The Multiscale Dynamics Underlying The Kuroshio Extension And Its Decadally And Seasonally Varying Eddy Kinetic Energy Field

As a direct continuation of Kuroshio,the western boundary current,the Kuroshio Extension exhibits a broadband of temporal variabilities,and is known as one of the regions with highest large-scale as well as mesoscale variabilities in the mid-latitude ocean.As these physical processes are highly nonlinear,multiscale interactive,and intermittent in time and space,studying their internal dynamics have become one of the major challenges in physical oceanography.Multiscale energetics analysis has long been recognized as a powerful tool to quantify the multiscale interactions,intrinsic instabilities and energy distributions of atmospheric and oceanic processes.For general nonstationary multiscale processes,such as instabilities,however,the interpretation and the diagnoses of multiscale energetics have long been recognized challenging in geophysical fluid dynamics;even in representing energy there exist misconceptions.A common practice,for example,is to square the filtered or reconstructed field as multiscale energy,while in fact multiscale energy is a concept in the frequency(or wavenumber)domain,which is connected to the physical energy through the Parseval identity.These issues have not been tackled until Liang and Anderson(2007)developed a new functional analysis tool,namely,the multiscale window transform(MWT).Using the MWT and MWT-based localized multiscale energy and vorticity analysis(MS-EVA),and MS-EVA-based canonical transfer theory(Liang,2016),this study investigates the intricate nonlinear mutual interactions in the Kuroshio Extension from a multiscale energetics view.Specifically,we aim to address the following challenges:1.How the eddies and other variabilities are generated in the Kuroshio Extension region,and how the generated eddies interact with the low-frequency fluctuations,and the mean flow?Where are the sources of variabilities?Using the outputs from an eddy-resolving multidecadal hindcast simulation(OFES),we reconstruct the Kuroshio Extension system into three subspaces called scale windows,i.e.,the decadally modulating mean flow window,the interannual low-frequency fluctuation window,and the transient eddy window.Then,this study investigates the intricate nonlinear interactions among these scale windows in this region.In the upstream,strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed.Besides fueling the eddies,the unstable mean jet also releases energy to the interannual low-frequency processes.Between 144°-154°E,both transfers from the mean flow and the low-frequency variabilities are important for the eddy development.Further downstream,eddies are found to drive the mean flow on both the kinetic energy and available potential energy maps.Similar upscale energy transfers are also observed in the northern and southern recirculation gyre regions.In these regions,the low-frequency fluctuation-eddy interaction exhibits different scenarios:The eddies lose kinetic energy to the low-frequency processes in the northern RG region,while gaining kinetic energy in the southern RG region.2.It is found that in the upstream Kuroshio Extension,the stronger the baroclinic jet,the weaker the eddy kinetic energy(on a decadal scale).What causes this phenomenon,which is counterintuitive to the classical linear stability theory?Using the state estimate from the "Estimating the Circulation and Climate of the Ocean,Phase ?"(ECCO2),this study investigates the physical mechanism responsible for such variations from a multiscale energetics perspective.For the first time,we find that the decadal modulation of EKE is mainly controlled by the barotropic instability of the background flow.During the high-EKE state,violent meanderings efficiently induce strong barotropic energy transfer from mean kinetic energy(MKE)to EKE despite the rather weak jet strength.The reverse is true in the low-EKE state.Although the enhanced meander in the high-EKE state also transfers a significant portion of energy from mean available potential energy(MAPE)to eddy available potential energy(EAPE)through baroclinic instability,the EAPE is not efficiently converted to EKE as the two processes are not well correlated at low frequencies revealed in the time-varying energetics.3.The EKE in the Kuroshio Extension differs from season to season.What controls the seasonality?Finally,using ECCO2 state estimate,the contribution of local wind forcing,nonlinear barotropic and baroclinic energy transfer,and nonlocal energy advection to the seasonal eddy variability in the Kuroshio Extension are investigated.The EKE in this area exhibits a clear seasonality,which is strong in summer and weak in winter.In contrast,the wind power input is strongest in winter and weakest in summer in the Kuroshio Extension region,implying that the eddies in this area are not directly generated through local wind forcing.Our results highlight the different oceanic dynamics controlling the seasonal eddy variability in the upstream and downstream region of the Kuroshio Extension.Both barotropic and baroclinic instabilities are responsible for the broad summertime EKE peak in the upstream region,while they are not related to the seasonal EKE variation in the downstream region.Rather,the seasonal cycle of the EKE in the downstream region is more connected to the nonlocal EKE advection from the upstream.