Impact of mesoscale dynamic and thermodynamic changes in sea ice on the development of low pressure systems in the Fram Strait

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In polar regions the exchange of heat and momentum between the ocean and the atmosphere depends on the sea ice distribution. Sea ice acts as an insulating layer between the relatively warm ocean water and the mostly cold air. The sea ice concentration can change on time scales from several hours to a few days due to dynamic processes like ice drift or breaking of the ice sheet and thermodynamic processes like melting and freezing. The influence of these mesoscale changes of the ice distribution on the heat exchange at the surface and on atmospheric processes is investigated by numerical simulations for the Fram Strait region. The studies are performed with the model system METRAS/MESIM (Dierer et al., 2005). It consists of the mesoscale atmospheric model METRAS (Lüpkes and Schlünzen, 1996; Schlünzen, 1990) that is interactively coupled with the mesoscale sea ice model MESIM (Birnbaum, 1998). The sea ice model is able to simulate dynamic and thermodynamic processes in the ice jointly and separately. Therefore, it is possible to evaluate the influence of these processes on the ice concentration and the resulting impact on low pressure systems in detail. Two periods are simulated: The first one from 05 to 07 March 2002, and the second one from 12 to 15 March 2002. During the first phase, a trough was passing from East to West through the Fram Strait region. During the second phase, three cyclones were passing in series over Fram Strait (Brümmer et al., 2008). In March 2002, the field experiment FRAMZY 2002 took place in the region of Svalbard and Fram Strait including ship measurements, aircraft measurements and drift buoys (Brümmer et al., 2005). The model results are compared with these measurements and the influence of changes in the sea ice cover on the exchange of heat and momentum at the surface and on the development of the trough (first phase) and the "cyclone family" (second phase) are investigated. For this development dynamically caused changes of the sea ice seem to be more important than thermodynamic changes. For sea ice export this is less clear. Zones of strong convergences and divergences in the wind field support break up of sea ice and the positioning of high wind regimes leads to important changes in ice drift velocities. Under these conditions, changes in ice concentration are mainly due to dynamic processes thus playing an important role for the exchange of heat and momentum between ocean, sea ice and atmosphere and for the development of low pressure systems. References: Birnbaum, G., 1998: Numerical modelling of the interaction between atmosphere and sea ice in the Arctic marginal ice zone. Reports on Polar Research, Vol. 268, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, 160 pp. Brümmer, B., Launiainen, J., Müller, G. and Schröder, D., 2005: FRAMZY 2002, Second field experiment on Fram Strait Cyclones and their impact on sea ice. Field report with Measurement Examples. Rep. 37, Zentrum für Meeres- und Klimaforschung der Universität Hamburg, Hamburg, Germany, 154 pp. Brümmer B., Schröder D., Müller G., Spreen G., Jahnke-Bornemann A. Launiainen J. (2008): Impact of a Fram Strait cyclone on ice edge, drift, divergence, and concentration: Possibilities and limits of an observational analysis. J. Geophys. Res. - Oceans, 113, C12, C12003, DOI: 10.1029/2007JC004149. Dierer S., Schlünzen K.H., Birnbaum G., Brümmer B., Müller G. (2005): Atmosphere- Sea Ice Interactions during a Cyclone Passage Investigated by Using Model Simulations and Measurements. Month. Wea. Rev., 133, No.12, 3678-3692. Lüpkes C. and Schlünzen K.H. (1996): Modelling the Arctic convective boundary-layer with different turbulence parameterizations. Boundary-Layer Meteorol, 79, 107- 130. Schlünzen, K.H., 1990: Numerical studies on the inland penetration of sea breeze fronts at a coastline with tidally flooded mudflats, Beitr. Phys. Atmosph., 63, 243-256.

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