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Cloud bands over southern Africa: seasonality, contribution to rainfall variability and modulation by the MJO

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Abstract

Tropical-extratropical cloud band systems over southern Africa, known as tropical temperate troughs (TTTs), are known to contribute substantially to South African summer rainfall. This study performs a comprehensive assessment of the seasonal cycle and rainfall contribution of TTTs by using a novel object-based strategy that explicitly tracks these systems for their full life cycle. The methodology incorporates a simple assignment of station rainfall data to each event, thereby creating a database containing detailed rainfall characteristics for each TTT. This is used to explore the importance of TTTs for rain days and climatological rainfall totals in October–March. Average contributions range from 30 to 60 % with substantial spatial heterogeneity observed. TTT rainfall contributions over the Highveld and eastern escarpment are lower than expected. A short analysis of TTT rainfall variability indicates TTTs provide substantial, but not dominant, intraseasonal and interannual variability in station rainfall totals. TTTs are however responsible for a high proportion of heavy rainfall days. Of 52 extreme rainfall events in the 1979–1999 period, 30 are associated with these tropical-extratropical interactions. Cut-off lows were included in the evolution of 6 of these TTTs. The study concludes with an analysis of the question: does the Madden-Julian Oscillation influence the intensity of TTT rainfall over South Africa? Results suggest a weak but significant suppression (enhancement) of intensity during phase 1(6).

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Acknowledgments

Benjamin Pohl is thanked for his preprocessing of the WRC station rainfall data. Interpolated OLR data was obtained from NOAA/OAR/ESRL PSD, Boulder, Colorado, USA through http://www.cdc.noaa.gov. The first author gratefully acknowledges funding through SANAP and a D & E Potter Foundation PhD Fellowship. Two anonymous reviewers are thanked for their suggestions.

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Authors and Affiliations

  1. Dept. Oceanography, MaRe Institute, University of Cape Town, Cape Town, South Africa

    Neil C. G. Hart & Chris J. C. Reason

  2. NIWA, Auckland, New Zealand

    Nicolas Fauchereau

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  1. Neil C. G. Hart
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  2. Chris J. C. Reason
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  3. Nicolas Fauchereau
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Correspondence to Neil C. G. Hart.

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Hart, N.C.G., Reason, C.J.C. & Fauchereau, N. Cloud bands over southern Africa: seasonality, contribution to rainfall variability and modulation by the MJO. Clim Dyn 41, 1199–1212 (2013). https://doi.org/10.1007/s00382-012-1589-4

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  • DOI: https://doi.org/10.1007/s00382-012-1589-4

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