The future North Atlantic jet stream and storm track: relative contributions from sea ice and sea surface temperature changes

Abstract. Using a novel set of coordinated simulations with four different models, the response of the wintertime (December–February) North Atlantic jet stream and storm track to prescribed sea surface temperatures and sea-ice loss is analysed. Three out of the four models show a southward shift of the upper-level jet stream with an increase in jet speed over Europe, where the contribution of sea surface temperatures dominates over the effects of sea-ice loss. However, the remaining model lacks the increase in jet speed over Europe, which originates from opposite responses of similar magnitude due to the future sea surface temperatures and sea-ice cover. The jet stream responses are primarily driven by the change in the meridional temperature gradient and, as a consequence, baroclinicity. At the same time, momentum flux convergence acts as a secondary amplifying and dampening factor. The same three models see a significant eastward shift of the extratropical cyclone track density, which is equally driven by changes to sea surface temperatures and sea ice cover. A consistent feature across all models is a decrease in the frequency of extratropical cyclones in the Mediterranean. The responses of extratropical cyclones to future sea-ice cover and sea surface temperatures do not exceed the inter-model climatological differences. Notable differences in the future response occur, and thus considerable uncertainty remains in how the European climate will respond to a warmer climate.