Global mesoscale variability from collinear tracks of SEASAT altimeter data

Abstract

In 1978, during the last 25 days of the SEASAT mission, the satellite ground tracks repeated within 2.5 km every 3 days. This yielded eight to nine sets of global collinear altimeter data with a cross‐track grid spacing of approximately 900 km at the equator and 600 km at mid‐latitude. Because the geoid is time‐invariant, such observations can reveal sea surface height variability due to dynamic ocean phenomena. We have solved for variations due to deep‐ocean mesoscale features by eliminating the longer wavelength deviations. Modeled tidal heights were first subtracted from the altimeter data, and linear trends were then removed from collinear segments approximately 2000 km in length. This second step eliminates relative orbit error together with any residual tide model errors. The resulting sea height profiles have a precision of a few centimeters, and the global variability map constructed from these data reveals a strikingly realistic view of mesoscale energetics. Maximum values of 20–40 cm rms variability are generated by meanders and eddies of five major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circumpolar, and Falkland/Brazil confluence. Perhaps a more significant discovery was the dominance of exceedingly small variability over vast regions of the oceans; approximately 70% of all global values were less than 5 cm. This category included the eastern North Pacific, eastern South Pacific, and almost the entire South Atlantic where values as small as 1–2 cm were common. With such a low level of background noise, even some currents with relatively small sea height signatures could be detected. In both the Atlantic and the Pacific, for example, the North Equatorial Current systems were clearly expressed as zonal bands of higher variability.