Weather Bee | Why did Australia's BOM declare the end of El Niño? - Hindustan Times

Weather Bee | Why did Australia's BOM declare the end of El Niño?

Apr 24, 2024 11:35 PM IST

The Australian Bureau of Meteorology recently announced the end of the El Niño weather pattern, diverging from other weather departments.

On April 16, the Bureau of Meteorology (BOM) of the Australian government declared that the El Niño weather pattern – a periodic warming of the equatorial Pacific Ocean – had ended. Curiously, most news reports did not refer to such a declaration by any other weather department. Why was this the case?

El Nino is a periodic warming of the equatorial Pacific Ocean. (Twitter) PREMIUM
El Nino is a periodic warming of the equatorial Pacific Ocean. (Twitter)

The simple answer to the riddle posed above is that the BOM uses a different criterion for determining the status of El Niño. However, this is a good opportunity to take an even more detailed look at the methods used for declaring the status of the Pacific than a previous edition of this column had done.

As explained in the earlier edition, the Pacific Ocean undergoes periodic warming and cooling. The former is called El Niño and the latter is La Niña. To be sure, there is a threshold for both, and a small degree of warming or cooling is considered a neutral condition. How small? This is where the BOM differs from the other big weather department – the National Oceanic and Atmospheric Administration (NOAA) of the United States government – in determining the status of the Pacific. The BOM considers a deviation – compared to a 30-year average updated every five years – of under 0.8°C in the equatorial Pacific as neutral, while this threshold for NOAA is 0.5°C. According to NOAA’s data, the weekly average of the deviation had changed from 0.9°C warming in the week ending April 10, to 0.7°C in the week ending in the week ending April 17, which satisfies BOM’s threshold, but not NOAA’s threshold.

To be sure, the status of the Pacific is determined on the basis of averages over a period longer than a week. According to NOAA, an El Niño or La Niña can only be declared if the 0.5°C threshold is met by the three-month running mean for five consecutive months. However, weather departments (even those other than the Australian one) may declare the end of El Niño when the weekly value meets the 0.5°C threshold. This is because every month, multiple models are used to generate a forecast for nine months; and these models currently show over 80% probability of neutral conditions for the April-June season.

While the above caveats clarify the BOM’s declaration of neutral conditions last week, they are not the only ones one must keep in mind while reading such statements. For example, the equatorial Pacific itself can be defined in multiple ways. Historically, this has been done either due to the impact for which the Pacific’s status is being studied or due to our lack of understanding of weather events in the past.

For example, the weekly averages of deviation shown in the chart above are for the Niño 3.4 region: 5°N-5°S, 120°-170°W. Similarly, there is a Niño 1+2 region (0-10°S, 90°W-80°W), a Niño 3 region (5°N-5°S, 150°W-90°W), and a Niño 4 region (5°N-5°S, 160°E-150°W). Moreover, sea surface temperatures in the Pacific are not the only way to track El Niño-La Niña events. The BOM, for example, also uses a more volatile Southern Oscillation Index (SOI) that tracks the atmospheric component of this weather cycle using air pressure.

The upshot of this discussion is that when one reads a statement declaring El Niño, La Niña, or neutral conditions; one must ask what metric was used to make that statement. Some weather departments may track indices that are only relevant to them. Others, like the Australian one, may modify thresholds of popular indices according to their own needs.

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.

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    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.

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