Large-scale circulation patterns favourable to tropical cyclogenesis over the western North Pacific and associated barotropic energy conversions

Using the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) reanalysis and the Joint Typhoon Warning Center (JTWC) best track data for the period 1991-2010, this study classifies five large-scale circulation patterns in association with the tropical cyclone (TC) formation over the western North Pacific (WNP): the monsoon shear (MS), the monsoon confluence (MC), the reverse-oriented monsoon trough (RMT), the monsoon gyre (MG), and the trade wind easterlies (TE). The first three patterns account for about 80% of tropical cyclogeneses. Through a diagnosis of energetics, it is found that the tropical cyclogenesis in the MS, MC, and RMT patterns is highly associated with the barotropic energy conversion. Analysis shows that the horizontal shear of basic zonal flow provides a favourable condition for the eddy kinetic energy (EKE) growth in the MS pattern. When a TC forms in the MC pattern, the horizontal shear and convergence of basic zonal flow are both important for the EKE growth. When the basic flow is the RMT pattern, in addition to the horizontal shear of basic flow, zonal and meridional wind convergence play an important role for tropical cyclogenesis over the WNP and the South China Sea, respectively. However, the barotropic energy conversion appears not to be a main mechanism for the EKE growth in the MG and TE patterns. Copyright (c) 2013 Royal Meteorological Society