Wednesday, 11:55 a.m.
I will take a break from my normal routine of discussing the immediate and short-term weather to look at the equatorial Pacific. Over the past couple of years, we've been in what has technically been considered ENSO neutral conditions - neither fully La Nina nor fully El Nino. The most recent definition of an El Nino (La Nina) is derived from the Oceanic Nino Index (ONI): Warm (cold) episodes based on the the threshold of +(-) 0.5 degrees Celsius of the Nino 3.4 region. To qualify for an El Nino (La Nina), the ONI must be at or above (below) the threshold for a minimum of five consecutive three-month averages. The last official La Nina under this definition ran from August through October of 2011 to February through April of 2012, which covered a span of seven consecutive three-month periods with an ONI of -0.5C or lower. The last official El Nino was from June through August of 2009 to March through May of 2010.
I won't get into what qualifies as a weak, moderate or strong El Nino here. Instead, I'll look at the most recent water conditions off the coast of South America stretching westward across the equatorial Pacific.
On the face of it, that doesn't look very impressive. And, quite honestly, it really isn't that the central Pacific IS warm, and HAS BEEN warm, but the eastern part of the basin between 5 degrees North and 5 degrees South latitude has been relatively cool. Let's look at it another way, chopping it up into the region blocks:
Again, only the central part of the basin appears to be warm in any way. However, let's look a little deeper - literally:
That shows the sub-surface water temperature anomalies, and it is quite revealing! Lurking just below the surface of the ocean lies some anomalously warm water, and if you follow the trend of this, you'll find that it has been expanding eastward into the eastern Pacific in recent weeks, while at the same time it has been expanding closer to the surface.
Assuming this trend continues, the cool signal over the eastern equatorial Pacific is likely to be reversed in the coming months, and it could do so in a hurry. All it would take is the typical trade winds to weaken in the coming months, and that warm water in the western and central part of the basin should expand eastward.
Most of the modeling that I've seen in recent weeks have been going more and more toward the idea of an El Nino blossoming sometime in the coming months. Again, for El Nino conditions to be considered present, all you need is to get that ONI to reach 0.5C or better for a three-month period. In order to qualify for a full-fledged El Nino, that 0.5C threshold must be met for five consecutive three-month periods, which means the earliest that we'd have a full-fledged El Nino would be late summer.
That said, what if the conditions do, indeed, change rapidly? El Nino conditions could appear much sooner that late summer or early fall, and given the water temperature profiles, that's a real possibility. I won't address that question here, but I will state three other things:
1) How strong will the El Nino become, assuming we're heading in that direction?
2) When will it develop?
3) Where will it be based?
The answer to all three of these questions will have an impact on how the atmosphere responds globally. It will have a decided impact on the summer temperature and precipitation pattern across North America, and it will likely play a significant role in what the hurricane season will look like, both in the Pacific and Atlantic basins. And while that third question seems like an odd one, look at it much like you would the North Atlantic Oscillation. When the NAO is negative, there tends to be blocking in the high latitudes of the Northern Hemisphere, but where that blocking is based has a huge impact on how the pattern sets up across North America and the Atlantic. In much the same way, having the core of the warmth in the equatorial Pacific in the central part of the basin, basin-wide, or in the eastern Pacific will all have different outcomes.
That's a lot to try to figure out when you're trying to come up with a summer seasonal forecast, or even putting out some initial thoughts on what next WINTER might look like! However, it all has to be factored in when coming up with the seasonal outlooks. Suffice it to say that while we're pretty certain El Nino is likely to develop, there's still many unanswered questions about its impacts on the weather and climate in the months to come!
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