Sahel rainfall variability and response to greenhouse warming

Abstract

[1] The NCEP/NCAR re-analyses as well as ensemble integrations with an atmospheric GCM indicate that interannual variations in Sahel rainfall are related to variations in the mean sea level pressure (MSLP) over the Sahara. In turn the MSLP variations are related to the global distribution of surface air temperature (SAT). An increase in SAT over the Sahara, relative to the surrounding oceans, decreases the MSLP over the Sahara, thereby increasing the Sahel rainfall. We hypothesize that through this mechanism greenhouse warming will cause an increase in Sahel rainfall, because the warming is expected to be more prominent over the summer continents than over the oceans. This has been confirmed using an ensemble of 62 coupled model runs forced with a business as usual scenario. The ensemble mean increase in Sahel rainfall between 1980 and 2080 is about 1 -2 mm day À1 (25 -50%) during July -September, thereby strongly reducing the probability of prolonged droughts. Citation: Haarsma, R. J., F. M. Selten, S. L. Weber, and M. Kliphuis (2005), Sahel rainfall variability and response to greenhouse warming, Geophys. Res. Lett., 32, L17702,

Studies of 21st‐century precipitation trends over West Africa

“…However, Figure 4 also shows some precipitation reductions over the southern edge of the Sahel. Haarsma et al (2005) conclude that the frequency of prolonged droughts will be reduced towards the latter part of the 21st century, in contradiction to conclusions based on the CM2 (Held et al, 2005). Note, however, that simulations of a 10-member ensemble of the CCM3, forced by observed SST between 1945 and 2000, imperfectly reproduce the observed interannual variability of Sahel rainfall.…”

“…Although the CSM seasonal march of Sahel precipitation simulated for the current climate is realistic, the mean summer monsoon rain band does not penetrate far enough inland, giving a dry bias in the northern Sahel relative to observations. In addition, CSM-simulated SST evolution in the Echoing the earlier studies by Maynard et al (2002), Haarsma et al (2005) and Kamga et al (2005), Hoerling et al (2006) also project a trend towards wetter Sahel summers, but in this case, the study refers only to the first half of the 21st century. The assessment is based on 42 IPCC AR4 A1B scenario climate change simulations of 18 AOGCMs ( Figure 5).…”

“…However, the relationships do not remain consistent for the projections of future climate. Haarsma et al (2005) integrate the CCM3 [Community Climate Model #3, National Center for Atmospheric Research (NCAR)] coupled with an ocean model, assuming the A2 emissions scenario for years 2000-2080. The ensemble of 62 parallel simulations shows Sahel July-September rainfall increases of 25-50% between 1980 and 2080 that they attribute to differential continental heating, because GHG warming should be more prominent over the summer continents than over the surrounding oceans.…”

Recent changes in precipitation, ITCZ convection and northern tropical circulation over North Africa (1979–2007)

“…Such a pattern produces a poleward expansion of the subtropical highs with a tendency for monsoonal circulations to result in increased precipitation due to enhanced moisture convergence (Christensen et al, 2007). Moreover, as pointed out by Haarsma et al (2005), any increase in surface air temperature over the Sahara, relative to the surrounding oceans, tends to decrease surface pressure over the Sahara, thereby increasing the Sahel rainfall. Using an empirical approach and case studies, Raicich et al (2003) also showed that the Mediterranean Sea area takes part in the tropical circulation regime which increases during summer in the African region through the low-level branch of the local Hadley cell.…”

“…For example, a future wetter Sahel has been conceived as the response to a projected warming of the North Atlantic Ocean compared with the South Atlantic Ocean, and a further warming of the Indian Ocean (Hoerling et al, 2006), due to either a reduction of aerosol loading or an oscillation of Atlantic Ocean variability with multi-decadal time scale (Maynard et al, 2002;Haarsma et al, 2005;Kamga et al, 2005). This last hypothesis is based on numerical simulations showing that a Using numerical experiments, Bader and Latif (2003 ) provided evidence of anti-correlations between the observed low-pass-filtered rainfall over the West Sahel and tropical Indian Ocean SSTs.…”