Investigating the Global Impacts of the Agulhas Current

Eos, Vol. 91, No. 12, 23 March 2010

VOLUME 91 NUMBER 12 23 MARCH 2010

EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 109–116

responses with potential consequences Investigating the Global Impacts for the Atlantic MOC [Biastoch et al., 2009; Knorr and Lohmann, 2003]. While the signif- icance of these mechanisms is increasingly of the Agulhas Current recognized, their dynamics and sensitivity are not well understood. The key ques- PAGES 109–110 The Agulhas Current infl uences climate tions that arise are as follows: (1) How does in two ways. First, the warm tropical waters the Agulhas Current react to shifts in wind The Agulhas Current is the major west- carried by the current stimulate convec- fi elds and regional ocean fronts? (2) How ern boundary current of the Southern Hemi- tion of the overlying atmosphere with direct do such changes affect the Agulhas leak- sphere [Lutjeharms, 2006] and a key com- consequences for regional weather sys- age into the Atlantic? (3) Does the leak- ponent of the global ocean “conveyor” tems [Reason and Jagadheesha, 2005]. Sec- age indeed perturb the Atlantic MOC? To circulation controlling the return fl ow to the ond, the shedding of Agulhas rings south answer these questions, paleoceanographic Atlantic Ocean [Gordon, 1986]. As such, it of Africa causes buoyancy anomalies in reconstructions and numerical modeling is increasingly recognized as a key player in the South Atlantic that stimulate dynamical are essential tools. ocean thermohaline circulation, with impor- tance for the meridional overturning circulation (MOC) of the Atlantic Ocean. Unusual dynamics pervade the motion of this warm- water current—as it moves west around the southern tip of Africa, it is retrofl ected back east by the Antarctic Circum- polar Current. Not all waters are captured by this sudden diversion of course—parts of the Agulhas Current leak away into the South Atlantic Ocean (Figure 1). New research is giving tantalizing sugges- tions that variations of the Agulhas leakage south of Africa are key players in global climate developments, and hence understanding their operation is important to climate projec- tions. To examine the possible relationships between the Agulhas system and climate, two investigative avenues are open: modeling and paleoceanographic reconstructions. A new international initiative, called Multi-Level Assessment of Ocean- Climate Dynamics: A Gateway to Interdisciplinary Training and Anal- ysis ( GATEWAYS), addresses both.

The Agulhas System and Impacts

The Agulhas Current has a volume fl ux of about 70 sverdrups (1 sverdrup = 106 cubic meters per second), a dimension similar to the Gulf Stream and the North Atlantic Cur- rent. South of Africa the current retrofl ects back into the South Indian Ocean. Before reaching this retrofl ection, meanders— triggered by anticyclonic eddies deriving from the Mozambique Channel—propagate downstream with the current. On reaching Fig. 1. The Agulhas Current and Indian-Atlantic water exchange. The snapshot is taken from a the area of retrofl ection, these eddies, called high-resolution Agulhas model (box outlined in gray) nested in a global ocean/sea ice model “Agulhas rings,” spin off into the South Atlan- [Biastoch et al., 2009]. Colors and shading indicate temperature and magnitude of currents. tic (Figure 1). This mechanism, and the vol- Symbols mark locations and sample materials used to reconstruct past variation of the current. ume of the Agulhas Current itself at any The data will be fed into models to assess the dynamics of the current and interocean water given time, constitute the primary factors transports, including the Atlantic meridional overturning circulation (MOC), under contrasting that defi ne the leakage. past climates. Eos, Vol. 91, No. 12, 23 March 2010

Documenting Past Changes conjunction with atmosphere- ocean sim- (ARI, grant HPRI- CT- 2001- 00120); and the Through Paleoreconstruction ulations will allow for the assessment of Netherlands Organization for Scientifi c impacts of the Agulhas regime on global Research. Efforts of chief scientists, mas- Paleoceanographic records spanning the oceanic and atmospheric circulation [Bias- ters, and crews of the RRS Charles Dar- past several hundred thousand years have toch et al., 2009; Park and Latif, 2008]. This win, R/V Marion Dufresne, R/V Meteor, R/V helped to demonstrate the Agulhas Current’s has important implications for climate stud- Pelagia, and R/V SA Agulhas are gratefully sensitivity to variable wind fi elds over the ies, and Earth system models of intermedi- acknowledged. South Indian Ocean and the migration of ate complexity (EMICs) [Knorr and Lohm- ocean fronts south of Africa. Paleoevidence ann, 2003] are valuable tools for studying References provides qualitative indications of past the relationships between different climate variations of the Agulhas Current and adja- components and for validating paleoceano- Bard, E., and R. Rickaby (2009), Migration of the cent ocean fronts [Bard and Rickaby, 2009; graphic data. Subtropical Front as a modulator of glacial cli- Peeters et al., 2004; G. Martínez- Méndez The new GATEWAYS project, sponsored mate, Nature, 460, 380–383. et al., A 345,000 year record of heat and salt by the 7th Framework Programme of the Biastoch, A., et al. (2009), Increase in Agulhas leakage due to poleward shift of Southern Hemi- European Community, takes up these chal- transports in the Agulhas corridor south sphere westerlies, Nature, 462, 495–499. of Africa, submitted to Paleoceanography, lenges through fostering collaborations Franzese, A. M., S. R. Hemming, and S. L. Goldstein 2009;] that suggest shifted wind fi elds and, among scientists from Spain, Germany, (2009), Use of strontium isotopes in detrital sedi- potentially, changes to the circulation in the Israel, Netherlands, United Kingdom, and ments to constrain the glacial position of the Agulhas corridor and the leakage. Radio- South Africa in an interdisciplinary partner- Agulhas Retrofl ection, Paleoceanography, 24, genic isotope signals point to past changes ship of physical oceanography, ocean and PA2217, doi:10.1029/2008PA001706. in Agulhas Current strength, while the posi- atmospheric numerical modeling, and pale- Gordon, A. L. (1986), Interocean exchange of ther- tion of the retrofl ection appeared reason- oceanography and paleoclimatology. Col- mocline water, J. Geophys. Res., 91, 5037–5046. ably stable according to these data [Franz- laboration between these disciplines ena- Knorr, G., and G. Lohmann (2003), Southern Ocean origin for the resumption of Atlantic ther- bles scientists to (1) test the sensitivity of ese et al., 2009]. The consequences of such mohaline circulation during deglaciation, Nature, scenarios for the Agulhas leakage have yet the Agulhas Current to changing climates 424, 532–536. to be tested. of the past, (2) assess the impact of the cur- Lutjeharms, J. (2006), The Agulhas Current, 329 New sediment cores have been collected rent on climates in southeastern Africa, and pp., Springer, Berlin. from three sectors of the Agulhas regime (3) examine the sensitivity of the Atlantic Park, W., and M. Latif (2008), Multidecadal and (Figure 1). These sectors are the south- MOC to Agulhas leakage. multicentennial variability of the meridional east African margin, to reconstruct the cur- GATEWAYS also offers training to young overturning circulation, Geophys. Res. Lett., 35, rent’s variability as it forms the South Indian researchers in paleoceanographic recon- L22703, doi:10.1029/2008GL035779. Ocean western boundary current; the Agul- structions and ocean and atmosphere Peeters, F. J. C., et al. (2004), Vigorous exchange between the Indian and Atlantic oceans at the modeling. The interdisciplinary transfer of has corridor south of Africa, to reconstruct end of the past fi ve glacial periods, Nature, 438, the Indian-to- Atlantic water transports; and knowledge is pivotal to gaining fuller under- 661–665. the Agulhas plateau, to capture migrations standing of the Agulhas Current as one of Reason, C. J. C., and D. Jagadheesha (2005), Rela- of the ocean subtropical front and their the most vigorous and complex current sys- tionships between South Atlantic SST variability impact on the gateway circulation. Analysis tems in the world ocean. and atmospheric circulation over the South of the sediment cores will be complemented More information about GATEWAYS can African region during austral winter, J. Clim., 18, by studies of data and materials from sedi- be found in the electronic supplement to this 3339–3355. ment traps and moorings. On going work on Eos issue (http:// www .agu .org/ eos_elec/). some of these materials and cores already —RAINER ZAHN, Institució Catalana de Recerca highlights the quality of the paleoprofi les Acknowledgments i Estudis Avançats (ICREA), Barcelona, Spain; and obtainable from this region. Institut de Ciència i Tecnologia Ambientals, Depar- GATEWAYS is sponsored by the 7th tament de Geología, Universitat Autònoma de Bar- Facing the Challenge Framework Programme of the European celona, Bellaterra, Spain; E-mail: rainer.zahn@ uab Community. Acquisition of materials is sup- .cat; JOHANN LUTJEHARMS, Department of Ocean- Integrating paleoceanographic recon- ported by the International Marine Global ography, University of Cape Town, Rondebosch, structions and numerical ocean model- Change Study ( IMAGES); Deutsche For- South Africa; ARNE BIASTOCH, Leibniz-Institut für ing is the only way to fully understand the schungsgemeinschaft, Germany; Natural Meereswissenschaften (IFM-GEOMAR), Kiel, Germa- ny; IAN HALL, School of Earth and Ocean Sciences, sensitivity of the Agulhas Current and the Environmental Research Council, United Cardiff University, Cardiff, UK; GREGOR KNORR, signifi cance of the interocean salt trans- Kingdom; Ministerio de Ciencia e Inno- Alfred-Wegener- Institut für Polar- und Meeresforsc- ports for the Atlantic MOC. Further, embed- vación, Spain; Institut Polaire Français Paul hung, Bremerhaven, Germany; WONSUN PARK, ding high-resolution Agulhas modules Émile Victor, France; European Commis- IFM-GEOMAR; and CHRIS REASON, Department of into global models (as seen in Figure 1) in sion Access to Research Infrastructures Oceanography, University of Cape Town