Influence of retreating Barents–Kara sea ice on the periodicity of El Niño–Southern Oscillation

The abrupt decline in sea ice in the Barents–Kara (BK) Sea because of global warming has been argued to influence not only higher latitudes but also the tropics. Using EC‐Earth model simulations, we demonstrated that the El Niño–Southern Oscillation (ENSO) period becomes longer when BK sea ice substantially decreases. As BK sea ice was forcibly reduced through nudging experiments, the mean Walker circulation shifted to the west, and the zonal sea surface temperature contrast in the tropical Pacific was enhanced. Consequently, the western Pacific mean thermocline became deeper, which reduced the sensitivity of oceanic wave response to wind forcing. Therefore, the oceanic Kelvin waves reflected by ENSO‐induced surface winds, a primary delayed negative feedback factor, were significantly weakened. Thus, ENSO phases could be sustained for longer. From EC‐Earth model simulations, we find that the period of ENSO becomes longer when BK sea ice is substantially reduced (a). The reduction of BK sea ice leads to the enhanced trade wind over the central‐to‐western Pacific resulting in deeper mean thermocline (b). As the deeper thermocline relaxes the sensitivity of oceanic wave response to wind, the reflected Kelvin wave, that is, a primary delayed negative feedback, is significantly weakened, and thus ENSO could sustain longer.