An assessment of the ECMWF reanalysis (ERA) air/sea fluxes using Wave and Ocean General Circulation Models

Een assessment van de heranalyse (ERA) van het ECMWF zee-oppervlak fluxen door middel van de Wave and Ocean General Circulation Modellen


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As a contribution to ongoing efforts in modelling of the coupled atmosphere/ocean system we have assessed global fields of air/sea fluxes of heat and momentum, produced as part of the European Centre for Medium-range Weather Forecast Reanalysis project (ERA). In the wave part of this project we made a 15-year simulation of the global wave climate, by forcing a reliable wave model with ERA winds, and by compiling a data base of wave observations. By comparing the wave model response with the observed wave heights we were able to assess the quality of the ERA surface winds, and associated momentum fluxes. In general, agreement was good, confirming the accuracy of ERA. However, high waves were underpredicted. We have given arguments that this underprediction results from the relatively low resolution of the atmospheric model. We have also studied the wind stress parameterization of the planned 40-year reanalysis (ERA40). In this reanalysis the aerodynamical roughness of the sea surface depends on the sea state which is calculated with the wave model. We have found that the ERA40 parameterization leads to a better representation of observed wind stress variability. In the ocean part of the project we assessed heat and momentum fluxes by forcing general circulation models of the ocean and by comparing the (upper) ocean temperature response with observations of the deeper ocean. Although the scope of our study is global we made a special study of the response of the Southern Ocean and of the Tropical Pacific. From our Southern Ocean study the main conclusion was that ERA has a realistic interannual variability in heat and momentum fluxes. In our Tropical Pacific study we developed a method which allows the improvement of both the fluxes and the ocean analysis.

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