Oceanic eddy variability measured by GEOS 3 altimeter crossover differences

Abstract

The last decade in oceanography has seen rapid advances in both in situ and remote instrumentation which have provided new insight into the physics of the sea. Satelliteborne instruments have been especially valuable in revealing complexities of the surface layer through infrared and multispectral imagery, precise global tracking of surface drifter fields, altimetric measurements of sea surface height, and radar observations of wind and wave patterns. This flood of information has opened new avenues of research and, in so doing, has significantly altered some of our conceptions of the general oceanic circulation. With improved observational techniques has come an appreciation of the highly variable nature of oceanic phenomena. This variability occurs on a wide range of scales, from microstructure to basinwide events, but particular attention has been directed toward the mesoscale. Current meanders, ring vortices, and midocean eddies comprise the ocean mesoscale variability, collectively known as the eddy field. The existence of oceanic eddies is not surprising, considering their well‐documented counterparts, the storms and weather patterns of the atmosphere. Yet their role in the maintenance of the general circulation is far from clear, and numerous expeditions have been dedicated to their exploration. Because these eddying motions often contain energy significantly greater than the mean flow itself, determination of eddy intensity and distribution is a necessary key to understanding global ocean dynamics.

Seasonal variability of the Gulf Stream from satellite altimetry

“…A commonly used approach is the so-called crossover difference method, by which the differences in altimetric measurements made at the intersections of satellite ground tracks, called crossovers, are related to oceanic variabilities. The method was first used to map the geographic distribution of oceanic mesoscale variability [e.g., Cheney and Marsh, 1981;Gordon et al, 1983]. Fu andChelton [1984, 1985] have demonstrated that the temporal history of sea level variability can be recovered from altimetric crossover differences.…”

“…The multiyear, nearly continuous GEOS 3 altimetric observations of the western North Atlantic, which happens to be the best covered ocean by GEOS 3 (see, for example, Chelton [1985], Figure 3), have provided an excellent opportunity for studying the Gulf Stream variability east of Cape Hatteras. GEOS 3, which operated from April 1975 to December 1978, was the first altimetric mission showing promise for oceanographic applications [e.g., Huang et al, 1978;Douglas and Gaborski, 1979;Mather et al, 1979;Cheney and Marsh, 1981;Robinson et al, 1983;Gordon et al, 1983]. Compared with Seasat, the first dedicated oceanographic satellite [Born et al, 1979], the advantage of GEOS 3 is its much longer duration (Seasat lasted for only 3.5 months: July-October, 1978), but the disadvantage is its much poorer accuracy and intermittent Copyright 1987 by the American Geophysical Union.…”

Calculation of sea level time series from noncollinear GEOSAT altimeter data

“…However, no use was made of the time varying oceanic signal in the residuals. Cheney and Marsh (1981) used crossover differences to determine the statistics of oceanic mesoscale eddy variability. At the same time, Douglas and Cheney (1981) used collinear nests of Geos 3 data to compute the same information.…”

Evaluation of GEOSAT data and application to variability of the Northeast Pacific Ocean

“…Marsh et al (1980) computed the mean sea surface in a variety of locales using GEOS-3 outputs, while Diamante and Nee (1981) determined tidal constituents from GEOS-3 derived sea surfaces. Cheney and Marsh (1981b) mapped mesoscale variability measured by GEOS-3, and others used the data to improve geoid models (e.g., Rapp [19791, Lerch et al [1979]). GEOS-3 was followed by SEASAT; which, along with four other microwave sensors, flew an improved altimeter similar to GEOSAT's.…”

“…Tai and Wunsch (1983) mapped the subtropical gyre in the north Pacific; the first direct measurement of such a feature not dependent on conventional hydrography. Fu and Chelton (1984) November 1986, and was optimized for the study of oceanic variability (Cheney et al, 1987 (Wunsch and Gaposchkin, 1980 To a good approximation, surface currents are in geostrophic equilibrium and hydrostatic balance (Wunsch and Gaposchkin, 1980 (Wunsch and Gaposchkin [1980], Stommel [1965] (Wunsch and Gaposchkin, 1980) Cheney and Marsh (1981b), and Tai and Wunsch (1983) (1980), and on SEAS AT data by Marsh and Martin (1982) (Douglas et al, 1984 Examples of the collinear technique may be seen in the work of Douglas and Gaborski (1979), Mather et al (1980), Cheney et al (1983), and Fu (1983 The spatial and temporal sampling which result from this scheme are fairly close to the 20-day repeat and 140 km equatorial groundtrack spacing suggested by Mitchell (1983) (Cole, 1985). The main reference on the GDR is the GDR User Handbook (Cheney et al, 1987 Jones et al [1987] and Cole [1985] for further discussion).…”