Interannual Variability of January Haze Pollution in the Sichuan Basin, China and Its Association With the SSTs Over the Southeastern Indian, Tropical Pacific and North Pacific Oceans

Haze pollution occurs in almost every winter month in the Sichuan Basin, with a higher frequency in January. The interannual and decadal variations of January haze are inconsistent with those in the other winter months, showing a noticeable sub-seasonal variability in haze pollution. The findings of this study indicate that the noticeably higher haze days over the Sichuan Basin in January are closely associated with the simultaneous sea surface temperature (SST) warming in the North Pacific Ocean (NPO) and tropical Pacific Ocean (TPO), and with the SST cooling in the southeastern Indian Ocean (SIO). When the SST in the NPO warms in January, the negative phase of the Arctic Oscillation is stimulated through a high-latitude path, the Middle East jet stream weakens, and the Siberian high weakens, and the Aleutian Low intensifies, indicating that cold air activities are restricted and their paths are eastward. This synoptic situation results in weaker cold air over the Sichuan Basin. Meanwhile, the southern branch of the Rossby wave train from the eastern Pacific Ocean propagates along Western Europe to the East Asian coast, leading to high pressure and anticyclonic circulation anomalies over the Sichuan Basin, resulting in reduced precipitation and more haze days in the Sichuan Basin. When the January SST shows warming in the TPO and cooling in the SIO, prominent anticyclonic anomalies strengthen over the Tibetan Plateau and its downstream regions, weakening the southwesterly warm moisture flow. This situation provides unfavourable conditions for precipitation occurrence, thereby facilitating haze pollution over the Sichuan Basin. Consequently, the January SST anomalies in the NPO, TPO and SIO may be associated with the anomalies of local meteorological elements, which restrict the vertical and horizontal dispersion of haze pollution over the Sichuan Basin in January.