Ocean Circulation, Part II

Now that you understand where deep and bottom water form "ocean circulation, part I", lets look at one ocean basin in particular: the Atlantic. Right now, I'm in a part of the Southern Ocean that could also be considered the tip of the South Atlantic. Those of you at DLMS are very near the North Atlantic. So if we took a north-south line from you to to me, what would the ocean look like in between us?



Here's a map showing salinity from -60° (that's me!) to 70° (further north than New York, which is at 40°) in the western Atlantic Ocean. The average salinity in the ocean is 35. The units of salinity are a little complicated - some people use what's called the practical salinity unit, or psu, and some people use per mil (0/00‰), which is like percent except that it's a fraction out of one thousand instead of one hundred.

For now, just concentrate on the colors in the map. Red and yellow represent the high salinities and blue and purple represent low salinities. The big mass of green is NADW. The blue is AABW, and the purple is AAIW. The red area near 30° of latitude is water that flows out from the Mediterranean Sea.

Saltier water is denser, but temperature has a bigger effect on density than salinity does. AABW isn't the saltiest, but it's the coldest and the densest. NADW comes next: it's not as cold, but it's very salty. AAIW is near the same temperature as NADW, but it isn't as salty so it's less dense. The water from the Mediterranean Sea is very salty but also very warm, so it's the least dense of all.

Question: Besides temperature and salinity, what else do you think might make water masses different?

Hydrophones.

Remember Dr. Erin Pettit from Girls on Ice ? She's working on Flask Glacier to the southwest of us right now, but before she left, she asked me and Yuribia Muñoz to help her out with a really cool project.

One of our goals down here is to understand how glaciers work. We're studying them lots of different ways. We put AMIGOS on them, we put GPS stations on them, we take ice cores, we take sediment cores, we look at satellite pictures - you get the idea. But Erin wants to try something else: listening to them.

Some of you may remember that I use sound to measure ocean currents. That's called active acoustics because I send out a sound and listen for the return. This is passive acoustics, where I put a special kind of microphone, called a hydrophone, into the water and listen.

What am I listening for? When ice melts, it makes sounds. It can sound like crackling or popping. When big chunks of ice fall off glaciers (called calving) it can sound like fireworks, at least in the air. Erin and other scientists think that they could use passive acoustic systems to monitor glacial melt rates in areas that are too hard to reach with other instruments.

To test that idea, we are making measurements of sound near any ice we can reach. Our goal right now is to gather preliminary data and also to test the instruments.



Yesterday, we got to go out on the ice again. The biologists took an ice core to look for algae and we used the hole that remained to put the hydrophones in to the water. Clockwise from the left, you can see Yuribia Muñoz, Kim Roe, me, and Laura Grange. We got some good data and identified some problems with one of the hydrophones. It isn't heavy enough, so it's hard to get it through the hole in the ice. Once it's in the water it doesn't fully straighten the wire it's attached to, so we can't be sure exactly how deep it is. We also had some interference on both hydrophones from noises that the ship makes.

It's really important to work out those kinds of problems, even if they sound small and unimportant compared to the complicated equations we use to turn sound from the water into usable data. Scientists need to be able to solve practical problems as well as intellectual ones. Did any of you at DLMS have similar problems with your science fair projects?

Ocean Circulation, Part I

Whenever I do a CTD cast, I get a very precise snapshot of the ocean. I know the temperature and salinity of the water underneath me and exactly how they vary with depth. But it's only one moment, in one place. For the data to make sense, I need to understand how the ocean works.

The ocean is mostly stable. The ocean is heated by the sun, so the water on top is warmer than the water in the bottom. If you remember what you learned about convection in sixth grade, you know that you only get all of that motion when heat is added to the bottom of a fluid. When it's added at the top, you get layers of lighter (less dense) water over layers of heavier (more dense) water.

Still, the oceans circulate. One reason is that the tropics get more heat than the poles do. Since it's warm near the equator and colder everywhere else, heat has to circulate. Some of that circulation is done by the atmosphere, but some is done by the ocean.

Water sinks in the ocean in two areas: the North Atlantic and the Southern Ocean around Antarctica. The water that sinks in the North Atlantic is called North Atlantic Deep Water (NADW) and is very salty. Some of the water that sinks in the Southern Ocean goes down to around 1000m and is called Antarctic Intermediate Water (AAIW). The really cold water from the Southern Ocean sinks all the way down to the sea floor and is called Antarctic Bottom Water (AABW).

NADW is around 2° to 4° C (35.6° to 39.2° F). That sounds pretty cold to me! But AABW is colder: -2° to 0° C (28.4° to 32° F). Based on these different temperatures, we can see how far they travel.



Everywhere the map shows red is NADW, and the blue is AABW. Now you can see why processes in Antarctica are so important: water that forms here spreads all over the world.

Science Fair, Antarctica style

It's science fair season at Dual Language Middle School, and I'm sorry I can't be there for it. But I've been working on my own science fair project out here and I'd like to report some results.

Topic: The speed of ocean currents in Antarctica

Question: How does the depth of the water affect its speed?

Hypothesis: Shallow water moves faster than deep water.

Background Information: Lowered acoustic Doppler current profilers (LADCPs) use sound energy to measure the speed and direction of ocean currents. Water in the ocean can be moved by wind, tides, and differences in pressure due to temperature or salt content.

Procedure: Use LADCP data to compare the depth of water (the independent variable) with the speed of the water (the dependent variable).

Results: I was able to collect data from 38 stations all around the Antarctica Peninsula. On average, the fastest water was at the surface as predicted (figure 1).

Discussion: Average speed does not tell the whole story. The maximum speed that was found in any location occurred at 60 m (figure 2), and the minimum was found at 175 m (figure 3). There is a lot of variability in water speeds, especially at depths above 100m. Below that, the speed changes very little but does not stay exactly steady.


These figures show average, maximum, and minimum water speeds. Please note the different scales.

Conclusions: On average, the fastest water is at the surface. However, there is variability between stations and there can be high speeds below the surface. Speed generally decreases with depth, but there are exceptions. Differences in tides, wind, and ice conditions might also effect water speed.

CTD, Part #1 "the basics of running the CTD"

Some of you might remember the CTD from my last trip to the Philippines. CTD stands for conductivity, temperature, and depth. There's a big frame, lots of bottles for holding water, and sensors for measuring a whole bunch of different things. It goes into the water on a thick cable attached to a winch. Here are Kim Roe and Yuribia Munoz opening the bottles so that the
CTD can go in the water:



While the CTD is in the water, a scientist sits at the computer and watches all the sensors. It's an important job, because that person is responsible for telling the winch operator how fast to go and when to stop, and for making sure that everything is working correctly, and for stopping the CTD to close the bottles at right the depths. While the CTD is in the water, everyone else is crowding around the CTD computer, asking to see this graph or that graph and arguing about where to sample water.

And do you know who is responsible for running the CTD? Me.

Well, it's me if it goes into the water between 8am and 8pm, otherwise it's done by Bruce Huber (my boss) or one of the electrical technicians on board. It's important enough that I'm going to do a few posts about the CTD. Next time I'll talk about the data that I get from the CTD, but for now we'll start with the basics. Here is my list of the top ten things you should remember if you ever find yourself running the CTD:

10. Ask questions! If you don't understand what to do, or how to do it, just ask someone who knows. Don't agree to do something if you don't understand why.

9. Remember to have fun. The job you're doing is pretty cool! Look at data from other CTD stations, make a hypothesis to explain what you're seeing, and try to understand what you're looking at.

8. Pay attention. It's not that your job is hard, it's that it requires a lot of concentration. No daydreaming! Conversations are okay, but you need to keep your eyes on the screen.

7. Go to the bathroom before you start. Once the CTD begins, you can't leave for even a minute.

6. Stay calm. Other people will get stressed as they try to figure out what they want you to do. That's okay. You can't control how other people react, but you can control yourself.

5. Take it slow. There is no prize for getting the job done a few minutes faster, and you're more likely to make a mistake if you hurry.

4. Be nice to your winch operator! He's the one who is really driving. If he does his job well, it makes you look good, so make it easy for him to do his job well.

3. Take very good notes. You think you'll remember little questions or comments that you had, but you won't.

2. Make sure you have good friends around. Sometimes Kim will bring me M&Ms, or Kathleen (my roomie and photographer extraordinaire) will go and find my mug of tea for me.

1. DON'T HIT THE BOTTOM. If you hit the seafloor, everyone will be really, really mad at you. You might break or lose the equipment, and it will take lots of time to fix. This is the worst thing that can happen to the CTD. So DON'T HIT THE BOTTOM.

Rothera Station

I boarded this ship in Chile, went to Antarctica, and stepped off in
England.

Well, not quite. But it sure seemed that way as our tour guide spoke with a British accent and then invited us all for tea and biscuits. We were at Rothera Research Station, a part of the British Antarctic Survey and now a part of LARISSA.

Four out of our five glaciologists were out in the field when the weather got bad, so our helicopter couldn't pick them up. Instead, the plane from Rothera was able to get them, and they'll be working out of Rothera for the next two weeks. But they needed all of their supplies (including the AMIGOS!), so the ship went to meet them.

I was so excited to get off the ship at Rothera! Working at sea is fun, but it's great to stretch your legs on dry land. We got a tour of the area and saw penguins and seals. In addition to the tea and biscuits (cookies) and a game of football (soccer), we had a big party and all got to relax a bit.

Now we're heading back to the eastern side of the peninsula. We'll have a lot of work to do, but we're all hopeful that the ice will be better and we'll be able to get all the way south.

A History Lesson

I was really lucky yesterday. I was lucky because my very kind boss, Bruce Huber, volunteered me as an assistant to Greg Balco. Greg is a geologist from Berkeley, and he's here studying the glacial history of this region.

Geology is like a big puzzle. The area where we worked, Duthier's Point, was once covered by a glacier. Now it isn't. But when did that happen? How can we find out?

Greg was looking for rocks that would give us clues. Since I was his assistant, I was there to help him look for rocks but mainly for safety. Greg's work involves a lot of climbing up and down rocks, and it would be too risky to send one person off alone, especially in Antarctica.

In search of clues, we took a helicopter to Duthiers Point. I loved riding in the helicopter and seeing the glaciers and icebergs from above. The ship looked so small! Once we were on land, we started looking.



We were looking for rocks that didn't belong, called glacial erratics. When glaciers expand, they pick up rocks along the way and drop them in new places. It's pretty easy to spot them. In this case, they were quartz, which is much shinier and lighter than the rocks that we were standing on. The problem is that Greg needs glacial erratics that haven't been covered by snow. If the glacier retreated but then snow covered the rock, Greg's techniques for determining how long the rock had been exposed wouldn't help. The rock can't tell the difference between being covered by a glacier and covered by snow! So we needed to find a glacial erratic that was perched high up, not in a hole or or protected by other rocks.

We didn't find any good rocks at our first site, so we started climbing. We climbed. And climbed. We made it down 300 feet of rock until we were almost at the sea. As we climbed, I though about two things.

The first was the penguins. They were everywhere, perched only a few feet away from me. They had their babies on their feet and they were ridiculously cute. We're not supposed to approach them, but they didn't seem to care about us at all.



The second was memories of the last time I had done any climbing at all, which was about 8 years ago and was not nearly as challenging. Greg is an experienced climber; I am not. I was terrified, hanging on to the edges of slippery, crumbling rocks for dear life. But I kept going, because I hate to admit that I can't do things and because every time I turned around, I saw views that were more beautiful than ever.



When we made it to the bottom, we were in the middle of a penguin colony that now surrounds a GPS station. Greg had to check on the station, which transmits data to scientists back home. The penguins have now moved in all around it and continued to ignore us.


After spending some time with the penguins, we headed back up the cliff. The plan was to pick up any good rocks that we had spotted on the way down - but we hadn't found any! Climbing up is always much easier than climbing down. I think that's because you're not looking in the direction that you might soon be falling.

At the top of the cliff, I rested for a bit while Greg called the ship and asked them to send the helicopter to pick us up. While we were waiting, we wandered around a little more and there it was: the perfect glacial erratic, perched in an unprotected location. Greg tried to break it so that we could bring home a smaller, lighter piece, but it was too hard and he decided to keep the whole thing.



The helicopter came and took us back to the ship. I had another ride with a great view. When I got back, I quickly showered, ate dinner, and collapsed into my bunk.