1997年艾凡和喬安雙超級颱風大氣海洋前置條件

Abstract

The nearly synchronizing genesis and intensification of the dual supertyphoons Ivan and Joan (1997) was a very distinctive event in history. Ivan and Joan formed within two days in the vast cloud cluster over the tropical Central Pacific (CP) during October 1997, the mature phase of the strongest El Nino on record. Afterward, they maintained 20° latitude apart and intensified to 160 kt within the same day prior to recurvature. Based on statistics, Ivan and Joan were two of the four 160 kt extremely intense TCs over the Western North Pacific (WNP) in the past 30 years. The time and space background of this special event is analyzed in this study. Result shows that the intense westerly wind events of the 20 to 25 day time scale variability during El Nino autumn provided a favorable environment for cyclogenesis over tropical CP. However, the mechanisms for intensification seem to differ from each other. Ivan was predominantly intensified by large divergence of the pre-existing weak upper level PV maxima while Joan principally intensified under very weak vertical wind shear. The fast enough translation speed of Ivan and Joan was also remarkable for the TCs intensification through a shallower upper warm ocean layer in WNP during El Nino. Interestingly, Ivan and Joan were characterized by their relatively small size in horizontal. The result also indicates that the radial relative humidity distribution may be critical to the radial distribution of the convective activities. And it may in turn affect the intensity and intensity change. Ivan and Joan are suggested to be the integrated products which were contributed from interannual to intraseasonal variability, as well as to certain lucky coincidence.