Distinctive MJO Activity during the Boreal Winter of the 2015/16 Super El Niño in Comparison with Other Super El Niño Events

Abstract

Many previous studies have demonstrated that the boreal winters of super El Niño events are usually accompanied by severely suppressed Madden-Julian oscillation (MJO) activity over the western Pacific due to strong descending motion associated with a weakened Walker Circulation. However, the boreal winter of the 2015/16 super El Niño event is concurrent with enhanced MJO activity over the western Pacific despite its sea surface temperature anomaly (SSTA) magnitude over the Niño 3.4 region being comparable to the SSTA magnitudes of the two former super El Niño events (i.e., 1982/83 and 1997/98). This study suggests that the MJO enhanced over western Pacific during the 2015/16 super El Niño event is mainly related to its distinctive SSTA structure and associated background thermodynamic conditions. In comparison with the previous super El Niño events, the warming SSTA center of the 2015/16 super El Niño is located further westward, and a strong cold SSTA is not detected in the western Pacific. Accordingly, the low-level moisture and air temperature (as well as the moist static energy, MSE) tend to increase in the central-western Pacific. In contrast, the low-level moisture and MSE show negative anomalies over the western Pacific during the previous super El Niño events. As the MJO-related horizontal wind anomalies contribute to the further westward warm SST-induced positive moisture and MSE anomalies over the western tropical Pacific in the boreal winter of 2015/16, stronger moisture convergence and MSE advection are generated over the western Pacific and lead to the enhancement of MJO convection.

摘要

此前的观测表明, 1982/83 和 1997/98 年的超强厄尔尼诺发生时, 由于沃克环流异常导致西太平洋地区上升气流减弱, MJO 的活跃性会强烈地受到抑制. 本研究发现, 尽管 2015/16 年超强厄尔尼诺冬季 Niño 3.4 区域的异常暖海温强度与前两次超强厄尔尼诺年十分相似, 但西太平洋地区MJO对流的活跃程度反而增强. 通过进一步分析发现, 2015/16年厄尔尼诺冬季, 海温异常分布与前两次厄尔尼诺事件有较大的不同. 与前两次超强事件相比, 2015/16年冬季异常的暖海温向西延伸, 中西太平洋低层水汽与气温升高, 造成西太平洋地区背景态低水汽与湿静力能 (MSE)增强. 在与MJO相关的季节内尺度水平风场的作用下, 西太平洋地区的水汽辐合与MSE输送也相应增强, 从而导致了2015/16年冬季西太平洋地区MJO的活跃性加强.