Parameterization of vertical mixing in the Weddell Sea |
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| Institution: | 1. National Center for Atmospheric Research, Boulder, CO, USA;2. Canadian Centre for Climate Modeling and Analysis, Environment and Climate Change Canada, Victoria, British Columbia, Canada;3. Atmospheric, Oceanic, and Planetary Physics, Department of Physics, University of Oxford, Oxford, UK;1. Department of Physics, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand;2. Geophysical Institute, University of Alaska Fairbanks, P.O. Box 757500, Fairbanks, AK 99775, USA;3. National School of Surveying, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand;4. Department of Chemistry, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand;5. National Institute of Water & Atmospheric Research Limited (NIWA), 301 Evans Bay Parade, Hataitai, Wellington 6021, New Zealand;6. Callaghan Innovation, P.O. Box 31-310, Lower Hutt 5040, New Zealand |
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| Abstract: | A series of vertical mixing schemes implemented in a circumpolar coupled ice–ocean model of the BRIOS family is validated against observations of hydrography and sea ice coverage in the Weddell Sea. Assessed parameterizations include the Richardson number-dependent Pacanowski–Philander scheme, the Mellor–Yamada turbulent closure scheme, the K-profile parameterization, a bulk mixed layer model and the ocean penetrative plume scheme (OPPS). Combinations of the Pacanowski–Philander parameterization or the OPPS with a simple diagnostic model depending on the Monin–Obukhov length yield particularly good results. In contrast, experiments using a constant diffusivity and the traditional convective adjustment cannot reproduce the observations. An underestimation of wind-driven mixing in summer leads to an accumulation of salt in the winter water layer, inducing deep convection in the central Weddell Sea and a homogenization of the water column. Large upward heat fluxes in these simulations lead to the formation of unrealistic, large polynyas in the central Weddell Sea after only a few years of integration. Furthermore, spurious open-ocean convection affects the basin-scale circulation and leads to a significant overestimation of meridional overturning rates. We conclude that an adequate parameterization of both wind-induced mixing and buoyancy-driven convection is crucial for realistic simulations of processes in seasonally ice-covered seas. |
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