The impact of Climate Change on the Western Pacific Subtropical High and the related ozone pollution in Shanghai, China

Severe ozone (O(3)) episodes occur frequently in Shanghai during late-summers. We define geopotential height averaged over the key area region (122.5°E-135°E, 27.5°N -35°N) at 500 hPa as a WPSH_SHO(3) index which has high positive correlation with surface O(3) concentration in Shanghai. In addition, the index has a significant long-term increasing trend during the recent 60 years. Analysis shows the meteorological conditions under the strong WPSH_SHO(3) climate background (compared to the weak background) have several important anomalies: (1) A strong WPSH center occurs over the key area region. (2) The cloud cover is less, resulting in high solar radiation and low humidity, enhancing the photochemical reactions of O(3). (3) The near-surface southwesterly winds are more frequent, enhancing the transport of upwind pollutants and O(3) precursors from polluted regions to Shanghai and producing higher O(3) chemical productions. This study suggests that the global climate change could lead to a stronger WPSH in the key region, enhancing ozone pollution in Shanghai. A global chemical/transport model (MOZART-4) is applied to show that the O(3) concentrations can be 30 ppbv higher under a strong WPSH_SHO(3) condition than a weak condition, indicating the important effect of the global climate change on local air pollution in Shanghai.