[M-IS24] Geophysical fluid dynamics-Transfield approach to geoscience

[MIS24-03] Cyclonic-anticyclonic asymmetry on maintained vortices near the westerlies focused on external vortices

*Koryu Yamamoto¹, Keita Iga¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:cyclonic-anticyclonic asymmetry, potential vorticity, trajectory analysis

The weather in mid- and high-latitude area including Japan is affected by the meandering westerlies. They occasionally result in vortices such as blocking anticyclones and cut-off cyclones. It is suggested by several previous studies that the way how anticyclones and cyclones sustain themselves for some days and maintain their position can be different between polarity. Few studies, however, have clarified the cyclonic-anticyclonic asymmetry by comparing them comprehensively. In the present study, we focus on maintained vortices near the westerlies, and we also analyzed their migration and how the potential vorticity flowed into them which can be one of the reasons of the migration. First, maintained anticyclonic and cyclonic non-dipole vortices are extracted from a global reanalysis as we symmetrically applied the previous and objective method to extract blocking anticyclones to cyclones. The analysis period is for 61 years, from March 1958 to February 2019. Geopotential height on 500hPa surface is used to extract them. A comparison of the anticyclones and cyclones extracted shows that several characteristics such as migration length of their center are suggested to be different with statistical significance. Furthermore, detailed examination of their migration during particular periods confirms that the cyclones exhibit larger movement in the east-west direction but smaller in the north-south direction than the anticyclones do. Next, composited maps which show how the potential vorticity into the vortices are made and analyzed in order to discover the differences in the migration. The results show that the maps are consistent with the migration. It is confirmed that air with high potential vorticity flows into the cyclones mainly from the west, whereas air with low potential vorticity doesn't show such a tendency for the anticyclones. Finally, in order to confirm the inflow and investigate the origins of the air into the vortices, we conducted a backward trajectory analysis. The result shows that the origins are migrating disturbances in the westerlies and vortices which existed in the previous time. Diabatic effects also contribute to the anomalous potential vorticity around the center of the vortices. It is confirmed that almost all the cyclones are accompanied with an area where the potential vorticity decreases due to strong convection to their south side, whereas the anticyclones are not. The present study has confirmed that anticyclones and cyclones can have different mechanisms for maintenance near the westerlies.