QBO/Solar Influences on the Tropical Madden-Julian Oscillation: A Mechanism Based on Extratropical Wave Forcing in Late Fall and Early Winter

Possible sources of the observed modulation of the tropical Madden-Julian oscillation (MJO) by the stratospheric quasi-biennial oscillation (QBO) and the 11-year solar cycle are investigated using 41 years of reanalysis data and archived climate model data. Larger upward fluxes of extratropical planetary-scale waves, leading in some cases to sudden stratospheric warmings (SSWs), are observed in late fall and early winter during the easterly phase of the QBO than during the westerly phase (the "Holton-Tan effect"). A similar but smaller increase occurs, on average, during solar minima relative to solar maxima. In addition to the warming at high latitudes, extratropical wave forcing events produce cooling and reduced static stability in the tropical lower stratosphere. Here, it is found that if SSWs occur in early winter (before similar to mid-January), the reduced static stability produces, on average, a statistically significant, lagged strengthening of the MJO. This therefore represents a possible mechanism for producing, or at least enhancing, the observed QBO and solar modulations of the MJO in boreal winter. An initial analysis of archived climate model data shows that at least one model version with realistic QBO and solar forcing and with 4 x CO2 forcings partly simulates both of these characteristics (QBO/solar modulation of early winter wave forcing and lagged strengthening of the MJO following early winter SSWs). However, the modeled MJO is insufficiently sensitive to QBO-induced static stability reductions, precluding simulation of the QBO-MJO connection. Statistical studies have shown that upper atmospheric conditions significantly modulate the occurrence rate and amplitude of the wintertime Madden-Julian oscillation (MJO), which is a tropical convective disturbance with important consequences for weather events at northern latitudes. Specifically, easterly and westerly winds in the tropical and subtropical stratosphere driven by the quasi-biennial oscillation (QBO) and the 11-year solar cycle influence the amplitude of the MJO in northern winter. Here, it is first found using reanalyzed meteorological data that a statistically significant, lagged strengthening of the MJO is produced, on average, by large-scale waves propagating up from the troposphere in late fall and early winter, leading to a cooling and destabilization of the tropical lowermost stratosphere. Second, more such wave forcing events occur during the easterly phase of the lower stratospheric QBO and at solar minimum. A survey of archived climate model data from a recent model intercomparison project shows that at least one model version with realistic QBO and solar forcing (and high greenhouse gas concentrations) is able to partly simulate both of these characteristics. However, the model fails to simulate the observed stratospheric QBO modulation of the MJO, apparently because the modeled MJO is insufficiently sensitive to stability reductions in the tropical lower stratosphere.