Different Effects Of ENSO On Meteorological-scale Temperature Variability In Eastern China Before And After Winter And Possible Mechanism

Based on the daily averaged air temperature data from China’s first-generation global atmosphere reanalysis product(CRA),reconstructed monthly sea surface temperature data from the National Oceanic and Atmospheric Administration(NOAA),and the National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR)atmospheric circulation reanalysis data,this paper examines the impact of El Ni?o-Southern Oscillation(ENSO)on the sub-seasonal synoptic-scale air temperature variability.The study highlights the differences of ENSO impact on the synoptic-scale air temperature variability between the early and late winter over eastern China.The possible physical mechanisms are also discussed.The results are as follows:(1)In winter,the synoptic-scale temperature variability show obvious spatial and temporal variations over China,with large variance in the east/south and small variance in the west.ENSO has a significant impact on the winter synoptic-scale air temperature variability over eastern China,particularly in the middle-lower reaches of the Yangtze River and the Hetao regions.These regions display a positive interrelation between the synoptic-scale temperature variability and ENSO.The specific performance is that in the winter of El Ni?o,the synopticscale temperature variability in eastern China is larger than the climatic state;In the winter of La Ni?a,the synoptic-scale temperature variability in eastern China is relatively small compared to the climate state.(2)The impacts of ENSO on synoptic-scale temperature variability over eastern China differ remarkably between early and late winters.Specifically,the synoptic-scale air temperature response is weak in early winter but strong in late winter.In late winter,a significant positive correlation exists between ENSO and the synoptic temperature variability in the middle-lower reaches of the Yangtze River in eastern China.It suggests that the synopticscale air temperature variability and temperature fluctuation in El Ni?o(La Ni?a)late winters are usually stronger(weaker)compared to normal years.In late winter,ENSO can modulate the atmospheric baroclinicity by changing the north-south temperature gradient over Eurasia,which affects the synoptic-scale variation of atmospheric circulation in East Asia region and thus the synoptic-scale air temperature variability over eastern China.In El Ni?o late winters,the north-south temperature gradient is larger and the corresponding atmospheric baroclinicity is stronger,which could lead to more active meridional wind activity and thus more frequent cold air activity.Opposite mechanisms apply during La Ni?a late winters.However,in early winter,ENSO has a weak influence on the meridional temperature gradient in the middle and high latitudes of Eurasia,thus exhibiting a minor effect on the synoptic-scale temperature variability over eastern China.(3)The impact of two types of ENSO on the synoptic-scale air temperature variability over eastern China differs significantly.There is a clear linear relationship between EP ENSO events and synoptic-scale air temperature variability over eastern China in late winter,while CP ENSO events have a weak relationship.During EP El Ni?o(La Ni?a)events,the synoptic-scale air temperature variability over eastern China in late winter increases(decreases)significantly,with a symmetrical effect in space.The mechanism is that two types of ENSO modulate the synoptic-scale variability of the atmospheric circulation in the downstream East Asia region by regulating the synoptic-scale transient eddy activity,and then affect the synoptic-scale air temperature variability over eastern China.The results can enrich our understanding of the impact of ENSO on China’s air temperature variability,and have reference value for improving the seasonal prediction of wintertime air temperature over China.