Surface Cyclolysis in the North Pacific Ocean. Part I: A Synoptic Climatology

Abstract

A continuous 11-yr sample of extratropical cyclones in the North Pacific Ocean is used to construct a synoptic climatology of surface cyclolysis in the region. The analysis concentrates on the small population of all decaying cyclones that experience at least one 12-h period in which the sea level pressure increases by 9 hPa or more. Such periods are defined as threshold filling periods (TFPs). A subset of TFPs, referred to as rapid cyclolysis periods (RCPs), characterized by sea level pressure increases of at least 12 hPa in 12 h, is also considered. The geographical distribution, spectrum of decay rates, and the interannual variability in the number of TFP and RCP cyclones are presented. The Gulf of Alaska and Pacific Northwest are found to be primary regions for moderate to rapid cyclolysis with a secondary frequency maximum in the Bering Sea. Moderate to rapid cyclolysis is found to be predominantly a cold season phenomena most likely to occur in a cyclone with an initially low sea level pressure minimum. The number of TFP?RCP cyclones in the North Pacific basin in a given year is fairly well correlated with the phase of the El Niño?Southern Oscillation (ENSO) as measured by the multivariate ENSO index. An analysis of the composite structure and evolution of cyclones containing a single RCP reveals that, on average, such storms undergo a significant period of cyclogenesis immediately prior to the commencement of decay. This cyclogenesis is characterized by the development of a sharply curved and progressively more negatively tilted upper-level shortwave trough. The sudden deamplification of this feature, which results in an abrupt reduction in the mid- and upper-tropospheric mass divergence above the surface cyclone, is the underlying physical factor responsible for the concurrent rapid cyclolysis at the surface. A complementary overview of this evolution from a potential vorticity (PV) perspective demonstrates that rapid cyclolysis at the surface is coincident with a weakening of the upper and lower PV anomalies and the acquisition of a downshear phase tilt to the anomalies. Thus, it is suggested that rapid cyclolysis events, though influenced by friction in the boundary layer, are initiated and largely controlled by synoptic-scale dynamical processes.