|Towering ice bergs. One of my favorite photos from this cruise, courtesy of Hannah Dawson.|
A couple posts ago, I discussed the Kerguelen (Ker-gway-len) Plateau and why it was important to the Southern Ocean. In this post, I would like to step back a bit further and talk about why we care about the Southern Ocean in the first place. Here, I will try to describe the uniqueness of the Southern Ocean and the important role it plays in the global climate system. My hope is to provide readers, especially those who don’t have a background in oceanography, a bit more context for the work that we do.
When oceanographers refer to the Southern Ocean, they usually mean the oceanic region south of about 45 degrees south. One key feature of this ocean is that it spans all longitudes. As you can observe from the schematic below, there are no land boundaries in the latitudinal bands of the Drake Passage (the opening between South America and Antarctica). This allows for the existence of the Antarctic Circumpolar Current (ACC), a strong eastward current system that circles around Antartica.
In terms of volume transport, the ACC is the most powerful ocean current in the world. Even though its average speed is relatively small (about 0.2 meters per second), its depth varies between 2000-4000m and is over 3000 km at its widest point.
One of my undergraduate professors loved to describe the ACC as an "egg-beater". In that sense, the Southern Ocean is a giant mixing bowl and the ACC acts as a mixer that blends water masses1 from all the other oceans. The ACC mixes water masses from the Pacific, Indian and Atlantic ocean basins and creates new water masses, which it then exports back into the global ocean. So that's the first major feature of the Southern Ocean. It's the ocean that links all the other major oceans and it is the main cog in the global ocean circulation system.
The ACC is driven by strong westerly (eastward flowing) winds. These westerlies mainly lie in the 40-60S latitude band and are some of the strongest surface winds in the world. Early navigators of the Southern Ocean referred to these winds as the Roaring Forties and Furious Fifties. Those winds lived up to their names during the first 10 days of our cruise.
|Even on the calmest days, the winds of the Southern Ocean are still quite gusty.|
Due to the Coriolis effect, these eastward flowing winds push the surface waters of the Southern Ocean northwards - away from Antarctica. This effect is due to the Earth's rotation about its axis. The result is a counterintuitive phenomenon where (large-scale) surface currents2 in the ocean move 90 degrees relative to the direction of the surface winds. In the northern hemisphere, the Coriolis force deflects ocean currents 90 degrees to the right; in the southern hemisphere, the same force deflects ocean currents 90 degrees to the left.
This large-scale motion of surface water away from Antartica is a really big deal. In fact, it is perhaps the most important feature of the Southern Ocean. Since winds are moving surface water out of the Southern Ocean, there needs to be a compensating flow to bring water to the surface. This compensating flow comes from the great depths of the global ocean. Thus, the Southern Ocean features large-scale upwelling of deep water to the surface.
|A schematic of deep water upwelling and formation in the Southern Ocean. Source: http://dimes.ucsd.edu/en/|
The surface layers of the open ocean are generally nutrient deplete. Most of the nutrients needed to sustain marine life are stored in the dark depths of ocean, beyond the reach of most organisms. However, due to the strong and persistent deep water upwelling, the surface layers of the Southern Ocean are flush with nutrients, which form the basis of a flourishing and vibrant marine ecosystem.
|Giant petrel in flight. One of the many sea birds that call the Southern Ocean home.|
|Another Giant Petrel. This one flew within 10 feet of the bridge.|
|A wandering albatross in flight.|
The Southern Ocean is arguably the breadbasket of the global ocean. The waters in this region are stocked with phytoplankton and krill, which form the base of the marine food pyramid. The Antarctic Krill, in particular, is one of the most abundant animal species on the planet in terms of biomass. Because of this surplus of food, migratory animals such as blue whales, orca and albatrosses travel thousands of kilometers to feed in the Southern Ocean. Additionally, the nutrients that don’t get consumed in the Southern Ocean are exported by ocean currents to the other parts of the ocean. Much of the nutrients that get consumed in the rest of the ocean are first upwelled in the Southern Ocean.
|A wandering albatross taking a break from flying. Photo courtesy of Hannah Dawson.|
|A small penguin decided to investigate our ship. This was the highlight our day.|
In addition to supporting deep water upwelling, the Southern Ocean is also key location for deep water formation. Surface conditions around Antarctica are below freezing for most year, which leads to the formation of sea ice. When sea ice forms, it pulls freshwater from the surface ocean, leaving behind a salty solution in a process called brine rejection.
The rejected brine solution from ice formation is relatively salty and cold, and is more dense than the surrounding surface water. Under the influence of gravity, this water sinks to the interior of the ocean. The densest of these deep waters is Antarctic Bottom Water (AABW). The AABW flows away from Antarctica to occupy the deepest layers of the Pacific, Atlantic and Indian oceans.
Putting everything together, we can start to appreciate the Southern Ocean’s central role in not only the global ocean circulation but also the global climate system. When AABW is formed, it brings with it enormous quantities of heat and carbon to the deep ocean for longterm (not permanent) storage. In this sense, the Southern Ocean acts as a huge buffer for the climate system. Because of this, we are being spared from the most detrimental consequences of global warming in the short term. On the flip side, since deep water also upwells in the Southern Ocean, it is also release valve for heat and carbon stored in the deep ocean. The net effect of these processes remains poorly understood.
Even though the Southern Ocean is a central cog in the earth’s climate system, it is still a largely unexplored frontier. Due to insufficient data, many fundamental scientific questions remain unanswered. Much of this is due to the region's remoteness and harsh climate. Research ships like ours can only access the Southern Ocean in austral summer when sea ice is at its minimum. Even then, we can only sample relatively small portion of the ocean for a relatively short of period of time.
|Let me give you a hand. Alison and Natalie tag teaming to cock the bottles on our rosette.|
This urgent lack of data is what helped to spur the creation of the SOCCOM program. A major goal of SOCCOM is to expand our current observational network of the Southern Ocean. With our next generation Argo floats, we will be able to remotely track the physical and biogeochemical processes that occur in the ice-covered parts of the Southern Ocean throughout the year. This is something we have not been able to do until now.
|Navis SOCCOM floats in the UW float lab being prepped for deployment.|
So, this is my summary of why the Southern Ocean is important. It is by no means an exhaustive review of Southern Ocean oceanography, but I believe I covered many of the major points. In a future post, I hope to go discuss my own research focus in more detail.
Until next time,
1 A water mass is how oceanographers refer to ocean water that originates from a particular region with certain identifiable properties. For example, North Atlantic Deep Water is a type of water mass that originates from the North Atlantic and is known to be relatively warm and salty. ↩