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The Walker circulation, an atmospheric circulation pattern in the tropics, has accelerated in recent years, puzzling climate scientists who had anticipated the opposite. Researchers from the Max Planck Institute for Meteorology and the University of Tokyo have found out why by revealing the competing effects between the global warming and sea surface temperature pattern effect.
Some of the climate system’s responses to global warming are rather surprising: Contrary to scientists’ expectations, the Pacific Walker circulation – a large-scale circulation in the tropical atmosphere – has strengthened in recent decades. Why this is the case, and how the Walker circulation might develop in the future, are urgent questions. After all, it impacts weather patterns far beyond the tropics: This can be witnessed during La Niña and El Niño conditions, phenomena that are known to cause extreme weather in various regions of the world with the former being linked to a strengthening and the latter to a weakening of the Walker circulation.
The Walker circulation forms over the tropical Pacific, where the Western Pacific is typically warm with low sea level pressure and the Eastern Pacific is cooler with high pressure. Warm, moist air ascends over the West Pacific, while cooler, dry air descends over the Eastern Pacific. Near-surface equatorial trade winds, blowing east to west, complete the circulation loop.
What if…? Experiments with a circulation model
A new study led by Sarah Kang, director at the Max Planck Institute for Meteorology (MPI-M) provides an explanation of the unexpected recent behavior of the Walker circulation and was now published in the journal Geophysical Research Letters. The team, which also included Masahiro Watanabe from the University of Tokyo and MPI-M researcher Veronika Gayler, compared dedicated simulations of the general circulation atmospheric model ECHAM6.3 over the past 35 years, which assumed different warming amplitudes and sea surface temperature (SST) patterns: What happens for a certain temperature increase given the observed SST pattern? And would it be any different if the SST pattern were reversed?
The researchers examined both the Walker circulation – defined as the difference in sea level pressure between the West and East Pacific – and the subgrid-scale convective mass flux, a direct measure of convection strength. The convective mass flux is expected to decrease with increasing temperatures because the atmosphere becomes more stable due to amplified warming in the upper tropical troposphere under global warming. This reasoning is in agreement with observations.
A weakening is still likely in the long term
A weakening of the convective mass flux has been commonly used to argue that the Walker circulation will also slow down. But the observations were telling a different story. Kang and her colleagues found out why: the Walker circulation is not as tightly coupled to the convective mass flux as previously assumed. It does weaken due to global warming, but unlike the convective mass flux, several factors can counterbalance this tendency – most notably the SST pattern, as the study showed. “Despite global warming, the Walker circulation can strengthen if the difference in SST between the West and East Pacific is sufficiently large”, says lead author Sarah Kang. “This not only explains the recent strengthening of the Walker circulation, which coincides with cooling in the East Pacific, but also suggests that it could continue to strengthen for some time, as long as the increasing zonal SST gradient persists.”
However, climate scientists might be proven right in the long term: As global warming continues, the SST gradient is projected to decrease, and the SST pattern effect will then reinforce the global warming effect, leading to a weakening of the Walker circulation. Therefore, while the Walker circulation may strengthen in the short term, it is likely to slow down in the long run. The study highlights the need for a better understanding of the mechanism behind the tropical Pacific warming pattern.
Prof. Dr. Sarah Kang, Max Planck Institute for Meteorology, sarah.kang@mpimet.mpg.de
Kang, S. M., Watanabe, M., & Gayler, V. (2025). Common and distinct drivers of convective mass flux and Walker circulation changes. Geophysical Research Letters, 52, e2024GL111897. https://doi.org/10.1029/2024GL111897
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