The Stability of Loop Current Eddies

The following images show time-evolution of Loop Current Eddy-like structures over a slope representative of the northern Gulf of Mexico. In the three cases shown below, the three-dimensional, primitive-equation calculations were initialized with a model LCE centered approximately over the 2000m isobath, but the initial rotation differed. In Case 1, the upper and lower (at 800m) layers rotated in the same sense (i.e. anticyclonic), the maximum current speed at 800m being 25% that at the surface. In Cases 2 and 3, both layers rotate in the opposite sense, but Case 3 has a lower-layer counter-rotation that is much stronger – 60% versus 10% for Case 2.

In Case 1, the eddy was stable throughout the 40day integration shown (the calculation was extended to 120 days and the eddy remained stable). In Case 2, the eddy became weakly unstable, while in Case 3 it was explosively unstable and resulted in eddy-spliting.

Conclusion: counter-rotating eddies tend to be unstable, while co-rotating eddies are stable.