Ocean Science www.ocean-sci.net 1812-0784 1812-0792 4 3 2008 10.5194/os-4-223-2008 http://www.ocean-sci.net/4/223/2008/ http://www.ocean-sci.net/4/223/2008/os-4-223-2008.html http://www.ocean-sci.net/4/223/2008/os-4-223-2008.pdf 223 237 2008-09-30 Agulhas ring injection into the South Atlantic during glacials and interglacials V. Zharkov D. Nof nof@ocean.fsu.edu Geophysical Fluid Dynamics institute, Florida State University, USA Department of Oceanography, Florida State University, USA Recent proxies suggest that, at the end of the last glacial, there was a significant increase in the injection of Agulhas rings into the South Atlantic (SA). This brought about a dramatic increase in the salt-influx (from the Indian Ocean) into the SA helping re-start the then-collapsed meridional overturning cell (MOC), leading to the termination of the Younger Dryas (YD). Here, we propose a mechanism through which large variations in ring production take place. <br><br> Using nonlinear analytical solutions for eddy shedding, we show that there are restricted possibilities for ring detachment when the coast is oriented in the north-south direction. We define a critical coastline angle below which there is rings shedding and above which there is almost no shedding. In the case of the Agulhas region, the particular shape of the African continent implies that rings can be produced only when the retroflection occurs beyond a specific latitude where the angle is critical. During glaciation, the wind stress curl (WSC) vanished at a latitude lower than that of the critical angle, which prohibited the retroflection from producing rings. When the latitude at which the WSC vanishes migrated poleward towards its present day position, the corresponding coastline angle decreased below the critical angle and allowed for a vigorous production of rings. <br><br> Simple process-oriented numerical simulations (using the Bleck and Boudra model) are in satisfactory agreement with our results and enable us to affirm that, during the glacials, the behavior of the Agulhas Current (AC) was similar to that of the modern East Australian Current (EAC), for which the coastline slant is supercritical. Andrews, J. C. and Scully-Power, P.: The structure of an East Australian Current anticyclonic eddy, J. Phys. Oceanogr., 6, 756–765, 1976. Arruda, W. Z., Nof, D., and O'Brien, J. J.: Does the Ulleung eddy owe its existence to \textit$\beta $ and nonlinearities?, Deep-Sea Res., 51, 2073–2090, 2004. Bard, E.: Climate shock: abrupt changes over millennial time scales, Phys. Today, 55, 32–38, 2002. Berger, W. H. and Wefer, G.: Expeditions into the past: paleoceanographic studies in the South Atlantic, pp. 35–156, Springer-Verlag, Berlin-Heidelberg, 1996. Bleck, R. and Boudra, D.: Wind-driven spin-up in eddy resolving ocean models formulated in isopycnic and isobaric coordinates, J. Geophys. Res., 91, 7611–7621, 1986. Bostock, H. C., Opdyke, B. N., Gagan, M. K., Kiss, A. E., and Fifield, L. K.: Glacial/interglacial changes in the East Australian current, Clim. Dynam., 26, 645–659, 2006. Byrne, A. D., Gordon, A. L., and Haxby, W. F.: Agulhas eddies: A synoptic view using geosat ERM data, J. Phys. Oceanogr., 25, 902–917, 1995. Chassignet, E. P. and Boudra, D. B.: Dynamics of the Agulhas retroflection and ring formation in a numerical model. II. Energetics and ring formation, J. Phys. Oceanogr., 18, 304–319, 1988. de Ruijter, W. P. M.: Asymptotic analysis of the Agulhas and Brazil Current systems, J. Phys. Oceanogr., 12, 361–373, 1982. 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.: Indian-Atlantic interocean exchange: Dynamics, estimation and impact, J. Geophys. Res., 104, 20 885–20 910, 1999. Esper, O., Versteegh, G. J. M., Zonneveld, K. A. F., and Willems, H.: A palynological reconstruction of the Agulhas Retroflection (South Atlantic Ocean) during the Late Quaternary, Global Planet. Change, 41, 31–62, 2004. Gasse, F.: Hydrological changes in the African tropics since the Last Glacial Maximum, Quat. Sci. Rev., 19, 189–211, 2000. Goni, G. J., Garzoli, S. L., Roubicek, A. J., Olson, D. B., and Brown, O. B.: Agulhas ring dynamics from TOPEX/POSEIDON satellite altimeter data, J. Mar. Res., 55, 861–883, 1997. Gordon, A. L., Lutjeharms, J. R. E., and Gründlingh, M. L.: Stratification and circulation at the Agulhas Retroflection, Deep-Sea Res., 34, 565–599, 1987. Howard, W. R. and Prell, W. L.: Late quaternary surface circulation of the southern Indian Ocean and its relationship to orbital variation, Paleoceanogr., 7, 79–117, 1992. Kawagata, S.: Tasman front shifts and associated paleoceanographic changes during the last 250,000 years: foraminiferal evidence from the Lord Howe Rise, Mar. Micropal., 41, 167–191, 2001. Knorr, G. and Lohmann, G.: Southern Ocean origin for the resumption of the Atlantic overturning circulation during deglaciation, Nature, 424, 532–536, 2003. Lutjeharms, J. R. E.: The exchange of water between the South Indian and South Atlantic Oceans, in: The South Atlantic: Present and Past Circulation, edited by: Wefer, G., Berger, W., and Sielder, G., pp. 125–162. Springer-Verlag, Berlin-Heidelberg, 1996. Lutjeharms, J. R. E.: The Agulhas Current, Springer-Verlag, Berlin-Heidelberg-New York, XIV, 330 p., 2006. Martinez, J. I.: Late Pleistocene palaeoceanography of the Tasman Sea: implications for the dynamics of the warm pool in the western Pacific, Palaeogreogr. Palaeoclimatol. Palaeoecol., 112, 19–62, 1994. Martinez, J. I., de Deckker P., and Barrows, T. T.: Palaeoceanography of the western Pacific warm pool during the last glacial maximum: long-term climatic monitoring of the maritime continent, in: Bridging Wallace's line, edited by: Kershaw, P., Bruno, D., Tapper, N., Penny, D., and Brown, J., Adv. Geoecol., 34, 147–172, 2002. McDonagh, E. L., Heywood, K. J., and Meredith, M. P.: On the structure, paths, and fluxes associated with Agulhas rings, J. Geophys. Res., 104, 21 007–21 020, 1999. Nilsson, C. S., Andrews, J. C., and Scully-Power, P.: Observation of eddy formation off East Australia, J. Phys. Oceanogr., 7, 659–669, 1977. Nilsson, C. S. and Cresswell, G. R.: The formation and evolution of East Australian Current warm-core eddies, Progr. Oceanogr., 9, 133–183, 1981. Nof, D.: On the migration of isolated eddies with application to Gulf Stream rings, J. Mar. Res., 41, 399–425, 1983. Nof, D.: The momentum imbalance paradox revisited, J. Phys. Oceanogr., 35, 1928–1939, 2005. Nof, D. and Pichevin, T.: The ballooning of outflows, J. Phys. Oceanogr., 31, 3045–3058, 2001. Nof, D. and Van Gorder, S.: Did an open Panama Isthmus Correspond to an invasion of Pacific water into the Atlantic?, J. Phys. Oceanogr., 33, 1324–1336, 2003. Olson, D. B. and Evans, R. H.: Rings of the Agulhas Current, Deep-Sea Res., 33, 27–42, 1986. Peeters, F. J. C., Acheson, R., Brummer, G.-J. A., de Ruijter, W. P. M., Schneider, R. R., Ganssen, G. M., Ufkes, E., and Kroon, D.: Vigorous exchange between the Indian and Atlantic oceans at the end of the past five glacial periods, Nature, 430, 661–665, 2004. Pichevin, T., Nof, D., and Lutjeharms, J. R. E.: Why are there Agulhas Rings?, J. Phys. Oceanogr., 29, 693–707, 1999. Rau, A. J., Rogers, J. Lutjeharms, J. R. E., Giraudeau, J., Lee-Thorp, J. A., Chen, M.-T., and Waelbroeck, C.: A 450-kyr record of hydrological conditions on the western Agulhas Bank Slope, south of Africa, Mar. Geol., 180, 183–201, 2002. Sandal, C. and Nof. D.: A new analytical model for Heinrich events and climate instability, J. Phys. Oceanogr., 38, 451–466, 2008. Schouten, M. W., de Ruijter, W. P. M., van Leeuwen, P. J., and Lutjeharms, J. R. E.: Translation, decay and splitting of Agulhas rings in the south-eastern Atlantic ocean, J. Geophys. Res., 105, 21 913–21 925, 2000. Schouten, M. W., de Ruijter, W. P. M., and van Leeuwen, P. J.: Upstream control of the Agulhas ring shedding, J. Geophys. Res., 107, doi:10.1029/2001JC000804, 2002. Shi, C. and Nof, D.: The destruction of lenses and generation of wodons, J. Phys. Oceanogr., 24, 1120–1136, 1994. Sokolov, S. and Rintoul, S.: Circulation and water masses of the southwest Pacific: WOCE Section P11, Papua New Guinea to Tasmania, J. Mar. Res., 58, 223–268, 2000. Speich, S., Lutjeharms, J. R. E., Penven, P., and Blanke, B.: Role of bathymetry in Agulhas Current configuration and behavior, Geophys. Res. Lett., 33, L23611, doi:10.1029/2006GL027157, 2006. Tilburg, C. E., Hulburt, H. E., O'Brien, J. J., and Shriver, J. F.: The dynamics of the East Australian current system: the Tasman front, the east Auckland current and the East Cape current, J. Phys. Oceanogr., 31, 2917–2943, 2001. van Aken, H. M., van Veldhoven, A. K., Veth, C., de Ruijter, W. P. M., van Leeuwen, P. J., Drijfhout, S. S., Whittle, C. P., and Rouault, M.: Observations of a young Agulhas ring, Astrid, during MARE in March 2000, Deep-Sea Res., 50, 167–195, 2003. van Ballegooyen, R. C., Gründlingh, M. L., and Lutjeharms, J. R. E.: Eddy fluxes of heat and salt from the southwest Indian Ocean into the southeast Atlantic Ocean: A case study, J. Geophys. Res., 99, 14 053–14 070, 1994. Weijer, W., de Ruijter, W. P. M., Dijkstra, H. A., and van Leeuwen, P. J.: Impact of interbasin exchange on the Atlantic Overturning Circulation., J. Phys. Oceanogr., 29, 2266–2284, 1999. Weijer, W., De Ruijter, W. P. M., and Dijkstra, H. A.: Stability of the Atlantic overturning circulation:~ Competition between Bering Strait freshwater flux and Agulhas heat and salt sources, J. Phys. Oceanogr., 31, 2385–2402, 2001. Zharkov, V. and Nof, D.: Retroflection from slanted coastlines – circumventing the "vorticity paradox", Ocean Sci. Discuss., 5, 1–37, 2008.