A New Method to Objectively Classify Extratropical Cyclones for Climate Studies: Testing in the Southwest Pacific Region

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Extratropical cyclones can vary widely in their configuration during cyclogenesis, development mechanisms, spatial and temporal characteristics, and impacts. An automated method to classify extratropical cyclones identified in ERA-Interim data from 1979 to 2010 in the Australia and New Zealand region has been developed. The technique uses K-means clustering on two upper-tropospheric flow fields at the time of cyclogenesis and identifies four distinct clusters. Composites of these clusters are investigated, along with their life cycles and their spatial and temporal variability. The four clusters are similar to a previous manual classification. Cluster 1 develops in the equatorward entrance region of the subtropical jet, clusters 2 and 4 develop in the poleward exit region of the subtropical jet but with different relative positions of the upper-level trough and jet streak, and cluster 3 resembles secondary cyclogenesis on a preexisting front far poleward of the subtropical jet. The clusters have different impacts in terms of their precipitation (cluster 1 has the highest average precipitation), different seasonal cycles, and different preferred genesis locations. Features of the composite cyclones resemble extratropical cyclones from other regions, indicating the utility of the method over larger regions. The method has been developed to be easily applied to climate model output in order to evaluate the ability of models to represent the full range of observed extratropical cyclones.