Assessing the Variability of Heavy Rainfall during October to December Rainfall Season in Tanzania

Lovina Peter Japheth,Agnes Lawrence Kijazi,Guirong Tan,Ladislaus Benedict Chang’A,Kantamla Biseke Mafuru,Isack Yonah

doi:10.4236/acs.2021.112016

Abstract

Heavy rainfall is one of the primary causes of flood during rainy season in Tanzania leading to severe socio-economic impacts. The study aimed at assessing and characterizing the variability of Heavy Rainfall Events (HREs) using Empirical Orthogonal Function (EOF), Mann-Kendal (MK) trend test, Correlation and Composite analysis methods. Based on the daily-observed precipitation and reanalysis data sets for the October to December (OND) rainfall season of 35 years (1981-2015), the spatial and temporal characteristics of HREs in Tanzania are studied. The relationship between heavy rainfall (HR) and large-scale circulation anomalies including the Indian Ocean dipole (IOD) and El Ni?o southern oscillation (ENSO) indices was assessed. The study found that, approximately 590 HREs were concentrated over northern sector and coastal belt of Tanzania. The monthly variability indicates that HREs are more pronounced in December followed by November while October being the least affected. The occurrence of HREs over the Lake Victoria, Kigoma and Tabora is largely attributed to low-level convergence of westerlies and enhanced moisture from Congo basin accompanied by a pronounced rising limb of Indian Walker circulation cell. A time-series analysis of HRE exhibits an inter-annual variation characterized by a slightly increasing trend, though the computed trends were not statistically significant at 95% confidence level. In most part of Tanzania HREs were positively correlated with both ENSO and IOD indices, underscoring the critical role of ENSO and Indian Ocean dynamic in modulating rainfall variability over the region. In general, it has been found that most of the HREs are generally triggered or amplified by large-scale circulation patterns such as ENSO and IOD.

Highlights

Recent reports from the Intergovernmental Panel on Climate Change [1] [2] have indicated the observed increase in the frequency and intensity of extreme events across much of the world
Based on the threshold set for characterizing Heavy Rainfall Events (HREs), a total of 590 HREs have been identified for the period 1981-2015 for the 18 selected synoptic stations
It has been observed that, total OND rainfall is much influenced by the numbers of heavy rainfall events in respective areas during the month of November and December

Summary

Introduction

Recent reports from the Intergovernmental Panel on Climate Change [1] [2] have indicated the observed increase in the frequency and intensity of extreme events across much of the world. Several studies have attempted to map and characterize specific extreme event with the purpose of determining the mechanisms involved in its evolution and dynamic, for instance, the El Niño related extreme rainfall events that occurred in southern Africa [6], the drought in 2005 [7] and floods in 1994 [8] and 1997 [9] over East Africa (EA). The rainfall patterns over the East Africa( EA) region are complex, are characterized by stronger spatial-temporal variability and are influenced by multiple factors including El Niño Southern Oscillation (ENSO) and the ocean dynamics over the Indian Ocean [10] [11]. Studies by [13] [14] and [15] provide detailed information and mechanism on how ENSO influences the climate of Tanzania, whereby the above (below) normal rainfall occurs during El Niño (La Niña) episodes

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