PLANETARY-SCALE AND SYNOPTIC-SCALE CHARACTERISTICS OF EXPLOSIVE WINTERTIME CYCLOGENESIS OVER THE WESTERN NORTH-ATLANTIC OCEAN

The planetary- and synoptic-stale environment for explosive wintertime cyclogenesis over the western North Atlantic Ocean is documented usin g a series of 6-day composites based on a 12-season sample of 42 cyclo nes. This research extends earlier studies in that it focuses on plane tary-scale Row features and their relationship to synoptic-scale upper -tropospheric precursors to surface cyclogenesis. Notable features of the explosive cyclone composite include (i) a negative 500-hPa geopote ntial height anomaly over the North Pacific, indicating strengthening and a southward shift of the pacific jet stream (relative to a 25-yr c limatology); (ii) a quasi-stationary troposphere-deep ridge over weste rn North America; (iii) two predecessor upper troughs that cross the e ast coast of North America approximately 72 and 36 h prior to the onse t of surface deepening; (iv) a cyclogenetic mobile upper trough that b ecomes organized in northwesterly Row over central North America 24-36 h prior to surface deepening and crosses the East Coast at the onset of surface deepening; and (v) a middle- and upper-tropospheric ridge t hat develops east of the surface cyclogenesis region 24-48 h after the onset of surface deepening. The composite analyses reveal a jet strea k at the 250-hPa level located immediately upstream of the cyclogeneti c upper trough 24 h prior to coastal crossing. The three mobile synopt ic-scale upper troughs [(iii) and (iv)] that appear downstream of the western North American ridge are characterized by an approximate wavel ength of 2000 km, a period of 36-48 h, and a phase speed of 12-15 m s( -1). A control composite, based on a nine-season sample of 25 nonexplo sive cyclones, does not exhibit prominent anomalies corresponding to t he Pacific trough, western ridge, and predecessor troughs observed in the explosive composite. This comparison between the explosive and con trol composites suggests that the planetary-scale Row anomaly configur ation characteristic of the explosive composite may represent a dynami cal signal for explosive western North Atlantic cyclogenesis that is s ubstantially reduced in nonexplosive cases. The sequential formation o f geopotential height anomalies of alternating sign downstream of the initial Pacific-North American anomaly couplet is consistent with the concept of downstream development. The explosive composite evolution d iffers from the classic downstream development picture in that the ups tream anomaly centers maintain nearly constant amplitude and are quasi -stationary during the 6-day evolution, suggesting an orographic influ ence. The nearly constant amplitude and quasi-stationarity of the Paci fic trough and western ridge are particularly pronounced in a composit e based on a subset of 23 temporally clustered explosive cyclones. A s ubset composite of 19 temporally isolated explosive cyclones is charac terized by lower-amplitude, shorter-lived, and more mobile flow anomal ies relative to the clustered explosive cyclone composite. This compar ison of the clustered and isolated explosive cyclone composites sugges ts that although the same features are present in both composites, the planetary-scale Bow anomalies play a more important role in explosive western North Atlantic cyclogenesis when such events occur in success ion rather than individually.