The Deformation Mechanism Of Kuroshio In The Luzon Strait And Its Interaction With Rossby Waves And Eddies

Kuroshio’s pathway in the Luzon Strait when losing western boundary support isan important scientific issue in western boundary current dynamics and has drawn alot of oceanographers’ attentions, as it plays a very important role in determining themomentum, vorticity as well as mass exchange between Pacific and South China Sea(SCS). In view of existing debates, in this paper, we analyzed the observations, did aseries of numerical experiments, and applied instability and energy conversiondiagnosis using2.5-layer quasi-geostrophic (QG) model, Reynold-averaging methodas well as wavelet-based Muti-Scale Energy and Vorticity Analysis (MS-EVA)method. Finally, we clarified the debates and arrived at some innovative conclusionsas follows.This paper proved again that the mean-state Kuroshio has a gap-leaping pathwayin the Luzon Strait. We pointed out that the topography, especially the bottomtopography and islands near Balintang Channel, constrains the initial incident angle ofKuroshio. When Kuroshio develops into an inertial jet after losing support from thewestern boundary, in the absence of external vorticity forcing, the conservation ofabsolute vorticity determines the gap-leaping pathway of Kuroshio in the Luzon Strait.This result also explains the physical essence for Kuroshio looping into SCS in lots ofnumerical simulations which do not fully resovle the bottom topography and islandsnear Balintang Channel.We first pointed out that the Kuroshio pathway transition to looping state inwinter is dominated by the Ekman transport and negative wind stress curl exerted bywinter Monsoon. This result corrects the previous viewpoint that Kuroshio loopinginto SCS in winter is determined by the week inertial effect associated with smallKuroshio transport in winter.Our research proved the gap-leaping Kuroshio and corresponding westwardshoaling of thermocline in Luzon Strait builds up very intense zonal potential vorticity(PV) gradient, which is one order higher than-induced planetary PV gradient.According to QG dynamics, this zonal PV gradient will be a strong constraint forRossby wave and eddy propagation, preventing them penetrating through Luzon Strait.While in looping state, Kuroshio’s blocking effect vanishes.Through instability and energy diagnosis using numerical model results, we pointed out that baroclinic energy conversion from mean flow to eddy/perturbationfield is the energy source of anticyclonic eddy shedding and associated cyclonic eddygrowth west of Kuroshio front. High baroclinic energy conversion locates upstream ofthe anticyclonic loop on the western flank of Kuroshio axis. During Kuroshiodeformation and anticyclonic eddy growing stage, the energy source straddles boththe anticyclonic and accompanied cyclonic eddies, and the mean flow suppliespotential energy to both of them for growth; during the anticyclonic eddy sheddingstage, the energy source moves upon cyclonic eddy, and the mean flow suppliespotential energy only to the cyclonic eddy, makeing it strong enough to cut off theanticyclonic eddy from Kuroshio.