The seasonality of the Northern Hemisphere storm tracks
Shaffrey, Leonard Christopher
PublisherUniversity of Reading
MetadataShow full item record
The aim of this thesis is to increase the understanding of the mechanisms that maintain the observed Northern Hemisphere storm tracks, and in particular their seasonality, by investigating the sensitivity and formation of storm tracks within a simple GCM (Global Circulation Model). This simple GCM maintains an observed zonal temperature field in the model by restoring the model back to an empirically determined zonal temperature field and includes simple representations of diabatic heating, orography and surface friction. The local orography and diabatic heating are prescribed individually for the Atlantic and Pacific storm tracks, and for two different seasons, winter and autumn. The model is capable of capturing some of the seasonality of the observed storm tracks, especially the seasonal change in magnitude. The sensitivity of the model is investigated and latent heating is seen to play a pivotal role in the maintenance of the storm tracks, not only by increasing the growth rates of individual baroclinic systems but also by enhancing the baroclinicity of the time-mean flow at the beginning of the storm tracks. The modelled Pacific storm track is stronger in the autumn than in winter, a consequence of the increase in the latent heating parameter, which in itself is due to warmer observed autumnal sea surface temperatures. The Atlantic storm track is weaker in autumn than in winter even though the latent heating parameter is increased. The autumn Atlantic storm track is located northwards of its winter position, and so subsequently there is an absence of latent heating over the Gulf of Mexico which is found in the winter Atlantic storm track. This extra latent heating strongly enhances the baroclinicity at the beginning of the winter storm track, as well as increasing the amount of latent heating in developing cyclones. The seasonal changes in the observed diabatic heating are examined and it is seen that the equinox diabatic heating is weak in the Atlantic storm track while the equinox Pacific diabatic heating remains strong relative to its wintertime value. Furthermore, the regions of cyclogenesis remain relatively stationary during the seasonal cycle of the Pacific storm track, while the equinox cyclogenesis in the Atlantic storm track moves northwards of its DJF position, implying that some of the changes seen in the GCM may occur in the real atmosphere. However, over the Pacific the equinox diabatic heating is weaker than in winter, suggesting a difference between model and observations and that the changes in diabatic heating cannot totally explain the observed seasonality of the Pacific storm tracks.