Instabilities and Multiscale Interactions Underlying the Loop Current Eddy Shedding in the Gulf of Mexico

Using a recently developed tool namely multiscale window transform (MWT) and the theory of canonical energy transfer, the role of multiscale interactions and instabilities in the Gulf of Mexico loop current (LC) eddy shedding is investigated in a three-scale energetics framework, in which the LC system is reconstructed onto a background flow window, a mesoscale eddy window and a high-frequency frontal eddy window. It is found that, quite different from the traditional point of view, the eddy shedding is mainly attributed to a barotropic instability of the background flow. During the shedding events, the unstable LC along the Campeche Bank efficiently transfers kinetic energy to the mesoscale field, which is dominated by a pair of anticyclonic and cyclonic eddies developing and propagating northwestward along the eastern basin. The well-developed cyclonic eddy then squeezes the neck of the unstable LC, facilitating the detachment of the anticyclonic eddy. Unlike barotropic instability, whether baroclinic instability exists during the shedding is case dependent; in some opposite cases, the system can even exhibit strong negative baroclinic canonical transfer, implying inverse cascades of available potential energy. Different from that between the background flow and mesoscale windows, the canonical energy transfer between the mesoscale and high-frequency frontal eddy windows makes only a small fraction in the mesoscale eddy energy balance, which generally acts as a damping mechanism for the mesoscale eddies. A budget analysis reveals that the mesoscale eddy energy gained through the instabilities is redistributed mainly via advection and pressure work.