The decadally modulating eddy field in the upstream Kuroshio Extension and its related mechanisms
Both the level of the high-frequency eddy kinetic energy (HF-EKE) and the energy-containing scale in the upstream Kuroshio Extension (KE) undergo a well-defined decadal modulation, which correlates well with the decadal KE path variability. The HF-EKE level and the energy-containing scales will increase with unstable KE path and decrease with stable KE path. Also the mesoscale eddies are a little meridionally elongated in the stable state, while they are much zonally elongated in the unstable state. The local baroclinic instability and the barotropic instability associated with the decadal modulation of HF-EKE have been investigated. The results show that the baroclinic instability is stronger in the stable state than that in the unstable state, with a shorter characteristic temporal scale and a larger characteristic spatial scale. Meanwhile, the regional-averaged barotropic conversion rate is larger in the unstable state than that in the stable state. The results also demonstrate that the baroclinic instability is not the dominant mechanism influencing the decadal modulation of the mesoscale eddy field, while the barotropic instability makes a positive contribution to the decadal modulation.
KeywordsKuroshio Extension mesoscale eddy decadal modulation baroclinic instability barotropic energy conversion rate nonlinear eddy-eddy interaction
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