Decadal Variability of Eddy Characteristics and Energetics in the Kuroshio Extension: Unstable Versus Stable States. Issue 9 (17th September 2018) - Yang, Haiyuan
Abstract: Using the Estimating Circulation and Climate of the Ocean Phase II product, this study investigates the eddy characteristics and energetics in two dynamical regimes within the Kuroshio Extension (KE) region. Based on the empirical orthogonal function analysis, it is found that the decadal evolution of eddy kinetic energy in the KE region is characterized by a delayed negative correlation between the upstream and downstream. Besides the out‐of‐phase change in eddy activity levels, eddy characteristics and energetics in the upstream KE are also different from those in the downstream. In the upstream region, eddies are stretched in the zonal direction and the unstable state is characterized by strong eddy‐shedding processes. During these processes, cyclonic eddies are found to be formed in the first quasi‐stationary meander and finally pinched off from the KE jet. Energy budget illustrates that the eddy shedding processes are triggered by baroclinic instability, while barotropic instability becomes the dominated energy source after the meander is fully developed. Accompanied by the generation of cyclonic eddies, significant inverse energy cascade is detected. When the upstream KE is stable, the eddy activity is dominated by baroclinic instability and the inverse energy cascade occurs similarly. Distinct from the upstream, eddies in the downstream KE tend to be stretched in the meridional direction and the decadal variability of eddy kinetic energy in this region is mainly regulated by the upstream through lateral advection. Plain Language Summary: Based on classic physical oceanography theory, oceanic eddies are generated mainly through the vertical shear of background mean flow, i.e. baroclinic instability (BC). A stronger mean flow should favor BC and lead to more eddy generation. However, at decadal timescales, the correlation between the strength of the Kuroshio Extension (KE) jet and the regional eddy kinetic energy (EKE) level is negative, therefore the decadal variability of mesoscale eddy field in the KE system cannot be simply explained by the classic instability theory. This study explores in great depth the dynamical processes in the unstable state in the upstream KE and analyzes the evolution of eddy energetics during these processes. It is found that unstable events in the upstream KE are characterized by strong eddy‐shedding processes. During these events, cyclonic eddies are found to be formed in the first quasi‐stationary meander and finally pinched off from the KE jet. During these processes, both BC and barotropic energy transfer are important in the generation of eddies. This study will be a step toward the understanding of low‐frequency variability in strong oceanic current regions in the global ocean. Key Points: Energy‐weighted length scale of eddies in the KE region is about 250 km; in the upstream (downstream) region, eddies tend to be stretched in the zonal (meridional) direction In the upstream, the unstable state is characterized by strong eddy‐shedding processes; strong cyclonic eddies are found to be pinched off from the first quasi‐stationary meander and are controlled by both baroclinic and barotropic instabilities In the downstream, eddy activity is mainly regulated by the upstream through lateral advection