Evaluating Northern Hemispheric Polar Vortex Variability Using Reanalysis and Satellite Observation: A Case Study Approach

Previous studies have investigated the performance of reanalyses during Northern Hemisphere winters using several approaches over the period 1980-present. This study evaluates the performance of three reanalyses (ERA-5, MERRA-2, and JRA-3Q) in representing stratospheric conditions over the two most recent Northern Hemisphere winter periods (2023-2024 and 2024-2025) using temperature and zonal wind across the high-latitude stratosphere. The statistics analysis indicates that ERA-5 exhibits a warmer upper and middle stratosphere, whereas JRA-3Q shows a warmer lower stratosphere. However, for the circulation, regardless of the month and region in the stratosphere, it is observed that ERA-5 relatively shows wind values closer to easterly circulation. As stated in previous literature, the variability in both temperature and zonal wind is relatively highest in the upper stratosphere and lowest in the lower stratosphere. A monthly climatological comparison between MLS and the three reanalyses indicates that MERRA-2 is a better candidate for representing thermal conditions in the upper stratosphere (1 hPa). Additionally, zonal wind is identified as a more suitable parameter in defining SSW rather than temperature. The analysis identifies reversal and warming events originating in the upper stratosphere, highlighting the need for multi-level diagnostics that extend beyond the conventional single-level zonal wind method. The investigation further reveals that, rather than relying on the traditional 10-hPa reference in determining the SSW, these early SSW-like events occurring in the upper stratosphere initiate atmospheric-ionospheric coupling sooner. Therefore, considering the upper stratosphere helps to identify the actual onset of atmospheric-ionospheric coupling during SSW more accurately.