Coherent changes of southeastern equatorial and northern African rainfall during the last deglaciation

Abstract

During the last deglaciation, wetter conditions developed abruptly ~14,700 years ago in southeastern equatorial and northern Africa and continued into the Holocene. Explaining the abrupt onset and hemispheric coherence of this early African Humid Period is challenging due to opposing seasonal insolation patterns. In this work, we use a transient simulation with a climate model that provides a mechanistic understanding of deglacial tropical African precipitation changes. Our results show that meltwater-induced reduction in the Atlantic meridional overturning circulation (AMOC) during the early deglaciation suppressed precipitation in both regions. Once the AMOC reestablished, wetter conditions developed north of the equator in response to high summer insolation and increasing greenhouse gas (GHG) concentrations, whereas wetter conditions south of the equator were a response primarily to the GHG increase.

Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania)

“…Results from climate model studies suggest a north-south antiphase relation in African annual precipitation in response to North Atlantic cooling, consistent with latitudinal migrations of the ITCZ's annual mean position (e.g., Lewis et al, 2010;Kageyama et al, 2013). In line with this hypothesis, the arid phase recorded in our data during H1 has (within age model uncertainties) a pronounced wet counterpart in the Zambezi region (Schefuß et al, 2011;Otto-Bliesner et al, 2014). Therefore, we suggest that the observed H1 dry conditions in the uplands are part of a north-south dipole rainfall anomaly over eastern Africa and the Indian Ocean, which corroborates the seesaw hypothesis supported by further climate model studies (Claussen et al, 2003) and which is consistent with a southward shift of the ITCZ annual mean position in response to Northern Hemisphere cooling .…”

“…Therefore, our data suggest that H1 had a greater influence on eastern African hydrologic conditions than the YD, another North Atlantic cold event that likely, due to its shorter duration and weaker Northern Hemisphere cooling compared to H1, did not displace the annual mean ITCZ as far south as H1, thus causing these ambiguous signals. In addition, it has recently been suggested that gradually increasing greenhouse-gas forcing through the last glacial termination resulted in increasingly wetter conditions in tropical Africa (Otto-Bliesner et al, 2014), leading to a generally higher precipitation in the Rufiji region during the later stages of the deglaciation compared to H1.…”

Scientific Drilling at Lake Tanganyika, Africa: A Transformative Record for Understanding Evolution in Isolation and the Biological History of the African Continent, University of Basel, 6–8 June 2016

“…James Russell (Brown University) discussed the potential of Lake Tanganyika as a long paleoclimate record of the Afrotropics. Lake Tanganyika's sensitivity to regional/global climate variability is well documented (Tierney et al, 2008;Otto-Bliesner et al, 2014), and this variability also forces changes in the lake's ecosystem over long timescales. Walter Salzburger (University of Basel) focused on the cichlid fish fauna of the lake, which, while not the most diverse in terms of numbers of species within a single African lake, undoubtedly contains the greatest breadth of morphological, ecological and behavioral variation of any lake in the world, rendering Lake Tanganyika's cichlids iconic examples of evolution in isolation and adaptive radiation (Fryer and Iles, 1972).…”

The Effects of Younger Dryas Orbital Parameter and Atmospheric pCO<sub>2</sub> Changes on Radiative Forcing and African Monsoonal Circulation

“…Additional studies indicate declining greenhouse gases cause precipitation increases in the semi-arid, northern tropical and southern Sahel regions, as well as declines in precipitation due to insolation decreases associated with orbital forcing changes (Otto-Bliesner et al, 2014;Renssen et al, 2006). Cooling in the northern hemisphere could have led to weakening of the African monsoonal system (Alley and Clark, 1999).…”

“…Decline in greenhouse gas induced radiative forcing and associated tropospheric cooling could lead to higher aridity by decreased precipitation (due to the temperature dependence of vapor saturation pressure). The CO 2 radiative effect (cooling and increase in salinity) influences convection in the GIN Sea and the AMOC strength (Broecker and Denton, 1989;Rooth, 1982 Decrease of greenhouse gas radiative forcing during the last deglaciation may have altered African monsoonal circulation by precipitation increases in the southern area of the Sahel whereas with orbital forcing changes, precipitation increases over the entire Sahel region and expands northward over the northern area of the African continent (Otto-Bliesner et al, 2014).…”