El Niño, meaning "the Christ child", is so-called because the first signs of its appearance are marked by a warm current off the coast of Ecuador just after Christmas. These rising sea temperatures are related to a weakening of the trade winds that usually transport warm surface waters to the western margin of the Pacific. During an ENSO phase – which occur every 2–7 years – these warmer waters accumulate in the eastern tropical Pacific.

Individual ENSO episodes can last up to two years and lead to severe flooding in Latin America and droughts in South East Asia. One extreme cycle in 1997–1998 had far-reaching consequences, including extensive fires in the Indonesian rainforests and mudslides in California. Another impact of El Niño is that the accumulated warm water acts to block cold-water currents, which usually transport nutrients from the deep ocean to ecosystems along the Latin American coast. This can have a devastating effect on the fish stocks that form an important part of the economy in countries such as Peru and Colombia.

Mysterious origins

Despite El Niño's familiarity, scientists still do not fully understand what triggers these events, how they are sustained or what finally causes an ENSO cycle to subside. One thing that has been noted is that once ENSO episodes are under way, they all tend to follow a similar pattern of developing during summer or autumn in the northern hemisphere, then peaking during the northern winter. This interaction between ENSO and the annual cycle has now been more firmly established by a numerical study by Karl Stein and his colleagues at the University of Hawaii at Manoa.

Stein’s team has analysed observations of sea-surface temperature from the UK Met Office Hadley Centre spanning the period 1964–2007 and covering 20°S–20°N and 120–290°E. The extensive numerical analysis showed that ENSO events and the annual variation in temperature in the eastern Pacific are synchronized in a "2:1 Arnold tongue". In simple terms, this means that during a positive phase, ENSO and the annual cycle run according to the same beat but the seasonal cycle is moving twice as fast as ENSO.

Stein told physicsworld.com that one of the ultimate goals in characterizing ENSO is to develop a means of predicting when the next large warming event might occur. "Understanding the relative importance of amplitude versus phase modulation should lead to a better understanding of the physics involved in synchronizing ENSO to the annual cycle, which should hopefully lead to better predictions," he says. Stein believes that given the complexity of the climate system, realistically we could hope to predict the state of the equatorial Pacific only months or a year ahead of time at best.

Numerical connection

K V Ramesh, a climate scientist at the Centre for Mathematical Modelling and Computer Simulation in Bangalore, India, is impressed by the fact that the new research establishes a quantative relationship between ENSO and the annual cycle. However, he feels that to gain a better understanding of El Niño will also require improved climate monitoring. "The main limitations to our understanding come from the various practical constraints in setting up well-distributed observing systems capable of making measurements continuously," he says.

Stein says that he and his colleagues intend to develop their research by investigating the influence that the tropical convergence zones have on the timing of ENSO events. He believes that the main outstanding questions relate to how ENSO will respond to future changes in the global climate. "The ENSO cycle is always going on; right now, we're observing La Niña [cold] conditions that are likely to persist through the winter," he says.

This latest research in published in Physical Review Letters.