Introduction

Tornadoes are amongst the most unpredictable and violent weather phenomena in Canada and pose significant economic and public safety risks. Approximately one-half of tornadoes in Canada originate from supercell thunderstorms, which are more typically observed across the Prairies, Ontario and Quebec, but they can occur elsewhere if atmospheric conditions are favourable to their development.[1] With climate change, the atmospheric ingredients that combine to form these types of severe, rotating thunderstorms are changing.[2] These climatic changes could alter both the frequency and intensity of tornadoes and expand their geographical extent.

This article builds upon our previous article on Thunderstorms and Climate Change in Canada. It is recommended you read the Thunderstorm article first.

What are tornadoes and how do they form?

Tornadoes are vortices of air that extend from a thunderstorm cloud to the surface. They often occur within low pressure systems, particularly severe thunderstorms known as supercells, which feature a deep, persistently rotating updraft called a mesocyclone.[3] Other types of vortices exist, including landspouts, waterspouts and even pyrotornadoes (tornadoes generated by fire; Box 1). However, this article will focus on supercell tornadoes, as these vortices cause the most destruction and thus pose the greatest risk to human safety and infrastructure.

Tornado formation, also called tornadogenesis, is an active area of research. Tornado formation requires highly specific atmospheric conditions. This is demonstrated by the fact that, although most violent tornadoes originate from supercell thunderstorms, a U.S. study found that fewer than 20% of supercell storms result in a tornado.[4] Tornado researchers do, however, know the basic building blocks of tornadoes:

1. Atmospheric Instability: The atmosphere’s change in temperature with height determines whether the atmosphere will be stable or not. An unstable atmosphere occurs when temperatures drop off quickly with height. This causes parcels of relatively warm air to rise, which can lead to cloud formation and storms. Conversely, if the atmosphere cools slowly or if its temperature increases with height, the atmosphere is stable, preventing upward movement of air. Atmospheric instability is enhanced when cold, dry air overlies hot, humid surface air. Such conditions are often found during the summer in the lee of the Rocky Mountains, where air, having been forced to rise, cool, and dry over the mountains, moves over the relatively hot, humid air of the central American Plains and Canadian Prairies. A highly unstable atmosphere promotes the development of thunderstorms. To learn more about atmospheric instability and its role in creating thunderstorms, read our previous article on Thunderstorms and Climate Change in Canada.
2. Moisture: The more moisture available, the more intense a thunderstorm can become. This is because a large amount of energy is released when water vapour condenses into liquid droplets. This energy, in turn, keeps a parcel of air warmer than it would otherwise be, enhancing its buoyancy, which in turn, increases atmospheric instability.
3. Wind Shear: This term refers to the change in wind speed or direction as you go higher in the atmosphere. When wind speed increases with altitude, it can create rolling motions in the air, like a rolling pin moving along a surface. If the wind direction changes with height, it can introduce horizontal rotation. When these rotating winds interact with a storm’s updraft, they can cause the entire storm to rotate. If this rotation continues and strengthens, it can lead to the formation of a long-lasting supercell thunderstorm.

Predicting tornado formation is difficult because weather models lack the fine-scale resolution needed to represent the complex movement of air within convective storms. However, advancements in data assimilation techniques and high-resolution modeling are helping to overcome these limitations. That said, it is still difficult to predict more than a few hours in advance exactly when and where a tornado will develop, how strong it will be, or the precise path it will follow.

Tornadoes in Canada

Canada experiences approximately 60 tornadoes per year[5], making it one of the most tornado-prone countries in the world.[6] Most tornadoes are observed in the Prairies, southern Ontario, and southern Quebec. These regions experience a tornado season that extends from April through September, with the highest activity in the summer months, corresponding to peak severe thunderstorm activity. While most Canadian tornadoes are small and short-lived, some are very powerful and destructive (Box 2).

How will climate change impact tornadoes in Canada?

The influence of climate change on tornado formation and intensification is a complex and actively researched topic. As the climate continues to warm, changes in the key drivers of tornadoes—instability, moisture, and wind shear—are anticipated. These changes could potentially alter tornado frequency, intensity, and geographical distribution, though the extent and nature of these changes remain highly uncertain.

Rising surface temperatures may enhance atmospheric instability, potentially leading to more frequent and severe thunderstorms. Warmer air holds more moisture, resulting in higher humidity levels. This increase in moisture can provide more energy to thunderstorms, possibly leading to more intense storm development under the right conditions. Wind shear patterns are also likely to be affected by climate change. A possible decrease in wind shear due to a reduced temperature gradient between the poles and the equator has been cited as a factor that could mitigate the formation of severe thunderstorms in some regions of North America.

Additionally, changes in jet stream patterns driven by climate change can influence tornado activity, potentially altering the geographical distribution and timing of tornadoes. For example, the tornado season could start earlier than usual. More northernly parts of Ontario, Quebec, and the Prairies might become more prone to tornado occurrences as weather patterns shift northward along with rising temperatures. That said, the cumulative impact of these changes on tornado formation is still not well understood.

Several other factors, beyond changes in the drivers of severe weather, complicate the determination of how climate change will affect tornado activity, namely:

1. Limited Data: The scarcity of long-term tornado records makes it difficult to establish clear trends in tornado frequency or intensity (Box 3).
2. Modeling Limitations: Due to their coarse spatial resolution, climate models do not capture the small-scale and rapidly evolving atmospheric conditions that drive tornado formation.
3. Complex Atmospheric Interactions: Tornadoes result from the interaction of multiple atmospheric elements. Modelling these interactions remains a significant challenge.

References