An ERA5 climatology of synoptic-scale negative potential vorticity–jet interactions over the western North Atlantic

Abstract. Recent numerical modeling 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 has been further postulated that negative potential vorticity (NPV) may be relevant for the large-scale circulation, as it has been observed to grow in scale into elongated mesoscale bands when in close proximity to the jet stream, accelerating jet stream winds and degrading numerical weather prediction skill. However, these findings are largely confined to case studies. Here, we use a climatological and composite perspective to evaluate the occurrence of elongated bands of NPV over the northwest Atlantic and its implications for jet stream dynamics. This research focuses on synoptic-scale bands (>1650 km) of NPV that are in close proximity (< 100 km) to the jet stream (termed NPV–jet interactions) using ERA5 data from January 2000 to December 2021. Climatological characteristics show that NPV–jet interactions occur most frequently over the coastal western Atlantic during boreal winter along 40° N. This latitude band has also seen an 11 % increase (relative change) in NPV–jet interactions over the 22-year time period. Separating NPV–jet interactions into three distinct large-scale flow patterns using k-means clustering conceptually illustrates the evolution of NPV features from their initial formation along the westward flank of the ridge to the eastern flank of the ridge. The large-scale environment of NPV–jet interactions is characterized by 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 (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 of the three clusters, we showed that the presence of NPV near the jet stream adiabatically enhances wave activity flux due to NPV mutually strengthening momentum transport and the ageostrophic flux of the geopotential. The results show that the close proximity of synoptic-scale NPV to the jet stream is conducive to the occurrence of wind speed maxima and could be dynamically relevant in enhancing downstream development despite NPV's theorized origin from submesoscales.