Future Changes in ENSO Governed by Eastward Shift in Air-Sea Coupling

El Niño-Southern Oscillation (ENSO) dynamics have been studied focusing on the Niño3.4 region - the region of strongest air-sea coupling. Studying future ENSO changes focusing on the fixed Niño3.4 region could be problematic because the location of coupling could change, altering the balance of processes driving the onset of El Niño events. For instance, under global warming, climate models project the Pacific cold tongue to warm more than other regions making the atmosphere in this region closer to the threshold of convection and thus more responsive to variations in sea-surface temperature. Here we show that this enhanced equatorial warming produces a robust eastward shift in the coupling region of ENSO across CMIP models. We found that as the air-sea coupling region shifts eastward into the shallower climatological thermocline of the eastern Pacific, thermocline variability is able to positively feedback on the atmosphere. This stronger thermocline feedback favors the growth of El Niño initiated by westerly wind bursts. We also expect El Niño events to become stronger as more mechanisms become involved in the growth of El Niño. Lastly, it would favor the onset of El Niño after La Niña leading to better predicatibilty. Our results show that these changes in ENSO variability, instead of driven by more responsive winds to SSTs, arise from coupling shifting towards the eastern Pacific. The shallower climatological thermocline there, makes the thermocline feedback play a bigger role in the onset of El Niño.