Revisiting the relationship between jet position, forced response, and annular mode variability in the southern midlatitudes

Climate models exhibit a wide range in latitudinal position of the Southern Hemisphere westerly jet. Previous work has demonstrated, in the annual mean, that models with lower latitude jets, exhibit greater poleward jet shifts under climate forcings. It has been argued that this behavior is due to stronger eddy/mean flow feedbacks in models with lower latitude jets, as inferred from the timescale of the Southern Annular Mode (SAM). Here we revisit this question with a focus on seasonality. Using a larger set of models and forcing scenarios from the Coupled Model Intercomparison Project, phase 5, we find that the jet position/jet shift relationship is strong in winter but insignificant in summer, whereas the model spread in SAM timescales arises primarily in summer, with winter timescales similar across models. The results, therefore, question previous interpretations and motivate an improved understanding of the spread in model behavior. Key Points The SH jet position/jet shift correlation is highly seasonal in CMIP5 models It is strong in JJA but weak in DJF, opposite to the spread in SAM timescales Proposed theories for the jet position/jet shift relationship appear inadequate to explain this seasonality