On the importance of intermediate water flows for the global ocean overturning

Abstract

A steady state inverse global ocean model is used together with the available original, historical hydrographic database to study and quantify the large-scale global ocean circulation. The model has a variable resolution grid with grid sizes as small as 2.5° longitude by 2° latitude along boundaries, straits or over steep topography and a default resolution of 5° by 4° in "quiet" open ocean regions. The model has 26 vertical levels with 60 m resolution near the surface. The adjoint method is applied to drive the model to the hydrographic data and to optimize horizontal flows, air-sea heat fluxes, and mixing coefficients in an iterative way. Mass, heat, and salt budgets are satisfied exactly by the model. After assimilation, both simulated temperature and salinity fields are in good agreement with observations. Sensitivity experiments show that different circulation patterns with varying relative importance of intermediate water versus warm water transports and varying warm water inflow from the Indian Ocean into the Atlantic are consistent with the hydrographic data. However, for all solutions we find that the water mass that dominates the compensation of North Atlantic Deep Water export is Antarctic Intermediate Water. The northward transport rates of intermediate water in the South Atlantic and South Pacific in our model solutions range between 10 and 15 Sv in each ocean and are considerably larger than previously published values. Copyright 1999 by the American Geophysical Union.