Long-term austral summer wind speed trends over southern Africa.

ABSTRACT A comprehensive analysis of near surface (10-m and 850 hPa) long-term (1980-2015) austral summer mean wind speed trends over southern Africa and the surrounding oceans (10°-40°S and 10°W-50°E) is carried out. The climate forecast system reanalysis and the climate forecast system reanalysis version 2, the modern era retrospective-analysis for research and applications version 2, and the ERA-Interim products are used for the analysis. Trends are estimated using the Theil-Sen slope estimator and their significance tested with the Mann-Kendall test. Attribution of the wind speed trends to large scale circulation systems is determined from: (1) trends in 850 hPa geopotential height, surface pressure, and surface skin temperatures, (2) changes in the annual frequency of archetypal synoptic states generated by self-organizing maps from geopotential height data, and (3) the correlations of the wind speeds to the southern annular mode ( SAM) and El Niño Southern Oscillations ( ENSO) indices. There is an overall decline in wind speeds evident in all the reanalyses. Wind speeds reduce by an average 0.7 and 1.3 m s⁻¹ at 10 m and 850 hPa, respectively, over the study period. Deceleration in midlatitude westerly and Atlantic south easterly winds are associated with a poleward shift in the subtropical anticyclone, a positive trend in the annual frequency of summer circulation weather types, and the weakening of the subtropical continental heat low. The SAM and ENSO are also found to have a statistically significant linear association with the observed wind speed trends. [ABSTRACT FROM AUTHOR]