Nonlinear Interaction Between Cold Tongue Mode and El Niño: Weakening of El Niño Bjerknes Feedback

An increase in Central Pacific (CP) El Niño events over past decades is linked with, and possibly driven by, an eastern Pacific cooling trend known as the Cold Tongue Mode (CTM). Our results strengthen this hypothesis, using an atmospheric general circulation model to show a nonlinear strengthening of Walker Circulation (WC) during El Niño events that occur within strong CTM climatology. As other studies have shown a potential causal link between global warming and the recent emergence of the CTM, a more robust WC could imply that a strong CTM, forced by global warming, may continue to increase CP El Niño frequency. However, our model results also demonstrate that a strong CTM corresponds to increased atmospheric stochasticity in the western equatorial Pacific, which could enhance Westerly Wind Burst (WWB) activity that is both inherently stochastic and necessary for generating ocean waves that trigger larger eastern Pacific (EP) El Niño events.