The Maintenance of Low-Frequency Atmospheric Anomalies

A linear budget is used to ascertain which of several proposed processes are most important in maintaining four prominent low-frequency perturbation patterns in a perpetual January general circulation model simulation with fixed ocean temperatures. In the budget technique the time-average model equations are separated into those terms that are in common with the steady, adiabatic linearized form of the model equations and the remaining terms, which are treated as forcing terms. By examining the linear response to each of these forcing terms for prominent episodes of the low-frequency patterns, the terms that are most important in maintaining the low-frequency patterns can be determined.The results indicate that interactions between the low-frequency patterns and the time-mean zonal asymmetries of the model climate are crucial to the maintenance of the patterns. Of equal importance are anomalous fluxes from transients, without which the low-frequency anomalies would not be maintained. Vorticity fluxes due to bandpass (1–7 days) fluctuations ranging over broad sectors of the globe are found to be the most important components of the maintaining flux anomalies. Of secondary importance are nonlinear interactions of the low-frequency deviations with themselves. For some patterns this interaction acts to skew the distribution of observed amplitudes. It is also found that the influence of the zonal-mean component of low-frequency perturbations on the remaining low-frequency perturbation components can be appreciable.Charts of the ability of point sources of heat and vorticity to excite the low-frequency patterns are used to interpret the budgets. These plots indicate that, in a system with time-dependent ocean temperatures, diabatic anomalies could be more instrumental in maintaining low-frequency anomalies than they are in the currently studied model. Midlatitude ocean temperature appears to be especially important in this regard. However, the collocation of regions with pronounced low-frequency anomalous vorticity fluxes from high-frequency transients and regions from which the low-frequency patterns are easily stimulated means that even if varying bottom boundary conditions are present, low-frequency maintenance by transients should continue to be important. The point source results also show that the maintaining transients are not configured optimally for forcing the low-frequency patterns. This indicates that some organizing mechanism must be affecting the maintaining transients.