Poetry (Not) In Motion

“Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.”
—Lines from Samuel Taylor Coleridge’s 1834 poem, “The Rime of the Ancient Mariner,” describing the “doldrums”—an area circling the Earth near the equator where sailors would become stalled due to little or no wind. Largely forgotten in the modern age of engine-driven ships, the doldrums—part of the global equatorial trough that includes the intertropical convergence zone (ITCZ)—were the subject of recent research published in Geophysical Research Letters refuting the commonly held belief that they are caused by converging and rising air masses. In fact, the study found that the opposite is true: the doldrums are generated by large areas of sinking air that diverge at the surface. According to the study's author, Julia Windmiller of the Max Planck Institute for Meteorology, the problem with the original theory of rising air is that it can only be true when averaged over longer time scales (days or weeks). Windmiller studied 20 years of ITCZ meteorological and buoy data for the Atlantic Ocean to define the edges of the ITCZ and identify low wind speed events (winds blowing slower than three meters per second for at least six hours). She discovered such events occurred in the absence of precipitation and with changing air temperatures, suggesting a connection to sinking air masses diverging at the surface rather than rising air. “Most of the air inside the Intertropical Convergence Zone is actually going down rather than up,” Windmiller explains. “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.” She also found that the low wind speed events primarily occur in the inner regions of the ITCZ and vary based on the season and the region of the Atlantic. It’s not clear what causes the large regions of sinking air, but Windmiller notes that most air in the tropics is slowly sinking, and suggests they could also be caused by large convective systems that leave downdrafts in their wakes, or humidity gradients that cause local air to cool and sink. [Source: American Geophysical Union]

PHOTO CREDIT: Photo Credit: iStock.com/Jay Austin