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Planetary- and Synoptic-Scale Characteristics of Explosive Wintertime Cyclogenesis Over the Western North Atlantic Ocean

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  • 1 Department of atmospheric Science, State University of New York at Albany, Albany, New York
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Abstract

The planetary- and synoptic-scale environment for explosive wintertime cyclogenesis over the western North Atlantic Ocean is documented using a series of 6-day composites based on a 12-season sample of 42 cyclones. This research extends earlier studies in that it focuses on planetary-scale flow 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 geopotential height anomaly over the North Pacific, indicating strengthening and a southward shift of the Pacific jet stream (relative to a 25-yr climatology); (ii) a quasi-stationary troposphere-deep ridge over western North America; (iii) two predecessor upper troughs that cross the east coast of North America approximately 72 and 36 h prior to the onset of surface deepening; (iv) a cyclogenetic mobile upper trough that becomes organized in northwesterly flow 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 that develops east of the surface cyclogenesis region 24–48 h after the onset of surface deepening. The composite analyses reveal a jet streak at the 250-hPa level located immediately upstream of the cyclogenetic upper trough 24 h prior to coastal crossing. Ale three mobile synoptic-scale upper troughs [(iii) and (iv)] that appear downstream of the western North American ridge are characterized by an approximate wavelength 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 nonexplosive cyclones, does not exhibit prominent anomalies corresponding to the Pacific trough, western ridge, and predecessor troughs observed in the explosive composite. This comparison between the explosive and control composites suggests that the planetary-scale flow anomaly configuration characteristic of the explosive composite may represent a dynamical signal for explosive western North Atlantic cyclogenesis that is substantially reduced in nonexplosive cases.

The sequential formation of 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 differs from the classic downstream development picture in that the upstream anomaly centers maintain nearly constant amplitude and are quasi-stationary during the 6-day evolution, suggesting an orographic influence. The nearly constant amplitude and quasi-stationarity of the Pacific trough and western ridge are particularly pronounced in a composite based on a subset of 23 temporally clustered explosive cyclones. A subset composite of 19 temporally isolated explosive cyclones is characterized by lower-amplitude, shorter-lived, and more mobile flow anomalies relative to the clustered explosive cyclone composite. This comparison of the clustered and isolated explosive cyclone composites suggests that although the same features are present in both composites, the planetary-scale flow anomalies play a more important role in explosive western North Atlantic cyclogenesis when such events occur in succession rather than individually.

Abstract

The planetary- and synoptic-scale environment for explosive wintertime cyclogenesis over the western North Atlantic Ocean is documented using a series of 6-day composites based on a 12-season sample of 42 cyclones. This research extends earlier studies in that it focuses on planetary-scale flow 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 geopotential height anomaly over the North Pacific, indicating strengthening and a southward shift of the Pacific jet stream (relative to a 25-yr climatology); (ii) a quasi-stationary troposphere-deep ridge over western North America; (iii) two predecessor upper troughs that cross the east coast of North America approximately 72 and 36 h prior to the onset of surface deepening; (iv) a cyclogenetic mobile upper trough that becomes organized in northwesterly flow 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 that develops east of the surface cyclogenesis region 24–48 h after the onset of surface deepening. The composite analyses reveal a jet streak at the 250-hPa level located immediately upstream of the cyclogenetic upper trough 24 h prior to coastal crossing. Ale three mobile synoptic-scale upper troughs [(iii) and (iv)] that appear downstream of the western North American ridge are characterized by an approximate wavelength 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 nonexplosive cyclones, does not exhibit prominent anomalies corresponding to the Pacific trough, western ridge, and predecessor troughs observed in the explosive composite. This comparison between the explosive and control composites suggests that the planetary-scale flow anomaly configuration characteristic of the explosive composite may represent a dynamical signal for explosive western North Atlantic cyclogenesis that is substantially reduced in nonexplosive cases.

The sequential formation of 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 differs from the classic downstream development picture in that the upstream anomaly centers maintain nearly constant amplitude and are quasi-stationary during the 6-day evolution, suggesting an orographic influence. The nearly constant amplitude and quasi-stationarity of the Pacific trough and western ridge are particularly pronounced in a composite based on a subset of 23 temporally clustered explosive cyclones. A subset composite of 19 temporally isolated explosive cyclones is characterized by lower-amplitude, shorter-lived, and more mobile flow anomalies relative to the clustered explosive cyclone composite. This comparison of the clustered and isolated explosive cyclone composites suggests that although the same features are present in both composites, the planetary-scale flow anomalies play a more important role in explosive western North Atlantic cyclogenesis when such events occur in succession rather than individually.

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