Interannual Variability of Mesoscale Eddy Activity in the South Pacific Subtropical Countercurrent

Interannual variability of eddy activity in the western, subtropical Pacific is observed using the past two decades of satellite altimetry data. Between of 21°-29°S, there is a band of heightened eddy activity. In this region, the eastward South Pacific Subtropical Countercurrent (STCC) overlays the westward South Equatorial Current (SEC). This vertically sheared STCC-SEC system is subject to baroclinic instabilities. Using the European Centre for Midrange Weather Forecasts (ECMWF) ocean reanalysis data (ORAS4), and verified by gridded Argo float data, low-frequency variations in the state of the ocean in this region are investigated. It is found that the low-frequency changes in the shearing and stratification of the STCC-SEC region both work to modulate the strength of baroclinic instabilities, as measured through the baroclinic growth rate. These changes in the strength of the instabilities consequently modulate the observed eddy activity. Using a linearization of the baroclinic growth rate, it is found that the contribution to the variability from the changes in shearing and stratification are roughly equal. However, the changes in these two parameters do not occur concurrently, and can alternately work to negate or augment each other.