Quasi-biennial Variability of Indian Ocean Subtropical Mode Water Subduction Driven by Atmospheric Circulation Modes during the Argo Period

Abstract Argo data were used to investigate interannual variability in the subduction rate of the Indian Ocean Subtropical Mode Water (IOSTMW). The IOSTMW subduction rate in the southwestern part of the southern Indian Ocean subtropical circulation varies substantially from year to year without significant trend during the Argo period 2005–2019. The variability of the IOSTMW subduction rate is quasi-biennial and dominated by the lateral induction term associated with the wintertime mixed layer depth (MLD). The contribution of vertical pumping to the change of subduction rate is relatively small. This variability in the subduction rate directly contributes to IOSTMW volume with a quasi-biennial variation. The quasi-biennial variations in the IOSTMW subduction and volume reflect the variability of the overlying atmosphere. The wintertime Mascarene High (MH) modulates the winter MLD in the subduction area through changes in heat fluxes and wind forcing. The quasi-biennial variability in MH is associated with the Southern Annular Mode, but it was disturbed during two El Niño events in 2009 and 2015–2016. Variability of the wintertime MH plays a key role in driving the quasi-biennial variability of the IOSTMW subduction during the Argo period.