Collaborative Research: Estimating the Indian Ocean overturn and diapycnal mixing

Collaborators:
Lynne Talley and Julie McClean

WOCE hydrographic transect station locations overlaid on Indian Ocean bathymetry. (L. Talley)

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. We propose 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 (east-west and basin-oriented) distributions of mixing and overturn, defined by the complex basin geometry, and forcing that is strongly variable in both space and time, and 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).

We are using three complementary approaches 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 32°S 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.