The Doldrums: How the ITCZ Stalls Sailors, Spawns Hurricanes, Shifts Rain

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Coleridge's painted ship upon a painted ocean made the doldrums a byword for stagnation, but the region is the Intertropical Convergence Zone, a band where the northeast and southeast trade winds collide near the equator and are forced upward as the rising branch of the Hadley cell. Horizontal wind dies at the surface while violent convection builds towering thunderstorms, which is why sailing ships sat becalmed for days or weeks in heat and humidity. Meteorologists called it the intertropical front from the 1920s to the 1940s before renaming it once they found the converging air masses barely differ in temperature. The zone follows the thermal equator, bringing the wet and dry seasons of the tropics.

Over land it distorts, and Asia's summer heating pulls it toward the Himalayas to drive the monsoon, while the South Pacific Convergence Zone can break apart during El Nino. When the band drifts at least five degrees from the equator, the Coriolis force can spin its storms into tropical cyclones. In 2009 Air France Flight 447 flew into ITCZ storms at night, iced its pitot tubes, and crashed with 228 aboard. Titanium in Cariaco Basin sediment off Venezuela records past positions of the zone, which shifted south during Heinrich events and again from the 1950s to the 1980s, likely forced by northern aerosol pollution.

• That southward shift is widely believed to have driven the 1970s and 1980s Sahel droughts.
• Since at least 1979 the Pacific ITCZ has been narrowing and intensifying.
• Seawater under its drying fringes is growing saltier while water under its core freshens.
• CMIP6 models suggest a northward push over the Indian Ocean and East Africa and a southward shift over the eastern Pacific and Atlantic.
• The name inspired the Doldrums where Milo gets stuck in The Phantom Tollbooth.