Explosive Cyclogenesis in the West-Central North Atlantic Ocean, 1981–84. Part I: Composite Structure and Mean Behavior

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Abstract

Tracks and central values of surface low-pressure centre and 500-mb absolute vorticity maxima were gathered from operational analysis for 48 casts of expressive cyclogenesis in the west-central North Atlantic. The cases were stratified into relatively strong moderate and weak categories. Tracking was done from 36 h before to 24 h after the time of most rapid surface deepening.

The surface center often first appeared only 12 to 24 h before maximum deepening, but the upper vorticity maximum was present 36 h or more in advance, often many days. Mean motion of the surface low was rapidly northeastward from the southeastern states, or just offshore, to Newfoundland. Intense deepening occurred in a period of no more than 24–36 h. Strong cyclones moved most rapidly and most meridionally, traveled farther over warm water, and deepened dramatically in a single 12-h period while crossing the closely spaced isotherms of sea surface temperature north of the Gulf Stream. The upper vorticity center moved rapidly eastward from an initial position far northwest of the surface low to a final position close by to its south. Modest intensification of this center occurred during overtaking as the surface cyclone deepened explosively. Detailed study of two cases illustrates the range of behaviors as well as problems of oceanic analysis.

A high correlation was found, for the sample means, between upper-level cyclonic vorticity advection over the surface cyclone and simultaneous surface-deepening rate. Thus the explosive maritime cyclone appears to be a fundamentally baroclinic disturbance in which the low-level response to a given upper-level forcing is dramatically large.

Abstract

Tracks and central values of surface low-pressure centre and 500-mb absolute vorticity maxima were gathered from operational analysis for 48 casts of expressive cyclogenesis in the west-central North Atlantic. The cases were stratified into relatively strong moderate and weak categories. Tracking was done from 36 h before to 24 h after the time of most rapid surface deepening.

The surface center often first appeared only 12 to 24 h before maximum deepening, but the upper vorticity maximum was present 36 h or more in advance, often many days. Mean motion of the surface low was rapidly northeastward from the southeastern states, or just offshore, to Newfoundland. Intense deepening occurred in a period of no more than 24–36 h. Strong cyclones moved most rapidly and most meridionally, traveled farther over warm water, and deepened dramatically in a single 12-h period while crossing the closely spaced isotherms of sea surface temperature north of the Gulf Stream. The upper vorticity center moved rapidly eastward from an initial position far northwest of the surface low to a final position close by to its south. Modest intensification of this center occurred during overtaking as the surface cyclone deepened explosively. Detailed study of two cases illustrates the range of behaviors as well as problems of oceanic analysis.

A high correlation was found, for the sample means, between upper-level cyclonic vorticity advection over the surface cyclone and simultaneous surface-deepening rate. Thus the explosive maritime cyclone appears to be a fundamentally baroclinic disturbance in which the low-level response to a given upper-level forcing is dramatically large.

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