Tropopause folds and cross‐tropopause exchange: A global investigation based upon ECMWF analyses for the time period March 2000 to February 2001

A new methodology is proposed to identify folds of the dynamical tropopause (taken as the 2 potential vorticity (PV) units (pvu) isosurface) from global analysis data sets from the European Centre for Medium‐Range Weather Forecasts (ECMWF). It consists of a three‐dimensional subdivision of the atmosphere into stratospheric and tropospheric parts and a subsequent examination of multiple tropopause crossings in vertical profiles of the analyses. The method is applied to a 1‐year period starting in March 2000. Seasonal mean fold frequency distributions show that folds occur preferentially in the subtropics, with maximum frequencies of about 30%. Pronounced maxima are located in the subtropical bands that extend from 20°E to 120°E on both hemispheres during summer. Generally, subtropical folds are comparatively shallow. Deep folds occur most frequently in midlatitudes during winter (over eastern North America and the western North Atlantic), with maximum frequencies of about 1%. Detailed investigation of the relationship between individual folds and cross‐tropopause exchange events reveals that, on the average, individual folds in the extratropics are associated with larger exchange mass fluxes compared to the subtropics. However, due to the much higher frequency of subtropical folds, the percentage of exchange events that are associated with tropopause folds is larger near 20°–40° latitude (50–70%) than further poleward (20–30% during winter and 10–20% during summer). This indicates that tropopause folds are the key feature for cross‐tropopause exchange in the subtropics, whereas in the extratropics other tropopause structures are at least of equal importance.