Synoptic-Scale Disturbances near the Equator

Abstract

A spectral analysis of winds analyzed and initialized at the European Centre for Medium-Range Weather Forecasts reveals an abundance of power in the 850 mb meridional wind along the equator with periods near four days. The power is mostly in the westward propagating component. Using high-pass filtered data it is shown that the waves have westward phase and eastward group propagation relative to the mean wind. The longest wavelengths are found over the Pacific Ocean, while the shortest are found over the convectively variable regions of Indonesia, South America, and Africa. Mean phase speeds at 850 mb are positively correlated with the mean wind on the equator at 500 mb and below, and negatively correlated with the mean wind above that level. The effective advecting zonal wind of the disturbances seems to be the density weighted average of the lower troposphere. The structure of the disturbances bears resemblance to the expected structure of an equatorially trapped mixed Rossby-gravity wave over the central Pacific and Atlantic oceans, although the anomalies, while statistically significant, are extremely small. The outgoing longwave radiation (OLR) pattern is consistent with the flow field, suggesting that the waves are not merely a model artifact. Over the Atlantic there is a mode well defined by the zonal wind at the equator, but the OLR pattern is not consistent. Over the far western Pacific, there is evidence of meridional propagation from Northern Hemisphere midlatitudes. North of the equator there is meridional propagation at every longitude. The strongest disturbances are primarily confined to the lower half of the troposphere, but at many longitudes there is evidence of a weak first baroclinic-mode structure within the troposphere. North of the equator the structures are barotropic. Effective equivalent depths are estimated by comparing dispersion characteristics with mixed Rossby-gravity dispersion curves. Where the assumption of a mixed Rossby-gravity mode is believed to be valid, the equivalent depths are found empirically to lie between 1?60 m.