The Loop Current (LC) enters the Gulf of Mexico (GoM) at the Yucatan Peninsula and sometimes flows around Cuba and goes directly into the Florida Straits (FS). Then, as time progresses, the LC intrudes increasingly into the GoM and makes a right-hand U-turn to return and connect with the FS. This trombone-shaped path can extend several hundred kilometers toward New Orleans and sometimes can come uncomfortably close to New Orleans. This poses a hurricane danger to the Gulf Coast because deep warm water accumulates inside the loop and can supply energy to hurricanes that pass through the loop. We know this water is warm because radar altimeter maps of the sea surface show peak elevations 70 cm or more. These raised portions are there for periods of many months, and this requires that they contain warmer, lighter water.
The LC flows around the periphery of the hill of warm water at approximately the 20 cm elevation contour. The Coriolis force pushes the moving water to the right with a force equal to the density times the velocity times the Coriolis parameter. The Coriolis parameter is twice the Earth rotation rate times the sine of the latitude. This sidewise force is balanced by the density times the acceleration of gravity times the slope of the sea surface. On May 29, 2011, the LC surface was sloped upward to the right by 9.5 parts per million. The Coriolis parameter was 5.93E(-5)/s. The balance of sidewise forces, which is called geostrophic equilibrium, requires that the surface velocity be 1.57 m/s, which is about what we expect.
The LC is about 700 m deep because the Florida Current path is this deep. The water is stratified with the coldest, most dense water at the bottom, and there is no flow below 700 m depth. So the velocity near the bottom of the LC flowing around the warm water hill is low and there is little Coriolis sidewise force. But geostrophic balance is preserved because the greater depth of water on the right is composed of warmer, lighter water, so there is little sidewise force from gravity. So the LC water hangs together and flows around the hill of warm water. I used to think that the LC piled up the water inside the loop by means of the Coriolis effect. Now I realize that this is not true. The hill of warm water moves into the GoM, possibly due to wave action driven by the easterly winds. I am only beginning to learn about long waves in the tropical ocean, so I am just guessing. I do know from observation of sea surface height (SSH) maps from 1993 to the present that these masses of warm water just keep coming into the Gulf. They never retreat back into the Caribbean. This seems to be a movement separate from the LC, which transports an average of about 25 million cubic meters per second. The LC could fill the entire 700 km-long hill present on May 29, 2011 in about half an hour, but this is not what happens. The LC is forced to flow around the hill. Note that the hill is hardly noticeable. It is less than 1 meter high and extends more than 200 km in width. Nevertheless, as the hill moves into the Gulf, the LC is forced to travel a longer path until the power available from the difference in sea level at the entrance compared to the outlet can no longer supply the power dissipated in the turbulent friction of the longer path.
When the sea level difference is no longer able to supply the power dissipated in the longer path, a piece of the loop pinches off and may separate and drift westward across the Gulf. Before I realized that the intruding hill of water could stretch the LC path, I looked for events of decreasing sea level difference to initiate loop pinch-offs. Sometimes I found them and sometimes I did not. But now i see that pinch-off can be initiated by the intruding hill stretching the LC path. I used to think that the LC path increased because there was more power to dissipate. Not so. More likely, not enough power is available to drive the increased length, so pinch-off is initiated. A closed recirculating ring separates and drifts off, along with some of the warm water hill. Then the remaining warm water hill continues to move into the GoM and what is left of the driven LC is forced to stretch around it.
If the foregoing speculation proves to be correct, we may still have a valid method of using floating turbines to reduce hurricane intensity on the Gulf Coast.
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