A relationship between planetary waves and persistent rain- and thunderstorms in China

Abstract

A spectral study of long planetary waves indicates that major and persistent heavy rains and severe thunderstorms in China during the past 50 years were closely related to specific anomalous large-scale circulation patterns, such as blocking highs over East Asia. As the heavy rains of Meiyu occur, such blocking develops at high latitudes, in particular over the Sea of Okhotsk and over Lake Baikal. A southward shift of the westerlies allows upper troughs to pass over the Tibetan Plateau and low pressure disturbances, generated over the plateau area, successively move along the Yangzi River valley, causing prolonged and heavy rainfall. The establishment of an upper trough over the Bay of Bengal also provides an important condition for heavy rainfall in South China. In North and Northwest China, large-scale circulation patterns responsible for heavy rainfall events are characterized by the establishment and slow regression of a high-pressure system over the Seas of Japan and Okhotsk, or by the rapid amplification of long-wave ridges into blocking ridges near the position of seasonal-mean troughs. Owing to the blocking action of these ridges, the rain-bearing synoptic systems (typhoons, upper troughs and low-level vortices) often undergo sudden changes in their tracks or tend to slow down, becoming nearly stationary, thus enhancing and prolonging the heavy rainfall events. At the same time, a well-developed low-pressure zone in the tropics and subtropics helps to establish a strong, low-level easterly wind, which often assumes the intensity of a low-level jet. This flow configuration offers a major moisture supply for heavy rains. The study also reveals that ultralong planetary waves during heavy rainfall are more stationary than under climatological mean conditions. The departures from normal of their phase angles appear to be an important factor in generating heavy rain and severe thunderstorm episodes.