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University of Nairobi (2003)

Climate anomalies over Eastern Africa associated with various ENSO evolution phases

Mutemi, Joseph N

Titre : Climate anomalies over Eastern Africa associated with various ENSO evolution phases

Auteur : Mutemi, Joseph N

Université de soutenance : University of Nairobi

Grade : Doctor of Philosophy in Meteorology 2003

Résumé partiel
The occurrence of extremes in East African seasonal rainfall cause severe social economic hardships to the communities and governments. Timely long-range rainfall prediction and early warning products can be used to mitigate the negative impacts, and also to take maximum advantage of the positive impacts. Previous studies have shown that the El Nino/Southern Oscillation (ENSO) introduces worldwide signals in the general circulation that persist for several months and have great potentials as predictors of seasonal rainfall. Most studies in the region have largely addressed simple linkages between ENSO and seasonal rainfall. This study investigated more complex rainfall-ENSO linkages with a specific focus on the impacts of the evolution of the specific phases of ENSO for the specific rainfall seasons, with a prime goal of improving the long range prediction potential over region. The specific objectives that were addressed to achieve the overall objective included diagnosis analyses to enhance the knowledge base on the teleconnections between East Africa rainfall,and the specific ENSO evolution phases including the Onset, Maturity and Withdrawal of both cold and warm events, and an investigation of the temporal stability of the derived ENSO teleconnections. The core of the study involved general circulation model (GCM) simulations of El Nino and La Nina signals for the specific rainfall seasons in an attempt to study the physical reality of the ENSO-Seasonal rainfall linkages that were derived from diagnosis studies. The ECHAM GeM was adopted in the study and the physical reality of the ECHAM GCM was first investigated by simulating some known seasonal and interannual climatologies. Attempts were also made in the study to investigate whether the ECHAM model could capture the major spatial patterns over the region that were derived from Empirical Orthogonal Functions (EOFs) analysis. The data used in the study were monthly station rainfall observations, gridded rainfall data set, global SSTs, and southern oscillation index (SOl) for the 50 years from 1950 to 1999.In order to achieve the overall and specific objectives of the study, the data were subjected to various analyses including quality control and standardization to enable realistic time and space comparisons to be made. The standardized rainfall indices were used to identify all wet and dry events at the specific locations and for both long and short rainfall seasons. Pearson correlations between rainfall and ENSO indices were then computed for the evolving ENSO modes at the onset, peak, and withdraw of both warm and cold ENSO events. The stability of the derived ENSO-rainfall linkages were further investigated using time lagged correlation techniques that included cross-validation technique.


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