GCM Simulations of Intraseasonal Variability in the Pacific/North American Region

Abstract

General circulation model (GCM) simulations of low-frequency variability with time scales of 20 to 70 days are analyzed for the Pacific sector during boreal winter. The GCM's leading mode in the upper-tropospheric zonal wind is associated with fluctuations of the East Asian jet; this mode resembles, in both structure and amplitude, the Pacific/North American (PNA) pattern found in the observations on these time scales. In both the model and observations the PNA anomaly is characterized by 1) a linear balance in the upper-tropospheric vorticity budget with no significant Rossby wave source in the tropics, 2) a barotropic conversion of kinetic energy from the time mean Pacific jet, and 3) a north/south displacement of the Pacific storm track. In the GCM, the latter is associated with synoptic eddy heat flux and latent heat anomalies that appear to contribute to a strong lower-tropospheric source of wave activity over the North Pacific. This is in contrast to the observations, which show only a weak source of wave activity in this region. The GCM produces 60% of the total observed Pacific sector low-frequency zonal wind variance. About one-third of the missing variability appears to be due to unrealistic simulations of the Madden-Julian oscillation; the remainder is characterized in the variance spectrum as a deficit in the overall level of ?background? variability. The nature of this missing background variability is unclear.