On the weakened connection between ENSO SST and warm water volume along the equatorial Pacific

The El Niño−Southern Oscillation (ENSO) is the strongest interannual variability in tropical oceans and the premier source of global climate predictability. Since 2000, ENSO has shifted to a regime with smaller variability and higher frequency, as a result, ENSO prediction skills degraded due to the weakened connection between ENSO sea surface temperature and warm water volume (WWV) along the equatorial Pacific. That is connected with the enlarged zonal contrast in the mean sea surface temperature across the tropical Pacific and the westward shift of the dominant region of the atmosphere-ocean coupling associated with ENSO. We noted that the recharge/discharge processes associated with ENSO vary with decade, time scale, and longitude. The weakening of the recharge/discharge processes since 2000 is mainly at the interannual time scale without significant changes at the intraseasonal and interseasonal time scales. The weakened connection between the WWV and the Niño3.4 indices since 2000 is mainly due to the weakened connection with the western tropical Pacific WWV (WWVw). It is hypothesized that the enlarged mean state contrast across the equatorial Pacific with enhanced thermocline slope and strengthened easterly trade wind made the WWV less efficient in El Niño growth.