Climate change causes land-falling hurricanes to stay stronger for longer
By Dani Ellenby
Okinawa Institute of Science and Technology
Graduate University
Climate change is causing hurricanes that make landfall to take more time to weaken, reports a study published today in leading journal, Nature.
The
researchers showed that hurricanes that develop over warmer oceans carry more
moisture and therefore stay stronger for longer after hitting land. This means
that in the future, as the world continues to warm, hurricanes are more likely
to reach communities farther inland and be more destructive.
“The implications are very important, especially when considering policies that are put in place to cope with global warming,” said Professor Pinaki Chakraborty, senior author of the study and head of the Fluid Mechanics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST).
“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.”
Many
studies have shown that climate change can intensify hurricanes – known as
cyclones or typhoons in other regions of the world – over the open ocean. But
this is the first study to establish a clear link between a warming climate and
the smaller subset of hurricanes that have made landfall.
The
scientists analyzed North Atlantic hurricanes that made landfall over the past
half a century. They found that during the course of the first day after
landfall, hurricanes weakened almost twice as slowly now than they did 50 years
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,” said Lin Li, first author and PhD student in the OIST Fluid Mechanics Unit.
The scientists tested the link between warmer sea surface temperature and slower weakening past landfall by creating computer simulations of four different hurricanes and setting different temperatures for the surface of the sea.
Once
each virtual hurricane reached category 4 strength, the scientists simulated
landfall by cutting off the supply of moisture from beneath.
Li
explained: “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.
“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.”
The
researchers found that even though each simulated hurricane made landfall at
the same intensity, the ones that developed over warmer waters took more 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?” said
Prof. Chakraborty.
Using
additional simulations, the scientists found that “stored moisture” was the
missing link.
The
researchers explained that when hurricanes make landfall, even though they can
no longer access the ocean’s supply of moisture, they still carry a stock of
moisture that slowly depletes.
When
the scientists created virtual hurricanes that lacked this stored moisture
after hitting land, they found that the sea surface temperature no longer had
any impact on the rate of decay.
“This
shows that stored moisture is the key factor that gives each hurricane in the
simulation its own unique identity,” said Li. “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.”
The
increased level of stored moisture also makes hurricanes “wetter” – an outcome
already being felt as recent hurricanes have unleashed devastatingly high
volumes of rainfall on coastal and inland communities.
This
research highlights the importance for climate models to carefully account for
stored moisture when predicting the impact of warmer oceans on hurricanes.
The
study also pinpoints issues with the simple theoretical models widely used to
understand how 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,” said
Li.
The
researchers now plan to study hurricane data from other regions of the world to
determine whether the impact of a warming climate on hurricane decay is
occurring across the globe.
Prof.
Chakraborty concluded: “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.”