Abstract

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