Both differential and equatorial heating contributed to African monsoon variations during the mid-Holocene
Introduction
Extensive geophysical data indicate that the Sahara was significantly wetter and greener during the early and mid-Holocene (11-5 kya) and during multiple earlier interglacial periods (deMenocal et al., 2000). Yet most modern climate models underestimate the past greening of the Sahara, suggesting critical physical processes are misrepresented in these models (Harrison et al., 2014, Claussen et al., 2017). The changes in the Saharan precipitation are generally understood to be paced by variations in Earth's orbital parameters, which regulate the differential heating of the hemispheres that drives the seasonal migrations of the tropical rain belt. However, the relation of the zonally uniform changes in insolation caused by orbital variations to regional precipitation variations is not well understood (e.g., Liu et al., 2017). Here, using energetic constraints on the position of the tropical rain belt, we provide a conceptual framework for understanding the relation of orbital variations to different climatic states of the African monsoon.
During the wet phase of the Sahara in the early and mid-Holocene, known as the African Humid Period (AHP), parts of the Sahara were vegetated, contained permanent lakes, and sustained human populations in regions that are uninhabitable in the present climate (deMenocal et al., 2000, Kuper and Kröpelin, 2006). In part, the AHP is coeval with wet conditions in eastern Africa (the East African Humid Period; Gasse, 2000, Tierney et al., 2011), which, like the AHP, are not well understood and not well captured by climate models (Liu et al., 2014, Liu et al., 2004; Tierney et al., 2011). The drying of the Sahara began around 6 kya. It is generally understood to have occurred alongside cooling in the northern hemisphere (though the extent and timing of the cooling trend remain unclear, Renssen et al., 2012, Marcott et al., 2013, Liu et al., 2014, Marsicek et al., 2018), as evidenced by various proxies of Sahara and Sahel precipitation (Marcott et al., 2013, Shanahan et al., 2015) and by archaeological findings (Manning and Timpson, 2014). The extent and distribution of the greening of the Sahara during the AHP remains a matter of some debate (see Quade et al., 2018 for a review). Nevertheless, most modern climate models underestimate even the low end of plausible greening estimates (Joussaume et al., 1999); they are an order of magnitude short of estimates at the upper end (Armitage et al., 2007, Drake et al., 2011). While the mechanisms behind the discrepancies between models and proxy data remain unclear, improved agreement between simulations and data in some models with vegetation and dust feedbacks suggest surface feedbacks are important for reproducing the greening of the Sahara (Claussen and Gayler, 1997, Levis et al., 2014, Patricola and Cook, 2007, Swann et al., 2014, Pausata et al., 2016, Gaetani et al., 2017).
Due to the tendency of the tropical rain belt to migrate toward and intensify in a differentially warming hemisphere (Chiang and Friedman, 2012, Schneider et al., 2014), the greening of the Sahara is generally attributed to Earth's orbital precession, which modulates seasonal differential heating. However, recent theory, modeling studies, and paleorecords indicate that precession alone cannot explain the greening of the Sahara (Claussen et al., 2017). First, regional energy fluxes are not clearly related to the zonally-uniform heating associated with orbital variations (Adam et al., 2016b, Roberts et al., 2017). Second, modern climate models forced by early- to mid-Holocene insolation (with up to 7% precession-driven increase in boreal summer insolation) tend to underestimate Saharan precipitation relative to proxy reconstructions (Joussaume et al., 1999, Claussen et al., 2017), even when ocean and land feedbacks are included (e.g., TEMPO, 1996, Kutzbach and Liu, 1997). Third, wet Saharan episodes are suggested to have existed during intervals of relatively low boreal summer insolation (e.g., Lézine and Casanova, 1991, Castañeda et al., 2009).
Here we examine how the regional precipitation and atmospheric energy balance are affected by mid-Holocene orbital changes in models participating in the third phase of the Paleo Model Intercomparison Project (PMIP3, Braconnot et al., 2011), which is the most extensive modeling study of mid-Holocene conditions to date.1 Our analysis sheds light on the discrepancy between models and reconstructions of the greening of the Sahara and provides a regional perspective on how orbital variations may drive African monsoon climate variations. In particular, our results suggest that, in addition to the commonly invoked differential heating of the hemispheres, equatorial heating of the atmosphere is an important factor controlling African monsoon variations.
Section snippets
Data
Our analysis is based on monthly data from 12 PMIP3 models (Fig. 1) and uses simulations of mid-Holocene and preindustrial conditions. In models for which an ensemble of runs exists, only the first realization of each experiment is analyzed. Seasonal and annual-mean climatologies are calculated from the first available 100 years, interpolated to a horizontal grid. The boundaries of the African sector are defined as 20∘W–40∘E. The sector-mean results were found to be qualitatively
The energy flux equator
The ITCZ marks the location of maximal surface mass convergence, and therefore, by mass conservation, the location of the rising branch of the mean meridional overturning circulation (Schneider et al., 2014). Since energy transport in the deep tropics is dominated by the mean meridional overturning circulation, the ITCZ also approximately marks the latitude of the atmospheric energy flux equator (EFE), where total atmospheric energy transport (AET) vanishes and diverges (i.e., flows away from) (
Regional energetic constraints
As expected, a clear northward shift of the annual-mean precipitation over Africa is captured in the ensemble mean of PMIP3 models (Fig. 1a; see Harrison et al., 2015, Chevalier et al., 2017 for a detailed analysis). However, all models severely underestimate reconstructions of the minimal increase in precipitation required to sustain a Saharan steppe: simulated changes do not exceed 100 mm yr−1 in the mid Sahara, in disagreement with evidence of widespread grasslands that require an increase of
Zonal- and sector-mean variations in PMIP3 models
A comparison of the zonal-mean and African sector-mean fractional changes in and is shown in Fig. 7. Even though regional cross-equatorial energy fluxes are not clearly related to the zonally-uniform orbital forcing, the zonal-mean and sector-mean fractional changes in are strongly correlated (Fig. 7a, ). As expected from orbital forcing, the mean fractional change in the zonal-mean is 10%, compared with 15% in the African sector. However, these changes vary
Discussion and conclusions
African monsoon variations are commonly conceptualized as meridional shifts of the continental ITCZ in the African sector. Based on the tendency of the ITCZ to migrate toward the differentially warming hemisphere, these shifts and similar shifts in the tropical rain belt outside the African sector are traditionally associated with changes in the inter-hemispheric differential heating of Earth. We have shown that changes in the equatorial heating of the atmosphere, which modulates both
Acknowledgments
The PMIP3 data was downloaded from Earth System Grid Federation (ESGF). Ori Adam acknowledges support by the Israel Science Foundation grant 1185/17.
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