The Polar Stratosphere as an Arbiter of the Projected Tropical Versus Polar Tug of War

This study explores the tug of war between the effects of Arctic amplification (AA) and upper-troposphere tropical warming (UTW) on the response of the future North Atlantic atmospheric circulation. The late 21st century AA and UTW temperature anomalies are imposed in a high-top atmospheric model by nudging the temperature. Two sets of experiments are performed, with and without feedback of the polar stratosphere to highlight its role in the response to UTW, AA, and both combined. With interactive polar stratosphere, UTW forces an equatorward shift of the eddy-driven jet that reinforces the response to AA. However, when the polar stratosphere feedback is suppressed, the response to UTW is opposite and reflects the previously identified tug of war between the effects of UTW and AA in midlatitudes. This study highlights that the polar stratosphere is a key component for future changes in the North Atlantic atmospheric circulation and that it must be accurately represented in climate change scenarios. Plain Language Summary The jet streams are bands of strong westerly winds that drive weather patterns in midlatitudes. With climate change, they are expected to migrate poleward in association with an expansion of the tropical belt and amplified upper-level tropical warming (UTW). However, recent studies have pointed out that the fast warming in the Arctic (or Arctic Amplification, AA) may counteract this effect in winter. In this study we explore the respective and combined influence of UTW and AA by prescribing temperature anomalies in an atmospheric model that resolves the stratosphere. The focus is on the North Atlantic sector, where previous work had found a pronounced tug of war effect. In fall, in the absence of any significant response in the polar stratosphere, we retrieve well-established results, that is, a poleward shift of the midlatitude jet in response to UTW and an equatorward shift in response to AA. However, in winter UTW induces a robust weakening of the stratospheric polar vortex that shifts the jet equatorward and reinforces the effect of AA. Our study highlights that the polar stratosphere may significantly modulate the tropics-Arctic tug of war in the future, advocating for an accurate representation of stratospheric processes in climate models.