OCEAN SURFACE-TOPOGRAPHY MEASURED BY THE GEOSAT RADAR ALTIMETER DURING THE FRONTAL AIR-SEA INTERACTION EXPERIMENT

An objective analysis (OA) technique was used to analyze Geosat Geodetic Mission residual sea height anomalies (RSHA) for the Frontal Air-Sea Interaction Experiment (FASINEX), conducted in the western North Atlantic. Preprocessing of the Geosat data included removal of a mean sea surface and orbit error and corrections for tides, wave height, inverse barometer, and ionospheric and dry tropospheric effects. Geosat data were not corrected for water vapor. Central to the analysis was the computation of a spatially anisotropic, time-dependent autocorrelation function directly from the Geosat data. Results show a single autocorrelation function peak, having a diameter of approximately 270 km at 0-day time lag. A 15-month time series of RSHA topography was constructed at 10-day intervals, forming a set of 47 OA maps. These maps show a field of topographic features propagating to the west at approximately 4 km d-1. The features exhibited spatial and temporal scales of approximately 200 km and approximately 50 days, respectively. The root-mean-square (rms) of the OA-derived feature amplitudes fluctuated slowly between 0.05 and 0.09 m with a period of approximately 200 days over the 15-month interval. These fluctuations appeared to be related to the passage of large-scale, northeast-southwest trending sea surface temperature (SST) anomaly features reported for FASINEX, which were surface manifestations of large-scale, first-mode baroclinic Rossby waves. The rms amplitude of the OA-derived features was maximal (minimal) during periods of negative (positive) SST anomaly, when the depths of the mixed layer and main thermocline in the upper ocean shoaled (deepened). The fluctuations also appeared to be correlated with the structure of the field. Periods of decreased features amplitudes corresponded to a "closely packed" structure, while during times of increased amplitudes, the field structure became "zonal" with regions of alternating sign. During FASINEX (January-June 1986) the field was characterized by the closely packed structure and decreasing feature amplitudes. This corresponded to the period of positive SST anomaly and high upper ocean heat content reported for FASINEX.