A Climatological Analysis of Upper-Level Velocity Potential Using Global Weather Reanalysis, 1959–2020

Upper-level (200 hPa) velocity potential (VP200) is useful in identifying areas of rising or sinking atmospheric motions on varying temporal scales (e.g., weekly, seasonal, interannual) especially in the global tropics. These areas are associated with enhancement (rising motion) or suppression (sinking motion) of tropical convection and subsequent weather phenomena dependent on these processes (e.g., tropical cyclones). This study employed commonly used global weather reanalysis datasets to calculate and compare VP200 on interannual through multidecadal temporal scales and quantify any differences that existed between them from 1959 to 2020 over four key regions of tropical variability (Equatorial Africa, Amazon Basin, Equatorial Central Pacific, and Equatorial Indonesia). To supplement this analysis, the highly correlated variables to VP200 of outgoing longwave radiation (OLR) and daily precipitation rate were used and directly compared with independent OLR and precipitation datasets to determine the reanalysis' level of agreement with the independent data. The ECMWF ERA5 held the highest agreement to these data over all regions examined and was reasoned to have the highest confidence in accurately capturing the variability of VP200 fields for the study period. Confidence was decreased in the usefulness of the NCEP/NCAR Reanalysis 1 as it consistently performed poorly over much of the study domain. The results of this study also emphasised the usefulness in ensemble-based approaches to assess climate variability and understanding of potential biases and uncertainties that are inherent in these data sources.