The Instabilities and Multiscale Energetics Underlying the Mean–Interannual–Eddy Interactions in the Kuroshio Extension Region

Abstract

Using a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 144° and 154°E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual variabilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.

On the Decadal Variability of the Eddy Kinetic Energy in the Kuroshio Extension

“…Ever since the Lorenz (1955) energy cycle was introduced in the atmosphere science, energetics analysis has become a powerful tool to diagnose the intrinsic and external energy sources and sinks and flow instabilities. Thus, it is natural to evaluate the decadal EKE variations in the Kuroshio Extension from the perspective of time-dependent energetics analysis, which, to our knowledge, has not been documented before [what Yang and Liang (2016) investigated is long-term mean energetics in this region]. In this section, we present a brief introduction of the localized MS-EVA and the MS-EVA-based theory of finiteamplitude baroclinic and barotropic instability; for details, see the references cited thereof.…”

“…Baroclinic and barotropic instabilities have been considered as the major energy sources for mesoscale eddies in the Kuroshio Extension (Hall 1991;Waterman et al 2011;Yang and Liang 2016). In this subsection, we focus on the two instabilities, which are measured by the canonical baroclinic and barotropic transfer (see Liang 2016), respectively.…”

“…We will hence use the available reanalysis data instead. Based on an eddy-resolving global hindcast model, the longterm climatology of the multiscale energetics in the Kuroshio Extension was recently addressed by Yang and Liang (2016). They found that the baroclinic energy pathway, transferring mean APE (MAPE) to eddy APE (EAPE) and finally converting to EKE, together with the barotropic energy pathway, transferring mean kinetic energy (MKE) to EKE, are equally important for the growth of the EKE in the upstream region of Kuroshio Extension.…”

“…Thus, quantifying baroclinic instability and barotropic instability from the data is the key to uncovering the underlying dynamics of the decadal EKE modulation in the Kuroshio Extension system. Recently, Robinson (2005, 2007) developed a localized, finite-amplitude instability analysis, which is based on the localized multiscale energy and vorticity analysis (MS-EVA); it proves to be very efficient in diagnosing the nonlinear eddy-mean flow interactions and multiscale instabilities in real oceanic and atmospheric processes (Liang and Robinson 2009;Yang and Liang 2016). In the formalism, contributions from baroclinic and barotropic instabilities to the eddy energy generation as well as other sources, sinks, and redistributions of eddy energy can be explicitly computed.…”

Decadal Variability of Eddy Characteristics and Energetics in the Kuroshio Extension: Unstable Versus Stable States

“…Based on both observations and an idealized model study, Waterman and Jayne () and Waterman et al () indicated that the KE jet is subject to a mixed instability mechanism. Using a multiscale energy and vorticity analysis method, Yang and Liang () and Yang et al () suggested the important role of barotropic instability (BT) in the eddy energy budget within the KE region. They claimed that both BT and BC are important for the growth of EKE in the time‐mean sense, while the decadal modulation of EKE is mainly controlled by BT.…”