Unraveling the Submesoscale Nature of the Subtropical Mode Water in the South Atlantic

We investigate the mechanisms involved in the formation of the subtropical mode water in the South Atlantic. The distinct characteristic of the mode water is the homogeneity of parameters such as temperature and salinity, in both vertical structure and along its horizontal extension. The subtropical mode water occupies a region between 30ºS and 40ºS across the whole extension of the basin. It is formed between July and October as a consequence of the ocean and atmosphere interaction processes. Here we present the results from a survey done in the region of formation between 37ºW and 32ºW and 34ºS and 37ºS from July to October, 2018, using high resolution measurements obtained from a glider.

The formation of the mode water seems to be triggered by atmospheric events, mostly cold fronts advecting sub-polar air masses to the region. The mode water development is initially observed as localized chimney-like patterns with horizontal scale of 1 to 5 km, associated with the oceanic convective mixing processes. Over time, these submesoscale structures tend to expand and dominate the region as a large volume of mode water. These vertical structures present highly homogeneous temperature and salinity values. The observed potential vorticity was on the order of 5.0 x 10^-11 m^-1 s^-1 which is one order of magnitude smaller than the preexistent mode water at the lower layers. Comparisons of the sea surface temperature measured from satellite (GHRSST MUR) collocated with the glider's measurements showed a very good agreement. This validates the use of these satellite data to monitor mode water formation at submesoscale.