Remote impacts from the tropical Indian Ocean on haze pollution in January over the Yangtze River Delta

Haze events in the Yangtze River Delta (YRD) region have recently been occurring more frequently and with dramatic damages inflicted on human and ecosystem health.In this study,observational analyses and numerical experiments are used to investigate the meteorological conditions associated with haze pollution,with the main emphasis on the impacts of the preceding sea surface temperature (SST) in the tropical Indian Ocean (TIO).The results show that the December SST in the TIO has a significant positive correlation with the number of haze days in January over the YRD,especially during 1999-2017.In December,the positive SST anomalies in the TIO heat the overlying air,and then in the following January provoke a Matsuno-Gill-like pattern and a series of Rossby wave-like trains in the upper troposphere,transmitting signals to the YRD and downstream through the Sea of Japan and Aleutian Islands.The cyclonic anomalies in the YRD seem to significantly weaken the East Asian jet stream by means of anomalous easterlies,and subsequently affect the climate in the region.Near the surface,the increased surface air temperature and southerly winds,along with the decreased surface wind speed,accompanied by influences from upstream areas,are conducive to the occurrence of haze.These observational results were also reproduced well in CESM-LE simulations.