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

A steady state inverse global ocean model is used together with theavailable original, historical hydrographic database to study andquantify the large-scale global ocean circulation. The model has avariable resolution grid with grid sizes as small as 2.5\degree\longitude by 2\degree\ latitude along boundaries, straits or oversteep topography and a default resolution of 5\degree\ by 4\degree\ in``quiet'' open ocean regions. The model has 26 vertical levels with 60m resolution near the surface. The adjoint method is applied to drivethe 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 arein good agreement with observations. Sensitivity experiments showthat different circulation patterns with varying relative importanceof intermediate water versus warm water transports and varying warmwater inflow from the Indian Ocean into the Atlantic are consistentwith the hydrographic data. However, for all solutions we find thatthe water mass that dominates the compensation of North Atlantic DeepWater export is Antarctic Intermediate Water. The northward transportrates of intermediate water in the South Atlantic and South Pacific inour model solutions range between 10 and 15 Sv in each ocean and areconsiderably larger than previously published values

Indian-Atlantic interocean exchange: dynamics, estimation and impact

Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3 s?1). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales. Modeling studies suggest that the Indian-Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Currents' retroflection. State-of-the-art global and regional “eddy-permitting” models show a reasonably realistic representation of the mean Agulhas system; but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement. In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchanges dynamics and forcing, its representation in state-of-the-art models, and, finally, the impact of the Indian-Atlantic fluxes on regional and global scale both within the Atlantic Ocean and in interaction with the overlying atmosphere