Changes of wind speed at different heights over eastern China during 1980-2011

Long-term decreasing trends of near-surface wind speed (SWS) have been well demonstrated in China, and at the same time the factors causing the slowdown in SWS have also been explored. However, changes in wind speed and pressure gradient force (PGF) at higher elevations have received little attention. In this paper, the spatio-temporal changes of wind speed and PGF at different heights were explored in eastern China during the period 1980-2011, and the potential causes inducing the changes of wind speed were analysed. The results included that the reduction in wind speed emerged from the near surface to 300 hPa, while the PGF also exhibited a synchronous reduction. The influence of PGF on the decrease of wind speed was relative smaller at near surface than that at higher levels (850-300 hPa). At the near surface, 71.8% of changes in annual mean SWS could be attributed to the changes of PGF, in which the seasonal difference of the influence of PGF on wind speed was less than 7.0%. At the higher levels, most of the changes in wind speed (>90%) was induced by PGF, in which the seasonal difference of the influence of PGF was less than 1.0%. Furthermore, the changes of PGF in eastern China could be mainly influenced by both the Northern Hemisphere annular mode (NAM) and the Pacific Decadal Oscillation (PDO). The NAM could affect the inter-annual and inter-decadal changes of PGF, and the effects of NAM played more significant role on the inter-annual scale than that on the inter-decadal scale. At the same time, the correlation between NAM and PGF increased with height. Otherwise, the PDO mainly affected the inter-decadal changes of PGF, and their correlation decreased with height.