URI talk focuses on ocean flow, climate, weather patterns
URI talk focuses on ocean flow, climate, weather patterns
May 2, 2017 06:46AM
By Cynthia Drummond
Sun Staff Writer
NARRAGANSETT — A visiting scientist told an audience at the University of Rhode Island last month that the widely-accepted “conveyor belt” theory used to describe ocean currents does not accurately reflect their extreme variability, and more data is needed to understand a highly changeable and complex global system that affects our weather and our climate.
While most people are familiar with the natural ocean processes of waves and tides and wind, there are also deep ocean currents that transport water from the coldest parts of the planet to warmer regions.
Ocean currents have an impact on every part of the world, including southern New England, so it is important to understand how they function and how that might be altered by climate change. Ocean circulation is also central to studies of how the ocean is acting as a “carbon sink,” absorbing carbon dioxide created by human activity, or anthropogenic CO2.
Presenting the fifth annual Scott W. Nixon lecture on April 13 at the Graduate School of Oceanography, physical oceanographer Susan Lozier of Duke University suggested that scientists take what she called a “21st century approach” to studying large-scale ocean circulation. “The estimates have been that 35 percent of the anthropogenic CO2 is stored in the ocean,” she said. “When I started graduate school, there was a big question ‘where has all the carbon gone’ because the estimate of the anthropogenic CO2 that was in the atmosphere did not equal the estimate of the CO2 that had been released. We now understand that the ocean has been an important reservoir.”
Lozier explained that ocean circulation was first documented by a British scientist in 1800.
“Count Rumford described what we know as the ocean overturning circulation,” she said. “The simple idea then is a simple idea now. At high latitudes, the water in the wintertime becomes very dense, because it moves all this heat to cold, overlying atmosphere. Those dense waters sink and spread through distant parts of the ocean. And then we have another surface current that returns the water to formation sites.”
It is important to understand how currents move water around the planet, Lozier said, because of what could happen if the warm water currents that moderate temperatures in Europe simply stopped, something that has happened in our distant past. Studies of ice core samples in Antarctica and Greenland have revealed historic changes in global air temperatures that occurred over relatively short periods; a few years to a decade.
“Scientists attribute those changes to changes in overturning circulation,” Lozier said. “So in 2002, there was a lot of concern about abrupt climate change. A lot of people on both sides of the Atlantic were talking about abrupt climate change, the possibility that the overturning circulation could diminish or even shut down in a couple of years.”
In 2004, a team of researchers from the United States and the United Kingdom installed an array of instruments across the North Atlantic ocean basin to provide long term measurements of the overturning circulation. The results, published a year later, showed dramatic variations in circulation over much shorter periods than was previously believed.
“This one year of data where the overturning circulation was directly measured really transformed our understanding of overturning, because...it varies by a factor of five or six over the course of one year,” Lozier said.
The measuring initiative, known as “rapid array,” showed that rather than traveling on conveyor belt-like pathways, cold deep water spreads in a much more irregular fashion.
“What we see is something very different than this traditional view of the water being channeled along in the coastal boundary currents,” she said.
Ocean circulation is also affected by winds and upwelling, where deep waters are brought to the surface. Ocean-atmospheric processes are still poorly understood, and Lozier is currently leading the Overturning in the Subpolar North Atlantic observation program, a long term data collection initiative involving several countries.
“Our conceptual understanding of the ocean’s overturning circulation has advanced over the past decade, and I think it’s primarily advanced because of observations,” she said.
URI oceanography professor and hurricane forecasting expert Isaac Ginis, who attended Lozier’s presentation, agreed that more data would be necessary to understand how ocean currents function. Ginis is particularly concerned with ocean temperature because of how it influences the intensities of hurricanes. He is not prepared to reject the conveyor belt model, but he said the process is more complex than the traditional model implies.
“We have some basic understanding of how the heat transfers on a large scale, and this concept of a conveyor belt conceptually is what I think, essentially, is working in nature,” he said. “We know that the cold water that is produced in the Arctic, because of the sinking of the cold and dense water to the bottom, does move to the south near the border, and then there is the opposite current, the warm current, near the surface, moving back from the tropics to the North. So generally it is true how the planet is able to maintain its heat balance overall, but the details — this is what is still unclear.”
Ginis said the conveyor belt model oversimplified a complex and highly variable process.
“Regionally, it’s much more complex,” he said. “I think the point she was trying to make is that there is much more complexity of exactly how the currents move and spatially are distributed, especially in the Atlantic. It’s significantly more complex than just a simple diagram represents.”