A decadal change on relationship between winter rainfall over Southern China (SCWR) and Western Pacific warm pool heat content (WPHC) was detected by a sliding correlation analysis. The decadal turning point occurred around 1993. Before 1993, the relationship between SCWR and WPHC was quite weak with a correlation coefficient of −0.17. During this epoch, the WPHC‐related anomalous troposphere circulation, sea surface temperature (SSTA) and outgoing longwave radiation (OLRA) were mainly confined to the Pacific Ocean. Therefore, the corresponding climate system anomalies cannot influence SCWR. However, after 1993, the relationship between SCWR and WPHC became significant and reached a correlation coefficient of −0.81. The troposphere circulation anomalies along with the SSTA and OLRA pattern that were associated with WPHC during this epoch significantly shifted northwestward and intensified, resulting in a direct impact on SCWR. A possible mechanism was put forth to explain the decadal enhanced relation between SCWR and WPHC. In the context of the increased and northwestward‐shifted climatological maximum WPHC, the concurrent intensified and northwestward‐shifted SST and atmospheric circulation anomaly pattern contributes to the decadal enhanced relation between SCWR and WPHC. On one hand, because of the increased and northwestward‐shifted maximum sea surface temperature, the convection became more sensitive to the SSTA over Indian Ocean and Maritime Continent, thus dragged the Philippines anticyclonic/cyclonic anomaly northwestward via Kevin wave and Rossby wave response, which can directly affect SCWR. On the other hand, the increased and northwestward‐shifted SSTA anchored the convection anomalies in South China Sea (SCS) and Maritime Continent, impacting the SCWR via anomalous descending/ascending motion of the anomalous local meridional cell.
The interannual relationship between the spring sea surface temperature over the western tropical Indian Ocean (WTIO SST) and summer water vapor content over Tibetan Plateau (TPWVC) enhances significantly after 1992/1993. The regressed atmospheric circulation against WTIO SST index (WTIO SSTI) for two periods is explored to explain the interdecadal variation. During ID1 (1979–1991), the center of the anomalous anticyclone is generally located eastward and the weak easterly anomalies on its southern flank transport moisture from the western Pacific to Southeast China with no effects on TPWVC. In ID2 (1994–2017), the Northwest Pacific anticyclone, the anomalous easterlies, and the subtropical high at 500 hPa all move westward and enhance significantly; thus, it forms a westward moisture transport pathway delivering the water vapor from the western Pacific into Tibetan Plateau. A possible mechanism is raised. On the one hand, the SST anomalies (SSTA) related to WTIO SSTI extend eastward from spring to summer in ID2. With the increased mean SST in the Indo-western Pacific Ocean under the global warming and the stronger mean summer SST in the eastern Indian Ocean, the positive SSTA induce the enhanced Kelvin waves and Northwest Pacific anticyclone with strong easterly anomalies during ID2. But in ID1, the SSTA related to WTIO SST confined in the western-central Indian Ocean from spring to summer excite the decreased Kelvin waves with less significant easterly anomalies due to the weaker mean SST. On the other hand, the eastward shift of tropical summer SSTA generates the increased convection and rising motion over the Southeast Indian Ocean in ID2. They enhance the easterly anomalies on the southern flank of the Northwest Pacific anticyclone and induce anticyclonic shear through the meridional circulation. As a result, the easterly anomalies shift westward to transport more moisture into Tibetan Plateau. However, in ID1, the easterly anomalies of the anticyclone cannot be strengthened with no westward shift. Therefore, the above reasons lead to the interdecadal enhancement of relationship between the spring WTIO SST and summer TPWVC.
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