Can Extratropical Cyclones Increase Baroclinicty? A Pathway to Cyclone Clustering

The existence of cyclone clustering, the succession of multiple cyclones in a short amount of time, indicates that the baroclinicity feeding these storms undergoes episodic cycles. With the generally accepted paradigm of baroclinic instability for extratropical cyclones, one would anticipate that clustering coincides with increased baroclinicity, though simultaneously individual cyclones reduce baroclinicity to maintain their growth. This apparent contradiction motivates our hypothesis that some cyclones increase baroclinicity, which could be a pathway for cyclone clustering.

Using a new cyclone clustering diagnostic based on spatio-temporal distance between cyclone tracks, we analyse cyclone clustering for the period 1979 until 2016. We complement this analysis with a baroclinity diagnostic, the slope of isentropic surfaces. With the isentropic slope and its tendencies, the relative roles of diabatic and adiabatic effects associated with extra-tropical cyclones in maintaining baroclinicity are assessed. We first present a case study, for which a sequence of cyclones culminated in severe cyclones due to the fact that one of the storms significantly increased the background baroclinity along which the succeeding storms evolved. The life cycle of these storms is discussed in terms of how the storm changes and uses its environment to attain its intensity. We compare these findings to composites of clustered and non-clustered cyclones to quantify how consistent the proposed clustering-mechanism is.