Mechanisms of Rainfall Biases in two CORDEX-CORE Regional Climate Models at rainfall peaks over Central Equatorial Africa

Two regional climate models (RCMs) participating in the CORDEX-Coordinated Output for Regional Evaluations (CORDEX-CORE) project feature a dipole-type rainfall bias during March-May (MAM) and September-November (SON) over Central Equatorial Africa (CEA), consisting in positive bias in West Central Equatorial Africa (WCEA) and negative bias in East Central Equatorial Africa (ECEA). One is the REgional MOdel version 2015 (REMO2015), and the other is the fourth version of the Regional Climate Model (RegCM4-v7). RCMs are nested in three Earth System Models (ESMs) from the Coupled Model Intercomparison Project phase 5 (CMIP5), and in the reanalysis ERA-Interim, at ~25 Km spacing-grid resolution. This study highlights misrepresented underlying physical processes associated with these rainfall biases through a process-based evaluation. Both RCMs produce a weaker Congo basin cell, associated with a weaker land-ocean zonal surface pressure gradient. Consequently, less water vapour enters the region, and little amount is transported from WCEA to ECEA, resulting in higher moisture availability in the west than in the east. This leads to an unevenly distributed moisture across the region, favouring a stronger atmospheric instability in WCEA where the moist static energy (MSE) anomalously increases through an enhanced latent static energy (LSE). Moisture arrives at a slower pace in ECEA, associated with the weak cell's strength. The intensity of ascent motions in response to the orographic constraint is weak to destabilise atmospheric stability in the lower layers, necessary for initiating deep convection. Therefore, the convection is shallow in ECEA related to underestimating the MSE due to the reduced LSE.

Climate of Western and Central Equatorial Africa

Western and Central Equatorial Africa (WCEA), home to the Congo rainforests, is the green heart of the otherwise dry continent of Africa. Despite its crucial role in the Earth system, WCEA’s climate variability has received little attention compared to the rest of Africa. Climate variability in the region is a result of complex interactions among various features acting on local and global scales. The mesoscale convective systems (MCSs) that have a preferentially westward propagation and present a distinct diurnal cycle are the main source of rainfall in the region. As a result of strong MCS activity, WCEA stands out as a convective anomaly within the tropics and experiences the world’s most intense thunderstorms as well as the highest lightning flash rates. The moisture of the region is supplied primarily from the Atlantic Ocean, with additional contributions from local recycling and East Africa. WCEA, in turn, serves as a moisture source for other parts of the continent. One striking characteristic of WCEA is its intrinsic heterogeneity with respect to interannual variability of rainfall, resulting in delineation of the region primarily in the zonal direction. This is in contrast to the meridionally oriented spatial variability of the annual cycle and underlines the fact that driving factors of the two can be quite different. The annual cycle is mainly determined by the seasonal excursion of the sun. However, the interannual and intraseasonal variability of the region are modulated by remote forcings from all three oceans, reflected via zonal atmospheric cells and equatorial wave dynamics. The local atmospheric jets and regional Walker-like circulations also contribute to WCEA’s climate variability by modulating the moisture transport and vertical motion. The region has experienced an increasing rate of deforestation in recent decades and has made a significant contribution to the global biomass burning emissions that can alter regional and global circulation, along with energy and water cycles. The mean annual temperature of the region has increased by about 1°C in the past 70 years. The annual rainfall over the same period presents a negative trend, though that is quite negligible in the eastern sector of the region.