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Indian-Atlantic interocean exchange: dynamics, estimation and impact

Indian-Atlantic interocean exchange: dynamics, estimation and impact
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.
0148-0227
20885-20910
de Ruijter, W.P.M.
85dd535e-71a9-4397-b3d7-b1c61dfd9758
Biastoch, A.
fa7df768-2a47-4071-aafc-30f36910def8
Drijfhout, S.S.
a5c76079-179b-490c-93fe-fc0391aacf13
Lutjeharms, J.R.E.
0f364c0b-c97e-4252-a826-4283b2101171
Matano, R.P.
e4e5bc85-d13b-462d-937d-0f6c846e54a7
Pichevin, T.
3a86f40a-18ea-4357-9a8f-e4424e33bdf0
van Leeuwen, P.J.
52371b83-1a99-4b6d-9c80-757a379a9bb3
Weijer, W.
3edc6c54-4df9-4d8c-814e-924a8a1fb8d3
de Ruijter, W.P.M.
85dd535e-71a9-4397-b3d7-b1c61dfd9758
Biastoch, A.
fa7df768-2a47-4071-aafc-30f36910def8
Drijfhout, S.S.
a5c76079-179b-490c-93fe-fc0391aacf13
Lutjeharms, J.R.E.
0f364c0b-c97e-4252-a826-4283b2101171
Matano, R.P.
e4e5bc85-d13b-462d-937d-0f6c846e54a7
Pichevin, T.
3a86f40a-18ea-4357-9a8f-e4424e33bdf0
van Leeuwen, P.J.
52371b83-1a99-4b6d-9c80-757a379a9bb3
Weijer, W.
3edc6c54-4df9-4d8c-814e-924a8a1fb8d3

de Ruijter, W.P.M., Biastoch, A., Drijfhout, S.S., Lutjeharms, J.R.E., Matano, R.P., Pichevin, T., van Leeuwen, P.J. and Weijer, W. (1999) Indian-Atlantic interocean exchange: dynamics, estimation and impact. Journal of Geophysical Research, 104 (C9), 20885-20910. (doi:10.1029/1998JC900099).

Record type: Article

Abstract

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.

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Published date: January 1999
Organisations: Ocean and Earth Science

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Local EPrints ID: 349198
URI: http://eprints.soton.ac.uk/id/eprint/349198
ISSN: 0148-0227
PURE UUID: be85a562-bc18-4279-90cd-b7065c80814e

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Date deposited: 26 Feb 2013 12:12
Last modified: 08 Jan 2022 00:15

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Contributors

Author: W.P.M. de Ruijter
Author: A. Biastoch
Author: S.S. Drijfhout
Author: J.R.E. Lutjeharms
Author: R.P. Matano
Author: T. Pichevin
Author: P.J. van Leeuwen
Author: W. Weijer

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