Relationship between Typhoons with concentric eyewalls and ENSO in the western North Pacific basin

The typhoons with concentric eyewalls (CE) over the western North Pacific in different phases of the El Niño-Southern Oscillation (ENSO) between 1997 and 2012 are studied. They find a good correlation (0.72) between the annual CE typhoon number and the oceanic Niño index (ONI), with most of the CE typhoons occurring in the warm and neutral episodes. In the warm (neutral) episode, 55% (50%) of the typhoons possessed a CE structure. In contrast, only 25% of the typhoons possessed a CE structure in the cold episode. The CE formation frequency is also significantly different with 0.9 (0.2) CEs per month in the warm (cold) episode. There are more long-lived CE cases (CE structure maintained more than 20 h) and typhoons with multiple CE formations in the warm episodes. There are no typhoons with multiple CE formations in the cold episode. The warm episode CE typhoons generally have a larger size, stronger intensity, and smaller variation in convective activity and intensity. This may be due to the fact that the CE formation location is farther east in the warm episodes. Shifts in CE typhoon location with favorable conditions thus produce long-lived CE typhoons and multiple CE formations. The multiple CE formations may lead to expansion of the typhoon size.