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Study challenges long-held explanation for Doldrums, the equatorial low-wind region
Windmiller's explanation of the doldrums is that large regions of sinking air near the equator causes long-lasting regions of low-to-no wind speeds at the surface. Although the doldrums typically occur between 5 degrees North and South of the equator, those boundaries can shift considerably around the tropics. Credit: Mary Heinrichs, AGU
Study challenges long-held explanation for Doldrums, the equatorial low-wind region
by Robert Schreiber
Berlin, Germany (SPX) Sep 17, 2024

During the Age of Sail, mariners crossing the equator feared the doldrums - a region notorious for its lack of wind that could leave ships stranded for weeks. For over a century, scientists believed that the doldrums' windless conditions were caused by converging air masses rising upwards. However, a recent study suggests the opposite may be true.

"The idea of what causes the doldrums came from a time where we didn't know a lot about how air actually moves in the tropics," explained Julia Windmiller, an atmospheric scientist at the Max Planck Institute for Meteorology and lead author of the study. "We have forgotten about the doldrums to such a degree that nobody has taken the trouble of thinking through this original argument again."

Windmiller proposes that rather than rising air, it is the sinking and diverging air at the surface that creates the calm and windless days associated with the doldrums. This new perspective challenges the long-standing explanation for this tropical oceanic phenomenon that stranded sailors for centuries and has largely been absent from scientific discussion in recent years.

Traditionally, the windless conditions in the doldrums, also known as the Intertropical Convergence Zone (ITCZ), were explained by converging trade winds and rising air, which created low-pressure systems. While this theory can account for calm winds when averaged over time, it doesn't fully explain large, windless areas that last for days. The new research indicates that these conditions are more likely caused by sinking air masses that diverge at the surface, rather than rising air.

The study was published in 'Geophysical Research Letters', a peer-reviewed journal of the American Geophysical Union (AGU), which specializes in short-format, high-impact reports on Earth and space sciences.

Unraveling the Doldrums
The doldrums, named by sailors in the 19th century, often left ships immobile due to the absence of wind. The region was so frustrating for mariners that it even made its way into poetry, such as Samuel Taylor Coleridge's 1834 work, 'The Rime of the Ancient Mariner':

"Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean."

The ITCZ is typically described as an area near the equator where trade winds converge, causing air masses to rise, form clouds, and produce storms. This process, known as Hadley Cell circulation, involves the air eventually descending near 30 degrees latitude north and south. The calm conditions under these rising air masses were thought to explain the doldrums.

However, according to Windmiller, this explanation falls short when considering shorter timescales and larger windless areas. "There's this fascinating break in reasoning because this upward circulation of air doesn't work for short time scales and large areas of still wind," she said. "To some degree, because we've historically forgotten about the doldrums, this flaw in the logic never really came up."

Using meteorological data from the ITCZ over the Atlantic Ocean between 2001 and 2021, along with buoy data from 1998 to 2018, Windmiller investigated low wind speed events in the region. These events are defined by wind speeds below three meters per second (about five knots) lasting for at least six hours. Analyzing data on multi-day and hourly timescales, she found that low wind speed events aligned with clear weather, cooler temperatures, and no precipitation, all indicating sinking air rather than rising air.

Windmiller's research shows that these events are most common in the inner ITCZ and occur in around 5% of the region at any given time, but can increase to 21% in the eastern Atlantic during the Northern Hemisphere's summer. The locations and frequency of these low wind events also vary with the season and specific areas of the Atlantic.

"Most of the air inside the Intertropical Convergence Zone is actually going down rather than up," Windmiller said. "It's not just on average that we have low wind speeds in this region, but that we have these moments in time when the wind has just gone away over very large areas."

This observation is echoed in another verse from Coleridge's famous poem:

"Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink."

A Shift in Understanding
Windmiller has spent years questioning the traditional explanation for the doldrums. "They would start to explain this upward circulation of air, but as they were explaining it, they often realized it didn't actually make sense," she said. "I was always surprised. It's such a basic phenomenon, so why wouldn't we have a theory for it?"

Some mysteries remain, though. Windmiller is still unsure what causes the large areas of sinking air in the ITCZ. While most air in the tropics tends to sink slowly, this effect may not be strong enough by itself to cause the doldrums. Other factors, such as downdrafts from large convective systems or humidity gradients that cool and sink the air, could also contribute.

While modern mariners have diesel engines to avoid getting trapped in the doldrums, understanding the phenomenon could still have real-world impacts. High-resolution climate models struggle to simulate low wind speed regions accurately, and better understanding the doldrums could improve predictions of weather patterns and precipitation.

"We can no longer explain these low wind speed events in the way we've done before," Windmiller said. "I hope that this is something that people will see and read, and realize that the explanation is really upside down from what we've had."

Research Report:The calm and variable inner life of the Atlantic Intertropical Convergence Zone: the relationship between the doldrums and surface convergence

Related Links
Max Planck Institute for Meteorology
Earth Observation News - Suppiliers, Technology and Application

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