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. The Madden–Julian oscillation (MJO) is a major source ofintraseasonal variability in the troposphere. Recently, studies haveindicated that also the solar 27-day variability could cause variability inthe troposphere. Furthermore, it has been indicated that both sources couldbe linked, and particularly that the occurrence of strong MJO events could bemodulated by the solar 27-day cycle. In this paper, we analyze whether the temporal evolution of the MJO phasescould also be linked to the solar 27-day cycle. We basically count theoccurrences of particular MJO phases as a function of time lag after thesolar 27-day extrema in about 38 years of MJO data. Furthermore, we develop aquantification approach to measure the strength of such a possiblerelationship and use this to compare the behavior for different atmosphericconditions and different datasets, among others. The significance of theresults is estimated based on different variants of the Monte Carlo approach,which are also compared. We find indications for a synchronization between the MJO phase evolution andthe solar 27-day cycle, which are most notable under certain conditions: MJOevents with a strength greater than 0.5, during the easterly phase of thequasi-biennial oscillation, and during boreal winter. The MJO appears tocycle through its eight phases within two solar 27-day cycles. The phase relationbetween the MJO and the solar variation appears to be such that the MJOpredominantly transitions from phase 8 to 1 or from phase 4 and 5 during thesolar 27-day minimum. These results strongly depend on the MJO index usedsuch that the synchronization is most clearly seen when using univariateindices like the OLR-based MJO index (OMI) in the analysis but can hardly beseen with multivariate indices like the real-time multivariate MJO index(RMM). One possible explanation could be that the synchronization pattern isencoded particularly in the underlying outgoing longwave radiation (OLR)data. A weaker dependence of the results on the underlying solar proxy isalso observed but not further investigated. Although we think that these initial indications are already worth noting, we do not claim to unambiguously prove this relationship in thepresent study, neither in a statistical nor in a causal sense. Instead, wechallenge these initial findings ourselves in detail by varying underlyingdatasets and methods and critically discuss resulting open questions to lay asolid foundation for further research.

Indications for a potential synchronization between the phase evolution of the Madden–Julian oscillation and the solar 27-day cycle