Mechanisms On Evolution Asymmetry Between El Ni(?)o And La Ni(?)a

Observed SST anomaly(SSTA)in equatorial eastern Pacific exhibits an asymmetric evolution characteristic between typical El Ni(?)o and La Ni(?)a.While typical 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,typical La Ni(?)a is characterized by a weaker decay after its peak and a re-intensification of cold SSTA in the second year.The relative roles of dynamic(wind field)and thennodynamic(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 western Pacific,whereas the latter is associated with asymmetric cloud-radiation-SST and evaporation-SST feedbacks.A strong negative SSTA tendency occurs during typical El Ni(?)o decaying phase,compared to a much weaker positive SSTA tendency during typical La Ni(?)a decaying phase.Such a difference leads to SSTA sign change for typical El Ni(?)o but no sign change for typical La Ni(?)a 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 typical El Ni(?)o,but the re-occurrence of a La Ni(?)a episode by end of the second year for typical La Ni(?)a.The overall heat budget analysis during the entire ENSO evolutions indicates the thermodynamic process is as important as the dynamic process in causing the typical El Ni(?)o-La Ni(?)a evolution asymmetry.The fundamental difference of the current result with previous theories is further discussed.1986 El Ni(?)o experienced distinctive evolution feature compared to the typical El Ni(?)o events.The 1986 El Ni(?)o persisted for more than two years,whereas the typical El Ni(?)o transitioned into a cold episode in the following winter.The physical mechanisms that caused the distinctive evolution features are carried out by an oceanic mixed-layer heat budget analysis.For 1986 El Ni(?)o,major differences with typical El Ni(?)o lie in wind induced anomalous zonal advection and thermocline feedback terms.In the developing year mature winter,there is an anomalous cyclone over the western North Pacific in the 1986 El Ni(?)o,in contract to an anomalous anticyclone in the typical El Ni(?)o.Due to both the seasonal mean westerly anomaly south of the western North Pacific cyclone and continuous high frequency westerly wind event forcing in the equatorial western Pacific,the 1986 El Ni(?)o decayed slowly and retained a positive SSTA throughout the second year.1988 and 2005 La Ni(?)a also experienced distinctive evolution feature compared to the typical La Ni(?)a events.The typical La Ni(?)a persisted for more than two years,whereas the 2005 La Ni(?)a transitioned into a warm episode in the following winter.Through an oceanic mixed-layer heat budget analysis,it is found that major differences between typical La Ni(?)a and 2005 La Ni(?)a also lie in wind induced anomalous zonal advection and thermocline feedback terms.The SSTA center of the 2005 La Ni(?)a shifted eastward compared to the typical La Ni(?)a.As a result,there is an anomalous western North Pacific cyclone,with pronounced westerly anomalies at the equator in its mature phase.This led to a rapid decay of the 2005 La Ni(?)a,and in the following boreal summer,a positive SSTA formed.The strong positive air-sea feedback in boreal fall further strengthens the warming,leading to a transition from La Ni(?)a to El Ni(?)o in the following winter.The physical mechanisms that caused the distinctive evolution feature of 1988 La Ni(?)a are also investigated.The result shows that,the special 1988 La Ni(?)a has a fast decay after its peak,and recovers to the climatological mean state in the end of the second year.The heat budget analysis result indicates that,it is the anomalous zonal advection term related to the wind forcing that contributed to the distinctive decaying rates between the typical and special 1988 La Ni(?)a.In the decaying period,there is an anomalous cyclone over the western North Pacific in the special 1988 La Ni(?)a,and anomalous westerly wind south of the cyclone can induce downwelling Kelvin waves propagating eastward,that help the special 1988 La Ni(?)a decay fast.As the anomalous cyclonic circulation can persist in later of the second year,1988 La Ni(?)a recovers to the mean state.