A new hypothesis on the intrusion of the Loop Current

Abstract

The unsteady and inviscid shallow water equations of motion and continuity for a northward moving Loop Current (LC) on the f-plane were integrated to derive an equation for the rate of LC area increase. The dA/dt, where A is the LC surface area, is a function of kinetic energy change across the LC and absolute vorticity. This equation agrees with previous results and is proposed as a new hyphothesis to explain the LC intrusion: a reduction in kinetic energy from west to east across the LC causes mass to be stored inside the Gulf of Mexico (GOM) because the outflow is smaller than the inflow and is manifested as a positive dA/dt indicating a growing and intruding LC into the GOM. The necessary kinetic energy reduction for driving the LC intrusion is caused by relative vorticity and dissipative energy mechanisms. Observations from the intruding LC showed a horizontal reduction of kinetic energy as suggested herein. We discussed that relative vorticity, dissipative forces, and baroclinic and baratropic instabilities induce a kinetic energy reduction across the LC causing an intrusion. Because of the possibility of several kinetic and potential energy-changing mechanisms acting jointly, finding a single mechanism or “trigger” to explain the LC intrusion seems difficult. Additionally, it seems that the relative vorticity values within the LC act to keep dA/dt close to its steady state value. The proposed hypothesis sheds light on the correlation between the LC intrusion and relative vorticity found by Candela et al. (2002). This hypothesis is diagnostic in the sense that it suggests a mechanism or mechanisms that induce a kinetic energy reduction across the LC and the intrusion. Numerical models are ideally suited to evaluate such mechanism(s), to verify the hypothesis, and help design field experiments to test it in the real ocean.