| Abstract: |
Climate models project that the Arctic Ocean could see an ice‐free summer by the middle of this century. Through coordinated simulations, the Polar Amplification Model Intercomparison Project (PAMIP) aims to elucidate the causes and consequences of future Arctic sea‐ice loss. There is particular interest in understanding the mechanisms by which midlatitude weather and climate may be impacted, including via the "stratospheric pathway"; Arctic sea‐ice loss and associated warming is proposed to induce a wave‐driven weakening of the wintertime stratospheric polar vortex, which could subsequently impact tropospheric circulation. However, this is not well understood: studies do not so far find a robust stratospheric response to Arctic sea‐ice loss in either strength or sign. Here, we conduct novel analysis of the stratospheric response in thirteen PAMIP‐contributing models, looking beyond the typical time‐ and zonal‐mean diagnostics. Although our results overall confirm the lack of robust response, one model, HadGEM3‐GC31‐MM, has a statistically significant equatorward shift in vortex latitude, deceleration of vortex winds, and increase in sudden stratospheric warmings. Its response is found to be highly state‐dependent, significant only in the easterly phase of the quasi‐biennial oscillation (QBO). Though we cannot comprehensively conclude why models simulate this range of responses, our analysis does highlight areas for consideration in future work to better constrain the stratospheric response to Arctic sea‐ice loss. We explore the role of ensemble size, resolution and basic state, including zonal‐mean winds in the polar and midlatitude stratosphere and upper troposphere, as well as the QBO. Plain Language Summary: Since the 1980s, the Arctic has warmed much faster than the rest of the world and summertime sea‐ice has halved. This dramatic change is thought to have far‐reaching impacts. It may, for example, affect the jet stream and the weather systems it carries over northern America, Asia and Europe. One suggested pathway for impacts goes via the stratosphere: Arctic change could trigger atmospheric disturbances that disrupt the typically strong and stable band of winds circling the north pole in the winter, known as the "stratospheric polar vortex." We use climate models to study changes in the characteristics and behavior of the vortex in response to possible future Arctic sea‐ice loss. We find no consistent change in the average state nor variability of the vortex across models. However, one model does show a significant weakening and shift of the vortex. Though it is tricky to unpick why certain models differ from the others, our findings highlight that future studies into the response to Arctic sea‐ice loss may benefit from careful consideration of model resolution, ensemble size, and representation of the stratosphere's basic state, including the polar vortex, tropical lower stratosphere, and midlatitude upper troposphere/lower stratosphere winds. Key Points: The stratospheric polar vortex response to Arctic sea‐ice loss is weak and not statistically significant in most modelsDiverse stratospheric responses across models arise partly due to differences in the models' simulated basic statesHadGEM3‐GC31‐MM, with a realistic basic state, simulates a robust weakening and shift of the polar vortex, with more frequent sudden warmings [ABSTRACT FROM AUTHOR] |