JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 86, NO. C5, PAGES 4193-4197, MAY 20, 1981 Seasonalityof SouthernOcean Sea Ice ARNOLD L. GORDON Lamont-DohertyGeological Observatory of ColumbiaUniversity, Palisades, New York 10964 The seaice coverof the SouthernOcean undergoes a rapid decreasefrom mid-Novemberto mid-Janu- ary. The atmosphere-to-oceanheat flux is insufficientto accountfor thismelting, even in the presenceof the high percentageof open water characteristicof the SouthernOcean seaice. It is estimatedthat sea-air heatexchange in the 60ø to 70øSzone can account for roughly50% of the requiredspring heating. The remaindermust be suppliedby the relativelywarm deep water, residing below the SouthernOcean pyc- nocline.Deep-to-surface water heat flux is accomplishedby upwellingof the pycnoclinedue to the re- gionalEkman divergenceof the surfacelayer and by cross-pycnoclinemixing. It is estimatedthat the re- quiredvertical mixing coefficient for the pycnoclineis 1.5 cm2/s.This valueis consideredas realisticin view of the relativelyweak pycnocline which is a consequenceof low levelsof freshwater input to the surfacelayer. It is suggestedthat the magnitudeof freshwater input is a majorfactor in determiningthe degreeof seasonalityof SouthernOcean sea ice. 1. INTRODUCTION new ice. In this way new seaice forms in the interior of the sea The sea ice surrounding Antarctica undergoeslarge sea- ice fieldsrather than being accretedto the outer edge.While sonalchanges in areal extent.This is evidentfrom both histor- some addition at the edge may be expected,the northward ical observations and recent satellite sensing [Mackintosh, migration of the ice edge may be primarily governedby the 1946, 1972;Heap, 1964;Zwally and Gloersen,1977; S. F. Ack- Ekman velocity. ley, unpublishedmanuscript, 1980]. The Zwally et al. [1979] As ice is carried northward, the local heat balance would review of the sea ice areal cover as observedby satellitesfor induce melting, and a steady-stateice cover is attained, as 1973-1975 for greater than 15% and greater than 85% (repro- may be the casefrom August to October. During this period duced as Figure 1) show a sea ice maximum cover in Septem- ice forming in the south is balancedby melting in the north. ber and Octoberof approximately20 x 106km 2, with a mini- Using Ekman drift velocitiespresented by Taylor et al. [1978], mum cover in February of approximately 3 x 106 km2. the sea ice northward transport across60øS amountsto 9.62 x Therefore,17 x 106km 2 of seaice, an arealarger than Antarc- 10•3 W polewardheat flux, for eachmeter thicknessof ice. tica, is within the seasonalsea ice zone. As Zwally et al. (Fig- Ice melting in the springwould also be influencedby Ek- ure 1) indicate,much of this cover(about two thirds) has con- man divergence.The atmosphere-to-oceanheat flux would be centrations between 15% and 85% of full ice concentration. If greatestwithin the low albedo leadsgenerated by the diverg- the ice were compactedto 100%concentration, its area would ence. The leads act as heat collectors, which accelerate melt- be approximatelythree fourthsof the presentcover with leads ing of the surroundingpack. Langleben[1972] also stresses the (C. Parkinson,personal communication, April 1980). importanceof radiational heatingwithin leadsin regardto sea The growth and decayof the seaice covershown in Figure ice melting in the Arctic, but points out that radiational heat- 1 for both the 15%and 85% concentrationcurves are not sym- ing of the sea ice should not be neglected. metric about the maximum ice cover. The growth rate is less The ability of the sea-airheat flux to accountfor the spring than the decayrate by nearly a factor of two. The major por- retreatof the seaice is now tested.The ice chartsof the Navy- tion of the decay occursin the period mid-November to mid- NOAA ice center indicate that during the mid-November to Januarywith a decreasefrom 17.5x 106km 2 to 6.5 x 106km 2. mid-Januaryperiod sea ice virtually disappearsover the deep Decay of the greaterthan 85% coveroccurs earlier (mid-Octo- ocean.The additional ice melt from mid-Januaryto mid-Feb- ber to mid-November) and is even more spectacularin areal mary is mainly over the continental margins of Antarctica. decrease,retreating from the near annual maximum cover to The major exceptionis the 0.8 x 106km 2 (aboutone third of only 25% of that in the one month period. which is over the deep ocean)field of perennial seaice east of the Antarctic Peninsula. Walsh and Johnson[1979] area versusmonth plot for Arctic Southern Ocean seasonal sea ice is estimated at 1.0 to 1.5 m sea ice cover (their Figure 3) showsthat the ice growth rate is faster than the ice retreat rate. This contrast with the Southern [Untersteiner,1966]. The heat requiredto melt the averagesea Ocean situation suggeststhat fundamental differences exist ice thicknessof 1.25 m with a latent heat of fusionof 292 J/g between the heat budgetsof Arctic and Antarctic sea ice. (Table 4.3 from Neumannand Pierson[1966]) is 3.3 x 108J/ During the growth period for Southern Ocean sea ice a m2. Sincemost of the openocean ice is removedin the 60-day simple Ekman divergencemodel yields realistic results[Gor- period from mid-November to mid-January,the daily heat re- don and Taylor, 1975; S. F. Ackley, unpublishedmanuscript, quirementis 552 x 104J/m 2 d or 64 W/m 2. This representsa 1980]. The conceptis as follows: The cyclonicwind field in- minimum heat requirement since a higher value would be ducesa divergentEkman transportin the surfacelayer of the necessaryif significant warming of the surface water occurs ocean. Sea ice on the ocean also diverges, thus generating before mid-January or if the heat required to melt icebergsis open water or leads.In the winter when the ocean-atmosphere taken into account.While somewarming is observed,and ice- heat flux is directedtowards the atmospherethe leads fill with berg melting in early summeroccurs, both are neglectedin the calculations.The question is: Can the atmosphere-to-ocean heat flux provide enough heat to explain the sea ice decay Copyright¸ 1981by the AmericanGeophysical Union. even if no other demands are placed on the available heat? Paper number 80C 1623. 4193 0148-0227/81/080C- 1623501.00 4194 GORDON: SEASONALITYOF SOUTHERN OCEAN SEA ICE 25. tion. Trenberth[1979], on the other hand, determinesa 100 x 10•3 W flux across60øS, closer to the open oceanvalue and i• 20 about twice that for the realistic sea ice condition (Table 4). f 15- Newton[1972, Appendix C] gives29.3 x 10•3 W oceanheat _o _ flux across60øS, approximatelytwice Hastenrath'svalue, half of the Table 4 value for realistic ice cover, and one third of Trenberth's values. The one order of magnitude range in heat flux estimates across60øS indicatessome of the difficultiesin determining O FMAMJ d A SOND this importantnumber. Before it is possibleto identify the pri- MONTH mary mechanismresponsible for meridional heat flux in the Fig. l. Areal extent of Southern Ocean waters with sea ice con- Southern Ocean, it is necessaryto establish reliable sea-air centration at least 15% and with sea ice concentration at least 85% in heat flux estimates as a function of latitude for the Southern 1973, 1974, and 1975 [Zwally et al., 1979]. Ocean. In this presentstudy the mode in which the heat flux into the 60ø-70øS zone is accomplishedis important only in that it is assumed the annual heat loss of the surface water to 2. SEA-AIR HEAT EXCHANGE IN THE BAND 60ø-70øS the atmosphereis balancedby vertical heat transferacross the Estimates for sea-air heat exchange for the 60ø-70øS lati- pycnocline,rather than lateral poleward heat transferwithin tude band are made usingmonthly climaticdata (Table 1) for the surfacelayer itself. Since the Ekman drift is directed to- ice free (Table 2), full ice and partial ice cover(Table 3). Ad- wards the equator, this assumptionis justified. mittedly, the errors associatedwith these calculations are 3. DISCUSSION large, due to inadequate meteorological data set, use of monthly meansrather than shorterterm means,uncertainty in It is unlikely that the atmosphere-to-oceanheat flux given the exchangecoefficients for the non-radiative heat flux terms in Tables 2 and 3 is adequateto accountfor the massiveice and from uncertaintyin the exact amount of seaice insulation melt from mid-November to mid-January. For the realistic of the ocean(the R term in Table 3) and ratio of ice cover to ice coverthe averageheat gain for thisperiod is 34 W/m2, open ocean (P value in Table 3). slightlyover 50% of the requiredminimum heat flux. Even the The requiredpoleward oceanic heat across60øS for balance higherestimate of 47 W/m 2 for a no-icecondition is not suf- of oceanheat lossbetween 60ø-70øS (16.8 x 1016cm 2) is de- ficient and the full ice covervalue of 7 W/m 2 is grosslyin- termined for each of the ice concentrationsituations (Table sufficient.This suggeststwo things:leads induced by the gen- 4). Oceanic heat flux across60øS estimated by Hastenrath eral Ekman divergence must be an important factor by [1980]is 13 x 1013W. Hastenrath'sheat flux estimateacross increasing ocean warming to a point where atmosphere-to- 53øS,which marks the mean circumpolarposition of the polar ocean heat flux can play more than a minor role in the spring front, is 35 x 1013W (his Figure 9), similarto the value sug- ice cover retreat and that an additional source of heat is gestedby Gordon[1975]. It is surprisingthat his 60øSvalue is needed to balance the surfacewater heat balance during the so much lower than his 53øSvalue. It implieslarge oceanheat springmelting as well as on an annual basis(both valuesare lossin the 53ø-60øSzone (13 W/m 2) where other estimates about 30 W/m2). [see Taylor et al., 1978] actually suggesta small annual heat Heat flux into the surfacelayer from the sub-pycnoclinewa- gain by the oceanin this zone.