Different ENSO impacts on early and late winter synoptic-scale air temperature variability over eastern China and possible mechanisms

Based on daily mean air temperature data from the first-generation global atmosphere reanalysis product (CRA) of China, the reconstructed monthly sea surface temperature data from National Oceanic and Atmospheric Administration (NOAA), and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) atmospheric circulation reanalysis data, the present study investigates influences of El Niño-Southern Oscillation (ENSO) on the early and late winter synoptic-scale air temperature variability over eastern China and possible mechanisms. It is revealed that the ENSO impacts differ remarkably between early and late winter. That is, the synoptic-scale air temperature response to ENSO over eastern China is weak in early winter but strong in late winter. In late winter, there is a significant positive correlation between ENSO and the synoptic temperature variability in the middle and lower reaches of the Yangtze river of eastern China. It suggests that the synoptic-scale air temperature variability and temperature fluctuation in late winters of El Niño (La Niña) years are usually stronger (weaker) than that in normal years. In late winter, ENSO can modulate the atmospheric baroclinicity by changing the meridional temperature gradient in the middle and high latitudes of Eurasia, which affects the synoptic-scale variation of atmospheric circulation in East Asia and subsequently affects the synoptic-scale air temperature variability over eastern China. Specifically, in late winters of El Niño years, the north-south temperature gradient is larger and the corresponding atmospheric baroclinicity is stronger, which could lead to more active meridional wind activities and more frequent cold air activities. Roughly opposite mechanisms apply during late winters of La Niña years. However, in early winter, ENSO has a weak influence on the meridional temperature gradient in the middle and high latitudes of Eurasia, and thus exhibits minor effects on the synoptic-scale temperature variability over eastern China. The results can enrich our understanding of the ENSO impact on air temperature variability in China, and provide references for improving seasonal prediction of wintertime air temperature over China.