El Niño update explains impact on Canada in months to come

Friday, January 16, 2015, 8:22 AM - As colder than seasonal temperatures have taken hold across Canada this winter, you may be asking, “Where is El Niño?” The answer is that El Niño conditions are already in place in the Pacific, but not all El Niños are created equal. Analysis of current conditions shows how this year’s unique flavour of El Niño will help keep some parts of Canada warm, while others will remain in the deep freeze.

WHAT WE KNOW SO FAR

El Niño is a coupled system of interactions between the atmosphere and the ocean along the equatorial Pacific which has far reaching effects on weather around the globe. To understand the current conditions associated with this pattern, you have to consider both sides of the coin – the ocean and the atmosphere. The oceanic component of El Niño occurs when a pool of warmer than normal water exists along the equator in the eastern and central Pacific. We can gauge the characteristics of an El Niño by looking at exactly how much warmer than normal the water is, and where the warmest water is centered.

This image, from the “Super El Niño” of 1997-1998 is an example of the classic sea surface temperature pattern associated with strong El Niño.

Notice the pool of very warm water along the equator, centered in the Eastern Pacific near the coast of South America. This is the pattern which produces our most pronounced El Niño weather impacts. Now compare that image with the current conditions pictured below, and you will notice some significant differences.

In this image, you can see that while there is a large amount of warm water to be found in the Pacific this winter, it is less concentrated along the equator. It is also shifted more westward, so that rather than hugging the South American coast, the core of the warmth is closer to the International Dateline in the central Pacific.

When we break down these temperature anomalies by region, we can see that the warmest water is in fact to be found near the International Dateline (the top graph), where temperatures are about 0.8°C warmer than average. Each successive graph represents the temperature somewhat farther east, and so you can see that by the time you reach the bottom graph – which represents the coast of South America – water temperatures are at or even slightly below normal.

This tells us that this event is much more focused on the Central, rather than the Eastern Pacific, which will make an important difference on its impacts for Canada, as we’ll see. These data also show that this El Niño event is on the weak side, with the strongest temperature anomalies only approaching 1°C. Compare that with the 1997-1998 event, where anomalies were greater than 5°C.

Just as we look at sea surface temperatures to analyze the oceanic component of El Niño, we can look at air pressure to analyze the atmospheric component. In particular, we look at the difference in air pressure between two locations in the Pacific: the island of Tahiti, and Darwin, Australia. This gives us a value known as the Southern Oscillation Index, or SOI.

When the SOI is negative, this means that there is relatively higher pressure in Darwin, in the far eastern Pacific. This high pressure is a signal of El Niño conditions, and causes the easterly trade winds, a persistent feature at these latitudes, to slow down. These slower trade winds in turn allow the subtropical jet stream to become stronger, which drives many of the impacts of El Niño on North America.

SOI values of -8.0 or less indicate the presence of El Niño conditions, and as this chart shows, we have been at or near this value since late 2014. SOI values persisting very close to the threshold is another clue that we are currently experiencing a weak El Niño event.

So we’ve established that weak El Niño conditions are occurring in the Central Pacific. What does this mean for Canada’s weather, and how will it be different from a more typical El Niño year? We can take a look at data from similar events in the past to get an idea. This image is produced by Environment Canada, and shows the changes in average surface temperatures during a typical El Niño winter.

As you can see, in a typical El Niño winter, much of Canada can expect warmer than seasonal temperatures to persist, with only the B.C. coast experiencing near normal conditions. This next image, on the other hand, shows the changes in average surface temperature during central Pacific El Niño events like the one we are currently experiencing.

Notice how there is much more pronounced warming along the Pacific coast, compared with the previous image, while some slightly warmer than average values occur in Atlantic Canada as well. The most striking change however, is a large area of colder than normal conditions, particularly in the central Prairies and around Hudson Bay, but also extending into southern Ontario and Quebec.

These two images are climatological analyses based on historical data from previous El Niño events, but now let’s take a look at what has actually happened across Canada over the past few months. This image shows the temperature departure from average for the last 120 days.

The pattern is very similar to what we would expect from a Central Pacific El Niño, with warmth in British Columbia and southern Alberta, as well as in the Maritimes, while colder than seasonal temperatures dominate the from east of the Rockies to Labrador. This similarity with the expected pattern gives us good confidence that the Central Pacific El Niño has been an important driver of our weather patterns through the late fall and early winter, and should continue to be influential as long as it persists.

So the question then becomes, how long can we expect this pattern to continue? The latest forecast guidance suggests that El Niño conditions will continue through the spring of 2015. This image shows that all of the forecast models expect Pacific sea surface temperatures to remain warmer than average (above the center black line) for at least the next few months.

IN CONCLUSION

El Niño is a major player on the global weather stage, and even relatively weak events such as this one can do a lot to shape the weather that we experience on a day to day basis. As we move into the second half of winter, Weather Network meteorologists will continue to monitor these conditions and how they might impact your forecast. But hopefully analyses like these give you some insight into the forecasting process, and how these fascinating and complex interactions around the globe act to influence our weather.