Global Warming is Affecting Present-Day Hurricanes

According to a study recently published in a prominent journal, Nature, climate change is causing landfalling hurricanes to weaken more slowly. This means present-day hurricanes weaken more gradually than hurricanes did in the past.

(Left) The graph shows that on average, present-day hurricanes weaken more slowly than hurricanes did 50 years ago. (Right) This slowing of intensity means that on average, present-day hurricanes are penetrating further inland at greater intensities. The graph assumes that the hurricanes are hitting land head-on and move forward at a typical speed of 5 meters per second. Image Credit: Okinawa Institute of Science and Technology Graduate University.

The investigators demonstrated that more amounts of moisture are carried by hurricanes that develop across warmer oceans and, hence, present-day hurricanes stay stronger for longer periods of time after hitting the land.

This implies that in the days to come, as the globe continues to warm, hurricanes would probably reach communities that are residing further inland and cause more destruction.

The implications are very important, especially when considering policies that are put in place to cope with global warming. We know that coastal areas need to ready themselves for more intense hurricanes, but inland communities, who may not have the know-how or infrastructure to cope with such intense winds or heavy rainfall, also need to be prepared.

Pinaki Chakraborty, Study Senior Author and Professor, Fluid Mechanics Unit, Okinawa Institute of Science and Technology Graduate University

Several studies have demonstrated that climate change can increase hurricanes, called typhoons or cyclones, in other parts of the world—across the open ocean. However, this is the first-ever study to determine a clear relationship between the smaller subset of landfalling hurricanes and a warming climate.

The researchers examined the North Atlantic landfalling hurricanes over the past 50 years. And they observed that during the duration of the first day following landfall, present-day hurricanes weakened almost twice as slowly as they did half a century ago.

When we plotted the data, we could clearly see that the amount of time it took for a hurricane to weaken was increasing with the years. But it wasn’t a straight line—it was undulating—and we found that these ups and downs matched the same ups and downs seen in sea surface temperature.

Lin Li, Study First Author and PhD Student, Fluid Mechanics Unit, Okinawa Institute of Science and Technology Graduate University

The researchers examined the association between slower weakening past landfall and warmer sea surface temperature by producing computer simulations of four varying hurricanes and setting varying temperatures for the sea surface.

When all the virtual hurricanes reached the category 4 strength, the team replicated landfall by stopping the supply of moisture from underneath.

Hurricanes are heat engines, just like engines in cars. In car engines, fuel is combusted, and that heat energy is converted into mechanical work. For hurricanes, the moisture taken up from the surface of the ocean is the “fuel” that intensifies and sustains a hurricane’s destructive power, with heat energy from the moisture converted into powerful winds,” explained Li.

Making landfall is equivalent to stopping the fuel supply to the engine of a car. Without fuel, the car will decelerate, and without its moisture source, the hurricane will decay,” Li further added.

The investigators noted that although every replicated hurricane made landfall at the same level of intensity, the ones that developed across warmer oceans took a longer time to weaken.

These simulations proved what our analysis of past hurricanes had suggested: warmer oceans significantly impact the rate that hurricanes decay, even when their connection with the ocean’s surface is severed. The question is—why?” Professor Chakraborty stated.

The researchers used more simulations and found that the missing link was the “stored moisture.”

They explained that when hurricanes make landfall, although they cannot access the sea’s supply of moisture anymore, they still carry some amounts of moisture that depletes gradually.

The researchers subsequently produced virtual hurricanes that did not contain this stored moisture after hitting land and observed that the temperature of the sea surface no longer had any effect on the decay rate.

This shows that stored moisture is the key factor that gives each hurricane in the simulation of its own unique identity. Hurricanes that develop over warmer oceans can take up and store more moisture, which sustains them for longer and prevents them from weakening as quickly.

Lin Li, Study First Author and PhD Student, Fluid Mechanics Unit, Okinawa Institute of Science and Technology Graduate University

Moreover, the high level of stored moisture renders hurricanes “wetter”—a consequence that is already being felt as recent hurricanes have resulted in devastatingly high amounts of rainfall on both inland and coastal communities.

The new study emphasizes the significance of climate models to cautiously consider the stored moisture when estimating the effect of warmer oceans on hurricanes. The research work also highlights problems relating to the basic theoretical models that are extensively utilized to figure out the way hurricanes decay.

Current models of hurricane decay don’t consider moisture—they just view hurricanes that have made landfall as a dry vortex that rubs against the land and is slowed down by friction. Our work shows these models are incomplete, which is why this clear signature of climate change wasn’t previously captured,” added Li.

At present, the researchers have planned to analyze the hurricane data collected from other parts of the globe to find out whether the effect of a warming climate on hurricane decay is taking place across the world.

Overall, the implications of this work are stark. If we don’t curb global warming, landfalling hurricanes will continue to weaken more slowly. Their destruction will no longer be confined to coastal areas, causing higher levels of economic damage and costing more lives,” concluded Professor Chakraborty.

Journal Reference:

Li, L & Chakraborty, P (2020) Slower decay of landfalling hurricanes in a warming world. Nature. doi.org/10.1038/s41586-020-2867-7.

Source: https://www.oist.jp/

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