Antarctic Intermediate Water Formation in a High-Resolution OGCM

1. EGU2010-12311 OS7 – Z94 Antarctic Intermediate Water Formation in a High-Resolution OGCM Antonio F.H. Fetter1,2, Victor Zlotnicki1, and Michael P. Schodlok1,3 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena; 2 Universidade Federal de Pernambuco, Recife, Brazil; 3JIFRESSE, University of California Los Angeles, Los Angeles • Motivation: • changes in the heat content of the Southern Ocean are linked to changes in the formation area of AAIW, which in turn is linked to changes in the size of the area of intermediate water mass formation bounded by fronts and to changes in eddy variability in that region preconditioning the source water masses. • very little is known about the spatial and temporal variability of these ventilation processes in the formation regions because in-situ observations, in particular in winter, are scarce. Therefore, we want to: • assess ECCO2 ability to reproduce the observed rates and patterns of AAIW formation and circulation • estimate the changes in the Antarctic Circumpolar Current (ACC) frontal locations in ECCO2 and altimetry Figure 1: High resolution global ocean circulation model ECCO2 (Estimating the Circulation and Climate of the Ocean, Phase II: High-Resolution Global-Ocean and Sea-Ice Data Synthesis). Syntheses of all available global-scale ocean and sea-ice data at resolutions that start to resolve ocean eddies and other narrow current systems, which transport heat, carbon, and other properties within the ocean. (http://ecco2.org) • ECCO2 model set up (CS84) • 18 km global resolution • 50 z-layers • 16 yrs integration 1992-2007 • surface forcing: ERA40/ECMWF with modifications • Initial conditions: optimization from 70+ sensitivity studies (a) Figure. 2: Deep Winter Convection as seen by the mixed layer depth for March (boral winter, left) and August (austral winter, right). The thickness of the mixed layer depth is according to the models KPP criteria. The latitudinal zonation of the density in the mixed layer reveals the intense ocean-atmosphere interaction. The densest southern hemisphere mode water is in the southern part of the mixed layer, in particular the south-eastern Pacific Ocean, the region of AAIW formation and in good agreement with observations (e.g. Talley et al., 200x). Figure. 5: The ACC transport through Drake Passage in neutral density space averaged (γn) over the integration period (a), and the seasonal variability of a climatological year (b). The mean ACC transport of 145 Sv is in agreement with previous estimates. The mean transport of AAIW is slightly higher compared to estimates from observations (34.8 Sv vs 28 Sv; Sloyan and Rintoul, 2001). A large fraction of the seasonal variability of the ACC transport happens in the AAIW density class of 27.4 kg/m3. Maximum AAIW transport across Drake Passage occurs in February/March associated with the deepest mixed layer depths in the south eastern Pacific Ocean. (b) Figure 3: Climatological Winter Salinity section along 80oW Antarctic Intermediate Water (AAIW) in ECCO2. The subsurface layer of salinity minimum shows the formation of AAIW at the end of the austral winter. The AAIW is usually found at the 27-27.4 kg/m3 neutral density class. In ECCO2, however, the AAIW is located at higher values, in the 27-27.8 kg/m3 class. • Conclusions: • ECCO2 produces AAIW in the south eastern Pacific Ocean at higher neutral density classes • 34.8 Sv AAIW are exported through Drake Passage with maximum values in Winter • water mass conversions occur in the Atlantic and Indian Oceans • major ACC fronts are well represented in the ECCO2 model configuration Figure. 6: Water mass transformation integrated over the Indian, Pacific and Atlantic sectors of the Southern Ocean south of 30S. It shows the inflow of AAIW and NADW in the South Atlantic and their conversion into surface and AABW waters. In the Indian Ocean bottom and heavy intermediate waters are converted into a lighter intermediate water. In the Pacific Ocean AAIW and AABW are produced. Note: positive values denote entering, negative leaving the domain. Figure 7: Location of the major ACC Fronts according to Orsi et al. (1995) and the location derived from the ECCO2 model run. The location of the Polar Front (PF), the Subantarctic Front (SAF) and the Subtropical Front (STF) compare well with major deviations in the SAF between model and observations in the Brazil/Malvinas Confluence • Outlook: • investigate to what extent the rate of formation of AAIW is related to the meridional migrations of the Subantarctic Front • elucidate the diapycnal processes involved in the formation of AAIW (e.g., Ekman contribution, eddy fluxes, and air-sea fluxes) Figure. 4: Mean Meridional overturning stream function in Neutral Density space. It shows the shallow and deep branches of the MOC. The southward flow of the NADW (~12 Sv), the northward flow of the AAIW, the upwelling at the equator and the dense northward flow of the AABW at the bottom (12 Sv) are well defined. The Deacon cell is present in neutral density space. References: Orsi, A. H., T. Whitworth III, and W. D. Nowlin, Jr., On the meridional extent and fronts of the Antarctic Circumpolar Current, Deep-Sea Res. I, 42, 641-673, 1995. Sloyan, B.M., and S.R. Rintoul, Circulation, renewal and modification of Antarctic mode and intermediate water, J. Phys. Oceanogr., 31, 1005-1030, 2001. Talley, L., T. Chereskin, J. Holte, Y-D. Lenn, Subantarctic Mode Water and Antarctic Intermediate Water formation near the Subantarctic Front in the southeast Pacific in winter 2006. Ocean Sciences 2006,session OS075, 2006. Acknowledgements: ECCO2 is a contribution to the NASA Modeling, Analysis, and Prediction (MAP) program. We gratefully acknowledge computational resources and support from the NASA Advanced Supercomputing (NAS) Division and from the JPL Supercomputing and Visualization Facility (SVF).AF is funded by a NASA-ROSES-OSTST grant.