As summer slowly begins to wind down, with weather across the region being fairly uneventful, the question is, just what should I write about this week? We have looked back at the historic heat we’ve seen over the last year, we have gone into detail about the record-breaking heat over the central U.S., and we have explored some of the weather anomalies around the world over the last month or two. For me it is a little too early to start thinking about school, but the question I’ve been asked the most over the last few weeks is “What is causing all of this warm and dry weather?”
Sounds like a fairly simple and straightforward question doesn’t it? In reality it’s a very broad question that doesn’t have a simple or straightforward answer. When I hear this question two actual questions come to mind. The first question being, why have we now seen 13 months in a row with above-average temperatures? And the second question being, why have we seen such warm temperatures so far this summer? What I can do is break the question down into some manageable parts and try to answer those parts as simply as possible.
The first question is probably the toughest, and I think I’ll leave that for a future article. So that leaves the second question as to why we, and our neighbours to the south, have seen such warm temperatures so far this summer. Those of you who read my weekly forecast should have a pretty good idea of the culprit, as I have repeatedly mentioned it in those forecasts: the upper-level ridge.
To understand what an upper-level ridge is, we need to first come to an understanding of what constitutes a ridge. Simply put, the term ridge is used to depict a region of high pressure, with the term trough used to depict regions of low pressure. But it’s a little more complicated than that. Remember that in the Northern Hemisphere, an area of high pressure occurs when air over a large area is sinking toward the earth and spreading out or moving away from the centre. Areas of low pressure are the opposite.
For this article we will mostly discuss high pressure. As the air flows out of the area of high pressure it begins to curve to the right, due to the Coriolis force. If the air flowing out of a high-pressure system curves enough, it will eventually form a complete circle of air and we would now have what is known as an anticyclone, or simply, a region of high pressure. If you look at a typical surface weather map on any given day you can see regions of high and low pressure, or areas of anticyclones and cyclones (areas of low pressure).
What most of us don’t see, unless you know where to look online, are maps showing the pressure pattern higher up in the atmosphere. If you were to do this, one of the first things you’d notice is that there are much fewer areas of high and low pressure. Instead, there would be troughs and ridges. As we go up in the atmosphere, the closed circulations we see at the surface open up and the general flow becomes wave-like. If the flow dips southward, we have a trough and if the flow curves northward we have a ridge. Areas under a ridge tend to be warm and dry, while those under the trough are cooler and wetter. What controls the placement of these troughs and ridges is the jet stream. When the jet stream is strong (large difference in temperature from north to south) we see very small troughs and ridges (known as zonal flow) and the weather is usually not very active. When the jet stream is weak, the troughs and ridges grow to be fairly large (meridional flow) and the weather is usually very active.
So far this summer we’ve seen a predominantly meridional flow across North America, with a large ridge of high pressure situated over the central part. This has brought our region the warm and dry weather that accompanies such a ridge. Regions farther to our west have been on the edge of the ridge and have seen the active weather that is also associated with the pattern. The interesting part to this meridional flow is that while it is often unstable and can break down fairly quickly, it is also known to get “stuck” in what is known as a blocking pattern. This is what has happened during the first half of this summer. The question for forecasters is whether this pattern will remain essentially “stuck” for the rest of the summer and fall as they are currently forecasting — or will it break down, as it is trying to do now?