Tropical Pacific is major player in global ocean heat transport

Far from the vast, fixed bodies of water oceanographers thought they were a century ago, oceans today are known to be interconnected, highly influential agents in Earth’s climate system.

A major turning point in our understanding of ocean circulation came in the early 1980s, when research began to indicate that water flowed between remote regions, a concept later termed the “great ocean conveyor belt.”

The theory holds that warm, shallow water from the South Pacific flows to the Indian and Atlantic oceans, where, upon encountering frigid Arctic water, it cools and sinks to great depth. This cold water then cycles back to the Pacific, where it reheats and rises to the surface, beginning the cycle again.

This migration of water has long been thought to play a vital role in circulating warm water, and thus heat, around the globe. Without it, estimates put the average winter temperatures in Europe several degrees cooler.

However, recent research indicates that these global-scale seawater pathways may play less of a role in Earth’s heat budget than traditionally thought. Instead, one region may be doing most of the heavy lifting.

A paper published in April in Nature Geoscience by Gael Forget, a research scientist in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS) and a member of the Program in Atmospheres, Oceans,and Climate, and David Ferreira, an associate professor in the Department of Meteorology at the University of Reading (and former EAPS postdoc), found that global ocean heat transport is dominated by heat export from the tropical Pacific.

Using a state-of-the-art ocean circulation model with nearly complete global ocean data sets, the researchers demonstrated the overwhelming predominance of the tropical Pacific in distributing heat across the globe, from the equator to the poles. In particular, they found the region exports four times as much heat as is imported in the Atlantic and Arctic.

“We are not questioning the fact that there is a lot of water going from one basin into another,” says Forget. “What we're saying is, the net effect of these flows on heat transport is relatively small. This result indicates that the global conveyor belt may not be the most useful framework in which to understand global ocean heat transport.”

Updating ECCO

The study was performed using a modernized version of a global ocean circulation model called Estimating the Circulation and Climate of the Ocean (ECCO). ECCO is the brain child of Carl Wunsch, EAPS professor emeritus of physical oceanography, who envisioned its massive undertaking in the 1980s.

Today, ECCO is often considered the best record of ocean circulation to date. Recently, Forget has spearheaded extensive updates to ECCO, resulting in its fourth generation, which has since been adopted by NASA.

One of the major updates made under Forget’s leadership was the addition of the Arctic Ocean. Previous versions omitted the area due to a grid design that squeezed resolution at the poles. In the new version, however, the grid mimics the pattern of a volleyball, with six equally distributed grid areas covering the globe.

Forget and his collaborators also added in new data sets (on things like sea ice and geothermal heat fluxes) and refined the treatment of others. To do so, they took advantage of the advent of worldwide data collection efforts, like ARGO, which for 15 years has been deploying autonomous profiling floats across the globe to collect ocean temperature and salinity profiles.

“These are good examples of the kind of data sets that we need to inform this problem on a global scale,” say Forget. “They're also the kind of data sets that have allowed us to constrain crucial model parameters.”

Parameters, which represent events that occur on too small of a scale to be included in a model’s finite resolution, play an important role in how realistic the model’s results are (in other words, how closely its findings match up with what we see in the real world). One of many updates Forget made to ECOO involved the ability to adjust (within the model) parameters that represent mixing of the ocean on the small scale and mesoscale.

“By allowing the estimation system to adjust those parameters, we improved the fit to the data significantly,” says Forget.

The balancing act

With a new and improved foundational framework, Forget and Ferreira then sought to resolve another contentious issue: how to best measure and interpret ocean heat transport.

Ocean heat transport is calculated as both the product of seawater temperature and velocity and the exchange of heat between the ocean and the atmosphere. How to balance these events — the exchange of heat from the “source to sink” — requires sussing out which factors matter the most, and where.

Forget and Ferreira’s is the first framework that reconciles both the atmospheric and oceanic perspectives. Combining satellite data, which captures the intersection of the air and sea surface, with field data on what’s happening below the surface, the researchers created a three-dimensional representation of how heat transfers between the air, sea surface, and ocean columns.

Their results revealed a new perspective on ocean heat transport: that net ocean heat redistribution takes place primarily within oceanic basins rather than via the global seawater pathways that compose the great conveyor belt.

When the researchers removed internal ocean heat loops from the equation, they found that heat redistribution within the Pacific was the largest source of heat exchange. The region, they found, dominates the transfer of heat from the equator to the poles in both hemispheres.  

“We think this is a really important finding,” says Forget. “It clarifies a lot of things and, hopefully, puts us, as a community, on stronger footing in terms of better understanding ocean heat transport.”

Future implications

The findings have profound implications on how scientists may observe and monitor the ocean going forward, says Forget.

“The community that deals with ocean heat transport, on the ocean side, tends to focus a lot on the notion that there is a region of loss, and maybe overlooks a little bit how important the region of gain may be,” says Forget.

In practice, this has meant a focus on the North Atlantic and Arctic oceans, where heat is lost, and less focus on the tropical Pacific, where the ocean gains heat. These viewpoints often dictate priorities for funding and observational strategies, including where instruments are deployed.  

“Sometimes it’s a balance between putting a lot of measurements in one specific place, which can cost a lot of money, versus having a program that's really trying to cover a global effort,” says Forget. “Those two things sometimes compete with each other.”

In the article, Forget and Ferreira make the case that sustained observation of the global ocean as whole, not just at a few locations and gates separating ocean basins, is crucial to monitor and understand ocean heat transport.

Forget also acknowledges that the findings go against some established schools of thought, and is eager to continue research in the area and hear different perspectives.

“We are expecting to stimulate some debate, and I think it's going to be exciting to see,” says Forget. “If there is pushback, all the better.”

from MIT News - Oceanography and ocean engineering http://bit.ly/2Hj3YAy

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North Atlantic Ocean productivity has dropped 10 percent during Industrial era

Virtually all marine life depends on the productivity of phytoplankton — microscopic organisms that work tirelessly at the ocean’s surface to absorb the carbon dioxide that gets dissolved into the upper ocean from the atmosphere.

Through photosynthesis, these microbes break down carbon dioxide into oxygen, some of which ultimately gets released back to the atmosphere, and organic carbon, which they store until they themselves are consumed. This plankton-derived carbon fuels the rest of the marine food web, from the tiniest shrimp to giant sea turtles and humpback whales.

Now, scientists at MIT, Woods Hole Oceanographic Institution (WHOI), and elsewhere have found evidence that phytoplankton’s productivity is declining steadily in the North Atlantic, one of the world’s most productive marine basins.

In a paper appearing today in Nature, the researchers report that phytoplankton’s productivity in this important region has gone down around 10 percent since the mid-19th century and the start of the Industrial era. This decline coincides with steadily rising surface temperatures over the same period of time.

Matthew Osman, the paper’s lead author and a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, says there are indications that phytoplankton’s productivity may decline further as temperatures continue to rise as a result of human-induced climate change.

“It’s a significant enough decine that we should be concerned,” Osman says. “The amount of productivity in the oceans roughly scales with how much phytoplankton you have. So this translates to 10 percent of the marine food base in this region that’s been lost over the industrial era. If we have a growing population but a decreasing food base, at some point we’re likely going to feel the effects of that decline.”

Drilling through “pancakes” of ice

Osman and his colleagues looked for trends in phytoplankton’s productivity using the molecular compound methanesulfonic acid, or MSA. When phytoplankton expand into large blooms, certain microbes emit dimethylsulfide, or DMS, an aerosol that is lofted into the atmosphere and eventually breaks down as either sulfate aerosol, or MSA, which is then deposited on sea or land surfaces by winds.

“Unlike sulfate, which can have many sources in the atmosphere, it was recognized about 30 years ago that MSA had a very unique aspect to it, which is that it’s only derived from DMS, which in turn is only derived from these phytoplankton blooms,” Osman says. “So any MSA you measure, you can be confident has only one unique source — phytoplankton.”

In the North Atlantic, phytoplankton likely produced MSA that was deposited to the north, including across Greenland. The researchers measured MSA in Greenland ice cores — in this case using 100- to 200-meter-long columns of snow and ice that represent layers of past snowfall events preserved over hundreds of years.

“They’re basically sedimentary layers of ice that have been stacked on top of each other over centuries, like pancakes,” Osman says.

The team analyzed 12 ice cores in all, each collected from a different location on the Greenland ice sheet by various groups from the 1980s to the present. Osman and his advisor, Sarah Das, an associate scientist at WHOI, collected one of the cores during an expedition in April 2015.

“The conditions can be really harsh,” Osman says. “It’s minus 30 degrees Celsius, windy, and there are often whiteout conditions in a snowstorm, where it’s difficult to differentiate the sky from the ice sheet itself.”

The team was nevertheless able to extract, meter by meter, a 100-meter-long core, using a giant drill that was delivered to the team’s location via a small ski-equipped airplane. They immediately archived each ice core segment in a heavily insulated cold storage box, then flew the boxes on “cold deck flights” — aircraft with ambient conditions of around minus 20 degrees Celsius. Once the planes touched down, freezer trucks transported the ice cores to the scientists’ ice core laboratories.

“The whole process of how one safely transports a 100-meter section of ice from Greenland, kept at minus-20-degree conditions,  back to the United States is a massive undertaking,” Osman says.

Cascading effects

The team incorporated the expertise of researchers at various labs around the world in analyzing each of the 12 ice cores for MSA. Across all 12 records, they observed a conspicuous decline in MSA concentrations, beginning in the mid-19th century, around the start of the Industrial era when the widescale production of greenhouse gases began. This decline in MSA is directly related to a decline in phytoplankton productivity in the North Atlantic.

“This is the first time we’ve collectively used these ice core MSA records from all across Greenland,  and they show this coherent signal. We see a long-term decline that originates around the same time as when we started perturbing the climate system with industrial-scale greenhouse-gas emissions,” Osman says. “The North Atlantic is such a productive area, and there’s a huge multinational fisheries economy related to this productivity. Any changes at the base of this food chain will have cascading effects that we’ll ultimately feel at our dinner tables.”

The multicentury decline in phytoplankton productivity appears to coincide not only with concurrent long-term warming temperatures; it also shows synchronous variations on decadal time-scales with the large-scale ocean circulation pattern known as the Atlantic Meridional Overturning Circulation, or AMOC. This circulation pattern typically acts to mix layers of the deep ocean with the surface, allowing the exchange of much-needed nutrients on which phytoplankton feed.

In recent years, scientists have found evidence that AMOC is weakening, a process that is still not well-understood but may be due in part to warming temperatures increasing the melting of Greenland’s ice. This ice melt has added an influx of less-dense freshwater to the North Atlantic, which acts to stratify, or separate its layers, much like oil and water, preventing nutrients in the deep from upwelling to the surface. This warming-induced weakening of the ocean circulation could be what is driving phytoplankton’s decline. As the atmosphere warms the upper ocean in general, this could also further the ocean’s stratification, worsening phytoplankton’s productivity.

“It’s a one-two punch,” Osman says. “It’s not good news, but the upshot to this is that we can no longer claim ignorance. We have evidence that this is happening, and that’s the first step you inherently have to take toward fixing the problem, however we do that.”

This research was supported in part by the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), as well as graduate fellowship support from the US Department of Defense Office of Naval Research.

from MIT News - Oceanography and ocean engineering http://bit.ly/2V1ItYF

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Study demonstrates seagrass’ strong potential for curbing erosion

Most people’s experience with seagrass, if any, amounts to little more than a tickle on their ankles while wading in shallow coastal waters. But it turns out these ubiquitous plants, varieties of which exist around the world, could play a key role in protecting vulnerable shores as they face onslaughts from rising sea levels.

New research for the first time quantifies, through experiments and mathematical modelling, just how large and how dense a continuous meadow of seagrass must be to provide adequate damping of waves in a given geographic, climatic, and oceanographic setting.

In a pair of papers appearing in the May issues of two research journals, Coastal Engineering and the Journal of Fluids and Structures, MIT professor of civil and environmental engineering Heidi Nepf and doctoral student Jiarui Lei describe their findings and the significant environmental benefits seagrass offers. These include not only preventing beach erosion and protecting seawalls and other structures, but also improving water quality and sequestering carbon to help limit future climate change.

Those services, coupled with better-known services such as providing habitat for fish and food for other marine creatures, mean that submerged aquatic vegetation including seagrass provides an overall value of more than $4 trillion globally every year, as earlier studies have shown. Yet today, some important seagrass areas such as the Chesapeake Bay are down to about half of their historic seagrass coverage (having rebounded from a low of just 2 percent), thus limiting the availability of these valuable services.

Nepf and Lei recreated artificial versions of seagrass, assembled from materials of different stiffness to reproduce the long, flexible blades and much stiffer bases that are typical of seagrass plants such as Zostera marina, also known as common eelgrass. They set up a meadow-like collection of these artificial plants in a 79-foot-long (24-meter) wave tank in MIT’s Parsons Laboratory, which can mimic conditions of natural waves and currents. They subjected the meadow to a variety of conditions, including still water, strong currents, and wave-like sloshing back and forth. Their results validated predictions made earlier using a computerized model of individual plants.

Researchers used a74-foot-long wave tank at MIT, loaded with simulated seagrass plants, to study how seagrass acts to attenuate waves under various conditions. In this video, the simulated plants are exposed to strong waves.

In further tests in the MIT tank, simulated seagrass plants are subjected to very low-velocity waves.

The researchers used the physical and numerical models to analyze how the seagrass and waves interact under a variety of conditions of plant density, blade lengths, and water motions. The study describes how the motion of the plants changes with blade stiffness, wave period, and wave amplitude, providing a more precise prediction of wave damping over seagrass meadows. While other research has modeled some of these conditions, the new work more faithfully reproduces real-world conditions and provides a more realistic platform for testing ideas about seagrass restoration or ways of optimizing the beneficial effects of such submerged meadows, they say.

To test the validity of the model, the team then did a comparison of the predicted effects of seagrass on waves, looking at one specific seagrass meadow off the shore of the Spanish island of Mallorca, in the Mediterranean Sea, which is known to attenuate the force of incoming waves by a factor of about 50 percent on average. Using measurements of meadow morphology and wave velocities collected in a previous study led by Professor Eduardo Infantes, currently of Gothenburg University, Lei was able to confirm the predictions made by the model, which analyzed the way the tips of the grass blades and particles suspended in the water both tend to follow circular paths as waves go by, forming circles of motion known as orbitals.

The observations there matched the predictions very well, Lei says, showing the way wave strength and seagrass motion varied with distance from the edge of the meadow to its interior agreed with the model. So, “with this model the engineers and practitioners can assess different scenarios for seagrass restoration projects, which is a big deal right now,” he says That could make a significant difference, he says, because now some restoration projects are considered too expensive to undertake, whereas a better analysis could show that a smaller area, less expensive to restore, might be capable of providing the desired level of protection. In other areas, the analysis might show that a project is not worth doing at all, because the characteristics of the local waves or currents would limit the grasses’ effectiveness.

The particular seagrass meadow in Mallorca that they studied is known to be very dense and uniform, so one future project is to extend the comparison to other seagrass areas, including those that are patchier or less thickly vegetated, Nepf says, to demonstrate that the model can indeed be useful under a variety of conditions.

By attenuating the waves and thus providing protection against erosion, the seagrass can trap fine sediment on the seabed. This can significantly reduce or prevent runaway growth of algae fed by the nutrients associated with the fine sediment, which in turn causes a depletion of oxygen that can kill off much of the marine life, a process called eutrophication.

Seagrass also has significant potential for sequestering carbon, both through its own biomass and by filtering out fine organic material from the surrounding water, according to Nepf, and this is a focus of her and Lei’s ongoing research. An acre of seagrass can store about three times as much carbon as an acre of rainforest, and Lei says preliminary calculations suggest that globally, seagrass meadows are responsible for more than 10 percent of carbon buried in the ocean, even though they occupy just 0.2 percent of the area.

While other researchers have studied the effects of seagrass in steady currents, or in oscillating waves, “they are the first to combine these two types of flows, which are what real plants are typically subjected to. Despite the added complexity, they really sort out the physics and define different flow regimes with different behaviours,” says Frédérick Gosselin, professor of mechanical engineering at Polytechnique Montréal, in Canada, who was not connected to this research.

Gosselin adds, “This line of research is critical. Land developers are quick to fill and dredge wetlands without much thinking about the role these humid environments play.” This study “demonstrates how submerged vegetation has a precisely quantifiable effect on damping incoming waves. This means we can now evaluate exactly how much a meadow protects the coast from erosion. … This information would allow better decisions by our lawmakers.”

The work was funded by the U.S. National Science Foundation.

from MIT News - Oceanography and ocean engineering http://bit.ly/2J9qiOw

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