Abrupt changes in the extratropical circulation of the atmosphere: Dynamic vs. Thermodynamic Regimes


A virtual presentation this year. A pre-recorded talk plus 4 minutes of discussion at 11:30 pm, EST. O brave new world, with such meetings in it!

Abrupt changes in the atmospheric circulation in response to continuous forcing pose a threat in the context of climate change. Such regime transitions are possible in the tropics, as observed in the annual cycle of the monsoonal circulation, but are rapid changes of the climatological circulation possible in the extratropics as well? We investigate changes in the jet streams and associated storm tracks with a hierarchy of atmospheric models, focussing on the circulation response to the tropopause height, one of the most robust responses to global warming.

As well documented in the literature, dry dynamical cores – primitive equation solvers on the sphere, driven by an idealized forcing – exhibit abrupt regime shifts between a merged and split jet states over a critical range of the tropopause height. The merged jet state is similar to the observed North Pacific, where the eddy driven jet (characterized by surface westerlies) is co-located with the subtropical jet (characterized by upper level winds and the tropopause break). In the split jet state, the eddy driven jet pushes substantially farther poleward and separates from the subtropical jet, more similar to the observed state of the austral hemisphere.

More advanced models that account for radiative and moist processes, however, do not as readily simulate these abrupt reorganizations of the extratropical jet, suggesting that the regimes in dry dynamical cores may be an artifact of their idealized nature. We find that abrupt transitions are still possible, but they appear to be more critically linked to thermodynamic processes and coupling to the surface. To differentiate these regime transitions, we contrast the purely dynamic regimes transitions from transitions that fundamentally involve coupling with the thermodynamics. The key is a matter of time scale: dynamic transitions are set exclusively by the synoptic time scale of eddy momentum transport, while the coupled transitions additionally depend on the thermal inertia in the system.