Abstract
Seasonal trends in low and total cloud cover, as well as for associated climate variables diurnal temperature range (DTR) and number of rain days, are investigated for South Africa. It is also investigated whether the observed trends and variability in cloud cover could be related to the El Nino-Southern Oscillation (ENSO) phenomenon, which has a major influence on the variability of summer rainfall in South Africa. These trends have not been investigated recently and in such detail. In the light of the climate change debate, updated studies of historical climate change are important, especially for regions and climate variables of which such studies are not published often. Seasonal trends of daily means were examined from quality-controlled data time series of 28 climate stations over South Africa, for the period 1960 to 2005. Regional trends could be determined by averaging series of stations showing similar trends, within areas delineated in such a way that the trend of the averaged series would be statistically significant. In this way the intra-seasonal spatial variability of same trend regions, as well as the spatial relationships between trends of the different climate variables under discussion, could be established. The main results, taking all seasons into account, is a general decrease in mean daily low cloud cover, and to a lesser extent total cloud cover, over most of South Africa, but an increase in the south and south-west of the country. However, the sizes of same trend regions show considerable variability between seasons. While trends in DTR and rain days are the opposite and the same, respectively, of trends in cloud cover in most cases, it is shown that this is not always the case. A region covering the northern, central and western interior of South Africa, with late-summer (JFM) cloud cover negatively correlated with equatorial Pacific sea-surface temperatures (SSTs), shows only a non-significant decrease in total and low cloud cover for JFM, which corresponds to a non-significant increase in equatorial Pacific SSTs during the same period.
Highlights
In the light of the climate change debate, assessments of historical climate change over regions of which such studies are not published often, such as sub-Saharan Africa in this case, could be regarded as important
For Southern Africa this study indicates a decrease in the annual mean of mean daily total cloud cover, but an increase in a 10° x 10° grid box south-west of the continent which includes a small part of the south-western Cape
Reliable cloud observation values should usually have a negative relationship with diurnal temperature range (DTR), and a positive relationship with the number of rain days (Dai et al, 1997; Dai et al, 1999; Rebetez and Beniston, 1998)
Summary
In the light of the climate change debate, assessments of historical climate change over regions of which such studies are not published often, such as sub-Saharan Africa in this case, could be regarded as important. The detailed spatial and temporal trends and variability of low and total cloud cover, as well as of related climate variables diurnal temperature range (DTR) and number of rain days, over the period 1960 to 2005, are investigated for South Africa. This is the first study of trends in cloud cover which focuses only on South Africa, and aims to present more detailed results than already published global studies of cloud-cover trends. From the above-mentioned studies it is clear that historical trends in cloud cover are not uniform for the globe
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