The north-south symmetry of the low level near-equatorial atmospheric flow is examined over the equatorial oceans with the aid of conventional and satellite observations and is compared to output of a climate model. The qualitative agreement between the observations and the simulations at low levels is very good. For the month to seasonal time scale during boreal summer, the criterion for symmetric stability of the mean atmospheric flow is met only in the Central Pacific. In order to explain the instability apparent in other regions and to extend the results to additional vertical levels, the climate model simulation results are used to investigate the two-dimensional, diabatic, frictional linear stability problem as formulated by Stevens (1983). It is apparent that the criterion for symmetric instability is met throughout a deep layer of the equatorial troposphere. The low level mean flow is unstable north of the equator and the flow aloft is unstable south of the equator. Two possible processes that can maintain the basic state against the apparent symmetric instability in the equatorial low-level troposphere are discussed.
A UNIX-style compressed Postscript file (728K) for the paper text , figures 1-3 , and figure 4 .