Climatic Anomalies and their influence on Rainfall Trends in Vhembe District South Africa
Abstract
The principal aim of this study was to relate the large-scale meteorological systems to the local seasonal rainfall characteristics (especially rainfall onset and variations/trends). In addition, the study established the relationship between the seasonal rainfall onset, mean climatological systems and anomalies and their respective dynamics in the north east of South Africa (Vhembe District). The two most significant forcings in Vhembe District that influence rainfall are the topographic effects and the large scale circulation, which are mutually interactive in some cases. Four steps of analyses were involved. In the first step humid and semi-arid climatic subdivisions of Vhembe District were assumed from earlier studies. The rainfall characteristics and trends for the local area were then analysed based on these climatic groups. This first step also included the task to determine seasonal rainfall onset changes over different decades and within decades
from 1980 to 2009 for both humid and semi-arid areas. The results in this step were obtained using two techniques, namely Mann Kendal trend analysis and Sen's slope estimator, which were employed to test for the presence of statistically significant trends and magnitude respectively. The second step was to establish the degree to which the local gauge data captures the signal of large-scale climate forcing of the study area. This step was accomplished by examining rainfall estimated by global precipitation datasets (in this case Climate Prediction Centre (CPC)'s Merged Analysis of Precipitation (CMAP)) in order to compare and validate the seasonal rainfall onset obtained based on the analysis of local gauge data. The third step was to establish relationships between seasonal rainfall onset and change and large-scale mean atmospheric circulation. This was accomplished by exploring meteorological systems and indicators of moisture transport and accumulation (specific humidity, moisture flux and vertical motion), that feature during seasonal rainfall onset. The final step, explores the time-varying large-scale meteorological anomalies
associated with the evolution of local seasonal rainfall characteristics. The third and fourth steps used the data products of NCEP/NCAR reanalysis output. The study has established differences and common denominators between humid and semi-arid zones during early and late seasonal rainfall onset in Vhembe District. For example, no large-scale moisture supply or convective activities are needed in the humid
zone for seasonal rainfall to commence. While in the semi-arid zone, large-scale moisture supply and large-scale convective activities initiates seasonal rainfall onset.
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