Identification and ERA-15 Climatology of Potential Vorticity Streamers and Cutoffs near the Extratropical Tropopause

Abstract

A novel approach is introduced to identify potential vorticity (PV) streamers and cutoffs as indicators of Rossby wave breaking near the extratropical tropopause and to compile climatologies of these features on different isentropic surfaces. The method is based on a contour searching algorithm that identifies the dynamical tropopause [2 potential vorticity units (PVU; PVU ≡ 1 ? 10?6 K kg?1 m2 s?1) isoline] on isentropic surfaces. The contour is then analyzed to search for cutoffs and filament-like streamers. Whereas the identification of cutoffs is unambiguous, the one for streamers requires the specification of two parameters that determine the width and length of the contour feature to be classified as a streamer. This technique has been applied to the PV distribution in the Northern Hemisphere on isentropes from 295 to 360 K during the time period from 1979 to 1993 using the 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15). The climatology reveals a pronounced zonal asymmetry in the occurrence of PV streamers and cutoffs. On all isentropes considered there are clear frequency maxima whose location changes with altitude. For instance, in winter and on the 300-K isentrope, stratospheric streamers and cutoffs occur most frequently near 50°?60°N over the western side of Canada and Siberia. On higher isentropes, the maxima are located farther south and at the downstream end of the storm-track regions. Considering continental areas, the Mediterranean appears as a region with particularly abundant PV features. As noted in previous studies, there is a significant seasonal cycle if considering the frequency of PV features on individual isentropes. It is shown that this is mainly due to the seasonal cycle in the location of the isentropes themselves. Comparing the streamer and cutoff frequencies during different seasons on isentropes that are comparably located in the zonal mean yields a fairly robust pattern with almost no seasonal cycle. This indicates on the one hand that care should be taken when considering the seasonal cycle of dynamical processes on isentropes and on the other hand that Rossby wave breaking occurs year-round with almost constant frequency. A quantitative statistical analysis of individual PV features reveals that stratospheric and tropospheric streamers often occur in pairs.