The changes in ENSO-induced tropical Pacific precipitation variability in the past warm and cold climates from the EC-Earth simulations

Abstract

The El Nino-Southern Oscillation (ENSO) is one of the most significant climate variability signals. Studying the changes in ENSO-induced precipitation variability (ENSO precipitation) in the past climate offers a possibility to a better understanding of how they may change under future climate conditions. This study uses simulations performed with the European community Earth-System Model (EC-Earth) to investigate the relative contributions of dynamic effect (the circulation anomalies together with the climatological specific humidity) and thermodynamic effect (the specific humidity anomalies together with the climatological circulation) on the changes in ENSO precipitation in the past warm and cold climates, represented by the Pliocene and the Last Glacial Maximum (LGM), respectively. The results show that the changes in ENSO precipitation are intensified (weakened) over the tropical western Pacific but weakened (intensified) over the tropical central Pacific in Pliocene (LGM) compared with the pre-industrial (PI) simulation. Based on the decomposed moisture budget equation, these changes in ENSO precipitation patterns are highly related to the dynamic effect. The mechanism can be understood as follows: the zonal gradient of the mean sea surface temperature (SST) over the tropical Indo-Pacific is increased (reduced) during the Pliocene (LGM), leading to the strengthening (weakening) of Pacific Walker Circulation as well as a westward (eastward) shift. In the Pliocene, the westward shift of Walker Circulation results in an increased (decreased) ENSO-induced low-level vertical velocity variability in the tropical western Pacific (central Pacific), and, in turn, favoring convergent (divergent) moisture transport through a dynamic process, and then causing intensified (weakened) ENSO precipitation there. The opposite mechanism exists in LGM. These results suggest that changes in the zonal SST gradient over tropical Indo-Pacific under different climate conditions determine the changes in ENSO precipitation through a dynamic process.