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Formation and Maintenance of a Polynya in the Weddell Sea

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Formation and Maintenance of a Polynya in the Weddell Sea Formation and Maintenance of a Polynya in the Weddell Sea Ralph Timmermann Alfred-Wegener-Institut fur Polar- und Meeresforschung, Bremerhaven, Germany Peter Lemke Institut fur Meereskunde an der Universitat Kiel, Germany Christoph Kottmeier Institut fur Meteorologie und Klimaforschung, Universitat Karlsruhe, Germany ; accepted Received Short title: 2 Abstract. A dynamic-thermo dynamic sea ice-mixed layer mo del for the Weddell Sea is complemented by a simple, diagnostic mo del to account for lo cal sea ice - atmosphere interaction. To consider the atmospheric in uence on the o ceanic mixed layer, the pycno cline upwelling velo city is calculated using the theory of Ekman pumping. In several exp eriments, formation and conservation of a p olynya in the Weddell Sea are investigated. Intrusion of heat into the lower atmosphere ab ove the p olynya area is assumed to cause a thermal p erturbation and a cyclonic thermal wind eld. Sup erp osed with daily ECMWF surface winds this mo di ed atmospheric forcing eld intensi es o ceanic upwelling and induces divergent ice drift. Simulation results indicate that in case of a weak atmospheric cross p olynya ow the formation of a thermal wind eld can signi cantly extend the life-time of a large p olynya. The rep eated o ccurrence of the Weddell Polynya in the years 1974 to 1976 thus app ears to b e an e ect of feedback mechanismsbetween sea ice, atmosphere and o ceanic mixed layer. 3 Intro duction During the Southern Hemisphere winter up to 20 million square kilometres of the Antarctic o cean are ice-covered. Scattered patches of op en water within the sea ice region consist of narrow leads of 1 to 10 km length and wide op enings called p olynyas. Although they comprise only a few p ercent of the ice-covered area, leads and p olynyas in the winter pack ice are imp ortant for a wide range of physical and biological pro cesses. Here, the relatively warm water at freezing temp erature is in contact with the cold air ab ove, thus causing large upward turbulent uxes of heat and water vap our. Coastal p olynyas are a few hundred meters to 100 km wide and can b e explained by prevailing winds and o cean currents at the margins of the Antarctic continent Kottmeier and Engelbart, 1992. In the Weddell Sea, they contribute signi cantly to the formation of deep and b ottom water which is the ma jor source of the b ottom water of the world o cean Fahrbach et al., 1995. O shore p olynyas are far less frequent. The most striking eventwas observed in the Weddell Sea for the entire winters of 1974 { 1976 and is called \Weddell Polynya". 2 Within an area of ab out 300 000 km between 64 S and 69 S near the Greenwich meridian, the ice concentration did not exceed 15 while the surrounding area was nearly completely ice covered. According to the analysis of remote sensing data collected by the Electrically Scanning Microwave Radiometer ESMR, the p olynya did not emerge from an already existing ice cover; the ice cover rather grew around the p olynya nally creating a secluded op en water area Zwally et al., 1981. Di erent explanations for the o ccurrence of such an extended and long-lived p olynya were given. The simplest mechanism which might cause the formation of a large area of op en water would b e the exp ort of ice due to a divergent wind eld. However, the rate of ice exp ort required to maintain nearly ice free conditions in the p olynya area would b e of the order of meters p er second. This would require wind sp eeds averaging over 50 1 ms which is clearly to o large to b e reasonable Martinson et al., 1981. 4 From 1974 to 1976, the p olynya shifted westward with a mean velo city of 1 cm/s which is consistent with the mean o cean current Carsey, 1980. From this p oint, the phenomenon app ears to b e related to o ceanic pro cesses. Comparison of water temp eratures within 200 to 2700 m depth b etween the years 1973 and 1977 shows substantial co oling after the p olynya's o ccurrence. Gordon 1982 explained this by extensive heat loss to the atmosphere, connected with low stability of the water column and deep convection. Martinson et al. 1981 prop osed that when surface co oling and ice formation reduce the temp erature and increase the salinity of a shallow mixed layer, static instability with intense vertical mixing can o ccur. The upwelled warm, salty deep water can supply enough heat to melt the ice or prohibit its formation. To induce a shallow mixed layer, Martinson et al. 1981 suggested that upwelling may raise the pycno cline until convection can o ccur. Actually, Gordon and Hub er 1984 detected large warm subsurface eddies which travelled from the east into the Weddell Sea. These warm water cells lifted the mixed layer base signi cantly and increased the pycno cline temp erature, leading to more intense entrainmentofwarm and saltywater. Using a one-dimensional, purely thermo dynamic, sea ice - mixed layer mo del, Lemke 1987 indicated that b oth a divergent drift of sea ice and the pycno cline upwelling prop osed by Martinson et al. 1981 can induce the formation of a p olynyain three subsequentyears: A divergent ice drift reduces the mean ice thickness and the ice concentration during the season of sea ice formation. This leads to a higher freezing rate and thus to increased brine rejection. Increased upwelling leads to enhanced entrainmentofwarm, salty deep water into the mixed layer. While heat is lost to the atmosphere, salinity of the mixed layer is preserved if the e ects of di usion and advection are neglected. 5 In b oth cases, increased density of the mixed layer induces deep convection with 2 entrainment heat uxes of the order of 150 W/m Lemke, 1987. To reduce the formation of sea ice signi cantly, it is essential that during the p erio d of increased upwelling the o cean is not yet ice-covered. In case of an existing ice cover, melting of sea ice due to entrained heat creates a fresh water ux which stabilizes the upp er o cean and prohibits convection. In a similar simulation using a dynamic-thermo dynamic sea ice-mixedlayer mo del Lemke et al., 1990 the system's resp onse was limited to the rst winter. Apparently, advection of sea ice into the ice free area and subsequent melting i.e. stabilization of the water column reduces the lifetime of a p olynya signi cantly. In order to maintain a p olynyaover a longer p erio d a few years two prerequisites seem to b e necessary: 1. a suciently weak o ceanic strati cation, and 2. no in ow of ice into the p olynya. This pap er aims to address the second p oint through examination of the interactions between sea ice, o ceanic mixed layer and lower atmosphere which can explain the formation and conservation of an op en o cean p olynya similar to the 1974 { 1976 observations. As already mentioned, the upward heat uxes over a p olynya far exceed the values over a solid ice cover. Assuming that this causes lo cal warming of the lower atmosphere, a low pressure system asso ciated with cyclonic thermal wind will b e forced to develop. This will havetwo e ects on sea ice conditions Fig. 1: 1. The surface wind stress will induce a divergent ice drift. 2. The curl of the surface wind stress will according to the theory of Ekman pumping enhance the o ceanic upwelling velo city. Through the exp ort of ice out of the p olynya, this lo cal wind system may b e able to maintain itself - as long as the surrounding area is ice-covered and the large-scale 6 pressure gradients are relatively small. To fo cus on this item, a state-of-the-art sea ice mo del is coupled to a diagnostic mo del of the thermal wind system whichisintro duced in the following section. Results of the coupled simulations are presented and consequences for the maintenance of the p olynya are discussed. Mo del description Sea ice-mixed layer mo del The sea ice dynamics is based on the dynamic-thermo dynamic sea ice mo del presented by Hibler 1979. Thermo dynamic ice growth rates are derived from a surface energy balance Parkinson and Washington, 1979 using the Semtner 1976 zero-layer approach for heat conduction. The incoming longwave and shortwave radiation uxes are treated with standard formulations based on Idso and Jackson 1969 and Zillmann 1972. A prognostic snowlayer Owens and Lemke, 1990 considering the e ect of snow/ice conversion Lepparanta, 1983; Fischer, 1995 is included. To determine the o ceanic heat ux prognostically, the mo del is coupled to the one-dimensional o ceanic mixed layer-pycno cline mo del of Lemke 1987, as describ ed by Lemke et al. 1990. For this study, a diagnostic calculation of the pycno cline upwelling velo city is added using the theory of Ekman pumping. If A denotes the ice concentration, then the surface stress on top of the o cean is ~ =1 A~ + A~ ; 1 a iw where ~ and ~ represent the atmospheric surface stress and the shear stress b etween a iw ice and water, resp ectively. The upwelling velo city is calculated from curl ~ using the standard formula.
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