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Moisture sources for precipitation in Southwest China in summer and the changes during the extreme droughts of 2006 and 2011

Chi Zhang

Published 2020 in Journal of Hydrology

ABSTRACT

Abstract Two record-breaking summer droughts hit Southwest China (SWC) in 2006 and 2011, causing severe losses to the society and economy. Moisture sources for precipitation in SWC and changes accompanied with the extreme droughts are thus investigated using a moisture tracking model, the Water Accounting Model-2layers (WAM-2layers), with observation-constrained data. SWC is influenced by three major circulations, namely, the Indian monsoon, the East Asian summer monsoon (EASM), and the westerlies. The representative source regions, the Southwest, Southeast, and West, contribute around 50.8 ± 3.9, 13.3 ± 3.8, and 16.4 ± 2.1% of the tracked moisture climatologically. The westerlies turn out to be the second-largest moisture contributor to SWC over the EASM. In 2006 when the western Pacific subtropical high (WPSH) intruded to the west, the influence of the EASM was intensified profoundly as it increased to be the second largest contributor to contribute 21.2% of the moisture. But this change induced circulation anomalies that hindered moisture transport from the West and Southwest. Their moisture contributions were substantially reduced, resulting in an overall moisture deficit for SWC. In 2011, the contributions from subregions were universally reduced. The moisture conveyor belt in the Southwest released moisture early before it reached SWC, thus resulting in a shortage in moisture supply to SWC. The precipitation recycling ratio in SWC is around 15.7 ± 1.9%. However, the ratio increased during both droughts to 18.6% in 2006 and 17.7% in 2011. Further analyses reveal that the external moisture decreased heavily, thus making local moisture more important to result in a marked increase in the recycling ratio.

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