Interannual Variation of Multiple Tropical Cyclone Events in the Western North Pacific

The interannual variability of occurrence of multiple tropical cyclone (MTC) events during June--October in the western North Pacific (WNP) was examined for the period 1979--2006. The number of the MTC events ranged from 2 to 9 per year, exhibiting a remarkable year-to-year variation. Seven active and seven inactive MTC years were identified. Compared to the inactive years, tropical cyclone genesis locations extended farther to the east and in the meridional direction during the active MTC years. A composite analysis shows that inactive MTC years were often associated with the El Nino decaying phase, as warm SST anomalies in the equatorial eastern-central Pacific in the preceding winter transitioned into cold sea surface temperature (SST) anomalies in the concurrent summer. Associated with the SST evolution were suppressed low-level cyclonic vorticity and weakened convection in the WNP monsoon region. In addition to the mean flow difference, significant differences between active and inactive MTC years were also found in the strength of the atmospheric intraseasonal oscillation (ISO). Compared with inactive MTC years, ISO activity was much stronger along the equator and in the WNP region during active MTC years. Both westward- and northward-propagating ISO spectrums strengthened during active MTC years compared to inactive years. The combined mean state and ISO activity changes may set up a favorable environment for the generation of MTC events.