Millennial to multi-decadal variability in the climate of southern africa

摘要

Wavelet analysis has been used to determine changes in the variability of southern Africa climate over the last three millennia. The structure of the variability in a 3000-year δ~(18)O record has been compared with that in tree ring data for the last 600 years and both to the variability in annual rainfall and runoff during the 20th century. Preferred modes of variability are shown to have occurred in all the data sets. For overlap periods, the variability modes are in phase where the data are for a common seasonal rainfall regime. From the combined proxy and instrumental data it appears that an ～1500-year oscillation in rainfall variability may have been present during the late Holocene. Considerable variability in South African climate has been shown to reside in the bands 500-800 and 60-120 years. the latter is manifest in stable isotope and tree ring proxy data and in measured rainfall and runoff, and shows a distinctive latitudinal variation over southern Africa. The ～80-year variability has been present for at least 600 years in the tree ring data and at least 3000 years in the stable isotope data. During the 20th century, the variability in the 60-120-year range was confined south of 20°S in southern Africa. The existence of considerable multi-decadal climate variability in the range 16-20 years has been confirmed south of 15°S. Tree ring data show that the ～18-year oscillation in climate has been present for at least 600 years. The 500-800-year, ～80-year and ～18-year variability has been both frequency- and amplitude-modulated over time, but never to the extent that the variability bands have lost their integrity. Their persistence is a striking feature of southern African climate. The millennial, centennial and multi-decadal variability in climate over southern Africa finds counterparts with temporal variability in similar time ranges reported elsewhere. Spatial teleconnections in variability likewise have been shown to occur. The changes in climate variability over the subcontinent are part of variability changes affecting the Earth system in many other regions.