Comparison of wave packets associated with extratropical transition and winter cyclones

Abstract

Differences in the development of wave packets associated with midlatitude cyclones and the extratropical transition (ET) of tropical cyclones in the western North Pacific (WNP) and Atlantic basins are diagnosed observationally by compositing Climate Forecast System Reanalysis (CFSR) data over a 32-yr period and applying the null hypothesis of no difference in the development, structure, propagation, or downstream extent. While the development of midlatitude cyclones during the fall (August-October) and winter (November-March) amplifies a preexisting wave packet moving through the midlatitude storm track, ET is associated with the quasi-stationary amplification of the midlatitude flow. The ET cases involving the interaction of the decaying TC with a preexisting midlatitude trough are associated with a greater downstream amplitude and longer-lasting downstream response than ET cases that do not involve the interaction with a trough. In the WNP, ET wave packets have greater amplitude than those associated with winter midlatitude cyclones, but are similar to those associated with fall midlatitude cyclones. Moreover, ET events are associated with larger wavelengths and a statistically significant meridional wind anomaly farther downstream. By contrast, ET wave packets in the Atlantic basin have less amplitude and do not reach as far downstream as wave packets associated with fall and winter cyclones. WNP ET is characterized by larger integrated moisture flux convergence and, thus, latent heat release relative to its midlatitude counterpart, while Atlantic basin ET has smaller moisture flux convergence compared to midlatitude cyclones, which could explain why Atlantic ET is associated with less-amplified wave packets.