Examining the Subtropical Mode Water in the Southwestern Atlantic From in Situ Observations

Abstract

We investigated the formation and evolution of the South Atlantic subtropical mode water using data from profiling conductivity, temperature, and depth sensors (CTD) deployed in April–May 2015 and from two customized Argo floats that drifted from April 2015 to June 2017. From the CTD data, we observed a mode water layer below the seasonal thermocline that deepened from the southern side of the area to the north. The two Argo floats remained in the proximity of the cruise area for 2 years. Their slow displacement and recirculating patterns allowed us to observe the changes in the temperature and salinity structure before and after the formation period. We observed that the potential vorticity of newly formed mode water was O[10−1 to 10−2] of the mean value found in the whole mode water layer. There is a significant correspondence between the phases of the time integral of surface heat fluxes and the sea surface temperature. Mode water is observed to form at the integrated heat flux minimum phase. The relationship between the air-sea fluxes and sea surface temperature promotes the necessary preconditioning for the mode water formation. Once this was established, the outcropping of the mode water, that was at about 100 m depth, coincided with the passage of an atmospheric cold frontal system. This event suggests that the mode water formation can be triggered by the passage of cold fronts.