Chinese Academy of Sciences
During summer 2020, the Yangtze River basin experienced persistent, record-breaking meiyu rainfall, and suffered from severe flooding and water damage. Regions outside the meiyu rain belt received significant summer rainfall as well.
Typically, an above average meiyu rainfall season follows a strong El Nino during the previous winter. However, summer 2020 followed a neutral El Nino-Southern Oscillation (ENSO) event.
A group of scientists from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences and the UK Met Office Hadley Centre tried to find clues as to why the 2020 summer meiyu season was so unique.
They explored many dynamic components of this outlying season, and why some predictability signals, like the phase of ENSO, did not verify well for summer 2020 rainfall forecasts across the Yangtze River basin.
Their findings were published in Advances in Atmospheric Sciences on June 28.
Despite the poor ENSO signal, archived model data and observations agreed that the Met Office GloSea5 operational forecast system provided an accurate above average summer 2020 rainfall projection.
Researchers attributed the reliable forecast to a good reproduction of the notably anomalous western North Pacific subtropical high (WNPSH) pressure system. The model used local and tropical Indian Ocean sea surface temperature (SST) data to predict the size and placement of the WNPSH throughout the summer season.
"These results support that the anomalous WNPSH change and associated local air-sea interactions could exist without the impact of ENSO," said Dr. LI Chaofan, the lead author of the study.
Collaborators also explored which extratropical dynamics may have driven extreme heavy rainfall. They found that the East Asian westerly jet (EAJ) was significantly accelerated in observed data, but not nearly as intense in model simulations.
While this scenario may favor more meiyu rainfall, accurately predicting the placement and speed of the EAJ through the summer season is more difficult. That said, the EAJ likely boosted seasonal Yangtze River rainfall against weaker model projections, hindering precision forecasting and disaster mitigation.
"Unfortunately, the forecast Yangtze River rainfall anomaly was weaker compared to that observed," added LI, "But the forecast members are useful to identify predictability and dynamical causes for this exceptional rainfall."
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