INTERDECADAL MODULATION OF TROPICAL BIENNIAL OSCILLATION AND EL NINO-SOUTHERN OSCILLATION

利用美国国家环境预报中心/国家大气研究中心的再分析，在海洋环流模式的辅助下，确定了厄尔尼诺-南方涛动(ENSO)对南亚夏季风年际变化的主要影响。它具有赤道对称的结构，在20世纪60年代至70年代中期的夏末ENSO增长期最为明显，此时类对流层两年振荡(TBO)的ENSO占主导地位。随着ENSO的发展，热带印度洋和太平洋上的异常Walker环流系统从夏季的单单体环流系统转变为秋季的双单体环流系统。同时，热带印度洋上空出现赤道对称结构的降水异常，并伴有对流层低层南北孪生环流异常。北半球环流与印度异常季风降水有动态联系，尤其是在夏末。结果表明，印度洋的风-蒸发反馈和热带太平洋的海洋动力共同作用，对与类TBO的ENSO相关的异常Walker环流系统的形势转换是至关重要的。由于20世纪70年代末前后ENSO循环的季节性变化，ENSO增长期和衰退期的不同影响可能影响了ENSO与季风的关系，并引起了年代际变化。

A dominant impact of El Nino-Southern Oscillation (ENSO) on the South Asian summer monsoon interannual variability is identified using the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis aided by an ocean general circulation model. It has an equatorially symmetric structure and is most pronounced at the growth phase of ENSO in late summer during the period from the 1960s to mid-1970s when tropospheric biennial oscillation (TBO)-like ENSO dominates. As ENSO develops, an anomalous Walker circulation system over the tropical Indian and Pacific Oceans changes from a single-cell regime in summer to a double-cell regime in fall. In the meantime, rainfall anomalies of an equatorially symmetric structure are induced over the tropical Indian Ocean, accompanied by north-south twin circulation anomalies in the lower troposphere. The northern circulation is dynamically linked with anomalous monsoon rainfall over India especially in late summer. It is suggested that a combination of the wind-evaporation feedback in the Indian Ocean and ocean dynamics in the tropical Pacific is crucial for the regime transition of the anomalous Walker circulation system associated with the TBO-like ENSO. As the seasonality of ENSO cycle changes before and after the late 1970s, the difference in ENSO impacts between its growth and decay phases may have influenced the ENSO-monsoon relationship and caused its interdecadal change.