Interactions between synoptic, intraseasonal and interannual convective variability over Southern Africa
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After removing the annual cycle, a principal component analysis is applied to the daily outgoing longwave radiation anomaly field, used here as a proxy for atmospheric convection. The analysis is carried out over the southern African region (7.5°E–70°E, 10°S–40°S) for austral summer (November through February) for the period 1979–1980 to 2006–2007. The first five principal components (PC) are retained. The first two PCs describe spatial patterns oriented north-west to south-east from tropical southern Africa (SA) to the mid-latitudes. They are interpreted to be different possible locations for synoptic-scale tropical–temperate troughs (TTT), one dominant rainfall-producing synoptic system in the region. The phase relationship between these two PCs describes a tendency for these TTT to propagate eastwards from SA to the Mozambique Channel and southern Madagascar. The next three PCs describe convective fluctuations, respectively, located over the north-west, the south and the centre of SA. Their time series are significantly associated with Madden–Julian oscillation (MJO) activity in the tropics. However, we find that TTT systems are statistically independent of the MJO, i.e. they are equally liable to occur during any phase of the MJO. Three PCs out of five also show a significant association with El Niño southern oscillation, confirming that El Niño years mostly coincide with suppressed convection at the intraseasonal time-scales, a result consistent with its impact on seasonal averages diagnosed in previous studies.
KeywordsOLR Atmospheric convection Summer rainfall Southern Africa Principal component analysis Tropical–temperate troughs Madden–Julian oscillation El Niño southern oscillation
This work was carried out at the University of Cape Town as part of the PICS PESOCA exchange programme co-funded by France and South Africa. Benjamin Pohl thanks the staff of the Department of Oceanography at UCT for its hospitality. Nicolas Fauchereau and Mathieu Rouault thank funding from WRC project K5/1747. The authors thank Matt Wheeler (Centre for Australian Weather and Climate Research) and two anonymous reviewers for helpful comments on the manuscript.
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