On the mechanisms of Atlantic Niño/Niña decadal variability

The Atlantic Niño/Niña is a dominant climate variability, exerting substantial climate impacts. Besides interannual variability, the observed Atlantic Niño/Niña also demonstrates robust variations at decadal timescale (decadal ATL). The underlying mechanisms, however, remain unclear. Here, we conduct a 300yr picontrol experiment using CESM that reasonably captures mean climate of the Atlantic cold tongue and decadal ATL. A warming of the Atlantic cold tongue weakens St. Helena anticyclone via triggering atmospheric Rossby wave, which decreases the subtropical cell and suppresses the equatorial upwelling, amplifying the initial warming. Meanwhile, the weakened anticyclone enhances wind speed over the southwestern Atlantic and cools local SST. Such cooling propagates with mean current toward east, driving an eastward propagation of negative wind stress curl anomalies and thus a cooling along thermocline over 5S-12S, with a cross basin time of 6yr. This cooling is further advected with mean current at thermocline to reach the equator, after which it develops following the Bjerknes feedback and shifts the phase of decadal ATL.