Wednesday, 13 January 2016
The rainfall in Eastern Africa is a lifeline to the millions of people living around and downstream of the large lakes scattered across the area. The rainfall in the region is highly variable on all timescales and is influenced by many factors and teleconnections. The interannual variability of the short rains (October to December, OND) has been extensively studied and has been shown to be primarily influenced by the El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). However the variability of the long rains (March to April, MAM) is not well understood, and given that the short rains provide the greater rainfall, this is an area in need of improvement. Recently it has been shown that the Atlantic Multidecadal Oscillation (AMO) is influential on the decadal timescale for the long rains, in particular for the cessation in the month of May. The influence on the region is propagated through AMO-induced stationary Rossby Waves. The placement of the high and low pressures, as a result of the Rossby wave, influences the flow over the region and the corresponding rising and sinking motion influences the locations that receive the most rainfall. Variations in the positions determine which regions see more or less rainfall. It is proposed that this influence could also be important on an interannual basis. We hypothesize that the indices that produce the waves, such as the AMO, also influence the regional climate over shorter timescales. We envisage that the mechanisms responsible for the longer time-scale variability also acts on a day to day, month to month and year to year basis during which the index changes albeit on a much smaller scale. The interannual variation of AMO and rainfall for May are analysed for the period, 1950-2010. These results showed that the role of AMO-induced teleconnections may be more complex on the interannual time-scale than on the decadal time-scale. For example, the Rossby wave may not be treated as a stationary phenomena thus explaining why the simple correlation analysis, which we previously adopted in our multi-decadal study, cannot detect a significant relationship; however a visible wave pattern is still apparent over the region and appears to originate in the Atlantic Ocean. It is further hypothesized that the SST in the Atlantic Ocean still plays an important role in modulating the interannual variability of East African rainfall. Additionally, it is expected that the interactions of individual teleconnections on an interannual timescale are different to that of the decadal timescale and that these interactions are highly influential in determining the amount of rainfall East Africa experiences each year. Consequently this project identifies the indices which are responsible for the teleconnections that interact to either reinforce or counter influences in the East African region. This is done through the computations of correlations and combined EOFs. The mechanisms by which these indices influence the region are analysed using a regional model in order to gain an understanding of the physical processes at play over the region. A greater understanding of the interactions of teleconnections and their influence over the region should lead to a greater ability to understand what is likely to happen in the future.
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