Transition Between Forced and Oscillatory ENSO Behavior Over the Last Century

As the largest mode of coupled climate variability, the El Niño Southern Oscillation (ENSO) carries consequences for weather patterns worldwide. Because of its impacts, and the subsequent importance of predicting when ENSO might occur, there has been lengthy research into precursor mechanisms that initiate ENSO events. In this paper, thanks to the length of the SODAsi.3 data set, we study the relation between ENSO and a subset of known precursors over 140 years (1871-2011). We uncover that the influence of North Pacific Oscillation (NPO) related precursors—namely the Trade Wind Charging and North Pacific Meridional Mode (TWC/NPMM)—upon ENSO is nonstationary. The TWC/NPMM ENSO coupling is strong from 1871 to 1920, then weakens before regaining significance from 1960 onward. Importantly, in the intervening period between 1920 and 1960, not only does the TWC/NPMM ENSO connection disappear, there are also no other wind related drivers preceding ENSO events during this period. We find that in the absence of wind driven precursors during this intervening period the temporal characteristics of ENSO variability itself change, as the signal oscillates within a relatively narrow 6-7 year periodicity band. These features set this intervening period apart from what we see during the first and last periods when the ENSO signal is noisier, and its power is distributed over a wider range of periodicities spanning from 2 to 6 years. These results lead us to hypothesize that, during the last 140 years, ENSO shifted between a stochastically forced interannual mode of variability, to a multiannual, quasi regular one with a self sustained oscillation.