INTRASEASONAL OSCILLATIONS OVER THE ATLANTIC

Winds at low (near-surface) and 200-mb levels from National Hurricane Center objective analyses are used to elucidate the structure and dyna mics of the tropical and subtropical intraseasonal oscillations for th e North Atlantic/northeast Pacific regions, including over the contine nts, for the years 1980-1989. The intraseasonal oscillations are broke n into three bands, with long (50-85 day), intermediate (30-55 day), a nd short (13-29 day) periods. Winter and summer seasons are analyzed s eparately. A complex empirical orthogonal function technique is used t o derive the dominant modes of intraseasonal variability over the regi on, including their propagation characteristics. Statistically distinc t modes of variability are found only during the winter and only for t he long-period and short-period bands. The dominant mode of coupled 20 0-mb low-level long-period variability during winter has a dipole stru cture. It has a substantial equivalent barotropic component in the sub tropics, as well as a baroclinic structure consistent with quasigeostr ophic midlatitude systems. Negative outgoing longwave radiation anomal ies tend to be in phase with a low-level convergence-upper-level diver gence couplet, which lies approximately one-quarter wavelength to the east of the cyclonic vorticity centers. The long-period oscillations d uring 1981-1988 are dominated by three events, with periods between ab out 60 and 70 days. There is a negative correlation, explaining about 50% of the variance, between the magnitude of the mode and an index of El Nino based on sea surface temperatures in the eastern equatorial P acific. The dominant modes of short-period variability during winter a ppear as zonally oriented wave trains similar to those found by previo us investigators of global-scale fluctuations. Rotation of the modes o f 200-mb variability effectively separates them into their propagating and standing components. Approximately one-half of the variance in th e meridional wind near teleconnection centers of action is found in th e eastward propagating component. The dominant mode of coupled 200-mb/ low-level variability propagates to the east, and has a vertical struc ture similar to that in the long-period band. It has a predominant per iod near 18 days.