Wednesday, March 22, 2017

Kurt`s Mooring

Our first major science task on the cruise was to deploy a mooring for Kurt Polzin from Woods Hole. Kurt is in hopes of placing the mooring in a region of active deep ocean turbulence. For this purpose, he chose a spot at 60S near the South Orkney Islands. If this is indeed a region of active turbulence, the measurements will help to understand how waters mix in the deep ocean, and how this mixing transforms (changes) water properties (e.g., temperature, salinity, carbon, oxygen) as the water moves northward into the South Atlantic.

Basics of oceanic internal waves

Much of the deep ocean turbulence is initiated by astronomical tidal forcing that move columns of seawater across rough bottom passages, such as in the South Orkney Island chain. As this "barotropic" motion (coherent motion from top to bottom of the ocean, like a 5km skyscraper of water) encounters the rough ocean bottom, some of the barotropic energy is converted to "baroclinic" gravity waves through interactions (i.e., bumping into) the ocean bottom topography.There has been extensive research into this process during the past few decades. But there remains a lot of mystery, largely due to the extreme difficulties of direct ocean measurements of the processes.

Baroclinic gravity waves are akin to surface waves we see from a boat, only they are inside the ocean interior.  They are undulations of the stratified density surfaces, and they oscillate due to gravitational forces acting on different layers of seawater buoyancy. Imagine oil on top of water, and gently shake the container. You have just created internal gravity waves in the kitchen! For this reason, some like to call these waves "internal gravity" waves, which sounds a bit less sophisticated than the rather technical term ``baroclinic''. But they are really the same thing, just different names. 

Internal gravity waves generally interact with one another, as they are created all over the place, propagating around the ocean interior. Imagine waves emanating from two pebbles thrown into a pond, with the interacting waves creating a mishmash of constructive and destructive interfering waves. Besides interacting with one another (``wave-wave interactions``), internal waves also bounce off the bottom topography, and that can further impact their properties (e.g., wave length, amplitude).  

When internal waves steepen, they can break in a way that creates turbulent mixing. Think of the breaking white caps on the ocean surface, or breaking waves on the beach, only now these internal waves are 1000s of meters in the ocean interior. Or think of the oil/water bottle, and shake it vigorously.  When internal gravity waves break, they can mix very very large masses of water, like mixing a huge pot of the oceanic soup.

The dynamics of internal gravity waves and the mixing associated with gravity wave breaking are very interesting physical phenomena that intrigue physical oceanographers. Their impacts are also quite important for how the ocean works, such as how much heat and carbon are mixed into the ocean interior.  Hence, these processes have a very important role in how the ocean fits into the global climate system.  

To help better understand the physics of deep ocean mixing, Kurt has placed some ultra-sensitive devices along his mooring to measure flow and various turbulent fluid properties. He hopes to measure some of these "mixing events" occurring when waves break. The bulk of Kurt's instruments are within the bottom 1000m-2000m of the ocean bottom, where most of the interesting turbulent mixing is presumed to exist. Results from Sonya's computer model provided nice confirmation of the deep mixing in the proposed mooring location.
Kurt Polzin on the back deck watching the deployment of his mooring. Note the yellow plastic balls near his feet, which are floats to be strung near the top of the mooring.

What is a mooring?

A mooring is basically a long cable with any number of instruments attached at selected vertical positions. It extends from the ocean bottom (anchored with a huge weight, such as a piece of iron) to near the surface. There are floats (glass balls surrounded by hard yellow plastic containers) attached to keep the mooring vertically positioned. Kurt is very proud of his mooring given its very high precision instruments to measure current properties. He has 21 velocity sensors and 11 temperature sensors strung along the mooring, extending from the ocean bottom to around 2000m above the ocean bottom.

We reached the mooring deployment site on Tuesday, 21 March  near the South Orkney Islands. The mooring will stay in the ocean for about 40 days, after which time we will recover it on the way north near the end of the cruise. One of the uncertainties in this science is whether all of the measurement devices will stay in working order throughout the deployment, and how readily we can recover the information saved when sitting on the ocean bottom.

Deploying Kurt's mooring

Back deck with the mooring floats (yellow balls) and the crew getting things organized. Kurt is on the right (yellow jacket) and Andy is near the centre with the red jacket. They sky was clear but the winds were blowing and the air near freezing.
The process of deploying a mooring is nontrivial to say the least. It involves many of the ship's most experienced crew, as well as a top-notch ocean tech, Andy Davies, from Woods Hole who came on the cruise with Kurt. Povl Abrahamsen from BAS is also onboard.  He is an expert at deploying moorings, having been the principle scientist on the previous DynOPO cruise two years ago where he organized the deployment of other moorings that we will recover later in this cruise.

The overall aim is to have instruments placed at selected positions on a cable that is about 4000-5000m long. That is 4-5km!! It is a very long cable, taking many hours of careful winch operation, instrument placement, float placement, etc., all while standing on the back of a ship exposed to the rolling sea. We had plenty of waves on the ocean surface during deployment due to a strong wind (a rather normal occurrence for this part of the world). At least the sky reasonably clear.

Those near to the back deck doing the bulk of the mooring work wear safety cables in case they are swept into the ocean. At our location, the sea surface temperature is very close to freezing. Unprotected, a person will live no longer than a 3-5 minutes in that water. So it is simply NOT an option to fall overboard. Hence, there are heaps of safety precautions, including the cables attached to the work clothes. I took GoPro time lapse of some portions of the mooring deployment, at least for as long as I could withstand the winds and cold standing outside the UIC lab.

After a number of hours, Andy, Povl, and the crew successfully deployed the mooring. Now, we hope it will take a full suite of measurements for the next 40 days, with a successful recovery toward the end of the cruise. Fingers crossed!

Andy (red jacket and gray hat) leading the mooring deployment.  Note the train of yellow floats now in the water. At this point, the ship steamed ahead for about 1km, extending more cable out. Then, instruments were placed at selected positions along the cable. When all the instruments were attached, the anchor was then attached and thrown over. As the anchor fell to the bottom, the mooring line became vertical, with the yellow floats about 2km below the ocean surface. In 40days, we will return to this location to recover the mooring instruments.  We will do so by sending an acoustic signal to the anchor hook, releasing the anchor so the floats will rise to the surface. 

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