Weakened Seasonality of the Ocean Surface Mixed Layer Depth in the Southern Indian Ocean During 1980-2019

Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980-2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin-wide shoaling trend that is absent in the other basins within 40 degrees S-40 degrees N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward-shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons. The ocean surface mixed layer (ML) is a well-mixed layer with uniform physical and chemical properties and is key for the ocean in exchanging materials and energy with the atmosphere. The present study shows that during 1980-2019, the Southern Indian Ocean (SIO) ML depth (MLD) displays a basin-wide decreasing trend, which is absent in the other basins within 40 degrees S-40 degrees N. The SIO MLD shoaling primarily appears in austral winter when the climatology ML is deep but is insignificant in summer, substantially weakening the MLD seasonality. The SIO MLD changes are primarily explained by the ocean dynamical adjustment driven by the surface zonal wind changes. Specifically, the strengthened Southern Annular Mode in recent decades drives southward shifts of the subtropical anticyclonic winds and ocean gyre year-round. However, the poleward-shifted ocean gyre preferentially decreases the SIO winter MLD as the meridional MLD gradient is sharp and thus efficiently reduces the deep ML water converging from the Southern Ocean into the SIO. In contrast, the SIO MLD displays negligible change in summer when its meridional gradient is flat. The results highlight that despite under nearly identical southward-shifted subtropical winds, the winter and summer MLD responses are distinct due to different background oceanic conditions. The seasonality of the Southern Indian Ocean surface mixed layer (ML) depth prominently weakens during 1980-2019 The weakened seasonality mainly results from a pronounced winter ML shoaling The southward shift of the subtropical ocean gyre driven by the strengthened Southern Annular Mode dominates the ML shoaling