Horizontal and vertical structure of observed tropospheric equatorial Rossby waves

Equatorial Rossby (ER) waves are identified in an 8‐year data set of National Meteorological Center operational analyses. The westward moving waves have symmetric circulations about the equator and eastward energy dispersion, with maximum zonal wind perturbations along the equator and meridional wind maxima roughly 10°–15° off the equator. They have maximum amplitude in the lower troposphere and are associated with convective signals at roughly the mean latitude of the tropical convergence zones. The circulations thus possess many of the features of the equatorially trapped n = 1 Rossby modes derived by Matsuno 1966. They are generally around zonal planetary wavenumber 6 scale and have a deep, nearly equivalent barotropic structure up to 100 mbar. This is in contrast with the Madden‐Julian Oscillation and mixed Rossby gravity waves, which generally have a first baroclinic mode vertical structure. During the intensive observing period (IOP) of the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) and in other years, westward propagating ER disturbances along the equator are observed by using 6‐ to 30‐day filtered circulation data at 850 mbar. Zonal wind anomalies at 850 mbar are preferentially associated with like‐signed anomalies in a deep layer at this frequency, as was observed during COARE. The ER waves are most common during northern winter in the central Pacific, although they can be detected sporadically over the other ocean basins and in other seasons as well. One source for the equatorial Rossby waves appears to be upper level extratropical Rossby wave activity propagating into the eastern Pacific, although the equatorial waves are sometimes present during the northern summer when the extratropical source is absent. We cite evidence that propagation of ER wave activity from the central into the western Pacific is related to occurrences of westerly wind bursts.