For the better part of a century, researchers have been focussing on shifts in conditions of the tropical Pacific Ocean to try to predict the rhythms of El Niño, the most powerful source of seasonal variation on the planet. For millions of people, the weather impacts of its changing ocean temperatures can bring on droughts and floods and hurricanes — or chase them away. Now comes a study suggesting that El Niño forecasters have been looking in the wrong place.
Takeshi Izumo and colleagues in Tokyo and Paris report in the current issue of the journal Nature Geoscience that whether or not a warm El Niño or a cool La Niña comes along depends on events in the Indian Ocean that are meteorologically "upstream" of what's going on in the Pacific any given year.
By monitoring changes in these upstream events — especially a similar temperature oscillation known as the Indian Ocean Dipole (IOD) — they say forecasters could predict what is technically called the El Nino-Southern Oscillation (ENSO) more than a year before it arrives — a major advance over the current state of the science.
In 1992, researcher Peter J. Webster at Georgia Tech and colleagues identified a stubborn spring "predictive barrier" that prevents forecasters from knowing before the month of May whether El Niño is likely to develop and change the impending Atlantic hurricane season or patterns of temperature and precipitation the following winter. This spring barrier — when it looks like ENSO conditions could go either way — means that the forecast comes too late for insurance companies to adjust rates of their hurricane coverage, for instance, or for farmers to change their plans.
Izumo and co-authors say their 1981-to-2008 statistical analysis of the intricate links between the Indian Ocean Dipole and El Niño — detailed here in this diagram courtesy of Nature Geoscience — "clearly identifies the IOD as an important precursor of ENSO."
"It highlights that the Indian Ocean is not completely enslaved to the powerful ENSO cycle from the neighbouring Pacific: it has degrees of freedom on its own and even partially controls the ENSO state the following year," they write.
For his part, Webster, who wrote a commentary on the study, sounds interested but not convinced. He put it this way:
New physical understanding, and a prediction methodology that would allow longer-range forecasts of La Niña and El Niño by somehow transcending the spring predictability barrier, could help with monsoon predictions and with climate forecasts in other regions where ENSO has a strong influence.
The study may be "merely reflecting the statistics of the strong two-year rhythm of the Indian and Pacific oceans," Webster writes, and researchers need to extend their analysis farther back because the Indian Ocean oscillation has changed over time.
Still, a "clear message emerges," in the study, he writes:
If the prediction horizon of El Niño and La Niña is to be extended, then both the Indian and Pacific ocean basins must be included in empirical and dynamical forecasting schemes.