The poleward motion of extratropical cyclones from a PV tendency analysis

The direction of midlatitude storm tracks impacts significantly midlatitude climate, as storms are the main distributers of heat and precipitation in the extratropics. The poleward tendency of extratropical cyclones and the corresponding downstream poleward tilt that characterizes the Pacific and Atlantic storm tracks is studied from a potential vorticity (PV) perspective. A tracking algorithm is combined with an idealized zonally symmetric moist GCM, to perform a detailed PV tendency analysis of cyclone composites. This reveals the important role of the upper level PV and diabatic heating associated with latent heat release (LHR) in the poleward motion of the cyclones. During the growth stage a classic picture of baroclinic instability emerges, with an upper level PV to the west of a low level cyclone. This configuration promotes, in addition to intensification, a poleward tendency that results from the nonlinear advection of the surface anomaly by the upper level PV. The PV analysis also emphasizes the important role of LHR, which tends to maximize on the northeastern side of the cyclone, where warm and moist air ascents, cools and condenses. As the air condenses it releases latent heat, which strengthens the PV perturbation at lower levels. Hence a strong positive PV tendency that acts to propagate the anomaly eastward and poleward is produced at lower levels. We explore the sensitivity of the poleward tendency to variations in atmospheric moisture as well as to increasing global mean surface temperatures, which has important implications in a changing climate. Finally, the differences between cyclones and anticyclones are investigated by performing a similar PV tendency analysis for anticyclone composites. The latter are characterized by a slight equatorward tendency, consistent with both equatorward winds induced from the upper level PV, and the lack of LHR in regions of descending air associated with anticyclones.