Multiscale interactions driving the devastating floods in Henan Province, China during July 2021

The rare extreme flooding event in Henan Province, China, during July 2021 (referred to as the “21.7” flooding event) was attributable to persistent heavy rainfall boosted by an enhanced moist southeasterly flow and substantial moisture convergence. Based on analyses of the scale-decomposed moisture budget equation, we show that the 21.7 flooding event was a result of scale interactions between the monsoon mean field, intraseasonal oscillation, and synoptic disturbances. The two distinct modes of the boreal summer intraseasonal oscillation (10–30 and 30–90 days) unusually had a crucial combined role in moisture convergence, aided by the increased seasonal-mean moisture content, maintaining persistent rainfall from the development (July 17–19) to decaying (July 21–23) phases of the 21.7 flooding event. In contrast, synoptic-scale moisture convergence was the leading process during the peak phase of the flooding event (July 20), contributing to the extreme values. The state-of-the-art subseasonal-to-seasonal prediction models showed limited skills in predicting this extreme event one to two weeks in advance, partly because of their biases in representing the intraseasonal oscillation and multiscale interactions.