A Case Study of Rossby Wave Breaking along the Subtropical Tropopause

Abstract

Global analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) are used to analyze the sequence of events leading to the creation of a large-scale reversal of potential vorticity (PV) gradients across the subtropical tropopause on 10 June 1992 over the North Pacific. The ?reversed? (i.e., southward directed) PV gradients across the tropopause initially appeared at 1200 UTC on 10 June, the time herein designated as the ?onset? of this event, at 25°N near the date line. Maps of PV are shown to highlight the evolution of a mobile Rossby wave packet during its migration across eastern Asia toward the subtropical Pacific during the several days prior to onset. With explicit regard to the validity of linear theory and the Wentzel?Kramers?Brillouin?Jeffreys approximation, wave activity flux diagnostics are employed to assess the propagation characteristics of this disturbance during that time. Following its departure from eastern Asia, much of the wave activity associated with the packet traveled southeastward along a quasi-horizontal path to the subtropical Pacific, above 300 hPa. A Hovmöller diagram of PV at 350 K is used to estimate the eastward phase speed (cx) of the PV anomalies in the wave packet during its upper-level transit. The spatial distribution of u ? cx, where u symbolizes the basic-state zonal wind, suggests that the generation of reversed PV gradients over the subtropical Pacific in this case was a result of the wave packet's proximity to a critical line. The evolution of this disturbance, and its associated production of reversed PV gradients across the tropopause, is viewed in light of previous climatological studies of Rossby wave breaking (RWB) to offer insight into the origin and maintenance of the ?surf zone? that resides near the subtropical tropopause over the North Pacific during summer. Composites of PV-anomaly patterns and wave activity fluxes, over a set of 33 RWB events that occurred during the 10 boreal summers from 1986 to 1995, imply that this feature is intimately related to repeated critical-layer interactions of the kind exhibited by the 10 June case. Moreover, the entrainment of Asian monsoon outflow into the 10 June PV-gradient reversal (as shown by air-parcel trajectories during the week prior to onset), when placed in the context of previous climatological statistics of the participation of Asian monsoon outflow in similar events, suggests that this surf zone delineates a sector of the atmosphere wherein tropical tropospheric air from the Asian monsoon and relatively high PV from the extratropical stratosphere are united during critical-layer interactions over the subtropical Pacific. These results provide a dynamical framework for understanding complex transport phenomena associated with the outflow of biomass burning and pollution from subtropical continental regions.