Surface Cyclolysis in the North Pacific Ocean. Part II: Piecewise Potential Vorticity Diagnosis of a Rapid Cyclolysis Event

Abstract

Employing output from a successful numerical simulation, piecewise potential vorticity inversion is used to diagnose a rapid surface cyclolysis event that occurred south of the Aleutian Islands in late October 1996. The sea level pressure minimum of the decaying cyclone rose 35 hPa in 36 h as its associated upper-tropospheric wave quickly acquired a positive tilt while undergoing a rapid transformation from a nearly circular to a linear morphology. The inversion results demonstrate that the upper-tropospheric potential vorticity (PV) anomaly exerted the greatest control over the evolution of the lower-tropospheric height field associated with the cyclone. A portion of the significant height rises that characterized this event was directly associated with a diminution of the upper-tropospheric PV anomaly that resulted from negative PV advection by the full wind. This forcing has a clear parallel in more traditional synoptic/dynamic perspectives on lower-tropospheric development, which emphasize differential vorticity advection. Additional height rises resulted from promotion of increased anisotropy in the upper-tropospheric PV anomaly by upper-tropospheric deformation in the vicinity of a southwesterly jet streak. As the PV anomaly was thinned and elongated by the deformation, its associated geopotential height perturbation decreased throughout the troposphere in what is termed here PV attenuation. The analysis reveals that neither surface friction nor latent heat release played a primary role in this case of rapid surface decay. It is suggested, therefore, that rapid surface cyclolysis is unlikely to occur except in large-scale environments that promote rapid erosion of the upper-tropospheric PV anomalies associated with synoptic-scale short waves.