Losing the fat in the chilly north Atlantic

The weather in the North Atlantic has a significant impact on millions of people living across Europe and eastern North America. These effects are largely driven by recurring large-scale atmospheric circulation patterns known as atmospheric regimes. One of the most influential among them is the North Atlantic Oscillation (NAO), which governs wind strength, storm tracks, and temperature patterns across the region.

Despite its influence, a long-standing question has remained: How does human-induced climate change affect these atmospheric regimes and their variability?

To address this question, a recent study published in Communications Earth & Environment examined the role of anthropogenic warming in reshaping the North Atlantic's atmospheric circulation patterns. The researchers employed a statistical changepoint detection method, which involved analyzing 100 simulations from the Community Earth System Model Large Ensemble (CESM2-LE).

Each simulation shared the same external forcing factors—greenhouse gases, aerosols, and volcanoes—while beginning from slightly different initial conditions. This approach enabled the researchers to distinguish human-driven changes from the climate system's inherent variability.

The study found that anthropogenic climate change began to imprint itself on North Atlantic atmospheric circulation patterns around 1995. When comparing atmospheric regimes before and after this date, the researchers discovered a striking contrast in structure and organization. While the number of regimes remained constant, their spatial patterns underwent significant reorganization.

The centers of high and low-pressure systems shifted northward, intensifying circulation in some regimes and altering the balance between different patterns. For instance, a prominent example showed a clear northward displacement of the low-pressure region in the post-1995 period, indicating a shift in the dominant atmospheric regime.

However, when the researchers focused solely on internal variability without considering anthropogenic forcing, the picture changed dramatically. The number of regime states decreased after 1995, and one dominant regime became more persistent. This suggests that global warming suppresses some aspects of natural atmospheric variability, leading to fewer extreme swings between positive and negative phases of the North Atlantic Oscillation (NAO).

Positive NAO phases, associated with stronger westerly winds and milder winters in northern Europe, have shown a subtle but detectable shift toward more positive phases during most of the 21st century. In contrast, the overall variability of the NAO has declined, resulting in fewer strong swings between positive and negative phases.

Interestingly, the trend appears to partially reverse toward the end of the century, with low-amplitude negative NAO phases becoming more frequent. This nuanced behavior highlights the importance of using a large-ensemble approach to detect such changes. The study also linked these atmospheric regime changes to the mid-tropospheric jet stream, finding that warming strengthens the jet in NAO-positive regimes while weakening it in NAO-negative ones.

These changes in atmospheric circulation patterns have significant implications for seasonal prediction, climate risk assessment, and adaptation planning, particularly in regions like Europe, eastern North America, and the Arctic.