Could we swing from three years of La Nina to El Nino this summer? That is the question being asked by weather forecasters as we head towards the second half of autumn.

The Bureau of Meteorology (The Bureau) has an El Nino WATCH currently which means there is a 50 percent chance of an El Nino event occurring.

As we know, the rain in Lismore and the Northern Rivers has been lower than the average since late 2022. So far in 2023, all months have produced less rainfall than the mean and well below 2022 levels. April's mean is 111mm, to date we have recorded 7mm.

The Bureau says "The ENSO Outlook remains at El Niño WATCH. This means that while the El Niño–Southern Oscillation is currently neutral, there is approximately a 50% chance that El Niño may develop later in 2023. This is about twice the normal likelihood. A significant amount of warmer-than-average water exists in the sub-surface of the western and central tropical Pacific Ocean, and warm sea surface temperatures (SST) anomalies continue to strengthen in the eastern tropical Pacific."

"The Southern Oscillation Index (SOI) has decreased to negative values over the past fortnight. Warming ocean temperatures in the eastern and central tropical Pacific and decreasing SOI values can be a precursor of El Niño development.

El Niño WATCH is not a guarantee that El Niño will occur, rather an indication that some of the typical precursors are currently occurring. All climate models surveyed by the Bureau suggest that El Niño thresholds are likely to be approached or exceeded during the southern hemisphere winter."

The Bureau can give a clearer indication of the potential El Nino weather pattern towards the end of autumn.

Meanwhile, the long-range forecast overview is for below-average rainfall for Lismore as the drier trend continues.

There is a 43% chance of above-median rainfall and a 26% chance of an unusually dry May to July.

The weather in the coming three days is for a possible shower with up to 1mm forecast. However, the chance of showers increases as we head towards the weekend.

For those that would like to know more about El Nino, read on....

What is it?

The term El Niño refers to the extensive warming of the central and eastern tropical Pacific Ocean which leads to a major shift in weather patterns across the Pacific. This occurs every three to eight years and is associated with a weaker Walker Circulation (see diagram below) and drier conditions in eastern Australia. El Niño Southern Oscillation(ENSO) is the term used to describe the oscillation between the El Niño phase and the La Niña, or opposite, phase.

In the eastern Pacific, the northward flowing Humbolt current brings cooler water from the Southern Ocean to the tropics. Furthermore, along the equator, strong east to southeasterly Trade winds cause the ocean currents in the eastern Pacific to draw water from the deeper ocean towards the surface, helping to keep the surface cool.

However in the far western Pacific there is no cool current, and weaker Trades mean that this "upwelling" effect is reduced. Hence waters in the western equatorial Pacific are able to warm more effectively under the influence of the tropical sun. This means that under "normal" conditions the western tropical Pacific is 8 to 10°C warmer than the eastern tropical Pacific. While the ocean surface north and northeast of Australia is typically 28 to 30°C or warmer, near South America the Pacific Ocean is close to 20°C. This warmer area of ocean is a source for convection and is associated with cloudiness and rainfall.

However, during El Niño years, the trade winds weaken and the central and eastern tropical Pacific warms up. This change in ocean temperature sees a shift in cloudiness and rainfall from the western to the central tropical Pacific Ocean.