On quantifying the sinuosity of typhoon tracks in the western North Pacific basin

This paper analyses the available typhoon data for the western North Pacific between 1951 and 2008, archived by the Regional Specialized Meteorological Centre in Tokyo. GIS methodology was used to extract key spatial characteristics of 1533 individual typhoons, including origin and destination coordinates and intervening positions, in order to investigate the nature of typhoon-track sinuosity over the past six decades. Temporal analysis illustrates strong inter-annual variability as the principal feature of sinuosity change, although not necessarily driven by ENSO, and allows anomalous years (e.g. 1991, 1995 and 1999) and the present quiescent period to be identified. Yet, typhoon-track sinuosity has not experienced any long-term trend during the study period, implying the limited effects (so far) of climate change on this aspect of typhoon behaviour. Spatial patterns in sinuosity are distinguished through track visualisation and zoning. UTM zones 49–51 (108–126°E) have a higher tendency to produce straighter-tracking typhoons, which make landfall in South East Asia. Nearly two thirds (64%) of typhoons following convoluted track shapes with high sinuosity values (sinuosity ≥2) originated within a 30° longitudinal band in the mid–eastern portion of the study area (UTM zones 54–58, 138–168°E). Temporal analysis reveals that later months of the NW Pacific typhoon season (September to December) are characterised by greater proportions of sinuous tracks. Quartile division of the positively-skewed sinuosity distribution enables the designation of typhoon tracks into four ordinally-named sinuosity categories: ‘straight’, ‘quasi-straight’, ‘curving’ and ‘sinuous’. This system facilitates the conceptualisation of track style and thereby improves on traditional binary groupings for track shape description.