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GFI/BCCR Seminar | Chris Weijenborg: Maintenance of baroclinicity by extratropical cyclones

Maintenance of baroclinicity by extratropical cyclones

Chris Weijenborg (University of Bergen, GFI, and Bjerknes centre of climate research)

Abstract
The positioning and maintenance of mid- and high-latitude baroclinicity and associated storm tracks is still not fully understood. In particular, the relative role of surface inhomogeneities, such as land-sea contrasts and fronts in sea surface temperature versus the role of diabatic and adiabatic processes in the free troposphere, are still debated. A seminal paper pointed to the self-maintenance of storm tracks via diabatic heating of the storms themselves, which would imply that the storms not only live off the baroclinicity, feeding from its available potential energy, but would actually also act to resupply baroclinicity for subsequent development. Using a recently introduced diagnostic analyzing the slope of isentropic surfaces and its tendency, the relative roles of diabatic and adiabatic effects associated with extratropical cyclones in maintaining baroclinicity in the lower and upper troposphere can be assessed. The presentation is twofold.

Firstly, I present a case study proving the concept and value of our diagnostic in interpreting baroclinic development. I will show that a particular sequence of storms culminating in a severe cyclone is due to the fact that the previous storms act to maintain or increase the background baroclinity along which the succeeding storms evolve. I separately quantify the diabatic and adiabatic effects and thereby assess their relative importance and discuss the buildup of the baroclinicity as well as the relative contributions to its tendency. In addition, the life cycle of these storms is discussed in terms of how the storm changes and uses its environment to attain its intensity. I will also compare the slope diagnostic to more traditional measures, such as baroclinic energy conversion, pinpointing advantages and caveats of the method.

Secondly, I present a global climatology of baroclinicity for summer and winter, measured by the slope of isentropic surfaces. As expected, the storm tracks are clearly visible in the mean baroclinicity, as well as their seasonal progression. Furthermore, the previously claimed self-maintenance of storm tracks is confirmed, with the diabatic tendencies balancing the adiabatic tendencies in the time mean, where the diabatic tendencies are mostly associated with extratropical cyclones and act to increase baroclinicity. In addition, certain differences between the Atlantic and Pacific storm tracks in terms of their maintenance by the aforementioned processes are highlighted. I will also contrast the two main storm tracks in the Northern Hemisphere to the storm track in the Southern Hemisphere. In addition, the potential role of sea surface temperature and land-sea contrasts in this framework for baroclinicity is discussed.