Climatic changes in the subtropical Southeast Atlantic: the St. Helena Island Climate Index (1893–1999)

Abstract

The Benguela upwelling system in the subtropical Southeast Atlantic is subject to dramatic interannual fluctuations sometimes termed ‘Benguela Niño’ events. The South Atlantic Anticyclone (SAA) is assumed to be the responsible climatic ‘activity centre’ for the south-east trade winds driving the upwelling processes along the Namibian and South-west African coasts. Air temperature and humidity signals of this region, modulated by upwelling-controlled sea surface temperature (SST), are carried by the trades towards St. Helena Island. The island’s 1893–1999 century-long monthly weather records of temperature, pressure and rainfall have been assembled and homogenized. They exhibit trends for decreasing precipitation (10 mm/100 year), increasing air temperature (0.9 °C/100 year), and decreasing air pressure (0.6 hPa/100 year). Their first empirical orthogonal eigenfunction (EOF) covers 46% of the total variance; its associated temporal coefficient is proposed as a ‘St. Helena Island Climate Index (HIX)’. Austral winter HIX has a 42% correlation with a remotely sensed SST-derived Benguela upwelling index, called intense Benguela upwelling, for the time period 1982–1999. Not yet identified Benguela Niños (1895, 1905, 1912, 1916, 1946) and years of strong Benguela upwelling (1911, 1922, 1967, 1976) are newly suggested by the HIX.