The Importance of Central Pacific Meridional Heat Advection to the Development of ENSO

The relationship between the warm water volume (WWV) ENSO precursor and ENSO SST weakened substantially after similar to 2000, coinciding with a degradation in dynamical model ENSO prediction skill. It is important to understand the drivers of the equatorial thermocline temperature variations and their linkage to ENSO onsets. In this study, a set of ocean reanalyses is employed to assess factors responsible for the variation of the equatorial Pacific Ocean thermocline during 1982-2019. Off-equatorial thermocline temperature anomalies carried equatorward by the mean meridional currents associated with Pacific tropical cells are shown to play an important role in modulating the central equatorial thermocline variations, which is rarely discussed in the literature. Further, ENSO events are delineated into two groups based on precursor mechanisms: the western equatorial Pacific type (WEP) ENSO, when the central equatorial thermocline is mainly influenced by the zonal propagation of anomalies from the western Pacific, and the off-equatorial central Pacific (OCP) ENSO, when off-equatorial central thermocline anomalies play the primary role. WWV is found to precede all WEP ENSO events by 6-9 months, while the correlation is substantially lower for OCP ENSO events. In contrast, the central tropical Pacific (CTP) precursor, which includes off-equatorial thermocline signals, has a very robust lead correlation with the OCP ENSO. Most OCP ENSO events are found to follow the same ENSO conditions, and the number of OCP ENSO events increases substantially since the start of the twenty-first century. These results highlight the importance of monitoring off-equatorial subsurface preconditions for ENSO prediction and to understand multiyear ENSO.

Automated summary

The relationship between warm water volume (WWV) ENSO precursor and ENSO SST has weakened after 2000, coinciding with reduced dynamical model ENSO prediction skill. Monitoring off-equatorial subsurface preconditions is crucial for ENSO prediction and understanding multiyear ENSO events. The research shows that off-equatorial thermocline temperature anomalies play a significant role in modulating central equatorial thermocline variations, particularly in off-equatorial central Pacific (OCP) ENSO events.