The lack of La Niña is a red herring…
Back in the late spring there were signs of La Niña. Some networks stopped just short of forecasting a La Niña event, while others were fairly bullish about the impending event. However, over the past couple of weeks, there have been many articles claiming that La Niña isn’t actually going to happen. Even the National Weather Service’s Climate Prediction Center has adjusted its forecast to ENSO neutral, which is a fancy way of saying nothing. Now that summer is (almost!) over, it’s worth looking at what happened with La Niña, and why we should care.
What is ENSO and what is La Niña?
ENSO refers to the oscillation of the temperature of the surface of the water located between Indonesia and the west coast of South America, typically right along the equator. La Niña is said to occur when the water along the equator in the eastern Pacific are colder than average.
For a much more detailed explanation of ENSO and its interactions with the atmosphere, take a look at my post from May.
Why don’t people think La Niña is on its way?
First, the SST anomalies in the eastern Pacific just aren’t that cold. You can see in the image below that the SST anomalies (bottom panel) are around 0.5°C below average. The aforementioned Climate Prediction Center considers La Niña to occur if the equatorial SSTs between 170°-120°W are more than 0.5°C below average for 3 consecutive months.
Secondly, the computer models aren’t showing more than a 50% chance of being below that magical threshold of -0.5°C for 3 months in a row.
Let’s put all this aside though. The raw observations above show that the equatorial Pacific ocean is colder than average. It doesn’t, and likely won’t, cross the arbitrary threshold set by the CPC, but that doesn’t really matter. Let’s look at what happens to Winter if the SST anomalies stay around -0.5°C.
Looking to the past
Here’s a crude, but interesting, experiment: let’s find all the months since 1980 that had SSTs in the aforementioned region between 0°C and -0.5°C during winter (Dec/Jan/Feb). Then, we’ll remove all of the “official” La Niña events from that list. We’re left with months that fall in these years: 1982, 1997, 2002, and 2013. Here’s what their composites look like, courtesy of the awesome plotting tools at NOAA ESRL:
Compare the top row (not La Niña) to the bottom row (canonical La Niña, ONI < -1 sigma). The La Niña winters obviously have much colder SSTs and a much stronger cold signal in Northwest North America. However, there’s still quite a bit of cold from Alaska southeast to North Dakota in the non-La Niña years. There’s also still a warm signal in the southeastern United States. In fact, the northern east coast is quite a bit warmer during the non-La Niña years.
Some might argue that the warmth along the East Coast is more associated with the long-term global warming trend than with those cold SSTs. This might be true, although another crude composite of the past 10 years of winter anomalies reveals it’s probably not:
So what’s the tl;dr here? Don’t discount the effects of the ocean just because it doesn’t fit into the arbitrary definitions for a La Niña. The ocean is still important and can have an impact on our winter. If I were a betting man, I’d put my odds on a warm winter for the East Coast.