Weakened Eddy Activities in the South Pacific Subtropical Countercurrents Under Global Warming

Abstract

The subtropical countercurrent (STCC), located near the South Pacific boundary, exhibits a band of high eddy activities. These eddies have significant implications on the exchanges of heat, salt, volume, and biogeochemical properties. Yet their response to greenhouse warming change remains unknown. Using an ensemble of long-term high-resolution climate simulations under a high carbon emission, we show that eddy kinetic energy (EKE) in the STCC is projected to be significantly reduced in a warming climate. This reduction primarily stems from weakened baroclinic instability of the STCC-South Equatorial Current (SEC) system, which is associated with a projected decrease in vertical velocity shear and an increase in upper ocean stratification. The former majorly results from an enhanced meridional salinity gradient due to the reduction in subtropical precipitation, whereas the latter is caused by an enhanced thermal stratification associated with a faster surface warming in a warming climate. The findings of this study will contribute to a better understanding of the impact of climate change on ocean motions as well as their climate impact near the western boundary.