Comparison of a Barotropic Blocking Theory with Observation

Abstract

In an effort to explain observed blocking phenomena, the work of Charney and DeVore (1979) and Hart (1979) has been extended to incorporate observed zonal topography in a barotropic nonlinear channel model. Multiple stationary equilibria are obtained, one of which, for an appropriate forcing, corresponds exactly to the ?normal? winter flow predicted by Charney and Eliassen (1949) from the linearized version of the model. When this forcing is applied in the nonlinear model, other equilibria, related to resonances with the wevenumber 2 and 3 Fourier components of the zonal topography, occur. Wavenumber 1 and 4 resonances could also have occurred with slight modifications of the model. For comparison with observation, semi-objective criteria are adopted for identifying blocking events from daily 500 mb observations of 15 consecutive winter seasons. Following Dole (1979), we demand that there exist sufficiently large geopotential height anomalies for a sufficient length of time. Numerical values of the anomaly and duration criteria are determined from physical characteristics of observed blocks. Altogether, 34 blocking events were found by this process, and the hemispheric patterns associated with 19 of these appear to be explainable qualitatively as one or another of the calculated equilibria. Five of the remaining blocking events might also have been explained if the forcing and geometry were some-what altered. What is not explained is the localized character of the blocking ridge (or trough) and the mechanism of transition to and from a blocking configuration. The failure to explain the localized properties is attributed in part to the exclusion of longitudinal variations of forcing and dissipation and in part to limitations on north-south structure in the model. It is suggested that the generation and decay of blocks may occur by changes of external factors driving the flow closer to or farther from topographic resonance, or by strong, large-scale cyclonic development. Another possibility is that when the flow is driven into a superresonant configuration, form-drag instability may transform it either into a subresonant blocking configuration or a nonblocking configuration.