the inflow of thermally stable subsiding air masses from the Indian Ocean. This therefore increases low level moisture convergence, decreases lower troposphere stability and so increase cloudiness and precipitable water anomalies during the short OND rainy season (McHugh 2006). A second mode that has a significant impact on East African interannual precipitation variability, particularly in the short OND rains, is the Indian Ocean dipole (IOD). Ummenhoffer et al. (2009) conclude that anomalously high rains during OND are primarily associated with local warm SSTs in the western Indian Ocean, with the cooler eastern part of the dipole much less crucial. This occurs because the warm sea surface temperature anomalies induce a co-located area of low sea level pressure (SLP) over the western Indian Ocean and East African coast. These SLP anomalies induce enhanced westerlies over central Africa and onshore anomalies that converge over equatorial East Africa. These influences combine to increase latent heat flux, atmospheric moisture content and local convection (Ummenhofer et al. 2009). These events are known as a positive IOD mode. Conversely, during the negative IOD mode East Africa receives less precipitation than normal during the OND rainy season (Behera et al. 2005). In contrast to the short OND rains, the long MAM rains show much weaker teleconnections to external modes of variability, partially due to the incoherent spatial and temporal precipitation anomalies seen during this period, although there may be connections on a more local scale. For example, eastern Ethiopian precipitation is negatively correlated with southwest Indian Ocean tropical cyclone activity and northern Ethiopian and Eritrean precipitation in spring may be connected to movement of mid latitude ridge-trough systems (Riddle and Cook 2008). 2.3.2 Decadal variability and observed trends Global mean temperatures have increased by 0.74ºC ±0.18ºC over the past century (1906– 2005) with an accelerated rate of warming of 0.07ºC/decade over the past 50 years on average (IPCC AR4 2007). With this in mind and the current focus on anthropogenic climate change it is important to evaluate trends in the climate over East Africa that may be attributable to increases in greenhouse gases over recent decades. Christy et al. (2009) analyse 20th century station temperature over Kenya and Tanzania and conclude that from 1946–2004 trends in maximum temperature are near zero, whereas minimum temperature sees a significant positive trend over the same period. This shows a decrease in the diurnal temperature range,
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