副热带北太平洋SST在ENSO冷暖位相转变中的作用

The Influence of SST in Subtropical North Pacific on the Warm-Cold Phase Transition of ENSO

2015/2016年发生的极端El Ni?o事件,与1997/1998年El Ni?o事件具有可比拟的强度,但是2016年事件转变为弱La Ni?a,而1998年事件则为强La Ni?a。本文通过对比这两次极端El Ni?o事件,揭示其转变为不同强度La Ni?a事件的物理机制。混合层热收支分析的结果表明,在El Ni?o衰减年的4~11月,2016年平流反馈和温跃层反馈相对较弱,混合层温度衰减速率慢,其产生的主要原因是赤道中西太平洋的东风异常较弱。进一步分析表明,赤道中西太平洋的东风异常与副热带东北太平洋的海表温度异常(SSTA)有关,该地区的SST在1998年表现为冷异常,2016年为暖异常。副热带东北太平洋冷的SSTA有利于信风加强,从而加强中西太平洋的东风异常;而暖的SSTA使得赤道以北出现西南风异常,从而削弱中西太平洋的东风异常。此外,合成分析也表明,副热带东北太平洋SSTA与转变的La Ni?a的强度具有关联,El Ni?o转变为强La Ni?a的情况在位相转变期伴随着副热带北太平洋冷的SSTA,而El Ni?o转变为弱La Ni?a的情况没有明显的冷SSTA。

The strength of the extreme El Nino episode during 2015/2016 is comparable to that of the 1997/1998 super El Nino.However,the 2016 event was followed by a weak La Nina,while the 1998 event was followed by a strong one.This paper tries to reveal the causes for La Ni?a events that have different strengths by comparing the two extreme El Nino cases.The mixed layer heat budget analysis shows that during April-November of the El Nino decaying year,the zonal advection feedback and thermocline feedback in 2016 were relatively weak,which weakened the mixed layer temperature decaying rate.The weaker than normal easterly wind anomalies over the western-central equatorial Pacific are the main reasons for the above phenomena.Further analysis reveals that the easterly anomalies over the western-central equatorial Pacific are related to sea surface temperature anomalies in the subtropical Northeast Pacific(STNEP).While the SSTA in STNEP was negative in 1998,it was positive in 2016.Negative SSTA in the STNEP is favorable for the strengthening of trade winds,and thus the strengthening of easterlies over the western-central equatorial Pacific.In contrast,positive SSTA in the STNEP induces southwesterly wind anomalies through the WES(Wind-Evaporation-SST)feedback and thus suppresses the easterly over the western-central equatorial Pacific.In addition,composite analysis also confirms that the SSTA in the subtropical Northeast Pacific is related to the phase transition of ENSO,that is,the case of El Ni?o-strong La Nina is accompanied by negative SSTA in the STNEP during its phase transition period,while no obvious negative SSTA in STNEP is observed during the case of El Nino-weak La Nina.