Sea Surface Temperature Influences On Interannual Variability Of March To May Rainfall Over Tanzania And Its Inter-decadal Changes

Tanzania is a region of vulnerability,with high interannual and interdecadal long rains(March to May rainfall)variability combined with rain-dependent agriculture.The dominant modes of Tanzania’s long rains(TLR)together with the underlying physical mechanism and non-stationarity behavior instigating this variability are investigated during the period 1950-2018,as acquired knowledge is essential for adapting and mitigating the socio-economic distress associated with variability.This thesis uses a combination of gauge,gauge-satellite and reanalysis data to explore the nature,mechanism,and sea surface temperature(SST)forcing of TLR at interannual and interdecadal time scales.(1)The spatial nature of interannual TLR,demonstrated through empirical orthogonal function(EOF)analysis,shows three dominant modes of TLR.The modes account for 30.0%,21.3%,and12.3% of long rains variance for the first(EOF1),second(EOF2)and third(EOF3)dominant mode of TLR,respectively.EOF1 displays a monopole pattern over Tanzania.EOF2 show a meridional pattern in which negative(positive)anomalies are observed over southern(northern)Tanzania.EOF3 depict a zonal pattern characterized by positive(negative)anomalies over eastern(western)Tanzania.(2)The mechanisms of interannual EOF1,EOF2,and EOF3 variability show links to southeast Indian Ocean(SEIO),equatorial central Pacific Ocean(CP)and mega-El Ni?o Southern Oscillation(ENSO)-like SST anomalies(SSTAs),respectively.Warm SEIO SST is accompanied by an anomalous lower-level cyclone and upper-level anticyclone over the SEIO.Notable upperlevel easterly winds anomalies flow from the SEIO toward Tanzania.The anomalous vertical movements over Tanzania concur with a remarkable increase in upper-level convergence and lower-level divergence.The former leads to subsidence,which contributes to dry anomalies over Tanzania.Alongside stronger than normal surface southerly winds anomalies indicate an early arrival of the Southeast Asian Monsoon,which is also linked to dry conditions over Tanzania.The equatorial CP-like SSTA is related to convection over the CP associated with upper-tropospheric outflow,leading to upper-level convergence over southern Tanzania.The adiabatic cooling accompanied by suppressed convection excites strong southerly winds anomalies over southern Tanzania.Concurrently,the ascent branch of reversed western Indian Walker circulation(WC)is accompanied by anomalous moist northeasterly winds from the western Indian Ocean to northern Tanzania.Anomalous cool southerly(moist northeasterly)winds are associated with dry(wet)conditions over southern(northern)Tanzania.The convection anomaly over the western Pacific K-shaped region related to mega ENSO-like SSTA forms strong low-level westerly tendencies over the eastern Indian Ocean concomitant with the intensification of the WC over the ocean.The WC appeared to shift eastward,concurring with the formation of the western Indian Walker cell with an ascending branch near eastern Tanzania.The ascending motion is closely related to wet conditions in eastern Tanzania.A novel aspect of this study is identified climatological features and SSTA associated with variability of the Intertropical Convergence Zone(ITCZ)across Tanzania.The SEIO SST triggered the fast northward movement of the ITCZ.The equatorial CP SST locate ITCZ over northern Tanzania.The mega-ENSO SST shift the meridional arm of ITCZ eastward.(3)Using sliding correlation,we identify that the TLR-SST South-central tropical Indian Ocean(SCTIO)relationship undergoes an interdecadal shift around the early 1990 s.Prior to the early1990 s,we found upper-level meridional atmospheric anomalies,including cyclonic over Tanzania and anticyclonic over the southeast of SCTIO.Upper-level cyclone anomaly over Tanzania is associated with subsidence,which causes dry anomaly over Tanzania.In contrast,the anomalous atmospheric circulation features linked to SCTIO SSTA disappeared after the early 1990 s.We found that the change in the TLR-SCTIO SST connection is ascribed to a change in the amplitude of SCTIO SST forcing due to enhanced coupling between the Interdecadal Pacific Oscillation(IPO)and SCTIO SST after the early 1990 s.Prior to the early 1990 s,the SCTIO SST variance was substantially higher than it was following those years.Our study suggests that the IPO may act as a crucial modulator for the TLR-SCTIO SST relationship.