The day to recover the mooring has arrived. But before I get to the description of the mooring recovery operations, which is very exciting in itself, I want to explain what is a mooring and why we need moorings to study the oceans.
The purpose of the cruise is to analyze seawater properties. We can see how much these properties change in space by sampling with the rosette at different spots along the cruise track, like we have been doing so far. It is like taking a cruise-long snapshot of a vertical slice of the ocean and observe its property distributions. We assume that the properties do not change during the two months that takes us to complete the cruise.
We know that seawater properties change with time and that these changes happen with different time scales. You are familiar with these types of changes because they happen in a similar way in the atmosphere. The temperature and rain in your town can change from day to day, but they also change according to the seasons. They even change from year to year or decade to decade. This winter might have been colder than the winter 20 years ago, for example.
In an attempt to understand changes the happen in long periods of time, our cruise is following the same stations sampled 19 years ago to quantify the changes over that period of time.
Alex is also interested on studying changes in the oceans that happen within a year. In order to resolve this faster changes, we would need to park our ship on the same spot and repeat the sampling and analysis every day. This becomes impractical when we want to analyze changes during one year. Imagine being on this boat on the same spot for a whole year? Where would we get the food, fuel and all other necessities?
At one point someone thought of leaving the instruments in the water for a whole year and said, 'Hey, we could anchor a cable to something very heavy in the bottom of the sea and attach floats to the other end so the cable stays vertical. We can then attach different instruments on the cable for them to measure the specific properties that we want. We will attach a recorder on each instrument so we can retrieve the data when we come a year later to recover the mooring'. And that is what they did.
Dr. Alejandro Orsi has specific objectives he wants to achieve with the two moorings he deployed a year ago: 'the project will investigate what processes control the flow of warm Circumpolar Deep Water onto the Antarctic continental shelf in the eastern Ross Sea'. I will write later why this particular problem is of interest. (read the journal entry of March 9 to know what is the Circumpolar Deep Water).
There are many types of moorings with different type of instruments. The bigger ones have a large buoy sticking out of the water with meteorological instruments and oceanographic instruments on the cable. Some of these are capable of sending information to a satellite in real time. The moorings we are recovering today are smaller, but still very efficient. Let us take a closer look at these moorings. Here is a diagram of the mooring design.
Diagram of the moorings we are to recover on the East side of the entrance to the Ross Sea.
As you can see on the diagram, the mooring is anchored by a 1500 pound death weight. A 5 meter chain connects the anchor to the two acoustic releases. Their job is to open the link between the anchor an the rest of the mooring when we send a signal, so the buoys can float to the surface. The releases are also equipped with a transponder that communicates with the boat; we send a radio signal from the boat and the transponder will reply to it. We can tell how far we are from the mooring by timing the time it takes the radio signal to go back and forth.
The two current meters measure the speed and direction of the ocean currents (velocity) like the radar gun that police uses for measuring your car's speed on the highway. The CTD sensors are just like the ones we lower with the rosette. CTD means, as you might remember, Conductivity, Temperature Depth. We use the electrical conductivity of water to infer the salt content of the water, so with the CTD data we can know the salinity, temperature and pressure, and ultimately calculate density (see journal entry March 6).
There are two sets of glass floats. Each float has two half spheres of 1 inch thick glass on a plastic enclosure called 'yellow hats'. All floats need to be able to sustain the high pressure of being 800 m below the sea surface, so even the foam floats are made of an extra rigid plastic that prevents them from imploding.
You can see that the last float is about 340 meters below the surface. This is what we call a sub-surface mooring. How do we recover such mooring? As said before, the two acoustic releases receive the order from the ship to open the link between the cable and the anchor, and let go of the mooring above. That is when the hunt begins. All eyes will gaze the surface waters hoping to find the buoys.
The deck on the stern (aft deck) was cleaned and prepared for the recovery. Everybody is getting anxious for the new event of the cruise. This type of sporadic events help break the monotony of the every day CTD casting.
Brett makes the deck safer by removing the slippery ice in preparations for the mooring recovery
We arrived to the location a few hours ago and found it covered by pieces of ice known as pancake ice. It is an awesome view, but it is not helping us locate the mooring.
A field of pancake ice makes a beautiful view, but does not help when trying to locate the mooring.
Usually we would trigger the releases when in the vicinity of the mooring because we could locate its beacon and radio transmitter from miles away. This time we have decided not to trigger the releases until we are certain that we are very close to where it will surface; we do not want to miss the beacon if the buoys remain under the ice. We faced another challenge with the first mooring. The transponder in the releases indicate the mooring was not in the exact location where it was left. We have been triangulating the ice patch to narrow down the location, and found that it has somehow drifted 1 mile from its original position.
Wake left by the Palmer as we triangulate the pancake field in search of the mooring.
Mike, Buzz and Jim listening to the signal from the transponder on our instruments trying to find the exact position where the mooring is now.
There is no more sunlight, and we will not recover the mooring in the dark. We are planning on finding the exact location of the mooring, and then do five quick CTD casts with the rosette on the shelf slope. We are only interested on the salinity and temperature data from the casts, so we only need to use four bottles from the rosette for calibrating the CTD data. We will go back to the mooring location tomorrow morning and retrieve it with full sunlight.
Here is a question for you. If we are only interested on four bottles out of 36, do we need to leave only four bottles open as the rosette is lowered to 800 m, or should we open them all? Send me your reasoning for doing it one way or the other.