Assimilating XBT Data into HYCOM

Abstract

A scheme is presented for assimilating expendable bathythermographic data into HYCOM, an oceanic circulation model featuring a hybrid vertical coordinate. The scheme is fully multivariate, using observations of temperature to correct density, pressure, salinity, and momentum, in addition to temperature. Central to the scheme is the estimation of companion profiles of salinity and potential density. The potential density profiles are used to estimate the thicknesses of the model's layers, so that layer-averaged values of potential density and potential temperature can be computed. These derived data and the derived layer thicknesses are assimilated via optimal interpolation. Salinity corresponding to the corrected potential density and potential temperature fields is determined by the equation of state of seawater, and corrections to the momentum field are computed geostrophically from the corrections to the pressure field. The scheme is illustrated using data from March 1995 in the Atlantic Ocean.