Has La Niña started?

NOAA have declared that a La Niña event has now started. As a programmer I like hard and fast rules that I can set in code before I can scan monthly and weekly SST’s from the central Pacific to find if SST has been met. This latest flowchart from the Americans makes the process even more subjective, and I don’t particularly like it in that regard. Because these events in the Pacific have an effect globally many countries have an interest on the state of ENSO and they all seem to have different rules they use to decide if a La Niña or El Niño has started or is in progress.

Currently, each country has a different threshold for what constitutes an El Niño event, which is tailored to their specific interests. For example, the Australian Bureau of Meteorology looks at the trade winds, SOI, weather models and sea surface temperatures in the Nino 3 and 3.4 regions, before declaring an El Niño. The United States Climate Prediction Center (CPC) and the International Research Institute for Climate and Society (IRI) looks at the sea surface temperatures in the Niño 3.4 region, the tropical Pacific atmosphere and forecasts that NOAA’s Oceanic Niño Index will equal or exceed +.5 °C (0.90 °F) for several seasons in a row. However, the Japan Meteorological Agency declares that an El Niño event has started when the average five month sea surface temperature deviation for the NINO.3 region, is over 0.5 °C (0.90 °F) warmer for six consecutive months or longer. The Peruvian government declares that a coastal El Niño is under way if the sea surface temperature deviation in the Niño 1 and 2 regions equal or exceed 0.4 °C (0.72 °F) for at least three months.


The Australian BOM doesn’t agree with NOAA but expects thresholds to be met in October.

Japan’s JMA consider that a La Niña event started in August.

To investigate this more closely I smartened up my ENSO application and downloaded the latest weekly SST data from NOAA, and there is no doubt that weekly SST have been falling in the central Pacific in the last few months. On the 2nd of September in ENSO region 3.4, there have been six consecutive weeks with SST anomalies below -0.5°C, and the 13 weekly moving average is -0.46°C. Using the consecutive weeks method makes it mid October before we accumulate thirteen of them, and to me the thirteen week moving average is the better indicator, and that could be at or below -0.5°C by next week. It’s a shame that all the world’s meteorological agencies can’t all share the same data and decide on a method that they all can agree on.

Personally, for what it’s worth, I side with the BOM on this one, although I do think a La Niña event will commence in the next few weeks. What we really need is a world wide agency, a bit like the United Nations if you like, who can take the lead in global warming and the sharing of climate and observational data, and bash a bit of some sense into these agencies and Met Services.

*As the Wikipedia article clearly states there is thought to be no direct link with either El Niño or La Niña events and global warming.
*Apologies for using yesterday’s satellite picture of five tropical cyclones in the Atlantic – I just couldn’t help myself.

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