INTRASEASONAL OSCILLATIONS IN A BAROTROPIC MODEL WITH ANNUAL CYCLE, AND THEIR PREDICTABILITY

Observational and modeling studies have shown that intraseasonal, 40-d ay oscillations over the Northern Hemisphere extratropics are stronges t around the winter season. To explore intraseasonal variability in th e presence of the annual cycle, an eigenanalysis method based on Floqu et theory is used. This approach helps us determine the stability of t he large-scale, midlatitude atmospheric how's periodic basic state. It gives information about the growth rate of the unstable, intraseasona l eigenmode and confirms the atmosphere's preference for intraseasonal activity during the winter months, as the annual cycle modulates the eigenvector field. This eigenmode solution, furthermore, provides a ba sis for making extended-range (40-day) streamfunction-anomaly forecast s on a set of intraseasonal oscillations whose amplitude and phase dep end on the season. A simple autoregressive model is developed to shed light on the seasonal dependence of predictive skill for the intraseas onal signal.