During the Age of Sail, sailors navigating the trade winds near the equator feared getting stuck in the doldrums, a unique meteorological zone in the deep tropics. For over a century, scientists believed that the stillness of the doldrums was due to colliding, rising air masses. However, recent studies indicate that this understanding might be incorrect.
During the Age of Sail, sailors navigating the trade winds near the equator feared getting stuck in the doldrums, a unique meteorological zone in the deep tropics. For over a century, scientists believed that the stillness of the doldrums was due to colliding, rising air masses. However, recent studies indicate that this understanding might be incorrect.
Julia Windmiller, an atmospheric scientist at the Max Planck Institute for Meteorology and the lead author of the study, stated, “The classic explanation for the doldrums stems from a time when our understanding of tropical air movement was limited. We’ve overlooked the doldrums to such an extent that no one has revisited this original assumption.”
Windmiller suggests that the calm conditions in the doldrums are actually caused by vast regions of descending air that spread out at the surface, resulting in clear, windless days. Her findings contradict the long-standing explanation behind this tropical phenomenon that has led to shipwrecks and inspired literary works, yet has been largely neglected by scientists.
Historically, the low wind areas around the equator were attributed to converging and rising air masses. Although these rising air masses create zones of low pressure and gentle winds at the surface, this explanation only accounts for the doldrums when averaged over extended periods. In the short term, converging air masses don’t cover sufficient areas to create these extensive windless regions that can persist for days, as seen in the doldrums.
This research was documented in Geophysical Research Letters, a leading open-access journal published by AGU, known for disseminating impactful, concise reports relevant to Earth and space sciences.
Understanding the doldrums
The doldrums, officially referred to as the Intertropical Convergence Zone, received its name from 19th century sailors who found themselves stranded in a sea devoid of wind. The term was originally defined to signify a sense of gloom or despair, and it now describes the sometimes-stormy, sometimes-quiet equatorial band. This maritime zone is even mentioned in the poem “The Rime of the Ancient Mariner” by Samuel Taylor Coleridge, published in 1834:
Day after day, day after day, We stuck, nor breath nor motion; As idle as a painted ship Upon a painted ocean.
Typically, the Intertropical Convergence Zone is depicted as a region where trade winds converge and air masses rise near the equator. These air masses, warmed by the sun’s heat, rise like balloons, leading to cloud formation and storms. They eventually descend at about 30 degrees North and South of the equator, completing a cycle known as Hadley Cell circulation. Historically, it was assumed that the converging, rising air caused the doldrums, generating pockets of low or no wind directly beneath.
However, little contemporary research has focused on identifying the true cause of the doldrums. According to Windmiller, the widely accepted explanation may not hold unless the regions of uplifting air are considered over longer durations.
Windmiller remarked, “There’s an intriguing gap in reasoning here because this upward flow of air doesn’t apply at shorter timeframes and in large areas where the winds are still. Our historical neglect of the doldrums has led to this oversight in logic.”
By analyzing meteorological records from the Intertropical Convergence Zone in the Atlantic Ocean from 2001 to 2021, along with buoy data from 1998 to 2018, Windmiller aimed to define the boundaries of the Intertropical Convergence Zone and explore occurrences of low wind speeds in this area. These low wind events are identified by winds blowing at less than three meters per second (five knots) for at least six hours. Windmiller scrutinized the data across multi-day, hourly, and minute-by-minute intervals to understand how low wind events developed over time.
Her results showed that low wind speed occurrences were linked to clear weather, lower air temperatures, and minimal precipitation—conditions suggesting that it is descending air that creates the stillness, rather than rising air. Windmiller also discovered that these low wind speed events primarily happen in the core regions of the Intertropical Convergence Zone and occur only about 5% of the time in that area, but can reach as high as 21% in the eastern Atlantic during the summer months of the Northern Hemisphere. Additionally, the locations of low wind speeds were influenced by both seasonal changes and different parts of the Atlantic.
“In fact, the majority of air in the Intertropical Convergence Zone is descending rather than rising,” said Windmiller. “It’s not just that we observe low wind speeds on average, but there are significant moments when winds have completely vanished over expansive areas.”
Her theory is backed not only by scientific findings but also by the next lines in Coleridge’s poem that famously depict a ship stuck in a windless, rainless expanse of the doldrums:
Water, water, everywhere, And all the boards did shrink; Water, water, everywhere, Nor any drop to drink.
Challenging an old theory
For years, Windmiller has asked fellow atmospheric scientists about the phenomena of the doldrums: What truly causes the winds to sometimes vanish near the equator?
“Often, they would start explaining the upward flow of air, but as they spoke, they’d begin to realize it didn’t really add up,” Windmiller recalled. “I was consistently surprised. It’s a fundamental phenomenon, so why is there no clear theory?”
Nevertheless, some uncertainties remain. Windmiller is unsure about the reasons behind the large areas of sinking air within the Intertropical Convergence Zone. While much of the air in the tropics gradually descends, this alone may not suffice to explain the stagnation. Other contributing factors might include large convective systems that leave turbulent wake patterns, or gradients in humidity that cool local air and prompt it to sink, she noted.
Even though today’s sailors are less likely to get trapped in the doldrums thanks to engines, grasping the true cause of these regions could still be relevant. New climate models with advanced resolution struggle to accurately represent areas of low wind speeds, so better understanding the doldrums could enhance our predictions regarding precipitation and wind patterns.
“We must reevaluate how we explain these low wind speed occurrences,” Windmiller urged. “I hope this revelation encourages others to perceive that our previous explanation was fundamentally flawed.”
