Climatology of Cyclogenesis over the East China Sea

Abstract

Extratropical cyclogenesis occurs off the coast of China, over the East China Sea and surrounding areas, as a result of cold-air outbreaks from the mainland about every five days, on the average, during winter months. Many of these storms subsequently propagate northeastward into the Pacific, and in the process they have a substantial impact on the weather of highly populated areas of northeast China, the Koreas and Japan. This paper presents a climatological analysis of this cyclogenesis for the period 1899?1962 based on U.S. Weather Bureau historical weather maps. The averaged seasonal variability during the period is discussed in some detail with comparisons to sea-surface temperature variability, taken from the Historical Sea?Surface Temperature Project data, revealing that the meridional gradient of sea surface temperature across the East China Sea plays a significant role in the annual cycle of cyclogenesis. In contrast, wind speed, air?sea temperature difference and averaged heat fluxes show little relationship to the cyclogenesis. This suggests that the surface baroclinicity associated with this temperature gradient triggers baroclinic instability on the subtropical jet stream as it enters and merges with the Aleutian Low to the east. Interannual variability of frequency of storm formation is shown to be linked to variations in the surface pressure at Darwin, Australia and, by inference, to the Southern Oscillation. This appears to be a manifestation of the West Pacific teleconnection in which warm equatorial temperatures and anomalous deep convection patterns in the Central Pacific are associated with a deepening and broadening to the west of the Aleutian Low, thus increasing the potential for storm formation over the East China Sea. Variability of storm formation at the frequency of the quasi?biennial oscillation is also suggestive of this mechanism.