Forced Response of North Atlantic Jet Variability and Associated Climate Extremes in Models and Reanalysis

Climate extreme events including heatwaves and drought impact human lives and disrupt economic and ecological systems; anthropogenic forcing of the climate system exacerbates future impacts, as warming increases the frequency of extreme heat events and increases atmospheric evaporative demand. Summer extremes of heat and drought events occur in association with atmospheric circulation often characterized by persistent anomalies in the strength and position of the midlatitude jet streams. Midlatitude circulation under anthropogenic forcing is subject to a complex of competing influences of tropical and polar changes, so there is considerable uncertainty surrounding the future expression of circulation features which contribute to extreme events. We asses the response of the North Atlantic jet to natural and anthropogenic external forcing in the Community Earth System Model Last Millennium Ensemble (CESM-LME) simulations, which include single-forcing, all-forcing, and pre-industrial control runs, comparing model-projected change under continued anthropogenic forcing to past internal variability and past changes under natural external forcing. To quantify NAJ variability in reanalysis (ERA-20c) and model (CESM-LME, CMIP5) output, we take monthly indicators of jet strength and position over the North Atlantic basin, computed from the zonal wind at levels 925-700 hPa over the North Atlantic basin. To assess NAJ response to forcing, we compare monthly climatologies of the statistical moments of each indicator across the CESM-LME single-forcing, all forcing, and control runs; between sub-periods of strong and weak forcing; and between reanalysis and models. Compared to reanalysis, CESM-LME exhibits consistent biases of jet speed for months October-February, when mean speed is high relative to reanalysis; and also jet position in months January-March, for which the model produces a jet that is biased poleward relative to reanalysis. Variance of jet speed and latitude indicators are consistently underrepresented in the model runs. Changes in jet position associated with projected 21st-century anthropogenic forcing under RCP8.5 are greater than those associated with past forcing over the last millennium and include poleward shift of mean jet position that is most pronounced for months May, June, August and September. The poleward trend in projected jet position over the 21st century is greater than any 100-year or 50-year trend over the preceding millennium, but 30-year trends opposing the long term trend continue to intervene. We discuss the different markers of jet variability which has under anthropogenic forcing exited the envelope described by internal climate variability and past natural external forcing over the last millennium. We contextualize changes to jet variability with composite examples of surface conditions attending relevant NAJ extremes.