El Nio and how it affects Atlantic Canada's weather

ElNio, a weather anomaly in the Pacific Ocean that causes unusual weather conditions worldwide, is back. But what does that mean for us here on the East Coast?

ElNiooften hasa profound impact on weather and climate conditions across the globe and this latest ElNioevent is the first time it's occurred since March 2010. Even here in Atlantic Canada, where its effects are more subdued, we feel the influence.

Butwhat is ElNio?

ElNiois a phase of the southern oscillation (a.k.a. different temperature patterns in the ocean waters) experienced in the eastern Pacific.Most often the trade winds, that blow east to west, near the equator in the Pacific drag the sun-warmed surface water along with them, pooling that warm water near southern Asia.

As a result, colder, nutrient-richwater flows up from the depths near continental South America in order to replace the warmer surface water that is being pushed west this is what gives the Pacific Coast it's abundant ocean life.When this is occurring the southern oscillation is declared to be a "neutral" or LaNiaphase.

Sometimes conditions in the ocean or the atmosphere are enough to weaken or evenreverse the trade winds.When this happens the warmer surface water stretches back east, towards continental South Americathat is an ElNiophase.

The oceans and the atmosphere are a coupled system that work to balance the inequality of solar heating experienced near the equator compared to the poles.The result of that system at work is the large-scale weather patterns across the globe.

With any significant change in the overall pattern of temperature in the atmosphere or the oceans (such as during an ElNio) come changes to those large scale weather patterns.

So how about Atlantic Canada?

Here on Canada's East Coast, wemost often feel the impacts of an ElNioin the winter months.Over the course of the winter season it has been found that during ElNioour temperatures trend a bit milder and precipitation a bit less compared to climate normals.

Now we can certainly still experience prolonged stretches of cold weather it just means that more swings in temperature from cold to mild are likely.

That pattern, in itself, can create tricky forecasts as it lends itself to more weather systems that mix snow and rain.It could also contribute to more episodes of freezing rain.

Another impact of ElNiois on the Atlantic hurricane season.

During an ElNiophase, more wind shear (wind changing speed and direction moving up through the atmosphere) is generated in the southern Atlantic basin.

The increased wind shear can help disrupt tropical weather systems before they get organized enough to develop into tropical storms or hurricanes.

Typically, thismeans fewerhurricanes are expected in the Atlantic.

That doesn't mean we won't see any, however, and each hurricane/tropical storm needs to be assessed on a one-by-one basis.We all remember Arthur, the first named storm of last season, directly hitting the Maritimes.

The current El Nio is being given a 90 per centchance of lasting through the summer and 80 per centchance of sustaining through 2015.