Salt Sinking in the Upper South Pacific Subtropical Gyre From 2004 to 2016

Abstract

The salinity in the 50–300-m water column of the South Pacific subtropical gyre (10°S–30°S) increased from 2004 to 2016. The observed changes are primarily associated with changes in the South Pacific Tropical Water (SPTW) volume, which increased at a rate of (3.17 ± 0.25) 1014 m3 per decade in the region of 10–30°S and 150°E–90°W. The increases in the SPTW volume are caused by increased SPTW production, which increased at a rate of 3.45 ± 2.65 Sv per decade. The temporal variability in the mixed layer accounts for 75 ± 32% of the observed increase in SPTW production. The increases in temporal changes in the mixed layer are due to a deeper mixed layer depth (MLD) in austral winter and a shallower MLD in austral spring. The possible drivers of MLD changes at the air-sea surface are examined. Among the wind stress curl, buoyancy fluxes, and the turbulence induced by wind stress, changes in the surface buoyancy fluxes play the most important role in the deepening of the MLD during austral winter, and changes in the wind stress play a leading role in the shoaling of the MLD during austral spring. Changes in the buoyancy fluxes during winter and changes in the wind stress in spring can be attributed to increases in the Southern Annular Mode from 2004 to 2016. This study highlights the strong modulation of SPTW formation by decadal climate variability over the subtropical South Pacific.