Asymmetry of the winter extra-tropical teleconnections in the Northern Hemisphere associated with two types of ENSO

Asymmetric atmospheric responses to ENSO are revisited after dividing it into two types: eastern-Pacific (EP) and central-Pacific (CP) ENSO. The EP ENSO triggers two obvious asymmetric atmospheric teleconnections: One is the Pacific-North American-like teleconnection. Its asymmetry is characterized by weaker amplitudes during the EP La Nia than EP El Nio, which is caused by a much weaker EP La Nia tropical forcing and the resultant weaker extra-tropical vorticity forcing. The other is the Atlantic-Eurasian teleconnection with negative height anomalies in the subtropical Atlantic and Eurasia and positive anomalies in the high-latitude Atlantic and northeast Asia, which appears during the EP La Nia but not during the EP El Nio. The background state plays a vital role in this asymmetry. The EP La Nia-type basic state is more conducive to propagation of the wave rays into the Atlantic-Eurasian region compared to EP El Nio situation. In contrast, the CP ENSO yields an Arctic Oscillation-like teleconnection, presenting an appreciable asymmetry in the subtropical amplitudes that are stronger during the CP El Nio than during the CP La Nia. In this case, the distinct effects of the different background state on the equatorward wave rays are responsible for this asymmetry. Under the CP El Nio-type background state, the equatorward wave rays tend to be reflected at the latitudes where the zonal wind equals zero (U = 0), and then successfully captured by the subtropical westerly jet. However, under the CP La Nia-type background state, the equatorward wave rays disappear at U = 0 latitudes.