Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies

Abstract

The transport of warm and salty Indian Ocean waters into the Atlantic Ocean-the Agulhas leakage-has a crucial role in the global oceanic circulation and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC). There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages and model studies that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies. A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing, but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a high-resolution ocean general circulation model to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contributed to the observed salinification of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC.

Contribution of Increased Agulhas Leakage to Tropical Atlantic Warming

“…Concurrent to the tropical Atlantic warming, Agulhas leakage-estimated using an established method Biastoch et al 2009)-has increased by about 45% from roughly 13 Sv (1 Sv 5 10 6 m 3 s 21 ) in the early 1960s to roughly 19 Sv in the early 2000s in the hindcast experiment (Fig. 4a).…”

“…This is consistent with Schmidtko and Johnson (2012), who attributed a warming of the AAIW core between 308 and 208S to increased Agulhas leakage via changes in the southern annular mode (SAM) index, and with McCarthy et al (2011), who showed that a salinification of the AAIW layer has its origin in the Indian Ocean. It is important to note that half of the AAIW immediately west of the Agulhas retroflection has its origin in the Agulhas Current (Biastoch and Böning 2013).…”

“…In this study, we explore the possibility that remote forcing from outside the tropical Atlantic Ocean, namely, because of increased Agulhas leakage transport south of Africa (Rouault et al 2009;Biastoch et al 2009), has led to the warming of the subsurface tropical Atlantic. These tropical Atlantic subsurface waters, which are warmer than in previous decades, are subsequently upwelled to the surface by the increased trade winds, resulting in the observed warming trend at the surface.…”

“…The strength of Agulhas leakage is controlled by the intensity and location of the Southern Hemisphere westerlies . In response to changes in those winds, model hindcast simulations suggest that Agulhas leakage increased over the past decades (Biastoch et al 2009;Durgadoo et al 2013). Trends in sea surface height show an increase in eddy kinetic energy in the Agulhas retroflection region, which has been used as an indicator of increase in Agulhas leakage (Backeberg et al 2012).…”

Investigating the Global Impacts of the Agulhas Current

“…First, the warm tropical waters carried by the current stimulate convection of the overlying atmosphere with direct consequences for regional weather systems [Reason and Jagadheesha, 2005]. Second, the shedding of Agulhas rings south of Africa causes buoyancy anomalies in the South Atlantic that stimulate dynamical responses with potential consequences for the Atlantic MOC [Biastoch et al, 2009;Knorr and Lohmann, 2003]. While the significance of these mechanisms is increasingly recognized, their dynamics and sensitivity are not well understood.…”