CLUSTER-ANALYSIS OF EASTERN NORTH PACIFIC TROPICAL CYCLOGENESIS PRECURSORS

Tropical cyclogenesis in the eastern North Pacific is investigated by applying an objective cluster analysis technique to wind and geopotential height data from the 1985 to 1989 hurricane seasons. The first 20 principal components derived from the 850 hPa European Centre for Medium-Range Weather Forecasting global analysis data are used to separate tropical cyclogenesis events into three relatively homogeneous groups. Composites of the height and wind fields for the days prior to cyclogenesis reveal distinct differences in the environment for the three groups. Two of the groups are associated with large-scale height and wind anomalies which lie upstream of the genesis region on the days before tropical cyclones are formed. The remaining group is associated with anomalies to the west of Mexico which provide a favorable environment for tropical cyclogenesis. An examination of the initial locations of the circulations which grew into named storms reveals that the groups associated with the upstream anomalies are characterized by tropical cyclones which formed over a broad range of longitudes, while the group associated with the anomalies in the Pacific contains storms which formed very near the coast of Mexico.