Subantarctic Mode Water Variations in the Three Southern Hemisphere Ocean Basins During 2004–2019

Abstract

AbstractSubantarctic mode water (SAMW) variations and their forcing mechanisms in the three Southern Hemisphere ocean basins are studied using Argo data and reanalysis products during the 2004–2019 period. In the Indian and Pacific Oceans, SAMW is characterized by potential vorticity (PV) minimums lower than 0.5 × 10−10 m−1 s−1. In the Atlantic Ocean, the PV minimum is generally lower than 1 × 10−10 m−1 s−1. The variations in SAMW thickness are significantly correlated with changes in the mixed layer depth (MLD) and show large differences among different ocean basins. The winter MLD in the Indian Ocean SAMW formation region is significantly positively correlated with the local wind stress curl (correlation coefficient = 0.61; 95% confidence level). In the Pacific and Atlantic SAMW formation regions, the variations in the winter MLD have significant negative correlations with the buoyancy flux; the correlation coefficients reach −0.57 and −0.67 (95% confidence level), respectively. These different mechanisms of the MLD variations in SAMW in the three ocean basins are related to the Southern Annular Mode (SAM) variation in winter. During positive (negative) SAM index periods, a strong positive (negative) wind stress curl anomaly occurs in association with a meridionally asymmetric westerly wind anomaly distribution. This phenomenon results in a corresponding MLD anomaly over the Indian Ocean SAMW formation region. In the Pacific and Atlantic SAMW formation regions, a negative (positive) westerly wind anomaly induces buoyancy gain (loss), leading to mixed layer shallowing (deepening) during positive (negative) SAM periods.