LOW-FREQUENCY SYNOPTIC-EDDY ACTIVITY IN THE PACIFIC STORM TRACK

The nature of low-frequency variations in synoptic-eddy activity over the North Pacific is examined in a general circulation model (GCM). A comparison with observations reveals that the GCM produces realistic t ime mean and low-frequency synoptic-eddy forcing of the 200-mb zonal w ind. In the time mean, this forcing, which is computed as the divergen ce of the extended Eliassen-Palm (E-P) flux, shows an east-west dipole structure that tends to reduce the zonal wind over the western North Pacific and tends to enhance it to the east. This structure is consist ent with the general picture of the life cycle of baroclinic waves, wh ich show strong upward and eastward propagation in the western and cen tral Pacific and meridional propagation to the east. The western and c entral Pacific synoptic-eddy forcing is dominated by the convergence o f the baroclinic component of the E-P flux divergence, while over the eastern Pacific the divergence of the barotropic component is importan t. The dominant component of the low-frequency ''envelope'' (periods > 10 days) of synoptic-eddy forcing, computed as the first empirical or thogonal function (EOF) modulates the time mean synoptic-eddy forcing. This modulation is associated with low-frequency changes in the inten sity of the synoptic-eddy activity and is only weakly tied to fluctuat ions in the low-frequency flow. Composites of the hemispheric distribu tion of synoptic-eddy forcing in the GCM, based on the extremes of the dominant Pacific EOF, show a seesaw behavior with enhanced eddy forci ng in the North Pacific basin associated with suppressed forcing in th e North Atlantic basin, and vice versa. The link between the Pacific a nd the Atlantic basins appears to be due to the presence of eastward-t raveling baroclinic wave packets that travel around the globe with a p eriod of about 10 days. Some evidence is found for a similar behavior in the observations.