Feedback of Mixing to ENSO Phase Change

Sally J Warner, Brandeis University, Environmental Studies Program & Physics Department, Waltham, MA, United States and Jim Moum, Oregon State Univ, Corvallis, OR, United States
A decade-long time series of mixing in the cold tongue of the equatorial Pacific 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 La Niña when sea surface temperature (SST) cools and weakest during the perturbation from the neutral state to El Niño when SST warms. Intermediate levels of mixing occur during the relaxations back to the neutral state. Throughout the record, the upper equatorial ocean is at a state of marginal instability to mixing, but that state differs between El Niño and La Niña. Transitions to and from La Niña exist at higher shear and stratification due to the shallowed thermocline and equatorial undercurrent. 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.