On the low-level circulation over the western north Pacific in relation with the duration of El Nino

We investigate how the low-level circulation centered at 10 degreesN over the western north Pacific differs between the shorter (SHORT-E) and longer (LONG-E) El Nino events based on their duration time. One major difference is the intensity of this circulation. This difference is due to the greater contribution of the 2 - 3 year quasi-biennial (QB) signal for the SHORT-E; strength of the 3 - 6 year interannual signal is almost the same in a mean sense between the two event types. Further, the amplitude of the QB signal is in phase with that of the interannual signal during the SHORT-E so that the circulation intensifies and evolves rapidly. The enhanced anticyclonic circulation during the mature stage of SHORT-E results in shallower thermocline in the equatorial western Pacific, and exerts influence on the duration of the event. During the LONG-E, the mentioned feature diminishes because of the weak QB. Our analysis further indicates a significant correlation between the zonal SSTA gradient over the Indian Ocean and the intensity of the western north Pacific circulation. A complete understanding of the related mechanism needs further study.