An algorithm for identifying the initiation of synoptic‐scale Rossby waves on potential vorticity waveguides

Abstract

In this study, a novel method is presented that automatically identifies the initiation of synoptic‐scale Rossby waves (RWIs) on potential vorticity (PV) waveguides. RWIs are identified based on geometry changes of the 2 PVU (where PVU is Potential Vorticity Units) contour on two isentropic levels (320 and 340 K). The 2 PVU contours are hereby regarded as proxies for the position and shape of the extratropical and subtropical waveguide, respectively. A RWI is recorded in a zonally aligned (i.e. wave‐free) longitudinal contour segment if it becomes wavy over time and, additionally, the respective 2 PVU contour is wave‐free upstream of the segment. To illustrate the potential of the method, three example RWIs are presented, which reveal distinct initiation mechanisms. In the first RWI, a large convective system triggers a Rossby wave on the extratropical waveguide. In the second case, the two waveguides are vertically co‐aligned and a mesoscale lower‐stratospheric high‐PV anomaly leads to the initiation of a Rossby wave on both waveguides. The third case illustrates the interaction between the two waveguides. A breaking wave on the extratropical waveguide steers a lower‐stratospheric high‐PV anomaly into close proximity with the undisturbed subtropical waveguide and induces a wave on the latter. The method is applied to ERA‐Interim data from 1979–2014, in order to produce a feature‐based climatology of RWIs on the two waveguides in the Northern Hemisphere during winter. The three main regions where RWIs occur are the Northwestern Pacific, North America and the North Atlantic, as well as North Africa and the Middle East.