NASA study adds a pinch of salt to El Niño models

Posted: April 8, 2020 by oldbrew in data, ENSO, Forecasting, modelling, Ocean dynamics

Credit: concernusa.org


Accurate ENSO forecasts without salinity data only extend out 4 months, while those with it cover 7 months, researchers believe.
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When modeling the El Niño-Southern Oscillation (ENSO) ocean-climate cycle, adding satellite sea surface salinity—or saltiness—data significantly improves model accuracy, according to a new NASA study.

ENSO is an irregular cycle of warm and cold climate events called El Niño and La Niña, says Phys.org.

In normal years, strong easterly trade winds blow from the Americas toward southeast Asia, but in an El Niño year, those winds are reduced and sometimes even reversed.

Warm water that was “piled up” in the western Pacific flows back toward the Americas, changing atmospheric pressure and moisture to produce droughts in Asia and more frequent storms and floods in the Americas.

The reverse pattern is called a La Niña, in which the ocean in the eastern Pacific is cooler than normal.

The team used NASA’s Global Modelling and Assimilation Office (GMAO) Sub-seasonal-To-Seasonal (S2S) coupled ocean/atmosphere forecasting system (GEOS-S2S-2) to model three past ENSO events: The strong 2015 El Niño, the 2017 La Niña and the weak 2018 El Niño.

Pulling from NASA’s Soil Moisture Active Passive (SMAP) mission, the past NASA-CONAE (Argentinian Space Agency) Aquarius mission and the European Space Agency’s Soil Moisture Ocean Salinity (SMOS) mission, they compared the forecast model’s accuracy for each of the three events with and without assimilating SSS data into the models’ initialization.

In other words: One model run’s initial conditions included SSS data, and the other did not.

Adding assimilation of SSS data to the GEOS model helped it to depict the depth and density of the ocean’s top layer more accurately, which led to better representations of large-scale circulation in response to ENSO.

As a result, the models’ predictions for the three case studies more closely reflected actual observations, compared to what forecasting models predicted at the time.

“In our three case studies, we examined different phases of ENSO,” said Eric Hackert, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and the study’s lead author.

“For the big El Niño in 2015, assimilating the salinity data damped the signal—our original model was overestimating the amplitude of the event. For the other two ENSO events, the forecasts originally predicted the wrong sign: For example, in 2017, the model without salinity data forecasted an El Niño, while the real ocean produced a La Niña. However, for each case we examined, adding satellite salinity to the initialization improved the forecasts.”

The study is one of the first to incorporate SSS data into forecast initialization for a global coupled model of interactions between the ocean, atmosphere, land, aerosols and sea ice. GEOS and other models used to help predict ENSO events do not typically include SSS.

However, ocean surface salinity plays an important role in ocean currents, evaporation and interaction with the atmosphere, and heat transfer from the tropics to the poles. Colder, saltier water is denser and heavier than warmer, fresher water, and the large-scale temperature and precipitation shifts of ENSO events change ocean circulation and interactions between the water and atmosphere.

Both phases of the ENSO cycle affect ecosystems, economies, human health, and wildfire risk—making ENSO forecasts vital for many people around the world, Hackert said.

Full article here.

Comments
  1. Damian Scott says:

    The Mayans recorded this. They watched the stars and when they twinkled more they new that the growing season would be affected.

  2. ren says:

    The subsurface temperature in the equatorial Pacific Ocean is falling.

  3. oldbrew says:

    Nothing for ‘planet on fire’ squealers to salivate over.

  4. Tom Williams says:

    Thank you for yet another brilliant scientific analysis. I can only imagine how much the “man made” climate change cult members will seize on the coronavirus as an excuse for any decrease in temperatures on land or sea. The timing of the prior eruptions (800 years ago) seems to coincide with the onset of the Little Ice Age afterward. The Little Ice Age wasn’t caused by people taking their cars off the road!

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