The Climate Prediction Center issued a La Niña Advisory on the 14th of December 2017. It said that a La Niña event is likely (exceeding ~80%) through the Northern Hemisphere during the winter of 2017-18, with a transition to ENSO-neutral most likely during the mid-to-late spring.
I personally find the whole ENSO alert system a little confusing, because if you look at the latest evidence, the SST conditions are still ENSO neutral, and it will require four further months of SST anomalies of -0.5°C or less before a La Niña event can be declared – which means February 2018 at the earliest. This page on the climate.gov website does clarify things.
Saying when an El Niño or its cold counterpart a La Niña event has commenced is rather a complicated business. In my ENSO application I stick by the American rules, which are 5 consecutive months with a SST anomaly of +/- 0.5°C in the Niño 3.4 region. Things are further complicated because these monthly SST are in fact a composite of a centred three-month rolling average. Not only that, even if these conditions are met, if there is insufficient evidence that a strong Walker circulation isn’t present, then an El Niño or a La Niña event is still not in progress (fig 2).
That’s why my application is showing a La Niña event happening during the last five months of 2016 (fig 1), although the values indicate that a short-lived La Niña was in progress, the Walker circulation was probably weak. As the Wikipedia article points out most countries around the rim of the Pacific ocean have a different way of declaring an event 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 Nino 3.4 region, the tropical Pacific atmosphere and forecasts that NOAA’s Oceanic Niño Index will equal or exceed +0.5 °C for several seasons in a row. However, the Japan Meteorological Agency declares that an El Niño event has started when the average 5 month sea surface temperature deviation for the NINO.3 region, is over 0.5 °C (0.90 °F) warmer for 6 consecutive months or longer. The Peruvian government declares that a coastal El Nino is under way, if the sea surface temperatures in the Nino 1 and 2 regions, equal or exceed +0.4 °C for at least 3 months”