Science continues aboard the N.B. Palmer, and today we had our first science lecture by Dr. Jim Swift, the chief scientist of the cruise. We plan to have a science talk every Saturday at 12:30 pm when both watches are awake. I am very lucky; this is an amazing opportunity for me to hear different scientific topics by experts. I will try to do my best to report the main ideas of these talks.
People who were free from essential duties gathered at the conference room to hear Jim's presentation titled 'The Oceans of the Polar Regions: A Comparison of the Northern and Southern Oceans and Their Relationships to the World Ocean'.
First science talk aboard the Palmer by Jim
What a great way to learn about how our work here fits in a larger picture, and hear about the questions that remain to be answered. So what are the main differences between the Arctic and Antarctic waters?
In the Arctic region, the seas are small and surrounded by continents, while in the the Antarctic region we have land surrounded by very large extensions of water. This is an important difference as we will see below.The Arctic seas have poor communication with the rest of the oceans; they only communicate with the deep North Atlantic by a couple of passages between Greenland and Norway. The Antarctic seas, on the other hand, have large communication with all of the major oceans: Atlantic, Pacific and Indian.
Slide form Jim's presentation showing that the Northern Ocean is constrained by the continents while the Southern one is not
Both polar areas areas are responsible for the formation of most of bottom waters by the process that I have explained before. For water to sink, it needs to be more dense that the surroundings. Water becomes more dense when the salinity increases and/or when the temperature decreases. Both the salinity increases and temperature decreases in the polar areas when sea water freezes, as it leaves behind its salts to the surrounding cold water. The Antarctic regions produces the Antarctic Bottom Water (AABW) while the Arctic produces the North Atlantic Deep Water (NADW). A process I have not talked about is becoming more salty by evaporation. This happens in the Mediterranean sea, which pours some very salty and warm water to be mixed with the cold and fresh water from the Arctic.
There is a factor that determines how much water will be formed that I have not explained yet. The difference in density between the surface waters and bottom waters. It is easier to produce bottom waters when this difference is small than when it is big because the water requires less modification in order to sink. This factor benefits the Antarctic over the Arctic ocean. The Antarctic has a 'fantastic potential' for bottom water formation because this difference is very small. The Arctic ocean, on the other hand, has a very large density difference in the vertical except on the Labrador and Greenland seas, where the difference is small.
Density difference between bottom waters and surface waters fro the Southern Ocean and Northern Ocean. Red indicates a big difference, as in the Arctic region, while blue indicates a small difference, as in the Antarctic region.
Another factor that affects bottom water formation is the depth of the basin. Seawater freezes on continental shelves because they are shallower. This means that the whole water column needs to loose less heat to the atmosphere than when it is deeper. The Antarctic seas are deeper than the Arctic, where about 40% of its surface is on a continental shelf. This factor benefits the Arctic seas over the Antarctic ones.
Another difference is the amount of gases that are transferred from the atmosphere to the oceans. The water carries these gases when it sinks. We observe higher concentrations of CFC in the Arctic, but in a much smaller area than the Antarctic. The net result is that that a greater amount of gases are absorbed in the Southern Seas than the Northern counterparts. This is particularly important for the absorption of atmospheric CO2. We are trying to understand where does the CO2 that we put in the atmosphere goes, and we believe the Southern Seas play a major role.
At the end, the Antarctic bottom waters have more influence over the deepest part of the global oceans than the Arctic because it is less confined. Studies have shown that the Antarctic Bottom Water spreads over 58 % of the ocean's bottom compared to 26% of the North Atlantic Deep Water. This is 2.2 times more for the Antarctic than the Arctic. As Jim said, if we find changes in the deep waters of the world we first need to see what is happening in the Antarctic region.
During the 1990' s there was a major field campaign around the globe that allowed us to analyze seawater properties at a global scale. This cruise is part of an effort to repeat the analysis in some of the same areas, with the objective to observe changes over time of seawater properties (does this sound familiar? check the name of our project). Jim mentioned that as very preliminary results we have found a small global increase in deep water temperatures in the stations that we have done, which is consistent with other cruises in other parts of the world. You should take this information with a grain of salt, as they are very preliminary and could change after further analysis.
Preliminary results showing a comparison of our bottom water measurements with two previous cruises. The vertical axis is depth in meters while the horizontal axis is temperature in Celsius.The panel on the right shows the same data for just the bottom waters. You can see a warming trend with time for waters deeper than 3300 meters.
We do not have a good explanation for this global temperature change in bottom waters. It is tempting to say that perhaps the water being formed on the continental shelves in the polar regions is warmer, and therefore ends up warming the bottom waters as it sinks, but it takes hundreds of years for new bottom water to flood all of the oceans' bottom. This mechanism with a time scale of nearly 1000 years cannot explain all changes seen in the past 20 years. It was great to see scientists on a friendly debate as they try to come up with scientific models that are supported by the data.
As Jim said, scientist on shore will have fun with our data trying to explain these types of questions. I do not know about them, but I am certainly having fun being part of the team that collects the data.
It was a great talk. I look forward to next week's topic.