For folks who follow weather and climate, I’m sure you have heard plenty about the current El Niño over the last year or so. As a quick reminder, El Niño, or the “warm” phase of the El Niño Southern Oscillation (ENSO), refers to the anomalous warming of sea surface temperatures (SSTs) across the central and eastern Tropical Pacific Ocean. Despite a “false start” in 2014, El Niño has been strengthening rapidly since the end of this past winter. As of right now, it appears as though this El Niño could be one of the strongest events on record.
While above average ocean temperatures have been present in the Tropical Pacific since the middle of last year, it wasn’t until March of 2015 that El Niño really started taking off. This is because El Niño is not only characterized by warmer than usual SSTs, but also a response from the atmosphere. This feedback finally came into fruition during March, as a westerly wind anomaly occurred across the Equatorial Pacific. Typically winds in this region blow in an easterly direction (east to west). This shift in the winds was significant, as it allowed warmer ocean water to move eastward from the Western Pacific towards South America. As a reference, the aforementioned westerly wind anomalies during March were the largest since the strong El Niño event of 1997-98.
When discussing the strength of El Niño, we typically use the Oceanic Niño Index (ONI), which measures SSTs anomalies in the Tropical Pacific. El Niño is broken down into weak (at least five consecutive overlapping 3-month periods of SST anomalies between +0.5°C and +0.9°C), moderate (SST anomalies between +1.0°C and +1.4°C) or strong (SST anomalies greater than +1.5°C) events. As of right now, SSTs departures across the Central Pacific are right around the +1.5°C threshold, but have climbed to near +2.5°C off the coast of South America!
In addition to the ONI, a relatively new tool called the Multivariate ENSO Index (MEI) can give a more in-depth look into what is happening at the Equatorial Pacific. The MEI is useful because it monitors six different variables (SSTs, sea level pressure, surface winds, cloud cover, and air temperature) across the Tropical Pacific. The MEI is computed for bimonthly periods, and like the ONI, positive values signify an El Niño. Based off of the recent values of the MEI, it is clear that the current El Niño is very healthy. The MEI value of +1.972 during the June / July period is the second highest ever for this time frame. Only the event in 1997-98, one of the strongest on record, had a larger readings at this juncture. Additionally, a majority of recent model guidance predicts that El Niño will continue to strengthen through the end of the year.
Model predictions of SSTs across the Tropical Pacific. Courtesy of the Climate Prediction Center (CPC)
El Niño generally has its greatest impacts on our weather during the colder months, so what exactly would a strong event mean for the upcoming winter? As we learned last year, El Niño can be very unpredictable, so it is still too early to be making any bold prediction. But hypothetically speaking, what could we expect if we had an event similar to the ones in 1982-83 and 1997-98? Overall, the two winters were relatively mild along the northern tier of the country, as temperatures across the Northeast, Mid-Atlantic and Midwest were 3 to 5°F above normal.
While a very strong event would seemingly favor a milder winter, it doesn’t mean that we won’t get any snow. It is important to remember that there are still other factors at play (let’s not forget the warm pool in the Gulf of Alaska is still alive and kicking). What is certain is that we will be watching El Niño very closely, so make sure you stay tuned!
.jpg)
Temperature anomalies across the US during the winters of 1982-83 and 1997-98. Courtesy of Earth System Research Laboratory.
