An ERA5 Climatology of Synoptic-Scale Negative Potential Vorticity-Jet Interactions over the Western North Atlantic

Abstract. Recent numerical modelling and theoretical work deduce that potential vorticity (PV) can turn negative in the Northern Hemisphere as a result of localized, convective heating embedded in vertical wind shear. It is further postulated that negative potential vorticity (NPV) may be relevant for the large-scale circulation as it has been observed to grow upscale into mesoscale bands when in close proximity to the jet stream, accelerating jet stream winds and degrading numerical weather prediction skill. However, these observations are largely confined to case studies. A composite investigation is proposed to evaluate whether strengthening of the jet stream is a robust response to NPV. This research focuses on synoptic-scale bands (>1650 km) of NPV that are in close proximity (<100 km) to the jet stream (NPV-jet) using ERA5 data from January 2000 – December 2021. Climatological characteristics show that NPV-jet interactions occur most frequently over the Western Atlantic (>1.2 % occurrence per year at particular grid-points) during Boreal Winter along 40° N. This latitude band has also seen a 11 % increase (relative-change) in NPV-jet interactions over the 22 year time-period. Separating NPV-jet interactions into distinct, large-scale flow patterns using K-means clustering illustrates the presence of a trough-ridge couplet adjacent to positive integrated vapor transport (IVT) anomalies, conducive to warm-conveyor belts and mesoscale convective systems. Even when NPV is positioned in a more adiabatic environment (i.e., far away from regions of strong IVT anomalies), robust, positive PV gradient and wind speed anomalies exist along the jet stream. Inspecting three detailed case-studies that serve as archetypes to the clusters, it is shown that the presence of NPV near the jet stream enhances wave activity flux due to NPV mutually strengthening momentum transport and the ageostrophic flux of geopotential. The results show that NPV-jet interactions can in-situ strengthen the mid-latitude jet stream and could be dynamically relevant in enhancing downstream development, despite NPV's theorized origin from sub-mesoscales.