Origin and ages of mode waters in the Brazil‐Malvinas Confluence region during austral winter 1994

Abstract

The ventilation of the main thermocline associated with the formation of Subtropical Mode Water (STMW) and Subantarctic Mode Water (SAMW) within the Brazil/Malvinas Confluence region is investigated using satellite data, hydrographic data, and dissolved CFCs as transient tracers. During the winter of 1994, two types of STMW were observed. Both types, 26.24 σθ and 26.46 σθ were formed within the warm pool of the South Atlantic Central Water (SACW). The SAMW is found on the 27.10 σθ density surface. This surface lies in the minimum salinity layer of the Antarctic Intermediate Water (AAIW). The mode waters are dated using CFC ratios. To our knowledge, this is the first estimate of the age of the mode waters in the region using the CFC‐113:CFC‐11 ratio method. The lighter STMW was being formed during the cruise. The heavier STMW was found to be 6±3 years old and is thought to have circulated in the subtropical gyre without losing its primary properties. The SAMW, or the lightest AAIW has an age of 3±2 years. This water is very young compared with the oldest (10±5 years) and heaviest component of the AAIW, which circulates within the subtropical gyre. The formation of the STMW in the confluence region is dependent on the southward extension of the Brazil Current.