The competitive effects of global warming and sea surface temperature explain the recent strengthening of Walker circulation

Walker circulation, a tropical atmospheric circulation pattern, has accelerated in recent years, baffling climate scientists who had anticipated opposition. Researchers at the Max Planck Institute for Meteorology and the University of Tokyo have discovered the reason by revealing the competitive effects between global warming and the sea surface temperature pattern effects.

Some of the climate system’s response to global warming is rather surprising. Contrary to scientists’ expectations, the Walker circulation in the Pacific (a large circulation in the tropical atmosphere) has been strengthened in recent decades.

Why this is true and how the Walker cycle will develop in the future is an urgent question. After all, it affects weather patterns well beyond the tropical. This can be witnessed in the conditions of La Niña and El Niño, known to cause extreme weather in various parts of the world. The former is related to reinforcement, and the latter is related to weakening of pedestrian circulation.

The Walker circulation forms over the tropical Pacific Ocean. In the tropical Pacific, the western Pacific is usually warmed by low sea level pressures, while the eastern Pacific is cooled by high pressure. Warm, humid air rises over the Western Pacific, while cool, dry air descends over the East Pacific. The equatorial trade winds near the surface blow from east to west, completing the circulation loop.

What if…? Experiments using a cyclic model

A new study led by Sarah Kang, director of the Max Planck Institute for Meteorology (MPI-M), provides an explanation of unexpected recent behaviors in the Walker cycle. This study is published in Geophysical Research Papers.

Additionally, a team including Watanabe Sabe from the University of Tokyo and researcher Veronika Gayler, a researcher at MPI-M, compared dedicated simulations of the total circulation atmospheric model Echam6.3 over the past 35 years. ) Pattern: What happens for a particular temperature rise, taking into account the observed SST patterns? If the SST pattern is reversed, is that different?

The researchers examined both Walker circulation, defined as the difference in sea level pressure between the Western and Eastern Pacific Oceans, and subgrid-scale convection mass flux, a direct measure of convection intensity. As the atmosphere becomes more stable due to amplified warming in the upper tropical troposphere under global warming, convection mass flux is expected to decrease with increasing temperatures. This reasoning is consistent with observation.

The weakening is likely to still be in the long term

Weakening of convective mass flux has been commonly used to claim that Walker circulation also slows down. But the observation told a different story. Kang and her colleagues found a reason: Walker circulation is not so tightly coupled to convective mass flux, as previously assumed. Although weakened due to global warming, unlike convective mass flux, several factors can offset this trend, particularly the SST pattern.

“In spite of global warming, if the difference between the SST between the Western and Eastern Pacific Oceans is large enough, the Walker circulation is strengthened,” Kang says. “This not only explains the recent strengthening of Walker circulation, consistent with cooling in the Eastern Pacific, but also suggests that it can continue to be strengthened for some time as long as the zone SST gradient increases.”

However, climate scientists may prove to be correct in the long run. As global warming continues, the SST gradient is predicted to decrease, and the SST pattern effect enhances the global warming effect, leading to weakening of pedestrian circulation. Therefore, Walker circulation may be strengthened in the short term, but may be slower in the long term.

This study highlights the need to better understand the mechanisms behind tropical Pacific warming patterns.