Huge impact of global warming on Southern Ocean

In 1990-2000 the Polar Front shifted southward twice as much as in the previous 900 years, potentially altering ocean temperatures and ecosystem functions.

By Heitor Shimizu, in Montevideo  |  Agência FAPESP – The growing concentration of greenhouse gases in the atmosphere has raised average temperatures around the world, leading to global warming. More than 90% of the heat trapped by greenhouse gases is absorbed by the oceans, where much of it stays in the upper layers but a significant proportion finds its way to depths below 700 m.

Since 1955 the oceans have absorbed 20 times more heat than the atmosphere. They are warming at an ever-faster rate because global temperatures are rising. Scientists’ projections point to stronger change at high latitudes.

According to Ilana Wainer, an Associate Professor in the Department of Physical Oceanography of the University of São Paulo’s Oceanographic Institute (IO-USP), the oceans can be compared to an “air conditioning plant for the entire planet” and the circumpolar sea, which some scientists call the Southern or Antarctic Ocean, is a key element of climate change, as it has no land barriers but has intense currents in response to strong winds.

“The Southern Ocean links the Pacific, Atlantic and Indian Oceans together. It’s also responsible for communication between the ocean depths and the atmosphere, as it allows temperature anomalies to be carried down from the surface to deeper layers,” Wainer said. “The global climate depends directly on water masses and the Southern Ocean plays an important role in climate change.”

Wainer was a speaker at FAPESP Week Montevideo, which took place in the Uruguayan capital on November 17-18, 2016. The symposium was organized by FAPESP in collaboration with the Montevideo Group Association of Universities (AUGM) and UDELAR.

One of the main components of the Southern Ocean is the Antarctic Circumpolar Current, the longest and strongest oceanic current on earth. The ACC is accompanied by several fronts, including the Polar, Subantarctic and Southern fronts. Furthermore, the waters of the Southern Ocean are separated from warmer and saltier subtropical waters by the Subtropical front.

Wainer analyzed changes in this system in the periods 1050-1950 and 1970-2000, using simulations run by the Community Earth System Model (CESM) at the National Center for Atmospheric Research (NCAR) in the US.

“The mean findings across all experiments show that the Polar Front moved 0.7 of a degree southward in the period 1990-2000 compared with the mean for 1050-1950. That’s statistically significant because it’s twice the standard deviation for variability in 1050-1950,” Wainer said.

Wainer specializes in ocean-atmosphere interaction and climate, and is co-principal investigator for the FAPESP-funded Thematic Project “Impact of the Southern Atlantic on the global overturning circulation and climate”.

“This effect is caused by the ACC’s southward motion, which in turn is caused by a southward shift of the latitude at which the zonal wind attains its highest shear value,” Wainer explained.

Wind shear is a variation in wind velocity occurring along a direction at right angles to the wind’s direction and tending to exert a turning force.

“Our correlation analysis showed that the southward shift in the position of maximum zonal wind shear correlates closely with changes in the ACC in the period 1970-2000. The same correlations are far weaker for the period 1050-1950,” Wainer said.

These variations have a significant environmental impact, affecting sea levels and temperatures, for example, as well as carbon sequestration and ecosystem functions.

Wainer is a member of the steering committee of the National Science & Technology Institute (INCT) on the Cryosphere, one of the INCTs headquartered in Rio Grande do Sul State, Brazil, and supported by the National for Scientific & Technological Development (CNPq) and FAPERGS, the state research funding agency. Her other research for the institute includes climate modeling to understand the role of Antarctic sea ice and ice platforms in ocean circulation and climate variability. Global warming has fueled an increase in the flow of ice into the oceans.

Pollution in the Plata Basin

Juan Carlos Colombo, director of the Environmental Chemistry & Biogeochemistry Laboratory, part of the School of Natural Sciences at Argentina’s La Plata National University (UNLP), delivered a presentation at FAPESP Week Montevideo on research findings identifying pollutants in the vast River Plate estuary formed by the confluence of the Uruguay and Paraná rivers in the southeastern portion of South America.

Colombo and collaborators installed biogeochemical markers to monitor this region of the coast of Argentina and Brazil. More than 55 million cubic meters of sediment sweep down every year into the estuary from Argentina’s northern provinces and Brazil’s southern states.

The markers enabled the researchers to identify a great many organic materials, heavy metals, organochlorine pesticides, and persistent contaminants.

Manuel Pulido, a professor of physics at Argentina’s Northeastern National University (UNN), spoke about the challenges of using data assimilation to refine atmospheric and climate models. Data assimilation is a statistical technique for combining observations from a wide variety of sources and forecast output from a weather prediction model. The resulting analysis is considered to be the best estimate of the state of the atmosphere at a particular time.

Read more about FAPESP Week Montevideo: www.fapesp.br/week2016/montevideo.