Finally, a week later, I present to you the materials from Dr. Alex Orsi's science talk. This might be the most technical entry of the journal, but it is also the most important as it presents preliminary results of the science being done aboard. It has more acronyms that I would like, so I ask you to be patient to the text and to read it a couple of times to fully understand the main idea from the cruise, which is spelled on the title.
Anne Marie, a PolarTREC teacher that finished her expedition two months ago along the very same waters we have been navigating, asked me a very hard question to answer. Before I get to her question, let me recommend you read her journal so you can make a simple comparison and observe the most obvious changes that two months brought to these waters. Her expedition was on a much bigger Swedish icebreaker, the Oden, since they were navigating through thick ice. The scientists aboard came out of the ship while on station to sample the ice, not the liquid water like we are doing.
Back to Anne Marie's question: 'It's been over 2 months since I finished my expedition on the Oden and I am grappling with the challenge of educating my students, parent, teachers, and the public about Antarctica and going beyond the 'penguins, seals, and icebergs' topics. What do you think is the most important information, or the 'Big Idea' that you will strive to share when you get home? What do you consider the 'key message' that you want people to learn about Antarctica and your research?'
I give thanks to Anne Marie for making me think about this very important question before I find myself facing that problem two months from now; after all, the whole reason for me being in this expedition is to do outreach for the science that is being done aboard, to 'educate my students, parent, teachers, and the public about Antarctica' as Anne Marie well wrote. There are many TV shows and magazines with professional photographers that provide you with much better images of penguin seals and icebergs than mine. So, what are the 'Big Idea' and the 'key message'?
On my vague response to Anne Marie I said the following 'the big idea I want to share about Antarctica is its fragility and the importance of scientific research to understand it. I wish I would know more about the results of our measurements and be able to speak more knowledgeably about the changes that the processes that provide sea water with its properties are going through.' The science talk of last week gave me a lifeline.
I am by no means an expert but Dr. Alejandro Orsi, a Professor of Oceanography at Texas A&M University, is. He helped me today by teaching us about some of the links between climate change and the Antarctic Ocean. Our Co-Chief Scientist gave a great talk entitled 'The Southern Pacific Ocean: an Alley of Global Change?'
Dr. Alex Orsi, professor in Oceanography at Texas A and M University, giving a science talk aboard the Palmer
The suggestive title of his talk comes from the evidence that the Southern Ocean is an active player in climate change. The basic premise of his talk, as I understand it, is: climate anomalies that arrive to the Southern Oceans from the north generate new anomalies by altering the multiple air-sea-ice interactions taking place over the vast Antarctic oceanic and continental shelf regimes. The new anomalies are then exported back to the north by Antarctic waters involved in the global Meridional Overturning Circulation (MOC). This north-south exchange of anomalies constitutes the "Alley of Global Change" that Alex referred to in his talk.
I will first present a basic description of Southern Ocean circulation and then explain what anomalies come from the north to disrupt this setting. Here is a map of Antarctica with some place names that I will refer to in the text.
Map of Antarctica with the names of the places used in the text
Winds and currents
Some of the strongest prevailing winds in the world blow around Antarctica from west to east. These southern polar westerlies are particularly strong because there are no continental barriers to slow them down like other northern atmospheric jet streams are around the world.**
There is a similar ocean current around Antarctica that also flows uncontained by continents on its path. This is the Antarctic Circumpolar Current (ACC) that flows from west to east. The current is actually made out of different jets. We can identify at least three of them within this current system.
Jets that form the Antarctic Circumpolar Current (ACC)
Another strong current in the Southern Ocean is found farther to the south of the ACC and flowing in the opposite direction, that is toward the west and nearly along the continental shelf break. This is the Antarctic Slope Current (ASC), it is not as strong as the ACC and it does not continuously circumnavigate Antarctica: it collides with the ACC after turning around the tip of the Antarctic Peninsula from the Weddell Sea.
Water masses
We have seen in this journal that we can use the seawater properties along our cruise to identify three basic water masses. We have identified the Antarctic Surface Water (AASW) as fresh and cold water. This water mass shows a great deal of variability reflecting seasonal influences, like the summer warming and freshening caused by the melting of sea ice.
Over the largest Antarctic continental shelves near-freezing surface waters increase their salinity due to extensive brine release during sea ice formation and become much denser than AASW, the so called Shelf Water. At a few sites SW nears the shelf break from the south and sinks to great depths entraining less dense ambient waters, thus forming the familiar Antarctic Bottom Water (AABW). Northward outflows of AABW along Deep Western Boundary Currents fill most of the abyssal layer of the world ocean. A voluminous water mass of the Southern Ocean is found sandwiched between the AASW and AABW is the Circumpolar Deep Water (CDW). I have previously mentioned that this water mass is the long term mixing product of AABW and deep waters entering the ACC from the north in each ocean basin, that is the North Atlantic Deep Water (NADW), Pacific Deep Water (PDW) and Indian Deep Water (IDW) [see schematic from Schmitt paper used in the blog before].
Let us focus on the fate of the NADW in the Southern Ocean, since it is the only source of salt for the CDW that reaches the Antarctic shelf break, where it is also warmer than the AASW above and AABW below. Saline CDW would first need to cross the ACC, and that is not an easy task because the current is very strong. The picture below is a map of the warm and salty signal of the NADW arriving to the Antarctic Circumpolar Current in the southwestern Atlantic Ocean. The orange shades (tongue) indicate how the NADW becomes less salty as it travels towards the east around Antarctica progressively mixing with the fresher waters above (IDW and PDW) and below (AABW). The lower portion of this water (the Lower CDW) crosses the ACC and shoals towards Antarctica, e.g. some of it arrives to the shelves of the Amundsen, Ross and Weddell seas. This image is from the amazing WOCE Southern Ocean Atlas. I have used several of the images below. It is great as a classroom resource.
Map of the warm and salty signal of the NADW arriving to the Antarctic Circumpolar Current in the southwestern Atlantic Ocean. The orange shades (tongue) indicate how the NADW becomes less salty as it travels towards the east around Antarctica progressively mixing with the fresher waters above (IDW and PDW) and below (AABW)
There are four mayor areas where Antarctic Bottom Water is produced: the Weddell Sea, on the eastern side of the Antarctic Peninsula, is thought to be the biggest formation site. It produces a very cold and relatively fresh type of AABW. The Ross Sea, where we began our cruise, is the second most important producer of AABW, and it has historically been known to produce cold and very salty type. The Adélie Sea and the Prydz Bay in the Indian Ocean sector also produce fresh AABW.
The extremely opposite scenario is seen over the continental shelves of the Amundsen Sea and West Antarctica Peninsula (WAP). Warm LCDW floods those margins either delaying or completely preventing the formation of sea ice in the winter. That is also the reason why the western side of the Antarctic Peninsula is one of the few parts of the entire coastline with no large ice shelves.
Changes in the Southern Ocean
Alex Orsi was so generous with his time that he wrote what follows. Observational atmospheric studies have shown that the Antarctic westerlies have intensified and moved southward in past decades as a result of global warming. This is a climate anomaly arriving to the Southern Ocean from the north. Consequently the Antarctic Circumpolar Current has also shifted polewards in response to that large-scale atmospheric change. It is well-known that prominent bathymetric ridges in the Southern Ocean effectively constrain the path of the ACC, for example the Southwestern Pacific Ridge limits the poleward extent of that current. But in the southeastern Pacific there are no such limiting topographic features, and the ACC is "free" to fluctuate in the north-south direction. This oceanic response to wind patterns can be inferred in the data collected during our cruise.
New position of ACC as derived from our data
The southward motion of the ACC may increase the availability of relative warm deep water (LCDW) to the shelves further to the south, thus altering melt rates of glaciers at their floating base and grounding line. This has been recently observed in the Pine Island Glacier (PIG) at the southern edge of the Amundsen Sea, where we are headed today.
There is a series of troughs in this shelf that helps to channel the LCDW inflow to the south. Here the bottom water is warm and flows inshore, and it is able to reach and melt the base of the PIG. Basal melt produces a very cold but also anomalously fresh type of AASW that is less dense than LCDW. As this locally produced surface water moves away from the ice shelves and reaches the shelf break, the strong westward-flowing Antarctic Slope Current (ASC) described above carries the local fresh water anomaly downstream, towards the Ross Sea.
The arrival of any type of fresh water anomaly to the Ross Sea has important consequences, because that shelf is an important formation site of AABW. Even if the total amount of AABW formed in this area each year were unaffected by this anomaly, the characteristics of the end mixing-product will most likely reflect the change in one of its source water masses. In fact observations from this cruise show that one of the outflows of newly formed Ross Sea Bottom Water (RSBW) is much fresher than it used to be in previous decades. The figure below shows two cross-slope vertical sections of salinity occupied just downstream of Cape Adare; the one on the left is from 1992 (WOCE S4P) and the other one from the beginning of this CLIVAR repeat cruise. The vertical axis has the depth in meters and the horizontal axis is the distance traveled by the ship in kilometers. Note the overall salty ("orange") deep and bottom layers in the older section, compared to the much fresher ("bluish") picture at present time.
Cross-slope vertical sections of salinity occupied just downstream of Cape Adare; from the WOCE S4P cruise in 1992 (left) and this CLIVAR S4P repeat cruise. The vertical axis has the depth in meters and the horizontal axis is the distance traveled by the ship in kilometers. High salinity in range and low salinity in blue.
It is clear that new bottom waters sinking near the shelf break of the western Ross Sea are fresher than they were almost 20 years ago. This change in water properties will have a significant impact on the rest of the abyssal ocean, since AABW outflows to the north are the primary suppliers to the lower limb of the global Meridional Overturning Circulation (MOC) mentioned before. As this Antarctic anomaly propagates to the north it completes the exchange along the "Alley of Global Change".
The Ross Sea could experience a shift in modes of circulation in the future, at least in terms of cross-slope exchange with the deep ocean. Hypothetically the Ross Sea could become the next Amundsen Sea, if at some point LCDW is to become more available to the base of the Ross Ice Shelf (RIS) and begins to melt it. In February 2010 our team from Texas A&M University sailed on the Oden icebreaker to deployed two moorings along the first likely entry point of LCDW to the eastern Ross Sea shelf, just west of Cape Colbeck. From the analysis of the multiple 1-yr time series of ocean currents and seawater temperatures and salinities that were recovered on this cruise (see journal March 20), we might find evidence of warm water pulses toward the RIS and ensuing "super-cooled" outflows. Let us hope that this is not the case, and that the world's largest ice shelf remains stable for a few more centuries.
The big idea
This is the big idea I want to communicate to my students, their parents and the general pubic about Antarctica. Antarctica is remote but fragile. We need to understand the processes that create and shape this unique environment, and how these processes respond to global changes originated very far away. We only have one Antarctica. It is through the work of dedicated scientist like Alex and others aboard the ship, that we will be better prepared to face future challenges in this corner of the world. Specifically, we have found that the properties of deep seawater formed in the Ross Sea are changing due to the displacement to the south of the ACC that is a consequence of the displacement of the South Polar Westerlies also to the south in response to climate change. The change in properties of deep waters, freshening in this case, will very likely have an impact on the world's abyss.
Alex's talk covered a lot more material that what is presented here, which I unsuccessfully tried to soak up like a sponge. I plan to keep on asking him questions around this topic during the upcoming couple of weeks; you should do the same. Take the opportunity to talk to an expert oceanographer, send me your questions and I would ask Alex about the extremely important work he is doing.
I would like to thank Alex for his unlimited patience that allowed me to understand what I present here.