Influences of the equatorial waves on multiple tropical cyclone genesis over the western North Pacific

  • 84.1% of TC genesis occurs within the active phase of CCEW
  • The contribution of CCEW to the convection is more significant in active MTC phase
  • ER waves produce better humidity and convective conditions than TD and MRG waves

The statistical features of tropical cyclone (TC) genesis (TCG) related to the convectively coupled equatorial waves (CCEW), including Kelvin, Equatorial Rossby (ER), mixed Rossby-Gravity (MRG), and tropical depression (TD)-type waves, are examined during the TC season (May to October) in the western North Pacific (WNP) for the period of 1979 - 2015. The result indicates that 84.1% of TC genesis occurs within the active phase of CCEW. Among them, about 71.3% concurs with a single wave or two coexisting waves. The contribution of each tropical wave shows notable seasonal dependence. The relative roles of individual tropical waves related to multiple tropical cyclone (MTC) events are examined. The CCEW provide strong (weak) favorable environmental conditions to result in more (less) TC genesis. In the active and normal MTC phases, ER waves play a more important role to TC genesis, while in the inactive MTC phase, TC genesis is primarily affected by TD waves and secondly by MRG waves. The contribution of CCEW to the OLR variance is more significant in the active MTC phase, especially in the region east of 150°E. Among them, ER waves make the contribution. Composite analyses reveal that all types of CCEW produce consistent forcing mechanisms for TCG with positive vorticity anomalies in the lower level and divergence anomalies in the upper level. Compared to the synoptic-scale waves (TD and MRG waves), the long-lasting ER waves produces more favorable humidity and convective conditions for MTC formation.

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