In 2011, a record-breaking marine heat wave off the west coast of Australia wreaked ecological havoc, bleaching coral reefs and causing massive fish die-offs. Scientists identified the cause of the destruction: Ningaloo Niño, a warm surge within an ocean current that flows south from Indonesia. Now a new study reveals that Ningaloo Niño forms a positive feedback loop in the Pacific Ocean, a discovery that could help scientists predict the El Niño-Southern Oscillation (ENSO): periodic changes in sea temperature that can cause droughts, floods, and heat waves.
Scientists already knew that La Niña, the cold phase of ENSO, plays a key role in triggering Ningaloo Niño by stirring up winds and ramping up the flow of water from the Pacific Ocean into the Indian Ocean. They also knew that, sometimes, local events also produce a Ningaloo Niño, entirely independent of La Niña. But they didn’t know how the abnormally high temperature of this warm current influences currents in the tropical Pacific or how those currents, in return, affect Ningaloo Niño.
To find out, Zhang and Han looked at data from the Hadley Centre Global Sea Ice and Sea Surface Temperature resource, a record of monthly satellite and in situ measurements of sea surface temperatures and sea ice concentrations around the globe. The researchers focused on the years 1940–2016 and also examined wind records from the same time period. Then, they ran three computer simulations: a control, which used the climatological sea surface temperatures, and two experiments that mimicked sea surface temperatures to resemble Ningaloo Niño and Pacific La Niña.
The models showed that the warmer waters in the southeast Indian Ocean strengthened the western tropical Pacific trade winds and cooled the waters of the equatorial Pacific regions. This cooling lowered atmospheric pressure at sea level and caused cyclonic winds to form over the southeast Indian Ocean. In turn, the stronger trade winds and cooler Pacific waters enhanced the strength of Ningaloo Niño’s warm current: a feedback loop.
These results indicate that Ningaloo Niño may play an unappreciated role in building up the strength of La Niña and has likely contributed to shaping the sea surface temperature patterns of several so-called super El Niños, including in 1982 and 1997. According to the authors, ENSO forecasts could be more accurate if they took Ningaloo Niño into account. (Geophysical Research Letters, https://doi.org/10.1029/2018GL078579, 2018)
—Emily Underwood, Freelance Writer