Doldrums debunked: what really causes still water?

The windless equatorial region of the ocean, known as “the doldrums”, became so famous for stranding boats in the Age of Sail that the word has become part of our vernacular.

But recently, research interest in the doldrums has been – well, stuck in the doldrums.

And according to a German scientist, the current explanation for why the doldrums exist doesn’t make sense.

“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,” says Dr Julia Windmiller, an atmospheric scientist at the Max Planck Institute for Meteorology.

“We have forgotten about the doldrums to such a degree that nobody has taken the trouble of thinking through this original argument again.”

The current explanation, touted by places like the US National Oceanic and Atmospheric Administration, is that hot rising air around the equator causes a region of low pressure and thus surface wind.

But in reality, according to Windmiller, this upward rising doesn’t cover enough area in the short term to create these big windless regions.

“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.”

Windmiller says she’s been asking other atmospheric scientists about the doldrums for years.

“They would start to explain this upward circulation of air, but as they were explaining it, they often realised it didn’t actually make sense,” she says.

“I was always surprised. It’s such a basic phenomenon, so why wouldn’t we have a theory for it?”

Windmiller has suggested a new explanation for the doldrums in a study published in Geophysical Research Letters.

She analysed meteorological data, collected by satellites and ocean buoys, in the Atlantic Ocean from 1998-2021.

She examined the link between wind speeds and the Intertropical Convergence Zone (ITCZ) – a band of low-pressure air near the equator, which is caused by rising hot air.

The results showed that low wind speeds occurred in about 5% of the region at any given time, although their frequency varied by season.

These low wind speeds also coincided with clear, rainless, weather and cooler temperatures – conditions that imply sinking air masses, rather than rising air.

“Most of the air inside the Intertropical Convergence Zone is actually going down rather than up,” says Windmiller.

“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.”

In her paper, Windmiller points out that this was a well-known feature of the doldrums, as outlined by Samuel Taylor-Coleridge’s poem The Rime of the Ancient Mariner.

“Day after day, day after day,
We stuck, nor breath nor motion;
[…]
Water, water, every where,
Nor any drop to drink.”

If sinking air is causing the doldrums, then what’s causing the sinking air?

Windmiller is unsure, but has a few theories and is planning to work with colleagues to investigate the phenomenon further.

The team will be using data from the ORCESTRA campaign, a mission currently using a number of vessels and technology to glean meteorological information from the Atlantic Ocean, as well as climate models, to test the hypothesis further.