Ryan D. Torn1 and Gregory J. Hakim2,
Monthly Weather Review, 141, submitted. (full paper)
1Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY
2Department of Atmospheric Sciences, University of Washington,Seattle, WA
Developing wave packets associated with the extratropical transition (ET) of tropical cyclones and winter cyclones in the Western North Pacific (WNP) and Atlantic basins are diagnosed observationally by compositing reanalysis data over a 32 year period. While the development of winter cyclones amplify a weaker wave packet moving through the midlatitude storm track, there is no indication of an upstream disturbance during ET; thus, on average, the wave packet is generated by the process of ET. In the WNP, ET and winter cyclone wave packets have comparable group velocity and amplitude relative to climatology; however, ET wave packets are characterized by longer wavelength and have a detectable signal further downstream. By contrast, the wave packets associated with winter cyclones in the Atlantic basin have greater amplitude and have a detectable signal further downstream relative to those associated with ET. Near the surface, winter cyclones are characterized by larger horizontal heat fluxes relative to their ET counterparts. WNP ET cyclones are characterized by larger horizontal moisture flux convergence and thus latent heat release relative to their winter counterparts, while Atlantic basin ET and winter cyclones have similar moisture flux convergence. This result suggests that the wave packets associated with ET cyclones are related to diabatic processes and could explain why the amplitude of Atlantic basin ET wave packets are smaller than winter cyclones. Finally, the greater baroclinicity present in the winter does not seem to influence the relative amplitude wave packets in either basin.