Variational Assimilation of Tao and Xbt Data in the Hope Ogcm, Adjusting the Surface Fluxes in the Tropical Ocean

A four-dimensional variational method has been developed that assimilates XBT and TAO sub-surface temperature data into the Hamburg Ocean Primitive Equation model. The method decreases the misfit between model and observed ocean temperatures by adjusting the surface forcing. The main goal of the assimilation scheme is to improve ocean analyses in the tropical Pacific. As a first study, only wind stress is adjusted in the assimilation. In two identical-twin experiments it is demonstrated that the scheme works well in the equatorial Pacific. The scheme is capable to reduce errors in the ocean analysis which originate either from the wind-stress forcing or the initial state. The impact of model errors on the data assimilation is investigated in an experiment with real observations. In this experiment, the temperature innovations near the equator are comparable to those of an Optimal Interpolation data-assimilation scheme.