从有效位能变化来分析ElNino的年代际变化

利用近50a的美国Maryland大学全球海洋同化分析资料和NCEP全球大气再分析资料，分析了与ElNino相联系的热带太平洋有效位能（APE）的时空演变过程，从有效位能变化角度揭示了50年来ElNino的年代际变化特征。结果表明，热带太平洋ElNino年代际变化特征主要表现为在20世纪70年代末期以前与其后的显著不同。在有效位能异常变化的振幅和频率方面，20世纪70年代末期以后，振荡的振幅显著增,大直频率变慢。在有效位能异常变化的振幅和频率方面，70年代末期以前，ElNino事件的先兆是有效位能先在热带西太平洋异常积累，然后这些积累的有效位能沿赤道快速向东传播并释放能量，使得东太平洋出现能量的异常，东太平洋的有效位能减少，产生ElNino事件；70年代末期以后，ElNino事件的建立没有表现异常的有效位能在热带西太平洋的积累，其建立表现为赤道中太平洋直接出现的局地有效位能的异常，然后表现为不稳定海气耦合模态增幅性地缓慢向东传播，使得东太平洋的有效位能减少，发生ElNino事件。

INTERDECADAL CHANGE OF EL NINO AS SEEN FROM VARIATIONS OF AVAILABLE POTENTIAL ENERGY

Based on two datasets of the Simple Ocean Data Assimilation (SODA) analysis generated by Maryland University and the NCEP/NCAR atmospheric reanalysis,temporal and spatial evolutions of the available potential energy (APE) during the El Nino episodes in the tropical Pacific are studied,and possible interdecadal changes of El Nino over the last 50 years are examined by making use of the APE.It shows that the characteristics of ENSO has an obvious shift around the end of 1970s.Since the end of 1970s,the El Nino variability in the El Nino phase also experiences a change before and after the end of 1970s.Before the end of 1970s,prior to onset of an El Nino event,the available potential energy anomalously accumulates in the western tropical Pacific.Then the accumulated APE propagates eastward and releases potential energy under the effect of free Kelvin waves,which makes the energy anomalies occur in the eastern equatorial Pacific,and an El Nino event occurs.On the contrary,after the end of 1970s,there is no APE accumulation in the west prior to onset of an El Nino.An El Nino event develops with an APE anomaly firstly from the central equatorial Pacific as an unstable coupled air-sea mode,which migrates eastward and finally results in a well-defined El Nino event.