Statistics of Binary Tropical Cyclones in Western North Pacific during 1950-2012

Statistics of binary tropical cyclones (TCs) in western North Pacific (WNP) during 1950-2012 are investigated by using best track and reanalysis data. The binary TC is defined in this study when two TCs coexist within 1600 km for more than 24 hours with intensity of larger than 17.2 m s-1. In WNP during the period of 1950-2012, 106 binary TCs occurred with an annual average of 1.68. Occurrence frequency of binary TCs shows significant year to year variation, and there are two peaks of activity in the mid-1960s and early 1990s. By applying clustering analysis technique, three representative trajectories of western and eastern TC are obtained. Transitional speed and recurving location are significantly different with respect to the clustered type. The trajectory of each type is strongly related to the location and strength of North Pacific high. The effects of various environmental factors on the tracks of clustered binary TCs are investigated. The relationships between the occurrence of binary TCs and El Niño-Southern Oscillation (ENSO) and Quasi-Biennial Oscillation (QBO) are investigated. More binary TCs occur in the El-Niño phase than La-Niña phase. This is likely due to longer lifetime of TC in El-Niño phase caused by the eastward shift of genesis location of TC. On the contrary, there is no significant difference in the occurrence frequency between westerly- and easterly-phase of QBO.