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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 ∼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. 

The tropical Madden–Julian oscillation (MJO) excites a northward propagating Rossby wave train that largely determines the extratropical surface weather consequences of the MJO. Previous work has demonstrated a significant influence of the tropospheric El Niño–Southern Oscillation (ENSO) on the characteristics of this wave train. Here, composite analyses of ERA-Interim sea level pressure (SLP) and surface air temperature (SAT) data during the extended northern winter season are performed to investigate the additional role of stratospheric forcings [the quasi-biennial oscillation (QBO) and the 11-yr solar cycle] in modifying the wave train and its consequences. MJO phase composites of 20–100-day filtered data for the two QBO phases show that, similar to the cool phase of ENSO, the easterly phase of the QBO (QBOE) produces a stronger wave train and associated modulation of SLP and SAT anomalies. In particular, during MJO phases 5–7, positive SLP and negative SAT anomalies in the North Atlantic/Eurasian sector are enhanced during QBOE relative to the westerly phase of the QBO (QBOW). The opposite occurs during the earliest MJO phases. SAT anomalies over eastern North America are also more strongly modulated during QBOE. Although less certain because of the short data record, there is some evidence that the minimum phase of the solar cycle (SMIN) produces a similar increased modulation of SLP and SAT anomalies. The strongest modulations of SLP and SAT anomalies are produced when two or more of the forcings are superposed (e.g., QBOE/cool ENSO, SMIN/QBOE, etc.).