Further Observational Characteristics of Bimodal Planetary Waves: Mean Structure and Transitions

Abstract

Further results concerning the mean gates of the bimodal wavnumber 2 to 4 amplitude probability density distribution are presented followed by composites of transitions from one side of this distribution to the other. The data used are ECMWF analyses from the four winters from 1980/81 to 1983/84. Cross sections of the mean states associated with the two modes reveal that both modes exhibit a baroclinic vertical structure, but that the difference between the two is more nearly equivalent barotropic. The composite transitions between the low-amplitude and high-amplitude states indicate that the transition time for the onset or decay of the large amplitude waves is about 4 days. The kinetic energy and available potential energy of wavenumbers 2 to 4 increases (or decreases) by 50 percent in this same time interval during the onset (or decay) of the large amplitude state. Nonlinear interaction with intermediate-scale waves is the only apparent source for the observed kinetic energy tendency during the transition from the low amplitude to the high amplitude mode. Thus, the growth of the 1arge amplitude events does not strictly resemble that of a classical baroclinic instability. During the decay of the large amplitude waves, nonlinear interaction between the wavenumber 2 to 4 ensemble and wavenumber 1 accounts for the decline in kinetic energy, while nonlinear interaction between wavenumbers 2 to 4 and smaller-scale waves accounts for the decline in available potential energy. Examples of individual cases are presented to corroborate the composite results. Finally, a case study of the synoptic evolution of a large-amplitude event is presented to illustrate the event's life cycle.