Seasonal Modulations in the Eddy Field of the South Pacific Ocean

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Abstract
Decade-long satellite altimeter data from the Ocean Topography Experiment (TOPEX)/Poseidon and European Remote Sensing Satellite-1 and-2 (ERS-1/2) missions are analyzed to investigate the eddy signals in the South Pacific Ocean. High–eddy kinetic energy (EKE) bands with well-defined annual cycles are detected along the eastward-flowing surface currents of the South Tropical Countercurrent (STCC) between 21°–29°S and the South Equatorial Countercurrent (SECC) centered near 9°S. Overriding the westward-flowing South Equatorial Current (SEC), the STCC layer has the sign of its mean potential vorticity gradient opposite to that in the SEC layer, subjecting the vertically sheared STCC–SEC system to baroclinic instability. In austral winter, the STCC–SEC system is baroclinically more unstable than in other seasons because of the large vertical shear and weak stratification. This seasonal variation in the intensity of baroclinic instability is responsible for the seasonal modulation of the STCC's EKE field with a November–December maximum and a June–July minimum. The large deformation radius in the low-latitude SECC region, on the other hand, prevents the vertically sheared SECC–SEC system from becoming baroclinically unstable. With the broad, westward-flowing SEC weakening the stabilizing planetary β effect, the high EKE level observed along the SECC is found to result from the barotropic instability associated with the horizontal shear of the SECC–SEC system. Together with an analysis of energetics, it is shown that the seasonal variation in the intensity of barotropic instability accounts for the seasonal modulation of the SECC's EKE field, with a maximum in April and a minimum in August.