Discovery of Quasi‐Stationary Equatorial Waves Trapped in Stratospheric QBO Westerly and Easterly Jets

Abstract

AbstractQuasi‐stationary planetary Rossby waves forced in the extratropical troposphere propagate upwards into the stratospheric vortex and can also be refracted toward the equator. To the extent that such waves penetrate the low latitudes they will be observable as quasi‐stationary zonal eddies in the equatorial middle stratosphere. These eddies should be strongly modulated by the phase of the Quasi‐Biennial Oscillation (QBO) of the mean zonal wind. In this project we used global reanalysis data to examine the quasi‐stationary waves in the monthly mean wind and geopotential fields near the equator and their relation to the QBO mean winds and the extratropical planetary waves. We discovered quasi‐stationary mid‐stratospheric eddy circulations near the equator that often take the form of equatorial planetary modes. Notably a quasi‐stationary equatorial Rossby mode structure can develop during westerly QBO phases, especially when the westerly mean jet is almost symmetric about the equator and deep in the vertical (often in November). More surprisingly, during the easterly QBO phase (EQBO), a modest amplitude quasi‐stationary equatorial Kelvin mode structure develops. Stationary equatorial waves forced in the troposphere will encounter critical surfaces at one or more levels below the mid‐stratosphere, and so presumably cannot account for our findings. A linear, steady model in an equatorial channel that was forced by observed fields specified at extratropical lateral boundaries also cannot fully reproduce the equatorial mode structures, especially for the Kelvin wave during EQBO. Other factors such as transient wave‐mean flow interaction or some dynamic instability might be responsible.