Subtropical mode waters in the South Atlantic Ocean

This systematic study of the subtropical mode waters in the South Atlantic is based on hydrological data (the basin-wide South Atlantic Ventilation Ex periment, Oceanus, and Ajar sections, and the World Ocean Circulation Exper iment (WOCE) sections in the Brazil-Malvinas Confluence region), the WOCE e xpendable bathythermograph data set, and the atmospheric forcing produced b y the National Centers for Environmental Prediction reanalysis. The main ve ntilation window of the subtropical gyre lies in the southwest and correspo nds to the southern part of the recirculation region and to the overshoot o f the Brazil Current (BC). Some ventilation occurs along the southern branc h of the gyre through deep mixed layers in harsh winters and in the east th rough the rings shed by the Agulhas Current. However, the ventilation from the eastern border concerns a small volume of water with highly variable ch aracteristics. Three main types of subtropical mode water are observed: the lightest around the 26.2 density surface and the second and third around t he 26.5 and 26.7 density surfaces, respectively. The 26.2 subtropical mode water (STMW1) corresponds to potential temperatures between 16 degrees and 18 degrees C and salinities between 35.9 and 36.2. The STMW1 thickness does not exceed 150 m. The volume of the 26.5 mode water (STMW2) is larger. It is produced through the formation of deep mixed layers (reaching 300 m or m ore) in the active ventilation window over the BC overshoot. Its potential temperature ranges between 14 degrees and 16 degrees C, and its salinity ra nges between 35.5 and 35.9. Its thickness can be above 300 m. The last comp onent observed around the 26.7 density surface (STMW3) has a potential temp erature between 12 degrees and 14 degrees C and a salinity of about 35.2 to 35.5. It is formed on the eastern flank of the BC overshoot and, during ha rsh winters, in the center of the southern branch of the gyre. Within the m ain ventilation window the STMWs exhibit quite a significant variability in characteristics and volume. There is a large interannual variability in th e atmospheric forcing over the BC recirculation and BC extension, and the o bserved variations of the subtropical mode waters are qualitatively correla ted to the atmospheric forcing variations.