Increase in Surface Friction Dominates the Observed Surface Wind Speed Decline during 1973-2014 in the Northern Hemisphere Lands

During 1973-2014, a reduction trend in the observed surface wind speed (10 m) in the Northern Hemisphere lands has been widely reported; this reduction is referred to as global stilling. The primary determining factors of global stilling include atmospheric circulation, turbulent friction, and surface friction when ignoring the vertical influencing factors. Most of the existing studies on the attribution of global stilling do not take changing surface friction into account. In addition, there are other changes in the climate system, such as aerosol loading, which could have an impact on atmospheric circulation, but are not included in the majority of current models either. Here, we developed a novel approach based on modeled winds calculated from sea level pressure observations and applied the method to approximately 4000 weather stations in the Northern Hemisphere lands from 1973 to 2014 to attribute the stilling in the three factors. In our methods, we neglected the vertical influencing factors on surface wind speed but took the aerosols' changes on atmospheric circulation and gradual urbanization effect on surface wind speed into account. We found that atmospheric circulation has dictated the monthly variation in surface wind speed during the past four decades. However, the increased surface friction dominates the long-term declining trend of wind stilling. Our studies had uncertainties while neglecting the influence of vertical factors on surface wind stilling, despite most of the existing studies showing their effect was minor compared to the three factors explored in our study.