Decreasing wind speed and weakening latitudinal surface pressure gradients in the Tibetan Plateau

The Tibetan Plateau, with an average elevation of over 4000 m above sea level, is the highest and most extensive highland in the world. Between 1980 and 2005, the annual mean temperature has warmed at the rate of 0.38 degrees C decade(-1). However, little attention has been paid to the variation of wind speed, the most important factor controlling evapotranspiration in the Tibetan Plateau. Here we used monthly mean wind speed from the Chinese Meteorological Administration data set to examine the spatial and temporal variability of wind speed at 71 stations (with elevations above 2000 m above sea level) in the eastern and central Tibetan Plateau during 1980-2005, and compared wind speed climatology and their trends with NCEP and ERA-40 reanalyses in the same domain. Compared with surface stations, NCEP overestimates wind speed and ERA-40 underestimates it, with mean annual biases of 0.93 m s(-1) for NCEP and -0.75 m s(-1) for ERA-40. Both surface stations and NCEP reanalysis show significant decreasing trends, at rates of -0.24 and -0.13 m s(-1) decade(-1), respectively, mainly evident in spring and summer. ERA-40 fails to capture any decrease. The above results indicate that NCEP captures wind speed better than ERA-40. We speculate that the most likely cause of diminishing wind speed are the asymmetrically decreasing latitudinal surface temperature and pressure gradients over the Tibetan Plateau, which may be part of a large-scale atmospheric circulation shift.