Abstract/Summary

Extratropical cyclones (ETCs) are three-dimensional synoptic systems in mid- and high latitudes. Previous studies on ETC propagation have typically focused on cyclones identified at a single level. In this work, we study the movement of wintertime extratropical cyclones by classifying North Pacific ETCs into deep cyclones, shallow low-level cyclones and shallow upper-level cyclones. By tracking the cyclones at different vertical levels, we identify different characteristics and mechanisms for the movement of deep and shallow ETCs from a Lagrangian potential vorticity (PV) perspective. A PV tendency analysis of cyclone-tracking composites reveals that for deep cyclones, the diabatic heating at 850 hPa and the horizontal advection by the stationary flow at 500 hPa are the main contributors to the poleward movement. For shallow cyclones, the nonlinear advection terms play a dominant role in their meridional motion, advecting shallow low-level cyclones poleward but shallow upper-level cyclones equatorward. A piecewise PV inversion analysis suggests that the nonlinear advection by winds induced from upper-level PV anomalies is responsible for the different performance of nonlinear advection terms for shallow low-level and upper-level cyclones.