It's going to be another rough season
By Roberto Molar Candanosa, NASA's Earth Science News Team
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| The map above shows sea surface temperature anomalies on July 14, 2020 indicating how much the water was above or below the long-term average (2003-2014) temperature on that same day. Credits: NASA's Earth Observatory |
The 2022 Atlantic hurricane season has officially started, and NASA scientists are working with partners at NOAA, FEMA and other organizations to help communities prepare for these storms and respond to their aftermath. To gain a better understanding of how hurricanes are intensifying and becoming stronger in the face of climate change, NASA is developing technology and missions to study our home planet as a complex, dynamic system.
But
what’s the recipe for hurricane formation, and how is the behavior of these
storms changing as Earth’s long-term warming trend
continues? NASA research answers these questions to help you understand how
excess heat in Earth’s oceans is changing the way hurricanes form and evolve.
1. What’s the recipe for a hurricane?
The
general name for “hurricanes” is tropical cyclones. Outside the United States,
people also call these storms typhoons or simply cyclones. In short, tropical
cyclones are storms with winds swirling rapidly around a center of warm air.
Hurricanes depend on four main ingredients to form.
First, they need heat or energy stored in the upper layer of the ocean. This ocean heat content powers a storm similar to how fuel powers an engine.
Second, they need high humidity in the air, achieved by evaporation of ocean waters above 79°F (26°C). As this humid air rises and interacts with cooler air above, it creates and grows increasingly larger clouds and thunderstorms.
Third, hurricanes need favorable winds. At different heights in the atmosphere, these winds need to be weak enough to avoid ripping the storm apart.
Fourth, hurricanes need background rotation to
organize scattered thunderstorms into one larger storm that spins increasingly
faster into a cyclone’s characteristic spiral form. Some of this spin comes
from Earth’s own spin as it rotates around its axis.
2. How does climate change interact with a hurricane’s ingredients?
Ocean
heat, air humidity, wind — all these ingredients factor into hurricane
formation. And all are affected by climate change.
The
ocean has absorbed 90% of the warming that has
occurred in recent decades due to increasing greenhouse gasses,
and the top few meters store as much heat as Earth's entire atmosphere. But the
way winds interact with this heat or energy in the ocean also plays an
important role in the fate of a storm, explained Scott Braun, a research
meteorologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
“If
you get a storm moving over a shallow layer of warm water, the stronger winds
can mix up the ocean enough that it pulls up that deeper, colder water,
and reduces the energy available for the storm,” Braun said. “However, if the
depth of the warm layer is fairly large, the storm can't really tap into that
cooler water, so there's less chance of that cooler water acting to weaken the
storm.”
Changes
in wind speed and direction at different heights, called vertical wind shear,
can make or break a hurricane. It can impede storm formation by dispersing heat
and moisture. If it’s strong, it can also break apart an existing storm by
blowing its top away from its bottom.
“Let's
say you've got an environment where at low levels, the winds are out of the
east at five miles an hour, and at the top, they're out of the west at five
miles an hour,” Braun said. “You've got winds trying to move the storm in different
directions as a function of height, and that tends to tilt over the storm and
potentially rip it apart.”
In
future climate projections of hurricane behavior, wind shear is the biggest
wildcard, as it may weaken or strengthen storms in different regions. Research
suggests some areas of the world might get stronger winds than others as a
result of global warming, but it is uncertain how that will play out.
3. Does climate change mean more hurricanes each
season?
No.
Even though a warm ocean is a key ingredient for hurricane formation, research
shows this warming has not significantly influenced the number of Atlantic
hurricanes that form each year.
However,
because projections suggest that as the ocean warms the atmosphere will also
hold more moisture to form clouds and feed storms, scientists are anticipating
stronger and wetter hurricanes in coming years. That is, although oceanic
warming isn’t yet increasing the frequency of hurricanes, this excess heat
appears to be affecting different characteristics of hurricanes that do form.
This
is why Dr. Mayra Oyola-Merced, an atmospheric physicist at NASA’s Jet
Propulsion Laboratory in Southern California, encourages people living in
places constantly exposed to hurricanes to follow guidance from local
authorities before a storm hits.
“We
know that in terms of a warming climate with increasing sea surface
temperatures, we have a higher probability of hurricanes in the extreme portion
of the scale,” Oyola-Merced said. “Sometimes even though it's a Category 1
storm, you can get a lot of damaging rain, winds and storm surges that can put
lives and property in danger.”
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| The map above shows the tracks of all 30 Atlantic storms in 2020, highlighting a few of the named storms. Three of them—Eta, Iota, and Delta—saw their winds intensify by at least 80 miles (130 kilometers) per hour in 24 hours. Credits: NASA's Earth Observatory |
4. How is climate change affecting hurricane intensity?
Research
suggests that with rising temperatures, it will be more likely that storms will
undergo rapid
intensification, which is defined as an increase in hurricane wind
speeds by 35 mph (or about 56 kilometers per hour) within 24 hours. These quick
changes can leave local authorities and communities in the path of a hurricane
without time to prepare.
Already,
rapid intensification appears to be one of the major influences of a warming
ocean on hurricane behavior. While rapidly intensifying storms occurred less
frequently in past decades, the Atlantic is now producing about one of these
storms per season, Oyola-Merced said. And as this phenomenon has happened more
often in recent decades, scientists are just beginning to understand how to
predict it.
“The
reason we don't know much about that is because until around 2005, this was a
very rare thing to happen,” Oyola-Merced said. “This gets tricky because if you
have a forecast when something like this happens, and the model is not able to
capture this rapid intensification, it means that you have the wrong
information, and you're giving the public the wrong information.”
5. Is climate change slowing down hurricanes?
Yes.
In recent decades, hurricanes have been stalling more as they approach coasts,
dropping more rainfall over confined locations. Research shows Atlantic
hurricanes are experiencing a reduction of roughly 17% in forward motion speeds
than in previous decades, which translates into an increase of about 25% in
rainfall, Oyola-Merced explained.
“The
longer a system spends over the ocean getting warm by now this extremely heated
water, you're also going to increase its rainfall,” Oyola-Merced said. “When
you have land that's already saturated, where it’s constantly raining, and then
you come with another storm that's packed with precipitation, it's a recipe for
disaster.”
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| Natural color image of Hurricane Iota in the Gulf of Mexico from NOAA's GOES-16 geostationary satellite. Credits: NOAA/NASA Earth Observatory |
With data from more than 20 satellites, NASA plays a foundational role in hurricane science. Before, during, and after a hurricane strikes, NASA satellites are in a prime position to identify impacts and help communities prepare, respond, and recover. With tools like the Disasters Mapping Portal, the agency supports regional governments and disaster management agencies.
When
it comes to operational forecasting, NASA’s main role is through its crucial
partnership with NOAA. NASA designs, builds, and launches NOAA’s suite of
satellites that provide the data that specifically feed numerical weather
prediction models.
