Covariability of Subantarctic Mode Water and the Southern Branch of the Subtropical Indian Ocean Countercurrent in Argo Observations

Abstract

The Subantarctic Mode Water (SAMW) forms in the deep mixed layer north of the Antarctic Circumpolar Current and spreads northward into the subtropical gyre. The subtropical South Indian Countercurrent (SICC) flows eastward on the north flank of the thick SAMW layer within 22° – 32°S from south of Madagascar at around 25°S, 50°E toward western Australia. The dynamical relation of the SAMW and the southern branch of the SICC (30° – 32°S) is investigated in this work based on the monthly mean Argo data from 2004 to 2019. The physical properties of the SAMW and its pathway from the formation region are described. Most of the SAMW in the Indian Ocean sector originates from the deep mixed layers of the southeastern Indian Ocean (about 40°S, 85° – 105°E) and moves along the subtropical gyre. It takes around ten years to arrive east of Madagascar Island preserving its low potential vorticity characteristics. As a thick layer with homogeneous vertical properties, the SAMW forces the upper pycnocline to shoal, and the associated eastward shear results in the surface-intensified SICC. The SAMW forms a tongue-shaped thickness pattern, which influences the southern branch of the SICC above the northern flank of the thickest SAMW layer between 24°S and 32°S. The seasonal, interannual, and decade variations of the southern branch of the SICC are closely related to the meridional gradient of the underlying SAMW thickness. The SAMW thickened and strengthened from 2005 to 2015, thereby anchoring a strengthened SICC. The interannual covariability of the SAMW and SICC further supports the SAMW’s role in driving SICC variability.