Contribution of variations in Northern Hemisphere annular mode to the near-surface wind speed changes over Eastern China for 1979-2017

Abstract

Studies have shown that large-scale ocean-atmosphere circulations (LOACs) played the major role to the near-surface wind speed (NWS) changes over China; however, the mechanisms whereby LOACs influences NWS to have received little attention. In this study, the processes of the Northern Hemisphere annular mode (NAM) influencing the NWS changes are revealed over eastern China for 1979–2017. The results showed a slowdown in NWS, at a rate of − 0.09 ± 0.01 m s− 1 decade− 1; meanwhile, this decline could be partly driven by the weakening of the zonal wind component. When the NAM exhibits positive phases, the zonal-mean westerly weakens at the low-to-mid-latitudes (10°–40°N); meanwhile, in the troposphere descending flows prevail near 40°N and ascending flows prevail near 65°N, and in the lower troposphere there are northerly anomalies at the low-to-mid-latitudes and southerly anomalies at mid-to-high latitudes (40°–70°N). The anomalous meridional flows transport heat from lower latitudes to higher latitudes and weaken the north–south air temperature gradient. The decreased air temperature gradient over East Asia reduces the pressure-gradient near the surface in eastern China, thereby decreasing the NWS. Furthermore, the effects of NAM on NWS changes are more significant at interannual scale than decadal scale. 32.0 ± 15.8 % of the changes in the annual mean NWS are caused by the variations in NAM; meanwhile, the NAM contribution to the interannual changes in the zonal component of NWS reach 45.0 ± 12.9 %.