VARIATIONAL ASSIMILATION OF ALTIMETER DATA IN A MULTILAYER MODEL OF THE TROPICAL PACIFIC-OCEAN

A four-dimensional variational method is used to examine the extent to which a time sequence of altimeter measurements can determine the sub surface flow in a linear multilayer model of the tropical Pacific Ocea n. The experiments are all of the identical-twin type. Complete maps o f sea level extracted from the model in a control integration play the role of the altimeter observations in the assimilation experiments. T he results of the experiments indicate that, over timescales of months , the sea level information can be effectively propagated into the sub surface, particularly in the dynamically active equatorial region. Sev eral degrees off the equator, however, where waves propagate more slow ly, the recovery of the subsurface Bow in models containing more than two vertical modes is significantly more difficult. The sensitivity of these results to the lengths of the data sampling and assimilation pe riods is discussed.