Frequency Change of Clear-air Turbulence over the North Pacific under 2 K Global Warming – Ensemble Projections using a 60-km Atmospheric General Circulation Model

Future changes in the climatological distribution of clear-air turbulence (CAT) and its seasonality over the North Pacific are estimated on the basis of an ensemble of climate projections under warming for the globally averaged surface air temperature of 2 K relative to preindustrial levels, which includes over 3000 years of ensembles using a 60 km atmospheric general circulation model (AGCM). The AGCM outputs are interpolated to a 1.25 degrees horizontal resolution, and the climatological CAT frequency is computed. The CAT broadly decreases in the midlatitude central to western North Pacific along with the anticyclonic (south) side of its present-day high-frequency band extending from Japan to the eastern North Pacific. Meanwhile, large relative increases are found outside the band, implying an increased risk of CAT encounters. Uncertainty in future CAT changes due to uncertainties in the spatial pattern of sea surface temperature (SST) change is addressed for the first time using six selected Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models. The uncertainty is greatest in the boreal winter and spring over the central North Pacific and is associated with an uncertainty in future changes in the jet stream and upper-level synoptic-scale disturbances.