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

国家环境预测中心/国家大气研究中心在海洋环流模型的辅助下进行再分析，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。它具有赤道对称结构，在 20 世纪 60 年代至 1970 年代中期夏末 ENSO 增长阶段最为明显，此时类似对流层两年期振荡 (TBO) 的 ENSO 占主导地位。随着 ENSO 的发展，热带印度洋和太平洋上空的异常沃克环流系统从夏季的单细胞状态转变为秋季的双细胞状态。与此同时，热带印度洋上空诱发赤道对称结构的降雨异常，并伴有对流层低层南北双环流异常。北部环流与印度异常季风降雨动态相关，特别是在夏末。研究表明，印度洋的风蒸发反馈和热带太平洋的海洋动力学的结合对于与类似 TBO 的 ENSO 相关的异常沃克环流系统的状态转变至关重要。由于ENSO周期的季节性在1970年代末前后发生变化，其增长期和衰退期对ENSO影响的差异可能影响了ENSO与季风的关系并导致其年代际变化。

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.