Response of the midlatitude jets and of their variability to increased greenhouse gases in the CMIP5 models

This work documents how the midlatitude, eddy-driven jets respond to climate change using output from 72 model integrations run for the Coupled Model Intercomparison Project, Phase 5 (CMIP5). We consider separately the North Atlantic, the North Pacific and the Southern Hemisphere jets. We do not limit our analysis to only the annual mean changes in the latitude and speed of the jets, but also explore how the daily variability of each jet changes with increased greenhouse gases. We demonstrate that the structure of the jet variability is a strong function of the jet position in all three sectors of the globe. For the Southern Hemisphere and the North Atlantic jets, the variability is less of a north-south wobbling (i.e. an `annular mode') for higher-latitude jets. In contrast, in the North Pacific, the jet variability is less of a pulsing and more of a north-south wobbling for jets at higher latitudes. In spite of these differences, we are able find a mechanism (based on Rossby wave breaking) that is able to explain many of the changes in jet variability within a single theoretical framework. Our findings suggest that: (1) GCM biases in the mean jet position may directly relate to biases in the GCM's atmospheric variability, given the dependence of the variability pattern on jet latitude; (2) since most GCMs have large equatorward jet biases in the North Atlantic and Southern Hemisphere, our results suggest that GCMs may also exhibit strong biases in their tropospheric variability; (3) some models project very large responses in the jet position (i.e. shifts of up to 5 degrees poleward), and so it is possible that the leading pattern of variability in the future may be different from the familiar north-south wobble of the jet.