The flow field of the subtropical gyre of the South Indian Ocean

The mean state of the transport field of the subtropical gyre of the South Indian Ocean has been derived for the upper 1000 m from selected historical hydrographic data. The subtropical gyre in the southwestern Indian Ocean is stronger than the flow in the other two oceans of the southern hemisphere. Most of the water in the South Indian gyre recirculates in the western and central parts of the basin. In the upper 1000 m the eastward transport of the South Indian Ocean Current starts with 60 Sv in the region southeast of South Africa. Between the longitudes of 40° and 50°E about 20 Sv of the 60 Sv recirculates in a southwest Indian subgyre. Another major diversion northward occurs between 60° and 70°E. At 90°E the remaining 20 Sv of the eastward flow splits up, 10 Sv going north to join the westward flow and only 10 Sv continuing in a northeastward direction to move northward near Australia. Near Australia, there is indication of the poleward flowing Leeuwin Current with a transport of 5 Sv. In the central tropical Indian Ocean between 10°S and 20°S, about 15 Sv flows to the west. The western boundary current of this subtropical gyre consists of the Agulhas Current along the east coast of southern Africa. Its mean flow is composed of 25 Sv from east of Madagascar and 35 Sv from recirculation in the southwest Indian subgyre south of Madagascar, with only 5 Sv being contributed from the Mozambique Channel. A net southward transport of 10 Sv results for the upper 1000 m of the South Indian Ocean. In contrast to the triangular shape of the subtropical gyre in the South Atlantic, probably caused by the cross‐equatorial flow into the North Atlantic, the area influenced by the subtropical gyre in the South Indian Ocean is more rectangular

Translation, decay and splitting of Agulhas rings in the southeastern Atlantic Ocean

All Agulhas rings that were spawned at the Agulhas retrofiec- tion between 1993 and 1996 (a total of 21 rings) have been monitored using TOPEX/Poseidon satellite altimetry and followed as they moved through the southeastern Atlantic Ocean, decayed, interacted with bottom topography and each other, or dissipated completely. Rings preferentially crossed the Walvis Ridge at its deepest parts. After having crossed this ridge they have lower translational speeds, and their decay rate decreases markedly. Half the decay of long-lived rings takes place in the first 5 months of their lifetimes. In addition to the strong decay of rings in the Cape Basin, about one third of the observed rings do not seem to leave this region at all but totally disintegrate here. The interaction of rings with bottom topography, in particular with the Verna Seamount, is shown frequently to cause splitting of rings. This will enhance mixing of the rings' Indian Ocean water into that of the southern Atlantic. This localized mixing may well provide a considerable source of warm and salty Indian Ocean water into the Atlantic overturning circulation

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