A Northern Jet Helped Drive South China's Record Rain

A new analysis of the record rainfall that soaked South China in April and May 2024 puts a midlatitude jet stream among the causes, and finds that a forecast model that handled the tropics well still underestimated the rain.

Xuefeng Liu and Zhiwei Zhu of Nanjing University of Information Science and Technology, with colleagues at Sun Yat-Sen University and the Chinese Academy of Sciences, published the work Nov. 1, 2026, in Communications Earth & Environment. The result matters for forecasting: flow well outside the tropics has to be captured before an extreme driven from the tropics can be predicted.

Rainfall that heavy over South China is usually attributed to warm tropical seas. Using observations, reanalysis data (a reconstruction of past weather) and numerical experiments, the team traced the April and May downpours to a trough of low pressure caught between two anticyclones, or regions of high pressure. One sat over the tropical western North Pacific and pushed moisture into South China. The other sat over Northeast Asia and stopped that moisture from leaving, so it built up over the region.

The two highs had different origins. The tropical one was forced by unusual sea surface temperatures in the Indian Ocean. The Northeast Asian high formed inside a chain of waves in the upper-level flow, a Rossby wave train, set off by storm activity over the tropical Atlantic. The polar front jet, the fast high-altitude wind that runs along the edge of cold polar air, steered that chain of waves.

The authors report that the European Centre for Medium-Range Weather Forecasts model reproduced the tropical side of the mechanism but underestimated the rainfall, and they attribute the shortfall to biases in how it represented the polar front jet.

The paper is open access. It was published as an author-accepted version carrying a permanent DOI ahead of the journal's final version, with the referee reports posted alongside it.