Coastal sea surface temperature variability along the south coast of South Africa and the relationship to regional and global climate

Abstract

The southern coastline of South Africa is approximately zonal, with a wide (up to 270 km) shelf region. Intense thermoclines are known to be established by insolation on the inner shelf region during summer, upwelling is generated by easterly-component winds, and occasionally Agulhas Current water can be advected close to the coast, particularly in the east. These processes induce daily and seasonal fluctuations of coastal sea-surface temperature (SST), but their influence over longer time scales (interannual) has not yet been tested. Here time series of SST ranging from 12 to 31 years are examined for inter-relationships with local and regional winds, and the southern oscillation index (SOI). The emphasis is on the summer period, and it is found that the correlation between SST and major axis wind anomalies can be improved substantially by considering the frequency of occurrence of winds above given thresholds. Moreover, winds and SSTs are also correlated with the SOI, such that fewer easterlycomponent winds are experienced at low phases (EI Nino) with consequent increases in coastal SST, and correspondingly more easterly-component winds at high phases (La Nina) result in decreased coastal SST; however, these relationships did not hold for a measuring site within a large open bay area. Long-term trends are also established, with substantial increases in SST (0.25°C/decade) in association with greater increases in air temperature (0.36°C/ decade)