Temporal evolution of the equatorial thermocline associated with the 1991–2006 ENSO

Abstract

An oceanic reanalysis data set for the period 1991–2006 has been used to investigate the temporal evolution of the thermocline depth during El Niño–Southern Oscillation (ENSO) cycles. We have examined the dynamical behavior of the thermocline depth in the central eastern equatorial basin by carrying out a detailed investigation into the water mass budget in a selected region lying east of the equatorial Pacific. The result shows that the time change in thermocline depth (i.e., the speed of the local vertical motion) caused by the meridional movement of warm water is consistent with the recharge‐oscillator theory. On the other hand, the time change associated with Bjerknes feedback via zonal mass movement seems to be linked to the development of the El Niño as often suggested by previous observational studies. The speed of the local vertical motion estimated from the meridional mass transport is negatively correlated with that estimated from the zonal mass transport and lags in time by three months. This relationship stems from the fact that rapid adjustments to changes in the surface zonal pressure gradient are observed in the equatorial basin, whereas delayed adjustments through the propagation of baroclinic Rossby waves take place in the central eastern equatorial basin. This time‐lagged relationship introduces a phase shift in the temporal evolution of the thermocline depth. The implications of these results for a better understanding and more accurate prediction of the ENSO time evolution are discussed.