The interdecadal enhancement in the interannual variability of summer monsoon meridional circulation (SMMC) over the South China Sea around the early 1990s is investigated. Results show the change in the SMMC variability may arise from the interdecadal shift in the leading modes of low-level geopotential height over East Asia–Australia and Indo–Pacific sea surface temperature anomalies (SSTAs) in boreal summer. Before the early 1990s, the leading mode of Indo–Pacific SSTAs shows a zonal tripole pattern, with abnormally warm eastern Pacific and northern Indian Ocean and cold western Pacific. At the lower level, the western North Pacific cooling and northern Indian Ocean warming generate an anticyclonic anomaly over western North Pacific, while the cooling over the Maritime Continent and east of Australia favors an abnormal anticyclone over Australia. Hence region-wide positive geopotential height anomalies cover East Asia–Australia, which resemble the major mode of geopotential height and generate weak south–north pressure gradient and SMMC variability. After the early 1990s, the leading SSTAs mode shifts to a zonal dipole with abnormally cold western Pacific and warm equatorial central–eastern Pacific. The central Pacific warming induces an anomalous low-level cyclone over Philippines and it is further maintained by the Maritime Continent cooling. Meanwhile, the cooling over the east of Australia and Maritime Continent favors an abnormal Australian anticyclone. The low-level geopotential height thus shows south–north dipole anomalies over East Asia–Australia, resembling its major mode and generating obvious meridional pressure gradient and SMMC variability. The atmospheric responses to different SSTAs modes are confirmed by CAM4 experiments.

研究了1990年代初南海夏季风季风环流（SMMC）年际变化的年代际增强。结果表明，SMAC变异性的变化可能是由北亚夏季东亚-澳大利亚和印度-太平洋海表温度异常（SSTA）的低水平地势高度的主导模式年代际变化引起的。在1990年代初之前，印度-太平洋SSTA的主导模式显示出带状三脚架模式，东太平洋和北印度洋异常温暖，而西太平洋则较冷。在较低的水平上，北太平洋西部的降温和北印度洋的变暖在北太平洋西部产生了反气旋异常，而海洋大陆和澳大利亚东部的降温则有利于反异常的反气旋。因此，整个区域的正地势高度异常覆盖了东亚－澳大利亚，这与主要的地势高度模式相似，并且产生了弱的南北压力梯度和SMMC变异性。在1990年代初之后，主要的SSTA模式转变为纬向偶极子，其中西太平洋异常寒冷，而赤道中东太平洋异常温暖。太平洋中部变暖在菲律宾上空引发了一次异常的低空旋风，海事大陆的降温进一步保持了这一状态。同时，澳大利亚东部和海事大陆的降温有利于异常的澳大利亚反气旋。低空位势高度显示出东亚-澳大利亚的南北偶极子异常，类似于其主要模式，并产生明显的子午压力梯度和SMMC变异性。CAM4实验证实了大气对不同SSTAs模式的响应。