The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation. Part II: The Boreal Autumn

Abstract

This paper examines the mechanisms controlling the year-to-year variability of rainfall over western equatorial Africa during the rainy season of October–December. Five regions with distinct behavior are analyzed separately. Only two show strong associations with the ocean and atmospheric features in the global tropics. These two regions, in the east (the eastern Zaire basin) and west (Angolan coast) of the study area, respectively, demonstrate strikingly opposite relationships with the anomalies of sea surface temperatures (SSTs), sea level pressure (SLP), and east–west atmospheric circulation. The wet (dry) conditions in the eastern Zaire basin are associated with El Niño(La Niña)–like phases. The inverse pattern is apparent for the Angolan coast. The other three regions, lying between these two poles of variability, represent a transition zone with a weak linear relationship to the circulation features. The vital impact of the east–west circulation cells on rainfall variability results in a stronger association with zonal wind than with SSTs or SLP. In addition to the zonal shift, changes in intensity of the zonal cells also play a crucial role. Variability in both magnitude and location of the circulation cells appear to be modulated by the remote forcing from the Pacific via an atmospheric bridge. However, the eastern sector is impacted mainly when synchronous changes occur in the Indian Ocean, and the western sector is impacted mainly when synchronous changes occur in the Atlantic Ocean.

Regions Subject to Rainfall Oscillation in the 5–10 Year Band

“…The phases where the anomalies reach their maximum are close, except in the south of Australia and in Eastern Europe where it is one or two years late compared to other anomalies. Referring to the work of various authors [41][42][43][44][45][46][47], RRH anomalies could originate from baroclinic instabilities above the oceans all the more so as SST anomalies that occur within this band are ubiquitous as shown below.…”

Variability of West African monsoon patterns generated by a WRF multi-physics ensemble

“…While not yet fully understood, the intensity and inter-annual variability of the TEJ is usually related to non-local phenomena: the strength of the Indian summer monsoon (Flaounas et al 2011b), the El Niño/Southern Oscillation (Chen and van Loon 1987), the intensity of the extratropical Southern Hemisphere westerlies (Dezfuli and Nicholson 2013), and latitudinal temperature gradients (Nicholson 2008). The impact of the TEJ on the WAM is mainly thought to be a causal one.…”

The Predictability of Rainfall over the Greater Horn of Africa. Part I: Prediction of Seasonal Rainfall

“…The statistical models exploit teleconnections between rainfall and other variables. Several studies (e.g., Segele et al 2009a,b;Hastenrath et al 2004;Dezfuli and Nicholson 2013;Nicholson and Dezfuli 2013) have shown that these parameters can govern interannual variability in equatorial Africa, independent of their relationship to SSTs. A notable exception is the forecast model of Camberlin and Philippon (2002), which includes among the predictors u and y winds and geopotential height.…”