Migration Of Mesoscale Eddies Through A Meridional Gap Leaped By A Western Boundary Current

A 1.5-layer quasi-geostrophic reduced gravity ocean circulation model is used to study the propagation of mesoscale eddies influenced by a western boundary current(WBC) of various states along the hysteresis loop of the flowing by a gap based on the ideal rectangle ocean basin numerical simulation.Both cyclonic and anti-cyclonic eddies migrate westward with amplitude decrease in an infinite β-plane. The amplitude decrease of cyclonic eddies is similar with anti-cyclonic eddies which is dependent with the initial radii of eddies. The eddies of larger initial radii lose less amplitude than the eddies with smaller radii. Eddies experience more blockage from the gappy boundary as they migrate through the gap.Both cyclonic and anti-cyclonic eddies migrate across the WBC of penetrating state in a gap accompanying with an opposite type of potential vorticity induced to the weakened(cyclonic) or strengthened(anti-cyclonic) penetration of intrusion branch of the WBC by the eddy. The eddy pair interacts with each other and migrate to the west together. Both type of eddies gained strength from the WBC in the course of the propagation across the WBC in the gap. Eddies approaching the gap from the upstream are found to migrate more easily into the western basin due to the advection of the WBC. The migration speed of the eddies are almost unchanged by the presence of the WBC in all experiments, which is 0.08 m/s.The steady gap-leaping WBC nearly blocks all eddies from propagating across it through the gap completely, keeping them confined in the eastern basin and moving northward downstream of the WBC along the western boundary.The interaction of the WBC in critical state of hysteresis loop and eddies is often in the wake of path transition of the WBC. when the WBC in the critical state from steady penetrating state to periodic eddy shedding state interacts with an eddy in the vicinity of the gap, path transition of the WBC happens whether the eddy is cyclonic or anti-cyclonic, cyclonic and anti-cyclonic eddies are shed alternately westward from the gap as a result. Both cyclonic and anti-cyclonic eddies migrates into the western basin through the gap completely or partly. With the bigger radius, the eddy has more resistance. Anti-cyclonic eddy has more resistance than cyclonic eddy with the same radius. The eddy located north has more resistance than the eddy located south. Both cyclonic and anti-cyclonic eddies obtain energy from the interaction with the WBC, and the energy is more than it from the interaction of eddies with the WBC in steady penetrating state.Both cyclonic and anti-cyclonic eddies induce the path transition of a critical WBC from the periodic eddy shedding state to the leaping state. The propagation of eddies is different for different type: the anti-cyclonic eddies are almost completely blocked and move northward along with the downstream of the WBC; the cyclonic eddies partly migrate into western basin through the gap.The path transition of the WBC from the leaping state to periodic eddy shedding state could be induced by cyclonic eddies, and the location of eddies is important. The cyclonic eddies are blocked by the WBC, while several anti-cyclonic potential vorticities induced in the west of the gap, all eddies propagate westward with amplitude development. Anti-cyclonic eddies cannot change the path of the WBC, the eddies are almost blocked completely and flow northward along with the downstream of the WBC, while a weak cyclonic eddy is induced in the west of the gap and propagate westward with amplitude decay.A 1.5-layer quasi-geostrophic reduced gravity model is also used to study the hysteresis of a periodic WBC flowing by a meridional gap or a WBC with leaking transport through the gap. When the periods of WBC variations are much longer than the Rossby adjustment time scale of the circulation in the vicinity of the gap, the path of the periodic WBC follows the hysteresis loop of a steady WBC. The critical Reynolds numbers of the regime transitions of the hysteresis are nearly unchanged by the periodic forcing, expect that the Hopf bifurcation during the Re-increase and Re-decrease loops are delayed to produce a new domain of regime shift. When the periods of the WBC are comparable to those of the Rossby wave adjustment time scales, the hysteresis domain of the Reynolds number increases for shorter periodic forcing. When the periods of the WBC are much smaller than those of the Rossby wave adjustment time scales, the WBC path in the vicinity of the gap becomes periodic without hysteresis. The intrusion of the WBC into the western basin generally gets smaller as the period decreases.Numerical experiments have also shown that the partial leakage of the WBC transport through the gap into the western basin has significant impact on the hysteresis loop of the WBC path. The smaller the throughflow transport, smaller the impact is.