Southern Ocean mixed‐layer depth from Argo float profiles

TLDR

Argo float temperature, salinity, and pressure profiles are used to determine mixed‑layer depth in the Southern Ocean, and variations in air‑sea heat exchange, wind stress, and curl across the Pacific and Indian basins suggest basin‑specific mode‑water formation processes. Mixed‑layer depth is calculated from individual profiles using potential density and temperature criteria, mapped monthly with an objective method, while wind mixing, Ekman transport of cold water, and upwelling‑induced cooling are proposed to precondition SAMW formation in the Indian Ocean and mode‑water formation in the eastern Pacific. Quantitative data are provided in the auxiliary material; spatial MLD structures are similar each month, with deep layers (≥400 m) just north of the ACC in the Pacific and Indian oceans, peaking from June to October, concentrated in a narrow surface density band within the SAMW density range, and associated with slightly fresher surface salinity than historical estimates.

Abstract

Argo float profiles of temperature, salinity, and pressure are used to derive the mixed‐layer depth (MLD) in the Southern Ocean. MLD is determined from individual profiles using both potential density and potential temperature criteria, and a monthly climatology is derived from individual MLDs using an objective mapping method. Quantitative data are available in the auxiliary material. The spatial structures of MLDs are similar in each month, with deep mixed layers within and just north of the Antarctic Circumpolar Current (ACC) in the Pacific and Indian oceans. The deepest mixed layers are found from June to October and are located just north of the ACC where Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are formed. Examination of individual MLDs indicates that deep mixed layers (MLD ≥ 400 m) from both the density and temperature criteria are concentrated in a narrow surface density band which is within the density range of SAMW. The surface salinity for these deep mixed layers associated with the SAMW formation are slightly fresher compared to historical estimates. Differences in air‐sea heat exchanges, wind stress, and wind stress curl in the Pacific and Indian oceans suggest that the mode water formation in each ocean basin may be preconditioned by different processes. Wind mixing and Ekman transport of cold water from the south may assist the SAMW formation in the Indian Ocean. In the eastern Pacific, the formation of mode water is potentially preconditioned by the relative strong cooling and weak stratification from upwelling.