Project summary (from NSF proposal)

Proposed work and intellectual merit (Criterion 1). 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. This proposal is part of a multi-institutional effort to characterize the processes responsible for the formation of Antarctic Intermediate water (AAIW) in the southeast Pacific. Results from this region will be relevant to SAMW formation in other regions. 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.

The proposal includes a winter hydrographic survey of the AAIW outcropping region and the fronts that bound it. Also proposed is a summer survey following the winter survey to study the evolution, restratification, and dispersal of the previous winter’s waters. The surveys proposed here are one part of a multifaceted approach to address this difficult question. This work will be coordinated with other U.S. and international studies involving carbon component and chlorofluorocarbon measurements, moorings, acoustic and profiling floats, and more limited hydrography in the study region (for mooring deployments). Here we cover the following components: CTD, salinity, oxygen, nutrients, XCTD, ADCP, LADCP, and shipboard meteorological measurements.

Broader Impacts (Criterion 2). Infrastructure: The high-quality data set with 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. The surveys will also serve as a means for making late winter carbon and CFC observations that will impact understanding of ocean carbon uptake. Education: 1 graduate student and 1 postdoc. International education: Work with Chilean and European Union graduate students, at no cost to this proposal. Talley taught a summer course at U. Concepcion in 2002 and advised a U. Copenhagen graduate student (Chilean nationality) for four months in 2002. The proposed work will strengthen collaboration between U.S. and Chilean scientists.