Variability of summer rainfall over southwestern Madagascar

Randriamahefasoa,Tsinampoizina Marie Sophie

Variability of summer rainfall over southwestern Madagascar

Master Thesis

2013

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

This study investigates the interannual variability of the frequency of wet days during the rainy season over southwestern Madagascar, and its associated regional and global circulation patterns. The number of wet days was counted for each summer season (December to March) of the period 1971 to 2000 at four rainfall stations in southwestern Madagascar. For each station, the frequency of wet days was correlated with ENSO indices, SIOD index, SST, 850 hPa geopotential height, zonal and meridional winds at the surface. Composite anomalies of moisture flux and moisture flux convergence at 850 hPa, 500 hPa omega and velocity potential at .995 sigma level, using NCEP/NCAR reanalysis data, associated with El Niño events, La Niña events that occurred during the period of study were examined. Also, circulation anomalies during neutral years having anomalously high frequency of wet days or anomalously low frequency of wet days were analysed separately. It was found that the number of wet days rarely exceeds half of the total days in summer for each station. Inverse relationship between the equatorial Pacific SST anomalies and the frequency of wet days anomalies was identified. Statistics showed that years characterised by high frequency of wet days often occur with La Niña seasons whereas years having low frequency of wet days tend to occur with El Niño seasons. A strong relationship between the southern Indian Ocean SST and the frequency of wet days was found at Ranohira (45.3° E, 22.5° S) and Toliara (43.72° E, 23.38° S). Increased number of wet days over southwestern Madagascar is associated with low atmospheric pressure over the southern Mozambique Channel. Neutral years having anomalously high (low) frequency of wet days are marked by northwesterly (southeasterly) moisture flux anomalies at 850 hPa over southwestern Madagascar.

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Includes bibliographical references.

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