Effects of Northern Hemisphere Annular Mode on terrestrial near-surface wind speed over eastern China from 1979 to 2017

Abstract

Large-scale ocean‒atmosphere circulations (LOACs) have a pronounced effect on the near-surface wind speed (NSWS). In this study, we discussed the contributions of zonal and meridional flows to NSWS changes and identify the possible association between the Northern Hemisphere Annular Mode (NAM) and the NSWS changes over eastern China from 1979 to 2017. Results show that the reduction in NSWS over eastern China was mainly dominated by the weakening of the zonal wind component. NAM has a considerable effect on the NSWS over eastern China. When the NAM exhibits positive phases, the zonal-mean westerly weakens at low-to-mid-latitudes (10°–40°N). Meanwhile, descending flows prevail near 40°N, and ascending flows persist near 65°N in the troposphere. In the lower troposphere, there are northerly anomalies at low-to-mid-latitudes and southerly anomalies at mid-to-high latitudes (40°–70°N). The anomalous meridional flows transport heat from low to high latitudes and weaken the north‒south air temperature gradient. The decreased air temperature gradient over East Asia reduces the pressure gradient near the surface, decreasing NSWS in eastern China. NAM variations could dominate (32.0 ± 15.8)% of the changes in the annual mean NSWS. Nevertheless, the contribution of NAM to the interannual changes of the zonal component in NSWS could reach (45.0 ± 12.9)%.

Keywords

• Near-surface wind speed
• Temperature gradient
• Pressure gradient
• Northern Hemisphere Annular Mode
• Large-scale ocean–atmosphere circulations