Title:
Maintenance of Baroclinicity by Extratropical Cyclones
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, we assess the relative roles of diabatic and adiabatic effects associated with extratropical cyclones in maintaining baroclinicity in the lower and upper troposphere.
We present a case study proving the concept and value of our diagnostic in interpreting baroclinic development. We 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. In addition, we 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.