The relationship between thermocline depth and SST anomalies in the eastern equatorial Pacific: Seasonality and decadal variations

Abstract

Even though the vital role of thermocline fluctuation in El Niño-Southern Oscillation (ENSO) cycle has been established previously, the direct relationship between the thermocline depth and sea surface temperature (SST) anomalies in the equatorial Pacific is yet to be fully understood, especially its seasonality. Thermocline depth anomalies have been found to lead SST anomalies in time with a longitude-dependent delay, but our study also suggests that the relationship shows considerable seasonal dependency and is strongest (weakest) during the boreal spring (summer). Over the eastern equatorial Pacific where there is least delay (compared to that in the western and central Pacific), the connection between thermocline and SST is the weakest during the boreal spring. This feature may be one of causes for ENSO spring persistence barrier. Furthermore, the thermocline-SST connections exhibit significant decadal variations, which are remarkably consistent with the decadal changes in the persistence barrier of SST anomalies over the eastern Pacific. It is also found that the decadal shift in the timing of the thermocline-SST connection barrier is caused by the changes in the seasonal cycle of tropical trade winds and thermocline depths.