Influence of convectively coupled equatorial Kelvin waves on March-May precipitation over East Africa

Ochieng Okello,Isaiah Nyandega,Victor Ongoma,Guirong Tan

doi:10.5937/gp25-31132

Abstract

Convectively coupled equatorial Kelvin waves (CCEKWs) are those types of equatorially trapped disturbances that propagate eastward and are among the most common intra-seasonal oscillations in the tropics. There exists two-way feedback between the inter-tropical convergence zone (ITCZ) and these equatorially trapped disturbances. Outgoing Longwave Radiation (OLR) was utilized as a proxy for deep convection. For CCEKWs, the modes are located over the West Atlantic, equatorial West Africa, and the Indian Ocean. The influence of other circulations and climate dynamics is studied for finding other drivers of climate within East Africa. The results show a positive relationship between Indian and Atlantic Oceans Sea Surface Temperatures and March-May rainfall over equatorial East Africa over the period of 1980 to 2010. This influence is driven by the Walker circulation and anomalous moisture influx enhanced by winds. Composite analysis reveals strong lower-tropospheric westerlies during the active phase of the CCKWs activities over Equatorial East Africa. The winds are in the opposite direction with the upper-tropospheric winds, which are easterlies. Singular Value Decomposition shows a strong coupling interaction between rainfall over equatorial East Africa and CCKWs. This study concludes that Kelvin waves are not the main factors that influence rainfall during the rainy season. Previous studies show that the main influencing factors are ITCZ, El-Nino Southern Oscillation (ENSO), and tropical anticyclones that borders the African continent. However, CCKWs are a significant factor during the dry seasons.

Highlights

The importance of rainfall over East Africa cannot be underscored
The main aim of this study was to determine the role of convectively coupled equatorial Kelvin waves and other circulation features on the precipitation over the Equatorial East African region
At frequencies less than about 0.3 cpd, there is a more significant occurrence of Convectively coupled Kelvin Waves (CCKWs) in Outgoing Longwave Radiation (OLR) Symmetric than OLR Anti-symmetric for almost all wavenumbers associated with the Kelvin waves

Summary

Introduction

The importance of rainfall over East Africa cannot be underscored. The rain exhibits high spatial andtemporal variability. Most of the recent studies in the region (Cattani et al, 2018; Ongoma & Chen, 2017) have focused on the past and future variability of rainfall, giving very little attention to the factors that modulate the rainfall. Pohl and Camberlin (2006a; 2006b) had reported a more significant effect of ISOs influence on the long rains. Pohl and Camberlin (2006b) described Phases 2 and 3 from the Wheeler-Hendon index, when the convective core is over, using the Intra-Seasonal Oscillations phases established. Phillip Okello Ochieng, GuirongTan, Victor Ongoma, Isaiah Nyandega by Wheeler and Hendon (2004). Enhanced precipitation over the East African mountains has been associated to Africa and the Indian Ocean

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