عنوان مقاله [English]
In the present study, using the ERA-INTERIM reanalysis data for geopotential height, horizontal wind speed and relative vorticity at 300, 200, 150, 100 and 50 hPa levels, the wave activity and wave activity flux for cyclonic and anticyclonic Rossby wave breaking events that occurred over Europe during the winter time 1979-2018, were calculated and analyzed. Results showed that in anticyclonic (cyclonic) wave breaking events, a narrow (wide) trough with north-east/south-west (north-west/south-east) axis and associated potential vorticity with values around 5-8 (4-6) 1e6 PVU are extended from Europe to the west (east) of Mediterranean.
In the anticyclonic wave breaking events, the wave amplitude and their associated potential voriticity are higher compared to cyclonic events and also the associated jet streams form in higher latitudes over Europe and the Mediterranean. It appears that intensification of the trough and its extention to lower latitudes over the Mediterranean is associated with intensification of the ridge over northwest of Europe, while in cyclonic wave breaking events the insification of the trough over the east of Europe is associated with intensification of the ridge in lower latitudes over the west of the Mediterranean. The waves in anticyclonic wave breaking events move to lower latitudes over the Mediterranean, while in cyclonic wave beakings, the waves move to north of Europe. Therefore, the anticyclonic (cyclonic) wave breaking events are generally associated with equatorward (poleward) flux of wave activity, downstream of the trough in latitudes lower (higher) than 40N, which causes the intensification (weakening) of synoptic waves over the Mediterranean and west of Asia. Also, the value of equatorward wave activity is 1.5 times more than those of poleward fluxes. It was found that in anticyclonic wave breakings, the downward wave activity flux is around twice, when compared to cyclonic wave breaking events.
Our results showed that during anticyclonic wave breaking, the equatorward of wave activity flux at 40N over west of Europe is larger than its value over the East Mediterranean and west of Asia, while during cyclonic wave breaking events the poleward flux of wave activity over west of Europe is negligible and it becomes equatorward over east of Europe and west of Asia. It seems that the reason is the existence of a ridge over Euroasia which results in the formation of a blocking over east of Europe and consequently causes the intensification of troughs in lower latitudes. As such, in addition to the lesser number of cyclonic wave breakings compared to those of anticyclonics, the cyclonic wave breaking events are relatively weaker than anticyclonic breakings over Europe.