Global mesoscale variability from the Geosat Exact Repeat Mission: Correlation with ocean depth

Abstract

We have developed a new technique for extracting global mesoscale variability from satellite altimeter profiles having large radial orbit error (∼3 m). Long‐wavelength radial orbit error, as well as other long‐wavelength errors (e.g., tides, ionospheric‐atmospheric delay, and electromagnetic bias), are suppressed by taking the derivative (slope) of each altimeter profile. A low‐pass filter is used to suppress the short‐wavelength altimeter noise (λ<100 km). Twenty‐two repeat slope profiles are then averaged to produce a mean sea surface slope profile having a precision of about 0.1 μrad. Variations in sea surface slope, which are proportional to changes in current velocity, are obtained by differencing individual profiles from the average profile. Slopes due to mesoscale dynamic topography are typically 1 μrad (i.e., a 0.1‐m change in topography over a 100‐km distance). Root‐mean‐square (rms) slope variability as low as 0.2 μrad are found in the southeast Pacific, and maximum slope variations up to 6–8 μrad are found in major western boundary currents (e.g., Gulf Stream, Kuroshio, Falkland, and Agulhas) and Antarctic Circum‐polar Current (ACC) systems. The global rms variability map shows previously unknown spatial details that are highly correlated with seafloor topography. Over most areas, the rms slope variability is less than 1 μrad. However at mid‐latitudes, areas of higher variability occur in deep water (>3 km) adjacent to continental shelves, spreading ridges, and oceanic plateaus. Variability is low in shallower areas (<3 km). Along the ACC, the meso‐scale variability appears to be organized by the many shallow areas in its path. We do not see convincing evidence that variability is higher downstream from topographic protrusions. Instead, the areas of highest variability occur in the deep basins (>4km).