Tropical Cyclone Activity in the Fiji Region: Spatial Patterns and Relationship to Large-Scale Circulation

Abstract

This study examines the variations in tropical cyclone (TC) genesis positions and their subsequent tracks for different phases of the El Niño?Southern Oscillation (ENSO) phenomenon in the Fiji, Samoa, and Tonga region (FST region) using Joint Typhoon Warning Center best-track data. Over the 36-yr period from 1970/71 to 2005/06, 122 cyclones are observed in the FST region. A large spread in the genesis positions is noted. During El Niño years, genesis is enhanced east of the date line, extending from north of Fiji to over Samoa, with the highest density centered around 10°S, 180°. During neutral years, maximum genesis occurs immediately north of Fiji with enhanced genesis south of Samoa. In La Niña years, there are fewer cyclones forming in the region than during El Niño and neutral years. During La Niña years, the genesis positions are displaced poleward of 12°S, with maximum density centered around 15°S, 170°E and south of Fiji. The cyclone tracks over the FST region are also investigated using cluster analysis. Tracks during the period 1970/71?2005/06 are conveniently described using three separate clusters, with distinct characteristics associated with different ENSO phases. Finally, the role of large-scale environmental factors affecting interannual variability of TC genesis positions and their subsequent tracks in the FST region are investigated. Favorable genesis positions are observed where large-scale environments have the following seasonal average thresholds: (i) 850-hPa cyclonic relative vorticity between ?16 and ?4 (?10?6 s?1), (ii) 200-hPa divergence between 2 and 8 (?10?6 s?1), and (iii) environmental vertical wind shear between 0 and 8 m s?1. The subsequent TC tracks are observed to be steered by mean 700?500-hPa winds.