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

Abstract

The monthly-mean atmospheric circulation in most atmospheric regions includes significant monsoonal flows and other quasi-stationary eddies ultimately caused by the geographic irregularities at the atmosphere's lower boundary. The equatorial stratosphere has been regarded as exceptional in this regard since the prevailing wind is zonally-symmetric, at least to first order. Indeed the stratospheric Quasi-Biennial Oscillation (QBO) has sometimes been treated approximately as a zonally-symmetric phenomenon (see Baldwin et al., 2001 for a review). The significant isolation of the equatorial middle stratosphere (e.g., 10 hPa level, which is the focus of this study) from the lower boundary effects is understandable, since at least in a linear theory, stationary equatorial waves forced in the tropical troposphere should not propagate upward far into the stratosphere as they will generally encounter a critical surface (or a layer of weak mean winds) near the tropopause and then a critical surface in a QBO transition shear layer (e.g., Andrews & McIntyre, 1976;Booker & Bretherton, 1967;Lindzen, 1970). In addition, the Walker circulations that are a prominent feature of the equatorial troposphere are found to decay rapidly in the lowermost equatorial stratosphere.