Weather Regimes in a General Circulation Model

Abstract

Perpetual January and July simulations each 1200 days long of the NCAR Community Climate Model (CCM0B) are investigated for the existence of large scale, midlatitude weather regimes. Four realizations of the midlatitude circulation were considered: Northern Hemisphere (NH) winter, Northern Hemisphere summer, Southern Hemisphere (SH) winter, and Southern Hemisphere summer. Statistically significant bimodality appears in the planetary-wave amplitude probability density distributions in the former three cases that is very similar to that observed in the atmosphere. The probability density estimation for SH summer in the model is also similar to observations in general, but a hint of a second mode also appeared on the high amplitude tail of the distribution. The fact that the bimodality is present in a fixed external forcing simulation implies that it is not connected to changes in boundary conditions, but rather that it is internally generated. The statistical flow regimes in physical space identified by the bimodal distributions are generally similar in the model to those in the atmosphere for NH winter and SH winter. Systematic errors in the model during NH summer preclude close comparisons to observations. Interestingly, the model's SH winter wavenumber 3 large-amplitude mode also shows bimodality in its phase. Partitioning the SH model days based on the amplitude and phase bimodality identifies three hemispheric-scale flow regimes in the SH during the perpetual July simulation. As with the observations, the time-mean circulation is not generally realized as a persistent weather regime in any of the bimodal CCM simulations.