The warm and cool phases of the El Niño Southern Oscillation (ENSO) in the tropical Pacific are part of the largest climatic phenomenon on earth with the greatest impact on our weather.

Since mid-May 2015 until May 2016, ENSO was in its warm phase – El Niño – which produced warmer-than-average conditions across much of the tropical eastern Pacific to the shores of South America.

The latest El Niño has been one of the strongest on record and this event has caused a significant disruption and socio-economic impacts with droughts in India and Africa, forest fires in Indonesia and floods in South America all arising because of the event. In addition its warming influence has extended to the global climate, boosting the global average temperature helping to push 2015 to be the warmest year on record – following 2014 which was also a record-warm year.

This diagram shows the average of sea surface temperature anomalies (difference from the 1961-1990 average) from 5°S to 5°N across the Pacific. Data are from the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) analysis. The horizontal axis represents longitude and the vertical axis represents time. The weak 2009-2010 El Niño can be clearly observed as well as the prolonged late-2010-2011 La Nina. At the bottom right of the diagram the currently developing cold anomaly can be seen clearly. This feature might become a La Niña.

The first four months of 2016 have been warmer than any equivalent period in the HadCRUT4 record. Climate experts anticipate that the legacy of the latest El Niño is that 2016 could be another record year for global average temperatures and the very high global temperatures in the first four months of the year seem to bear that out. However, ENSO is now transitioning out of its warm El Niño phase, and it is likely to move towards ENSO’s alter ego: La Niña.

If the predicted, cooler-than-average, conditions begin to dominate in the tropical Pacific, it will increasingly tend to suppress global temperatures. This cooling may not arrive sufficiently quickly to bring an end to the run of record-breaking years, however. As Jeff Knight of the Met Office Hadley Centre explains: “The El Niño phase of ENSO may well have run its course for now, but its effects on global temperatures will likely linger for a few months yet, and this, together with the high background level of global warming, could keep the global average temperature high enough to register another record in 2016.” Regardless of whether 2016 is another record-breaking year, it will likely be one of the warmest of a handful of years.

One consequence of El Niño is the suppression of the development of hurricanes in the tropical Atlantic, where La Niña conditions can enhance tropical storm activity. Jeff added: “Potentially, the transition from El Niño to La Niña could happen by the peak of the hurricane season.”

For about half the time, neither El Niño or La Niña are present and ENSO is said to be in a neutral phase. The latest El Niño is the strongest in nearly two decades and is one of the strongest on record.

To dsicover more about El Niño and La Niña, and to see a video animation of each phase of ENSO, please visit our dedicated page.