Extratropical cyclones over East Asia: climatology, seasonal cycle, and long-term trend

Extratropical cyclones (ETCs) in East Asia are automatically detected and tracked by applying a Lagrangian tracking algorithm to the 850-hPa relative vorticity field. The ETC statistics, which are derived from ERA-Interim reanalysis dataset from 1979 to 2017, show that East Asian ETCs primarily form over Mongolia, East China, and the Kuroshio Current region with a maximum frequency of six to seven cyclones per month. Both Mongolia and East China ETCs are initiated on the leeward side of the mountains. While Mongolia ETCs downstream of the Altai-Sayan Mountains develop slowly, East China ETCs downstream of the Tibetan plateau develop rapidly as they travel across the warm ocean. Both of them show a maximum frequency and intensity in spring rather than in winter. In contrast, oceanic ETCs across the Kuroshio Current and the Kuroshio-Oyashio Extension, where sea surface temperature gradient is sharp, reach a maximum frequency in winter although their intensity is still maximum in spring. On the decadal timescale, both ETC frequency and intensity exhibit insignificant trends. Exceptions are springtime East China and summertime Mongolia ETCs whose frequencies have slightly decreased since 1979. This declining trend is consistent with the enhanced static stability in the region.