Seasonal and intraseasonal variability of large-scale barotropic modes

The barotropic instability of time-dependent observed basic states that are periodic, with a period of 1 yr covering the complete annual cycle, is ana lyzed using Floquet theory. The time-dependent basic state is constructed f rom observed monthly averaged 300-mb streamfunction fields linearly interpo lated between the different months. The propagator over the 1-yr period is constructed, and its eigenvalues and some of the fastest-growing eigenvecto rs, termed finite-time normal modes (FTNMs), are calculated. The fast-growi ng FTNMs are large-scale modes with generally largest amplitudes in the Nor thern Hemisphere. They exhibit intraseasonal variability in their structure s, as well as longer period variations, and their amplification rates vary with time. The fastest-growing FTNM has its largest growth rate in early no rthern winter and its amplification has maximum cumulative effect in boreal spring when the equatorward penetration of this disturbance is also the la rgest. The other fast-growing FTNMs also have largest amplitudes during the first half of the year. In all months, there are fast-growing normal modes of the monthly averaged stationary basic states that have large pattern correlations with the faste st-growing FTNM for the time-dependent basic state. For some months the ind ividual normal modes experience dramatic local variations in growth rate; t hese bursts of relative growth and decay are associated with intramodal int erference effects between the eastward and westward propagating components of a single traveling normal mode. Both intramodal and intermodal interfere nce effects play significant roles in the evolution of the fastest-growing FTNM, particularly in boreal spring. The behavior of FTNM instabilities is also examined in simplified situation s including a semianalytical Floquet model in which the space and time depe ndencies of the stability matrix are separable. In this model, temporal var iations in growth rates are directly linked to seasonality in the intensity of the climatological state.