An Extreme Tropical Cyclone Silence in the Western North Pacific in July 2020

Abstract

ABSTRACT The western North Pacific (WNP) tropical cyclone (TC) number in July 2020 hit a record low since 1949, with no TC formation throughout July. It is shown that this record-breaking TC event can be largely attributed to the extremely strong lower-level anticyclone and subsidence over the TC-genesis-prone region (GPR), concurrent with an intensified western Pacific subtropical high (WPSH) there. This configuration is closely linked to both the air–sea interaction in the tropical Indo-western Pacific and the vigorous wave activities over the mid-high latitude Eurasia. The warming western Indian Ocean and the resultant active convection excited an anomalous lower-level anticyclone and descending atmospheric Kelvin wave over the GPR, together with a strengthened WPSH. Accompanied with this monthly background field, long-lasting Madden-Julian Oscillation was confined west of 90°E, which intensified the proposed anticyclonic vorticity and decreased the moisture condition in the South China Sea and tropical WNP. Meanwhile, two active wave fluxes prevailed across the jet streams over the mid-high latitude Eurasia with a quasi-barotropic cyclonic abnormity dominating from Northeast China to Japan. The resultant positive potential vorticity on its south excited the anomalous local ascending around 35°N, which favoured the upper-level convergence in the tropical WNP and reinforced the local subsidence through modulating the meridional circulation. This extratropical impact further aggravated the suppressed circulation condition for TC formation and increased the chance that the extreme tropical cyclone silence would happen.