Arctic airmass displacement and reduced midlatitudes wintertime temperature variability under climate change

Abstract:

An active current debate is whether polar amplification under a warming climate increases the waviness of the jet stream and hence the frequency of extreme temperatures in the midlatitudes. We approach this question by analyzing the statistics of near surface temperature in the Northern hemisphere wintertime from reanalysis products (ERA5) and climate model simulations (6 CMIP6 models under SSP585). Specifically, we compute an equivalent latitude of surface temperature, a rough measure of what latitude the surface air parcel traveled from under an adiabatic condition. By analyzing the local statistics of equivalent latitude, we can quantify the extent of latitudinal excursion of air under changing climate: a longer tail of equivalent latitude means extended latitudinal excursions of near-surface air parcels. We find that climate models reproduce well the patterns of wintertime midlatitudes temperature variations seen in reanalysis, with eastward-propagating disturbances of spatial wavenumber 6-8, consistent with Rossby wave influence, with propagation speeds typical of the background flow. In future projections, midlatitudes wintertime temperature variance decreases strongly by the end of the century, consistent with previous studies. The use of equivalent latitude allows us to partition this change into contributions due to either the weakening meridional temperature gradient or the changes in the distribution of airmass origin. We find no evidence of consistent broadening of distributions of equivalent latitude that would suggest increased future jetstream waviness. While these results do not directly address the dynamics of the jetstream, a simple metric based on surface temperature is suggestive of the consequences of dynamical changes, and is directly applicable to the socioeconomic impacts that drive concerns over climate change.