Interannual and Interdecadal Variations of the East Asian Summer Monsoon and Tropical Pacific SSTs. Part I: Roles of the Subtropical Ridge
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The interannual relationship between the East Asian summer monsoon and the tropical Pacific SSTs is studied using rainfall data in the Yangtze River Valley and the NCEP reanalysis for 1951-96. The datasets are also partitioned into two periods, 1951-77 and 1978-96, to study the interdecadal variations of this relationship.A wet summer monsoon is preceded by a warm equatorial eastern Pacific in the previous winter and followed by a cold equatorial eastern Pacific in the following fall. This relationship involves primarily the rainfall during the pre-Mei-yu/Mei-yu season (May-June) but not the post-Mei-yu season (July-August). In a wet monsoon year, the western North Pacific subtropical ridge is stronger as a result of positive feedback that involves the anomalous Hadley and Walker circulations, an atmospheric Rossby wave response to the western Pacific complementary cooling, and the evaporation-wind feedback. This ridge extends farther to the west from the previous winter to the following fall, resulting in an 850-hPa anomalous anticyclone near the southeast coast of China. This anticyclone 1) blocks the pre-Mei-yu and Mei-yu fronts from moving southward thereby extending the time that the fronts produce stationary rainfall; 2) enhances the pressure gradient to its northwest resulting in a more intense front; and 3) induces anomalous warming of the South China Sea surface through increased downwelling, which leads to a higher moisture supply to the rain area. A positive feedback from the strong monsoon rainfall also appears to occur, leading to an intensified anomalous anticyclone near the monsoon region. This SST-subtropical ridge-monsoon rainfall relationship is observed in both the interannual timescale within each interdecadal period and in the interdecadal scale.The SST anomalies (SSTAs) change sign in northern spring and resemble a tropospheric biennial oscillation (TBO) pattern during the first interdecadal period (1951-77). In the second interdecadal period (1978-96) the sign change occurs in northern fall and the TBO pattern in the equatorial eastern Pacific SST is replaced by longer timescales. This interdecadal variation of the monsoon-SST relationship results from the interdecadal change of the background state of the coupled ocean-atmosphere system. This difference gives rise to the different degrees of importance of the feedback from the anomalous circulations near the monsoon region to the equatorial eastern Pacific.In a wet monsoon year, the anomalous easterly winds south of the monsoon-enhanced anomalous anticyclone start to propagate slowly eastward toward the eastern Pacific in May and June, apparently as a result of an atmosphere-ocean coupled wave motion. These anomalous easterlies carry with them a cooling effect on the ocean surface. In 1951-77 this effect is insignificant as the equatorial eastern Pacific SSTAs, already change from warm to cold in northern spring, probably as a result of negative feedback processes discussed in ENSO mechanisms. In 1978-96 the equatorial eastern Pacific has a warmer mean SST. A stronger positive feedback between SSTA and the Walker circulation during a warm phase tends to keep the SSTA warm until northern fall, when the eastward-propagating anomalous easterly winds reach the eastern Pacific and reverse the SSTA.
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