Mid‐Holocene Sahara‐Sahel Precipitation From the Vantage of Present‐Day Climate

Abstract

To account for the wet mid‐Holocene (ca. 6 ka) Sahara‐Sahel region, we exploit teleconnections from four large‐scale ocean‐atmosphere‐land forcings: El Niño–Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO), hemispherical temperature difference, and net energy input at the equator; they offer theoretical bases for estimating effects of large‐scale forcing on mid‐Holocene Sahara‐Sahel rainfall. Together, these forcings explain 16–64% of historical rainfall variability, with the highest percentages in the Sahel region. With estimates for mid‐Holocene time, latitudes where rainfall today averages 1,000, 500, or 200 mm/year are reconstructed to have lain ~2.5° farther north. Calculated mid‐Holocene rainfall at 15°N, 17.5°N, and 20°N exceeds that today by ~250, 100, and 20 mm/year. These shifts account for more than half of paleoclimatic estimates of mid‐Holocene rainfall and are comparable with the General Circulation Model (GCM) runs that match paleorainfall estimates most closely. Mid‐Holocene radiation may have affected Sahel precipitation indirectly via teleconnections from oceanic regions.

Multiproxy Reconstruction of Pliocene North Atlantic Sea Surface Temperatures and Implications for Rainfall in North Africa

“…Furthermore, since 5 Ma the location of rainfall gradient has shifted ∼4°–5° southward. A warmer North Atlantic seems to contribute an important part in this shift, as inferred for Holocene time (Molnar & Rajagopalan, 2020). Between 10°N and 15°N in the Sahara‐Sahel region, the calculated annual rainfall decreased from ∼600 mm/year more than that today at 5 Ma to about 200 mm/year more at 2 Ma (Figure 7b).…”

“…Although the regression model relies upon the modern SST‐precipitation relationship, its application to the Pliocene provides a first‐order approximation of rainfall over the African Sahel. Thus, we use this regression model, exploited also by Molnar and Rajagopalan (2020).…”

“…Several aspects of our estimates of greater Pliocene than present‐day rainfall make them likely to be underestimates. First, the regressions that we use account for at most 64% of modern rainfall variability (e.g., Molnar & Rajagopalan, 2020). Second, the lake itself can affect rainfall on adjacent terrain, and GCM runs that include a Lake Chad show enhanced summer rainfall east of the lake (e.g., Coe & Bonan, 1997; Contoux et al., 2013; Krinner et al., 2012).…”

“…Specifically, the transition from cool mid‐Holocene SSTs in the tropical Pacific, which are typical of La Niña conditions, to warmer conditions and more El Niño events (e.g., Carré et al., 2014; Clement et al., 2000; Sandweiss et al., 1996, 2009) offers explanations for transitions from fewer to more frequent heavy rainfall events over the Andes (e.g., Moy et al., 2002; Rodbell et al., 1999), and for changing amounts and intensity of precipitation in the southeastern (Donders et al., 2005), midwestern (e.g., Knox, 2000), and southwestern USA (e.g., Ely, 1997; Ely et al., 1993). We have previously followed this latter approach of treating present‐day teleconnections as templates for understanding past climate (e.g., Gill et al., 2016, 2017; Molnar & Cane, 2002, 2007; Molnar & Rajagopalan, 2012, 2020; Wycech et al., 2020), and we do so again here. Our target is SST variation over the North Atlantic since 5 Ma, and possible impacts of those changing SSTs on African rainfall.…”

Wetlands of North Africa During the Mid‐Holocene Were at Least Five Times the Area Today

“…The spatial pattern of annual precipitation in the mid‐Holocene Sahara is still uncertain (Bartlein et al., 2010; Brierley et al., 2020; Hopcroft et al., 2017; Molnar & Rajagopalan, 2020; Tierney et al., 2017). To cover the precipitation uncertainty range in the mid‐Holocene Sahara, four precipitation levels were tested (Section 2.1), leading to 20 mid‐Holocene simulations with five wetland diagnostic models (Figures 4c and 4d).…”