Influence of Equatorial Waves on the Genesis of Super Typhoon Haiyan (2013)

Abstract

he influence of equatorial wave disturbances on the genesis of Super Typhoon Haiyan (2013) is investigated through spectral, composite, and ensemble sensitivity analysis of various observational datasets in combination with predictions from an operational ensemble. Under the favorable large-scale environmental conditions of the Asian monsoon combined with the Madden?Julian oscillation (MJO), the incipient Haiyan develops from a cyclonic disturbance that originates from a train of westward-propagating mixed Rossby?gravity (MRG) waves. Haiyan eventually develops in the monsoon trough region at the leading edge of the moist MJO phase that has strong low-level convergence, high moisture content, and weak shear, along with high sea surface temperature. These favorable environmental conditions promote the intensification of deep moist convection that facilitates the development of the cyclonic disturbance from an MRG wave into a tropical depression, which later intensifies rapidly into Super Typhoon Haiyan, one of the world?s strongest and most destructive tropical storms ever recorded. Results from ensemble sensitivity analyses are consistent with this finding and further show that the uncertainties in tropical waves and their interactions can impact the large-scale environment surrounding Haiyan?s precursor and therefore limit the predictability of tropical cyclone formation and intensity. The better-performing members tend to have a stronger initial MRG wave disturbance, which provides a stronger initial seed for the later development of the storm, as well as a stronger moist MJO wave in the tropical region, which not only promotes deep convection near the precursor location, but also reduces the environmental vertical wind shear by strengthening the tropical westerlies.