Mechanisms of midlatitude cross‐tropopause transport using a potential vorticity budget approach

Abstract

A potential vorticity (PV) budget method has been used to attribute vertical transport across the near‐tropopause (1 PVU surface) in extratropical weather systems to radiative, latent heating and cooling, and mixing processes. Sources and sinks of PV due to nonconservative processes are calculated online and advected as passive tracers. There is reasonable agreement between the spatial distribution of transport determined from the PV budget method and the transport across the 1–2 PVU zone from a passive tracer and trajectories, but different aspects of exchange can be diagnosed with each method. Stratosphere‐to‐troposphere transport occurred in the broad upper level PV anomalies and was attributed mainly to latent heating and cooling processes; troposphere‐to‐stratosphere transport occurred toward the tail of a PV filament and in a ridge region and was attributed mainly to radiative processes. The contribution of mixing processes to transport was comparatively small. Using the PV budget method, the domain integrated exchange across the 1 PVU surface was from stratosphere to troposphere, and the magnitude of 1 × 1015 kg over a 2 day winter integration in a large North Atlantic domain is consistent with stratosphere‐troposphere exchange calculations from other studies. This exchange arises from an approximate balance between the dominant stratosphere‐to‐troposphere transport due to latent heating and cooling processes and troposphere‐to‐stratosphere transport due to radiative processes. The direction of transport across the tropopause in a fold was found to be critically dependent on the PV surface considered to represent the tropopause.