Tropical and Subtropical North Atlantic Vertical Wind Shear and Seasonal Tropical Cyclone Activity

Given recent insights into the role of anticyclonic Rossby wave breaking (AWB) in driving subseasonal and seasonal North Atlantic tropical cyclone (TC) activity, this study further examines tropical versus subtropical impacts on TC activity by considering large-scale influences on boreal summer tropical zonal vertical wind shear (VWS) variability, a key predictor of seasonal TC activity. Through an empirical orthogonal function analysis, it is shown that subtropical AWB activity drives the second mode of variability in tropical zonal VWS, while El Nino-Southern Oscillation (ENSO) primarily drives the leading mode of variability. Linear regressions of the four leading principal components against tropical North Atlantic zonal VWS and accumulated cyclone energy show that while the leading mode holds much of the regression strength, some improvement can be achieved with the addition of the second and third modes. Furthermore, an index of AWB-associated VWS anomalies, a proxy for AWB impacts on the large-scale environment, may be a better indicator of summertime VWS anomalies. The utilization of this index may be used to better understand AWB's contribution to seasonal TC activity.