Analysis of the position and strength of westerlies and trades with implications for Agulhas leakage and South Benguela upwelling

Abstract

Abstract. The westerlies and trade winds over the South Atlantic and Indian Ocean are important drivers of the regional oceanography around southern Africa, including features such as the Agulhas Current, the Agulhas leakage, and the Benguela upwelling. Agulhas leakage constitutes a fraction of warm and saline water transport from the Indian Ocean into the South Atlantic. The leakage is stronger during intensified westerlies. Here, we analyze the wind stress of different observational and modeled atmospheric data sets (covering the last 2 millennia, the recent decades, and the 21st century) with regard to the intensity and position of the southeasterly trades and the westerlies. The analysis reveals that variations of both wind systems go hand in hand and that a poleward shift of the westerlies and trades and an intensification of westerlies took place during the recent decades. Furthermore, upwelling in South Benguela is slightly intensified when trades are shifted poleward. Projections for strength and position of the westerlies in the 21st century depend on assumed CO2 emissions and on their effect relative to the ozone forcing. In the strongest emission scenario (RCP8.5) the simulations show a further southward displacement, whereas in the weakest emission scenario (RCP2.6) a northward shift is modeled, possibly due to the effect of ozone recovery dominating the effect of anthropogenic greenhouse forcing. We conclude that the Agulhas leakage has intensified during the last decades and is projected to increase if greenhouse gas emissions are not reduced. This will have a small impact on Benguela upwelling strength and may also have consequences for water mass characteristics in the upwelling region. An increased contribution of Agulhas water to the upwelling water masses will import more preformed nutrients and oxygen into the upwelling region.

Twenty-first century Southern Hemisphere impacts of ozone recovery and climate change from the stratosphere to the ocean

“…Our analysis of the twenty first changes in the SH surface westerly winds supports previous findings (Shindell and Schmidt, 2004;Yin, 2005;Perlwitz et al, 2008;McLandress et al, 2011;Polvani et al, 2011a) emission scenario SSP5-8.5, FOCI projects that the GHG effect overwhelms the effect due to ozone recovery both during austral summer and in the annual mean, such that the westerlies shift farther poleward and the SAM shifts more towards its positive phase. This result is consistent with previous projections for the same high emission scenario (Bracegirdle et al, 2020) or for the similar scenario RCP8.5 (Eyring et al, 2013;Barnes et al, 2014;Gerber and Son, 2014;Iglesias-Suarez et al, 2016;Tim et al, 2019). However, it contrasts projections for low or moderate emission scenarios.…”

“…The latter result is supported by simulations with coupled climate models forced with the same scenario (Son et al, 2008(Son et al, , 2010Gerber and Son, 2014), the CMIP5 moderate scenario RCP4.5 (Eyring et al, 2013;Barnes et al, 2014;Gerber and Son, 2014), or the CMIP6 moderate scenario SSP2-4.5 (Bracegirdle et al, 2020). Under the low emission scenarios RCP2.6 for CMIP5 and SSP1-2.6 for CMIP6, coupled climate models and CCMs predict a negative SAM trend and an equatorward shift of the westerlies (Barnes et al, 2014;Eyring et al, 2013;Iglesias-Suarez et al, 2016;Tim et al, 2019;Bracegirdle et al, 2020). This highlights that the changes in the SH westerlies during the rest of the century will strongly depend on the future emissions of GHGs.…”

The impact of the Agulhas Current System on precipitation in southern Africa in regional climate simulations covering the recent past and future

“…Westerlies are projected to shift southward and strengthen, as previously found by e.g. Ivanciu et al (2022) and Tim et al (2019). This displacement and intensification is accompanied by a poleward shift and intensification of the high pressure systems of the oceans in austral winter.…”

Climatic change drives dynamic source–sink relationships in marine species with high dispersal potential

“…Ocean circulation is projected to change worldwide by the end of the century with changes in the intensity and position of western boundary currents including the Agulhas Current, the retroflection of which is projected to intensify and shift to the southeast (van Gennip et al., 2017; Popova et al., 2016). There is, however, considerably uncertainty regarding projected changes to circulation patterns in the Agulhas region (e.g., Tim et al., 2019), in part due to the relatively coarse spatial resolution of the presently available climate simulations. Nonetheless, an intensification of the Agulhas retroflection, in combination with a southwards shift in distribution of Jasus species, could result in reduced east to west connectivity of J. paulensis and J. tristani .…”

“…Ocean circulation is projected to change worldwide by the end of the century with changes in the intensity and position of western boundary currents including the Agulhas Current, the retroflection of which is projected to intensify and shift to the southeast (van Gennip et al, 2017;Popova et al, 2016). There is, however, considerably uncertainty regarding projected changes to circulation patterns in the Agulhas region (e.g., Tim et al, 2019) Accounting for such source-sink dynamics, for example by applying management measures to maintain particular levels of biomass in each population in relation to that population's likely role in supporting recruitment, may result in higher average recruitment and a more successful fishery.…”