Ocean Temperatures and the Developing El Niño Event

As ocean temperatures continue to warm through early 2026 across the eastern tropical Pacific Ocean, climate forecasters are calling this the start of what may be one of the strongest El Niño events in recent decades. The following will examine two previous "very strong" El Niño events and some of the similarities with the currently developing El Niño event over the Pacific Ocean.

What Is the El Niño–Southern Oscillation (ENSO) and an El Niño Event?

ENSO is a Pacific Ocean phenomenon that describes the variation in surface winds and sea surface temperatures (SSTs) across the tropical Pacific Ocean. There is a well-established connection between heat energy stored in the ocean and wind stresses in the lower atmosphere.

The ENSO cycle has three phases:

Neutral

La Niña

El Niño

In a Neutral ENSO phase, near normal SSTs see cool waters over the eastern tropical Pacific Ocean (and along the northwest coast of South America) and warmer waters over the western tropical Pacific Ocean (north and northeast of Australia) drive trade winds from east to west across the equatorial Pacific Ocean. Rising, moist air north of Australia leads to seasonal high rainfall, whereas descending air over the eastern tropical Pacific Ocean results in drier conditions.

During a La Niña, decreased SSTs across the eastern and central tropical Pacific Ocean, combined with increased SSTs over the western tropical Pacific Ocean, result in stronger trade winds blowing from the east across the tropical region. This leads to increased rainfall over the western Pacific and, in many cases, the northern and eastern regions of Australia during the spring and summer seasons.

Image: El Nino-Southern Oscillation (ENSO): La Nina Phase.  Source: Bureau of Meteorology

The El Nino phase. During this phase, warmer ocean waters — both at the surface and at depth — accumulate over the eastern and central tropical Pacific Ocean, while SSTs over the western tropical Pacific are relatively cooler than normal. Easterly trade winds weaken and, in strong El Niño events, can reverse direction, blowing from the west across the western tropical region. Heavier monsoon rainfall, which normally occurs over the western tropics, is suppressed by cooler SSTs and a broad region of background descending air. In contrast, warm, moist air rising over the central and eastern tropical Pacific brings enhanced rainfall and potentially heavy precipitation extending as far east as the west coast of South America.  

Image: El Nino-Southern Oscillation (ENSO): El Nino Phase.  Source: Bureau of Meteorology

When analysing SSTs across the tropical Pacific Ocean, forecasters examine four defined monitoring regions: Niño 1, Niño 2, Niño 3, and Niño 4. An additional region, Niño 3.4, is a hybrid zone that overlaps the eastern portion of the Niño 4 region and the western portion of the Niño 3 region. The Niño 3.4 region is used by the Australian Bureau of Meteorology and other international climate agencies as the benchmark region for classifying ENSO phases.

What forecasters are observing during 2026 is increasing SSTs across the eastern and central Pacific Ocean — specifically within the Niño 1, 2, and 3 regions — which have now surpassed the threshold defining an El Niño event. So how does early 2026 compare with previous strong El Niño events?

Analysis of SSTs, or more specifically, anomalous SSTs (warmer or cooler than normal), is just one aspect used to evaluate climate outlook forecasts. The following monthly anomalous SST charts show interesting similarities in the early stages of development between the 2026 event and the very strong El Niño events of 1997–98 and 2015–16.

Below are global anomalous SST maps for the month of May during 2026, 2015, and 1997. The Niño 3.4 region is highlighted within the outlined rectangle.

Above images: Sea surface temperature anomaly (°C) for May 2026 (top), May 2015 (middle), May 1997 (bottom). NINO 3.4 region is highlighted within the black rectangle. Ocean regions coloured as orange to dark brown represent regions of warmer than normal SSTs. Source: Climate Reanalyzer using the NOAA – Optimum Interpolation Sea Surface Temperature dataset. Climate Reanalyzer

In each event, SSTs near the Peruvian coast were significantly warmer than normal, with the warmer pool of ocean surface waters extending westward into the central Pacific. What followed during the remaining months of 1997 and 2015 was significant warming of ocean waters across the eastern and central tropical Pacific, resulting in very strong El Niño events. Anomalous SSTs — averaged over three months — within the Niño 3.4 region peaked at 2.40°C above normal during October–November–December 1997, and at 2.75°C above normal during November–December 2015 to January 2016. Rainfall across many areas of central and eastern Australia was below average from June through November during both 1997 and 2015, with 2015 proving to be a particularly dry year.

Above images: Rain deciles measured across the six-month period from June to November for 1997 (top) and 2015 (bottom). Below-average rainfall is represented on the red scale and above-average rainfall on the blue scale. Source: Bureau of Meteorology Australian rainfall deciles since 1900

For this current event, the Bureau of Meteorology has declared an El Nino event is underway and are forecasting further warming of tropical waters across the Pacific Ocean through to Spring and what may be the strongest El Nino event since 1950.

The rainfall outlook for August to October indicates a higher likelihood of below median rainfall across the eastern half of Australia.

Image: Bureau of Meteorology Long-range rainfall outlook – for August to October.  Source: Rainfall – Chance of above median for August to October - Long-range forecasts

You can find the latest rainfall outlooks across the Australian region at DTN/Weatherzone’s Climate Outlook (Climate Outlook - Australia).