Assessing the 21st century shift of ENSO variability

Sea surface temperature (SST) variability associated with El Niño and the Southern Oscillation (ENSO) intensified in the central Pacific Ocean but weakened in the eastern Pacific at the beginning of 21^st century. Here, we use a heat variance budget (HVB) analysis in the Niño3 (5°S-5°N, 150°W-90°W) and Niño4 (5°S-5°N, 160°E-150°W ) regions to quantify the relative contributions of various dynamical and thermodynamical feedbacks to this shift in ENSO variability. Each HVB term is calculated separately over the 1980-1999 and 2000-2010 time periods based on seven ocean reanalysis datasets. We find that in the 2000s, the decrease of Niño3 variability is mostly due to the reduced positive thermocline feedback (TCF) and the increase in the Niño4 region is due mainly to an increase in the positive zonal advective feedback (ZAF). This shift in the relative importance of key positive feedbacks leads to more frequent and greater amplitude central Pacific (CP) El Niño events during 2000s relative to the 1980-90s, which were dominated by eastern Pacific (EP) events. These results confirm those of a previous study using a different methodology based on the Bjerknes stability index. We also use the HVB to examine the important role of negative TCF and ZAF feedbacks in ENSO phase transitions. We find that with the shift of maximum ENSO variability to the central Pacific in the 2000s, the ZAF becomes more effective during the transition phases of the ENSO cycle than the TCF, which dominated in the 1980s and 1990s. This result is consistent with previous studies that suggested recharge and discharge of heat in the equatorial band is less effective in controlling ENSO transitions for CP vs EP events.