To discover the possible mechanisms resulting in the interdecadal variability of the relationship between PEC anomaly and SETIO SSTA, especially the closer relationship after 2005, anomalous atmospheric circulations associated with the SETIO SSTA are analyzed in the following.
5.1 Anomalous stream function and rotational wind
During 1979-2004, the 850-hPa stream function and rotational wind regressed from ISETIO (Figure 5a) shows that there exists a pair of anomalous cyclonic circulations and anomalous westerly flows over the equatorial Indian Ocean stretching from the tropical to subtropical regions, due to the Mastuno-Gill response to the SETIO SSTA (Mastuno, 1966; Gill, 1980). The anomalous cyclonic circulation in the northern hemisphere is more zonal-elongated (40°E-140°E). Eastern China is under the influence of this anomalous cyclonic circulation. Vice versa.
As shown in Figure 5b, the anomalous cyclonic circulation is located from the Bay of Bengal to the Indochina Peninsula and adjacent areas during 2005-2019, corresponding to the anomalous cyclonic circulation in the southern hemisphere. Due to the Kelvin wave response to SST warming over SETIO (Mastuno, 1966; Gill, 1980), the easterly flow anomalies over the western Pacific Ocean could enhance the western Pacific subtropical high and contribute to the anomalous southerly flows over eastern China.
Comparing the anomalous circulation associated with ISETIO during the two periods (1979-2004 vs 2005-2019), there both exist the response of Rossby wave associated with the positive SETIO SSTA. The anomalous cyclonic circulation is more zonal-elongated during 1979-2004, whereas is confined in relatively small areas over South China and Indochina Peninsula during 2005-2019, associated with the intensification and westward extension of the western Pacific subtropical high.
5.2 Anomalous water vapour transport
From 1979 to 2004, the integrated water vapour flux and its divergence regressed from ISETIO (Figure 6a) shows that there is anomalous water vapour transport from the west to the east in the tropical Indian Ocean, resulting in the anomalous water vapour convergence over SETIO. Part of the water vapour is transported from the tropical Indian Ocean, passing through Java Islands, the South China Sea and the Kuroshio area, then transported westward to eastern China, and finally to South Asia along the southern foot of the Tibetan Plateau. Anomalous water vapour divergence is distributed in some areas of eastern China, which is consistent with the anomalous circulation in the lower troposphere (Figure 5a).
As for the period of 2005-2019, a pair of cyclonic circulations is located along the equatorial Indian Ocean, corresponding to the eastward water vapour transport anomaly. Anomalous water vapour flux over the tropical western Pacific Ocean is transported to eastern China along the edge of the western Pacific subtropical high. The water vapour is anomalous convergence over eastern China, favoring the above-normal autumn precipitation over eastern China. Vice versa.
5.3 Vertical motion analysis
During 1979-2004, the positive SETIO SSTA corresponds to the anomalous lower-level convergence (Figure 7a), upper-level divergence (Figure 7b) and associated ascending motion (Figure 7c) over the Maritime Continent. To the opposite, the tropical western Indian Ocean and the African continent are regulated by anomalous lower-level divergence (Figure 7a), upper-level convergence (Figure 7b) and associated descending motion (Figure 7c). Whereas eastern China has no anomalous vertical motions (Figure 7c).
Similar to the period of 1979-2004, SETIO is featured with the anomalous lower-level convergence (Figure 7d), upper-level divergence (Figure 7e) and associated ascending motion (Figure 7f) during 2005-2019, corresponding to the positive SETIO SSTA. But the northwestern Pacific Ocean is characterized by anomalous divergence at the lower troposphere and the strengthened western Pacific subtropical high during 2005-2019 (Figure 7d). Noted that the anomalous convergence, divergence and ascending motions over SETIO during 2005-2019 are all stronger than that during 1979-2004. Eastern China is controlled by the anomalous convergence at the lower troposphere, divergence at the upper troposphere, and ascending motions, which is favorable for the above-normal autumn precipitation over eastern China.
The climatological mean equatorial zonal-vertical circulation over the tropical eastern Indian Ocean and Pacific Ocean (Figure 8a) indicates that areas from the tropical eastern Indian Ocean to the central Pacific Ocean are controlled by anomalous ascending motion, whereas the tropical western Pacific ocean is dominated by anomalous descending motion, that is known as the Walker Circulation. The difference of the unfiltered SSTA averaged over SETIO between the two periods, i.e. 2005-2019 and 1979-2004, is 0.2°C. Differences of the regressed equatorial zonal–vertical circulation with respect to the ISETIO for 2005-2019 and 1979-2004 (Figure 8b) present the strengthened anomalous ascending motion over tropical eastern Indian Ocean and anomalous descending motion over tropical western Pacific Ocean. This would produce the anomalous low-level divergence over the western Pacific Ocean, and strengthen the western Pacific subtropical high (Figures 5 and 7a-b). The integrated water vapour transported from the western Pacific Ocean to eastern China can subsequently be strengthened, providing abundant water vapour for the enhancement of the connection between SETIO SSTA and the anomalous autumn precipitation in eastern China.
As shown in Figure 8c, the ascending branch of eastern China is distributed between the equator to 20°N, with two descending branches locating at its north and south. The difference of regressed zonal-vertical circulation between the two periods demonstrates that the warming SETIO SSTA is corresponding to the enhancement of anomalous ascending motion over SETIO, weakening of the ascending motion at 5°N-20°N and subsequently strengthened ascending motion over eastern China (25°N-30°N). It shows a circulation pattern of strengthened local Hadley circulation over eastern China, providing favorable vertical motion conditions for the enhanced connection between SETIO SSTA and the anomalous autumn precipitation in eastern China.