Intermediate waters in the southwest South Atlantic

Abstract

The density (sigma-theta, σθ) interval 27.05-27.20 in the Subantarctic Zone of the northern Drake Passage is characterized by two water types with potential temperatures of 3.7 and 4.8°C, respectively, both with salinity of approximately 34.2. These major contributors to the low salinity intermediate water mass are advected northward along the continental slope of South America. The lower density water type enters the Argentine Basin both east and west of Burdwood Bank. Its thermohaline characteristics are modified by winter sea-air interaction near Burdwood Bank and mixing with the surrounding waters further north. The denser water type flows east of Burdwood Bank, undergoing salinity decrease, primarily by isopycnal processes. Low salinity water, derived from the Polar Front, is introduced into a still denser horizon (27.25 σθ), from along the axis of the cyclonic circulation feature described by the Malvinas Current and its return to the south. The thermohaline structure across the Malvinas Current is similar to the water mass zonation observed in the northern Drake Passage. In the vicinity of 38°S, the northward-flowing modified subantarctic water converges with subtropical thermocline water at the Brazil-Malvinas Confluence. The less dense subantarctic water spreads under the subtropical thermocline; however, the denser water (27.25 σθ) of Polar Front origin is not found under the subtropical thermocline in the western South Atlantic. In general the salinity at the salinity minimum increases rapidly across the Brazil-Malvinas Confluence, suggesting that the bulk of the subantarctic water advected into the region by the Malvinas Current turns towards the interior, spreading under the subtropical thermocline along a broader expanse of the South Atlantic.