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HHurricane Harvey, with its historical rainfall volume over Texas, followed by a series of hurricanes Irma, Jose and Katia in the North Atlantic Basin in 2017, sparked lengthy questions about the links between hurricanes and climate.
Can we really blame recent hurricanes on climate change? Or are they simply a coincidence of nature occurring every few decades, similar to the triple Hurricanes Beulah, Chloe and Doria in 1967?
See also: Hurricane Florence: arrival time, precipitation forecast, flood forecast
Answering these questions is at the heart of current research on the hurricane climate that atmospheric scientists are trying to understand. There are signs that climate change can affect hurricanes in different ways. However, these signals are inconclusive because of our insufficient understanding of the interaction of hurricanes with the environment.
Link with the temperature of the ocean
Just as an influenza virus transforms into different environments and becomes more infectious in cold weather, hurricanes depend on the surrounding environment for their existence and movement. To what extent the surrounding environment affects hurricane development is indeed among the most studied topics in hurricane research.
Evidence of the role of the environment in hurricane development has been noted since the early 1950s, but an important step was taken by Kerry Emanuel at MIT in his studies of hurricane dynamics in the late 1980s.
His idea was to consider hurricanes as heat engines capable of extracting heat from the surface of the ocean and depleting it in the upper troposphere. In this way, Emanuel was able to obtain a mathematical expression showing how the maximum potential intensity that a hurricane can achieve in a given environment depends on the temperature and temperature of the sea surface at about 14 kilometers overhead. the atmospheric troposphere. According to Emanuel's formulation, a higher temperature of the sea surface would result in a higher intensity.
In essence, Emanuel's relationship between hurricane intensity and sea surface temperature determines the strength of a hurricane for a given environmental condition. Numerous studies have subsequently confirmed the importance of sea surface temperature for controlling the maximum intensity of hurricanes and suggest a 2 to 3% increase in hurricane strength by 1 Celsius increase in sea temperature. in favorable conditions.
In this perspective, it is therefore very tempting to assert that hurricane intensity variations must be related to the global climate because of the essential role of ocean temperature in the development of hurricanes. Indeed, many studies on the climatology of hurricane intensity consider that the temperature of the ocean is the main indicator of the future evolution of the evolution of hurricane intensity.
The common consensus between these studies is the conclusion that future hurricanes will tend to be stronger than those of the current climate, assuming that the sea surface temperature will continue its current warming trend.
Look at the extremes for clues
Although we can expect an increase in hurricane intensity due to rising ocean temperatures, the way to interpret this result in a specific hurricane is very different.
For an intuitive illustration of the difficulty this may present, consider how climate change can affect certain aspects of our weather, such as the daily variation in temperature.
For example, a future warming of the air temperature of 0.5 degrees in the next 10 years would be mostly masked by any daily temperature variation, which is in the range of 10 degrees between day and night. night. In this sense, it would be hasty to conclude that the high intensity of hurricane Harvey or Irma is due to climate change, simply because fluctuations in local weather conditions could contribute much more than climate change.
In addition to daily fluctuations in intensity due to local environmental conditions, hurricanes can also have chaotic behaviors that vary their intensity considerably. A recent study has shown that internal variations in hurricane intensity can reach 10 to 18 miles per hour, which is more important than what climate change would cause.
On the other hand, one must naively deny any claim that the extreme impacts of Hurricane Harvey or Irma are symptoms of climate change.
Some research has indicated that global climate change could lead to a change in jet stream behavior in North America. The Harvey-related floods were unusual in part because the storm hit Texas much longer than any other hurricane. Thus, while our current knowledge does not allow us to relate Harvey's intensity to specific climate changes, Harvey's anomaly over a long period of time could be a manifestation of the change in global circulation in a more challenging climate. hot.
Similarly, the emergence of triple hurricanes in the Atlantic Basin in September 2017 may be another potential signal for more favorable conditions for hurricane formation through climate change.
From a climatological point of view, it is the frequency and magnitude of these abnormal extremes, such as the prolonged period during Hurricane Harvey or the extreme intensity of Hurricane Irma, that are often of interest to researchers. Indeed, these extremes are signals of climate change that can be distinguished from daily variations.
Limits of our understanding
In addition to the direct impacts of climate on hurricane intensity, another possible influence of climate on hurricanes is the change in hurricane trajectory in the future climate.
In principle, a change in the world's air circulation could affect the directional flows that guide hurricane movement, just like a leaf carried by a river. As such, changes in global circulation associated with climate change could introduce another degree of variability in the impacts of hurricanes that we must take into account.
A recent climatological study conducted by James Kossin at the University of Wisconsin suggested moving to the intersection of maximum hurricane intensity in a warming climate. But, unlike the relationship between hurricane intensity and the surrounding environment, the link between the change in global circulation and the hurricane movement is much more difficult to quantify at this time.
See also: Evacuation of Hurricane Florence: South Carolina has turned I-26 into a one-way street
Although hurricane research gives us a good idea of how hurricanes would change in a warmer climate, measuring this change and, in particular, associating a unique hurricane feature with climate change exceeds the current level of confidence. .
In reality, there are several other factors that could strongly interfere with the development of hurricanes, such as changing the atmospheric temperature with height. These factors directly affect the interaction of hurricanes with the environment. However, these are very difficult to quantify in the context of climate change because of the different time scales between hurricane development – measured in a few days and weeks – and climate change, which occurs over time. decades.
From the scientists' point of view, the lack of understanding of climate impacts on hurricanes is disappointing, even irritating. On the other hand, these uncertainties continue to prompt us to look for a possible link between hurricanes – including their intensity, frequency, formation time and location – and climate. A better understanding of the hurricane-climate relationship is necessary because ultimately this knowledge can help serve society.
This article was originally published on Chanh Kieu's The Conversation. Read the original article here.
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