The Importance of Critical Layer in Differentiating Developing from Nondeveloping Easterly Waves

Recently Asaadi et al. found that an easterly wave (EW) train over the Atlantic and eastern Pacific is oriented in a southeast–northwest direction because of the observed tilt in the easterly jet. This tilt results in only one out of four (~25%) waves to be located at the cyclonic critical layer south of the jet axis in a comoving frame, and they subsequently developed into named storms. Asaadi et al. suggested a geometrical view for developing disturbances, which is the coexistence of a nonlinear critical layer and a region of weak meridional potential vorticity (PV) gradient over several days. Asaadi et al. focused on the developing waves and did not investigate the nondeveloping ones. To determine whether the nondeveloping EWs are not associated with a critical layer, a simple objective tracking technique is used to identify EWs. Composite views of the large-scale structure and characteristics of nondeveloping EWs show that ~91% of nondeveloping waves are not located on a critical layer, while the remaining ~9% indicate characteristics similar to the developing waves. Examination of the composite Okubo–Weiss parameter indicates that the nondeveloping waves are characterized by larger negative values, implying that they are dominated by deformation, unlike developing waves, which tend to be more immune from the deformation.