Trends in Vertical Wind Shear and Clear-Air Turbulence Since 1979

Vertical wind shear and clear-air turbulence are projected to increase in future at flight cruising altitudes in response to climate change. This presentation examines whether these increases have already begun. We first analyse past trends in vertical wind shear over the North Atlantic in three different reanalysis datasets since 1979. We find that the shear at flight cruising altitudes has strengthened by 15%, and we show that this change is attributable to the thermal wind response to the enhanced upper-level meridional temperature gradient. We then analyse clear-air turbulence trends globally during 1979–2020 in a reanalysis dataset using 21 diagnostics. We find clear evidence of large increases around the globe at aircraft cruising altitudes. For example, at an average point over the North Atlantic, the total annual duration of light-or-greater turbulence increased by 17% from 466.5 hours in 1979 to 546.8 hours in 2020, with even larger relative changes for moderate-or greater turbulence (increasing by 37% from 70.0 hours to 96.1 hours) and severe-or-greater turbulence (increasing by 55% from 17.7 hours to 27.4 hours). Future projections using climate models indicate a 17–29% increase in vertical wind shear in the upper-level jet streams by 2100, as well as a possible tripling in the amount of severe clear-air turbulence. We conclude that the jet streams have already started to become more sheared because of climate change, generating more turbulence, with potentially important implications for the future of air travel.