Aspects of Geostrophic Adjustment during Tropical Ocean Data Assimilation

Abstract

Subsurface temperature data from the ship-of-opportunity network in the tropical Pacific Ocean was assimilated into a simple reduced-gravity model. A large initialization shock was found to occur in the model which takes the form of equatorially trapped waves. Observations in the western tropical Pacific are found to generate a more severe case of initialization shock than observations in the eastern half of the basin. In addition, the magnitude of the initialization shock was found to be dependent upon the strength of the sea surface forcing. Attempts to suppress the large amplitude equatorial Kelvin waves and Yanai waves excited as part of the initialization shock are partially successful, but the damage inflicted on the model first-guess fields by this procedure is greater than that which ensues if the Kelvin waves and Yanai waves are left unchecked. Despite the initialization shock, the model is able to predict the large-wale structure and variability of the major near surface currents in the currents in the central tropical Pacific. The spinup time of these currents was ?1 year.