Changes in ENSO Oscillatory Dynamics Associated with Zonal Shifts in Air–Sea Coupling Region

El Niño–Southern Oscillation (ENSO) oscillates between warm (El Niño) and cold (La Niña) phases, but observations indicate that this oscillatory behavior is irregular and asymmetric. Using the NCAR Community Earth System Model 1 (CESM), we investigate how the oscillatory dynamics of ENSO change across different climatic states. Our simulations of two glacial climates (Last Glacial Maximum, 21 ka; deglacial state, 15 ka), a preindustrial baseline (piControl), and two greenhouse warming scenarios (2xCO₂; 4xCO₂) reveal distinct dynamical regimes of oscillatory behavior driven by zonal shifts in air–sea coupling. Under preindustrial conditions, maximum wind–sea surface temperature coupling occurs near the Niño-3.4 region, and CESM reproduces the observed asymmetry of ENSO oscillatory behavior; El Niño events often rapidly decay into La Niña, while La Niña events rarely lead to El Niño but rather decay slowly back to neutral conditions. In simulated glacial climates, ENSO is nonoscillatory because the air–sea coupling region shifts westward of the Niño-3.4 region. In the western Pacific, a deeper thermocline weakens the delayed positive feedback needed for events to grow into the opposite phase, causing both El Niño and La Niña to decay back to neutral conditions. Conversely, in warmer climates, ENSO becomes more oscillatory and symmetric as the air–sea coupling region shifts eastward of the Niño-3.4 region. In the eastern Pacific, a shallower thermocline strengthens its positive feedback, enabling more frequent and consistent transitions between phases. This sensitivity of ENSO oscillatory behavior to the longitude of maximum air–sea coupling suggests that future changes in ENSO predictability in response to external forcings could be significant.