![]() ![]() Most previous studies of the movement of ocean eddies were based on visual tracking of identifiable features in time sequence of SSH maps. The velocity vectors represent the speed and direction of the horizontal propagation of ocean eddy variability. Superimposed on the eddy variability are the velocity vectors estimated from tracking the movement of eddies in the time series of SSH using the method described in Section 2. Displayed in color shades in Figure 1 is the standard deviation of SSH measured by the satellite altimeters, showing the intensity of eddy variability. Since the space and time lags of the correlation analysis are chosen for the mesoscales, the estimated velocities represent the speed and direction of the propagation of ocean eddy variability. The resulting velocities are associated with the variabilities that dominate the variance of the SSH time series at the scales of the correlation analysis. To focus on the mesoscale, the time lags were limited to less than 70 days and the dimension of the box was generally less than 400 km. The size of the box for computing the correlations was determined by the estimated speed and the time lag. The average velocity was then assigned to the eddy propagation velocity at the given grid node. Where C i is the maximum correlation at Δ T i, and Δ x i, Δ y i represent the location of the maximum correlation. Such space-time lagged correlation analysis was performed to estimate the speed and direction of the maximum correlations as they move in space and time. ![]() The correlations of the SSH anomalies with all the neighboring SSH anomalies at various time lags were computed. ![]() At a given location, the sea surface height (SSH) anomalies were computed as the residuals after a time mean was removed from the SSH time series. The propagation velocity of eddies is computed using a space-time lagged correlation analysis similar to the maximum cross correlation method. This data set, available form the French AVISO data center (, 2006), allows the tracking of large ocean eddies for studying their propagation velocity and pathways. When the data from the two satellites are combined with the use of an objective mapping technique, the resulting spatial resolution is approximately 150 km in wavelength, covering a substantial portion of the mesoscale spectrum (also see Chelton and Schlax for the estimate of a lower spatial resolution). Although the spatial resolution of a radar altimeter is typically 6–7 km along the satellite's ground tracks, the wide separation between the ground tracks, typically 100–300 km for a single altimeter, limits the resolution of eddy variability in the satellite's cross-track direction. The combined data from the T/P and ERS altimeters are the first decade-long record that is useful for tracking the movement of ocean eddies in two dimensions. The approach can be readily applied to the global oceans. The information describes a unique property of the ocean general circulation and serves as a basis for testing ocean models as well as for constraints in data assimilation and empirical prediction of eddy movement. In the present study, the combined data from the TOPEX/Poseidon (T/P hereafter) and ERS altimeters are used to construct a high-resolution map of the pathways of ocean eddies in the South Atlantic Ocean, showing the details of the interaction between mean flow and eddies with strong influence of bottom topography. The same method was applied to the North Atlantic Ocean by Brachet et al. provided a smoothed estimate of the eddy propagation velocity of the global ocean using a method of binned covariance. However, the patterns of the movement of eddies and their propagation velocity, or the pathways of ocean eddies, are more difficult to study due to the lack of long and global observations with sufficient spatial and temporal resolutions. ![]() The distribution of eddy energy in the ocean has been well mapped from space using satellite altimeter observations. They contain most of the kinetic energy of the circulation of the ocean and play important roles in determining the water properties of the ocean: temperature, salinity, dissolved gases, and nutrients. Eddies are the oceanic analog of weather in the atmosphere. The generic term “eddies” is used here to represent the various forms of ocean current variability at the mesoscale: vortices, fronts, planetary waves, and current meanders. The variability of ocean currents is dominated by the mesoscale eddies, characterized by a time scale on the order of 100 days and a spatial scale on the order of 100 km. ![]()
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