Relationship Between the Pacific‐North American Pattern and the Frequency of Tropical Cyclones Over the Western North Pacific

Abstract

AbstractThe frequency of tropical cyclones (TCs) over the western North Pacific during June–November has a significant inverse correlation with the concurrent Pacific‐North American (PNA) pattern from 1965 to 2016. During low PNA years, more TCs form north of 12.5°N, with significantly greater TC occurrences from 15 to 20°N, compared to high PNA years. The difference in TC genesis location can be explained by the differences in the genesis potential index derived from the environmental variables in both PNA phases. The PNA influences TC formation primarily by modulating large‐scale dynamic conditions, with thermodynamic conditions playing a lesser role. In low PNA years, low‐level anomalous cyclonic lows over the Philippines and in the subtropical central Pacific provide significant positive relative vorticity anomalies favorable for TC genesis. Additionally, there is also less vertical wind shear to the north of the Philippines due to enhanced winds at low levels and weaker winds at upper levels.