نوع مقاله : مقاله پژوهشی
نویسندگان
گروه فیزیک فضا، موسسه ژئوفیزیک دانشگاه تهران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
On isentropic surfaces, Rossby Wave Breaking (RWB) is characterized by the rapid and irreversible deformation of potential vorticity (PV) contours. Rossby waves are generated by the meridional PV gradient of the background flow and can be identified by their PV anomaly relative to that background flow. The present study is devoted to evaluating the distribution of RWB based on the method introduced by Pelly and Hoskins (2003; PH), known as the PV-θ method, and its associated index (BI), across the Northern Hemisphere storm tracks during the boreal wintertime over 65-year period from 1958 to 2023, using the JRA-55 reanalysis data with a horizontal resolution of 〖1.25〗^°×〖1.25〗^° at 00 UTC. Two quantities, the Direction of Breaking (DB) and the Intensity of Breaking (RI), were employed to identify four distinct types of Anticyclonic and Cyclonic Rossby wave breaking: Anticyclonic breaking with warm air intrusion (ABW), Anticyclonic breaking with cold air intrusion (ABC), Cyclonic breaking with warm air intrusion (CBW), and Cyclonic breaking with cold air intrusion (CBC). First, to evaluate the application of the PH method in identifying the occurrence of various wave breaking types, the distribution of BI, DB, and RI, were compared with the distribution of Potential Vorticity (PV) on the 330-K isentropic surface and potential temperature (θ) on the PV isosurface with 2 PVU value, alongside geopotential height at the 500 hPa level and wind speed at the 200 hPa level, for three case studies. The first case study corresponds to March 8, 1959, the second to January 6, 2008, and the third to January 27, 2018. In the next step, the climatological distributions of various types of wave breaking using the above quantities (BI, DB, and RI) for the winter season were examined and analyzed during the study period. Finally, wave breaking was examined in terms of its intensity and categorized into strong, moderate, and weak classes.
The evaluation of the PH method performance indicates that the higher the RI value, the more the overturning of θ contours, and a stronger geopotential height gradient at the 500 hPa level. The climatological distribution of the results demonstrates the predominance of anticyclonic wave breaking events over the eastern Atlantic Ocean, Europe, western Asia, the eastern Pacific Ocean, and western North America. The frequency of ABW is higher than ABC in these regions. The highest frequency of CBW is observed over the eastern Pacific Ocean and western Asia, while the maximum frequency of cyclonic wave breaking events dominated by CBC is found over the western Atlantic Ocean and the western Pacific Ocean. The frequency of CBW event is higher in East Asia than in other regions, while CBC events are more frequent in the western Atlantic and Pacific Oceans. Furthermore, the frequency of strong wave breaking is higher than that of weak wave breaking across all longitudes, though marginally.
کلیدواژهها [English]
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