Influence of Tropical Equatorial Waves on the Formation of Super Typhoon Megi (2010)

Based on the NCEP/NCAR reanalysis data and the filtering in the Wheeler-Kiladis wavenumber-frequency domain, this study examines the role of tropical equatorial waves in the formation of Super Typhoon Megi, the strongest tropical cyclone over the globe in 2010. It is found that the formation of Megi involved three westward-moving low-level disturbances with positive vorticity anomalies initially aligned with the spacing of ~1300 km in the area from 4 to 10oN and 145 to 180oE (about 6 days before Megi genesis on 13 October 2010). The easternmost disturbance moved faster and gradually caught up and merged with the middle disturbance forming the precursor of Megi. In the meanwhile, the westernmost wave disturbance intensified and subsequently induced the enhancement of the low-level southwesterly flow on its southeast side, i.e., on the southwest side of the precursor of Megi. Under the effect of the convergence caused by this southwesterly flow, the precursor of Megi strengthened gradually and finally developed into Megi. The aforementioned processes were greatly influenced by the large-scale tropical waves. Firstly, the Madden-Julian oscillation (MJO) caused remarkable low-level convergence in the west-to-mid tropical pacific that provided a favorable environment for the formation of Megi. As the mixed Rossby-gravity (MRG) wave that carried the easternmost two positive vorticity anomalies moved westwards, the spacing between the two anomalies reduced and their intensity strengthened gradually under the effect of the large-scale convergence associated with MJO and MRG wave. This process is quite similar to that described by the wave accumulation theory in tropical cyclogenesis. Secondly, the easternmost two positive vorticity anomalies encountered an eastward propagating Kelvin wave first and then caught up a westward-moving equatorial Rossby (ER) wave when they moved westwards. The former promoted the intensification and merger of the two anomalies while the later facilitated to the strengthening of the precursor of Megi. Finally, both the Kelvin and TD-type waves made contributions to the intensification of the westernmost positive vorticity anomaly, which subsequently strengthened the low-level southwesterly flow on its southeast side. The strengthened southwesterly flow facilitated the intensification and merger of the easternmost two anomalies via increasing the convergence in its fore part. Momentum diagnosis indicates that 50% percent of the enhancement of the low-level southwesterly was attributed to the tropical waves.