Drilling of gas and oil wells is done from the surface, using drill ships that sometimes look like conventional ships, and sometimes look like oil production platforms. The bottom may be 10,000 feet below the surface. The drill consists of a number of rods connected together, suspended and rotated from the top of a tower. The tower is built on the drill ship's deck, centered over a moon pool. The moon pool is a watertight passage through the ship's hull that allows the drill string to be lowered into the water from the top of the tower. Using GPS as a reference, the drill ship is held in position over the well by means of thrusters. This is called dynamic positioning. The drill bit is rotated from the top of the tower through the string of rods. As the bit drills into the bottom, more rod sections are hoisted into place and connected into the drill string.
When eddies detach from the Loop Current (LC), they can drift to the drilling locations. The rotating ring of water surrounding the eddy may have a velocity of 1 or 2 meters per second. Hydrodynamic drag on the drill string and vortices shed from it cause deflections and vibrations that can break the drill string. To avoid such a catastrophe, the drilling operation must be shut down when an eddy comes through the work area. Drill ships cost a lot to charter, so accurate knowledge and prediction of eddy activity has great economic importance.
The LC is 800 meters deep and transports an average of about 25 million cubic meters per second. The velocity ranges from zero at 800 meters depth to about 2 meters per second at the surface. The LC enters the Yucatan Channel and attempts to make a right turn from the far left lane in order to exit the Gulf of Mexico (GoM) via the Straits of Florida. The LC must cross lanes that are occupied by slow-moving masses of deep warm water that are surrounded by clockwise eddies with velocities up to 2 meters per second. Radar altimeter plots show that the sea surface height (SSH) is at least 50 cm on the right hand side of the Yucatan Channel due to the accumulation of these eddies. The LC cannot cross over these hills. It must intrude into the GoM so that it can cross the eddy traffic lanes before the eddies can occupy them. Sometimes the LC has to intrude past New Orleans to get ahead of the eddies. Then it has to come all the way back to exit via the Florida Straits.
The eddies form in the North Brazil Current near the mouth of the Amazon River. Jeffrey Early's website animation covers the period 14 October 1992 to 31 December 2008, about 194 months. I get a little bug eyed trying to count the number of clockwise eddies, but I guess it's about 90 to 100. They all go to the Yucatan Channel and get trapped in the LC. It is very difficult to count the eddies that break away from the LC in Early's animation, but these are easily observed and counted using printouts from the Gulf of Mexico Altimeter website. I count about 28 during the same time span. So the number of clockwise eddies drifting through the GoM is about 28% of the number formed in the North Brazil Current. Jeffrey Early's animation shows none leaving via the Florida Straits. So eddies are either morphing into fewer larger eddies or being annihilated. My impression is that they are being annihilated, and I think I have found a supporting observation.
I started a series of LC observations when a ring detached and drifted westward on 8/15/11. The altimeter SSH plots went through a number of changes and I noticed a strange feature on 12/21/11. Near the west end of Cuba there were almost straight and parallel SSH contours with heights up to 120 cm. These SSH contours were perpendicular to the coast and were about 100 km long. I interpreted this as two eddies trapped inside the Loop and being squeezed together. Picture two clockwise eddies approaching each other. The water circulating around one eddy is flowing in a direction opposite to the circulating water of the other eddy at the point of contact. As the eddies come together, the water must pile up in the collision. This could account for the unusually high SSH of 120 cm. There is a very steep downward SSH gradient toward the west from this peak, and a more gradual gradient toward the east, which is the downstream direction toward Florida. I think the water is flowing in the downstream direction. Thus, we have nulled out some vorticity. The 120 cm ridge was still present on 12/26/11, decreased to 110 cm on 12/28, and then disappeared. I interpret this as an eddy annihilation event lasting 8 days.
There is a wealth of information in these SSH contour plots. This should be a profitable study.
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