Model Diversity in Future Predicted Circulation Change in the Tropical and Extra-Tropical Pacific, and Their Connection

During Northern Hemisphere winter, there is considerable diversity in both the tropical and extra-tropical circulation changes that occur, under anthropogenic forcing, in the CMIP5 models. The multi-model mean shows that the extra-tropical westerly jet in the north west Pacific shifts poleward, and the tropical Pacific Walker circulation weakens in response to warming. However, this is an average over rather diverse behavior in both the tropics and extra-tropics with a strong connection between the modelled response in those two regions. Those models that exhibit a poleward shifting of the extra-tropical westerlies also exhibit a strong weakening of the Walker circulation whereas those that don't, exhibit very little change in the Walker circulation. In addition, there is a strong connection between a models present day climatological circulation and how it responds in the future, giving rise to a possible constraint on the real world response. The reasons behind these tropical/extra-tropical connections and their links to the present day climatological circulation will be assessed through idealized experiments. While the coupling between the tropics and the extra-tropics likely goes both ways, these idealized simulations suggest that the extra-tropics are ultimately the driving force behind a models overall response. Further experiments will be discussed that aim to understand the links between a models representation of the climatological circulation and how it responds in the future.