Revisiting the wintertime emergent constraint of the Southern Hemispheric midlatitude jet response to global warming

Abstract. Most climate models show a poleward shift of the southern hemispheric jet in response to climate change, but the inter-model spread is large. In an attempt to constrain future jet responses, past studies have identified an emergent constraint between the climatological jet latitude and the future jet shift in austral winter. However, we show that the emergent constraint only arises in the zonal mean, and not in separate halves of the hemisphere. This can be explained by the presence of a double jet structure in the Pacific region, making the zonal mean jet latitude a poorly defined quantity that does not represent the latitude of a zonally coherent structure during this season. The usefulness of the emergent constraint is therefore questionable. This finding can further explain the prior finding among CMIP5 and CMIP6 ensembles that the meridional structure of the zonal-mean zonal wind response does not change with climatological jet latitude but stays fixed.