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This study utilizes satellite-based rainfall CHIRPS to evaluate GCMs-CMIP6 models over Sudan from 1985 to 2014. Overall, the GCMs of BCC-CSM2-MR, CAMS-CSM1-0, CESM2, ECEarth3-Veg, GFDL-ESM4, MIROC-ES2L, and NorESM2-MM are well reproduced in the unimodal pattern of June to September (JJAS), and hence employed to calculate Multi-Model Ensemble (MME). Then, we examine the capability of the GCMs and MME in replicating the precipitation patterns on annual and seasonal scales over Sudan using numerous ranking metrics, including Pearson Correlation Coefficient (CC), Standard Deviation (SD), Taylor Skill Score (TSS), Mean Absolute Error (MAE), absolute bias (BIAS), and, normalized mean root square error (RMSD). The results show that the MME has the lowest bias and slightly overestimates rainfall over most parts of our study domain, whilst, others (ACCESS-CM2, BCC-CSM2-MR, CAMS-CSM1-0, CESM2, CNRM-CM6-1, CNRM-CM6-1-HR, CNRM-ESM2-1, FGOALS-f3-L, FGOALS-g3) consistently overestimate rainfall in referring to CHIRPS data, respectively, but FIO-ESM-2-0 underestimates bias value. Moreover, MIROC-ES2L and NorESM2-MM demonstrate better performance than the other models. Finally, we employed a bias correction (BC) technique, namely Delta BC, to adjust the GCMs model products through the annual and monsoon seasons. The applied bias correction technique revealed remarkable improvement in the GCMs against the observations, with an improvement of 0 - 18% over the original. However, MME and MIROC-ES2L show better performance after correction than other models. [ABSTRACT FROM AUTHOR]

Like most of the countries in the Great Horn of Africa, Tanzania is prone to floods and droughts. Anomalously wet years over the region are associated with low-level convergence, the advection of moisture from both the Indian Ocean and the Congo and an ascending limb of the Walker circulation cell centered over the western Indian Ocean (WIO). During positive Indian Ocean dipole (IOD) events, the Walker circulation cell over the Indian Ocean is well defined, and more so compared with observed during El Niño years. In El Niño years, the Walker circulation cell becomes weak compared with positive IOD events, where the zonal winds strengthen, resulting in active convective activities over the WIO. A key finding is that the Walker circulation cell developing over the Indian Ocean is strongly connected to IOD activity. The weakening of the Walker circulation cell is likely the reason for the decrease in rainfall during El Niño events compared with that of positive IOD events in recent years over Tanzania during the October–December season. The variability of the Walker circulation cell over the Indian Ocean can lead to extreme weather conditions, making the full understanding of the climate system essential. This is crucial for decision making about adaptation and mitigation measures for coping with climate change and for the minimization of huge socio-economic losses associated with extreme weather events in the region.

The study investigates the influence of the tropical Atlantic Ocean and its Walker circulation cell on October–December (OND) rainfall variability over Tanzania. The first two SVD modes contribute 95.3% of the total covariance with the strongest (positive) and significant correlation (r ≅.7) at the 99% confidence level showing a close relationship between OND rainfall and MAM tropical Atlantic SST anomalies. Significant positive running correlation observed between MAM Atlantic SSTs and OND rainfall indicates the variability of rainfall observed during the OND season is influenced by variability of Atlantic Ocean SSTs anomaly. Both SVD modes captured the positive phase of the South Atlantic Ocean Dipole (SAOD) which is characterized by warm sea surface temperature (SST) anomalies in the southwest pole (SWP) and cool SST anomalies in the northeast pole (NEP). The Walker circulation cell observed is due to inhomogeneous warming over the tropical Atlantic Ocean with ascending limb where it is relatively warm in the SWP and descending limb where it is relatively cool in the NEP. Nonsignificant low correlation is observed between the Atlantic Walker circulation index and the Oceanic Indices (dipole mode index [DMI] and Niño3.4). This indicates that the Atlantic Walker circulation cell is an independent system resulting from local ocean–atmosphere interaction and does not depend on the influence of the tropical Indian or Pacific Oceans. Above average rainfall in Tanzania during the OND season is associated with the ascending limb of the Atlantic Walker circulation cell which is enhanced by convection in the western Atlantic Ocean and the latent heat released through the condensation in the mid‐level which is advected by westerly winds aloft towards the study region, enhancing warming, and ultimately enhancing rainfall over Tanzania. [ABSTRACT FROM AUTHOR]

This work aims at detecting to what extent the El Niño Southern Oscillation (ENSO) modulation influences the upper warm temperature anomaly (UWTA) formation in Tanzania during March to May (MAM) season of 1980–2010. The UWTA has known to influence heavy rainfall events through enhanced vertical uplift and deepening surface low pressure. The study is thus critical towards improved early warning system through a better understanding of the mechanisms in the climate system responsible for the above average rainfall. The empirical orthogonal function analysis in this study, revealed the dominant interannual variability mode of the UWTA to be concentrated over the study region. The interannual variability of ENSO and the UWTA shows a moderate relationship during MAM (r =.5328) while a robust association (r =.7905) exists during the previous September to November (SON). Thus, the anomalous sea surface temperature anomaly (SSTA) of the central tropical Pacific (e.g., over Nino 3.4) leads 6 months before the UWTA of the study region and anomalous SSTA over the tropical Indian Ocean (TIO) peak up. Comparatively, the warm phase of ENSO during SON and December to February (DJF) is in phase relationship with the strengthened ascending branch of the Walker circulation over the warm TIO. Further analysis demonstrates that in SON the rising limb of the Walker circulation is the dominant source of uplift and convection enhancement over the warm TIO and later transports warm and moist air (i.e., diabatic heating) aloft for the UWTA formation during SON and DJF. Meanwhile, the Hadley circulation (HC) induced by the SSTA associated with ENSO exhibits the symmetric mode during SON and asymmetric mode in MAM. The asymmetric mode of the HC plays its role in transporting the warm and moist air from the warm TIO towards the study area for the UWTA formation during MAM. [ABSTRACT FROM AUTHOR]

This study analyses the spatial and temporal distribution of heavy rainfall events (HREs) and its associated circulation anomalies over Tanzania during March to May (MAM) rainfall season of 1980–2010. A total of 822 HREs were revealed, concentrated over the northern sector (NS) of the country. Years with anomalous HREs are associated with low-level westerly convergence, advection of moisture from both the Indian Ocean and Congo basin, an upper warm temperature anomaly (UWTA), intensified and well-positioned Intertropical Convergence Zone (ITCZ), and pronounced rising motion since the ascending limb of the Walker type of circulation is centered over Tanzania. The analysis of the UWTA in this study has brought a key factor in exploring the possible likely cause and improved early warning system for the HREs during the MAM rainfall season in Tanzania. Making use of the thermal wind equation and the velocity divergent form of the continuity equation (DFCE), we found that the UWTA results into an upper-level horizontal wind divergence which significantly accelerates vertical ascent, deepening the surface low pressure for an enhanced convective process and HREs formation. [ABSTRACT FROM AUTHOR]