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Relationships between the Madden–Julian oscillation (MJO) and the extratropical circulation have been studied extensively and applied in operational settings to improve subseasonal prediction. However, in certain situations, tropical cyclones (TCs), which often coincide with enhanced MJO activity, can interfere with MJO organization and common pathways through which the extratropics respond to the MJO, yielding unexpected extratropical circulation outcomes. A statistical experiment is developed to assess the extent to which the presence of west Pacific TCs in different parts of the basin during a given MJO phase are related to subsequent remote extratropical circulation outcomes and whether these outcomes can be explained by random chance. Results demonstrate that significant, high-amplitude remote circulation anomalies that align with or differ from those expected to lag a given MJO phase tend to develop in association with TCs that cluster in specific parts of the basin and at specific leads—in some cases, more than 2 weeks before a pattern emerges. These spatial and temporal clusters vary between MJO phases. Next, composite patterns of anomalous 200-hPa geopotential height associated with a set of nonrecurving TCs transiting the South China Sea during real-time multivariate MJO (RMM) phase 5 are examined relative to their full RMM phase-5 referencemore »
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A 200-hPa zonal momentum budget is performed to examine the role that western North Pacific tropical cyclones (TCs) play in helping to organize intraseasonal extratropical circulation anomalies that occur with the Madden–Julian oscillation (MJO). Zonal wind is linearly decomposed into components that occur on MJO time scales (i.e., 20–100-day periods), as well as those that occur with lower and higher frequency. Dates during Northern Hemisphere fall that feature nonrecurving TCs within a search radius centered on a South China Sea grid point when the MJO is convectively active over the Maritime Continent and west Pacific warm pool are used to generate composites of relevant budget terms. These composites are then compared to others that are based on the full list of dates that feature a convectively active MJO in the same location during NH fall without regard for TC presence. Composite results highlight the primary momentum sources that guide the evolution of the NH extratropical zonal wind and associated mass field in each event set. TCs help to accelerate the East Asian subtropical jet that evolves with the MJO by modulating the high-frequency subtropical circulation over Southeast Asia. The phasing of this circulation with its underlying MJO time-scale component enablesmore »
