Synergic influence of lower and upper troposphere large-scale circulations on tropical cyclone genesis over the western North Pacific

While influences of lower and upper troposphere large-scale circulations on tropical cyclone (TC) genesis in the western North Pacific (WNP) is generally well understood, their synergic influences on the WNP TC genesis and their relative contribution remain unclear. Here two leading modes of lower and upper troposphere large-scale circulation over the WNP are identified using multivariate empirical orthogonal function analysis on the low and upper-level large-scale parameters. The first mode features an anticyclonic circulation over the WNP, while the second mode is characterized by a cyclonic circulation in the tropical eastern WNP but an anticyclonic circulation to the north WNP. Both two modes have profound impacts on the WNP TC genesis, which can be reasonably interpreted by the collective influences of lower and upper-level large-scale parameters. Further diagnostic suggests that the lower troposphere large-scale circulations play a crucial role, while the upper troposphere large-scale circulations play a secondary but important role in TC genesis over the WNP. Moreover, it is found that the extremely favorable lower and upper large-scale conditions associated with the extremely negative phases of two leading modes led to the unprecedented WNP TC genesis frequency in 1994 since the geo-satellite era. The result indicates the important role of synergic influences of the lower and upper troposphere large-scale circulations in the interannual variability of TC genesis over the WNP.