Agulhas Leakage Explained

The Agulhas Leakage is an inflow of anomalously warm and saline water from the Indian Ocean into the South Atlantic due to the limited latitudinal extent of the African continent compared to the southern extension of the subtropical super gyre in the Indian Ocean. The process occurs during the retroflection of the Agulhas Current via shedding of anticyclonic Agulhas Rings, cyclonic eddies and direct inflow. The leakage contributes to the Atlantic Meridional Overturning Circulation (AMOC) by supplying its upper limb, which has direct climate implications.^[1]

Pathway

The Agulhas Current carries around 70 Sv southward towards 32°S. When the current passes the southern African tip, it changes direction and returns to the Indian Ocean. However, part of it (around 2-15 Sv) leaks into the Southern Atlantic.^[2] The leakage is mainly driven by large anticyclonic eddies shedded from the retroflection.^[3] The process is also induced by cyclonic eddies formed when the main current detaches from the continental shelf ^[4] and filaments peeled directly from the main current.^[5]

After reaching the Atlantic, the leakage enters the Cape Cauldron and the majority of the leakage propagates further north-westwards through the Benguela Current, South Equatorial Current, and finally crosses the equator along with the North Brazil Current.^[6] It then joins the Loop Current and the Gulf Stream. Part of the leakage follows the extension of the Indian-Atlantic supergyre to the Pacific Ocean.

A small part of it follows the "cold water route", by looping along the Antarctic Circumpolar Current and entering the Atlantic through the Drake Passage.^[7]

Dynamics of the leakage

The Agulhas Current represents a western boundary current which is primarily driven by a positive wind stress curl. The presence of the African continent allows for the southward flow of the current. Beyond Cape Agulhas, further southward propagation is no longer maintained by the western boundary. With large inertia, the current reaches the latitude of maximum westerlies (40°S) associated with neutral wind stress and loops back into the Indian Ocean (Agulhas retroflection). Without sufficient inertia, it turns westwards and leaks into the Atlantic Ocean.^{[8] [9]}

Variability of the leakage

The strength of the southward inertia and the position of the Subtropical Front (STF) are the key factors in the generation of the Agulhas leakage. Both of them are primarily controlled by the strength and the pattern of the wind field over the Indian Ocean.

• If the STF moves southward, the gap between South Africa and the westerlies becomes wider. The current thus needs more inertia to retroflect and more leakage occurs.
• If the wind field is weaker, the strength of the Agulhas Current is reduced. This leads to smaller inertial overshoot resulting in stronger leakage.

Generation of the Agulhas rings is also an important driver of the leakage. This depends on instabilities, topography, and mesoscale non-linear dynamics.^{[10] [11]}

Paleoclimate

The strength and location of the Agulhas Current, as well as the leakage can be reconstructed based on paleoceanographic data such as the provenance of sediments (presence of planktic foraminiferal species Globorotalia menardii,^[12] isotope ratio (87Sr/86Sr) in deep ocean cores,^[13] abundance of Agulhas fauna).

Paleoclimate observations allow for a reconstruction of the leakage for up to 1 350 000 years (mid Pleistocene). It has been shown that the leakage was more intense during interglacial periods.^[14] Those periods are characterized by a southward shift of the Subtropical Front associated with stronger leakage.

Paleoclimate data suggest that the strength of the leakage is positively correlated with the sea surface temperature, which is higher during interglacials. Moreover, the strength of the leakage was shown to be linked to the strength of the AMOC.

Climate change

There is evidence indicating that anthropogenic climate change causes southward expansion of the Indian Ocean subtropical gyre, which results in a southward shift of the westerlies. Simultaneously, no significant trend in wind curl value is observed. As a result, the latitude of the zero wind curl migrates towards the south and the leakage intensifies.^[15]

Moreover, there has been an increase in eddy kinetic energy in the Southeast Atlantic associated with more eddies and rings being formed leading to stronger leakage.^[16]

AMOC implications

The Agulhas leakage can potentially play a role in global climate because of its impact on the strength of the AMOC.

The leakage can modify the AMOC through:

• buoyancy forcing,
• wind stress changes,
• planetary wave perturbations.^[17]

The leakage brings relatively warm and saline water into the Atlantic basin which has two contrary effects on the density. Around the Southern tip of Africa, the heat input has a dominant effect resulting in a negative density anomaly. Further northward propagation leads to atmospheric heat loss and only the salinity anomaly remains which is manifested as a positive density anomaly. The associated buoyancy forcing enhances the Atlantic meridional density gradient giving rise to the North Atlantic Deep Water (NADW) formation which strengthens the AMOC.^[18]

The propagation of anticyclonic rings into the Atlantic leads to the density surfaces depression inducing planetary waves formation. This can result in AMOC oscillations on both short and interannual-to-decadal time scales.^[19]

See also

• Agulhas current
• AMOC
• Western boundary current

Notes and References

1. Schmidt . Christina . Schwarzkopf . Franziska U. . Rühs . Siren . Biastoch . Arne . 2021-08-16 . Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods . Ocean Science . English . 17 . 4 . 1067–1080 . 2021OcSci..17.1067S . 10.5194/os-17-1067-2021 . 1812-0784 . 238688605 . free .
2. Richardson . Philip L. . August 2007 . Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters . Deep Sea Research Part I: Oceanographic Research Papers . 54 . 8 . 1361–1389 . 10.1016/j.dsr.2007.04.010 . 2007DSRI...54.1361R . 1912/2579 . 140704046 . 0967-0637. free .
3. Schouten . Mathijs W. . 2002 . Upstream control of Agulhas Ring shedding . Journal of Geophysical Research . en . 107 . C8 . 3109 . 10.1029/2001JC000804 . 2002JGRC..107.3109S . 1874/2386 . 0148-0227. free . free .
4. Hall . C. . Lutjeharms . J.R.E. . March 2011 . Cyclonic eddies identified in the Cape Basin of the South Atlantic Ocean . Journal of Marine Systems . en . 85 . 1–2 . 1–10 . 10.1016/j.jmarsys.2010.10.003. 2011JMS....85....1H .
5. Lutjeharms . J.R.E. . Cooper . J. . February 1996 . Interbasin leakage through Agulhas current filaments . Deep Sea Research Part I: Oceanographic Research Papers . en . 43 . 2 . 213–238 . 10.1016/0967-0637(96)00002-7. 1996DSRI...43..213L .
6. Rühs . Siren . Durgadoo . Jonathan V. . Behrens . Erik . Biastoch . Arne . 2013-08-12 . Advective timescales and pathways of Agulhas leakage . Geophysical Research Letters . 40 . 15 . 3997–4000 . 10.1002/grl.50782 . 2013GeoRL..40.3997R . 55957449 . 0094-8276.
7. Sebille . Erik van . Beal . Lisa M. . Johns . William E. . 2011-05-01 . Advective Time Scales of Agulhas Leakage to the North Atlantic in Surface Drifter Observations and the 3D OFES Model . Journal of Physical Oceanography . EN . 41 . 5 . 1026–1034 . 10.1175/2011JPO4602.1 . 0022-3670. free .
8. De Ruijter . Will . April 1982 . Asymptotic Analysis of the Agulhas and Brazil Current Systems . Journal of Physical Oceanography . 12 . 4 . 361–373 . 10.1175/1520-0485(1982)012<0361:aaotaa>2.0.co;2 . 1982JPO....12..361R . 0022-3670. free .
9. de Ruijter . W. P. M. . Biastoch . A. . Drijfhout . S. S. . Lutjeharms . J. R. E. . Matano . R. P. . Pichevin . T. . van Leeuwen . P. J. . Weijer . W. . 1999-09-15 . Indian-Atlantic interocean exchange: Dynamics, estimation and impact . Journal of Geophysical Research: Oceans . 104 . C9 . 20885–20910 . 10.1029/1998jc900099 . 1999JGR...10420885D . 0148-0227. free .
10. Dijkstra . Henk A. . de Ruijter . Wilhelmus P. M. . October 2001 . On the Physics of the Agulhas Current: Steady Retroflection Regimes . Journal of Physical Oceanography . 31 . 10 . 2971–2985 . 10.1175/1520-0485(2001)031<2971:otpota>2.0.co;2 . 2001JPO....31.2971D . 1874/2203 . 0022-3670. free . free .
11. van Sebille . E. . Biastoch . A. . van Leeuwen . P. J. . de Ruijter . W. P. M. . February 2009 . A weaker Agulhas Current leads to more Agulhas leakage . Geophysical Research Letters . 36 . 3 . n/a . 10.1029/2008gl036614 . 2009GeoRL..36.3601V . 1874/43696 . 31561997 . 0094-8276. free .
12. Caley . Thibaut . Giraudeau . Jacques . Malaizé . Bruno . Rossignol . Linda . Pierre . Catherine . 2012 . Agulhas leakage as a key process in the modes of Quaternary climate changes . Proceedings of the National Academy of Sciences of the United States of America . 109 . 18 . 6835–6839 . 10.1073/pnas.1115545109. 22508999 . 3344998 . 2012PNAS..109.6835C . free .
13. Franzese . Allison M. . Hemming . Sidney R. . Goldstein . Steven L. . June 2009 . Use of strontium isotopes in detrital sediments to constrain the glacial position of the Agulhas Retroflection . Paleoceanography . 24 . 2 . n/a . 10.1029/2008pa001706 . 2009PalOc..24.2217F . 0883-8305. free .
14. Dickson . Alexander J. . Leng . Melanie J. . Maslin . Mark A. . Sloane . Hilary J. . Green . Joanne . Bendle . James A. . McClymont . Erin L. . Pancost . Richard D. . 2010-08-07 . Atlantic overturning circulation and Agulhas leakage influences on southeast Atlantic upper ocean hydrography during marine isotope stage 11 . Paleoceanography . 25 . 3 . 10.1029/2009pa001830 . 2010PalOc..25.3208D . 0883-8305. free .
15. Alory . Gaël . Wijffels . Susan . Meyers . Gary . 2007-01-20 . Observed temperature trends in the Indian Ocean over 1960–1999 and associated mechanisms . Geophysical Research Letters . 34 . 2 . 10.1029/2006gl028044 . 2007GeoRL..34.2606A . 129235410 . 0094-8276. free .
16. Rouault . Mathieu . Penven . Pierrick . Pohl . Benjamin . 2009-06-18 . Warming in the Agulhas Current system since the 1980's . Geophysical Research Letters . 36 . 12 . 10.1029/2009gl037987 . 2009GeoRL..3612602R . 73617925 . 0094-8276. free .
17. SCOR/WCRP/IAPSO Working Group 136 . Beal . Lisa M. . De Ruijter . Wilhelmus P. M. . Biastoch . Arne . Zahn . Rainer . April 2011 . On the role of the Agulhas system in ocean circulation and climate . Nature . en . 472 . 7344 . 429–436 . 10.1038/nature09983 . 21525925 . 2011Natur.472..429B . 4424886 . 0028-0836.
18. Weijer . W . November 2002 . Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy . Global and Planetary Change . 34 . 3–4 . 293–311 . 10.1016/s0921-8181(02)00121-2 . 2002GPC....34..293W . 1874/2604 . 0921-8181. free .
19. Biastoch . A. . Böning . C. W. . Lutjeharms . J. R. E. . November 2008 . Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation . Nature . en . 456 . 7221 . 489–492 . 2008Natur.456..489B . 10.1038/nature07426 . 0028-0836 . 19037313 . 4323345.