Detecting time-delayed causal interaction between Northern Hemisphere annular mode and winter surface air temperature over Northeast China: a case study of 2009/2010 winter

The extremely low value of the Northern Hemisphere annular mode (NAM) that occurred in the winter of 2009/2010 led to rare extreme low temperatures and heavy snow disasters over Northeast China. Studies on the effect of NAM on temperature often rely on correlation analysis, synthetic analysis, regression analysis, and other statistical methods. However, although the statistical method is instructive to the discussion of physical mechanisms, it cannot prove whether there is causality between variables. Here, we apply extended convergent cross mapping (ECCM) to detect the time-delayed causal interaction between NAM and winter surface air temperature (SAT) over Northeast China. Results indicate that the winter SAT information is encoded in the contemporaneous 1000 hPa NAM and vice versa, showing the contemporaneous bidirectional causality between them. In addition, we detected a unidirectional causality between 500 hPa NAM and winter SAT. NAM at 500 hPa can influence the SAT but not vice versa. Moreover, we examine the time-delayed causality between winter SAT and the stratospheric NAM at 50 hPa. Stratospheric NAM can influence the SAT approximately 10 days later. These causal relationships and feedback effects in dynamic systems have opened up the possibility of improving the prediction of winter SAT over Northeast China by using the NAM as an external factor.