Why this winter's El Nino was one of the strongest on record

Two years ago, climatologists were predicting a strong El Nino. But instead, we got a pretty average weather season during the 2014-15 winter.

It was actually this past winter that brought an unusually strong El Nino — and a warmer-than-normal winter in the Prairies, all the way east to Ontario, while the West Coast experienced above average rainfall.

But CBC Radio science columnist Torah Kachur says we shouldn't blame meteorologists for poor forecasting for an entire year, since the science behind El Nino is very complicated.

Why is El Nino such an important weather phenomenon?

El Nino, and the related La Nina, are two phases of a semi-regular temperature cycle in the Pacific Ocean. With El Nino, we see warmer-than-normal ocean surface temperatures in the eastern Pacific. We talk about it in Western Canada because it brings warm winters and warmer springs, like this year.

But it's much, much bigger than that. It's really a global weather driver everywhere from India to Peru, with huge economic consequences. 

The International Monetary Fund has estimated that an El Nino can boost the GDP of the United States by 0.5 per cent due to lower heating bills, while it can reduce the GDP of Indonesia by one per cent — because El Ninos can cause drought in areas of the Southern Pacific like Australia and Indonesia.

There have been effects right here at home. It's been suggested El Nino might have been one of the causes of the crippling 1997 ice storm in Quebec.

• El Nino winter may mean ice storms for southwestern Ontario

It's not just economics either — it also has major health impacts. El Nino, which happens every two to seven years, can cause warmer conditions in Central America and Eastern Africa, which means there is an increased risk of mosquito-borne diseases. There are outbreaks of malaria, dengue and yellow fever every time a strong El Nino sweeps across the world — and we can likely add Zika to that list now.

What causes an El Nino?

A lot of factors. It's an incredibly complex process that starts when a band of warmer water appears in the middle of the Pacific Ocean close to the equator. This seems to start right around Christmas — hence the name El Nino, meaning "the boy," and specifically referring to the baby Jesus.

That warmer water acts as a conveyor belt to pump hot air up north to the western coastal regions — the coasts of Peru, Chile and western North America.

The warmed-up water influences the weather, particularly the winds — sometimes causing them to switch from easterlies to westerlies. But most importantly, it changes how far north the jet stream lies, wreaking havoc for weather patterns around the globe.

How does this El Nino compare to other years?

This is a strong one. In fact, it's really in the top three ever recorded. And it can in part explain the terrible fire season in Alberta.

• Expect a rough fire season, says forecaster Claire Martin

This actually all started in 2014. Sea surface temperatures were building in fall of 2014, and climatologists were predicting a large El Nino that winter.

• El Nino forecast suggests next winter will be warm

But it seems to have arrived a bit late to the party, and this year we're experiencing a strong El Nino that would compare to those seen in in 1982 and 1997. 

There's actually an El Nino index to rank the strength of El Ninos, and this year is right up there with the strongest of them.

Why is this El Nino so strong?

That's what researchers from the National Oceanic and Atmospheric Administration (NOAA) in the United States were trying to figure out.

In a new paper presented in Geophysical Research Letters, they tried to solve an El Nino mystery — why the strong El Nino didn't start in 2014 when the conditions seemed to be ripe, with a warm ocean.

But then the winds changed, and instead of coming from the west, they started coming from the east.

Aaron Levine is a researcher with the NOAA. He said easterly winds in the summer of 2014 kept the warmer waters from travelling out from the equator — but those warmer waters were still there in the ocean's subsurface.

"So they stuck around through the winter of 2014-15, even though we didn't have the El Nino event then," he said. But then there were what he called "westerly wind anomalies" in the spring of 2015.

Those westerly winds, he said, had a larger heat reservoir to tap into, and that helped contribute to how strong the El Nino event was this year.

It's almost as if all the energy and heat that was accumulated last year was stockpiled to rain down on us this year. 

Can this research help us predict future El Ninos?

Unfortunately not. What we have to remember is that El Nino strength is multifactorial — there are many pieces of the puzzle that have to line up to get conditions that we see this year.

Most importantly, the winds that buffered the El Nino in 2014 and exacerbated it this year are random — at least daily winds are very hard to predict. There's really no way of knowing what those stochastic winds will do, because there are so many climatic variables that change the winds. And we simply don't have the ability to predict wind direction and strength in any accurate way.

Not only that, this false start a year before the strong El Nino did not result in the strongest on record — that was the winter of 1997-98. So it's part of the puzzle, just not the whole answer.

What does this mean for a coming La Nina?

At this point, it is predicted that La Nina — which brings the opposite forces and effects of El Nino — is going to deliver a big old-school winter to Western Canada in the 2016-17 season.

• Oh no! La Nina to bring Saskatchewan 'old fashioned kind of winter'

The West is expected to experience a colder-than-normal winter, the North Atlantic will see a strong hurricane season, and generally we're going to be pretty miserable.

As we found out in 2014-15, predictions aren't always right. But the NOAA's Climate Prediction Centre has just issued a report saying there's about a 75 per cent chance we'll see a La Nina this winter.