Observed and Simulated Processes Linked to the Recent Climate Variability and Changes Over the Greater Horn of Africa PDF Download

Author: Vincent O Otieno
Publisher:
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Category : Electronic dissertations
Languages : en
Pages :

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Book Description
The main objective of this dissertation is to document characteristics of the processes and mechanisms associated with 20th and 21st century spatio-temporal modes of climate variability and changes over the Greater Horn of Africa (GHA) region. This thesis research comprises three major parts. The first part used output from ten Earth System Models (ESMs) from the fifth phase of coupled model intercomparison project to characterize seasonal and annual mean precipitation cycle over the Greater Horn of Africa (GHA) region. Each ESM had at least 2 ensemble members. In spite of distributional anomalies of observations, ESM ensemble means were examined on the basis of gridded precipitation data. Majority of the ten ESMs analyzed correctly reproduce the mean seasonal and annual cycle of precipitation for the period 1979–2008 as compared to gridded satellite-derived observations. At the same time our analysis shows significant biases in individual models depending on region and season. Specifically, a modest number of models were able to capture correctly the peaks of bimodal (MAM and OND) and JJAS rainfall while a few either dragged the onset to subsequent months or displaced the locations of seasonal rainfall further north. Nearly all models were in agreement with their representation of the zonal orientation of spatial pattern of the leading EOF rainfall modes; more so, enhanced precipitation over the Indian Ocean and a dipole mode of precipitation pattern are captured in the first and second mode respectively. Further, the corresponding EOF time series of the ESMs rainfall modes were all in phase with observations. However, all models output were positively biased against observations, with large medians and varied range of anomalies. Therefore, caution needs to be taken when choosing models for applications over the region, especially when ensemble means have to be considered. The second part focused on the ESM projections under AR5 Representative Concentrations Pathways (RCP), 4.5 and 8.5 scenarios projections of the GHA Climate. Six Earth System Models (ESMs) from CMIP5 archive have been used to characterize projected changes in seasonal and annual mean precipitation, temperature and the hydrological cycle by the middle of twenty-first century over the GHA region. There is significant variation among models in projected precipitation anomalies, with some models projecting an average increase as others project a decrease in precipitation during different seasons. The ensemble mean of the ESMs indicates that the GHA region has been experiencing a steady increase in both precipitation and temperature beginning the early 1980s and 1970s respectively in both RCP4.5 and RCP8.5 scenarios. Going by the ensemble means, temperatures are projected to steadily increase uniformly in all the seasons at a rate of 0.3/0.50C/decade under RCP4.5/8.5 scenarios over northern GHA region leading to an approximate temperature increase of 2/3 0C by the middle of the century. On the other hand, temperatures will likely increase at a rate of 0.3/ 0.4 0C/decade under RCP4.5/8.5 scenarios in both equatorial and southern GHA region leading to an approximate temperature increase of 2/2.5 0C by the middle of twenty first century. Nonetheless, projected precipitation increase varied across seasons and sub-regions. Notably, as precipitation increases, the deficit (E-P) between evaporation (E) and precipitation (P) increased over the years, with a negatively skewed distribution. This generally implies that there is a high likelihood of an increased deficit in local moisture supply. This remarkable change in the general hydrological cycle (i.e. deficit in local moisture) is projected to be also coincident with intensified westerly anomaly influx from the Congo basin into the region. However, better understanding of the detailed changes in hydrological cycle will require comprehensive water budget analyses that require daily or sub-daily variables. The third part was the sub-regional analysis of precipitation and evaporation using high resolution Coordinated Regional experiments (CORDEX) output over the GHA sub region. Over southern GHA region, the number of wet and extreme wet days is projected to increase, expanding the length of a growing season during DJF. However during MAM, the number of wet days is projected to decrease by the middle of the 21st century over southern GHA region. Over equatorial GHA region, the number of wet days is projected to decrease during MAM and OND seasons. This might impact negatively on the agricultural activities in the region. Over the northern GHA region, significant increase/decrease in the number of wet/dry days is projected during September-October-November (SON) season. However, the number of extreme wet days is projected to increase during MAM, JJA and SON seasons while the number of extreme dry days is expected to remain relatively the same in all the seasons. The projected changes in precipitation distribution over the GHA region will have different impact on those sub-regions. For agricultural activities, causes of rain failure might be considered in terms of delayed onset of rains, an early withdrawal, or short but intense rainfall events separated by long dry spells. However, in this research the main focus was on the changes in distribution of rains. Other aspects such as onset and withdrawal and the gap between dry and wet spells also need to be documented for right choice of crop and optimum production.