This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The weather over the North Atlantic affects hundreds of millions of people. From winter storms over Europe to cold spells in eastern North America, many of these impacts are shaped by large-scale atmospheric circulation patterns that recur.
Climate scientists refer to these preferred patterns as atmospheric regimes. One of the most influential of these regimes is the North Atlantic Oscillation (NAO), a seesaw in atmospheric pressure between the subtropical Atlantic and Iceland that governs wind strength, storm tracks and temperature patterns across the region. Yet a fundamental question has remained open for decades: How does anthropogenic climate change affect these atmospheric regimes and their variability?
Our recent study, published in Communications Earth & Environment, set out to answer this question. One of the hardest problems in climate science is distinguishing human-driven change from the climate system's own internal variability. Atmospheric circulation naturally fluctuates on timescales from weeks to decades, even without any external forcing.
This makes it difficult to tell whether observed changes reflect global warming or just natural variability. Earlier studies often relied on single climate model simulations or short observational records. But these approaches can blur the signal, especially for phenomena like atmospheric regimes that are inherently noisy.
To overcome this, we used 100 simulations from the Community Earth System Model Large Ensemble (CESM2-LE). Each simulation experiences the same external forcing—greenhouse gases, aerosols and volcanoes—but starts from slightly different initial conditions. This allows us to cleanly separate: This distinction turned out to be crucial.
Rather than assuming climate change acts gradually, we asked a simple but powerful question: When does the imprint of anthropogenic warming become detectable in the North Atlantic circulation? Using a statistical changepoint detection method, we found a clear signal emerging around 1995 in mid-tropospheric circulation patterns. This motivated us to split our analysis into two periods: This allowed us to directly compare atmospheric regimes before and after the emergence of the climate change signal.
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