Extension and Validation of a Gulf Stream Geosat Synthetic Geoid

Abstract

An extended synthetic geoid for the western North Atlantic Ocean was constructed by employing Geosat altimeter data, concurrent dynamic model forecasts, and climatology. Estimates of the absolute dynamic topography from the altimeter were compared to estimates of the dynamic topography computed from independent in situ temperature measurements. The rms difference between the two topographic estimates was 0.104 m in the Gulf Stream meander and ring region, 0.081 m in the Sargasso Sea, and 0.098 m overall in this portion of the western North Atlantic ocean. The position of the mean Gulf Stream axis, the 1-σ width of the meander envelope, and the extremes of the meander envelope were determined from the altimetric data. The yearly mean AVHRR- (Advanced Very High Resolution Radiometer) derived surface north walls for 1987 and 1988 were approximately 30 km north of the yearly mean surface maximum velocity axes for the same years. This 30-km offset, however, is affected by the different satellite sampling schemes and the AVHRR data processing techniques. Separation distances derived from individual comparisons of nearly concurrent Geosat, AVHRR, and AXBT (air expendable bathythermograph) datasets result in an average (rms) offset of 17 (±12) km between the axis and the surface north wall, and an 11- (±8) km offset between the axis and the subsurface north wall. The individual axis/surface north wall offsets and their variability were found to increase with increasing anti-cyclonic curvature, consistent with the theoretical effect of centripetal acceleration on a meandering constant potential vorticity jet. This synthetic geoid, validated in portions of the western North Atlantic within a vertical accuracy of about 0.10 m, can be used with the planned Geosat Follow-On mission in the late 1990s to calculate the absolute dynamic topography in near real time.