Assimilation of TOPEX/Poseidon altimetric data in a primitive equation model of the tropical Pacific Ocean during the 1992-1996 El Nino-Southern Oscillation period

Four years of TOPEX/Poseidon data (October 1992 to October 1996) are assimilated into a high-resolution primitive equation model [Gent ann Cane, 1989] of the tropical Pacific Ocean. The assimilation method used is a degraded version of the Singular Evolutive Extended Kalman filter approach [Pham et al., 1998] in which the reduced order basis is fixed. The along-track TOPEX/Poseidon sea surface height residuals are introduced in the model every 3 days. Initialization of the model runs appears to be crucial. This is particularly true for the 1992-1996 period, which is characterized by several El Nino and La Nina events and exhibits quite anomalous climatological behavior. An in situ database dedicated to this period was built and used to initialize the model properly. The simulation results with and without assimilation were compared with expendable bathythermograph and Tropical Atmosphere-Ocean in situ observations. The impact of sea surface height assimilation is observed both on the surface and subsurface temperature and velocity fields. The deficiencies of the free model in terms of means and variability structures are significantly reduced. More importantly, the evolution of the model fields over time is considerably improved by the assimilation. These supposedly more realistic assimilated four-dimensional fields are used to investigate the mechanism responsible for the 1992-1996 warm pool replenishment and depletion. This approach provides insights into the combined role of zonal and meridional transports in balancing volume changes in the warm pool.