Impacts of atmospheric circulations on the interannual variability of springtime haze frequency in the Pearl River Delta region, China

The springtime haze events in Guangdong Province are mainly located in the Pearl River Delta (PRD) region, which has the second highest number of haze days among four seasons. The major regional-scale circulation patterns affecting the interannual spring haze days in the PRD region are closely associated with high pressure extending from Baikal Lake to Okhotsk Sea, the North Pacific low pressure, the East Asian Trough, low-level meridional wind over eastern China, and upper tropospheric East Asian Jet. We define a comprehensive circulation index (CCIDX) to express joint effects of major regional-scale circulations. The positive phase of the CCIDX is intimately linked to negative Scandinavian Peninsula (SCAND)-like and WP teleconnection patterns, which induces abnormal northerly winds, low temperatures, and less precipitation over the PRD region by modulating regional atmospheric circulations. In addition, the positive CCIDX favors the enhancement the negative 850 hPa Eady growth rate over the middle and high latitude areas in East Asia, thus weakening the atmosphere baroclinity in the lower troposphere and hampering transportation of pollutants north of southern China into the Pacific Ocean. As such, pollutants accumulated to the north of southern China are advected to the PRD region by pronounced low-level northerly airflows. The positive phase of the CCIDX can provide favorable local meteorological circumstances (i.e., enhanced vertical descending motion, reduced lower specific humidity, decreased surface wind speed, and enhanced low-level atmospheric inversion) required for the increased incidence of haze pollution over the PRD region. It is found that the increase of specific humidity with height, which is mainly contributed by horizontal moisture advection and apparent moisture sink, contributes largely to the sustainability of low-level atmospheric inversion in the PRD region via absorptions of long-wave radiation. Above 700 hPa, enhanced horizontal warm advection and diabatic heating also provide favorable conditions for maintenance of low-level atmospheric inversion.