Mechanisms associated with September to November (SON) rainfall over Uganda during the recent decades

Hamida Ngoma,Wang Wen,Rizwan Karim,Makula Kisesa,Brian Ayugi

doi:10.5937/gp25-29932

Abstract

This study revisits teleconnections associated with the anomalous events of September to November (SON) rainfall over Uganda during 1981-2019, owing to the recent intensification of extreme events. Empirical Orthogonal Function (EOF), Composite and Correlation analysis are employed to examine the variability of SON rainfall over the study domain and associated circulations anomalies. The first EOF mode (dominant mode) displays a positive monopole pattern and explains 67.2% of the variance. The results revealed that SON rainfall is largely influenced by a Walker circulation mode over the Indian Ocean, whereby, wet events are associated with an ascending limb of the Walker circulation on the western part of the Indian Ocean characterized by convergence at low levels and divergence at upper level. The study showed that SON rainfall is positively (negatively) correlated with Indian Ocean (Atlantic Ocean) sea surface temperatures (SST). Furthermore, Indian Ocean Dipole (IOD) events have impact on SON rainfall with strong positive correlation, whereas Southern Oscillation Index (SOI) revealed negative correlation. The results also reveal that there is a lag in ENSO and IOD episodes during wet/dry events over the region. ENSO and IOD also tend to extend the rainfall season of SON and thus study of extreme events may not be well captured by studies focusing on SON. Future studies might consider the season of October to December or December to February. These phenomena need to be closely monitored and considered when making seasonal forecasts.

Highlights

Uganda is an agricultural country and the economy largely depends on rainfed agriculture
This could be attributed to the characteristic of the episodes as reported by a study (Hoell et al, 2014) which found that the short rains are generally reduced during La Niña but that the degree and spatial consistency of the reduction depends on the nature of the episode
September to November (SON) rainfall displayed positive correlation with Dipole Mode Index (DMI) (SOI). This agrees with past studies which reported that Indian Ocean Dipole (IOD) has more influence on the interannual variability of SON rainfall over Uganda.The results further show that there is a lag in El-Nino Southern Oscillation (ENSO) events and anomalous wet and dry events of rainfall over the region

Summary

Introduction

Uganda is an agricultural country and the economy largely depends on rainfed agriculture. Rainfall over the region exhibits high spatial and temporal variability This is attributed to the complex topography, varying vegetation patterns and large inland water bodies which regulate the local climate (Basalirwa, 1995; Ogwang et al, 2014). Rainfall over the region is mostly influenced by the equatorial rain band which oscillates northwards and southwards throughout the year (Nicholson, 1996; 2018). The long rains occur between March to May (MAM) and short rains between September to November (SON) (Basalirwa, 1995) This tends to a unimodal pattern for areas away from the equator. Other factors influencing rainfall over the region include westerlies from Congo basin, monsoons, anticyclones; the Mascarene High, Arabian High, Azores High and the St Helena High, sea surface temperature (SST) anomalies, El-Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (Saji et al, 1999; Indeje et al, 2000; Ogwang et al, 2012; 2015)

Methods

Results

Discussion

Conclusion