Argo profiling floats document Subantarctic Mode Water formation west of Drake Passage

Subantarctic Mode Water (SAMW), the precursor of Antarctic Intermediate Water, which occupies the 600 to 1100 m layer of the water column in the oceans of the southern hemisphere, is formed in the southern oceans north of the Polar Frontal Zone (56 degrees- 60 degrees S), where saltier upper deep water is upwelled close to the surface. The surface layer there, in contrast, is determined by fresher water which occupies about the upper 100 m of the water column during austral summer. Surface cooling during austral winter initiate vertical convection which supposedly homogenizes several hundreds of meters of the upper water column, hereby transforming saltier upper deep water to lighter SAMW by incorporating fresh Subantarctic Surface Water. Here we show the formation process of SAMW west of Drake Passage as documented by autonomous profiling floats. Vertical convection exceeds 500 m during September and October but only south of 56 degrees S.