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Midlatitude atmospheric circulation is projected to shift poleward, yet the Northern Hemisphere jet shift is absent. Competing thermodynamic responses between tropical and Arctic warming have opposing influences on the jets and increase the uncertainties in future projections. This study shows, however, that sea surface temperature (SST) warming in the midlatitude is a major driver for the future midlatitude jet. Coupled Model Intercomparison Program phase 5 models indicate different SST warming between the midlatitude oceans, which induces a weakening of the North Pacific jet and a poleward shift of the North Atlantic jet. Our atmospheric model experiments enable to quantify the relative roles of Arctic, midlatitude, and tropical warming. The competing effects of midlatitude and tropical warming play a substantial role in the future midlatitude jet, hindering any poleward shift of the North Pacific jet, whereas for the North Atlantic jet, midlatitude SST warming is likely to win the competition.

Plain Language Summary Midlatitude weather and climate, including extreme events, are strongly influenced by changes in the jet stream and extratropical cyclones. The future midlatitude circulation in the Northern Hemisphere is considered to depend on the competing effects of tropical and Arctic warming, which have opposing influences on position and intensity of the midlatitude jet and, ultimately, increase the uncertainties in future projections. However, we find that ocean warming in the midlatitudes has a major influence on future midlatitude jet. In the North Pacific, sea surface temperature (SST) warming is the strongest in the north of the strong ocean currents, whereas the North Atlantic SST warming has a peak in the strong ocean currents. This different SST warming leads to the contrasting midlatitude jet responses between the oceans with jet cores. Despite a topic of much debate, the impact of Arctic sea ice loss on the jet is suppressed by midlatitude SST warming. We conclude that the competing effects of midlatitude and tropical SST warming will play a substantial role in the future midlatitude jet. The results here may help decrease the uncertainties in future projections and further our understanding of ongoing midlatitude climate change.