Barotropic Wave Propagation and Instability, and Atmospheric Teleconnection Patterns

A global barotropic model, linearized about the 300 mb climatological mean January flow, is perturbed by applying a series of localized forcings distributed throughout the tropics and subtropics. Structures which resemble the observed “Pacific/North American” and “East Atlantic” teleconnection patterns noted by Wallace and Gutzler (1981) tend to recur in the responses. Similar patterns are found to result from the dispersion of isolated initial perturbations placed at a variety of locations in the tropics and midlatitudes. It is shown that these structures are related to the most rapidly growing mode associated with barotropic instability of the zonally-varying climatological basic state. In the absence of damping, this mode has an e-folding time of about a week and a period close to 50 days. In localized regions the instantaneous growth rates can be competitive with those of baroclinic instability. These episodes of rapid local barotropic growth are interspersed with intervals in which the local perturbation relaxes as energy disperses throughout the hemisphere. Some of the less unstable modes exhibit a similar structure and time evolution, while others are spatially fixed and grow exponentially in time, without any periodic modulation. The dominant processes by which the growing disturbances extract kinetic energy from the basic state can be described in terms of two conversion terms: −u′v′∂ū/∂y and (v′2−u′2)∂ū/∂x, where u and v are the velocity components in the zonal (x) and meridional (y) directions, respectively, primes refer to the perturbations, and overbars to the basic state. In the fastest growing mode the dominant process contributing to the growth is found to be the second term, which is largest in the Pacific jet exit region where u′2 ≫ v′2. Nonlinear initial value calculations for weak damping rates exhibit strong low-frequency oscillations similar in structure and period to those of the most unstable mode. Nonlinear forced solutions show some sensitivity to the polarity of the forcing and they tend to show a larger response in the time average over the first month of the integration than in longer term means. Sensitivity to model resolution, dissipation, and choice of basic state is illustrated by means of selected experiments. On the basis of these results and recent work by Hoskins et al. (1983), it is suggested that much of the low-frequency variability of the Northern Hemisphere wintertime general circulation is associated with disturbances which derive their energy from the basic state through barotropic instability.