Ontario's changing weather - the jet stream is not what it used to be
Friday, December 5, 2014
With climate change, the weakening of the jet stream and the new importance of the Polar Vortex, Ontario can expect chillier winters and more 'gullywashers' – slower-moving storms that drop larger than normal precipitation totals along their path
by PHIL CHADWICK
My very first article for Better Farming in October 2013 was about the weakening of the jet stream. That was no accident. For me, this was the most blatantly obvious sign of climate change with the most immediate impact on Ontario farmers. As a meteorological instructor, I try describing a process in different ways until I find the best method to get the information across. Not everyone learns in the same way. Here is another analogy and it might better explain the coming winter – the new norm. (Figure 1)
Warming of the globe due to the excesses of greenhouse gases is certainly not uniform. Insolation and warming near the equator is by far the strongest, but other processes make certain that the impacts are stronger at the poles, resulting in roughly twice the rise in temperature compared to that observed at the equator.
Melting of snow and ice at higher latitudes changes the polar surfaces from shades of white to darker colours. These dark surfaces are much better absorbers of solar energy. More absorbed energy results in a greater warming of the polar surface, increased melting and even more warming. This "positive" feedback mechanism has been underestimated and will contribute to the melting of the permafrost and the release of polar methane gases. But that is another story and no one is quite certain yet just how that will play out.
The Canadian Arctic is now at its warmest temperature at any time in the least 40,000 years. This has caused the Arctic-wide melt season to lengthen at a rate of five days per decade (from 1979 to 2013) mainly due to a later autumn freeze-up. Sea ice changes have been clearly identified as a mechanism for this polar warming amplification.
The adjacent image (Figure 2) centered on the North Pole reveals where the average air temperatures (October 2010-September 2011) were up to three degrees Celsius above (shades of red) or below (blue) the long-term average (1981-2010). Red and abnormally warm temperatures dominate the high latitudes.
Initially, the Canadian Arctic was forecast to be ice-free by 2050. That forecast has been revised. Meteorologists are very accustomed to "amending" good forecasts that go bad. Scientists have noted that climate model predictions have tended to be overly conservative regarding sea ice decline. A 2006 paper by Marika Holland et al. predicted "near ice-free September conditions by 2040." Overland & Wang, in 2009, predicted that there would be an ice-free Arctic in the summer by 2037. A follow-up study by Overland & Wang in 2013 concluded with the possibility of major sea ice loss within a decade or two (2023 or 2033). The same year, Boé et al. found that the Arctic will probably be ice-free in September before the end of the 21st century. Climate models that best match historical trends project a nearly ice-free Arctic in the summer by the 2030s.
An "ice-free" Arctic Ocean has been defined as "having less than one million square kilometres of sea ice," since it is very difficult to melt the thick ice around the Canadian Arctic Archipelago. The International Panel on Climate Change Assessment Report 5 from 2013 ( IPCC AR5) defines "nearly ice-free conditions" as "when sea ice extent is less than 106 square kilometres for at least five consecutive years" and (for at least one scenario) estimates that this might occur around 2050.
To a new and keen meteorologist, as I was in 1977, a hot equator and a cold Arctic meant that the temperatures in the atmosphere were like surfaces tipped from high heights over the equator to very low levels over the poles. Pouring water down a steep, sandy slope with a corresponding tilt would result in a strong and straight torrent from the equator to the poles.
If we place this steep, sandy slope on a spinning sphere like the earth, the rotation of the sphere causes an apparent deflection of the current to the right – the so-called Coriolis force first described in 1835. The fast current of water flowing along the steep slope would be very similar to the westerly jet stream in the earth's atmosphere – a confined channel of strong winds blowing from west to east. The speed of these winds is directly related to the slope of the temperature surfaces in the same way that water flows down a steep slope.
The temperature surfaces have changed in the last 40 years. Today, the equatorial regions are certainly hotter, but the poles have increased in temperature at twice that rate. Imagine that the temperature surfaces between the equator and the poles are now at a much shallower slope. Water released at the equator down the shallow sandy slope now meanders back and forth as it weakly follows gravity down to the poles. The analogy to oxbow lakes, the U-shaped bodies of water that form when a wide meander from the main channel of a river is cut off. Blocked, oxbow flows in the atmosphere are now common, whereas in 1977 they were rare and hardly touched on during my meteorological instruction.
In the atmosphere, whenever the flow meanders poleward, the circulation is called a ridge. Warm air is directed toward the pole. A southward flowing meander is called a trough. Colder air reaches toward the equator in a trough.
Here is where the conservation of angular momentum comes into play. Figure skaters have long learned that they could rotate faster by pulling in their arms and legs and slower by stretching them outward. The atmosphere knows the very same science. A current of air approaching the Canadian Rockies from the west is confined and spreads out laterally as it is squashed between the rising foothills and the stable layer at the top of the atmosphere. The air must turn northward toward higher planetary rotation to compensate for the loss due to spreading outward, maintaining the same total of angular momentum. Looking down at the North Pole, a location near the pole has a smaller radius of rotation and higher planetary angular momentum than a location at the equator.
The opposite process occurs as the air crosses the peaks of the Rockies toward the plains and is deflected southward. An atmospheric ridge is thus formed as a result of the high Rockies over western North America. In contrast, the air tracking southward in the lee of the Rockies continues to do so and forms an atmospheric trough.
As a result of the preceding science, it might be clearer now why the jet stream meanders from west to east, roughly following the middle latitudes. Northward flows and ridges are favoured over north to south mountain ranges, while troughs are found downstream from the pronounced ridges of high pressure. The general weakening of the jet stream tends to make these meandering flows much more pronounced and herein can be found the problem.
A ridge tends to be warmer and drier than a trough. Conservation of angular momentum also plays a role in this process. Northward travelling air also tends to rotate clockwise (anticyclonic) to maintain a balance in the total angular momentum as it heads to higher values of planetary rotation. Such air must descend. Sinking air in the atmosphere must become warmer and drier with the creation of large areas of high pressure.
In this upper ridge portion of the meandering jet stream pattern, California remains hot and dry with record low relative humidity (as low as five per cent). During the winter of 2013-2014, Vancouver was sunnier and milder; Alaska was warmer with rain. Heavy rains, snows and warm temperatures helped to trigger a series of huge avalanches that blocked a 100-kilometre section of the Richardson Highway, the only road into Valdez, Alaska, about 200 kilometres east of Anchorage.
California has been in an historic "exceptional drought" since 2010. Wildfires have been constantly in the news. Farmers are resorting to drilling new wells for irrigation that are more than 2,000 feet deep as the groundwater is consumed. The consumption of these ancient groundwater supplies is clearly not sustainable.
California's dryness is only part of a longer-term, 15-year drought across most of the Western United States, one that bioclimatologist Park Williams said is notable because "more area in the West has persistently been in drought during the past 15 years than in any other 15-year period since the 1150s and 1160s" – 860 years ago.
The opposite process occurs in a trough. Southward travelling air rotates increasingly counter-clockwise to remain in total angular momentum equilibrium. The air rises in areas of counter-clockwise (cyclonic) rotation. Rising air causes the pressure at the surface to fall and thus helps to form low-pressure areas which are commonly referred to as "cyclones." Rising air also results in the formation of clouds and precipitation and the release of latent heat, which provides more energy for any cyclone. The intensity of the low-pressure area increases with the depth (southward extent) and size (breadth from west to east) of the trough. This trough originates from the pole and the rotating low-pressure area it contains has been simply called the "Polar Vortex." This term dates back to the 1950s but, with climate change, the weakening of the jet stream and the persistence of the vortex, the phrase "Polar Vortex" has gained new importance and media attention.
What does this mean for Ontario farmers? The winter of 2013-2014, which was dominated by thepolar vortex is likely to become the new norm. Blizzard-like conditions with lots of snow, freezing rain and cold air are what many will remember from last winter. Of course, these are the extreme conditions that were frozen into one's memory. There will still be "fair" winter weather, but expect it to be on the cloudier and cooler side with more precipitation in the form of snow. This is why my first article was about the jet stream and climate transformation. The weaker, meandering jet stream changes everything, the weather and climate included.
The good news is that Ontario is less likely to experience the prolonged droughts of the west coast. Ample precipitation and cloud cover should keep soil moisture at acceptable levels. The bad news is that this precipitation is more likely to come in "gullywashers" from slower-moving storms that drop larger than normal precipitation totals along their path. The air masses can hold about seven per cent more water vapour for each degree Celsius rise in temperature. This equates to one or two per cent more precipitation by volume.
The larger and stronger the polar vortex, the further south the chilly Arctic air is delivered by the jet stream. The polar vortex is strongest and thus more newsworthy in the winter when there is no sun and 24 hours of darkness in the far north. Last winter saw temperatures of minus 2 C in New Orleans, which was enough to chill out the preparations for Mardi Gras. Six inches of snow paralyzed Atlanta and the rare blue sky blizzard in southern Ontario was also a shocker. It is certainly paradoxical that warming in the Arctic can result in a chillier climate for Ontario. The weakening and meandering of the jet stream creates a home for the polar vortex over James Bay that in turn pumps the cold air further south. The world's weather and climate are still changing.
Meteorology has an even bigger role now to assist in facing the challenges of the future. The jet stream is perhaps the most important forecast tool. I have often said "Give me the jet stream and I can give you the forecast." My forecast friend, the jet stream, is not what it used to be. BF
Phil the Forecaster Chadwick has been a professional meteorologist since 1977, specializing in training, severe weather and remote satellite and radar sensing.
