The waters in the Pacific Ocean have reached the temperature required by the US government’s National Oceanic and Atmospheric Administration (NOAA) for an El Niño event, but an El Niño hasn’t been declared. What’s more, NOAA is saying the temperature will remain this high for the next few months, but confusingly it doesn’t expect an El Niño event to occur. El Nino is the warming of the surface water of the tropical Pacific Ocean, as detailed here. It can have a dramatic effect on the weather around the world, often bringing drought to parts of Australia and South Africa and flooding to South America and the Southern States of the USA. Therefore, it's important to know if an El Nino event is just round the corner.
There are several regions where El Niño scientists study the sea surface temperature of the tropical Pacific Ocean. The regions are called Niño 1, 2, 3 and 4, and their names correspond with the labels assigned to ships that crossed these regions. This labelling allowed historical records of El Nino to be studied as far back as 1949.
Niño 1 and 2 are generally studied as one area which lies by the coast of South America. Niño 3 is to the west, and Niño 4 crosses the date line. Nino 3 was once the primary focus for monitoring and predicting El Niño, but researchers later learned that the key region for the coupled ocean-atmosphere interactions of El Niño lay further west, in a region which was partly in Niño 3 and partly in Niño 4. Therefore region Niño 3.4 was born.
The temperature of Niño 3.4 is used to determine the all-important Oceanic Niño Index. The ONI is the 3-month mean of the average temperature of the Niño 3.4 region, compared with a 30-year average. The most recent ONI value, for the period October, November, December, is 0.5°C.
NOAA considers El Niño conditions to be present when the ONI is 0.5°C or higher, which indicates that the east-central tropical Pacific is significantly warmer than usual. Therefore if this one index were the only requirement, then NOAA would have already declared an El Niño event, but there is more to it than that.
To be certified as an El Niño event, the ONI must be more than 0.5°C for at least five 3-month periods, known as ‘seasons’, or it should be expected to stay above the threshold for this length of time in the future. This is where the El Niño conditions start to fall apart.
The ONI for September, October, December was 0.3°C, which clearly isn’t high enough for an El Niño event.
As for the forecast, although most of the current climate models do expect the ONI to remain above average for the next couple of 3-month periods, it doesn’t look like it will stay there for the next four which would be required for confirmation of El Nino. That’s because the current temperature increase seems to be a temporary, largely due to a wave of warmer water ‘sloshing’ east across the Pacific, known as a Kelvin Wave.
The prevailing winds in the tropical Pacific Ocean are easterly, pushing the warm surface water towards Indonesia. However, sometimes a huge wave, known as a Kelvin Wave will push the warm water back towards the east, changing the temperature of the ocean’s surface.
A Kelvin wave usually covers a vast area, often stretching across much of the Pacific Ocean. As the wave rolls east, it pushes down the boundary between the warm surface water and the colder, deeper water, known as the thermocline. This downwards motion earns the wave is title of a ‘downwelling’ Kelvin wave. An ‘upwelling’ sometimes occurs after the initial wave has passed.
The current downwelling Kelvin Wave is warming the surface of the tropical Pacific in the region known as Nino 3.4 and is expected to keep the temperature near the threshold of El Niño for the next few months. After that, however, the surface temperatures are expected to return to the average.
Therefore, although the threshold for El Niño currently appears to be met, it doesn’t look like the heat will remain in place for long enough to be classified as a true El Niño event. Average, or neutral, conditions are expected right through into the northern hemisphere summer. This is arguably good news for Australia, which normally sees reduced rainfall during an El Nino event and the country is still desperately fighting some of its worst wildfires in history. The announcement of El Nino certainly wouldn't be welcome.
This article was contributed to theWeather Club by our guest author, Steff Gaulter.