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Paleo Agulhas Rings Enter the Subtropical Gyre

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Paleo Agulhas Rings Enter the Subtropical Gyre EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Open Access Biogeosciences Biogeosciences Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Open Access Open Access Clim. Past Discuss., 9, 2095–2114, 2013 Climate www.clim-past-discuss.net/9/2095/2013/ Climate CPD doi:10.5194/cpd-9-2095-2013 of the Past of the Past Discussions © Author(s) 2013. CC Attribution 3.0 License. 9, 2095–2114, 2013 Open Access Open Access Earth System This discussion paper is/hasEarth been System under review for the journal Climate of the Past (CP). Paleo Agulhas rings Please refer to the correspondingDynamics final paper in CP if available. Dynamics Discussions enter the subtropical gyre Open Access Open Access Geoscientific Geoscientific P. Scussolini and Instrumentation Instrumentation E. van Sebille Paleo AgulhasMethods rings and enter the subtropicalMethods and Data Systems Data Systems Discussions Open Access gyre during the penultimateOpen Access deglaciation Title Page Geoscientific Geoscientific 1 2 Model Development Abstract Introduction P. Scussolini andModel E. van Development Sebille Discussions 1Earth and Climate Group, VU University, Amsterdam, the Netherlands Conclusions References Open Access 2 Open Access Climate Change ResearchHydrology Centre & ARCand Centre of Excellence forHydrology Climate System and Science, Tables Figures University of New South Wales, Sydney, Australia Earth System Earth System Received: 17 March 2013 – Accepted:Sciences 5 April 2013 – Published: 11 AprilSciences 2013 J I Discussions Open Access Correspondence to: P. Scussolini ([email protected])Open Access J I Published by Copernicus Publications on behalf of the European GeosciencesOcean Science Union. Ocean Science Back Close Discussions Full Screen / Esc Open Access Open Access Solid Earth Printer-friendly Version Solid Earth Discussions Interactive Discussion Open Access 2095 Open Access The Cryosphere The Cryosphere Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract CPD A maximum in the strength of Agulhas Leakage has been registered at the interface be- tween Indian and South Atlantic oceans during glacial Termination II (T II), presumably 9, 2095–2114, 2013 transporting the salt and heat necessary to maintain the Atlantic Meridional Overturn- 5 ing Circulation (AMOC) at rates similar to the present day. However, it was never shown Paleo Agulhas rings whether these were effectively incorporated in the South Atlantic gyre, or whether they enter the subtropical retroflected into the Indian and/or Southern Oceans. To solve this question, we investi- gyre gate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic basin than previously studied. Analysis P. Scussolini and 10 of a 20 yr dataset from a global ocean circulation model allows us to relate density E. van Sebille perturbations, at the depth of the thermocline, to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of δ18O variability of Globorotalia truncatulinoides single Title Page specimens, revealing high levels of pycnocline depth variability at the site, suggesting Abstract Introduction 15 enhanced numbers of Agulhas rings moving into the South Atlantic gyre around and before T II. Our record closely follows the published quantifications of Agulhas Leakage Conclusions References from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the Tables Figures South Atlantic circulation. This provides crucial support to the view of a prominent role of the Agulhas Leakage in the shift from a glacial to an interglacial mode of AMOC. J I J I 20 1 Introduction Back Close A transport of upper water masses takes place around the tip of South Africa, where the Agulhas Current retroflection spills Indian Ocean waters into the South Atlantic (Lutje- Full Screen / Esc harms, 2006; Beal et al., 2011, and references therein). This Agulhas Leakage (here- after AL) is an important component of the salt and heat balance of the AMOC (Gordon Printer-friendly Version 25 et al., 1992; Weijer et al., 2001; Dong et al., 2012). The receiver of this transfer is the Interactive Discussion 2096 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Cape Basin, in which a host of complex and turbulent phenomena occur (e.g. Olsen and Evans, 1986; Boebel et al., 2003; Doglioli et al., 2006). CPD Waters of Indian Ocean origin travel across the Cape Basin in a variety of forms 9, 2095–2114, 2013 and shapes. These can be broadly characterized as anticyclonic eddies (Agulhas rings 5 “sensu stricto”), cyclonic eddies, and filaments (or streamers, or liners) wound between the spinning eddies (Lutjeharms and Cooper, 1996; Treguier et al., 2003). It is not yet Paleo Agulhas rings clear how the AL is partitioned between these transport features (e.g. Doglioli et al., enter the subtropical 2006; van Sebille et al., 2010a), but a significant part of the AL is in the form of rings gyre (van Sebille et al., 2010a; Dencausse et al., 2010). P. Scussolini and 10 1.1 Agulhas rings and the thermocline E. van Sebille Agulhas rings have been described as quite regular features of mostly barotropic flow, and as the greatest and most energetic mesoscale eddies in the world (Clement and Title Page Gordon, 1995; Olson and Evans, 1986). Approximately six rings are spawned per year Abstract Introduction and they reach, though then very diminished, as far as 40◦ W (Byrne et al., 1995). Their 15 most conspicuous characteristic, upon entrance in the Cape Basin, is the dramatic de- Conclusions References pression they impose on isotherms, isopycnals and isohales (Arhan et al., 2011; Souza et al., 2011; Giulivi and Gordon, 2006), as they transport water that is much warmer Tables Figures and more saline than the surroundings, from the sub-surface to at least ∼ 1000 m depth (van Aken et al., 2003). J I 20 It is understood indeed that the bulk of the AL happens beyond the surface, at the J I depths of the thermocline (Donners and Drijfhout, 2004; Doglioli et al., 2006; Van Se- bille et al., 2010b). Observations of Gordon et al. (1992) show that across the Cape Back Close Basin thermocline, two thirds of the water between the 9 ◦C and 14 ◦C isotherms is of Full Screen / Esc Indian Ocean origin. The observational study of Richardson (2007) reveals that while 25 surface drifters adopt more northerly trajectories, closer to the Benguela Current, floats at ∼ 800 m travel right over the central Walvis Ridge. Souza et al. (2011) maintain that Printer-friendly Version the largest heat perturbation introduced by Agulhas rings occurs at depths comprised Interactive Discussion between 200 and 600 m. 2097 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1.2 In search for a missing paleo link CPD In the last decades, researchers have come to consider the AL mechanism to play an important role in the modes of AMOC circulation during Pleistocene climate shifts 9, 2095–2114, 2013 (e.g. Berger and Wefer, 1996). The hypothesis of reduced Indian to Atlantic transfer 5 during glacial periods is compatible with the northward shift of the oceanographic fronts Paleo Agulhas rings south of Africa (Hays et al., 1976; Be´ and Duplessy, 1976; Bard and Rickaby, 2009). enter the subtropical While paleoceanographic research on marine sediment has made a convincing case gyre that Agulhas Current (Hutson, 1980), and AL (Franzese et al., 2006) were reduced during glacial times, most of that evidence was gathered in the easternmost part of the P. Scussolini and 10 Cape Basin, depicting intense presence of Indian Ocean waters during glacial termina- E. van Sebille tions (Flores et al., 1999; Peeters et al., 2004) (Fig. 1). As modern observations show, Agulhas rings that reach such regions often bend south, to reach the Southern Ocean, or join the Agulhas retroflection (Dencausse et al., 2010; Arhan et al., 2011), where Title Page they likely fail to impact the AMOC. Therefore, a key question still remains on the table: Abstract Introduction 15 were the observed maxima in AL eventually incorporated in the South Atlantic gyre, thus forming the waters that subsequently cross the equator through the North Brazil Conclusions References Current? To postulate that stronger AL effectively influenced the AMOC strength de- Tables Figures mands this paleo connection, between the eastern Cape Basin and the South Atlantic circulation, to be drawn. J I 20 Here we test this case upon the penultimate deglaciation (termination II, T II), since it is characterized by a very distinguished peak in AL in the Pleistocene reconstruction of J I Peeters et al. (2004) (Fig. 3b). Our hypothesis is rationalized as follows. Regardless of the form that it assumes - either rings or filaments – the AL introduces anomalies in the Back Close temperature and salinity fields of the South Atlantic subtropical gyre. It is of high inter- Full Screen / Esc 25 est to both paleoclimatology and modern oceanography communities to know whether such perturbations were present over the central Walvis Ridge, almost 1800 km further Printer-friendly Version downstream into the Atlantic Ocean, and with which intensity, under different climatic frameworks. Interactive Discussion 2098 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Modern analytic techniques enable us to investigate such anomalies, by means of geochemical analysis of microfossils from marine sediment. By selecting the appropri- CPD ate species of planktic foraminifer, we aim to target water masses at the depth of the 9, 2095–2114, 2013 thermocline, where variability due to AL should be maximal (Souza et al., 2011).
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