There is a theory that you will be hearing a lot more on weather forecasts and climate change discussions. This is a gardening blog first and a weather blog second- the idea is to tie the two together. They go hand in hand. The more you garden the closer you watch the weather. In our mostly benign climate we like to grow as much as possible. That means the two overlap. Zonal denial made famous in our region in the last 15 years couldn’t be a more perfect example. The term you will be hearing is ‘The Lazy Jet Stream’- as opposed to the lazy gardener. Of which I am guilty.
Jet Stream 101
To understand how weather works you need (especially at our latitude) to understand the Jet Stream- what it is and what it does and a few facts about it that I will try to explain with as little scientific hoo haw as possible. The jet stream is a river of air that moves from the west to the east at our latitude and is responsible for virtually all of our weather. This is called the polar jet. It is formed because there is a temperature gradient that occurs world wide from the cold poles to the equatorial tropics. Cold air from the arctic does not stay in place, instead it migrates south. The same is true of tropical air- it migrates north. Where these air masses commonly collide is at the half way point (to be broad) which is us at just north of 45ºN in latitude- Hello Portland! This collision causes the jet stream and in concert with the rotation of the earth a river of wind in the upper atmosphere that moves rapidly from west to east. The jet stream is associated with (frontogenisis) the formation of storms and fronts (the boundary and movement of two air masses). Essentially, on the north side of the jet stream is colder air- which is more often associated with rain and snow- air from the poles. On the south side of the jet stream is ridging- most often associated with warm and dry conditions- air from the lower latitudes.
Zonal, Ridge/Trough- Speedy Gonzalez
The jet stream varies greatly in its strength depending upon the season. In winter there is a much greater temperature gradient between the arctic and the tropics. This energizes the jet and as I mentioned this creates storms and our famous rainy season. As you know in winter this can mean storms plow through every few days like Speedy Gonzalez. The jet stream is directly over us or slightly to our north or south. Its that steep pressure gradient that keeps things cruising along. When the flow is directly west to east (a horizontal line) its called zonal. But weather is anything but constant and the speed of the jet transverses high pressure ridges and low pressure ridges like a windy road. As the jet stream speeds up the acceleration/deceleration of the air flow induces areas of low/high pressure respectively, -we get a parade of ridges and troughs. This is called ridge and trough. Troughs dip down from the north and ridges impinge from the south. So, there is zonal flow- directly west to east and ridge/trough- high amplitude waves north to south in the Jetstream. In the summer when the temperature gradient from the warming poles of the north to the equatorial south is much less the jet stream decreases in velocity and the parade of storms and troughs becomes sluggish to non-existent- ridging takes hold. Our annual dry season. Well, what if the temperature gradient that fuels the speed of the jet stream was shall we say perturbed?
OMG did we kill Speedy?
We may actually be seeing what happens right before our very eyes. The theory- which makes a lot of sense is that because the poles are warming more rapidly than the lower latitudes the pressure gradient whose intensity speeds the jet stream along may be waning. A lazy jet stream. What is the big deal you ask? Well, when we hobble Speedy Gonzalas the jet stream begins to slow down and that has big implications. Instead of weather systems such as ridges and troughs moving steadily and quickly- and for instance the weather pattern moving as well- we get a slowdown in pattern changes. Weather conditions become more static and that can cause some real problems. If you are in a wet trough pattern- then you stay there not just for days- like we expect but for weeks- possibly months. If its raining, it will keep raining- this can obviously lead to flooding. If its hot under a ridge of high pressure then the stagnant conditions remain and heat intensifies.This can lead to flash droughts or long term droughts. This explains why weather events are not just strong- and they have always been, but now are persisting. This looks like a trend.
Typical heat waves are in and out of here.
One example of this- and believe me meteorologists took notice was our weather this past hottest June of all. Oregon has always had heat waves and they can be pretty intense as we well know. But there is a pattern and set up to these heat waves that naturally limits their length of time – a healthy Jet Stream, but not necessarily their intensity. A typical heat wave for us begins with a ridge of high pressure nosing north to cover our region (ridging). It moves north up the coast and atmospheric heating- warm air rising at the surface causes a heat low which is oriented as a north south trough located off the coast. The air from inland regions is pulled off the coast the first day down the mountains drying and heating as it goes. This can cause temperatures at the normally cool coast to soar- but then it is short lived. The second day the heat low moves east and orients itself north to south over the valleys of western Oregon. The coast cools off rapidly as the flow turns onshore. Hot air from south and east of the Cascades is drawn directly into the valley heating and drying as it goes. Inland, this is our second day of a typical heatwave and its often the hottest. The third day day the heat low moves over the Cascades and east of the crest they begin to heat up and our temperatures in the valley drop slowly at first then cool marine air rushes in and the heat wave is over. This is a pattern that repeats (in various intensities) a half dozen times a summer- and this is with a much less powerful jet stream that is primarily to our north in the warm season. Our most recent heat wave when we peaked at 103ºF and higher was a closer example of a typical heat wave. A few days in the 90’s to a day or two in the 100’s and then back down to seasonal averages. This is how we usually accumulate our 14 average days above 90 and one day a year of 100. I’ve watched this pattern my whole geek life.
But June? Why was June and the beginning of July so hot?
The difference in the heat we experienced for most of June into the beginning of July may be a local example of a Lazy Jet Stream. Instead of a fast forming and mobile heat wave this one stuck around. A persistent ridge which had no intention of moving on. And this pattern is being repeated all over the globe in different ways- aside from the typical press hyperbole its important to listen to the details of events. For instance just how long did that monsoonal rain in Myanmar fall? How long was the heatwave in the Middle East roasting those poor people? (Two recent events of note). And this could be happening and will on a more macro scale. The drought in California is associated with a persistent ridge of high pressure that is not just stagnant- it weakens for a bit and then bam! It reforms. The Eastern part of the country has had two winters where they were stuck on the wrong side of the Jet Stream. (Remember the Polar Vortex?) and intrusions of cold air one after another dove south. Eventually, leading to all time record snowfall in Boston among other places. Meanwhile we on the west coast basked in a persistent ridge for most of winter and spring. Our June heat which was so notable for its length and not necessarily intensity, PDX only got as high as 97ºF but there was a nearly uninterrupted string of 90ºF plus for almost two weeks- including the warmest over night lows ever for June. Could this be a view into our future?
I’m not saying that our local weather is going to change completely. And you should always have facts to back up any claims. What is most important is to pay attention to trends and events. Global Warming is a grand and scary experiment and as far as I can tell people are learning more and more as it intensifies. And, there is a reason so many long standing records have been broken in the 21st century. Could it be that we’ve broken Speedy Gonzalez?