Agulhas ring injection into the South Atlantic during glacials and interglacials

Abstract

Abstract. 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.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.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.

The Dynamical Subtropical Front

“…In the Indian Ocean this corresponds to the Agulhas Return Current, which is north of the super gyre boundary. It remains uncertain whether a change in the position of the Agulhas Retroflection and Agulhas Return Current would alter the volume of Agulhas Leakage [ Zharkov and Nof , ; van Sebille et al ., ; Le Bars et al ., ].…”

“…Climatologies therefore vary in the number of fronts defined and also the latitude of the STF (Figure ). As a result it is still unresolved whether the STF experiences a large or small seasonal cycle [ Deacon , ; Smythe‐Wright et al ., ; Wainer et al ., ; James et al ., ; Saraceno , ; Burls and Reason , ], and also how the position of the front is related to the wind stress curl field [ Deacon , ; Peeters et al ., ; Burls and Reason , ; Zharkov and Nof , ; Bard and Rickaby , ; Dencausse et al ., ]. The relationship between the position of the STF and wind stress curl is addressed in detail in a companion paper [ De Boer et al ., ].…”

Decoupling of the Agulhas Leakage from the Agulhas Current

“…Long-term variability in the exchange of these waters may have consequences for the interdecadal strength of the Atlantic meridional overturning circulation (Weijer et al 2002;Biastoch et al 2008b) and possibly plays a role in the termination of glacial phases (Peeters et al 2004). Leakage magnitude is mediated by the dynamics of the greater Agulhas system, under the influence of the large-scale winds (de Ruijter et al 1999;Biastoch et al 2008c;Rouault et al 2009;van Sebille et al 2009;Zharkov and Nof 2008). Reanalysis products show that Indian Ocean trade winds have recently intensified (Backeberg et al 2012).…”

The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front

“…A wind‐induced change in the position of the STF has therefore been suggested to indicate altered Indian‐Atlantic salt exchange that could be strongly influence the nature of climate transitions during the Quaternary [ Bard and Rickaby , ; Peeters et al ., ] and current Anthropocene [ Beal et al ., ; Biastoch et al ., ]. In these studies, the position of the STF is usually linked to the latitude of zero WSC so that a latitudinal movement of the wind field will cause an associated shift in the front [ Biastoch et al ., ; de Ruijter et al ., ; Dencausse et al ., ; Peeters et al ., ; Zharkov and Nof , ].…”

“…Hence, similar to Munk [] the boundaries of the gyre correspond to the zero WSC. The STF is also sometimes referred to as the Subtropical Convergence Zone which stems from its alternative perception as a feature coinciding with the maximum wind stress curl where the convergence is maximum [ Peterson and Stramma , ; Stramma , ; Zharkov and Nof , ]. The simple theory that relates the position of the STF to the zero WSC is powerful and has strong implications for our interpretations of climate responses to changes in the position of westerly winds.…”