Decadal Mesoscale Eddy Modulations in the South Pacific Subtropical Counter-Current

Abstract:

In this study, the annual cycle and low frequency variability of the mesoscale eddy variability in the western south Pacific (165°E-130°W, 21°-29°S) is investigated by using satellite altimetry data. By assuming that the South Pacific Subtropical Counter-Current and it's underlying undercurrent can be represented by a 2-1/2 layer system, a measure of baroclinic instability and it's growth rates can be calculated. This instability is related to the shearing and stratification of the system. The causes for the variability in this region are explored by using data from the European Centre for Mid-Range Weather Forecasts (ECMWF) Ocean Re-Analysis product. Low frequency changes in both the shearing and the stratification are found to have a measurable contribution to changes in the baroclinic instability. The fluctuations in the stratification are found to have the greatest impact on low-frequency changes to the baroclinic instability, and subsequently, the eddy variability. This stratification is altered primarily through a low-frequency warming of the upper layer in the region. The change in stratification leads to an increase in the baroclinic Rossby wave speed, and a reduction in the baroclinic instability. Fluctuations in the forcings on the system are analyzed in an effort to determine the low frequency variations in the shear and stratification.