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Mechanisms for El Niño and La Niña Evolution Asymmetry
Source: LASG    Viewed:  time(s)    Time: 2016-6-28
 Prof. Tim Li
University of Hawaii and NUIST
14:00 pm,28 June,2016
No.303,Keyan Building


Observed SST anomaly (SSTA) in the equatorial eastern Pacific (EP) 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 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 Niño decaying phase, compared to a much weaker positive SSTA tendency during La Niña decaying phase. Such a difference leads to a change of the SSTA sign for El Niño composite but no sign change for La Niña 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 Niña by end of the second year for El Niño composite, but the reoccurrence of a La Niña episode by end of the second year for La Niña 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 Niño-La Niña evolution asymmetry. The fundamental difference between the current theory and previous hypotheses is discussed.

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