Impact of surface wave mixing on sea ice and mixed layer depth

Duration: 20 mins 42 secs

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Description:	Rynders, S Friday 15th September 2017 - 10:20 to 10:40


Created:	2017-09-18 13:05
Collection:	Mathematics of sea ice phenomena
Publisher:	Isaac Newton Institute
Copyright:	Rynders, S
Language:	eng (English)


Abstract:	Co-authors: Yevgeny Aksenov (National Oceanography Centre), Daniel Feltham (University of Reading), George Nurser (National Oceanography Centre), Gurvan Madec (L’OCEAN Sorbonne Universités) Breaking waves cause mixing of the upper water column and present mixing schemes in ocean models take this into account through surface roughness. Sea surface roughness can be calculated from significant wave height, which is commonly parameterised from wind speed. We present results from simulations using modelled significant wave height instead, which accounts for the presence of sea ice and the effect of swell. The simulations use the NEMO ocean model coupled to the CICE sea ice model in a one degree configuration, with wave information from the ECWAM model of the European Centre for Medium-Range Weather Forecasts (ECMWF). It is found that in the simulations with modelled wave height mixing is reduced under the ice cover, since the parameterisation from wind speed overestimates wave height in the ice-covered regions. Decreased mixing decreases vertical heat fluxes to and from the sea ice, which in turn affects sea ice concentration and ice thickness. In the Arctic, ice thicknesses increase overall, with higher increases in the Western Arctic and decreases along the Siberian coast. In the Southern Ocean the meridional gradient in ice thickness and concentration is increased. The new mixing parameterisation improves sea ice volumes in the simulation, especially in the Southern Ocean, where the model has difficulty reproducing the winter sea ice volumes. The mixed layer depth under sea ice is also improved, without affecting mixed layer depth in ice-free regions. Wave and sea ice coupling will become more important in the future, when wave heights in a large part of the Arctic are expected to increase due to sea ice retreat and a larger wave fetch. Therefore, wave mixing constitutes a possible positive feedback mechanism for sea ice decline. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 607476.

Abstract:

Co-authors: Yevgeny Aksenov (National Oceanography Centre), Daniel Feltham (University of Reading), George Nurser (National Oceanography Centre), Gurvan Madec (L’OCEAN Sorbonne Universités)

Breaking waves cause mixing of the upper water column and present mixing schemes in ocean models take this into account through surface roughness. Sea surface roughness can be calculated from significant wave height, which is commonly parameterised from wind speed. We present results from simulations using modelled significant wave height instead, which accounts for the presence of sea ice and the effect of swell. The simulations use the NEMO ocean model coupled to the CICE sea ice model in a one degree configuration, with wave information from the ECWAM model of the European Centre for Medium-Range Weather Forecasts (ECMWF). It is found that in the simulations with modelled wave height mixing is reduced under the ice cover, since the parameterisation from wind speed overestimates wave height in the ice-covered regions. Decreased mixing decreases vertical heat fluxes to and from the sea ice, which in turn affects sea ice concentration and ice thickness. In the Arctic, ice thicknesses increase overall, with higher increases in the Western Arctic and decreases along the Siberian coast. In the Southern Ocean the meridional gradient in ice thickness and concentration is increased. The new mixing parameterisation improves sea ice volumes in the simulation, especially in the Southern Ocean, where the model has difficulty reproducing the winter sea ice volumes. The mixed layer depth under sea ice is also improved, without affecting mixed layer depth in ice-free regions. Wave and sea ice coupling will become more important in the future, when wave heights in a large part of the Arctic are expected to increase due to sea ice retreat and a larger wave fetch. Therefore, wave mixing constitutes a possible positive feedback mechanism for sea ice decline. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 607476.

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