Featured Stories, MIT | February 25, 2013

Highlights of the Southern Ocean Workshop at MIT

By Genevieve Wanucha

On January 28, 2013, the two-day Southern Ocean Workshop at MIT kicked off bright and early to accommodate the presentations of 32 leaders in oceanographic research. “It’s a meeting of some pretty hardcore ocean physics nerds who are obsessed with the ocean and climate as intellectual problems,” said recent MIT alum (PhD ’12) Ryan Abernathey, a physical oceanographer at Scripps Institution of Oceanography soon to take up a faculty position at Columbia University. At this meeting, the mysterious Southern Ocean proved worthy of the most dedicated oceanographer’s attention.

The importance of the Southern Ocean has been increasingly recognized over the past two decades. In particular, the Southern Ocean’s major role in global climate has risen to the fore. About 40% of the annual global CO₂ emissions absorbed by the world’s oceans enter through this region. But at the same time, the Southern Ocean’s overturning circulation brings up vast stores of dissolved organic carbon and heat stored in the deep ocean’s interior, some of which escapes into the atmosphere as CO₂. Even with the growing need to understand this spectacular window of interaction between ocean and atmosphere, the fact remains: “The Southern Ocean is certainly one part of the ocean we don’t understand,” as Carl Wunsch, one of the conference moderators and professor of physical oceanography at MIT, said. Indeed, the rough water that completely surrounds the relatively tiny landmass of Antarctica is the least observed of the world’s oceans.

This informal, open conversation amongst scientists from all over the world was arranged around 3 key science themes: the  Southern Ocean’s eddying and mixing processes; the Southern Ocean and its ice field response to changes in wind and buoyancy forcing; and the Southern Ocean’s role in modulating climate.

The most fascinating Southern Ocean puzzles discussed included: How will the documented shift in wind patterns affect the amount of carbon dioxide entering our atmosphere? Why is the Arctic sea ice dramatically melting while the Southern Ocean’s Antarctic sea ice hasn’t budged, even showing slight increases?

Relevant to the CO₂ question, Michael (Mick) Follows, Senior Research Scientist in MIT ‘s Program in Atmospheres, Oceans, and Climate (PAOC) presented recent findings from a simple model of the Southern Ocean’s carbon cycle. In this idealized system, Mick and his collaborators tweaked the wind stress over the Southern Ocean. And that change completely shook up the carbon cycle.

Increased wind stress affected the ocean mixing in a way that reduced the amount of time that the upwelled carbon-rich water stayed at the surface. This shortened residency time decreased the efficiency of the biological processes that export carbon down to deep ocean reservoirs. In turn, the ocean surface released an increased amount of carbon dioxide into the atmosphere.

This finding is interesting in light of the fact that, as will be discussed, the westerly winds have changed over the Southern Ocean and dramatically impacted its circulation. However, this model cannot predict how the real ocean operates. Follows emphasized that it can serve as a tool to quantify exactly how much carbon dioxide enters back into the atmosphere in different scenarios of climate-change related wind and circulation patterns.

One of the greatest ocean mysteries became a little less opaque with the talk by Columbia University’s Lorenzo Polvani. The mystery concerns the ozone hole, that gaping tear in Earth’s protective stratospheric lining over Antarctica. Now, more than 20 years after the Montreal Protocol phased out the ozone-killing chlorofluorocarbon-12, or CFC-12, the hole is slowly healing up, headed for a complete recovery by 2050 or 2060. The closing ozone hole, as Polvani’s talk related, figures into a current debate about future Antarctic sea ice extent.

As previously mentioned, while the Arctic sea ice has dramatically decreased, the Antarctic sea ice has slightly increased by 0.97% per decade since the late 1970s. A great deal of recent research shows that the ozone depletion caused a poleward shift in the cold winds over the Southern Ocean. Some have suggested that this new and strengthened wind stress has extended the reach of sea ice. Essentially, the claim is that human-borne ozone depletion is a contributing factor to the growth Antarctica sea ice. But Polvani’s research shows something quite different.

First off, the Antarctic sea ice isn’t growing everywhere. In actuality, sea ice trends vary over region (See graphic on right). While the sea ice has advanced in the Weddell and Ross Sea, it has declined in the Amundsen-Bellingshausen Seas. The modelling work of Polvani and his colleagues Karen Smith and Cecilia Bitz suggests that the slight positive trend in sea ice extent is likely due to natural climate variability, combined with suppressed greenhouse warming from strong ocean heat uptake in the Southern Ocean. But his new model goes even deeper into the problem.

Using a new model featuring interactive stratospheric chemistry, Polvani and his colleagues compared Antarctic sea ice scenarios with and without trends in ozone-depleting substances over the period 2001–2065. They also took into consideration the projected increase in atmospheric greenhouse gas concentrations during that same period.

Polvani found that in the case the ozone hole never closes, the global warming from the increased greenhouse gases actually results in Antarctic sea ice loss. However, ozone hole recovery cancels out the green house gas effect on sea ice. These results shed new light on the conundrum that will receive much study in the coming decades. It also drove home the idea that the coming ozone hole recovery is a very good thing indeed. In fact, it will not only reduce skin cancer cases caused by excess UV rays but also mitigate the ecosystem or ocean circulation disruptions of Antarctic sea ice loss.

Unsurprisingly, many participants acknowledged that sea ice and ocean-sea ice interactions must become a bigger focus of future research and workshops.

Another highlight of the meeting came with the opportunity to review some results from recent sea-going observational campaigns, particularly the DIMES Experiment led by Jim Ledwell, a chemical oceanographer at WHOI. Oceans at MIT recently reported on this international effort to determine the role of mixing in the dynamics and energetics of the Southern Ocean.

Teresa Chereskin (Scripps) and Randy Watts (University of Rhode Island) presented the lastest from cDrake, a project to quantify the transport and dynamic balances of Southern Ocean circulation. In the rough Drake Passage, they monitor the Antarctic Circumpolar Current‘s structure over time with acoustic tools to gain insight into why the Southern Ocean is particularly sensitive to climate change.

At meeting’s end, the conference’s organizer, John Marshall, oceanographer at MIT, reflected: “Many meetings we go to as oceanographers are huge, numbering up to 2,000 people. There’s never any time to discuss anything. Here, we tried to have a format that enabled people to talk about what they were doing in a wide ranging way.” Actually, too many people showed up to the event, making the Q&A sessions a bit unwieldy. But then again, how can we complain about too many people being interested in the Southern Ocean?