Relative Roles of Dynamic and Thermodynamic Processes in Causing Evolution Asymmetry between El Niño and La Niña*

Abstract

Abstract Observed SST anomaly (SSTA) in the equatorial eastern Pacific exhibits an asymmetric evolution characteristic between El Niño and La Niña. While El Niño is characterized by a rapid decay after its peak and a fast phase transition to a cold episode in the following winter, La Niña is characterized by a weaker decay after its peak and a reintensification of cold SSTA in the second year. The relative roles of dynamic (wind field) and thermodynamic (heat flux) processes in causing the asymmetric evolutions are investigated through a mixed layer heat budget analysis. The result shows both dynamic and thermodynamic processes contribute to the evolution asymmetry. The former is related to asymmetric wind responses in the western Pacific, whereas the latter is associated with asymmetric cloud–radiation–SST and evaporation–SST feedbacks. A strong negative SSTA tendency occurs during El Niño decaying phase, compared to a much weaker positive SSTA tendency during La Niña decaying phase. Such a difference leads to an SSTA sign change for El Niño but no sign change for La Niña by the end of summer of the second year. A season-dependent coupled instability kicks in during northern fall, leading to the development of a La Niña by end of the second year for El Niño, but the reoccurrence of a La Niña episode by end of the second year for La Niña. The overall heat budget analysis during the entire ENSO evolutions indicates the thermodynamic process is as important as the dynamic process in causing the El Niño–La Niña evolution asymmetry. The fundamental difference of the current result with previous theories is further discussed.