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The River Inside the Gulf · The Climate Translation

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In the summer of 1998, I spent a lot of time staring at the Gulf of Mexico.
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I was a television meteorologist in Corpus Christi, Texas, and within a few weeks we watched
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Tropical Storm Charlie come ashore near Port Aransas, followed in September by Tropical
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Storm Frances just north of the city.
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At the time, most of what I was showing viewers was happening at the surface, how warm was
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the water, where was the storm moving, how strong were the winds.
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But underneath that surface was another story.
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Today scientists are watching how a warming Gulf is changing the amount of heat that's
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stored there, how the heat can influence hurricanes and severe weather, and what happens
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when the water eventually leaves the Gulf and enters the Atlantic.
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Welcome back to the podcast, I'm Dr. Mac, and this is the Climate Translation.
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The Gulf of Mexico may look like a giant enclosed basin on a map, but it's not isolated.
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Warm tropical water from the Caribbean enters through the Yucatan channel, the narrow passage
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between Mexico's Yucatan Peninsula and Cuba.
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That water is already warm when it arrives, and it's moving.
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Once inside the Gulf, the current typically bends northward, before turning east towards
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Florida.
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It then squeezes through the Florida Straits between Florida and Cuba, where it becomes
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part of the Florida current and eventually feeds into the Gulf Stream that goes up the
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east coast of the United States.
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Because of that large looping path, meteorologists and oceanographers simply call it the Loop Current.
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But it's not a narrow river like the Mississippi, it's enormous.
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According to the National Oceanographic and Atmospheric Administration, or NOAA, the current can
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extend hundreds of miles into the Gulf and reach depths of nearly 2,600 feet.
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So, when we talk about a warm water in the loop current, we're not simply talking about
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a thin layer sitting at the surface.
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There can be a tremendous amount of heat stored below.
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And the shape of that current is constantly changing.
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Sometimes the loop current stays relatively close to the southern Gulf.
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At other times, it stretches north toward the central Gulf, occasionally even reaching
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areas south of Louisiana.
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As that loop becomes larger or more stretched out, something interesting happens.
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Part of it can become pinched off.
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Imagine stretching a loop of thick syrup flowing over your pancakes in the morning until part
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of it separates and forms its own circular blob.
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Ocean currents can behave the same way.
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When part of the current separates, it forms what oceanographers call a warm core eddy.
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These eddies can be hundreds of miles across.
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And once they break away, they slowly drift westward across the Gulf.
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And they carry warm water with them.
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They can survive for months.
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That means that the Gulf does not have one simple pool of warm water.
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Instead you can have the main loop current in one part of the basin and these giant spinning
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reservoirs of warm water moving through others.
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Oceanographers have been watching this system for decades using satellites and ships, drifting
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instruments, underwater floats and measurements from oil platforms.
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And one reason they watch it so carefully is that the loop current moves more than just
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heat.
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It transports salts, nutrients, marine organisms, even fresh water that enters the Gulf from
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rivers like the Mississippi.
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In other words, it helps to organize the physical and biological environment of the Gulf itself.
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But for meteorologists, there's another reason the current matters.
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The atmosphere is constantly exchanging energy with the ocean below it.
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Water evaporates, heat moves upward, moisture enters the atmosphere, and storms passing
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over the Gulf of Mexico can tap into that stored energy.
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That means two locations with the same surface temperature may not actually contain the
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same amount of usable heat.
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One might be warm only near the top.
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The other may sit above hundreds of feet of unusual warm water.
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And that difference becomes important when we start talking about what's been happening
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to the Gulf over the last several decades.
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Because the river inside the Gulf is still flowing, but the water around it is getting warmer.
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Now that we know how the loop current moves to the Gulf of Mexico, we can look at what
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has actually been changing.
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And this is where the story becomes less about the current itself and more about the water
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it's carrying.
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In 2023, NOAA scientists Zhankun Wang and his colleagues Tim Boyer, James Reagan, and Patrick
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Hogan, published a detailed study of the Gulf of Mexico warming and the Journal of Climate.
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Their team analyzed nearly 193,000 temperature profiles that had been collected between
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1950 and 2020.
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The measurements came from ships and floats and other instruments that recorded temperature
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not just at the surface, but deep in the water column.
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What they found was clear, between 1970 and 2020, the average sea surface temperature
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in the Gulf increased by one degree Celsius or 1.8 degrees Fahrenheit.
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That is a warming rate about twice as fast as the global ocean at the surface.
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But the surface temperature is only part of the story.
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The researchers found warming at nearly every depth they examined, and about 80 to 90%
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of that extra heat was stored in the upper 2,000 meters or 6,000 feet of the Gulf of Mexico.
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That's over a mile.
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And it was concentrated in the upper 1,000 meters or 3,000 feet.
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And that means the Gulf is not simply developing just a warmer skin.
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It's storing heat energy, deep below the surface.
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Imagine two swimming pools in a summer afternoon.
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Both have a surface temperature of about 86 degrees.
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One pool is a few feet deep, the other is 50 feet.
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All the water is warm.
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The thermometer at the surface tells you that the pools are identical, but they clearly
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do not contain the same amount of heat.
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There's a lot more energy in the deeper pool than the shallow one.
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The Gulf works the same way.
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As sea surface temperature map may show you warm water everywhere, but beneath the loop
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current and some of its warm core eddies, the warmth extends hundreds of feet downward.
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And that makes them enormous reservoirs of stored energy.
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The NOAA researchers estimated that the loop current transports about 40 terawatts of
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heat through the Gulf.
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That number is difficult to picture, but the most important part is that the current
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is constantly moving tremendous amounts of energy, from the Caribbean toward the
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Florida Straits.
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And the evidence does not show that the current itself has simply started racing faster
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because of climate change.
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Its speed and shape naturally change from year to year.
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Its northward reach changes, and the timing of eddies changes.
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The variations have always happened, but what's changed more clearly is the background
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environment.
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The Gulf surrounding the current is warmer, the warm layer is deeper, and there is more
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heat energy available for the current and its eddies to redistribute.
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Fresh water can make the storage even more complicated.
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Rivers like the Mississippi pour relatively fresh water into the Gulf.
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Because fresh water is lighter than salty water, it can sit near the surface and make it
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harder for the ocean to mix vertically.
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Scientists call this stratification.
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A more strongly layered ocean can sometimes keep the heat trapped near the top while warm
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water remains stored below.
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So climate change is not necessarily turning the loop current into a fast energy-rich river.
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It's changing the reservoir that the river itself flows through.
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That matters because the atmosphere above the Gulf is constantly reaching down and interacting
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with that stored heat.
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Up to this point we've been looking down on the Gulf.
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Let's look up, because all of the stored energy does not stay in the ocean.
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The Gulf of Mexico is constantly exchanging heat and moisture with the atmosphere above
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it.
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And for a meteorologist, that makes the Gulf much more than a body of water.
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It's part of a weather system.
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We see the most obvious example with hurricanes.
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As I discussed in the episode Fueling the Storm, hurricanes draw energy from warm ocean
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water.
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But the loop current adds an important detail.
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If a hurricane crosses water that's warm only near the surface, it's winds can churn
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colder water upward and begin reducing its own fuel supply.
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If it crosses the loop current or one of those deep warm core eddies, that becomes harder.
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The storm may keep finding warm water even as it mixes the ocean below.
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It doesn't guarantee that the storm will rapidly intensify.
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Wind shear, dry air, and the structure of the storm all play a role.
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But the deep ocean heat can remove a natural brake to the system.
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The Gulf of Mexico also affects storms much farther inland.
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In my year storm chasing across the Great Plains, we spent enormous amounts of time watching
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moisture.
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Where was the dry line?
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How high were the dew points?
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How much instability was developing?
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And often the moisture feeding the thunderstorms had traveled hundreds of miles away from the
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Gulf of Mexico.
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That connection has been studied directly.
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In 2016, atmospheric scientists Maria Molina, Reed Timmer, and John Allen published research
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in the geophysical research letters examining the relationship between the conditions in
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the Gulf and severe thunderstorms across the U.S.
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They found that during the spring, warmer Gulf of Mexico temperatures were associated with
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more hail and tornado activity across parts of the southern U.S.
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The reason was not that the warm Gulf water somehow created a tornado hundreds of miles
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away.
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The connection worked through the atmosphere.
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A warmer Gulf helped supply more low-level moisture.
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That moisture increased what meteorologists call specific humidity near the surface.
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And when the warm, humid air moved north, beneath colder air from a loft, the atmosphere
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became more unstable.
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During those warmer Gulf conditions, the researchers found higher levels of mixed-layer CAPE,
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convective available potential energy.
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It's the energy available to rising thunderstorms that allows them to explode with intensity.
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Even when they were moved years, strongly influenced by El Niño or La Niña, the relationship
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remained.
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This is important because it tells you the Gulf is acting as one piece of a larger, severe
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weather puzzle.
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You still need wind shear and lift, temperature stratification, and the right atmospheric pattern.
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But the Gulf helped supply the moisture and the energy storms work with.
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So when we talk about a warmer Gulf and deeper ocean heat energy, the effects don't stop
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at the shoreline.
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That energy moves upward into hurricanes.
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The moisture moves north into thunderstorms.
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And some of the water itself eventually leaves the Gulf all together.
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Because the loop current doesn't end near Florida.
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It keeps going.
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Eventually, water flowing through the loop current exits the Gulf.
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It turns south along Florida, passes through the Florida Keys and Cuba, and squeezes through
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the narrow Florida straits.
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At that point, oceanographers call it the Florida current.
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And it's not a small amount of water.
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As 1982, NOAA's Atlantic Oceanographic and Meteorological Laboratory has monitored
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the Florida current using ships and satellites and something surprisingly simple, a submarine
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telecommunications cable that stretches between Florida and the Bahamas.
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Because sea water conducts electricity, water moving through the Earth's magnetic field
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produces a tiny signal in the cable.
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And scientists use that signal to estimate how much water is flowing past.
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On average, the Florida current transports about 32 million cubic meters of water every
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second.
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Once it clears Florida, it moves north along the east coast as part of the Gulf Stream.
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And this is where the Gulf of Mexico becomes connected to the much larger climate system.
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The Gulf Stream carries warm tropical water northward along the United States before turning
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east into the North Atlantic.
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That transport helps move heat energy away from the tropics.
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It affects ocean temperatures, coastal sea level, marine ecosystems, and even the atmosphere
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above it.
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The sharp temperature difference between the warm Gulf Stream and the colder waters to the
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North helps to organize where storms develop and strengthen over the North Atlantic.
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But this is where we have to be careful.
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You may have heard that climate change is weakening the Gulf Stream, or that the Gulf Stream
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may collapse.
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The statements often mix together several parts of the ocean.
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The Gulf Stream is connected to the Atlantic Meridianal Overturning Circulation, or the
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AMOC, which we have discussed before on this podcast.
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But they're not the same thing.
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In fact, recent measurements have produced an interesting result.
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In 2024, a team led by Noah Oceanographer Dennis Volkov re-examined more than four
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decades of measurements from that submarine cable between Florida and the Bahamas.
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After correcting for changes in the Earth's magnetic field, they found that the Florida
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current had remained remarkably stable from 1982 through 2023.
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Then in August of 2026, another Noah team published a study in the geophysical research letters
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examining the Gulf Stream farther north after it separates from the coast.
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Using 31 years of satellite measurements, they found that this free flowing section of
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the Gulf actually became stronger beginning around 2014.
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It also shifted northward and became less meandering.
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The reason is not simply that the Gulf water is pushing it faster.
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The researchers connected much of that change to large-scale wind patterns over the North
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Atlantic that strengthened the subtropical ocean circulation.
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As you can tell, this is an important reminder of how complicated this system is.
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The Gulf is warming.
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The loop current is moving more heat energy.
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The Florida current has remained relatively stable.
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And farther downstream, part of the Gulf stream has actually strengthened because of changes
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of winds over the Atlantic.
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And there's another change happening even before that water leaves the Gulf.
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A 2024 study found that the sea level across the Gulf rose about 4.8mm, that's less than
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a quarter of an inch, per year between 1993 and 2020, faster than the global ocean over
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the same period.
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Much of that recent acceleration was connected to warm water that expands and to heat carried
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around the basin by the loop current eddies.
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So the changes occurring inside the Gulf don't stay hidden beneath their surface.
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They influence storms above it, the sea level around it, and eventually the enormous
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system of currents beyond it.
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The river inside the Gulf of Mexico eventually leaves home, but all that heat energy it
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carries keeps traveling.
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Today's technology allows us to monitor bodies of water like the Gulf of Mexico in unprecedented
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detail.
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We can track heat hundreds of feet below the water.
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We can watch the energy move through the loop current and warm core eddies and through
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the Florida Straight and into the Atlantic.
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And what we're finding is the Gulf of Mexico is not simply getting warmer, it's storing
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much more heat energy and redistributing that energy through connected system of currents.
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As we monitor the effects of climate change, that's an important variable to consider for
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tropical weather.
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For severe weather farther inland, for sea level along the Gulf coast and for the larger
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ocean circulation beyond Florida, the river inside the Gulf of Mexico is still flowing,
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but the warm water it's carrying is changing.
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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
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a differing opinion, I want to hear from you.
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I can use your viewpoints in future episodes.
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You can reach me at the climate translation at gmail.com.
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I'll see you next time.