Self-Organizing Maps and the Synoptic Climatology of Western North Atlantic Bomb Cyclones

Extratropical cyclones that undergo ‘bomb’ cyclogenesis (pressure drop generally ≥ 24mb in 24hrs) remain an under-predicted phenomenon in operational weather forecasts. This research aims to explore the climatology of bombs in the western North Atlantic and their associated circulation patterns. Sea level pressure data from the NCEP-NCAR Reanalysis (1948-2016) was used to create a bomb cyclone dataset of the western North Atlantic basin that retained genesis locations and the magnitude of pressure decrease at 24hr time intervals along the storm’s trajectory. Geopotential height at the 500hPa pressure level was used to compute monthly and seasonal blocking indices between 40° and 80° north latitudes across the North Atlantic basin. Implementing a circulation to environment approach, 500hPa geopotential height, relative humidity, and sea-level pressure patterns were defined by season across the western North Atlantic using self-organizing maps (SOMs). Blocking indices were mapped with their respective SOM patterns. The months of March-May exhibited the highest frequency of 5-day blocking across the entire North Atlantic. Furthermore, results of the SOMs illustrate the atmospheric patterns characteristic of bomb environments that include relatively low sea level pressure in the mid-Atlantic region of the United States with large amplitude, downstream ridging.