Response of an Ocean Mixed Layer Model in 30-Day December Forecasts with a Coupled Ocean-Atmosphere Model

Stephen Brenner Environmental Modeling Center, National Centers for Environmental Prediction, Washington, D.C.

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Abstract

A mixed layer model of the upper ocean was coupled to the global medium-range forecast model of the National Centers for Environmental Prediction. The ocean model is of the type commonly referred to as a bulk model and includes detailed parameterizations of wind mixing, convective overturning, heating and cooling by surface fluxes, heating by penetrative radiation, and large-scale vertical advection driven by the curl of the wind stress. It is run on the, Gaussian grid of the T62 version of the atmospheric model. The ocean and atmospheric models exchange information once every 3 h.

Results are presented from a series of nine 30-day December forecasts. Each of the nine cases consisted of a one-way interaction forecast (the atmosphere forces the ocean only) and a two-way interaction forecast (predicted sea surface temperatures fed back into the atmospheric model). In the one-way forecasts, the model's skill is better than persistence at all times and comparable to or better than a persistent initial anomaly forecast out to 3 weeks in the extratropics and out to 12 days in the Tropics. In general, the two-way feedback acts to reduce the magnitude of net surface heat flux, leading to a slight increase in forecast skill in the Tropics and the Southern Hemisphere.

Finally, a preliminary assessment of the effects of the initial conditions shows that the forecasts in the winter hemisphere are indeed sensitive to the specification of the initial subsurface temperature profiles since the rate of wintertime cooling of the sea surface is closely related to the heat content (i.e., vertical structure) of the upper water column. In net surface heating regimes (the Tropics and the summer hemisphere) the forecasts are not particularly sensitive to the initial profiles since here the predicted mixed layer depths are determined by the balance between the net surface heating and the downward transport of heat by the wind-induced mixing.

Abstract

A mixed layer model of the upper ocean was coupled to the global medium-range forecast model of the National Centers for Environmental Prediction. The ocean model is of the type commonly referred to as a bulk model and includes detailed parameterizations of wind mixing, convective overturning, heating and cooling by surface fluxes, heating by penetrative radiation, and large-scale vertical advection driven by the curl of the wind stress. It is run on the, Gaussian grid of the T62 version of the atmospheric model. The ocean and atmospheric models exchange information once every 3 h.

Results are presented from a series of nine 30-day December forecasts. Each of the nine cases consisted of a one-way interaction forecast (the atmosphere forces the ocean only) and a two-way interaction forecast (predicted sea surface temperatures fed back into the atmospheric model). In the one-way forecasts, the model's skill is better than persistence at all times and comparable to or better than a persistent initial anomaly forecast out to 3 weeks in the extratropics and out to 12 days in the Tropics. In general, the two-way feedback acts to reduce the magnitude of net surface heat flux, leading to a slight increase in forecast skill in the Tropics and the Southern Hemisphere.

Finally, a preliminary assessment of the effects of the initial conditions shows that the forecasts in the winter hemisphere are indeed sensitive to the specification of the initial subsurface temperature profiles since the rate of wintertime cooling of the sea surface is closely related to the heat content (i.e., vertical structure) of the upper water column. In net surface heating regimes (the Tropics and the summer hemisphere) the forecasts are not particularly sensitive to the initial profiles since here the predicted mixed layer depths are determined by the balance between the net surface heating and the downward transport of heat by the wind-induced mixing.

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