For this first issue of the new year, let’s finish up our El Nińo discussion.
In the previous articles we discussed how the “normal” flow across the tropical Pacific Ocean is from east to west. This flow results in water being pushed away from the eastern coasts of North and South America, where it travels across the Pacific, warming as it goes, then piles up on the western side.
Where the water is being pushed away from the coast, cold water from below moves up to replace it (upwelling). These general patterns result in wet conditions over western parts of the Pacific and generally dry conditions over the east. Figure 1 shows the typical or “normal” setup over the Pacific Ocean.
During an El Nińo event, for reasons still not fully understood, the pattern of winds and pressure over the Pacific Ocean changes. The easterly winds weaken, and in some cases, even reverse themselves. The cold upwelling is slowed or even blocked and warm water starts to build up in the eastern Pacific. This results in the movement of the area of clouds and precipitation from western regions to more-eastern locations. Figure 2 shows a typical El Nińo event.
During a La Nińa event the “normal” pattern is amplified, as colder-than-average water temperatures occur over eastern regions of the Pacific. So how does this changing of weather patterns over the Pacific Ocean affect us? Well, to make a long story short, the overall pattern of pressure across the Pacific is altered in an El Nińo or La Nińa year. This change in pressure patterns is reflected downwind, as the path of the jet stream is altered. When the jet stream is pushed northward, warm air is allowed to flood northward. When the jet stream is sent plunging southward, cold air follows right along.
MODERATE TO STRONG PATTERN
El Nińo patterns tend to bring warm, dry winters to the Prairies; overall summers are not affected as much. El Nińo is also the stronger of the two different patterns; that is it tends to have the greatest impact on our weather. La Nińa winters tend to be snowier, and again, like her big brother, the effect during summer is not as noticeable. That said, the current conditions over the Pacific are a moderate to strong El Nińo, so the chances are pretty good that we will see warmer-and drier-than-average conditions this winter. This doesn’t mean that we won’t see any cold weather outbreaks – just take a look at the weather in Alberta last month. What it does mean is that warm dry weather will be the more dominant weather pattern this winter.