Where the winds clash: what is really triggering El Niño initiation?

Abstract El Niño–Southern Oscillation (ENSO) is the Earth’s strongest source of year-to-year climate variability, whose center of action, although being in the tropical Pacific, influences the global climate, impacting also security aspects. ENSO is commonly described as an irregular oscillation of the coupled ocean-atmosphere system, which produces an oscillation in the sea surface temperature between a warm (El Niño) and a cold (La Niña) phase induced by internal waves propagating along the equatorial wave-guide and generated by the convergence between high-frequency westerlies and the quasi-stationary easterlies. Here we show that internal, downwelling Rossby and Kelvin waves are regularly formed beneath the Easterly/Westerly Wind Convergence Zone (EWCZ). When the longitude of EWCZ is located east of 175 o E, the Kelvin waves formed beneath the EWCZ reach the eastern Pacific boundary and determine an El Niño event and, the more the EWCZ elongates eastwards into the central Pacific, the more the observed El Niño event is classified as severe, suggesting that the position of the EWCZ is an important factor to trigger El Niño. Therefore, we study zonal shifts of EWCZ and show that the EWCZ and the deep atmospheric convection displace zonally in phase with each other and with the Southern Oscillation Index, indicating that the longitude of the clash between Easterlies and Westerlies determines the climate state of the Pacific and triggers the El Niño initiation.