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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Authors
Chang, C.-P.
Zhang, Yongsheng
Li, Tim
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2000-12
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Abstract
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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Meteorology
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2000 American Meteorological Society, Journal of Climate, Volume 13, pp. 4310-4325
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.