Influence of the stratospheric QBO on ENSO variability

Abstract

A hypothetical mechanism is described whereby the Quasi-Biennial Oscillation (QBO) of Zonal winds in the equatorial stratosphere actively modulates the timing of El Nino-Southern Oscillation (ENSO) events. The mechanism involves the meridional redistribution of deep convective activity throughout the tropical Pacific warm pool region in response to variable wind shear processes which are linked to the opposing phases of the QBO. Hydrostatic conditions favoring deep convective activity within approximately ±7°of the equator develop in response to the easterly shear phase of the QBO. At the same time, deep convection is inhibited in the monsoon-convergence zones farther off the equator (8-18° latitude) during the east phase of the QBO. The opposite trends occur during the westerly shear phase of the QBO wherein deep equatorial convection is suppressed while off equator monsoon convection is enhanced. It is shown that during the east phase of the QBO, the Pacific regional pressure and circulation anomalies which arise in response to QBO-linked trends in convective activity are consistent with conditions leading to warm ENSO events (i.e., El Nino). If the heat content of the warm pool is sufficient, a warm event will occur. Conversely, conditions favoring the development of cold (or La Nina) events tend to occur in association with the westerly phase of the QBO. Although several aspects of the hypothetical mechanism remain tentative, extensive empirical results present a compelling argument for the QBO as an active and fundamental component of ENSO variability