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Rare By Design · The Climate Translation

• 0:00
  If you plotted every tropical cyclone on a map of the world, a pattern would jump out
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  almost immediately.
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  The western Pacific is covered with storm tracks, so is the North Atlantic, the Eastern
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  Pacific, the Indian Ocean, and the waters around Australia.
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  But then you look at the South Atlantic.
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  Here there's almost nothing.
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  Since satellites began giving us reliable global coverage, only one storm there has ever
• 0:38
  reached hurricane strength.
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  It developed in an ocean basin that normally had several built-in defenses against hurricanes.
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  Water that's often too cool, harsh, upper level winds, and an atmosphere that does not
• 0:53
  regularly produce the kinds of disturbances that become hurricanes elsewhere.
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  So how did the storm manage to form?
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  And as climate change warms the South Atlantic, could storms like this become less rare in
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  the future?
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  Welcome back to the podcast.
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  I'm Dr. Mac, and this is The Climate Translation.
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  The stretch of ocean between South America and Africa, which we call the tropical and
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  subtropical South Atlantic, sits in the tropics, but it's almost empty of hurricanes.
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  When we think of the meteorological variables needed to help initiate hurricane formation,
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  however, the reasons aren't as mysterious.
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  Think of hurricane formation like baking a cake, cakes take a lot of ingredients to come
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  together.
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  You need more than just flour, you need eggs and sugar, heat, and the right amount of
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  time for everything to come together.
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  Hurricanes work the same way.
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  Warm ocean water is one important ingredient for tropical development, and parts of the
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  South Atlantic are often cooler than hurricane-producing regions farther north.
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  Along the eastern side of the basin, currents and coastal upwelling help bring relatively
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  cool water towards the surface.
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  The second variable, moisture, is abundant, but there has to be some kind of disturbance
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  to get thunderstorms organized in the first place, and because hurricanes rotate, they
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  generally need to be far enough away from the equator for the Earth's rotation to help
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  the circulation develop.
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  The North Atlantic also gets something the South Atlantic usually doesn't, a steady supply
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  of atmosphere disturbances.
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  During the Northern Hemisphere summer, a strong temperature contrast develops between
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  the blazing hot Sahara Desert and the cooler wetter regions farther south.
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  That helps to create a ribbon of winds called the African Easterly Jet.
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  Ripples develop along that flow and travel westward across Africa.
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  We call them African Easterly Waves, and when they emerge over the North Atlantic, some
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  are already containing clusters of thunderstorms and rotation.
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  Many never become anything more, but a large percentage of North Atlantic tropical storms
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  and hurricanes begin with one of these disturbances.
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  There's no equivalent assembly line operating over the South Atlantic.
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  The atmospheric circulation south of the equator is arranged differently, and the South
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  Atlantic usually lacks the broad zone of converging tropical winds and thunderstorms, so compared
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  with the North Atlantic, the South Atlantic gets far fewer opportunities to start building
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  tropical cyclones in the first place.
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  But there's another ingredient that's especially important.
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  The winds above the storm can't be fighting each other too much.
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  Meteorologists call this vertical wind shear.
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  Wind shear simply means the speed or direction of the wind changes as you move higher in
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  the atmosphere.
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  Imagine two children playing with blocks.
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  One child is trying to build a tower, but then the other keeps coming over to push the
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  tower over.
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  The first child might be able to get the bottom started, but with his companion poised
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  and ready to knock it over, once it gets too high, the child won't achieve too much height.
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  That's essentially what strong wind shear does to developing tropical cyclones.
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  Its thunderstorms may begin organizing around a area of low pressure near the surface, but
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  strong winds higher up can push those thunderstorms away from the center.
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  And the storm struggles to build vertically.
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  Across much of the South Atlantic, that shear is usually stronger than what tropical cyclones
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  like.
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  So the South Atlantic has many strikes against it.
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  Parts of the ocean are less favorable for tropical development.
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  The atmosphere rarely provides the organized disturbances that regularly seed hurricanes
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  that you see farther north.
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  And more importantly, the vertical wind shear is frequently strong enough that it tears
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  apart storms that try to organize.
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  That does not mean a hurricane is physically impossible here, but it does mean the necessary
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  ingredients almost never line up at the same place and the same time.
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  But in March of 2004, somehow they did.
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  A storm off the coast of Brazil found a narrow window where the South Atlantic stopped
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  saying, no.
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  And for the first time in the satellite era, meteorologists watched a true hurricane develop
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  there.
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  In March of 2004, a fairly ordinary area of low pressure developed off the southeastern
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  coast of Brazil.
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  And at first there wasn't anything tropical about it.
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  That's what meteorologists call an extra-tropical cyclone.
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  These are the same general family of low pressure systems that produce winter storms across
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  the United States.
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  They get much of their energy from the differences in temperature across the atmosphere.
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  Hurricanes work differently.
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  They develop a warm center and draw much of their energy from heat and moisture coming
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  from the ocean below.
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  But occasionally, a storm can change from one type to the other.
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  Meteorologists call this tropical transition, and that is what began happening off Brazil.
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  Normally the South Atlantic would have torn a developing tropical system apart, but the
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  atmosphere had temporarily rearranged itself.
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  A large blocking pattern had developed east of South America.
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  Now you can think of an atmospheric block like a traffic jam in the sky.
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  The normal west-to-east movement of systems slowed down and got redirected.
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  In this case, the block did two important things.
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  First, it stopped the original storm from being simply moved away into the open Atlantic.
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  Instead, the circulation eventually turned back toward Brazil.
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  Second, it created a large region where that normally strong vertical wind shear became
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  weak.
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  Remember our children building towers of blocks?
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  Well for once, the second child stopped knocking the tower over.
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  Thunderstorms were able to grow around the storm center and stay there.
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  And then something surprising happened.
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  The ocean underneath the storm was only around 25 degrees Celsius, that's about 77 degrees
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  Fahrenheit.
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  You may have heard the hurricanes require an ocean temperature of 26.5 Celsius or 80 degrees
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  Fahrenheit, but that's just a guideline, it's not a magic number.
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  What matters is not how warm the ocean is but how cold the atmosphere is above it.
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  In this storm's case, unusually cold air high in the atmosphere created a large temperature
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  difference between the warmer ocean surface.
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  That difference made the atmosphere unstable enough to allow air to rapidly rise and strong
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  thunderstorms to develop, even though the water was a little cooler than what we would normally
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  expect under a hurricane.
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  For several days, the storm changed.
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  Fronts disappeared, thunderstorms wrapped more evenly around the center, the core of
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  the storm became more warm, and satellite images began showing something no meteorologist
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  in history had ever confirmed in the South Atlantic, an eye.
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  By March 26, Caterina had reached Hurricane Strength.
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  Two days later, it moved ashore in the Brazilian state of Santa Caterina, with winds around 95
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  miles an hour.
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  And that name tells us how unexpected this was.
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  There is no South Atlantic hurricane list, there's never been one.
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  So the name Caterina was adopted from the Brazilian state, the storm struck.
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  For decades, meteorologists had understood why hurricanes almost never form in this
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  part of the world.
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  Caterina did not prove that understanding wrong, but it showed us that if the atmosphere
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  temporarily removes enough of the barriers, the South Atlantic is poised and physically
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  capable of producing one.
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  And that brings us to a climate question.
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  If climate change continues to warm our oceans and remove those barriers more often, does
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  storms like Caterina become less of an exception?
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  Given what we've discussed, the tempting answer to that question is yes.
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  After all, hurricanes need warm water.
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  Climate change is warming the ocean.
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  Therefore, that could lead to more hurricanes.
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  But the atmosphere is rarely that simple.
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  Remember that cake we talked about?
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  Climate change is not just adding more flour, it's changing the other ingredients at the
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  same time.
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  Scientists have actually tested the idea using another South Atlantic storm.
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  In March of 2010, a subtropical cyclone named Anita formed off the coast of Brazil.
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  Now it never became a hurricane, but it came close enough to developing a fully tropical
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  structure that researchers wondered what would have happened if the ocean below had been
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  just slightly warmer.
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  In 2019, Brazilian scientists at the Federal University of Itajubá recreated Anita in
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  a computer model.
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  First they simulated the storm using the conditions that actually existed in 2010.
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  Then they began turning up the ocean temperature.
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  When they made the sea surface just two degrees Celsius warmer, throughout Anita's entire life,
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  everything changed, in the model Anita completed to transition into a tropical cyclone.
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  But why?
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  In the model, the warmer ocean transferred just a little more heat and moisture into the
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  atmosphere.
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  That supported stronger thunderstorms around the center.
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  That caused the pressure to drop, the winds to strengthen, and the storm became increasingly
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  able to sustain itself using the energy from the ocean below.
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  So at first glance, that gives us an answer.
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  If you warm the South Atlantic enough, tropical cyclones can become easier to produce.
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  But there's a catch.
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  The experiment just changed one variable.
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  The climate change doesn't just warm the ocean, it warms the atmosphere, and changes other
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  elements as well.
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  So in 2021, another team at Niigata University in Japan ran a more complicated experiment.
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  They turned to Anita again, but this time they placed the storm into a simulated late
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  century climate based on our current climate change trajectories.
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  They changed the ocean temperature, but they also changed the temperature and the humidity
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  of the atmosphere, the air pressure patterns, and the winds based on projections from multiple
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  climate models.
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  The result was more complicated.
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  When researchers warmed only the ocean, Anita became stronger.
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  But when they also allowed the atmosphere above it to warm, some of that extra strengthening
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  disappeared.
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  It's because the hurricane didn't care just about the temperature of the ocean.
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  It also cared about that difference.
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  How cool it was above compared to how warm it was below.
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  That difference, that gradient, makes a huge change on whether a storm can develop.
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  Then you can change humidity, the wind shear, and changes in large-scale weather patterns.
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  Each variable has a different effect.
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  Some changes help a storm organize, others may work against it.
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  But even so, the full future climate simulations still suggested the South Atlantic sub-tropical
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  cyclones could become stronger and have an easier time making that transition into tropical
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  systems.
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  But it's not the same as predicting that the South Atlantic will suddenly become another
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  Caribbean.
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  We are studying a region where hurricanes are so rare, we have almost no real-world examples
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  to compare against.
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  And a computer experiment based on one storm can't tell you exactly how other future hurricanes
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  will form.
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  But it is telling us something more interesting.
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  One of the barriers protecting the South Atlantic is changing.
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  Because of human-induced climate change, the oceans are getting warmer.
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  Whether that's enough to overcome the other barriers will depend on what's happening
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  in the atmosphere above it.
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  And that's why the future of hurricanes in this almost hurricane-free ocean remains an
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  open question.
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  Caterina remains the only hurricane ever confirmed in the South Atlantic during the satellite
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  era.
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  But it's not the last tropical cyclone.
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  As mentioned in 2010, tropical storm Anita developed off Brazil, then came tropical storm
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  Iba in 2019.
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  And in February 2024, tropical storm Akará formed several hundred miles off the Brazilian coast.
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  Akará began along an old weather front, gradually developed a warmer center, and eventually
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  became fully tropical.
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  But it never became strong.
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  Once again, the South Atlantic had produced something that, not very long ago, many meteorologists
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  thought might be virtually impossible.
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  So are we watching the birth of a new hurricane basin?
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  There's no evidence for it right now.
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  And this is where science becomes more complicated than the simple idea that warmer water or changes
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  in different elements can produce more hurricanes.
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  As we mentioned, climate models generally agree on some things about tropical cyclones
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  in a warmer world.
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  On average, the storms that do form are expected to become somewhat more intense.
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  And they're expected to produce heavier rainfall because a warmer atmosphere can hold
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  more water vapor, and a greater percentage of tropical cyclones are expected to reach
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  the strongest categories.
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  But scientists have much less confidence that the total number of tropical cyclones around
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  the world will increase.
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  Many computer models are actually producing fewer storms overall.
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  And the South Atlantic is even harder to predict because there are so few tropical cyclones
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  there to begin with.
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  We've modeled and examined how cyclones might change later in the century.
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  Researchers have expected that a warmer climate might produce more storms, but we're not seeing
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  it just yet.
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  Models produce roughly the same number of tropical transitions in the future as they do in the
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  present-day climate, and that's an important result.
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  It reminds us that a warming ocean doesn't automatically remove all the other things that
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  the South Atlantic has in its defense.
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  The wind shear is still maybe too strong.
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  The atmosphere may still fail to provide enough disturbances, and the large-scale circulations
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  may prevent storms from organizing.
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  But remember, what we learned from the Anita experiments.
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  When the atmosphere's setup does cooperate, warmer water can help develop storms and
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  get them access to more energy.
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  So the future may not be one where Brazil suddenly experiences a hurricane season resembling
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  Florida or the Caribbean.
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  Instead the more realistic concern may be what happens during those rare occasions when
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  all the ingredients do line up.
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  A storm that manages to form in a warmer ocean may have the potential to become stronger
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  or produce heavier rainfall than similar storms in the past.
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  And there's another change that doesn't depend on whether the South Atlantic hurricanes
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  become common.
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  Sea level is rising.
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  Storm surge works by pushing ocean water towards the coast.
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  So if the ocean begins from a higher level, the same storms can push water farther inland.
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  So even if Catarina remains an extraordinary exception, a future Catarina-like storm would
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  occur along a coastline different from one that existed in 2004.
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  That might be the most important lesson from this nearly empty part of the hurricane map.
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  Climate change does not necessarily turn every unfavorable hurricane basin into a favorable
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  one.
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  It changes some of the ingredients, and scientists are still working out how those ingredients
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  will interact.
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  The South Atlantic may continue to be unfavorable for hurricane development in the future, but
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  Catarina showed us that there are exceptions to every rule.
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  The South Atlantic is still one of the least favorable places on Earth for hurricanes.
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  Most of the time, the ingredients simply don't come together.
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  But Hurricane Catarina proved that rare doesn't mean impossible.
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  As the ocean warms, one of those ingredients is changing.
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  And whether that's enough to make a future hurricane more common there remains uncertain.
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  But when the South Atlantic does say yes, the consequences may be greater than they
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  were before.
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  I'm Dr. Mac, and this has been The Climate Translation.
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  If you have a question about the climate you've been too afraid to ask, or if you have
• 19:54
  a differing opinion, I want to hear from you.
• 19:57
  I can use your viewpoints in future episodes.
• 20:00
  You can reach me at TheClimateTranslation at gmail.com.
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  I'll see you next time.