A new paradigm to interpret southern African palaeorecords: the Tropical-Temperate Interactions

Contrary to the current paradigm, the millennial-scale rainfall variability in both the Winter and Summer Rainfall Zones (WRZ and SRZ, respectively) of southern Africa is not always driven by changes in the dynamic and intensity of the regional dominant system (temperate systems in the WRZ, and (sub)tropical systems in the SRZ), but rather by the intensity of the weaker of these two systems.

This conclusion has been derived after many years of intense research in southern Africa to develop CREST [1] but also to develop Hyrax Middens as a reliable archives for palaeoclimatic indicators such as d15N (aridity proxy) [2]. In this study, we present a new composite d15N record from Seweweekspoort [3], along the south coast of South Africa, and located in a region clearly dominated by temperate systems [4]. However, the known variability of the Southern Westerlies alone couldn't explain the reconstructed variations. They could explain the first order of variation, with wetter conditions during the last glacial period, and reduced rainfall during the Holocene, but not the millennial-scale variability.

By detrending this new record and the regional rainfall stacks we have developed from the SRZ [5], we have been able to evidence that the millennial-scale variability in both the SRZ and the WRZ were positively correlated. During the last glacial maximum and the deglaciation, both records exhibit a sensitivity to the southern African monsoon (the weaker system at the time, top of the figure), while during the Holocene, the influence of the temperate systems (weaker because shifted poleward) can be felt in both the WRZ and the SRZ (bottom of the figure).

These results have important consequences for our understanding of past climates, as they suggest that records from the either the WRZ or the SRZ should never be interpreted individually, but rather in the light of what happens in the "opposite" zone.

Chase B.M., Chevalier, M., Boom, A. and Carr, A.: The dynamic relationship between temperate and tropical circulation systems across South Africa since the last glacial maximum. Quat. Sci. rev., 174, 54 – 62, 2017. https://doi.org/10.1016/j.quascirev.2017.08.011.

[1] Chevalier, M., Cheddadi, R. and Chase, B.M.: CREST (Climate REconstruction SofTware): a probability density function (PDF)-based quantitative climate reconstruction method, Clim. Past, 10(6), 2081 – 2098, 2014.

[2] Chase, B.M., Scott, L., Meadows, M.E., Gil-Romera, G., Boom, A., Carr, A.S., Reimer, P.J., Truc, L., Valsecchi, V., Quick, L.J., 2012. Rock hyrax middens: a palaeoenvironmental archive for southern African drylands. Quat. Sci. Rev. 56, 107– 125.

[3] Chase, B.M., Boom, A., Carr, A.S., Meadows,M.E., Reimer, P.J., 2013. Holocene climate change in southernmost South Africa: rock hyrax middens record shifts in the southern southern westerlies. Quat. Sci. Rev., 82, 199 – 205.

[4] Tyson PD, Preston-White RA. 2000. The Weather and Climate of Southern Africa. Oxford University Press Southern Africa: Cape Town.

[5] Chevalier, M. and Chase, B.M.: Southeast African records reveal a coherent shift from high- to low-latitude forcing mechanisms along the east African margin across last glacial-interglacial transition, Quat. Sci. Rev., 125,117 – 130, 2015.