Weather Bee | How strong is the current El Niño cycle?

The India Meteorological Department (IMD) forecast for the April-June season published on April 1 shows that almost the entire country is going to suffer a higher-than-usual number of heat waves. Experts have correctly attributed this partly to the ongoing El Niño cycle, which is a periodic warming of the equatorial Pacific Ocean that usually increases global temperatures. This cycle has also been blamed for the record margins by which the global average temperature beat previous records through the past year. Therefore, it is important to ask how strong the current El Niño cycle is.
Before one assesses its strength, a description of how an El Niño cycle is tracked is necessary. A common method for tracking the cycle is through values of the Oceanic Nino Index (ONI) in the Niño 3.4 region (5°N-5°S, 120°W-170°W). Simply put, this index measures deviations from normal sea surface temperatures (SST) for the particular part of the Pacific described (more on this later). When the three-month running mean of ONI is 0.5 or higher for five consecutive seasons (for example, from December-February to April-June), that cycle is termed an El Niño. Similarly, when the ONI is -0.5 or lower for five consecutive three-month seasons, the phenomenon is called La Niña. All other conditions are considered neutral.
An analysis of ONI values from January 1950 – the earliest available data – shows that there have been 24 El Niño events since then, including the present one. The 1.44 average ONI for the current El Niño up to February – it started in May 2023 according to the method described above – ranks second among these 24 events. The highest-ranked or strongest El Niño by this metric took place from May 1997 to May 1998, with an average ONI over this period of 1.71.
The relative strength of the current El Niño described above, however, needs some caveats. The first one of those is that the current El Niño is not over yet. The weekly values of ONI show that the current El Niño cycle peaked in late November and December and is on its way out now. However, the cycle is still not over. The latest update shows that ONI was 1.0 in the week ending March 27, still far above the threshold 0.5 value. This means that any analysis of the current cycle’s strength will overestimate it until the cycle is over. This is because we are yet to see the tail of the current cycle when ONI values will be lower than at the peak and decrease the average for the entire cycle.
 
The second caveat needed for measuring the strength of an El Niño cycle using ONI values is that the deviation computed for the index is not from a fixed base like other temperatures. The base or the normal for ONI calculation is updated every 5 years. For example, the normal for February was 26.75°C from 1996 to 2000 but changed to 26.66°C from 2001 to 2005. This is not usually the case with other temperatures, where a fixed base is used to calculate deviations for the entire period of the data. The peculiarity in ONI calculation means that a higher value of the index may not mean warmer temperatures in the Pacific.
To be sure, data is also available on the absolute temperatures in the Nino 3.4 region. This shows that the current cycle has an average temperature of 28.427°C, making it “only” the third warmest El Niño if it had ended in February. Clearly, the current El Niño has not led to as much warmth in the equatorial Pacific as ONI suggests. Why not always use absolute temperatures to measure the strength of El Niño instead of ONI then? That might defeat the purpose for which ONI has a shifting base. ONI tracks the additional heat that is being added or removed, on top of long-term changes in SSTs. This is also what is understood by El Niño, a cyclical change in SSTs in the equatorial Pacific.
Abhishek Jha, HT’s senior data journalist, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.
Abhishek Jha is a data journalist. He analyses public data for finding news, with a focus on the environment, Indian politics and economy, and Covid-19. …view detail