GeloriniPhD (PDF , 6973kb) - University of York

. 21 .
INTRODUCTION
Atlantic Ocean during the dry season in July-August, when the CAB moves eastwards over the Plateau (Fig. ii) (McGregor
and Nieuwolt, 1998). Whereas the long rains are more abundant, the short rains are particularly variable (Conway,
2002; Hastenrath, 2007). Moreover, circulation anomalies evoked by a powerful zonal–vertical circulation cell along the
Indian Ocean equator, may severely affect the magnitude of equatorial surface westerlies during the short rains, leading
to disastrous regional floods or droughts (Han et al., 2010; Hastenrath et al., 2010). The strong relationship between
monthly and seasonal rainfall patterns in East Africa and phases of ENSO also amplifies interannual rainfall variability, in
which ENSO warm events tend to be associated with above-average rainfall (i.e. El Niño years, e.g., 1997-1998) and ENSO
cold events with below-average rainfall (i.e. La Niña years, e.g., 1999-2000) during the short rainy season (Nicholson,
1996; Nicholson and Kim, 1997; Indeje et al., 2000; Mutai and Ward, 2000). Mean annual rainfall varies from ~200-400
mmyr -1 in the most arid regions (e.g., northeastern Kenya), and exceeds 1200 mmyr -1 in the most humid regions (e.g.,
Uganda). However, seasonal inter-annual rainfall patterns may differ distinctly within short distances (on the order of
tens of kilometers) depending on local topography (Nicholson, 1996); for instance, mountainous areas such as Mont
Elgon in Uganda, and large lake areas such as Lake Victoria experience highest rainfall, i.e. ~2500 mmyr -1 . Because of its
rich topography (e.g., mountains, valleys and lakes) and land cover, mean annual temperature in East Africa varies from
10-15 °C (T min ) to 21-26 °C (T max ). Although there seems to be a general warming trend since the 1940s, temperature close
to the coast and major inland lakes are gradually decreasing (King’uyu et al., 2000). Recently, evidence of accelerating
T min rise has also pinpointed significant human land development, possibly creating locally produced aerosols, as an
important temperature driver in East Africa (Christy et al., 2009).
a b
Fig. ii. General patterns of precipitation, trade winds, pressure and convergence over Africa during (a)
boreal winter (northeast monsoon) and (b) boreal winter (southeast monsoon), with special reference
to the East African region (CAB positions based on Nicholson, 1996; map modified from United
Nations Environment Programme (UNEP), 2007. (http://gridnairobi.unep.org/chm/EAFDocuments/
Maps_and_Data/maps_thumbnails/).