In recent years, global climate is becoming more extreme. The alternation of droughts and floods is intensifying, especially in the ecologically vulnerable mid-latitude regions. But what is driving this hydroclimate variability? Scientists have long debated the mechanism.

A research team led by Prof. LONG Hao from Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, drilled a 300.8-meter-long lacustrine sediment core in the Datong Basin, Shanxi, located in mid-latitude East Asia (Northern China). By tracing over 5.7 million years of Earth's history, researchers revealed that the "waviness" of the Westerly Jet Stream is the primary driver behind mid-latitude climate variability. The study was published in Nature Communications recently.   

This core is like a detailed "climate archive" documenting precipitation changes over the past approximately 5.7 million years (spanning the Pliocene and Pleistocene epochs). By analyzing chemical indicators within the core, scientists obtained a high-resolution record of ancient precipitation.

First, during the warm period before around 3 million years ago (the Pliocene), the precipitation variability in mid-latitude regions was significantly higher than during the subsequent Pleistocene epoch (the Ice Age era). Scientists attribute this drastic climate fluctuation to enhanced Westerly Jet waviness. Further idealized sensitivity simulations indicated that the force driving the Westerly Jet to become "wavy" was Arctic warming.

When the Arctic is cold, the polar vortex is strong, and the Westerly Jet remains stable and straight. Cold air is effectively contained in the north, and the mid-latitude climate is relatively stable.

When the Arctic warms, the polar vortex weakens, causing the Westerly Jet to become "sinuous". This fluctuation allows cold and warm air masses to frequently intermingle and penetrate north and south, leading to more frequent extreme wet and dry events in the mid-latitudes, and thus amplifying hydroclimate variability.

Also, the study ruled out atmospheric carbon dioxide (CO₂) concentration as the dominant driver of this variability, confirming that Westerly Jet waviness is the key factor.

“This study, based on ancient climate records, is crucial for our understanding of future climate change.” said by Prof. LONG Hao. “Given that future global warming is projected to lead to increased Westerly Jet waviness, it is reasonable to predict that mid-latitude regions will face more frequent hydroclimate extreme events, such as severe droughts and floods.”