The Indian Ocean has multiple roles in the global overturning circulation and carbon cycle, including throughput and transformation of Indonesian Throughflow Water, upwelling of deep and bottom waters from the Atlantic and Southern Ocean, and diffusive production of deep, intermediate and thermocline waters fed by this upwelling. Decadal change in South Indian ventilation and upper ocean overturn has been demonstrated. This project aims to: (1) Improve estimates of the Indian Ocean overturning circulation and diapycnal mixing, and the associated carbon and nutrient budgets. Modern, comprehensive data analysis and a high resolution GCM will be used to interpret and narrow the wide range of current estimates. (2) Characterize regional (basin-oriented, east-west, equatorial) distributions of mixing and overturn, affected by the complex basin geometry and by forcing that is strongly variable in both space and time; determine how these inhomogeneities impact carbon, anthropogenic carbon, oxygen, freshwater and heat transports and divergences. (3) Analyze changes in overturning circulation at 32°S from 1987 through 2009. Compute mass, heat, freshwater, CO2, oxygen and nutrient budgets from the 32°S section being collected in spring 2009; compare with previous occupations, and analyze in light of changes in the wind and buoyancy forcing (including the Southern Annular Mode).
Three complementary approaches will be used for estimating basin-wide circulation, overturn and mixing: (1) analysis of the Indian Ocean portion of two state estimates of the circulation, temperature and salinity (ECCO and the related high resolution SOSE); (2) analysis of the chemical and physical data from WOCE/JGOFS sections, and repeat sections at 32S for 1987, 2002, (1995) and 2009 using inverse techniques; and (3) analysis of overturn and diapycnal mixing in the high resolution POP model, which has recently been run with tracers. The project will include analysis of the mean and the seasonal (monsoonal) circulation. Carbon transports will be compared to previous observation-based results and to the results from OCMIP models.
Intellectual Merit: The proposed work is unique in that it will focus on the geographic distribution of diapycnal processes in the Indian Ocean, thus elucidating the dominant physical mechanisms. It will provide a comparison and combination of results from three different approaches: state estimation, suited to broad-scale, frequent sampling; analysis of highly-resolved synoptic sections with full chemistry, suited to detailing property budgets; and high resolution OGCM results using observed forcing, suited to studying dynamical processes. The project will be the first to use the WOCE/JGOFS fields to relate the details of ocean circulation, overturn and mixing to the location and magnitude of Indian Ocean CO2 uptake and outgassing, thereby connecting source/sinks regions to asymmetries in overturn and mixing. This project will also contribute to study of decadal change in the south Indian through analysis of the 2009 32S repeat hydrographic section, which is being collected by the U.S. Repeat Hydrography/CO2 program, which in itself does not provide funding for analysis.
Broader Impacts: Estimates of oceanic CO2 uptake are necessary for accurate prediction of future atmospheric CO2 content; direct estimates of inorganic carbon flux divergences will provide a constraint on the estimates made through surface pCO2 measurements and numerical models. Ventilation and overturn changes in the subtropical Indian Ocean have been demonstrated and this project will extend the decadal time series to 2009. A graduate student will be supported (SIO). A workshop on ocean circulation and climate change for 25 middle-school teachers will be developed (Talley). Oceanography projects with students and teachers at local schools will be continued (Macdonald). Ocean modeling and use in coupled climate models will be presented at a teachers' workshop at SFSU (McClean).
