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University of Cape Town (1999)

The relationship between ENSO, seasonal rainfall, and circulation patterns in South Africa

Kruger, A C

Titre : The relationship between ENSO, seasonal rainfall, and circulation patterns in South Africa

Auteur : Kruger, A C

Université de soutenance : University of Cape Town.

Grade : Master of Science (MS) 1999

Résumé
Relationships between the El Niño-Southern Oscillation phenomenon (ENSO), seasonal rainfall, and atmospheric circulation patterns at the 1000 hPa and 500 hPa levels are investigated. Firstly, correlations between early-summer (October to December), late-summer (January to March) rainfall over South Africa, and sea-surface temperatures in the NINO3 region in the equatorial Pacific Ocean were investigated, where the correlations in the case for late-summer showed much better spatial coherence than in the case for early-summer. Consequently, the study further concentrated only on late summer. The influence of the quasi 18-20 year oscillation of summer rainfall on the effect of El Niño and La Niña events was also investigated, and it was found that during an epoch of above-normal/below-normal rainfall a moderating effect is evident on the severity of El Niño/La Niña events so that on average even above-normal/below-normal rainfall is experienced during such events. Canonical correlation analysis (CCA) was applied to different combinations of years, to find associations between equatorial Pacific sea-surface temperatures and deviations in circulation patterns during certain years. Some at the above results were then verified by model runs, to further prove the relationships not to be coincidental, and to add more degrees of freedom. The results of CCA were then separately interpreted for each combination of seasons (e. g. El Niño during the above-normal phase at the 500 hPa level) with the aid of average circulation maps for different combinations of years. Above - or below-normal rainfall during such years could then be explained in terms of deviations of general synoptic features at the surface and 500 hPa levels.

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