In Lewis Carroll’s nonsense poem, The Jabberwocky, “the slithy toves did gyre and gimble in the wabe.” Most of these weird words were made up by Carroll, but “gyre” is a real one. It comes, according to the Merriam-Webster Dictionary, via the Latin gyrus from the Greek gyros, meaning "ring" or "circle" and is used to refer to a circular or spiral motion or form, particularly a giant circular oceanic surface current.
The term is used chiefly by oceanographers who apply it to large circular sea currents formed principally by global wind patterns blowing surface water, the force imparted by the Coriolis effect as the planet spins, and to the land formations and bathymetry the resulting currents interact with. There are many gyres, but the largest in scale are the North and South Pacific Subtropical Gyres, the North and South Atlantic Subtropical Gyres, and the Indian Ocean Subtropical Gyre.
The Central American Gyre
The term gyre is not, however, applied exclusively to ocean currents. It can also refer to vortexes in whirlpools, in tornadoes, and in the atmosphere.
Central American Gyres (CAGs), for example, are atmospheric gyres. CAGs form when winds flow cyclonically around a broad area of low pressure over the Central American isthmus. They are like broad monsoonal low-pressure systems in other oceanic basins, of which the Northwest Pacific monsoon gyre is perhaps the best-known.
From May to November Central America experiences a rainy season that coincides with the Atlantic hurricane season. During these months CAGs can form at any time, but they tend to form either at the beginning or the end of the season as the northern summer waxes and wanes and large-scale weather features readjust. CAGS that form early in the season typically lead to tropical cyclone development more often than late season CAGs do.
CAGs last on average for just over three days but can persist for much longer and there are on average 1.5 CAG events in a year. A 2017 study considers it, “likely that the higher topography of Central America plays a critical role in the generation and organization of convection and precipitation associated with CAG events.”
CAGs generally have relatively weak surface winds, but they move slowly and are known for widespread and often intense precipitation across Central America and the Caribbean and the devastating flooding that can result from it. CAGs can also contribute to the development of tropical cyclones in the Caribbean, the Gulf of Mexico, and the Eastern Pacific. The gyre’s rising air creates areas of low pressure and moisture that enable storms to develop, and tropical cyclones can form if those areas develop spin. Tropical cyclones tend to develop on the eastern side of about half of the CAGs that form, and to rotate counterclockwise around the larger circulation.
In 2020, for example, a CAG formed late in May and steered Pacific Tropical Storm Amanda across Guatemala and into the Bay of Campeche, where the system’s remnants regenerated to form Tropical Storm Cristobal on June 1. Cristobal made landfall on the Yucatan Peninsula on June 3 and again in southeastern Louisiana on June 7. A week of devastating precipitation deposited up to 20 inches of rainfall in parts of Central America and dropped the equivalent of a quarter of El Salvador’s average annual rainfall in just 70 hours. The flooding that ensued damaged about 900 homes and formed the worst natural disaster in the country since 1998. In the U.S., Cristobal passed directly over the New Orleans metro area. Rainfall and storm surge caused widespread flooding along the Gulf Coast and at least six tornadoes were confirmed in Florida from the storm’s outer rainbands.
Back in 2018 a disturbance in a CAG developed into a tropical depression, broke away, and later slammed into the Florida Panhandle as Hurricane Michael, a monster Category 5 tropical cyclone. Most recently it is possible that the CAG that formed at the end of May 2022 could have contributed to Hurricane Agatha’s remnants in the Eastern Pacific reorganizing into Tropical Storm Alex in the Atlantic Basin. The Central American Gyre is not something encountered every day, but when one hits the headlines it is something to pay attention to.