Explore the vast range of titles available.

The Congo Basin is one of three key convective regions on the planet which, during the transition seasons, dominates global tropical rainfall. There is little agreement as to the distribution and quantity of rainfall across the basin with datasets differing by an order of magnitude in some seasons. The location of maximum rainfall is in the far eastern sector of the basin in some datasets but the far western edge of the basin in others during March to May. There is no consistent pattern to this rainfall distribution in satellite or model datasets. Resolving these differences is difficult without ground-based data. Moisture flux nevertheless emerges as a useful variable with which to study these differences. Climate models with weak (strong) or even divergent moisture flux over the basin are dry (wet). The paper suggests an approach, via a targeted field campaign, for generating useful climate information with which to confront rainfall products and climate models.

The time-varying September-November relationship between the Indian Ocean Dipole (IOD) and Central African (CA) rainfall has strengthened since the 1990s, implying an increasing IOD influence over CA rainfall. Using observational and reanalysis datasets covering the 1980–2016 period, this study examines the CA circulation response associated with the Indian Ocean dynamics during the September-December IOD events, since this circulation constitutes a key moisture transport feature for CA rainfall variability. The results show that during positive IOD events (pIOD), the moisture transport drivers over CA and the Indian Ocean (IO) are synchronous, leading to an increase in CA rainfall, whereas the reverse pattern is observed during negative IOD events (nIOD). The equatorial easterly (westerly) moisture transport driven by the anticyclonic (cyclonic) circulation in the northern tropical IO and the weakening (intensification) of the African Easterly Jet’s northern component (AEJ-N), leads to an increase (decrease) in CA rainfall during pIOD (nIOD). Warm (cold) SST anomalies in the eastern Indian Ocean during nIOD (pIOD) event, intensify (weaken) the large-scale upward motion, strengthening (weakening) the cyclonic circulation in the mid-troposphere, thus favoring a significant westerly (easterly) circulation. The AEJ-N weakening during pIOD events is associated with a strengthening of the meridional pressure gradient and a westward shift in the Saharan high location at the AEJ-N’s northern edge. The results also reveal a significant influence of the Atlantic during pIOD events, induced by its teleconnection with the IO, whose effects are more modulated by the IOD’s western pole warming than by the IOD-related SST gradient.

We evaluate and compare the simulation of the main features (low-level westerlies (LLWs) and the Congo basin (CB) cell) of low-level circulation in Central Equatorial Africa (CEA) with eight climate models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) and the corresponding eight previous models from CMIP5. Results reveal that, although the main characteristics of the two features are reasonably well depicted by the models, they bear some biases. The strength of LLWs is generally overestimated in CMIP5 models. The overestimation is attributed to both divergent and rotational components of the total wind with the rotational component contributing the most in the overestimation. In CMIP6 models, thanks to a better performance in the simulation of both divergent and rotational circulation, LLWs are slightly less strong compared to the CMIP5 models. The improvement in the simulated divergent component is associated with a better representation of the near-surface pressure and/or temperature difference between the Central Africa landmass and the coastal Atlantic Ocean. Regarding the rotational circulation, and especially for HadGEM3-GC31-LL and BCC-CSM2-MR, a simulated higher 850 hPa pressure is associated with less pronounced negative vorticity and a better representation of the rotational circulation. Most CMIP5 models also overestimate the CB cell intensity and width in association with the simulated strength of LLWs. However, in CMIP6 models, the strength of key cell characteristics (intensity and width) are reduced compared to CMIP5 models. This depicts an improvement in the representation of the cell in CMIP6 models and this is associated with the improvement in the simulated LLWs.

This paper investigates and characterizes the control mechanisms of the low-level circulation over west equatorial Africa (WEA) using four reanalysis datasets. Emphasis is placed on the contribution of the divergent and rotational circulation to the total flow. Additional focus is made on analyzing the zonal wind component, in order to gain insight into the processes that control the variability of the low-level westerlies (LLW) in the region. The results suggest that the control mechanisms differ north and south of 6°N. In the north, the LLW are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the West African monsoon system. This northern branch of the LLW is well developed from June to August and disappears in December–February. South of 6°N, the seasonal variability of the LLW is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator, where ascent prevails. The heating contrasts lead to a Walker-type circulation with development of LLW as its lower branch. Thus, evidence is presented that the LLW are driven by differential heating. This contrasts with the traditional conceptual view that the Saint Helena high is the primary driver of low-level circulation off the Atlantic Ocean to WEA. Forest cover in WEA may modulate the latent heating that helps to drive the differential heating and maintain the LLW, and this interaction should be the focus of further study.

In this paper, the regional climate model REMO is used to investigate the added value of downscaling low resolutions global climate models (GCMs) and the climate change projections over Central Africa. REMO was forced by two GCMs (EC-Earth and MPI-ESM), for the period from 1950 to 2100 under the Representative Concentration Pathway 8.5 scenario. The performance of the REMO simulations for current climate is compared first with REMO simulation driven by ERA-Interim reanalysis, then by the corresponding GCMs in order to determine whether REMO outputs are able to effectively lead to added value at local scale. We found that REMO is generally able to better represent some aspects of the rainfall inter-annual variability, the daily rainfall intensity distribution as well as the intra-seasonal variability of the Central African monsoon, though few biases are still evident. It is also found that the boundary conditions strongly influences the spatial distribution of seasonal 2-m temperature and rainfall. From the analysis of the climate change signal from the present period 1976–2005 to the future 2066–2095, we found that all models project a warming at the end of the twenty-first century although the details of the climate change differ between REMO and the driving GCMs, specifically in REMO where we observe a general decrease in rainfall. This rainfall decrease is associated with delayed onset and anticipated recession of the Central African monsoon and a shortening of the rainy season. Small-scales variability of the climate change signal for 2-m temperature are usually smaller than that of the large-scales climate change part. For rainfall however, small-scales induce change of about 70% compared to the present climate statistics.

The characteristics of the main components of the water cycle over Equatorial Central Africa (ECA) were analysed using the 32-year period, spanning from 1968 to 2000, of the National Centers for Environmental Prediction-National Censearch (NCEP-) reanalysis project database. A special emphasis was given to identifying the causes of annual and interannual variability of water vapor flux and precipitation recycling. The results suggest that the first maximum of moisture convergence, during the rainy season MAM, comes from upper level moisture flux, related to the north component of the African Easterly Jet (AEJ-N). The second, and greatest, maximum in SON is found to be a consequence of low level moisture advection from the Atlantic Ocean. AEJ-N also drive the seasonal spatial pattern of moisture flux. The interannual variability of moisture flux is contributed mainly by the low level moisture advected from the Atlantic Ocean, underlying its crucial role for the regional climate. Studying the recycling ratio in ECA as a whole shows a low annual cycle whereas subregional scale analysis reveals high amplitude of the seasonal variation. Seasonal variability of the spatial gradient of precipitation recycling is regulated by both moisture flux direction and strength. The annual cycles of recycling ratio in the North and the South of ECA are regulated by both moisture transport and evapotranspiration.

In this paper, daily characteristics of the Central Africa rainfall are assessed using the regional model REMO in the framework of contributions to the CORDEX-Africa project. The model is used to dynamically downscale two global climate models (MPI-ESM-LR and EC-EARTH) for the present (1981–2005) and future (2041–2065, 2071–2095) climate under the Representative Concentration Pathway (RCP) 2.6, 4.5, and 8.5 emission scenarios. A substantial spatio-temporal variability of the daily precipitation characteristics is obtained, as well as varying inferences for individual indices. For the present days, both REMO’s runs capture reasonably well the mean seasonal rainfall, the frequency of wet days, the threshold of extreme rainfall, and the cumulative frequency of daily rainfall. The model better simulates the frequency of rainy days than their intensity. It is found that origins of model biases differ as a function of regions. Over the continent, boundary conditions tend to influence the spatial distribution of rainfall whereas over oceanic and coastal regions, REMO’s physics seems to dominate over the boundary forcing. The projected frequency of wet days shows a decrease along the twenty-first century over most part of the continent. Throughout the century, all scenarios of REMO decrease the rate of rainfall with increasing intensity, and which will be noticeable in the Sahelian region at late twenty-first century. Furthermore, the extreme event thresholds decrease over Sahelian regions and increase along the coastal regions.

This study examines the skill of the North American Multi-Model Ensemble (NMME) seasonal precipitation forecast and the influence of tropical sea surface temperature (SST) anomalies and their teleconnections on precipitation prediction skill over Central Africa (CA). The skill is assessed for December–February (DJF), March–May (MAM), June–August (JJA), and September–November (SON) seasons, at 0-, 3-, and 6- month lead time. Results show that for all seasons and at all lead times, models used in this study have tendency to overestimate the observed SSTs over the tropical areas. The multi-model ensemble mean (MME) generally succeeds in capturing the spatial differences in the seasonal mean climatology of precipitation and clearly determines the bi-modal and uni-modal natures of observed precipitation over CA. The El Ninõ-Southern Oscillation 3.4 index (Ninõ3.4), Indian Ocean Dipole (IOD) western pole index (IODWP), and IOD eastern pole index (IODEP) teleconnections with tropical SST are well represented by the MME at all seasons and lead times with a pattern correlation coefficient (PCC) >0.6. The quality of these teleconnections decreases when the lead time increases. The Ninõ3.4-induced precipitation’s teleconnection is better represented in MAM at all lead times, and it is found that precipitation is reinforced over northern CA during the El Ninõ years and weakened during the La Niña years. IODWP and IODEP teleconnections with CA precipitation are well represented in MAM and SON, with PCC > 0.8. The IODWP and IODEP could be a very good indicators to predict the increase or decrease of precipitation in CA during MAM and SON seasons.

Understanding the processes responsible for precipitation and its future change is important to develop plausible and sustainable climate change adaptation strategies, especially in regions with few available observed data like Congo Basin (CB). This paper investigates the atmospheric circulation processes associated with climate model biases in CB rainfall, and explores drivers of projected rainfall changes. Here we use an ensemble of simulations from the Swedish Regional Climate Model (RCM) RCA4, driven by eight General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5), for the [Formula omitted] and [Formula omitted] global warming levels (GWLs), and under the representative concentration pathways (RCPs) 4.5 and 8.5. RCA4 captures reasonably well the observed patterns of CB rainfall seasonality, but shows dry biases independent of seasons and large scale driving atmospheric conditions. While simulations mimic observed peaks in transition seasons (March-May and September-November), the rain-belt is misplaced southward (northward) in December-February (June-August), reducing the latitudinal extent of rainfall. Moreover, ERA-Interim reanalysis driven RCM simulation and RCM-GCM combinations show similar results, indicating the dominance of systematic biases. Modelled dry biases are associated with dry upper-tropospheric layers, resulting from a western outflow stronger than the eastern inflow and related to the northern component of African Easterly Jet. From the analysis of the climate change signal, we found that regional scale responses to anthropogenic forcings vary across GWLs and seasons. Changes of rainfall and moisture divergence are correlated, with values higher in March-May than in September-November, and larger for global warming of [Formula omitted] than at [Formula omitted]. There is an increase of zonal moisture divergence fluxes in upper atmospheric layers ( [Formula omitted]) under RCP8.5 compared to RCP4.5. Moreover, it is found that additional warming of [Formula omitted] will change the hydrological cycle and water availability in the CB, with potential to cause challenges to water resource management, agriculture, hydro-power generation, sanitation and ecosystems.