Record-breaking April–May 2024 rainfall over South China was driven by a midlatitude trough positioned between anomalous anticyclones over the tropical western North Pacific and Northeast Asia, according to a study that sheds new light on the limits of extratropical circulation modeling.
Beyond Tropical Forcing: Rethinking South China’s Deluge
Extreme precipitation events across South China have long been blamed primarily on tropical sea surface temperature forcing. Yet, the exact mechanics of how broader atmospheric patterns interact with these tropical drivers remained unclear until researchers applied observational and reanalysis data, Rossby wave propagation diagnostics, and numerical experiments to the rainfall of April–May 2024.
A Dual Atmospheric Setup
The analysis reveals that the tropical anticyclone enhanced moisture transport into South China. Simultaneously, a separate anticyclone over Northeast Asia suppressed moisture export, favoring moisture accumulation. This dual atmospheric setup created a trough that drove intense moisture convergence over South China.
Divergent Origins of the Anticyclones
While Indian Ocean sea surface temperature anomalies forced the tropical anticyclone, the Northeast Asian system originated differently. It developed within a midlatitude Rossby wave train. That wave train was initiated by tropical Atlantic convection and guided by a transient eddy-driven polar front jet.
Forecasting Shortfalls and Jet Stream Biases
These mechanics exposed notable limitations in current forecasting capabilities. When researchers examined output from the European Center for Medium-Range Weather Forecasts model, they found that while the system reproduced tropical forcing, it underestimated precipitation. The shortfall stemmed from polar front jet biases.
Integrating Extratropical Flow Dynamics
The findings highlight the importance of extratropical flow for predicting tropical-forced extremes. Without properly resolving the behavior of the polar front jet and midlatitude wave trains, weather models will continue to miss critical drivers of regional precipitation extremes.
The research demonstrates that regional flooding events cannot be fully understood by looking at the tropics alone. Integrating midlatitude circulation data into forecasting models remains an essential step for improving precipitation predictions.
