Climatological intraseasonal oscillation of the summertime haze-fog in eastern China

Abstract

Eastern China has recently experienced frequent haze-fog events that are widely thought to be modulated by atmospheric conditions. Using the climatological pentad mean occurrence frequency of haze-fog, for the first time, we found that the haze-fog frequency in eastern China (115°–125°E, 20°–40°N) presents a statistically significant climatological intraseasonal oscillation (CISO) with a period of approximately 50 days. The haze-fog CISO exhibits three cycles from May to September, with peaks occurring in mid-June, late July and mid-September. Among these three cycles, the first two have relatively large amplitudes and usually show northward phase propagation. A positive anomaly of haze-fog occurrence frequency propagates primarily from South China to North China, with a decreasing speed from 1.0 to 0.54° lat day−1. By dividing the cycle of haze-fog CISO into 8 phases, regional differences in haze-fog CISO are discovered. North China has a shorter “polluted” phase than the Yangtze River Basin and South China. Further analysis shows that the haze-fog CISO is connected to the summer monsoon via physical and chemical processes. The enhanced monsoon directly reduces haze-fog by active convection and strong winds, while the slowing horizontal wind speeds and anomalous sinking motion in the weakened phase of monsoon lead to an increase in haze-fog. In addition, the weakened monsoon will also indirectly increase haze-fog by enhancing the photochemical process.