It is another day for Science on the journal. So far we have covered,

    1. The rosette, how to collect ocean water at different depths 2. SamplingSampling refers to the process of selecting units or portions of a larger group that will be studied in order to answer questions about the larger group. The units can be people, water samples, ice cores, or any other appropriate object. Participants will explore the meaning of sampling and how it impacts experimental design and explore factors that define and limit sampling in the variety of projects visited during the expedition. They will consider how results from the chosen samples are used to describe the bigger target of a project's study. from the rosette 3. How density contributes to ocean circulation

    You now have an idea of what we work on during the cruise, and how carry it out, but I have not answered what should be the biggest question, 'why?' Why do we take all these measurements?

    Let us go back to the medical analogy, in which I compared our work on the ship to medical blood analysis. Your doctor has ideas of how blood characteristics relate to medical conditions. Her interpretation of the analysis will help her determine how healthy you are. For example, if she finds high glucose levels on your blood samples, she can diagnose diabetes. What would the interpretation of ocean water analysis tell us? What do we expect to learn from knowing the water properties in this region?

    Rosette recovery
    Rosette recovery. Why go through all this trouble to collect and analyze water from the depths?

    There are many answers to these questions. I present one of them here and will talk about some others later.

    One of the main goals of Physical oceanographers is to describe the circulation of the oceans and be able to accurately predict its evolution over time. Since it is impossible for us to simply place current meters all over the Ocean, oceanographers have come up with indirect ways to study the motion of the Oceans.

    We can take advantage of seawater properties, like salinity, temperature, dissolved oxygen, nutrients, etc., to know where a given water mass came from, just like we can use car plates on the highway to know from where they drove.

    Seawater properties are acquired in regions where physical and chemical processes imprint water property 'signature'. Most of the time this happens at the surface. Think about the changes in salinity of the ocean: evaporation, precipitation, freezing and melting, all happen near or at the surface. How about the temperature? The oceans gain thermal energy from the sun and exchange energy with the atmosphere. Where does it acquire dissolved gases, like CFCs? At he surface, by attempting to equilibrate with atmospheric concentration. We can use even oxygen, whose concentration is altered by the biology, because seawater gains oxygen at the surface where it is produced by phytoplankton and taken from the atmosphere. We will see later how it is affected below the surface by consumption by animals.

    Sunset in the Southern Seas
    Sunset in the Southern Seas. Water properties are mostly set at the sea surface and reflect local climate conditions.

    Water that is at the surface in the north Pacific will have different properties than surface waters around Antarctica. If water becomes denser at the surface, it carries these properties to different depths when it sinks (see journal entry for March 6 to see when surface waters sink).

    By precisely measuring seawater properties at the sea surface of source regions, oceanographers can interpret other observations made at different depth around the world ocean to infer the origin of particular water.

    This is similar to using car license plates to know where they came from. Imagine it is summer, when lots of people vacation by car, and you are driving on a highway around Wisconsin. You are coming out of the car at a rest stop and look at the other cars in the parking lot. You can roughly tell how far some of them have traveled by looking at the plates and knowing some geography. Let us say you see a car with Massachusetts' plates. You do not remember where is Massachusetts, so you look it up on your map. You can get a good idea of how far it has traveled (unless, of course, they have been doing loops all over the country, but water does not do that in the Oceans).

    Let us see how this applies to what we do in the boat. We can look at the results from a cruise that did the same type of analysis in this area 19 years ago. They also went from the west side of the Ross Sea to towards the Antarctic Peninsula.

    On the horizontal axis of the graphs below we have the distance traveled by the ship starting on the west (left) and ending in the east (right). The longitude is given above the graph. On the vertical axis we have depth in meters. The contours and colors give the values of the property that is measured, and the black along the horizontal axis corresponds to the mountains and valleys at the bottom of the ocean (called bathymetry).

    We will keep it simple and say there are three different water masses in the data below. oceanographers use acronyms to label them. The letters AA are used for Antarctic because A is used for Arctic. From the surface down, the water masses are Antarctic Surface Water (AASW), North Atlantic Deep Water (NADW), and the Antarctic Bottom Water (AABW).

    The temperature shown here is potential temperature, a concept I will cover another day. Think of it as regular temperature for the moment. It is measured in Celsius. The range goes from -0.8 C to 2 C (31.6 F to 35.6 F), pretty cold!

    Temperature vertical section for cruise S4 1992
    Temperature vertical section along the Akademik Ioffe S04 cruise (1992). Horizontal axis is distance in km from Cape Adaire or longitude. Vertical axis is depth in meters. The black shadow is the bathymetry. Adapted from Orsi, A., and Witworth III, T. 2005. WOCE Southern Ocean Hydrographic Atlas.

    Salinity is the amount of salts given in grams (g) that are dissolved in a kilogram of water (kg). Since 1000 g make up a kg, we can call it parts per thousand (ppt). A liter of water (about 1 kg) with 35 grams of salts gives us the average ocean salinity of 35 ppt.

    Salinity vertical section for cruise S4 1992
    Salinity vertical section along the S4 cruise (1992). Horizontal axis is distance in km from Cape Adaire or longitude. Vertical axis is depth in meters. The black shadow is the bathymetry. Adapted from Orsi, A., and Witworth III, T. 2005. WOCE Southern Ocean Hydrographic Atlas.

    Where does the Antarctic Bottom Water (AABW) comes from? Remember the explanation about density from March 6? Some of the surface water on the Ross Sea iceThere are terms for different types of ice. Shorefast ice forms along coasts and is attached to land. Pack ice is ice floating in open water. Multiyear ice is ice that has survived at least 1 summer. First year ice is ice that has not yet survived a melting season. shelf (towards the left on diagram) freezes as it loses energy to the air. Its salt is left behind in the surrounding water because it cannot dissolve in the ice. The density of the surrounding water increases as it gets very cold and very salty, so this surface water sinks all the way to the bottom forming the Antarctic Bottom Water. You can follow the cold and salty water on the diagram as it goes down on the west and spreads along the bottom even 2000 m away from where it was produced.

    The Antarctic Surface Water is created in the summer, when the winter ice and glaciers melt down. Since the ice has no salt, the resulting water is very cold (-0.8 C) but with low salinity (33.8 ppt). It is a low density water because of its low salinity, even if it is very cold, so it stays on the surface.

    Sandwiched between these two Antarctic water layers is water that is relatively warm and salty. We call this water Lower CircumpolarLocated or found within the Earth’s polar regions. Deep Water (LCDW). We can trace its origins to the water mass formed at the surface of the North Atlantic in the Labrador, Greenland and Norwegian Seas, mixed with the outflow of the Mediterranean Sea. This dense water mass flows along the bottom of the Atlantic Ocean, where we call it North Atlantic Deep Water (NADW).

    The NADW reaches the Southern Seas at the south west corner of the Atlantic ocean. It is then mixed by a strong current that flows around Antarctica (Antarctic CircumpolarLocated or found within the Earth’s polar regions. Current) which alters its temperature and salinity. It becomes the warm and salty Lower CircumpolarLocated or found within the Earth’s polar regions. Deep Water (LCDW) that we observe on the diagrams.

    We have learned today that we can estimate how far seawater masses have traveled in the oceans by looking at the water properties and their distribution. This is a first step for knowing how the oceans move due to density differences, but we are missing a very important piece of information that will allow us to predict where that water will be in the future. What is that?

    Let us go back to the rest stop in Wisconsin to continue with your trip. You finally reach a nice cabin on the shore of a small lake tucked in the northern woods. There are a few towns nearby and you want to order pizza. You look on the phone book and find many places that look promising. Let is assume they all take the same amount of time to make the pizza. You want to know who would deliver it first, since you are quite hungry. How can you figure that out?

    It would be good to understand the area's traffic (car circulation). You can use a map and the addresses from the yellow book to know which restaurant is closer, but that will not tell you all of the information you need for estimating which one will be delivered first. What other piece of information would help you understand the area's car circulation and predict which restaurant will deliver first? You have a distance, what else might you want to know? Send me an answer through the 'Ask the Team' tab if yo know this.

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