OCEANOGRAPHER LISA BEAL HAS LED A MULTITUDE OF VOYAGES TO THE INDIAN OCEAN, BRAVING SOME OF THE WORLD’S MOST MONSTROUS SWELLS.

It is there that Beal has deployed hundreds of ocean-monitoring instruments to learn more about how the Agulhas Current, one of the swiftest and strongest in the world, influences climate patterns around the world.

Denis Volkov grew up in southern Russia along the coast of the tiny and shallow Sea of Azov and on the Baltic Sea coast in Estonia. He always knew he wanted to study the oceans, and now he focuses his research on ocean circulation and sea levels.

William Johns researches the intricate relationship between the oceans and the atmosphere above it and how this interaction is impacting the global climate.

And Paloma Cartwright, after surviving a devastating hurricane in 2015 that destroyed her home in the Bahamas, is now a doctoral student devoted to a career in ocean science. She represents the next generation of researchers and scientists passionate about the oceans.

These people are part of the dedicated group of oceanographers and students at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science who delve into the dynamics of ocean currents, helping to deepen our understanding of the vital role they play in shaping the global climate system.

In constant motion, the oceans cover more than 70 percent of the Earth’s surface, yet as much as 80 percent of the ocean’s depths remain unexplored. Answers to critical questions on everything from sea level rise and marine heatwaves to the future of life in the ocean could be found in ocean currents.

The Lifeblood of Our Planet

Some travel only short distances. Others encircle the globe. But whatever path they take, ocean currents—like the 60,000 miles of arteries and veins that run throughout the human body—are the lifeblood of the planet. And in that regard, the Atlantic Meridional Overturning Circulation, or AMOC, is the aorta of the sea.

A complex system of currents, it is a global conveyor belt, distributing heat throughout the Atlantic by carrying warmer waters north and cooler waters south. “For the Earth’s climate to remain in equilibrium, there has to be a huge transport of heat from low to high latitudes by the combined atmospheric and oceanic circulations,” says Johns, a professor of ocean sciences, who uses long-term moored instrumentation to study ocean circulation. “In the northern hemisphere, the AMOC accounts for nearly 25 percent of that heat transport on a global basis. It is unique in the global oceans because the Atlantic is the only place where warm surface waters move northward all the way from the tropics to polar latitudes and are cooled and sink to great depths.” Those deep waters, he explains, then move southward underneath the warm layer, forming a meridional “overturning” circulation that scientists call the AMOC.

Its collapse would usher in a series of catastrophic climatic changes, but not quite on the timescale portrayed in the motion picture “The Day after Tomorrow.” The changes, explains Johns, would likely occur over many years to decades rather than just a few weeks. “The ocean has a lot of heat stored in it and that helps to buffer the climate system,” he says. “However, the impacts of an AMOC collapse or major slowdown are widely known and are generally robust outcomes of climate projection models.”

Those models project significant cooling in the North Atlantic and across the entire Northern Hemisphere as well as a pooling of heat in the tropical and South Atlantic, leading to more intense tropical storms, according to Johns. “There would also be major shifts in global precipitation patterns, which could create severe drought conditions in certain areas and devastating flooding in others,” he points out. “One of the most immediate and worrisome impacts would be a substantial sea level rise, of up to a foot or more, along the U.S. East Coast if the AMOC were to suddenly collapse.”

Over the past few years, Johns has participated in two major AMOC-related research endeavors. In the Rapid Climate Change-Meridional Overturning Circulation and Heat Flux Array (RAPID-MOCHA) project and the Overturning in the Subpolar North Atlantic Program, or OSNAP, he collaborates with teams of scientists from around the world, deploying deep-ocean moorings that monitor the strength of the AMOC.

“In RAPID-MOCHA, we have observed a general decline of the AMOC over the nearly 20 years we have been making those measurements,” he says, “but we are not certain yet how much of that is related to global warming versus natural variability on decadal time scales.”

Still, the future of the AMOC remains a concern, as both global warming and one of its major consequences—the increase in sea and land ice melting in the Arctic—will tend to slow it down, leading to an eventual tipping point, Johns warns.

Held in the Sea’s ‘Net of Wonder’

Long ago, the sea cast its spell on Volkov when he was a little boy growing up and going to school in southern Russia and Estonia, with the Sea of Azov and the Baltic Sea beckoning to him each day. And ever since, as legendary oceanographer Jacques Cousteau once said, he’s been held “in its net of wonder.”

“That’s why I pursued a career in earth sciences; I had absolutely no doubt that I wanted to become an oceanographer,” says Volkov, a physical oceanographer at the Rosenstiel School-based Cooperative Institute for Marine and Atmospheric Studies (CIMAS).

Volkov specifically focuses on regional sea level and ocean circulation changes. With Johns, fellow CIMAS scientist Marlos Goes, and others, he recently led a study that revealed that AMOC-induced changes in gyre-scale heat content, superimposed on global mean sea level rise, are already influencing the frequency of floods along the U.S. southeastern seaboard. Specifically, the investigators found that ocean heat convergence, being the primary driver for interannual sea level changes in the subtropical North Atlantic, accounted for 30 percent to 50 percent of the flood days from 2015–20.

He serves as the principal investigator of the National Oceanic and Atmospheric Administration (NOAA) Western Boundary Time Series, monitoring the volume transport and seawater properties of the western boundary currents in the subtropical North Atlantic. Those currents include the northward flowing Florida Current, which is part of the Gulf Stream as it passes through the Straits of Florida from the southernmost Florida Keys to the northernmost Bahamas Islands, the near-surface northward Antilles Current, and the southward Deep Western Boundary Current. The latter two are both found to the east of the Bahamas.

While his work often makes use of spaceborne observations such as satellite altimetry, Volkov also is a seagoing oceanographer who has sailed across four oceans. And he has encountered a multitude of challenges—but none quite like that of obtaining clearance for marine scientific research.

“Research ships often have to cross multiple exclusive economic zones (EEZs); and for each zone, we have to obtain permission to do any type of operation associated with measurements and sampling,” Volkov says.

During a 2018 cruise to the western part of the Indian Ocean—the first in 20 years to that region—Volkov and a team of other researchers aboard the NOAA ship Ronald H. Brown had to cross six EEZs. One of them, Tromelin Island, is a territory claimed by both France and Mauritius, which required the scientists to apply for two clearances to work near the island.

“While we had successfully obtained marine scientific research clearance from France, we were getting more and more anxious about not having a permit from Mauritius,” Volkov recalls. “We finally received it when we were only a day away from entering the Tromelin EEZ. But the situation became much worse with getting clearance from the [Indian Ocean island state] of Seychelles. By the time we reached the boundary between the Mauritius and Seychelles EEZs, the clearance was not issued. Whatever the bureaucratic reasons were, we were just stranded. And the situation was complicated by the fact that our Mauritius clearance was expiring,” he adds. “So, all science operations had to cease. We were just anchored at the boundary between two EEZs and already thinking of alternative routes. Fortunately, we finally received the clearance, but the delay cost us one full day at sea.”

Studying the Agulhas

From braving rogue waves to taking her turn on lonely night watches aboard ships, Beal can speak better than anyone on the challenges long scientific cruises can present, such as the 5,600-nautical-mile voyage from Africa to Australia that she took in 2003 aboard the RSS Charles Darwin. That cruise, which lasted for 47 days, was part of the Global Ocean Ship-based Hydrographic Investigations Program, or GO-SHIP, an international initiative to sustainably survey the ocean’s interior. It’s these taxing endeavors that test a researcher’s mettle.

“I stayed sane by breaking out some Michael Jackson moves on the flying bridge, listening on my fancy, new iPod,” she recalls of how she coped during the voyage.

“It’s this odd other existence away from your typical life, and it’s not for everybody,” explains the professor of ocean sciences at the Rosenstiel School. “You’re thrown together with other researchers and crew members you don’t know. So, you end up lacking the support of family and friends.”

Beal is an expert on the Agulhas system of currents. And through her many scientific voyages and publications, she has brought worldwide attention to the key role this remote system plays in a warming climate.

One of the swiftest and strongest in the world, the Agulhas is a western boundary current, flowing along the east coast of South Africa and transporting warm, salty water toward the Southern Ocean. “It’s the Indian Ocean’s version of the Gulf Stream, but it’s a bit of an enigma,” Beal says.

She has studied the Agulhas since her graduate school days at the University of Southampton in the United Kingdom, deploying to the Indian Ocean on a multitude of expeditions that lasted for weeks at sea. As chief scientist on many of those cruises, Beal quantified how much water, heat, and salt the Agulhas carries as an artery of Earth’s climate system.

Her Agulhas Current Time (ACT) Series of research cruises, funded by the National Science Foundation, produced some of the most captivating, and surprising, scientific data on the current. Combining shipboard measurements with more than 20 years of satellite data, she and her team discovered that the current has broadened, not strengthened, since the early 1990s. “We weren’t expecting that,” Beal reveals. “Based on observations from space, we’ve seen that these regions are warming at three times the rate of the rest of the world’s oceans. We also understand from atmospheric scientists that the world’s wind systems are intensifying and expanding poleward with climate change. So, we expected the Agulhas was intensifying over time. But that just isn’t the case. It appears to have broadened, largely because there are now more eddies. So, the current is more turbulent.”

On a later joint scientific mission with South African scientists, Beal investigated the heat carried by the Agulhas. The current dominates the heat budget of the Indian Ocean, impacting sea level rise, sea surface temperature, East African rainfall, and storm tracks. Some of the current’s waters even leak directly into the Atlantic Ocean. Combining moored measurements with a cluster of robotic instruments that drift throughout the ocean, Beal and her team were able to estimate for the first time how the heat transport of the Indian Ocean varies over time.

The role our oceans play in shaping global climate is as yet underestimated, according to Beal. “In terms of the global carbon cycle, our oceans have huge reserves of heat and carbon that they’re able to cycle back and forth with the atmosphere,” she says. “When and where will this heat and carbon be given up to the atmosphere? And how will those feedbacks from the ocean to the atmosphere affect the future of our climate? Those are the questions we must answer.”

The current has broadened, not strengthened, since the early 1990s.

We weren’t expecting that.

An Inseparable Link

You might say they are joined at the hip, inseparably linked. The ocean and atmosphere are a coupled system, playing important roles in climate variability and climate change. But does one play a bigger role than the other?

“The ocean is often thought of as being the long-term ‘memory’ of the system because of the large heat capacity of water compared to air. It can store vast amounts of heat for prolonged periods of time that can then feed back to the atmosphere, sometimes after traveling long distances in the ocean circulation,” explains Johns.

Ninety percent of the excess heat in the Earth system due to global warming has been absorbed by the oceans, he notes. “The atmosphere is just not able to store much heat, even though that is what we feel as humans when we talk about climate change. The atmosphere has many modes of short-term climate variability that are not strongly coupled to the oceans, but on longer time scales, the ocean’s role is critically important.”

As the ocean and atmosphere are a coupled system, to say which plays a bigger role is difficult according to Beal. “To follow the climate, you need to follow the carbon and the heat. About half the excess carbon and 95 percent of the excess heat from anthropogenic climate change goes into the ocean,” she points out. “The atmosphere is well-mixed—our carbon emissions are everyone’s carbon emissions, mixed around the globe within a few days. But the ocean takes hundreds to thousands of years to mix and adjust, so it holds a lot of memory about what the atmosphere was like many years ago and eventually feeds it back on the present-day atmosphere. That is a long feedback loop.”

The cryosphere—those portions of Earth’s surface where water is in solid form such as sea ice—has an even longer memory of climate, Beal notes. “The Antarctic ice sheet is millions of years old,” she says. “But in short, asking which is more important—the ocean or the atmosphere—is like asking whether the chicken or the egg is more important.”

Ben Kirtman, a professor of atmospheric sciences and the William R. Middelthon III Endowed Chair of Earth Sciences at the Rosenstiel School, calls ocean-atmosphere coupling a complex phenomenon. When it comes to which system is forcing which, “it can depend on location and all kinds of things,” he says. It critically depends on timescale and location and just where you are in the world,” he adds.

“In the Gulf Stream, the ocean is transporting huge amounts of warm subtropical and tropical water poleward, so the Gulf Stream is really warm and it’s really strongly forcing the atmosphere, even affecting where it rains,” Kirtman continues. “On the other hand, the North Atlantic subtropical high is this circulation that kind of steers our hurricanes. When that circulation retracts or expands, it affects where the Gulf Stream sits. So, the which is forcing which gets tricky.”

El Niño, the climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean, is the perfect example of the importance of coupled interactions, Kirtman points out.

At any rate, the nature of ocean-atmosphere coupling has given rise to collaborations between oceanographers and atmospheric scientists. Using atmospheric models and observations, Johns, for example, has collaborated with atmospheric scientists involved in estimating the heat exchange between the atmosphere and ocean through the sea surface.

“When we try to keep track of ocean heat content changes, we need to know how they were generated, how they move in the ocean circulation, and how they may be further affected by air-sea interaction along their pathways,” he says. “We do this by combining our ocean-heat transport measurements with estimates of atmosphere-ocean surface heat fluxes as well as estimates of regional ocean heat content derived from in-situ ocean observations.”

And while the ocean and atmospheric scientists examine these complex systems, they are sharing knowledge and mentoring the next generation of researchers at the Rosenstiel School.

Tomorrow’s Ocean Scientists Earn Their Stripes at Sea

In 2015, doctoral candidate Paloma Cartwright witnessed firsthand the power of a wind-sea interaction when Hurricane Joaquin, supercharged by warm ocean waters, swept through her home in Long Island, Bahamas, destroying nearly everything she owned. But the cyclone didn’t break her will to learn.

“I was old enough at 15 to understand the repercussions of what had happened and to realize that this was climate change in action,” says Cartwright, who is doing her graduate work in Beal’s lab. “I knew from that experience that I wanted to devote who I was to studying climate change and its consequences.”

For Cartwright, that meant studying one of the biggest influences on climate: the ocean. She is one of several students who have put out to sea on extended research expeditions, working alongside Rosenstiel School investigators to deploy special instruments to better understand the role the ocean plays in shaping weather and climate, including rising sea levels and sunny-day flooding in Miami and other coastal communities.

They are treated as equals, not underlings.

Samantha Medina, a third-year Ph.D. student in ocean sciences, has participated in a series of cruises as part of the $6.74 million Coastal Land-Air-Sea Interaction Experiment that will help the U.S. Navy improve its high-resolution weather forecast model—the Coupled Ocean/Atmosphere Mesoscale Prediction System. In the waters of Monterey Bay, California, and Pensacola, Florida, Medina helped assemble, deploy, and recover 40-foot- long buoys that collect critical information on near-shore wind and wave conditions.

“This project has cemented my passion for ocean sciences,” Medina says of the study, which is led by Brian Haus, professor and chair of ocean sciences. “I’m planning to use the data collected from instruments used in this project to help improve parameterizations for forecast models as well as improve our understanding of waves in coastal areas.”

Rachel Sampson, a doctoral candidate in meteorology and physical oceanography, spent 26 days in a region of the Southeastern Atlantic Ocean called the Cape Cauldron, deploying equipment that will measure the dynamic mixing of Agulhas Current waters into the Atlantic to learn more about weather and climate.

The expedition, led by a trio of female scientists, which included Beal, inspired Sampson. She calls it “an unmatched opportunity for my still-fledgling career.”

I wanted to devote who I was to studying climate change and its consequences.