The Relationship Between Potential Vorticity Streamer Activity and Tropical Cyclone Activity in the North Atlantic Basin

Potential vorticity streamers (PVSs) are elongated filaments of high potential vorticity (PV) air in the upper-troposphere that occur in North Atlantic (NATL) basin during the tropical cyclone (TC) season. PVSs occur in conjunction with anticyclonic Rossby wave breaking (AWB), where upstream low PV air is advected poleward over downstream high PV air. Repeated instances of AWB can often produce PVSs in the same location that result in an elongated, positively tilted upper-tropospheric trough downstream of the AWB axis. In the time-mean, this positively tilted trough resembles the Tropical Upper Tropospheric Trough (TUTT), also known as the mid-ocean trough. Prior research has shown that a stronger NATL TUTT negatively impacts overall TC activity in the North Atlantic basin due to increases in vertical wind shear (VWS) and decreases in moisture that occur in the main development region (MDR). This study is motivated by these apparent relationships between PVSs, the TUTT, and TC activity, where our main objective is to investigate is how and why changes in PVS activity affect changes in TC activity.

This study investigates PVSs in the NATL 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. PVS cases are then integrated by their area and intensity in order to create a metric that quantifies overall PVS activity during the North Atlantic basin TC season. This seasonal PVS activity metric is compared to a measure of TC activity known as accumulated cyclone energy (ACE) that integrates TC intensity by their duration. Seasonal ACE is calculated for NATL basin TC events using the International Best Track Archive for Climate Stewardship (IBTrACS). In general, seasonal PVS activity is negatively correlated with ACE, as increases in PVS activity enhance the time-mean TUTT, which enhances VWS and reduces moisture in the MDR. Interestingly, PVS activity on intraseasonal timescales peaks early in the TC season (July), with early season (June–July) PVS activity positively correlated to later season (August–November) PVS activity. Early season PVS activity may then be a useful predictor of TC activity later in the season since later season PVS activity is associated with increases in VWS and decreases in moisture in the MDR.