Past Tropical Climate Change Linked to Ocean Circulation

A new record of past temperature change in the tropical Atlantic Ocean’s subsurface provides clues as to why the Earth’s climate is so sensitive to ocean circulation patterns, according to climate scientists at Texas A&M University.

From left to right: Link Ji, Ping Chang, Matthew Schmidt, Jennifer Hertzberg

Geological oceanographer Matthew Schmidt and two of his graduate students teamed up with Ping Chang, a physical oceanographer and climate modeler, to help uncover an important climate connection between the tropics and the high latitude North Atlantic. Their new findings are in the current issue of PNAS (Proceedings of the National Academy of Sciences).

The researchers used geochemical clues in fossils called foraminifera, tiny sea creatures with a hard shell, collected from a sediment core located off the northern coast of Venezuela, to generate a 22,000-year record of past ocean temperature and salinity changes in the upper 1,500 feet of water in the western tropical Atlantic. They also conducted global climate model simulations under the past climate condition to interpret this new observational record in the context of changes in the strength of the global ocean conveyor-belt circulation.

“What we found was that subsurface temperatures in the western tropical Atlantic rapidly warmed during cold periods in Earth’s past,” Schmidt explains.

Image of a live foraminifera. Photo courtesy of Howard Spero at University of California Davis

“Together with our new modeling experiments, we think this is evidence that when the global conveyor slowed down during cold periods in the past, warm subsurface waters that are normally trapped in the subtropical North Atlantic flowed southward and rapidly warmed the deep tropics. When the tropics warmed, it altered climate patterns around the globe.”

He notes that as an example, if ocean temperatures were to warm along the west coast of Africa, the monsoon rainfall in that region would be dramatically reduced, affecting millions of people living in sub-Saharan Africa. The researchers also point out that the southward flow of ocean heat during cold periods in the North Atlantic also causes the band of rainfall in the tropics known as the Intertropical Convergence Zone to migrate southward, resulting in much drier conditions in northern South American countries and a wetter South Atlantic.

“Evidence is mounting that the Earth’s climate system has sensitive triggers that can cause abrupt and dramatic shifts in global climate,” Schmidt said.

“What we found in our subsurface reconstruction was that the onset of warmer temperatures, thought to reflect the opening of this ‘gateway’ mechanism, occurred in less than a few centuries. It also tells us that it might be a good idea to monitor subsurface temperatures in the western tropical Atlantic to assess how the strength of the ocean conveyor might be changing over the next few decades as Earth’s climate continues to warm.”

“One way to prepare for future climate change is to increase our understanding of how it has operated in the recent past.”

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 Contact: Matthew Schmidt at (979) 862-8342 or Keith Randall, News & Information Services, at (979) 845-4644 or Karen Riedel, College of Geosciences, at (979) 845-0910

Hurricanes That Pass Over Fresh Water Can Be Stronger

If a hurricane’s path carries it over large areas of fresh water, it will potentially intensify 50 percent faster than those that do not pass over such regions, meaning it has greater potential to become a stronger storm and be more devastating, according to a study co-written by a group of researchers at Texas A&M University.

Ping Chang, professor of oceanography and atmospheric sciences and director of the Texas Center for Climate Studies, along with his former student, Karthik Balaguru, now at the Department of Energy’s Pacific Northwest National Laboratory, are the lead authors of a paper in the current issue of PNAS (Proceedings of the National Academy of Sciences).

Their findings could benefit weather experts as they try to predict the path and strength of a hurricane, noting that about 60 percent of the world’s population resides in areas that are prone to hurricanes or cyclones.

Hurricanes can intensify if they pass over fresh water

Chang and Balaguru and their colleagues examined Tropical Cyclones for the decade 1998-2007, which includes about 587 storms, paying particular attention to Hurricane Omar. Omar was a Category 4 hurricane that formed in 2008 and eventually caused about $80 million in damages in the south Caribbean area.

They analyzed data from the oceanic region under the storm, including the salt and temperature structure of the water and other factors that played a part in the storm’s intensity.

“We tested how the intensity of the storm and others increased over a 36-hour period,” Chang explains.

“We were looking for indications that the storm increased in intensity or weakened and compared it to other storms. This is near where the Amazon and Orinoco Rivers flow into the Atlantic Ocean, and there are immense amounts of freshwater in the region. We found that as a storm enters an area of freshwater, it can intensify 50 percent faster on average over a period of 36 hours when compared to storms that do not pass over such regions.”

The researchers believe their results could help in predicting a hurricane’s strength as it nears large river systems that flow into oceans, such as the Amazon in the Atlantic, the Ganges in the Indian Ocean or even the Mississippi River into the Gulf of Mexico.

Hurricanes – called typhoons in the Pacific region and cyclones in the Indian region – are some of the most devastating natural hazards on Earth. A single storm, Cyclone Nargis in 2008, killed more than 138,000 people in Burma and caused $10 billion in damages.

“If we want to improve the accuracy of hurricane forecasting, we need to have a better understanding of not only the temperature, but also the salinity structure of the oceanic region under the storm,” Chang notes.

“If we know a hurricane’s likely path, we can project if it might become stronger when nearing freshwater regions. This is another tool to help us understand how a storm can intensify.”

The team’s work was funded by grants from the National Science Foundation, the Department of Energy and the National Science Foundation of China.

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About Research at Texas A&M University: As one of the world’s leading research institutions, Texas A&M is in the vanguard in making significant contributions to the storehouse of knowledge, including that of science and technology. Research conducted at Texas A&M represents an annual investment of more than $700 million. That research creates new knowledge that provides basic, fundamental and applied contributions resulting in many cases in economic benefits to the state, nation and world.

Media contact: Keith Randall, News & Information Services, at (979) 845-4644 or Ping Chang at (979) 845-8196

Study Shows Ocean Warming Has Accelerated The Past 100 Years

Increased warming from changing  ocean currents has accelerated over the past 100 years and could ultimately affect climate patterns over much of the world, according to research by two Texas A&M University oceanographers.

Ping Chang and Benjamin Giese, professors in the Department of Oceanography at Texas A&M, are part of a team that analyzed ocean temperature and current data since 1900. Their work is part of a multinational team that included researchers from the Ocean University of China, NOAA, the University of Hawaii, the University of Colorado, the Woods Hole Oceanographic Institution, the University of Tokyo, the Ocean University of China and scientists in Germany and Australia.  Results of the team’s study have been published in the journal Nature Climate Science.

A wall of warm air rises up from the Gulf Stream as it moves along the North American coastline

The Texas A&M research team found that parts of the world’s oceans are warming at an accelerated rate, showing a global warming “signature” in the ocean. The warming trend in some parts of the oceans is twice as large as the global average, the researchers have discovered.

“Certain parts of the oceans are getting warmer much faster than others,” Giese explains, “and it shows that there are significant regional differences in warming. The difference is from 0.5 to more than 1.5 degrees, and while that may seem small, it is a large change compared with the historical data over the past 100 years.”

Chang says the rising temperatures are possibly attributed to ocean circulation changes, which are likely caused by changes in atmospheric circulation, especially in the winds over the oceans.

“If this trend continues,” he says, “it could have a potential impact on the occurrence of extreme climate events, such as winter storms, in these regions because the atmospheric circulation is affected by sea-surface temperatures. These changes in ocean circulation could also have an impact on marine ecosystems.”

They say that the most severely affected areas of rising ocean temperatures are off the coast of Australia, near the Philippines, the Gulf Stream from Florida to New England, the Brazil current and the Kuroshio current, which is similar to the Gulf Stream but located in the Pacific Ocean near Japan.

The two Texas A&M researchers say the rising temperatures would probably not affect conditions of an El Niño or La Niña event.

“It is difficult to determine how these changes will affect global weather patterns,” Chang explains, “and it is more likely that regional climate extremes will be affected by these rising temperatures.”

The warming trend could pose problems for sensitive marine areas, Giese notes.

“People in Australia are worried about it because it could have an impact on its Great Barrier Reef,” he notes. “Any rise in temperature might damage the sensitive ecosystems of the reef.” At 1,800 miles long, the Great Barrier Reef is so large it can be seen from space.

The team’s work was funded by the China National Key Basic Research Project, the Australian Climate Change Science Program, the Southeast Australia Climate Initiative, the Japanese Ministry of Education, Culture, Sports, Science and Technology, NOAA (National Oceanic and Atmospheric Administration) and the National Science Foundation.

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About research at Texas A&M University: As one of the world’s leading research institutions, Texas A&M is in the vanguard in making significant contributions to the storehouse of knowledge, including that of science and technology. Research conducted at Texas A&M represents an annual investment of more than $630 million, which ranks third nationally for universities without a medical school, and underwrites approximately 3,500 sponsored projects. That research creates new knowledge that provides basic, fundamental and applied contributions resulting in many cases in economic benefits to the state, nation and world.

Media contact: Keith Randall, News & Information Services, at (979) 845-4644 or  Benjamin Giese at (979) 845-2306 or Ping Chang at (979) 845-8196