Abstract
Variability of and generation mechanisms for Madagascar rainfall are studied by conducting climatological, synoptic and mesoscale analyses. It is found the rainfall variability is highly sensitive to seasons with high variability in summer (Nov-Apr). The rainfall in summer is controlled by the Intertropical Convergence Zone (ITCZ) and orographic rainfall associated with tropical cyclones (TCs), while the rainfall in winter (May-Oct) is controlled by trade winds and local orographic rainfall along the eastern coast. Synoptic analysis reveals that major climate variations in summer are associated with ITCZ position, which is closely related to TC genesis locations and quasi-biennial oscillation (QBO). Linkages between El-Niño Southern Oscillation Index (ENSO) and Southern Oscillation Index (SOI) are identified as the cause of inconsistent dry or wet summers. Mesoscale analysis depicts the importance of the orographic effects on prevailing wind, which are controlled by the orography in both seasons. In winter, the prevailing trade winds over the Southwest Indian Ocean are from the east and are split to the north and south when it impinges on Malagasy Mountains. On the other hand, in summer the prevailing easterlies are weaker leading to the production of lee vortices, in addition to the flow splitting upstream of the mountain. Thus, the flow is classified into two regimes: (a) flow-over regime with no lee vortices under high Froude number (Fr=1.2-1.8) flow, and (b) flow-around regime with lee vortices under low Fr (=0.88-1.16) flow. A case study of TC Domoina (1984) indicates that the long-lasting heavy rainfall was induced by the strong orographic blocking of Madagascar. The shorter-term (e.g., 2 days) heavy orographic precipitation is characterized by large VH ∙Ñh which is composed by two common ingredients, namely a strong low-level wind normal to the mountain (VH) and a steep mountain slope (∇h).
Highlights
Rainfall features in the tropical region are mainly modulated by the intertropical convergence zone (ITCZ) which possesses a large-scale fluctuation characterized by the complex northeast and southeast trade wind systems accompanied with bands of convective clouds and rain
The rainfall maxima occur during the peak summer season (January-February) when the ITCZ is located at its southernmost about 10oS latitude enhancing the probability for a tropical cyclone to develop with respect to the favorable conditions described in different literatures (e.g., Lin, 2007)
The National Centers for Environmental Prediction (NCEP)/NCAR Reanalysis surface datasets of meteorological variables, such as 4x dailysurface wind (u), relative humidity (q) data at sigma level 995 mb at 2.5o resolution, that can be retrieved from the ESRL (2016), are widely utilized by other authors to study the heavy orographic rainfall induced by the TC Domoina on January 22, 1984
Summary
Rainfall features in the tropical region are mainly modulated by the intertropical convergence zone (ITCZ) which possesses a large-scale fluctuation characterized by the complex northeast and southeast trade wind systems accompanied with bands of convective clouds and rain (see e.g., Hastenrath & Lamb, 1978; Grodsky & Carton, 2003). Vol 5, No 2; 2016 episodes shows inconsistent relationship to Madagascar rainfall (Ingram & Dawson, 2005; Dunham et al, 2010), causing drought in some areas but wetter conditions into certain regions Assumptions such as above normal sea surface temperature (SST) and Quasi Biennial Oscillation (QBO) cyclical forcing can add to heavy TC rainfall enhancement (Jury et al, 1993; Fitchett & Grab, 2014). Another environmental factor, such as the orography, may play an important role in modulating the climate of a region.
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