The Complex Relationship Between ENSO And NAO And Its Mechanism

The relationship between ENSO and NAO are investigated by using monthly sea surface temperature (SST) from HadISST, reanalysis data from NCEP/NCAR, global land surface temperature and precipitation data from NOAA and the monthly/daily surface air temperatures grid data in China from CMA. The non-linear fearures of local air-sea coupling and extratropical atmospheric responses during two types of ENSO are demonstrated based on observed and simulated results. In particularly, the different climate responses associated with two-type ENSO events over Northern China are also addressed. The major conclusions are summarized as follows:(1) The ENSO exhibits a highly complex relationship with the NAO.Nonlinear ENSO/NAO relationship is displayed, which is closely related to different distributions of the tropical Pacific SST anomalies associated with the two types of ENSO. Linear NAO responses are detected for the central Pacific (CP) ENSO, while nonlinear NAO responses are observed for the eastern Pacific (EP) ENSO. Both EP and CP El Nino events are accompanied by a negative NAO-like atmospheric response. However, almost opposite NAO-like atmospheric anomalies appear during two types of La Nina winters.(2) Different extra-tropical atmospheric responses to different ENSO types are mainly attributed to nonlinear air-sea linkage over the tropical eastern Pacific. The local atmospheric response to the CP ENSO exhibits highly linear features since the air-sea action center is located near the eastern edge of the Pacific warm pool with a relatively high background SST. For the EP ENSO covered by a relatively low background SST, strong positive SST anomalies during the EP El Nino can result into positive convection anomalies; however,almost very weak convection anomalies are forced by the EP La Nina-related SST anomalies considering that the convection is dependent on the SST threshold. These different convection responses due to the nonlinear relationship between the SST and atmospheric anomalies, could result in very different teleconnection pattern over the extra-tropics. Therefore, the tropical SST anomalies and associated local atmospheric responses could be an important factor for their different NAO response to different ENSO types.These observed phenomena can be realistically reproduced by experiments.(3) The ENSO affects winter mean surface air temperature anomalies over northern Eurasian mainly through modulating the atmospheric circulations over mid-latitude North Atlantic and thus modifying the zonal temperature advection over northern Eurasia. During the two types of El Nino and EP La Nina winters, negative NAO-like atmospheric anomalies are evident over North Atlantic region , which tends to inhibit the warm and moist air transportation from Atlantic Ocean towards Eurasian continent and thus favors a colder than normal winter in northern China. In comparison, a positive NAO-like atmospheric anomaly pattern occurs over the North Atlantic region during the CP La Nina winter, while its positive anomaly center shifts to the westward. The significant temperature anomalies are mainly confined in Europe and little significant signal can be observed over downstream northern China. In addition, the daily temperature over northeastern China tends to be colder than normal during EP El Nino and CP El Nino winters, suggesting that the cold extremes occurred more frequently. However, the linkage between La Nina events and cold extremes seems to be weak.