological seasonal cycle (Figures 5aand 5b). Analysis of individual monthly means, however, indi-cates that the interannual variability of shelf water mass proper-

ties is significant. A time series of the simulated volume of salineshelf water (Figure 6) reveals a pronounced seasonal cycle mintedby the freezing and melting of sea ice in the southwesternWeddell Sea. However, HSSW is almost absent from late 1989until the winter of 1992. The change in shelf water mass structureis illustrated by the �-S diagrams of August 1991 and August1992 (Figures 5c and 5d): While no water with salinities exceed-ing 34.67 exists in the winter of 1991, a relatively large amountof HSSW with salinity up to 34.85 has been formed 1 year later.[14] A time series of the ocean surface freshwater flux on the

continental shelf (Figure 7) indicates the role of sea ice in thisvariability. In the 9 year average, 15 mSv of fresh water areextracted from the water column on the continental shelf. Theseasonal cycle of freezing and melting is well reproduced,modulated by a distinguished interannual variability. For theperiod 1985–1989 the time series features a quasi-stationaryseasonal cylce with a rather constant amplitude. The water massstructure in this period is very much resembled by the �-Sdiagrams in Figures 5a and 5b.[15] The situation changes drastically with the strong southward

wind stress anomaly found in 1990 (Figure 1), which leads to a

Figure 5. Simulated �-S diagrams of the inner Weddell Sea, i.e., the region south of the line Kapp Norvegia-Joinville Island as climatological monthly means of (a) March and (b) September and monthly means of (c) August1991 and (d) August 1992. Properties are marked according to the locations of grid points, i.e. ocean (blue) andcavities of Filchner-Ronne Ice Shelf (FRIS, red), Larsen Ice Shelf (LIS, yellow) and the Eastern Weddell Ice Shelves(EWIS, green). See color version of this figure at back of this issue.


pronounced ice export minimum (Figure 4). Sea ice formed on thecontinental shelf is unable to leave the area northward, so thatfurther sea ice formation is strongly reduced. Sea ice coverageduring the summer minimum 1990–1991 remains high, both inremote sensing observations [Heygster et al., 1996] and in modelsimulation. At the end, winter sea ice formation and thus fresh-water extract in the simulated years 1990–1991 is greatly reduced(Figure 7). In this period, HSSW gradually vanishes from thecontinental shelf because of mixing with (Modified) WDW(MWDW) and ISW. The emerging water mass is too light tocontribute to the formation of Weddell Sea Bottom Water(Figure 5c).[16] In contrast, 1992 is characterized by a maximum of

northward wind stress (Figure 1), which is linked to a positiveanomaly of sea ice export. Increased sea ice formation in thisand the subsequent winter provides enough salt input toreplenish the HSSW (Figure 5d) with salinities exceeding34.8. Hence water mass structure approaches the climatologicalmean (Figures 5a and 5b) again.[17] In reality, part of the newly formed saline water would

spread into the deep ocean and contribute to the formation ofWSBW. This spreading occurs mainly through isolated plumes,which are guided by deep ocean ridges or canyons [Jungclaus andBackhaus, 1994; Jungclaus et al., 1995] and have a typicalhorizontal scale of a few kilometers [Baines and Condie, 1998].Like every hydrostatic coarse-resolution model, BRIOS-2 is notable to resolve these processes and overestimates mixing withambient water masses. Thus density is reduced to a point thatmakes it impossible to sink down to the abyss; instead, thesewaters enter the Weddell Sea circulation at the Weddell Sea DeepWater (WSDW) level.[18] Another portion of the saline water leaves the continental

shelf and spreads northward along the western continental slope[Muench and Gordon, 1995]. In the BRIOS-2 simulations, cold,saline anomalies can be traced along the continental slope in thewestern Weddel Sea, gradually sinking down from 500 to 2000 m.About 12 months after their formation these anomalies reach the tip

of the Antarctic Peninsula and leave the Weddell basin through gapsin the South Scotia Arc. Thus surface forcing anomalies due to theACWand their response in the Weddell Sea’s outflow properties arecorrelated with a time lag of roughly 2 years, half the ACW period.[19] The remaining question is why a relation between ACW

and the production of HSSW is significant for 1990–1993 but notfor the period 1985–1989. As shown in section 1, anomalies ofnear-surface air temperature correlate with meridional wind fluc-tuations only for the period 1990–1993. In contrast to 1992, thepositive meridional wind stress anomaly of 1988 is not linked to acold anomaly of the 2 m temperature. Comparing monthly andannual means (Figure 2) shows that the 2 m temperature’sinterannual variability is determined by variations of the meanwinter temperature. While 1992 can be classified as an extremelycold winter, the winter of 1988 is specified as relatively warm.Therefore, compared to 1992, sea ice formation and export in 1988are reduced.[20] In addition to that, the negative anomalies of northward

wind stress differ in magnitude. In 1986 the annual mean windstress is still northward, while in 1990 the annual mean is directedsouthward. The latter is clearly visible in the winter months of1990 (Figure 1 (top)) and is responsible for the unusually low seaice export in 1990 and the reduced sea ice formation in thefollowing year.[21] In summary, the period 1990–1993 differs from the years

1985–1989 by higher extrema, causing more pronounced anoma-lies in sea ice formation and water mass production on thecontinental shelf. (An alternate hypothesis, that in our simulationthe ACW ran ‘‘out of phase’’ by repeating a 9 year period offorcing data, was rejected after a series of experiments omitting thedata from 1985 had produced the same results.) However, it shouldbe kept in mind that the ECMWF reanalysis data used to force thesimulation are a model output themselves. In the framework of thisstudy we are unable to distinguish whether the differences betweenthe two periods 1985–1989 and 1990–1993 are realistic or justdue to an insufficient data coverage in the earlier years. So, theresults we presented are not meant to be a hindcast of the Weddell


-50

0

50

100

mSv

15.2

FRIS

Figure 7. (left) Time series of sea ice-related freshwater fluxes on the continental shelf, as marked by the shadedarea on the (right) contour plot of bottom topography.

Jan.85 Jan.86 Jan.87 Jan.88 Jan.89 Jan.90 Jan.91 Jan.92 Jan.93 Jan.940

1.0•1013

2.0•1013

3.0•1013

4.0•1013

5.0•1013

m3

Figure 6. Time series of the simulated volume of saline shelf water, i.e., water with S > 34.65 and �2�C < � <�1.6�C.


Sea’s state in the years 1985–1993; instead, they are supposed toillustrate how the coupled ice-ocean system reacts on differentmodes of atmospheric forcing.

3. Ice Shelf-Ocean Interaction

[22] Models of ice shelf-ocean interaction [e.g., Hellmer andOlbers, 1989; Jenkins, 1991; Gerdes et al., 1999] indicate that iceshelf-ocean interaction is dominated by the pressure dependence ofthe in situ freezing point of seawater. Typically, large areas of basalmelting are found near the grounding line where the in situ freezingtemperature is significantly lower than the surface freezing point. Inregions with flow in the direction of decreasing ice shelf thickness,ice crystals can accumulate at the ice shelf base and form largebodies of marine ice [Engelhardt and Determann, 1987].[23] Corresponding to that, the modeled Filchner-Ronne sub-ice

shelf circulation in BRIOS-2 induces the formation of large areasof basal melting, especially near the grounding line. North of thecombined Henry/Korff Ice Rise complex, the long-term meanfeatures a region of basal freezing of up to 30 cm marine ice peryear [Timmermann et al., 2001]. Basal melting averaged over theFRIS area amounts to 29.8 cm yr�1.[24] Analyzing individual monthly means of sub-ice shelf basal

mass fluxes and ocean circulation, we found that the cavity beneaththe Filchner-Ronne Ice Shelf is not an independent system but isgreatly influenced by processes on the continental shelf north ofthe ice shelf edge. Sea ice plays an important part in this closelycoupled system as its formation determines the density distributionon the southwestern continental shelf.[25] From May to October, during the months of intense sea ice

formation, dense, saline water is formed on the continental shelf.

Portions of it are penetrating into the sub-ice shelf cavity. Dependingon where the densest water is located on the continental shelf, thesimulated sub-ice shelf circulation fluctuates between two modes.[26] The first mode (Figure 8 (top)) is characterized by the

density maximum located on the western Weddell shelf, near theAntarctic Peninsula. In this case, water from the open oceanpenetrates through the Ronne Depression into the Filchner-RonneIce Shelf (FRIS) cavity, leading to high melting rates at the base ofthe western Ronne Ice Shelf. Driven by the density gradient alongthe ice shelf edge, an anticyclonic circulation develops in theRonne cavity and around Berkner Island. In the Filchner cavity aweak cyclonic circulation exists that transports ISW northwardalong the east coast of Berkner Island, i.e., on the western slope ofthe Filchner Trough. This leads to basal freezing at the northwest-ern edge of Filchner Ice Shelf, in agreement with observations andprevious modeling [Grosfeld et al., 1998]. This process is subjectto a pronounced interannual variability with basal freezing ratesaround 1.5 m yr�1 in years with low HSSW formation.[27] The circulation is reversed in situations with the density

maximum located north of Berkner Island, which is typical foryears with a high formation rate of HSSW (Figure 8 (bottom)). Inthis mode, shelf water masses flow into the cavity directly west andeast of Berkner Island. Thus the highest melting rates are foundwest of Berkner Island and over the Filchner Trough. An anti-cyclonic circulation with a vertically integrated transport of up to2.5 Sv develops in the Filchner Trough, while a cyclonic circu-lation dominates beneath the Ronne Ice Shelf. Melting rates at theFilchner Ice Shelf base reach up to 2.5 m yr�1, while they do notexceed 1.5 m yr�1 in the ‘‘low HSSW’’ situation.[28] The FRIS cavity simulations of Gerdes et al. [1999], forced

with constant prescribed boundary conditions, feature a distribu-tion of freezing and melting regions that is similar to our low

BI

HK

ρ

BI

HK

Ψ

BI

HK

BI

HK

ρ

BI

HK

Ψ

BI

HK

27.5 27.55 27.6 27.65 27.7 27.75 27.8 27.85 27.9 27.95 28.0 kg/m3 -2. -1.6 -1.2 -0.8 -0.4 0 . 0.4 0.8 1.2 1.6 2. Sv -2.5 -2. -1.5 -1. -0.5 0. 0.5 1.0 1.5 2. 2.5 m/a

Figure 8. Characteristics of ice shelf-ocean interaction in situations with the maximum density located (top) on thewestern edge of the continental shelf (September 1989) and (bottom) north of Berkner Island (September 1992).Displayed are (left) monthly mean surface density, (middle) vertically integrated transport (stream function C), and(right) FRIS basal melting rates. Arrows indicate directions of the regional circulation. The bold black lines indicatethe ice shelf edge, BI is Berkner Island, and HK is Henry/Korff Ice Rise. See color version of this figure at back ofthis issue.


HSSW situation. One of the differences to our experiments is astronger cyclonic circulation in the Filchner Trough, which enhan-ces the region of basal freezing in the northwestern corner of theFilchner Ice Shelf. Another one is that the zone of high meltingrates beneath the central Filchner Ice Shelf is missing, which ismore pronounced in years with a high formation rate of HSSW butis also present in the low HSSW case.[29] Apparently, the circulation in the FRIS cavity changes as

part of an interannual variability, driven by fluctuations of density(salinity) distribution on the continental shelf north of the ice shelfedge. From analysis of water mass properties, Nøst and Foldvik[1994] concluded that south of Berkner Island, water is transportedfrom the Ronne into the Filchner cavity. However, Hellmer andOlbers [1991] already showed that the direction of flow south ofBerkner Island depends on the density distribution north of the iceshelf edge with a flow from the Ronne into the Filchner cavity inthe case of high-density water in the Ronne Depression. In BRIOS-2 this situation is typical for the years 1986–1989. During theperiod of low salt input on the continental shelf (1990–1991), flowacross the ice shelf edge is rather weak, and distinct circulationpatterns are hardly visible. During the period of high salt input,especially in the winter of 1992, a cyclonic circulation aroundBerkner Island with a transport from the Filchner into the Ronnecavity develops. The local wind field north of the ice shelf edgedoes not seem to play a major role in this process: a significantcorrelation between the wind direction and the sub-ice shelfcirculation was not found.[30] In an earlier BRIOS study [Beckmann et al., 1999] the

change from a cyclonic to an anticyclonic circulation in theFilchner Trough was described as a seasonal signal in a standaloneocean-ice shelf model (BRIOS-1). However, BRIOS-1 was forcedby quasi-climatological monthly means (no interannual variabil-ity), which were repeated for each year of integration. Forcing data

were derived from the integration of a stand-alone sea ice model(BRIOS-0), which was forced the same way as the coupled ice-ocean model BRIOS-2 (see Beckmann et al. [1999] for furtherdetails). Similar to the results presented here, net freezing rates andthus salt input north of Berkner Island in BRIOS-0 are high in thewinter months of 1992 and 1993. This signal is still present in theclimatological monthly means, so that BRIOS-1, forced with thesedata, reproduces an interannual variability as part of the seasonalcycle. If, as in BRIOS-2, the interannual variability of atmosphericboundary conditions is properly taken into account, seasonalvariations of the sub-ice shelf circulation are much smaller thanthe differences between the individual years of simulation.

4. Variability of the Freshwater Balanceof the Inner Weddell Sea

[31] According to the analyses of Timmermann et al. [2001] thefreshwater budget of the inner Weddell Sea can be described as along-term balance of freshwater loss due to sea ice formation andexport and freshwater gain due to precipitation and ice shelfmelting. Time series of monthly and annual means of eachcomponent (Figure 9) indicate that the long-term balance is theresiduum of large numbers different in sign. The seasonal andinterannual variability of the surface freshwater budget is domi-nated by fluctuations of sea ice formation both on a seasonal andan interannual timescale: In years with only moderate sea iceformation the surface freshwater budget is balanced or evenpositive. If sea ice formation is strong (1987, 1988, or 1993),however, an annual mean of up to 31 mSv of fresh water isextracted from the surface of the inner Weddell Sea. Thesefluctuations in the annual mean freshwater budget add up to astandard deviation of 13 mSv, which is considerably larger than the

ice shelves

sea ice

P-E

total Weddell Sea surface

1985 1986 1987 1988 1989 1990 1991 1992 1993-60

-40

-20

0

20

mSv

9.1

19.0

-33.7

-5.3


-200

0

200

400

mSv

Figure 9. Time series of (top) 30 day running mean and (bottom) annual mean freshwater fluxes from sea iceformation (solid), basal melting of ice shelves (dashed), and net precipitation (dotted) and the overall surface freshwaterfluxes in the inner Weddell Sea (bold). Numbers at the right axis indicate the 9 year averages; 1 mSv = 103 m3 s�1.


9 year mean of 5.7 mSv [Timmermann et al., 2001]. Therefore thenet southward advection of salt into the inner Weddell Sea is notsignificantly different from zero.[32] Compared to the fluctuations in sea ice formation, the

variability of sub-ice shelf melting is small. However, beneaththe Eastern Weddell Ice Shelves (EWIS), relatively warm waterfrom the coastal current is in direct contact with the ice shelf base.Variations of the water mass properties in the coastal current causefluctuations in the EWIS basal melt rates (Figure 10). Striking arethe high melting rates in the first quarters of 1986, 1990, and 1993,which are also found in the basal melt rates of the Larsen Ice Shelf(LIS) and FRIS. These anomalies are caused by very small summerice extents in 1986 and 1993, which can be found both in themodel and in the observations [Heygster et al., 1996]. In 1990,relatively large areas of the Weddell Sea remain ice covered, butthe simulation features open water along the eastern Weddell Seacoast and low ice concentration off FRIS and LIS. In these regionsthe ocean surface temperature increases by absorption of solarradiation. Downwelling transports this signal across the pycno-cline, and eventually, it is advected into the sub-ice shelf cavity,where it leads to an enhanced basal melting.[33] Similar to that, we have shown that variations of water

mass properties on the southwestern continental shelf affect thesub-ice shelf circulation of FRIS. During the period of vanishingHSSW (1990–1991), freshwater input by basal melting is reducedfrom typical values around 4 mSv to <2 mSv. Variability ofmonthly means is quite low during this period. Water massexchange across the ice shelf edge is reduced, so that sub-ice shelfcirculation and the ice-ocean interaction are virtually decoupledfrom varying boundary conditions at the ice shelf edge. With theformation of new HSSW, starting in 1992, the flow into the cavityincreases and basal melting recovers. The reversal of sub-ice shelfcirculation, however, has no significant impact on the area-aver-aged basal melt rates.

5. Conclusions

[34] Simulations with the coupled ice-ocean model BRIOS-2[Timmermann et al., 2002] indicate that large-scale atmosphericoscillations have a strong influence on bottom water production inthe Weddell Sea. Anomalies of meridional wind stress that are part

of the ACW cause significant fluctuations of the sea ice exportacross the line Kapp Norvegia-Joinville Island. Negative (i.e.,southward) wind stress anomalies are related to ice export minimaand lead to a reduced formation of sea ice and HSSW on thesouthwestern continental shelf. Positive anomalies of meridional(northward) wind stress, in turn, cause an enhanced ice export,resulting in a reduced summer sea ice coverage followed bystronger sea ice formation in the subsequent winter. HSSW, whichis formed during these periods, mixes with Modified CircumpolarDeep Water and is transported northward along the westerncontinental slope. Portions of this water can be traced as cold,saline anomalies that reach the tip of the Antarctic Peninsularoughly 12 months after their formation. Given a 4 year periodfor the ACW, wind stress anomalies and the response in theWeddell Sea outflow are phase-shifted by roughly half the ACW’speriod.[35] The production of Antarctic Bottom Water by mixing of

HSSW with MWDW at the slope front has been supposed to be aquite steady process because of the integration of properties in theshelf water [Gordon, 1991]. Our model results indicate, however,that this water mass formation may be quite an intermittentprocess, subject to the interannual variability of salt input on thecontinental shelf and the related changes in the volume of HSSW.[36] We have also demonstrated that sea ice formation on the

southwestern continental shelf affects the ocean circulation in theFilchner-Ronne Ice Shelf cavity significantly. An anticycloniccirculation in the Ronne cavity and around Berkner Island is thecommon situation if the maximum of shelf water density is foundon the western edge of the continental shelf. However, strong seaice formation in the southwestern Weddell Sea leads to a saltenrichment particularly on the Berkner Bank. With the densitymaximum to be found north of Berkner Island, sub-ice shelfcirculation reverses to an anticyclonic circulation in the FilchnerTrough and a cyclonic circulation beneath the Ronne Ice Shelf;however, area-averaged basal melt rates are hardly affected. Hence,by affecting sea ice formation and thus the density distribution ofthe continental shelf, signals of atmospheric variability propagateboth into the deep ocean and into the sub-ice shelf cavities.

[37] Acknowledgments. The authors would like to thank W. Cohrsand C. Lichey for preparing the ECMWF and NCEP atmospheric forcing

Jan.85 Jan.86 Jan.87 Jan.88 Jan.89 Jan.90 Jan.91 Jan.92 Jan.93 Jan.940

5

10

15

mSv

3.2

1.6

4.2

9.1

1985 1986 1987 1988 1989 1990 1991 1992 1993 0

5

10

15

mSv

3.2

1.6

4.2

9.1total

EWISFRISLIS

Figure 10. Time series of (top) monthly and (bottom) annual mean freshwater fluxes from total ice shelf melting(bold) and the contributions from FRIS (solid), EWIS (dashed), and LIS (dotted).


fields, which were received via the German Weather Service (DeutscherWetterdienst (DWD)) and the NOAA-CIRES Climate Diagnostics Center,Boulder, Colorado, using the Website http://www.cdc.noaa.gov/, respec-tively. Helpful discussions with E. Fahrbach are gratefully acknowledged.Constructive comments of J.R. Toggweiler and two anonymous reviewershelped to improve the manuscript.

ReferencesBaines, P. G., and S. Condie, Observations and modelling of Antarcticdownslope flows: A review, in Ocean, Ice, and Atmosphere: Interactionsat the Antarctic Continental Margin, Antarct. Res. Ser., vol. 75, edited byS. S. Jacobs and R. F. Weiss, pp. 29–49, AGU, Washington, D. C., 1998.

Beckmann, A., H. H. Hellmer, and R. Timmermann, A numerical modelof the Weddell Sea: Large-scale circulation and water mass distribution,J. Geophys. Res., 104, 23,375–23,391, 1999.

Comiso, J. C., and A. L. Gordon, Interannual variability in summer sea iceminimum, coastal polynyas, and bottom water formation in the WeddellSea, in Antarctic Sea Ice: Physical Processes, Interactions, and Varia-bility, Antarct. Res. Ser., vol. 74, edited by M. O. Jeffries, pp. 293–315,AGU, Washington, D. C., 1998.

Engelhardt, H., and J. Determann, Borehole evidence for a thick layer ofbasal ice in the central Ronne Ice Shelf, Nature, 327, 318–319, 1987.

Foldvik, A., T. Gammelsrød, and T. Tørresen, Circulation and water masseson the southern Weddell Sea shelf, in Antarctica Sea Ice: Physical Pro-cesses, Interactions, and Variability, Antarct. Res. Ser., vol. 43, edited byS. S. Jacobs, pp. 5–20, AGU, Washington, D. C., 1985.

Foster, T. D., and E. C. Carmack, Frontal zone mixing and Antarctic Bot-tom Water formation in the southern Weddell Sea, Deep Sea Res. Ocea-nogr. Abstr., 23, 301–317, 1976.

Gerdes, R., J. Determann, and K. Grosfeld, Ocean circulation beneathFilchner-Ronne Ice Shelf from three-dimensional model results, J. Geo-phys. Res., 104, 15,827–15,842, 1999.

Gordon, A. L., Two stable modes of Southern Ocean winter stratification,in Deep Convection and Deep Water Formation in the Oceans, editedby P. C. Chu and J. C. Gascard, pp. 17–35, Elsevier Sci., New York,1991.

Grosfeld, K., H. H. Hellmer, M. Jonas, H. Sandhager, M. Schulte, and D. G.Vaughan, Marine ice beneath Filchner Ice Shelf: Evidence from a multi-disciplinary approach, in Ocean, Ice, and Atmosphere: Interactions at the

Continental Margin, Antarct. Res. Ser., vol. 75, edited by S. S. Jacobs andR. F. Weiss, pp. 319–339, AGU, Washington, D. C., 1998.

Harms, S., E. Fahrbach, and V. H. Strass, Sea ice transports in the WeddellSea, J. Geophys. Res., 106, 9057–9074, 2001.

Hellmer, H. H., and D. Olbers, A two-dimensional model for the thermoha-line circulation under an ice shelf, Antarct. Sci., 1, 325–336, 1989.

Hellmer, H. H., and D. Olbers, On the thermohaline circulation beneathFilchner-Ronne Ice Shelf, Antarct. Sci., 3, 433–442, 1991.

Heygster, G., L. T. Pedersen, J. Turner, C. Thomas, T. Hunewinkel, H.Schottmuller, and T. Viehoff, PELICON: Project for estimation oflong-term variability in ice concentration, EC Contract Rep. EV5V-CT93-0268, Bremen, Germany, 1996.

Jenkins, A., A one-dimensional model of ice shelf-ocean interaction,J. Geophys. Res., 96, 20,671–20,677, 1991.

Jungclaus, J. H., and J. O. Backhaus, Application of a transient reducedgravity plume model to the Denmark Strait overflow, J. Geophys. Res.,99, 12,375–12,396, 1994.

Jungclaus, J. H., J. O. Backhaus, and H. Fohrmann, Outflow of dense waterfrom the Storfjord in Svalbard: A numerical model study, J. Geophys.Res., 100, 24,719–24,728, 1995.

Muench, R. D., and A. L. Gordon, Circulation and transport of water alongthe western Weddell Sea margin, J. Geophys. Res., 100, 18,503–18,515,1995.

Nøst, O. A., and A. Foldvik, A model of ice shelf-ocean interaction withapplication to the Filchner-Ronne and Ross Ice Shelves, J. Geophys. Res.,99, 14,243–14,254, 1994.

Timmermann, R., A. Beckmann, and H. H. Hellmer, The role of sea ice inthe fresh water budget of the Weddell Sea, Ann. Glaciol., 33, 419–424,2001.

Timmermann, R., H. H. Hellmer, and A. Beckmann, Simulations of ice-ocean dynamics in theWeddell Sea, 1,Model configuration and validation,J. Geophys. Res., 107(CX), 10.1029/2000JC000741, in press, 2002.

White, B. W., and R. G. Peterson, An Antarctic circumpolar wave in sur-face pressure, wind, temperature and sea ice extent, Nature, 380, 699–702, 1996.

��A. Beckmann, H. H. Hellmer, and R. Timmermann, Alfred Wegener

Institute for Polar and Marine Research, Postfach 12 01 61, D-27515Bremerhaven, Germany. ([email protected])


a)

WDW

WSDW

WSBW

SW

ISW

34.0 34.2 34.4 34.6 34.8 35.0-3.0

-2.0

-1.0

0.0

1.0

2.0

HSSW

b)

WDW

WSDW

WSBW

SW

ISW

34.0 34.2 34.4 34.6 34.8 35.0-3.0

-2.0

-1.0

0.0

1.0

2.0

HSSW

c)

WDW

WSDW

WSBW

SW

ISW

34.0 34.2 34.4 34.6 34.8 35.0-3.0

-2.0

-1.0

0.0

1.0

2.0

HSSW

d)

WDW

WSDW

WSBW

SW

ISW

34.0 34.2 34.4 34.6 34.8 35.0-3.0

-2.0

-1.0

0.0

1.0

2.0

HSSW

Figure 5. Simulated �-S diagrams of the inner Weddell Sea, i.e., the region south of the line Kapp Norvegia-Joinville Island as climatological monthly means of (a) March and (b) September and monthly means of (c) August1991 and (d) August 1992. Properties are marked according to the locations of grid points, i.e. ocean (blue) andcavities of Filchner-Ronne Ice Shelf (FRIS, red), Larsen Ice Shelf (LIS, yellow) and the Eastern Weddell Ice Shelves(EWIS, green).

11 - 4

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C3, 10.1029/2000JC000742, 2002

11 - 6

BI

HK

ρ

BI

HK

Ψ

BI

HK

BI

HK

ρ

BI

HK

Ψ

BI

HK

27.5 27.55 27.6 27.65 27.7 27.75 27.8 27.85 27.9 27.95 28.0 kg/m3 -2. -1.6 -1.2 -0.8 -0.4 0 . 0.4 0.8 1.2 1.6 2. Sv -2.5 -2. -1.5 -1. -0.5 0. 0.5 1.0 1.5 2. 2.5 m/a

Figure 8. Characteristics of ice shelf-ocean interaction in situations with the maximum density located (top) on thewestern edge of the continental shelf (September 1989) and (bottom) north of Berkner Island (September 1992).Displayed are (left) monthly mean surface density, (middle) vertically integrated transport (stream function C), and(right) FRIS basal melting rates. Arrows indicate directions of the regional circulation. The bold black lines indicatethe ice shelf edge, BI is Berkner Island, and HK is Henry/Korff Ice Rise.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C3, 10.1029/2000JC000742, 2002

Simulations of ice-ocean dynamics in the Weddell Sea 2...

Documents

Transcript of Simulations of ice-ocean dynamics in the Weddell Sea 2...