El Nino is coming: Epic event ahead?
Saturday, May 3, 2014, 7:50 AM -
El Nino is coming!
We are seeing a surge in the number of headlines about the developing El Nino, with many proclaiming that this will be a "Super El Nino" or an "epic event".
As we head into the summer, forecast confidence is high that an El Nino pattern is developing. However, will this pattern really become as extreme as many are saying?
You may be asking, "What is El Nino, and why should I care?" So, before looking ahead at the forecast, we will first take a look at some background information on El Nino.
EXPERT ANALYSIS: Dr. Doug Gilham appears on television as The Long Ranger and writes regularly for TheWeatherNetwork.com. His expertise can be read here
UNDERSTANDING THE DISCUSSION
Typically, discussions of El Nino focus on Pacific Ocean water temperatures near the Equator, especially in the region just to the west of South America. During El Nino events we find warmer than average sea surface temperatures in this region. The image below is from the strong El Nino event of 1997-1998. The yellow & red colours highlight the warmer than average sea surface temperature that were found at that time to the west (left) of South America.
This is in contrast to La Nina events which are associated with colder than average sea surface temperatures in the same region. The following map is from the 1999 La Nina.
Note the blue colours to the west of South America which represent colder than average sea surface temperatures in the same regions that were so warm during El Nino.
THE WATER TEMPERATURES
However, focusing on sea surface temperatures only tells us part of the story.
These temperatures change in response to changes in the atmosphere above the ocean. Typically the air flow in this region of the world is from east to west (often referred to as the tropical easterlies or the trade winds). This persistent flow pushes the warmest surface water to the west and causes deeper and colder water to come to the surface (referred to as upwelling) near South America.
Fluctuations in the strength of these easterly winds will cause changes in the strength of the upwelling. When the easterlies weaken, there is less upwelling and sea surface temperatures are able to warm. When the easterlies increase in strength, the upwelling will be even stronger and sea surface temperatures will be even colder than average.
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While the ocean water temperatures are affected by the flow of air in the atmosphere, changes in ocean water temperatures also impact the atmosphere. Therefore, El Nino and La Nina events do have far reaching impacts on general weather patterns over other parts of the world, including Canada.
TENDENCY TO OVERSIMPLIFY
However, there is also a tendency to oversimplify the impact of El Nino on the weather. As a child growing up in Canada (and a lover of snow and snow days), I remember El Nino being blamed for both snowy winters and winters that had little snow.
The nature of the impact for a given region is highly variable depending on the strength of the El Nino and on whether we are at the start or end of an El Nino event. The impact also depends on whether the warm water is found primarily over the eastern Pacific close to South America or whether the region of warmest water is found further to the west over the central Pacific.
NEXT PAGE: WHAT'S TO COME IN THE MONTHS AHEAD?
Now that we have a better understanding of El Nino, we will take a look at what we expect during the months ahead. The image below looks at how current sea surface temperatures compare to long term averages. In the circled region of the Pacific Ocean near the Equator, sea surface temperatures are near to slightly above average.
An interesting side note is the large region of warmer than average ocean water that is west of British Columbia. This relatively warm water contributed to the development of the persistent pattern in the upper atmosphere that we saw last winter which brought colder than seasonal temperatures to central and eastern Canada. This sea surface temperature pattern was also present during several of Canada’s coldest winters during the past century.
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As we look to the future, numerous models have been consistent in showing a steady progression to warmer than average sea surface temperatures in the Equatorial Pacific.
The image below is a model depiction of what this region may look like during the upcoming summer.
The black oval highlights the region to the west of South America where sea surface temperatures are forecast to rise to as much as 1 to 2 °C above average during June, July and August.
The trend towards warmer average temperatures is forecast to continue into autumn.
The map below shows a similar region of warmer than average sea surface temperatures during the months of September, October and November.
As we head into winter there is still a clear El Nino signature in the model depiction of sea surface temperatures, but several models show that sea surface temperatures will begin to cool slightly, with the warmest water found over the central Pacific rather than immediately to the west of South America.
FORECASTING VIA THE MODELS
Another way to look at the forecast for El Nino is the image below which shows sea surface temperature forecasts (relative to average) for the equatorial regions of the central Pacific from over 20 models. Each line represents a forecast from a different model. On the far left of the screen we start with sea surface temperatures from this winter. As we move to the right on the diagram we progress further into the future with the forecast. Without getting caught up in all the details, the key point is that all of the models show that sea surface temperatures will warm to above average as we head into the summer and fall (which indicates an El Nino), but there is considerable disagreement between the different models regarding how much warming will occur.
NEXT PAGE: HOW STRONG WILL THE UPCOMING EL NINO BE?
The image below also highlights the uncertainty regarding the strength of the upcoming El Nino. The solid blue line indicates the sea surface temperature relative to average for the past year. Each black line that diverges from the single blue line represents a variation of the Japanese model that forecasts sea surface temperatures in the Equatorial Pacific. The red line in the midst of the black lines represents an average of all of the model variations. While there is a clear warming trend as we head into the summer, most of the models show temperatures leveling off well below what we saw with the 1997 El Nino, followed by a cooling trend as we head into 2015.
While there are a few models that indicate the potential for a strong El Nino event, most models are more conservative in forecasting a moderate El Nino. This also fits best with the current overall pattern in the Pacific Ocean.
The Pacific Ocean as a whole goes through extended cycles of warming and cooling known as the Pacific Decadal Oscillation (PDO). We had a very strong El Nino during 1997-1998 and during the 1990s the Pacific Ocean was was at the end of its warm cycle. However, we are now in the cool phase of the Pacific Decadal Oscillation. As we look back through the past century, El Nino events during the cool phase of the PDO have tended to be weak to moderate and they have been considerably shorter in duration than the events that occurred during the warm cycle.
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Whether we have a moderate versus a strong El Nino and whether the warm water is concentrated in the central versus eastern Pacific will have a significant impact on future weather patterns across Canada, especially during next fall and winter. Moderate to strong El Nino events often bring milder winters to Canada, but weaker El Nino events with the warmest water in the Central Pacific often bring seasonal to below seasonal temperatures to much of Canada durng the fall and winter.
At this point it appears unlikely that the coming year will bring a "Super" or "Epic" El Nino event comparable to 1997-1998. There is very little model guidance pointing to an extreme event, and the models have had a recent tendency of being too warm with their forecasts. Therefore, a moderate El Nino looks much more likely, and this also better fits with the overall global pattern. However, we will continue to monitor the progress of El Nino and post updates as we develop our seasonal outlooks.