How will El Niño impact weather patterns?

This is a re-post from The Weather Network by Dr. Doug Gillham

Confidence remains high that El Niño is developing and that this will impact global weather patterns during the upcoming year. For more information on what El Niño is, see my previous article.

However, there is uncertainty regarding just how strong El Niño will become later this year. 

The ultimate strength of El Niño is critical to forecasting the impact on our weather across Canada as we head into the fall and winter. As we look to the past, we can see that a weak to moderate El Niño often has the opposite impact on some regions compared to what we typically see from a strong El Niño.

During the past few months there has been a steady trend towards warmer than average sea surface temperatures in the Equatorial regions of the Pacific Ocean. The black oval on the map below highlights the region of the Pacific Ocean to the west of South America that is evaluated to determine whether we are in an El Niño or La Niña pattern. The various shades of yellow, orange and red represent sea surface temperatures that are warmer than average, which is what we see with El Niño. White represents average water temperatures while blue represents colder than average water temperatures at the surface.

A comparison of the map above to the map below (from April 29) highlights the warming that has occurred during the past month. While the change is not dramatic, the warmer-than-average sea surface temperatures are becoming more widespread within that region.

While the sea surface temperatures are starting to take on the look of an El Niño pattern, we still have quite a ways to go before we will officially have El Niño.

The temperature anomaly for this region is calculated based on average water temperatures over a 90 day period. For El Niño, the 90 day average must be at least 0.5ºC above average for five consecutive months. So, we will likely be well into the autumn season before that criteria is fulfilled. However, the weather certainly will not wait until this official declaration before it is impacted by the warming of the ocean water temperatures in this region.

The models and the global pattern continue to support our forecast of a moderate El Niño event. There appears little support for the development of a “Super El Niño” which last occurred in 1997-1998. To be classified as a super El Niño, sea surface temperatures must warm to at least 2ºC above average across this region for at least three consecutive months.

The image below shows the forecasts for sea surface temperature anomalies through next winter from 24 models. Each line on the chart represents a forecast from a different model. The scale on the left shows sea surface temperatures relative to average, and as we move from left to right we are progressing further into the future with the forecast. The months of the year are given on the bottom in three month periods (MJJ is for May, June & July).


The above image is likely somewhat overwhelming at first glance, so here are the keys points to note. Nearly all of the models are in agreement in forecasting an increase in temperature through the summer and into the fall.  However, many of the models indicate that the warming will not exceed 1.0ºC above average across this region. While several of the most reliable models are forecasting the warmest temperatures, even these models are well below the +2.4ºC temperature anomaly that we saw with the 1997-1998 super El Niño. 

In addition, to the lack of model support for a super El Niño, long term cycles that we observe in this region also support this forecast.  The Pacific Ocean goes through cycles of warming and cooling.  This pattern is known as the Pacific Decadal Oscillation (PDO), and as the name suggests, the cycle occurs over the course of a few decades. The last two super El Niño events occurred during the “warm” or “positive” phase of this pattern, but the Pacific Ocean is now in the “cool” or “negative” phase of the cycle. 

As we look back over the past century we can see that El Niño events that occur during the cool phase of the PDO are typically weaker and last for a shorter period of time than the El Niño events that occur during the warm phase. Each of the El Niño events that have occurred since 2000 (during the current cool phase) have only been weak to moderate and each has been weaker than the models initially forecast. 

Nevertheless, the developing El Niño was an important consideration in producing our Summer Outlook. In addition, since El Niño typically has a greater impact on Canadian winters than it does on the summer season, we will continue to monitor El Niño’s progress during the upcoming months.

A more immediate impact is expected on the hurricane season in the Atlantic Basin. Typically, El Niño years are associated with fewer tropical storms and hurricanes in the Atlantic Ocean and Gulf of Mexico. In addition, sea surface temperatures this year in the tropical Atlantic between the Caribbean and Africa (where many tropical cyclones develop) are running at to below average. This is a notable pattern reversal from what we have seen during most years since the 1990s and should also contribute to fewer total storms for the upcoming hurricane season.

However, of concern for the Eastern U.S. is the fact that sea surface temperatures are warmer than average between the Caribbean and the Carolinas. This leaves the eastern United States vulnerable to storms that develop close to the coast as they have a higher threat of making landfall and to be intensifying as they come ashore.

It is important to remember that seasons with fewer storms can still have a high impact on Canada and the United States. For example, 1992 was a relatively quiet season in terms of the total number of storms, but that year also featured Hurricane Andrew which was one of the costliest natural disasters on record.

Please stay tuned to The Weather Network and check back online for updates on the developing El Niño and its potential impacts on our future weather patterns.

Posted by Guest Author on Friday, 20 June, 2014

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