Impact of ocean optical properties on seasonal SST: results with a surface ocean model coupled to the LMD AGCM

As the accuracy of ocean models improves, determination of the solar irradiance within the ocean may become important to simulate precisely the seasonal evolution of the SST. As ocean optical properties are not well documented in space and time, we have undertaken a sensitivity study to measure the corresponding SST uncertainties at a global scale using a model coupling the LMD AGCM with an integral mixed layer model and a thermodynamic sea ice representation. The downwelling irradiance formulation is that of Paulson and Simpson which has been tuned for the five water types of the Jerlov classification. Two sensitivity, and academic, experiments corresponding to a uniformly clear ocean or turbid ocean are carried out. Turbid waters exhibit, in general, a stronger seasonal cycle of the SST of about 2°C. The sensitivity is far from uniform, with a maximum in the subtropics and the mid-latitudes of the summer hemisphere. It corresponds precisely to the area in which the observed optical properties present a large temporal variability which is therefore likely to have an action on the seasonal cycle of the ocean surface temperatures. We perform a decomposition of the model sensitivity in four terms, corresponding to the direct impact of the water type change, feedback due to the mixed layer change, feedback due to the surface solar irradiance change, and feedback due to the non solar heat fluxes change. The first two terms dominate the SST change. The direct effect tends to increase the warming of the mixed layer. In addition, the mixed layer depth diminishes because of a higher stabilizing effect of solar radiation on the TKE budget. This tends to increase further summer warming of the SST as well as their winter cooling.