Subantarctic Mode Water and Antarctic Intermediate Water mass formation and distribution: an observational-model synthesis using a Southern Ocean state estimate

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Antarctic Intermediate Water (AAIW) is the densest of circumpolar Subantarctic Mode Waters (SAMW). AAIW fills the southern hemisphere and world tropical oceans at depths of 800 m to 1000 m. These waters are important in the global freshwater budget and may impact long-term climate variability through their influence on North Atlantic Deep Water (NADW) formation. Decadal freshening of Pacific AAIW may be associated with anthropogenic climate change. AAIW affects the stratification of the tropical ocean, and variability in AAIW may thus influence variations in the El Niño/Southern Oscillation. INTELLECTUAL MERIT. The central goal of this project is to characterize and quantify the formation of Subantarctic Mode Water and Antarctic Intermediate Water in an eddy resolving Southern Ocean State Estimate (SOSE), to describe their pathways in the SOSE ocean interior at eddy to global lateral scales, to compare the results with the observations, and to identify key processes and locations for SAMW and AAIW formation in support of organizing future fieldwork. The work will also include assessing the fidelity of estimated atmospheric forcing fields, especially at smaller (meso) scales. The investigators will synthesize the results of previous and ongoing projects, focusing on Southern Ocean mode and intermediate water observations and model results. Two years (2005 and 2006) of an eddy permitting (1/6° horizontal resolution) Southern Ocean State Estimate (SOSE) that assimilates observations, including those from two recently cruises in the southeastern Pacific mode water formation region, are available. SOSE permits comparison of the observational (cruise-based) picture of Southern Ocean water mass formation and spreading in the ocean interior with that in a numerical (assimilating) ocean of unprecedented lateral resolution and realism of air-sea forcing. SOSE assimilates surface fluxes, but adjusts them to be dynamically consistent with the oceanic fields, all with horizontal resolution of 1/6°. The investigators have completed a detailed comparison of large scale pattern of SOSE air-sea fluxes with those in the literature, finding them to be a notable improvement over previous flux products, and has also begun a study of water masses in SOSE, which shows that the SOSE fluxes more faithfully capture the density ranges within which SAMW/AAIW is formed than do the standard NWP flux products. A related study of mean flow and eddy dynamics in a different numerical model of Southern Ocean circulation will provides the opportunity to compare water mass properties and distributions in SOSE with those in a stand-alone ocean model of comparable resolution. The co-PI, Professor Talley will provide leadership in synthesizing SAMW and AAIW observations and model results. As a new post-doc at Scripps Institution of Oceanography funded through another project, Dr. Mazloff will continue the SOSE assimilation, extending it in time and data coverage. BROADER IMPACTS: This analysis will document in detail the hydrographic properties of the SOSE numerical representations of Southern Ocean circulation and of their corresponding ocean surface forcing fields in comparison with existing observations and literature. The results will be a foundation upon which to (i) plan and evaluate new observational studies, and to (ii) plan and evaluate new ocean and or ocean-atmosphere model studies, particularly studies intended to evaluate the effects of processes and/or changes in the Southern Ocean on climate. All SOSE fields are publicly available.