Cold-season extreme baroclinic growth processes over the North Atlantic and North Pacific basins

It is well accepted that explosive oceanic cyclogenesis occurs preferentially during the cold season, and in the vicinity of the western North Pacific and the western North Atlantic basins, where the respective Kuroshio and Gulf Stream Currents are located.

What is not well understood are the processes that facilitate the clustering of such events into atmospheric regimes that often persist for several weeks. Examples of such clustering of rapid cyclogeneses include the multiplicity of North American coastal and oceanic cyclones that were associated with historically extreme cyclogenesis and snowfalls during the winters of 2013-14 and 2014-15.

To improve our understanding of these processes, we produce a climatological analysis of cold-season (January, February, March) Eady baroclinic growth rate maxima for the period from 1950 through February 2015. These growth rates represent a synthesis of the crucial dynamic and thermodynamic processes that may be associated with explosive surface cyclogenesis.

We find that the baroclinic growth rates are clustered in time, similar to the clustering associated with surface cyclogenesis.

The clustering of growth rates is associated with planetary-scale circulation anomalies that include an anomalously cold upstream polar vortex, and an anomalously strong downstream tropopause jet. We examine the life cycles of each of these two phenomena through an examination of the thermodynamic processes associated with the evolutions of the arctic and subtropical air masses that flank the tropopause jet.

The dynamics of the genesis, maintenance and dissipation of long-lived circulation regimes are discussed in the context of explosive cyclogenesis processes. Implications for intraseasonal variability, and longer-term global climate change are discussed.