Spring rains, valued as "precious as oil" in ancient China, are crucial for ecosystems. In modern East Asia, "spring persistent rainfall" (SPR) brings about 600 mm of precipitation annually, making up over 35% of the region's rainfall. In contrast, North America lacks a similar rainy season despite being at a comparable latitude. A recent study in Communications Earth & Environment examines the 55-million-year history of SPR, exploring the relationship between paleogeography and climate change.

Researchers from the Institute of Atmospheric Physics of the Chinese Academy of Sciences studied the origins of SPR using time-slice climate simulations and proxy data. They found that SPR became prominent in East Asia during the Miocene (15–13 million years ago) when global temperatures were warmer. Previously, in the Eocene (around 55 million years ago), SPR was common in North America but absent in East Asia. This shift in rainfall patterns is mainly due to paleogeographic changes, with climate variability and CO₂-driven warming playing secondary roles.

The study highlights the uplift of the Qinghai-Tibet Plateau and the westward migration of the Rocky Mountains, driven by tectonic movements, as key factors in SPR evolution. These geological features influenced regional rainfall patterns in East Asia and North America and interacted to trigger a mid-latitude Rossby wave train, altering the position and intensity of subtropical high-pressure systems and reshaping spring rainfall distributions.

While paleogeography has been the dominant force in SPR evolution over geological timescales, the study highlights that future changes are likely to be driven primarily by climate warming. During the Eocene, when global temperatures were approximately 10°C higher than today, SPR rainfall increased in volume but decreased as a proportion of annual precipitation. These changes mirror contemporary observations in tropical regions under global warming, such as the "wet gets wetter" paradigm and the "seasonal delay" phenomenon. The findings suggest that, under extreme warming scenarios, East Asia's spring climate may transform akin to those currently observed in tropical regions.

This study provides a framework for predicting future changes in regional rainfall patterns under ongoing global warming.

Spring views in present-day southeastern North America (upper) and southeastern East Asia (bottom) (Images by SHEN Huimin)