Agulhas Leakage Predominantly Responds to the Southern Hemisphere Westerlies

Abstract

2 25 Abstract 26 27 The Agulhas Current plays a crucial role in the thermohaline circulation through its 28 leakage into the South Atlantic. Under both past and present climates, the trade winds and 29 westerlies could have the ability to modulate the amount of Indian-Atlantic inflow. 30Compelling arguments have been put forward suggesting that trade winds alone have 31 little impact on the magnitude of Agulhas leakage. Here, employing three ocean models 32 for robust analysis -a global coarse resolution, a regional eddy-permitting and a nested 33 high-resolution eddy-resolving configuration -and systematically altering the position 34 and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown 35 that the westerlies, in particular their intensity, control the leakage. Leakage responds 36proportionally to the westerlies intensity up to a certain point. Beyond this, through the 37 adjustment of the large-scale circulation, energetic interactions occur between the 38 Agulhas Return Current and the Antarctic Circumpolar Current that result in a state 39 where leakage no longer increases. This adjustment takes place within 1 to 2 decades. 40Contrary to previous assertions, our results further show that an equatorward (poleward) 41 shift in westerlies increases (decreases) leakage. This occurs due to the redistribution of 42 momentum input by the winds. It is concluded that the reported present-day leakage 43 increase could therefore reflect an unadjusted oceanic response mainly to the 44 strengthening westerlies over the last few decades. 45 46 3

Ocean currents generate large footprints in marine palaeoclimate proxies

“…We use two ocean models of contemporary circulation, which both include mesoscale eddies, to study the advection during the life span and the post-mortem sinking of foraminifera. Both models have a 1/10°horizontal resolution, but their domains differ: the INALT01 model 15 is focused around southern Africa in the Agulhas system and is among the best-performing models in that region 13,[15][16][17] , while the Ocean model For the Earth Simulator (OFES) model 18 is global in extent, allowing us to place these results in a wider context. In both models, we advect the virtual foraminifera as passive Lagrangian particles using the Connectivity Modeling System (CMS) 19 , simulating both their trajectories during their lifetime, as well as their post-mortem sinking.…”

Contribution of Increased Agulhas Leakage to Tropical Atlantic Warming

“…The LOCAL sensitivity experiment is considered in parallel with the last 30 years of the REF experiment. A more detailed description of the model and its experiments can be found in Durgadoo et al (2013).…”

“…On average, Agulhas leakage in LOCAL is significantly higher at the 99% confidence level by 2.4 Sv compared to REF over the entire 30-yr period, corresponding to a sustained increase of about 15% [ Fig. 4b; see also Durgadoo et al (2013, their Fig. 7)].…”

“…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).…”

The Agulhas Leakage: the missing link in the interhemispheric climate seesaw?

“…Nevertheless, questions remain on the postulated interplay between changing wind field and the AL strength. In fact, the scenarios inferred from the paleorecords seem to disagree with state-of-the-art numerical simulations, which, however, are only run with modern boundary conditions (Durgadoo et al 2013).…”