Differentiating average and extreme extra-tropical cyclones by their vertical tilt and potential vorticity structure

In this study, we compare the vertical tilt and potential vorticity (PV) structure of average and extreme extra-tropical cyclones (ETCs) in the Northern Hemisphere during the extended cold season (October--March) in ERA5 reanalysis. Enabled by a tracking method across pressure levels, the ETC vertical tilt is quantified using a novel two-component (directional and lateral) horizontal distance between the near-surface and upper-level ETC centres. The directional component of the tilt is parallel to the ETC travel direction at 850 hPa, while the lateral component is orthogonal to the ETC travel direction. During the ETCs life cycle, the typical tilt progresses from left and backwards leaning (negative directional and positive lateral tilt) to right and forward leaning (positive directional and negative lateral tilt). Compared to average ETCs, extreme ETCs exhibit an amplified tilt magnitude during the intensification phase and a change in the sign of the lateral tilt prior to the time of maximum relative vorticity.

Typically, studies that examine the vertical structure of PV in ETCs use the simplified approach of analysing the vertical profile of PV which is taken directly above the near-surface ETC location under the assumption the ETC is vertically aligned at the time of its maximum intensity, i.e. there is no vertical tilt. In our study, we challenge this assumption and demonstrates that neglecting the tilt of ETCs significantly underestimates the magnitude of potential vorticity associated with the ETC in the upper troposphere.

Furthermore, accounting for the tilted vertical structure, our study shows that average ETCs are associated with an upper-level positive PV anomaly, which is constant through the ETC lifecycle, while the PV intensifies from the bottom up. In contrast, extreme ETCs are characterised by an increasing magnitude of the upper-level PV anomaly, and a simultaneous intensification throughout the lower levels.