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Formation and Maintenance of a Polynya in the Weddell Sea

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  • 1 Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven, Germany
  • | 2 Institut für Meereskunde an der Universität Kiel, Kiel, Germany
  • | 3 Institut für Meteorologie und Klimaforschung, Universität Karlsruhe, Karlsruhe, Germany
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Abstract

A dynamic–thermodynamic sea ice–mixed layer model for the Weddell Sea is complemented by a simple, diagnostic model to account for local sea ice–atmosphere interaction. To consider the atmospheric influence on the oceanic mixed layer, the pycnocline upwelling velocity is calculated using the theory of Ekman pumping. In several experiments, formation and conservation of a polynya in the Weddell Sea are investigated. Intrusion of heat into the lower atmosphere above the polynya area is assumed to cause a thermal perturbation and a cyclonic thermal wind field. Superposed with daily ECMWF surface winds, this modified atmospheric forcing field intensifies oceanic upwelling and induces divergent ice drift. Simulation results indicate that in case of a weak atmospheric cross-polynya flow the formation of a thermal wind field can significantly extend the lifetime of a large polynya. The repeated occurrence of the Weddell polynya in the years 1974–76 thus appears to be an effect of feedback mechanisms between sea ice, atmosphere, and oceanic mixed layer.

Corresponding authors address: Ralph Timmermann, Alfred-Wegener-Inst. for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany.

Email: rtimmerm@awi-bremerhaven.de

Abstract

A dynamic–thermodynamic sea ice–mixed layer model for the Weddell Sea is complemented by a simple, diagnostic model to account for local sea ice–atmosphere interaction. To consider the atmospheric influence on the oceanic mixed layer, the pycnocline upwelling velocity is calculated using the theory of Ekman pumping. In several experiments, formation and conservation of a polynya in the Weddell Sea are investigated. Intrusion of heat into the lower atmosphere above the polynya area is assumed to cause a thermal perturbation and a cyclonic thermal wind field. Superposed with daily ECMWF surface winds, this modified atmospheric forcing field intensifies oceanic upwelling and induces divergent ice drift. Simulation results indicate that in case of a weak atmospheric cross-polynya flow the formation of a thermal wind field can significantly extend the lifetime of a large polynya. The repeated occurrence of the Weddell polynya in the years 1974–76 thus appears to be an effect of feedback mechanisms between sea ice, atmosphere, and oceanic mixed layer.

Corresponding authors address: Ralph Timmermann, Alfred-Wegener-Inst. for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany.

Email: rtimmerm@awi-bremerhaven.de

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