The Use of PIES Data to Observe South Atlantic Subtropical Mode Water

Subtropical mode water is a voluminous body of water in the ocean whose main feature is the homogeneity in both vertical structure and horizontal extension. The subtropical mode water (STMW) of the southwest Atlantic is formed between the months of July and October near the Brazil-Malvinas confluence and along the Brazil Current recirculation gyre. The formation region extends on the order of 3000 km zonally, from 20°W to 50°W, and 1000 km meridionally, from 30°S to 40°S , and it is typically about 170 m thick. In situ data from pressure-equipped inverted echo sounders (PIES) installed in the western portion of the basin, along 34.5°S, are available from 2009 to the present. These data when properly treated and calibrated can provide an unprecedented description of the STMW involving processes since its formation at the surface until the final stage of its residence in the interior of the ocean.

Temperature and salinity data estimated by the PIES are based on an empirical look-up tables that relate the acoustic travel time with the baroclinic structure of the ocean. This technique is known as the Gravest Empirical Mode (GEM) method, and here it is used to recognize profiles containing homogeneous segments of temperature and salinity that characterize the mode water. From the easternmost mooring data of the PIES array, the STMW was detected below the surface at depths ranging between 150 m to 500 m, with a typical layer thickness of 140 m, and temperature range between 14.1 and 15.9°C and salinity between 35.4 and 35.8. The GEM method will further be adapted to help us detect the STMW in its formation stage. The main hypothesis to be tested in this study is that variations at interannual scale in the formation of STMW are linked to variations in the intensity of its interaction with the Brazil Current.