Interannual Variation in Mainland China's Atmosphere Clearness Index Associated with El Niño-Southern Oscillation

Atmosphere clearness is the single most essential parameter determining surface solar radiation. However, few studies have investigated the interannual variations in China's atmosphere clearness and the impacts of El Nino-Southern Oscillation on it. This study aims to fill the knowledge gap by calculating the clearness index using the China Meteorological Forcing Dataset version 1.7 and then analyzing the correlations between the interannual anomaly and the Nino-3.4 index. The results show that there is a significantly negative correlation in the southeastern coastal regions, northern Xinjiang, northeastern Xizang, and areas from northern Hebei to middle Inner Mongolia. In these areas, the higher the ENSO index, the lower the clearness index, and, coincidingly, positive precipitation anomalies are reported in previous studies. The impacts of El Nino and La Nina vary with seasons. The ENSO events have generally opposite impacts in the seasons other than summer. El Nino tends to decrease the clearness index, while La Nina tends to decrease the atmosphere clearness in most territories of China. The impacts are different in the places of statistical significance. The negative impacts of El Nino are significant in the southeastern coastal regions of China in winter and in northeast China in autumn. The positive impacts of La Nina are significant in the southwestern and northeastern China in the autumn. In these seasons, ENSO's impacts scale with the strength of the event. Stronger events amplify the magnitude of the anomalies, while the spatial patterns of the anomalies are kept almost invariant. In summer, ENSO's impacts exhibit different characteristics than in the other seasons and between the El Nino and La Nina events. For the El Nino events, the impacts are insignificant in most territories of China, even for the strong ones. La Nina has a larger influence on the summertime clearness than El Nina, and the spatial pattern of the La Nina's impacts varies with the event strength. The anomalies during strong La Nina events have a tripolar pattern with a positive anomaly in south and north China and a negative anomaly in between. The pattern suggests that the relationship between the clearness index and precipitation is different during strong La Nina events. These findings would provide valuable insights into the interannual variations of atmosphere clearness in China and could be clues to further investigation. The revealed impacts of El Nino-Southern Oscillation should be important for developing solar energy.