John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
NOAA has issued a “La Niña Watch” for its probable arrival in summer or early fall.
The eastern half of the U.S. lucked out last year with a below-average Atlantic hurricane season—only four such tempests, compared to the historical average of six.
But with models forecasting the development of a strong La Niña, 2016 could ring in a higher number of hurricanes, delivering the usual problems like beach erosion and urban flooding.
Forecasters expect El Niño to perish sometime in the spring or early summer. La Niñas often, but not always, follow El Niños, and it’s that possibility that prompted NOAA yesterday to issue a “La Niña Watch” for late summer or early fall. If it does materialize, it’d mean the Atlantic would have weaker wind shear—or changes in the direction and speed of wind thousands of feet above the sea—which would give hurricanes an easier time forming.
NOAA has all the nerdy details:
During La Niña, the area of tropical convection and its Hadley circulation is retracted westward to the western Pacific and Indonesia, and the equatorial Walker circulation is enhanced. Convection is typically absent across the eastern half of the equatorial Pacific.
In the upper atmosphere, these conditions produce an amplified trough over the subtropical Pacific in the area north of the suppressed convection, and a downstream ridge over the Caribbean Sea and western tropical Atlantic. Over the central and eastern subtropical Pacific, the enhanced trough is associated with stronger upper-level winds and stronger vertical wind shear, which suppress hurricane activity. Over the Atlantic basin, the anomalous upper-level ridge is associated with weaker upper- and lower- level winds, both of which reduce the vertical wind shear and increased hurricane activity. La Niña also favors increased Atlantic hurricane activity by decreasing the amount of sinking motion and decreasing the atmospheric stability.
What else might a potential La Niña bring? Well, in winter months it can enforce drier, warmer weather in the South and colder temperatures in the Pacific Northwest and Midwest. (Also Alaska, which could use a respite from unusual heat.)
La Niña can also raise the temperature of parts of the Pacific Ocean, not a great thing for corals already suffering from the longest die-off in known history. “Coral can tolerate shorter periods of high temperatures, but global warming has raised the average temperature experienced by corals,” writes NOAA’s Emily Becker. “When the stress of El Niño or La Niña is added, the chance of widespread coral bleaching increases.”