Eastward-propagating interannual variability in Southern Ocean deep mixed layers

Argo data from 2004-2019 reveal strong interannual variability in deep winter mixed layer (MLs) and the related Subantarctic Mode Water (SAMW) thickness in an 11° band north of the Subantarctic Front (SAF) from the central Indian through the Pacific to Drake Passage. Anomalies exhibit a quasi-dipole structure in each ocean, and propagate eastward consistent with advection and winter-time reemergence. Mean September MLs are >500m depth with interannual anomalies of >300m. An EOF analysis of the monthly ML depth and SAMW anomalies reveals a circumpolar zonal wavenumber 3 (ZW3) structure, with a time-dependent, zonal dipole structure in the Indian and Pacific where winter MLs are climatologically deepest, and a weak dipole in the Atlantic where winter MLs are shallow. The first and second EOFs of MLs explain a similar fraction of variance but are shifted by a quarter-cycle zonally, reflecting the observed eastward propagation. The principal component is dominated by a 2-year time scale, and is correlated with a ZW3 structure in sea level pressure, meridional winds, and turbulent heat flux, with cool, southerly wind anomalies associated with deep ML and thick SAMW anomalies. A partial least squares regression model shows that the spatio-temporal patterns are correlated with the Southern Annular Mode (SAM) (dominant), and with ENSO and QBO (equal importance).

SAMW thickness anomalies persist after winter formation and propagate eastward at the mean advection speed within a 4° band north of the SAF. The dipole spatial structure of MLs and SAMW is consistent with downstream reemergence of anomalies the following winter when the mixed layer deepens into the SAMW layer, affecting the local, current SST anomaly and MLD anomaly (Verdy et al. 2006). Interannual variability of SAM, QBO, ENSO could reinforce the anomalies, leading to greater persistence than stochastic winter forcing. SST anomalies could also reinforce SLP anomalies through air-sea coupling (White et al. 1998).