Abstract
Both a quasi‐biennial variability and an overall linearly increasing trend are identified in the Sub‐Antarctic Mode Water (SAMW) subduction rate across the Southern Hemisphere ocean, using the Argo data during 2005–2019. The quasi‐biennial variability is mainly due to variability of the mixed layer depth. Variability of wind stress curl in the SAMW formation regions associated with the Southern Annular Mode plays a critical role in generating the quasi‐biennial variability of the mixed layer depth and consequently the SAMW subduction rates. The SAMW subduction rate across the Southern Hemisphere ocean, long‐term mean totaling 56 Sv, has increased at 0.73 ± 0.65 Sv year−1 over the past 15 years. The increase has directly contributed to the observed increase in the total SAMW volume. Much of this increasing trend can be explained by the deepening mixed layers, which in turn are primarily forced by the strengthening westerly winds under an increasing Southern Annular Mode.
Plain Language Summary
The upper ocean heat content has increased globally during the past decades. Recent studies have shown that this warming trend has concentrated in the extratropical Southern Hemisphere ocean and can be largely explained by the Sub‐Antarctic Mode Water (SAMW) variability. Analysis of the Argo data reveals an increasing trend of the SAMW subduction rate during the period 2005–2019. Superimposed with this increasing trend is a quasi‐biennial variability resulting from changes of the mixed layer depth. The increasing trend of the SAMW subduction rate directly contributes to the decade‐long increase of the total SAMW volume. A large portion of this increasing trend is due to deepening of the mixed layer. Enhanced downward Ekman pumping in the SAMW subduction regions associated with an increasing Southern Annular Mode is primarily responsible for the increasing trends of the mixed layer depth and consequently the SAMW subduction rate.
Key Points
A quasi‐biennial variability in the Sub‐Antarctic Mode Water subduction rate is revealed in response to the mixed layer depth variability
The Sub‐Antarctic Mode Water subduction rate has increased, contributing to the water's volume increase during the Argo period
Changes in wind stress curl associated with the westerly winds play a key role in modulating the Sub‐Antarctic Mode Water subduction rate
