Intraseasonal Oscillations over the Atlantic

Abstract

Winds at low (near-surface) and 200-mb levels from National Hurricane Center objective analyses are used to elucidate the structure and dynamics of the tropical and subtropical intraseasonal oscillations for the North Atlantic/northeast Pacific regions, including over the continents, for the years 1980?1989. The intraseasonal oscillations are broken into three bands, with long (50?85 day), intermediate (30?55 day), and short (13?29 day) periods. Winter and summer seasons are analyzed separately. A complex empirical orthogonal function technique is used to derive the dominant modes of intraseasonal variability over the region, including their propagation characteristics. Statistically distinct modes of variability are found only during the winter and only for the long-period and short-period bands. The dominant mode of coupled 200-mb low-level long-period variability during winter has a dipole structure. It has a substantial equivalent barotropic component in the subtropics, as well as a baroclinic structure consistent with quasigeostrophic midlatitude systems. Negative outgoing longwave radiation anomalies tend to be in phase with a low-level convergence-upper-level divergence couplet, which lies approximately one-quarter wavelength to the east of the cyclonic vorticity centers. The long-period oscillations during 1981?1988 are dominated by three events, with periods between about 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 Niño based on sea surface temperatures in the eastern equatorial Pacific. The dominant modes of short-period variability during winter appear as zonally oriented wave trains similar to those found by previous investigators of global-scale fluctuations. Rotation of the modes of 200-mb variability effectively separates them into their propagating and standing components. Approximately one-half of the variance in the meridional wind near teleconnection centers of action is found in the eastward propagating component. The dominant mode of coupled 200-mb/iow-level variability propagates to the east, and has a vertical structure similar to that in the long-period band. It has a predominant period near 18 days.