Northern Hemisphere atmospheric stilling partly attributed to an increase in surface roughness

Surface winds have declined in China, the Netherlands, the Czech Republic, the United States and Australia over the past few decades(1-4). The precise cause of the stilling is uncertain. Here, we analyse the extent and potential cause of changes in surface wind speeds over the northern mid-latitudes between 1979 and 2008, using data from 822 surface weather stations. We show that surface wind speeds have declined by 5-15 % over almost all continental areas in the northern mid-latitudes, and that strong winds have slowed faster than weak winds. In contrast, upper-air winds calculated from sea-level pressure gradients, and winds from weather reanalyses,exhibited no such trend. Changes in atmospheric circulation that are captured by reanalysis data explain 10-50 % of the surface wind slowdown. In addition, mesoscale model simulations suggest that an increase in surface roughness-the magnitude of which is estimated from increases in biomass and land-use change in Eurasia-could explain between 25 and 60 % of the stilling. Moreover, regions of pronounced stilling generally coincided with regions where biomass has increased over the past 30 years, supporting the role of vegetation increases in wind slowdown.