Linkages between Potential Vorticity Streamer Activity and Tropical Cyclone Predictability on Subseasonal Timescales


Potential vorticity streamers (PVSs) are elongated filaments of high PV air near the tropopause. In the warm season, anticyclonic Rossby wave breaking (AWB) produces enhanced PVS activity, which in turn modifies the equatorward tropical environment by enhancing vertical wind shear (VWS). This enhanced VWS can play an important role in suppressing nearby tropical cyclone (TC) activity. Given the important role that PVSs play in modifying their local environment, forecasts of PVS activity on subseasonal timescales may also influence forecasts of TC activity.
This study uses Navy Earth System Prediction Capability (Navy ESPC) 45-day forecasts initialized during Boreal summer 2009-2015 to investigate subseasonal predictability of PVSs and TCs in the North Atlantic. PVSs are identified on the 350-K isentropic surface bounded by the 2-PVU contour and defined as the high PV trough axis downstream of the AWB axis. TCs are identified in the forecasts using the TempestExtremes detection algorithm that tracks warm-core lows. PVS and TC activity metrics that sum the number and intensity of events for a given time period are also computed. We first use skill scores and mean state biases to determine the typical predictability of PVS activity, and then sub-select high and low PVS activity forecasts to determine how PVS forecast errors impact TC activity forecast errors. Results show that PVS activity can modulate TC activity at subseasonal timescales, with over-forecasted PVS activity corresponding to underestimated forecasts of TC activity and vice versa. This inverse correlation is consistent with enhanced VWS occurring equatorward of PVS troughs in the high PVS forecasts.