Variational Estimation of Temperature and Salinity With Vertical Coupled T-S Modes

A method for analyzing salinity, as well as temperature, is examined toward a development of a global ocean assimilation system in the Meteorological Research Institute in the Japan Meteorological Agency. The method is developed for correcting the model salinity field in the assimilation system. Correcting salinity is necessary for long term prediction, especially in the Western Equatorial Pacific and the boundary between subtropical and subpolar gyres where there is a great salinity variability. In the method, a variational method with vertical coupled temperature-salinity empirical orthogonal function (EOF) modes was used. We can handle covariances among state variables (i.e., temperature and salinity at all depths) through the EOF modes. The method was examined in the Equatorial and Northwest Pacific, adopting a climatology as the first-guess. The salinity field is analyzed without adopting salinity observation directly. Also, the interior structure of the ocean is effectively estimated from surface information alone (e.g., height, temperature and salinity of the sea surface). In the subtropical region, the analyses are more successful than another area. In the boundary between subtropical and subpolar gyres, the variability of salinity fields is estimated with some difficulties. The variability of the fresh water pool in the Western Equatorial Pacific is expressed. The results are validated by comparing with TOPEX/POSEIDON altimetry and other independent analyses.