Sensitivity of Extratropical Cyclone Poleward Motion to Low-Level Potential Vorticity

Extratropical cyclones frequently exhibit pronounced poleward propagation during their life cycle. This behavior is typically associated with the poleward advection of a low-level PV anomaly by an upper-level PV anomaly located to its west, which can be enhanced by diabatic production of positive low- to mid-level PV (LPV) through latent heat release. In CMIP6 models, the storm tracks tend to be too zonal, particularly in the North Atlantic, and the frequency and intensity of rapidly deepening cyclones are often underestimated. Such biases may partly arise from misrepresentation of the magnitude of diabatic processes and/or from the dynamical response of cyclone propagation to those processes.

The aim of this study is to assess the contribution of latent heating to the poleward propagation of extratropical cyclones and to evaluate how both the magnitude of LPV and the associated dynamical response contribute to the storm track biases in CMIP6 models. Using ERA5 reanalysis and CMIP6 model data for the period 1979–2014, this study applies ensemble sensitivity analysis and cyclone composite methods to quantify the sensitivity of cyclone poleward propagation, measured by the cyclone meridional velocity at the time of maximum intensity, to LPV associated with latent heating. The analysis is conducted over the North Atlantic and North Pacific basins, considering both western and eastern sectors.

In ERA5, preliminary results show that North Atlantic cyclones have larger LPV than North Pacific cyclones throughout the entire development phase. Within the North Atlantic, although latent heating is stronger in western cyclones than in eastern ones, the sensitivity of poleward propagation to LPV is largest for eastern cyclones. In contrast, in the North Pacific, cyclones in the eastern sector show slightly stronger latent heating than those in the western sector. However, the sensitivity of poleward propagation to LPV is largest for western cyclones.

The CMIP6 models evaluated so far are able to capture the overall structure of LPV and the sensitivity of poleward motion to latent heating in extratropical cyclones across both oceans and sectors, as well as the differences between them. However, model resolution appears to impact the accuracy in representing the magnitude of these sensitivities, particularly for eastern North Atlantic cyclones. This may help explain the reduced storm track biases found in higher resolution CMIP6 models.

These results suggest that the poleward motion of western North Pacific and eastern North Atlantic cyclones is more strongly responsive to diabatic forcing via latent heat release, even though the magnitude of latent heating is smaller in those sectors. In contrast, western North Atlantic and eastern North Pacific cyclones appear to be more directly controlled by dry baroclinic processes. Finally, improving the representation of moist processes and LPV generation in climate models is essential for reducing biases in storm track orientation, cyclone intensity, and associated uncertainties in future climate projections.