Causes of the upper-level warm temperature anomaly (UWTA) associated with March–May heavy rainfall in Tanzania

Abstract

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.