Antarctic Intermediate Water Formation in the Southeast Pacific

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0327544/TalleyIntellectual Merit: This project aims at characterizing the processes responsible for the formation of Antarctic Intermediate water (AAIW) in the southeast Pacific. The plan is to study (1) northward Ekman advection of Antarctic Circumpolar Current surface waters across the SAF, (2) convection driven by local air-sea fluxes, and (3) northward subduction of AAIW across the northern front bounding the deep mixing region. Antarctic Intermediate Water (AAIW) is a low salinity, high oxygen and low potential vorticity (thick) water mass that fills almost all of the southern hemisphere and the tropical oceans at about 800 to 1000 m depth. As the densest of the circumpolar Subantarctic Mode Waters (SAMW), AAIW is formed as a thick, outcropping mixed layer in the southeastern Pacific just north of the Subantarctic Front (SAF). SAMW and AAIW formation have a major impact on the oceanic sink for anthropogenic CO2, whose largest uncertainty is at intermediate depths. AAIW has a major role in southern hemisphere freshwater transport and as such, can impact global-scale ocean overturning processes. AAIW is the only intermediate-depth, large-scale water mass that has not been studied at its winter source, despite general knowledge of the location of the source region for several decades. The project consists of a winter hydrographic survey of the AAIW outcropping region and the fronts that bound and a summer survey following the winter survey to study the evolution, restratification, and dispersal of the previous winter's waters. Results from this region will be relevant to Subantarctic Mode Waters (SAMW) formation in other regions. Broader Impacts: The high-quality data set collected under this project with a comprehensive coverage in winter and summer will fill an important and large gap in global ocean observations for physical processes that impact long-term climate change. Training will be provided for a graduate student and a post-doc. The proposed work will strengthen collaboration between U.S. and Chilean scientists. The investigators will continue working with Chilean and European Union graduate students, at no cost to NSF.