Feedback of Mixing to ENSO Phase Change

A decade‐long time series of mixing in the equatorial Pacific cold tongue at 0°, 140°W reveals how mixing changes on El Niño–Southern Oscillation (ENSO) time scales. Separated into phase transitions to and from the neutral state, we find that mixing is most intense during the perturbation from the neutral state to peak La Niña when sea surface temperature cools and weakest during the perturbation from the neutral state to peak El Niño when sea surface temperature warms. Intermediate levels of mixing occur during relaxations back to the neutral state. Heating and cooling rates due to the divergence of turbulence heat flux across the mixed layer, where the net surface heat flux is the value of the turbulence heat flux at the sea surface, have the same amplitude and sign as sea surface heating and cooling rates during ENSO phase transitions. We suggest that the basic Bjerknes feedback must include mixing. Plain Language Summary Transitions to the peak El Niño state are accompanied by sea surface warming and to the peak La Niña state by sea surface cooling. Since, on a daily averaged basis, the sea surface at the equator is always heated by the atmosphere, thermodynamic cooling is only achieved by mixing of warm surface waters with cooler water from greater depths. Instruments that measure mixing have been deployed on long term equatorial oceanographic moorings for more than a decade and have now captured several El Niños and La Niñas. These show that mixing—or lack thereof—contributes to heating the sea surface during transitions to peak El Niño and cooling the sea surface during transitions to peak La Niña. Mixing therefore acts as a positive feedback mechanism to the development of El Niños and La Niñas. Key Points A decade of mixing measurements in the cold tongue of the equatorial Pacific now includes several ENSO events Heating/cooling rates due to mixing have the same amplitude and sign as observed sea surface heating/cooling rates on ENSO time scales Bjerknes feedback must include mixing in addition to upwelling