Feedback of Mixing to ENSO Phase Change

Abstract

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.