The relationship of rainfall variability in West Central Africa to sea‐surface temperature fluctuations

Abstract

Abstract:This study examines the interannual variability of rainfall in western equatorial Africa and its links to sea-surface temperatures (SSTs). Five geographical regions within the latitudes 10°N-5°S are delineated for the analysis. The links to SSTs in the tropical Atlantic, Pacific and Indian Oceans are examined via seasonal composites of wet and dry years and via linear correlations.The results show that interannual variability is extremely complex in this region and that several factors govern it. The most important include SST anomalies along the Benguela Coast, a general warming or cooling of the tropical oceans, Atlantic SSTs specifically, the contrast between the Atlantic and Indian Oceans, and the Pacific El Niño-Southern Oscillation (ENSO). These factors differ seasonally. In much of the region, rainfall variability is linked to the Pacific El Niño and the western Indian Ocean early in the year, but to the Atlantic during the boreal summer months. The Indian Ocean again becomes important in late summer/early fall.The role of the Atlantic appears to be the modulation of the north-south excursion of the Intertropical convergence Zone (ITCZ). Hence the polarity of the SST/rainfall association depends on location. The association between SSTs near the coast and rainfall is positive if the influence is direct but can be positive or negative for indirect influences. An opposition between the Indian and Atlantic Oceans appears to displace convection in an east-west direction.Our results suggest several generic conclusions concerning the link between SSTs and continental rainfall. One is that the influence of the three oceans is seasonally dependent. The impact of a specific SST anomaly is also seasonally dependent. The same SST pattern may enhance rainfall in one season, but reduce it in the following season. Finally, the SST/rainfall associations are generally not symmetric. That is, the factors producing wet conditions are not the reverse of those producing dry conditions. In order to understand these associations, the underlying mechanisms via the general atmospheric circulation must be determined.

A physical basis for the interannual variability of rainfall in the Sahel

“…The South Atlantic has been identified as a possible source of remote forcing of tropical Atlantic variability (Rajagopalan et al, 1998;Venegas et al, 1997;Tourre et al, 1999;Mo, 2000;Mo and Häkkinen, 2001;Hickey and Weaver, 2004) and African rainfall variability (Camberlin et al, 2001). It appears prominently in our reconstruction of links to the Pacific ENSO (Nicholson, 1997;Nicholson and Kim, 1997) and in our contrast between wet and dry years in western Central Africa (Balas et al, 2007) and Southern Africa (Nicholson, 1989). Warming in the South Atlantic precedes warming events in the tropical Atlantic (e.g.…”

ENSO and IOD teleconnections for African ecosystems: evidence of destructive interference between climate oscillations

“…Some even argue that the Indian Ocean influences the timing and intensity of ENSO (Annamalai et al, 2005;Wu and Kirtman, 2004). Though not the focus of study in this work, it is important to acknowledge other noteworthy phenomena driving regionally-important climate variability across Africa including the Benguela Niño (Boyer et al, 2000;Shannon et al, 1986), tropical Atlantic interdecadal oscillations (Giannini et al, 2003;Paz et al, 2003;Tourre et al, 1999Tourre et al, , 2006, and an equatorial, quasi-biennial Atlantic SST oscillation (Balas et al, 2007;Giannini et al, 2003;Hirst and Hastenrath, 1983;Latif and Groetzner, 2000;Tourre et al, 1999;Zebiak, 1993).…”

“…Examples come from continental analysis of gauge records (Nicholson and Kim, 1997;Nicholson, 2000Nicholson, , 2001 as well as more regional examinations in eastern (Birkett et al, 1999;Black et al, 2003;Black, 2005;Mapande and Reason, 2005;Marchant et al, 2007), western (Balas et al, 2007;Hirst and Hastenrath, 1983;Tourre et al, 2006) and southern Africa (Hachigonta and Reason, 2006;Reason, 2001Reason, , 2002Todd and Washington, 1998;Usman and Reason, 2004). Additionally, rainwater isotopic composition in east Africa (Vuille et al, 2005), continent-wide river discharge (Dube and Jury, 2003;Jury, 2003), dust production (Bryant et al, 2007;Prospero and Nees, 1986), and even the chemical composition of coral (Kayanne et al, 2006;Zinke et al, 2004) all indicate resonance of African rainfall with these ocean-atmosphere modes of variability.…”

Convective Squalls over the Eastern Equatorial Atlantic

“…Total numbers from all available stations per rainy season indicate somewhat less activity in 2007/08 (complete data coverage) than in 2006/07 (weakly active months of November and December missing) and in 2008/09 (most active months of April and May missing). Such variations could be related to changes in local sea surface temperatures, as investigated by Balas et al (2007). Figure 2a shows the average monthly distribution of squall days for WAG, MOHO, and DALIA.…”

“…To the best of our knowledge, there are currently no operational forecasts for severe convection for this region, despite the fact that convective squalls can pose a substantial threat inland, and to shipping and oil platforms over the adjacent eastern equatorial Atlantic Ocean. A good observational network is needed to monitor local triggers of convection such as orographic features and land or sea breezes, which often create distinct diurnal cycles (Jackson et al 2009;Laing et al 2011), and to understand the influence of seasonal changes in sea surface temperatures and regional circulation systems (McCollum et al 2000;Leroux 2001;Yang and Slingo 2001;Mori et al 2004;Balas et al 2007), and of equatorial wave phenomena such as the Madden-Julian oscillation (Nguyen and Duvel 2008;Laing et al 2011).…”