Revealing Ocean Dynamics Driving ENSO Phase Transitions

ENSO predictability relies largely on deterministic equatorial ocean dynamics, where wind variations during one phase trigger oceanic responses that favor a shift to the opposite phase. However, in observations, this deterministic response is obscured by air-sea coupled variations and stochastic Westerly Wind Bursts. Here, we present a method to isolate the ocean dynamics underpinning ENSO phase transitions using forced experiments with an Ocean General Circulation Model (OGCM). The control experiment is forced by interannually varying wind stresses, with thermal damping from air-sea heat fluxes computed interactively as relaxation to climatological Sea Surface Temperature (SST). This setup reproduces observed equatorial Pacific SST and heat content variations with high fidelity. To assess the role of ocean initial conditions, "memory" experiments branch from the control simulation every January 1^st, replacing wind stresses with climatological values (i.e., no interannual wind anomalies). In these experiments, interannual anomalies arise solely from the evolution of equatorial planetary waves in the initial conditions. The ocean memory index (OMI) derived from these experiments demonstrates hindcast skill for 1-year lagged ENSO peaks comparable to or exceeding traditional precursors like Warm Water Volume or western Pacific heat content. This highlights the effectiveness of our methodology in isolating the ocean dynamics driving ENSO phase transitions. Our findings emphasize the central role of low-order equatorial Rossby waves (meridional modes 1-3) in ENSO's oceanic memory via reflections at the Pacific western boundary and indicate that widely used indices such as Warm Water Volume orwestern Pacific heat content do not optimally capture these processes.