The 2010–2012 La Niña event caused extreme weather around the globe, from record snowfall in the northeast United States to severe drought in East Africa. It also resulted in unprecedented warming of ocean water off the coast of Western Australia in early 2011—an event termed the Ningaloo Niño—that killed fish and bleached coral in the region. Feng et al. have now shown that distant rainstorms intensified a major ocean current that contributed to the warming phenomenon.
From late 2010 to mid-2011, heavy precipitation lowered the salinity of surface ocean water in the maritime continent, a region that includes parts of Southeast Asia, Indonesia, and the Philippines. Using precipitation and upper ocean salinity data, together with modeling, the authors tracked the fate of this freshened water.
The researchers relied on free-drifting, battery-powered Argo floats, which repeatedly dive and resurface to relay temperature, salinity, and velocity data. They also used conductivity-temperature-depth measurements and mooring observations from a coastal monitoring station off Rottnest Island, located west of Perth. The station is part of Australia’s Integrated Marine Observing System.
The authors found that the Indonesian Throughflow current carried the warm and unusually fresh water westward. The South Equatorial Current carried some of it farther into the interior Indian Ocean, while the Leeuwin Current diverted some of it southward along the west coast of Australia, contributing to the Ningaloo Niño.
Using a modeling product called BRAN3p5, developed by Australia’s Commonwealth Scientific and Industrial Research Organisation, the scientists found that both temperature and salinity gradients generated a major increase in the volume of warm water transported by the Leeuwin Current. On its own, unusually low salinity was responsible for 30% of the increase in the current’s volume transport over the course of the Ningaloo Niño.
Peering into past records, the researchers uncovered similarly anomalous water freshening off the coast of Western Australia during previous La Niña events. Climate and marine ecosystems in the region are profoundly affected by heat transport of the Indonesian Throughflow and the Leeuwin Current, which makes it increasingly important for scientists to understand how La Niña affects their flow. (Geophysical Research Letters, doi:10.1002/2015GL065848, 2015)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), Distant rains contributed to La Niña ocean warming event, Eos, 97, doi:10.1029/2016EO045921. Published on 12 February 2016.