Upper-Tropospheric Environment–Tropical Cyclone Interactions over the Western North Pacific: A Statistical Study

Based on 25-year（1987–2011） tropical cyclone（TC） best track data, a statistical study was carried out to investigate the basic features of upper-tropospheric TC–environment interactions over the western North Pacific. Interaction was defined as the absolute value of eddy momentum flux convergence（EFC） exceeding 10 m s;d;. Based on this definition, it was found that 18% of all six-hourly TC samples experienced interaction. Extreme interaction cases showed that EFC can reach;20 m s;d;during the extratropical-cyclone（EC） stage, an order of magnitude larger than reported in previous studies.Composite analysis showed that positive interactions are characterized by a double-jet flow pattern, rather than the traditional trough pattern, because it is the jets that bring in large EFC from the upper-level environment to the TC center. The role of the outflow jet is also enhanced by relatively low inertial stability, as compared to the inflow jet. Among several environmental factors, it was found that extremely large EFC is usually accompanied by high inertial stability, low SST and strong vertical wind shear（VWS）. Thus, the positive effect of EFC is cancelled by their negative effects. Only those samples during the EC stage, whose intensities were less dependent on VWS and the underlying SST, could survive in extremely large EFC environments, or even re-intensify. For classical TCs（not in the EC stage）, it was found that environments with a moderate EFC value generally below;5 m s;d;are more favorable for a TC’s intensification than those with extremely large EFC.