A Statistical Analysis of Relationships between Western North Pacific Tropical Cyclones and Extratropical Circulation Patterns Accompanying the Madden–Julian Oscillation

Abstract

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 reference patterns. While both sets of patterns exhibit high correlation at early time lags, they quickly evolve out of phase with one another. It is suggested that the TCs featured in the TC-based composite contribute to this observed phase shift by modulating the RMM phase-5 extratropical response pathway.