Observation of South Atlantic subtropical mode waters with Argo profiling float data

Abstract

South Atlantic subtropical mode water (SASTMW) is detected using temperature and salinity profiles obtained by Argo floats. Marked by low potential vorticity within its extent, mode waters are identified by homogeneous temperature and salinity values, both horizontally and vertically. Their presence is evident in three regions: the Brazil Current recirculation gyre on the western side of the basin, the eastern side of the basin, and along the southern edge of the gyre. Formation occurs between winter and mid-spring in the regions of cumulative cooling of the ocean. We showed that Argo profilers do not preferentially sampled eddies. Yet, from the profiles that found SASTMW inside an eddy, 80% are anticyclonic. Furthermore, the mode water layer inside anticyclonic eddies is on average thicker than inside cyclonic eddies or anywhere else. We investigate the correlation between the presence of mode water with the eddy kinetic energy (EKE). The cross spectrum shows that the EKE and the layer thickness are out of phase in the annual period band. We addressed the role of negative surface heat fluxes during the SASTMW period of formation. Nearly all profiles that indicated mode water formation occurred at the time and within the region where the cumulative heat flux was negative.