The last deglaciation (~19 to 11.5 ka BP) represents the most recent major transition in Earth’s climate system, characterized by rapid warming and pronounced millennial variability. Understanding the behavior of the upper-level subtropical westerly jet over East Asia (EASWJ) during this period is of great significance for projecting midlatitude climate responses to future global warming.
Based on a set of transient climate simulations, the paleoclimate simulation team from the State Key Laboratory of Climate System Prediction and Risk Management, Nanjing Normal University revealed “dual characteristics” of the summer EASWJ during the last deglaciation: millennial-scale variability superimposed on a long-term weakening trend. More importantly, they found that orbital forcing and greenhouse gas (GHG) forcing drove the EASWJ in opposite directions—a finding that challenges the simple “warmer means weaker” assumption. The findings were recently published in Atmospheric and Oceanic Science Letters .
The research team analyzed the EASWJ changes in position, intensity, and width. The results showed that during the Heinrich Stadial 1 and Younger Dryas cold periods, the jet strengthened and shifted southward, whereas during the Bølling–Allerød warm period, it weakened and shifted northward. These variations were highly consistent with its seasonal behavior, i.e., “weaker north–stronger south”.
It was the orbital forcing that caused the long-term weakening trend, and surprisingly, the rising GHGs during the last deglaciation actually caused a strengthened EASWJ.
“Both forcings warmed the climate, but their warming structures differed dramatically,” explains Dr. Yan, corresponding author of the study. “Orbital forcing led to amplified warming at high latitudes, which reduced the meridional temperature gradient and weakened the jet. In contrast, GHGs induced stronger warming in the upper troposphere at low to mid latitudes, enhancing the temperature gradient and thus strengthening the jet. This means that under future global warming, the westerly jet might not necessarily weaken—it could instead become stronger.”
This finding provides important insights for projecting the climatic effects of future changes in GHGs.
This study also shows how the EASWJ reshaped the rainfall pattern in eastern China: the millennial variability of AMOC (Atlantic Meridional Overturning Circulation)-induced jet shifts tends to lead to a “wet north–dry south” or “wet south–dry north” dipole pattern; the long-term weakening trend would produce a tripole pattern—wet in the southeast and northeast, but dry in the Jianghuai River Basin. The width of the EASWJ also appears to influence uniform wet or dry conditions across northern China—a factor that has been largely overlooked.
In future works, the team plans to use multi-model ensembles and higher-resolution paleoclimate records to further diagnose the contributions of different forcing, and to study how the width of the EASWJ affects the regional hydroclimate. The study’s key takeaway is that when projecting future jet-stream changes, we must look beyond the magnitude of warming—its spatial structure matters just as much.
Atmospheric and Oceanic Science Letters