Influence of Tropospheric Water Vapor Corrections on Geosat Altimetry in the North Atlantic Ocean

Abstract

Geosat sea surface height (SSH) data in the tropical and midlatitude North Atlantic are analyzed with and without water vapor (WV) corrections to study the WV influence on along-track SSH anomaly profiles, mesoscale SSH variability, wavenumber spectra, and objectively mapped fields of SSH anomaly. Three different WV datasets were used, one from the Fleet Numerical Oceanographic Center (FNOC) model and two from the Special Sensor Microwave/lmager (SSM/1) based on different WV retrieval algorithms. These WV datasets show significant differences, in particular in the tropics. However, the method for deriving SSH anomalies from altimeter height data filters out much of the WV corrections. The residual WV effect on SSH anomaly is shown to be most significant in the seasonally migrating intertropical convergence zone of the tropical Atlantic: there the SSM I I corrections reduce the along-track mesoscale SSH variability by typically 1-1.5 cm. On seasonal timescales the maximum WV effect in this region is characterized by a 2-3-cm rms difference between SSH anomaly with and without SSM/ I WV corrections, whereas FNOC corrections have almost no effect. Inferred seasonal velocity variations in the North Equatorial Countercurrent core (4°-6°N) in the region of maximum WV influence (30°-40°W) are reduced by about 20o/o and 30o/o, depending on whether SSM/1 corrections by Emery or Wentz are used.