The key role of background sea surface temperature over the cold tongue in asymmetric responses of the Arctic stratosphere to El Niño–Southern Oscillation

The response of the Arctic stratosphere to ElNiño activity is strong but the response to LaNiña activity is relatively weak. The asymmetric responses of Arctic stratosphere to ElNiño and LaNiña events are thought to be caused by asymmetric ElNiño–SouthernOscillation (ENSO) teleconnections. Here, we suggest that the background sea surface temperature (SST) over cold tongue of tropical eastern Pacific may be an important contributor to the asymmetric ENSO teleconnections. The atmosphere is very sensitive to tropical SST variations in the range of 26 °C–30 °C.During ElNiño events, the background SST over cold tongue plus ElNiño SST anomalies typically falls into the range. Under these conditions, the atmospheric response to ElNiño SST anomalies is strong. During LaNiña events, the background SST plus LaNiña SST anomalies is typically below the range, which leads to aweak response of the atmosphere to SST anomalies. The proposedmechanism is well supported by simulations.