A New Detection Scheme of Wave-Breaking Events with Blocking Flow Configurations

Blocking flow configurations, which tend to accompany strong circulation anomalies and therefore can cause extreme weather conditions, have recently been studied in relation to large-scale wave breaking (WB). Although WB events have been detected often from an instantaneous morphology perspective, the present study proposes a new approach for the detection from a wave-activity perspective in focusing on its accumulation, saturation, and release. This evolution of wave activity is theoretically equivalent to anomalous potential vorticity (PV) flux with its sign changing from negative to positive, which is utilized in this study to detect WB events that accompany high-amplitude height anomalies and blocking flow configurations. As in previous studies, a given WB event is classified into a high pressure type or low pressure type depending upon the sign of the primary PV anomaly center and further into an eastward or westward type depending upon the longitudinal movement of that center. The new method applied to the wintertime Northern Hemisphere shows that a WB event with a blocking anticyclone is likely to accompany an eastward-moving PV anomaly center, occurring mostly under anticyclonic westerly shear. By contrast, a WB event with a strong cyclonic anomaly mostly accompanies the eastward-moving PV anomaly center under cyclonic westerly shear. Composite analysis confirms the consistency between the sign-changing anomalous PV flux and convergence/divergence of wave-activity flux of quasi-stationary Rossby wave trains around the WB region.