Interdecadal enhancement in the interannual variability of the summer monsoon meridional circulation over the South China Sea around the early 1990s

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.