Composite Trough Evolution of Selected West Pacific Extratropical Cyclones

Abstract

The observed vertical structures of the trough axes for 27 extratropical cyclones are presented. This study is motivated by results from a simple theoretical model. Two observing times during the cyclone life cycle are shown: prior to development and during the ?mature? but still amplifying stage. Prior to development, upper and lower troughs are present and separate, each has little or no tilt, the upper one is typically prominent down to 4-km elevation, and the separation between the lower and the upper features varies depending on where the approaching upper trough happens to be at the observing time. At the mature stage, upper and lower features are connected, a uniform tilt typically develops through the entire troposphere, the tilt is typically due west with height, and the tilt may have a preferred slope. An empirical orthogonal function (EOF) analysis finds that two modes account for more than 97% of the variance. The equivalent barotropic EOF has the most variance by far, though the fractional amount diminishes over time as this EOF also extends further downward. The first baroclinic EOF increases fractional amplitude in compensation.