A case study of an outbreak of twin tropical cyclones

Tropical cyclones occasionally form on opposite sides of the equator at nearly the same time and longitude. Previous studies of these twin tropical cyclones found that they typically occur 2 or 3 times a year. During October 1997, however, three sets of twin tropical cyclones developed in the Central Pacific within just one month.

All three twin tropical cyclogenesis events occurred within a broad and long-lasting region of warm water, low surface pressure, weak or easterly vertical shear, active convection, and low-level cyclonic vorticity. However, the convection and cyclonic vorticity, as indicated by brightness temperature (Tb) and 700-hPa equatorial zonal wind respectively, were also modulated by two primary modes. The first mode represents the low frequency variations of the background state associated with gradual changes in sea surface temperature, while the second mode is a convectively coupled equatorial Rossby wave packet.

Neither mode alone can account for all of the twin tropical cyclones. Instead, a favorable combination of the two modes appears to be a key ingredient for these twin tropical cyclogenesis events. The twin tropical cyclones only formed when the sum of the two modes produced equatorial westerlies in excess of 5 m/s and Tb below 270 K. By extrapolating the motion of these two modes and anticipating their combined effect, forecasters may be able to predict similar events in the future.