A Case Study of an Outbreak of Twin Tropical Cyclones

Previous studies have found that twin tropical cyclogenesis typically occurs 2–3 times a year in the Pacific Ocean. During October 1997, however, three sets of twin tropical cyclones developed in the central Pacific within a single month. Tropical cyclone archives indicate that this is the only such outbreak from 1969 to 2006. This case study explores the background and synoptic conditions that led to this unique event. All three twin tropical cyclogenesis events occurred within a broad and long-lasting envelope of warm water, low surface pressure, active convection, and weak or easterly vertical shear. Westerly winds at the equator and trade easterlies farther poleward created strips of cyclonic vorticity through a deep layer. A low-pass filter showed that these favorable conditions shifted eastward with time at 1–2 m s−1. In addition to the gradual eastward movement, the equatorial westerlies and convection were modulated by higher-frequency westward propagation. These anomalies appear to have been associated with convectively coupled n = 1 equatorial Rossby waves. The twin tropical cyclones formed only when the sum of the two modes produced equatorial westerlies in excess of 5 m s−1 and brightness temperature below 270 K. Applications of these results are proposed for the operational prediction of twin tropical cyclogenesis.