Winter observations of Antarctic Intermediate Water and Subantarctic Mode Water formation in the southeast Pacific

Talley, Chereskin, Sloyan and Holte

Subantarctic Mode Water and Antarctic Intermediate Water are major global water masses, responsible for much of the anthropogenic carbon inventory and freshwater transport of the southern hemisphere. The formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) was observed in late austral winter 2005, in the southeast Pacific west of Drake Passage. The new water masses formed as thick mixed layers (300 to 500 meters thick) in a 500 km band north of the Subantarctic Front. New SAMW and AAIW were clearly distinguished from each other. SAMW was formed in a meridional band characterized by relatively high surface salinity, apparently preconditioning the mixed layers to be the deepest in the region. The low potential vorticity SAMW layer spreading northward from the formation region was centered at potential dens ity 27.02 kg/m3. In contrast, AAIW was formed in deep mixed layers exactly adjacent to the Subantarctic Front (SAF), with the potential density of the AAIW salinity minimum core, 27.05 kg/m3, set by the isopycnal that outcropped at the SAF. The SAF is a relatively strong barrier to isopycnal spreading of properties from south of the front to north of the front. However, some cross-frontal mixing is indicated by somewhat reduced salinity north of the front compared with the high salinity SAMW band. Mixing is likely to occur through cross-frontal intrusions with vertical scales of 50 to 100 m, evident at the base of the winter mixed layer and throughout the surface layer in the subsequent austral summer 2006 survey.