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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]

This study diagnoses the cause of the upper-level warm temperature anomaly (UWTA) associated with March–May (MAM) heavy rainfall of 1980–2010 in Tanzania in terms of the empirical orthogonal function (EOF), singular value decomposition (SVD) and dynamical diagnosis. EOF reveals the dominance of enhanced UWTA over the entire study region with strong warming to the northern coast, central, south-western highlands (SWH) and western part of the country. SVD depicts a monopole positive co-variability between the UWTA of the entire study area and SSTA over the entire domain of the Indian Ocean with strong positive coefficients to the central equatorial and western Indian Ocean (WIO). The lead-lag correlation coefficients between the three months running mean for the normalized WIO SSTA and UWTA indices show that, the UWTA formation starts early before MAM with the highest peak in February–April (FMA). The ascending branch of the Walker circulation is the dominant primer for the vertical uplift of warm moist air from the WIO during MAM while convective coupled Kelvin waves (CCKWs) dominate in FMA season. In both seasons, strong positive zonal temperature advection anomalies over the WIO spread towards the study area thereby dominating diabatic cooling and further increase the upper-level tropospheric temperature. Thus, diabatic heating due to enhanced uplift of warm and moist air from enhanced warm SSTA of the WIO warms the upper-level troposphere (i.e., also the UWTA) of the study area through the zonal temperature advection. • The Upper-level Warm Temperature Anomaly (UWTA) formation peaks in February to April (FMA) • Enhanced warm Sea Surface Temperature is the key for the UWTA formation • Convective Coupled Kelvin Waves enhances convection for the UWTA formation in FMA • Walker Circulation enhances convection for the UWTA formation in March to May (MAM) [ABSTRACT FROM AUTHOR]