Guest commentary*

“Are we there yet?” yells the kid in the backseat of the car.  Well, not quite, but we appear to be almost there.

Like that long car ride, predicting the timing of El Niño onset can be an exercise in patience.  Most scientific journal articles, and even the official NOAA index for the El Niño-Southern Oscillation (ENSO), the Oceanic Niño Index (ONI), define ENSO events by the seasonal (3-month) average.  There are good reasons for this.  Unlike our day-to-day weather, ENSO is a climate phenomenon and it is best seen in the slower evolution of the tropical Pacific oceanic and atmospheric circulations.

NOAA/NWS Climate Prediction Center (CPC) and the International Research Institute (IRI) for Climate and Society issue monthly updates on ENSO and its outlooks.  CPC also provides weekly updates on conditions across the tropical Pacific Ocean, and just recently, the CPC, the IRI, and the NOAA Climate Program Office teamed up to begin a new ENSO blog.   All of these updates are provided more frequently than the aforementioned seasonal average.

So, how close are we to El Niño and when will we know?  And how strong is this El Niño likely to be?

In our June 5 ENSO diagnostics discussion, the forecasters decided to continue the El Niño Watch based on our criteria.  While weekly values of Niño-3.4 sea surface temperatures (a region in the east-central tropical Pacific that is strongly related to ENSO) are flirting around the threshold for El Niño (+0.5°C), the main sticking point is the atmospheric circulation.  ENSO is based on the coupling between the ocean and the atmosphere, so we need to observe both.   In particular, the pattern of tropical rainfall has not quite come together at this point (see figure below).  During El Niño, one would expect more anomalous rainfall near the Date Line (180°) and reduced rainfall over Indonesia.

The chance of El Niño developing this year is now quite high, with a 70 percent chance of El Niño during the Northern Hemisphere summer, and an 80 percent chance during the fall and winter.  The ultimate strength of this El Niño – if one forms – is where the forecast team is the least confident.  Looking purely at the model guidance, it appears that anything from a weak to a strong event is still in the cards (see figure below).  Yet, the forecast team tends to slightly favor a moderate-sized event (3-month average Niño-3.4 value, or ONI, between 1.0°-1.4°C) during the Northern Hemisphere fall and winter.  However, the typical model errors for those longer forecast ranges imply a weaker (0.5°-0.9°C) or stronger event (1.5°C+) is still well within reach.

Going forward, please continue to get the latest updates in the CPC/IRI ENSO Diagnostics Discussion and visit the new ENSO blog.

*The author, Michelle L’Heureux, is a climate scientist at NOAA’s Climate Prediction Center