Prediction of sea level anomalies using ocean circulation model forced by scatterometer wind and validation using TOPEX/Poseidon data

Uncertainties in the surface wind field have long been recognized as a majo r limitation in the interpretation of results obtained by oceanic circulati on models. It is especially true in the tropical oceans, where the response to wind forcing is very strong on short time scales. The purpose of this p aper is to show that these uncertainties can be greatly reduced by using sp aceborne wind sensors that provide accurate measurements on a global basis. Surface winds over the global oceans have been measured by scatterometry si nce the launch of the European Remote Sensing Satellite (ERS-1) in August 1 991 by the European Space Agency, Noordwijk, The Netherlands, and is curren tly provided by ERS-2, launched in April 1995. The ground track wind vector s are processed to compute mean weekly surface winds onto a 1 degrees squar e grid at the Institut Francais de Recherche pour 1'Exploitation de la Mer (IFREMER), Plouzane, France. These winds are validated by comparison with t he buoy array in the tropical Pacific ocean, showing good agreement. In ord er to further evaluate this wind field, the three dimensional (3-D) ocean m odel OPA7 developed at Laboratoire d'Oceanographie Dynamique et de Climatol ogie, Paris, Prance, is forced over the tropical oceans by the ERS-derived wind stress fields and by fields from the atmospheric model Arpege/Climat. Selected ocean parameters are defined in order to validate the ocean model results with measurements of the tropical ocean and global atmosphere (TOGA ) buoys in the Pacific ocean. The ability of the model to describe the shor t scale (a few weeks to a few years) oceanic variability is greatly enhance d when the satellite-derived surface forcing is used. In this paper, we present further comparison of the ocean model results wit h the TOPEX-Poseidon altimeter measurements. Simulated and measured sea lev el variability are described over the three tropical oceans. The annual and semi-annual signals, as well as the interannual variability, partly linked to the El Nino southern oscillation (ENSO) phenomenon, are well simulated by the OPA7 model when the satellite winds are used. Furthermore, it shows that the objective method, kriging technique, used to interpolate the mean ERS wind fields, dramatically reduces the effects of the satellite bandlike sampling. In the last part of this paper, we focus on the relationship bet ween the wind stress anomalies and the sea level anomalies in the case of t he 1997-1998 El Nino event. It clearly shows that sea level anomalies in th e eastern and western parts of the Pacific are strongly linked to wind stre ss anomalies in the central Pacific. The forthcoming scatterometers aboard the METOP and ADEOS satellites will provide a much better coverage. It will enable the wind variability spatial and temporal scares to be resolved bet ter, in order that wind uncertainties no longer blur the interpretation of ocean circulation numerical models results.