Abstract
The future response of the atmospheric circulation to increased anthropogenic forcing is uncertain, in particular due to competing influences of the large projected warming at the surface in the Arctic, and at upper-levels in the tropics. In the present study two ensembles of fully-coupled 21st century climate simulations are used to analyze changes in the wintertime eddy-driven jet in the North Atlantic and the relation to the well-defined thermal signatures of climate change. The models project a robust reinforcement of the eddy-driven jet and a decrease in waviness and blockings, that we attribute to a narrowing of the westerly flow in mid-latitudes. Composite analyses suggest that this signal is driven by the opposite influence of Arctic and tropical warming on each flank of the jet. We find that a significant portion of the multi-model spread in the jet metrics can be explained by the ratio between these two signals. The tug-of-war between the two effects influences by how much wintertime cold extremes diminish at the end of the 21st century. Models with dominant tropical warming (i. e. narrower and stronger eddy-driven jet) exhibit less decrease in cold extremes with climate change, due to the maintenance of cooler conditions in the subpolar North Atlantic and subarctic seas compared to models with a predominance of Arctic warming.
Highlights
How climate change will impact the densely populated areas in mid-latitudes depends in large measure on the response in the large-scale atmospheric circulation to increasing anthropogenic radiative forcing
In both Coupled Model Intercomparison Project phase 5 (CMIP5) and CESM-LENS, we identify a tripole of zonal wind anomalies in the lower troposphere, with decreased westerlies on both flanks of the jet, and increased westerlies at its core (figures 1(a), (b), (d) and (e))
It is only found in winter and it is more pronounced in OND in CMIP5, and JFM in CESM-LENS
Summary
How climate change will impact the densely populated areas in mid-latitudes depends in large measure on the response in the large-scale atmospheric circulation to increasing anthropogenic radiative forcing. The regional temperature response is inconsistent across the numerical studies, most agree on the response of the zonal-average circulation to a decline in Arctic sea ice, with weaker westerlies on the poleward flank of the eddy-driven jet and a southward shift of the jet/storm tracks. This response is robust in ocean-atmosphere coupled simulations with artificially-induced Arctic sea ice loss as projected at the end of the 21st century (Deser et al 2015, Blackport and Kushner 2017, Screen et al 2018)
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