Intensity Variations in Potential Vorticity Streamers: Development Pathways, and Environmental Impacts in the North Atlantic Basin

Potential vorticity streamers (PVSs) are elongated filaments of high potential vorticity (PV) air in the upper troposphere that can facilitate tropical-extratropical air mass exchange. PVSs often originate during anticyclonic Rossby wave breaking (AWB), where upstream low PV air is advected poleward over downstream high PV air in the upper troposphere. This flow evolution results in an elongated, positively tilted upper-tropospheric trough (i.e., PVS) downstream of the AWB axis. Subtropical PVSs modify the tropospheric environment by enhancing vertical wind shear (VWS) equatorward and reducing moisture anomalies along their axis. These changes in VWS and moisture can play an important role in enhancing or suppressing different tropical cyclogenesis pathways in the Atlantic basin. However, these environmental factors have not yet been examined when PVSs fluctuate in size, intensity, or tilt during the tropical cyclone (TC) season. Moreover, it is unclear what role the upstream ridge-induced AWB plays in the development of strong versus weak PVSs. This work is motivated by the variability in PVS intensity in the North Atlantic basin and how these intensity differences may alter environmental variables important to TC activity in the Atlantic basin.

This study investigates PVSs in the Atlantic basin using the ERA-Interim reanalysis from 1979–2015. PVSs are identified using an algorithm from June–November on the 350-K isentropic surface bounded by the 2-PVU contour. This algorithm identifies PVSs as the high PV trough that occurs downstream of the AWB axis. PVSs are then sorted into strong (top 20th percentile) and weak (bottom 20th percentile) intensity categories using standardized PV anomaly averaged within the PVS area. Strong and weak PVSs are composited prior to formation to reveal differences in diabatic heating, irrotational wind, and potential vorticity anomalies in the ridge upstream that induces AWB. Strong and weak PVSs are also composited after formation to reveal differences in VWS and precipitable water. Prior to formation, the strong PVS composite has more amplified upstream ridging associated with greater diabatic heating and negative PV advection by the irrotational wind. After formation, the strong PVS composite has increased VWS anomalies equatorward of its axis, but decreased precipitable water anomalies poleward of its axis. These environmental differences likely explain why TC seasons with more strong PVSs have decreased TC activity.