A climatology of mechanisms that generate intense extratropical cyclones in the Northern Hemisphere

Extratropical cyclones (ETCs) intensify due to three vertically interacting positive potential vorticity anomalies that are associated with warm temperature anomalies at the surface, condensational heating in the lower-level atmosphere, and stratospheric intrusion in the upper-level atmosphere. It remains unclear how much each mechanism contributes to ETC intensification, as results from case studies are conflicting and a climatological assessment has not yet been done. Such a climatology would be useful for identifying sources of ETC biases and uncertainties in global climate models (GCMs). To fill this gap, this study presents the first climatology of mechanisms that generate intense ETCs in the Northern Hemisphere for the period 1980 to 2016 (3273 ETCs). Using piecewise potential vorticity inversion, I show that the lower level contributes most to maximum ETC intensification (52%), followed by the upper level (26%), and the surface (22%). These values vary during the last 36 hours prior to maximum ETC intensity, with decreasing surface contributions (from 35% to 22%) and increasing upper-level contributions (from 13% to 26%). The dominance of the lower level applies to 74% of ETCs, followed by the upper level (18% of ETCs) and the surface (8% of ETCs). Upper-level contributions are stronger in eastern than in western ocean basins, while the opposite applies to surface and lower-level contributions. This is consistent with regional patterns of potential vorticity anomalies, which, as discussed, may be associated with Rossby wave breaking and western boundary currents. The ability of GCMs to reproduce the mechanisms quantified in this study remains to be assessed.

Bio: Christian is a research climatologist with the Pacific Climate Impacts Consortium (PCIC). His research focuses on the dynamics of coastal storms, how well they are reproduced by climate models, and how they are projected to change in the future. He joined PCIC in 2013 when he started working as a post-doctoral research fellow for the Marine Environmental Observation Prediction and Response Network. Christian is also an Adjunct Assistant Professor at the School of Earth and Ocean Sciences at the University of Victoria where he teaches a course on climate physics.