The Southern Ocean plays a fundamental role in the global climate as it provides the principal connections among the world’s major ocean basins and between the deeper and upper layers of the global ocean circulation. The connection between the deep ocean and the sea surface in the Southern Ocean allows water masses to interact with the atmosphere, exchanging heat and gases before being sent back to the ocean depths for decades to millennia.
It also contributes to moderate climate change by absorbing large proportions of heat and carbon that are associated with current climate change. Previous studies revealed that oceans, including the Southern Ocean, have absorbed 93% of the excess energy in the climate system arising from global warming. A 2018 study published in Oceanography reported that it also plays an important role in ventilating the world’s deep ocean and setting its characteristics.
According to the researchers, more than half of the world's ocean volume has had contact with the atmosphere in the Southern Ocean surface layer. As a result, a vast majority of the world’s oceans acquire its physical and biogeochemical characteristics in the Southern Ocean, which is why any change it experiences will have massive global ocean and climate consequences. At the same time, it has the ability to buffer terrestrial climate change that can influence its entire climate and ecosystem.
Warming of the Southern Ocean
The average global temperature recorded on the land surface has increased 1-degree centigrade (1.8 degrees F), and by 0.6 degrees (0.054 degrees F) across the ocean surface since the middle of the 19th century. Previous studies have shown substantial warming of global ocean temperature, averaging about 0.1°C between 1955 and 1995. Over the same period, the Southern Ocean warming also averages about 0.1°C
A 2002 study examined the sea temperature in the Southern Ocean by recording temperatures in depths between 700 and 1100 meters. One of the major findings showed that the temperature records are systematically warmer than earlier hydrographic temperature measurements from the region. This suggested that mid-depth Southern Ocean temperatures have risen 0.17°C between the 1950s and the 1980s. This recorded warming was faster compared to the global ocean, the scientists said.
According to Science Mag, the findings were based on the data collected during the 1990s by Autonomous Lagrangian Circulation Explorer (ALACE) floats. Throughout the 1990s, the ALACE floats were deployed from research ships as part of the World Ocean Circulation Experiment (WOCE) to predetermine pressure and follow mid-depth ocean currents for a fixed time interval of 10 to 25 days. The observations were not only used to identify the global sea temperature but also to study global and regional ocean circulation in a variety of ways.
The Southern Ocean is Actually Cooling
While the findings mentioned showed the increasing temperature in the Southern Ocean, it may be surprising to learn that the Southern Ocean is likely in a cooling period. The cooling trend was recorded in some parts of the Southern Ocean around the Antarctic continent, specifically in the area south of 55 degrees latitude, between 1982 to 2011. Scientists found out that the Pacific sector of the Southern Ocean has registered the strongest cooling, while the weakest are n the Indian and parts of the Atlantic sectors.
“The oceans around Antarctica — if you look at temperature trends over the last 50 years — that’s the one place that hasn’t been showing a lot of warming. It’s actually shown a little bit of cooling in the last 30 years,” Kyle Armour, professor of oceanography at the University of Washington, said.
According to Phys.org, an internet news portal that provides the latest news on science, the researchers showed that sea-ice changes are the most probable cause for the cooling of the surface waters in the Southern Ocean using a serious of simulations. The team was able to replicate the observed pattern of the temperature changes by incorporating the observed changes in sea ice into the model. The reduced exchange of heat between the deeper layers and the surface water and the reduction of the vertical exchange of heat in the ocean created the observed situation where the surface water cooled and the subsurface warmed.
A 2016 study suggested that this cooling trend in the Southern Ocean is because of the ozone hole. “Our study tries to address one of the most mysterious problems of recent historical climate change in the region because, in contrast to the strong global warming trend, we’ve seen persistent cooling in the Southern Ocean and sea ice expansion. And our study addresses some mechanisms that could be related to this persistent cooling trend,” lead author Yavor Kostov, an MIT graduate, said.
The researchers learned more about how the ocean, atmosphere, and ice interact together, which could lead sea surface temperatures to fall and sea-ice to expand around Antarctica, using results from computer simulations with models called coupled general circulation models (CGM) and observations. The findings suggest that the ozone-related changes in surface wind trends have resulted in the cooling and expanding of sea surface temperatures and sea-ice around Antarctica.
According to MIT News, an online site dedicated to communicating to the media and the public the news and achievements of the students, faculty, staff, and the greater MIT community, the ozone-related changes also encourage sea-growth because the strengthening of the westerly winds drives cold water equator-ward away from Antarctica. However, this doesn’t mean that the Southern Ocean will remain in the cooling trend for a long time.
“Our paper suggests that the first process — the northward transport of colder water — dominates this fast cooling response, but then over longer time scales, we have this build-up of heat below the surface that impacts the slow timescale of response — the gradual warming,” Kostov said.
While the cooling of the Southern Ocean over the past three decades is unusual, this shouldn’t be interpreted as a reduction of the long-term warming of the global climate system as a whole. The findings suggest that this is a redistribution of heat in the Southern Ocean from the surface to the deeper layers of the ocean. "We assume the strong winds pushing the sea ice in the Southern Ocean northward are potentially a side-effect of climate change. Climate change is clearly man-made and cannot be disputed simply because one area of the ocean shows signs of cooling,” ETH Professor Nicolas Gruber said.