ASSIMILATION OF TOPEX POSEIDON ALTIMETER DATA INTO A CIRCULATION MODEL OF THE NORTH-ATLANTIC/

Assimilation experiments were conducted using the first 12 months of T OPEX/POSEIDON (T/P) altimeter measurements in a multilayered quasigeos trophic model of the North Atlantic between 20 degrees N and 60 degree s N. These experiments demonstrate the feasibility of using T/P data t o control a basin-scale circulation model by means of an assimilation procedure. Moreover, they allow us to recreate the four-dimensional be havior of the North Atlantic Ocean during the year October 1992 - Sept ember 1993 and to improve our knowledge and understanding of such circ ulation patterns. For this study we used a four-layer quasigeostrophic model of high horizontal resolution (1/6 degrees in latitude and long itude). The assimilation procedure used is an along-track, sequential, nudging technique. The evolution of the model general circulation is described and analyzed from a deterministic and statistical point of v iew, with special emphasis on the Gulf Stream area. The gross features of the North Atlantic circulation in terms of mean transport and circ ulation are reproduced, such as the path, penetration and recirculatio n of the Gulf Stream, and its meandering throughout the eastern basin. The North Atlantic Drift is, however, noticeably underestimated. A no rthern meander of the north wall of the Gulf Stream above the New Engl and Seamount Chain is present for most of the year, while, just downst ream, the southers part of the jet is subject to a 100-km southeastwar d deflection. The Azores current is shown to remain stable and to shif t southward with time from the beginning of December 1992 to the end o f April 1993, the amplitude of the shift being about 2 degrees. The co mputation of the mean latitude of the Gulf Stream as a function of tim e shows an abrupt Shift from a northern position to a southern positio n in January, and a reverse shift, from a southern position to a north ern position, in July. Finally, some issues ape addressed concerning t he comparison of assimilation experiments using T/P data and Geosat da ta. The first results show that the T/P simulations are more energetic than the Geosat simulations, especially east of the Mid-Atlantic Ridg e, for every wavelength from 50 km to 500 km. This property is also ve rified in the deep ocean. The predicted abyssal circulation is indeed more energetic in the T/P case, which is more in accordance with what we know of the real ocean. Moreover, the good T/P altimeter coverage n ear the coasts greatly improves the model eddy kinetic energy levels i n these areas, especially east of 25 degrees W.