INFLUENCE OF TROPOSPHERIC WATER-VAPOR CORRECTIONS ON GEOSAT ALTIMETRYIN THE NORTH-ATLANTIC OCEAN

Geosat sea surface height (SSH) data in the tropical and midlatitude N orth Atlantic are analyzed with and without water vapor (WV) correctio ns to study the WV influence on along-track SSH anomaly profiles, meso scale SSH variability, wavenumber spectra, and objectively mapped fiel ds of SSH anomaly. Three different WV datasets were used, one from the Fleet Numerical Oceanographic Center (FNOC) model and two from the Sp ecial Sensor Microwave/Imager (SSM/I) based on different WV retrieval algorithms. These WV datasets show significant differences, in particu lar in the tropics. However, the method for deriving SSH anomalies fro m altimeter height data filters out much of the WV corrections. The re sidual WV effect on SSH anomaly is shown to be most significant in the seasonally migrating intertropical convergence zone of the tropical A tlantic: there the SSM/I corrections reduce the along-track mesoscale SSH variability by typically 1-1.5 cm. On seasonal timescales the maxi mum WV effect in this region is characterized by a 2-3-cm rms differen ce between SSH anomaly with and without SSM/I WV corrections, whereas FNOC corrections have almost no effect. Inferred seasonal velocity var iations in the North Equatorial Countercurrent core (4-degrees-6-degre es-N) in the region of maximum WV influence (30-degrees-40-degrees-W) are reduced by about 20% and 30%, depending on whether SSM/I correctio ns by Emery or Wentz are used.