Dynamical and Thermodynamic Elements of Modeled Climate Change at the East African Margin of Convection

Abstract

We propose a dynamical interpretation of model projections for an end‐of‐century wetting in equatorial East Africa. In the current generation of global climate models, increased atmospheric moisture content associated with warming is not the dominant process explaining the increase in rainfall, as the regional circulation is only weakly convergent even during the rainy seasons. Instead, projected wetter future conditions are generally consistent with the El Niño‐like trend in tropical Pacific sea surface temperatures in climate models. In addition, a weakening in moisture convergence over the adjacent Congo Basin and Maritime Continent cores of convection results in the weakening of near‐surface winds, which increases moisture advection from the Congo Basin core toward the East African margin. Overall confidence in the projections is limited by the significant biases in simulation of the regional climatology and disagreement between observed and modeled tropical Pacific sea surface temperature trends to date.

A tale of two futures: contrasting scenarios of future precipitation for West Africa from an ensemble of regional climate models

“…However, the 'wet-get-wetter' mechanism (e.g. Seager et al 2010) is counterbalanced by the dynamic component ∆DY, which shows strong negative values over the areas affected by the monsoon (figures 2(b), (e), (h) and (k)), consistent with weaker convergence (stronger horizontal divergence, see equation (4); Endo and Kitoh 2014, Giannini et al 2018).…”

“…See e.g. Giannini et al (2018). Note that during June-September, both dry and wet RCMs project increased precipitation intensity but decreased frequency over the Sahel (10 • N-15 • N) (compare figures 4(a), (b) and 3(g), (h)); however, the reduction in RR1 is much stronger for the dry models, accompanied by a weaker increase in SDII, which results in the opposite sign of mean precipitation signal between dry and wet models where present and future precipitation exceeds 4 mm day −1 ), dynamical term of the moisture budget (e ) and (f) and change in number of rainy days (∆RR1) (g, h, where contours denote the present climate value of RR1) for the 'Dry' RCMs (first column) and 'Wet' RCMs (second column).…”

“…The transient (∆TE) and residual (∆Res) terms in equation (1) are not considered here (similarly to e.g. Giannini et al 2018) as their contribution is usually small over the tropics (see also Endo andKitoh 2014, Lee et al 2017).…”

“…Poan et al 2016, Akinsanola and Zhou 2019b, Raj et al 2019: in particular, the analysis of atmospheric moisture fluxes and, in particular, the thermodynamic and dynamic components of the moisture budget has proved particularly helpful in understanding future changes in precipitation at global and regional scale (e.g. Endo and Kitoh 2014, Pomposi et al 2015, Giannini et al 2018. Lee et al (2017) and Tamoffo et al (2019) used RCMs to investigate the relative contribution of dynamic and thermodynamic moisture budget components over East Asia and Central Africa, respectively; however, their studies were based on the results of either several RCMs downscaling a single GCM, or a single RCM forced by several GCMs.…”

The effect of westerlies on East African rainfall and the associated role of tropical cyclones and the Madden–Julian Oscillation

“…Westerly wind anomalies have been associated with enhanced rainfall over East Africa (specifically, the highland region) in studies of flooding events (Anyamba, 1983), seasonal variability (Camberlin and Wairoto, 1997), the Madden-Julian Oscillation (MJO; Pohl and Camberlin, 2006;Berhane and Zaitchik, 2014;Hogan et al, 2015), and climate change (Giannini et al, 2018). However, the idea of absolute westerly flow from the Congo has been described to some extent in only the studies of the MJO.…”