A Nonlinear Response of Sahel Rainfall to Atlantic Warming

Abstract

The response over West Africa to uniform warming of the Atlantic Ocean is analyzed using idealized simulations with a regional climate model. With warming of 1 and 1.5 K, rainfall rates increase by 30%-50% over most of West Africa. With Atlantic warming of 2 K and higher, coastal precipitation increases but Sahel rainfall decreases substantially. This nonlinear response in Sahel rainfall is the focus of this analysis. Atlantic warming is accompanied by decreases in low-level geopotential heights in the Gulf of Guinea and in the largescale meridional geopotential height gradient. This leads to easterly wind anomalies in the central Sahel. With Atlantic warming below 2 K, these easterly anomalies support moisture transport from the Gulf of Guinea and precipitation increases. With Atlantic warming over 2 K, the easterly anomalies reverse the westerly flow over the Sahel. The resulting dry air advection into the Sahel reduces precipitation. Increased low-level moisture provides moist static energy to initiate convection with Atlantic warming at 1.5 K and below, while decreased moisture and stable thermal profiles suppress convection with additional warming. In all simulations, the southerly monsoon flow onto the Guinean coast is maintained and precipitation in that region increases. The relevance of these results to the global warming problem is limited by the focus on Atlantic warming alone. However, confident prediction of climate change requires an understanding of the physical processes of change, and this paper contributes to that goal.

Regional climate projections for impact assessment studies in East Africa

“…Projected long-term average changes in T-max (°C) in East Africa for 2020s, 2050s, and 2080s under RCP2.6, RCP4.5, and RCP8.5. Hemisphere. Compared to the baseline period, the western parts of Ethiopia will be drier during JJAS and OND and according to recent studies (Neupane and Cook 2013, Lyon 2014, Souverijns et al 2016, the drying is due to the projected warming of the Gulf of Guinea and changes in the low level of geopotential height. In Tanzania, precipitation during JF will increase in the 2020s, 2050s, and 2080s and this is in line with the observed increasing trend, which was also concluded as a signal of climate change (Dosio and Panitz 2016).…”

Characterizing half‐a‐degree difference: a review of methods for identifying regional climate responses to global warming targets

“…There has been limited work to test pattern scaling for other impacts relevant variables such as radiation, humidity, evaporation, and wind speed. Research using idealized experiments indicates the potential for nonlinear responses to CO 2 forcing and increasing sea surface temperatures (SSTs) …”

Impact of climate change on mid-twenty-first century growing seasons in Africa

“…However, Biasutti and Sobel (2009) find a delayed monsoon onset in many of the GCM simulations, consistent with the May reduction in rainfall simulated here for the western West African region. In regional-scale simulations for 2081-2100 under the stronger forcing of the SRESA2 Neupane and Cook (2012) find that Atlantic SSTAs of 2 K or less produce increased precipitation in the Sahel, while warming above 2 K produces drying. This is consistent with Vigaud et al (2011), who downscale GCM projections for 2032-2041 over northern Africa.…”