Abstract
During the boreal summer from June to August, rainfall in Southwestern China shows substantial interdecadal variabilities on timescales longer than 10 years. Based on observational analyses and numerical modeling, we investigated the characteristics of interdecadal Southwestern China summer rainfall (SWCSR) and its dynamic drivers. We find that the SWCSR is markedly impacted by the interdecadal Indian Ocean basin mode (ID-IOBM) of the sea surface temperature (SST), which may induce anomalous inter-hemispheric vertical circulation. During the cold phase of the ID-IOBM, an enhanced lower-level divergence and upper-level convergence exist over the tropical Indian Ocean. The simultaneous lower-level outflow anomalies further converge over the Indo-China peninsula, resulting in an anomalous ascending motion and a lower-level cyclone that contribute to strengthening the eastward moisture transport from the Bay of Bengal to Southwestern China. The joint effects of the anomalous ascending motion and the above-normal moisture transport play a key role in increasing the SWCSR. In summers during the warm phase of the ID-IOBM, the situation is approximately the same, but with opposite polarity. After the beginning of the 1970s, the impacts of interdecadal Indian Ocean dipole (ID-IOD) on SWCSR is strengthening. The anomalous vertical circulation associated with the positive (negative) phase of ID-IOD is in favor of decreased (increased) rainfall in SWC. However, the impacts of ID-IOD on SWCSR is relatively weak before the 1970s, indicating that the ID-IOD is the secondary driver of the interdecadal variability of SWCSR. Modeling results also indicate that the ID-IOBM of SST anomalies is the main driver of interdecadal variability of SWCSR.
Highlights
Southwestern China (SWC) is one of the most populated agricultural regions in China, providing approximately 16% of the national food supply [1]
One branch of the corresponding upper-level outflows converges over the Indo-China peninsula, which is favorable for anomalous descending motion and lower-level northeasterly anomalies, both of which favor decreased rainfall in SWC. This is consistent with the results revealed by Cao et al [2], who showed that the summer rainfall in SWC is negatively connected with the subtropical Indian Ocean dipole on interannual timescale
We studied the relationship between interdecadal Indian Ocean basin mode (ID-IO basin mode (IOBM)) and SECSR separately [31], which is beyond the scope of this work
Summary
Southwestern China (SWC) is one of the most populated agricultural regions in China, providing approximately 16% of the national food supply [1]. It contains the headwaters of many important rivers, including the Yangtze, Lancang, and Nujiang rivers, which provide as much as 46% of China’s available water resources [1]. The terrain in this area is very complex. 5 mm day−1 and 8 mm day−1 (Figure 1). Agriculture and ecosystems in this region are quite vulnerable to summer rainfall variability in low-latitude highlands [2,3]
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