AUTHOR=Liu Mingyue , Chen Ru , Guan Wenting , Zhang Hong , Jing Tian 

TITLE=Nonlocality of scale-dependent eddy mixing at the Kuroshio Extension

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 10 - 2023

YEAR=2023

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1137216

DOI=10.3389/fmars.2023.1137216

ISSN=2296-7745

ABSTRACT=Although eddy parameterization schemes are often based on the local assumption, previous studies indicate that the nonlocality of total eddy mixing is prevalent at the Kuroshio Extension (KE). For eddy-permitting climate models, only mixing induced by eddies smaller than the resolvable scale of climate models ($L^*$) needs to be parameterized. Therefore, here we aim to estimate and predict the nonlocality of scale-dependent eddy mixing at the KE region. We consider the separation scale $L^*$ ranging from $0.2^\circ$ to $2.5^\circ$, which is comparable to the typical resolution of the ocean component of climate models. Using a submesoscale-permitting model solution (MITgcm llc4320) and Lagrangian particles, we estimate the scale-dependent mixing (SDM) nonlocality ellipses and then diagnose the square root of the ellipse area ($L_{n,{particle}}$).  $L_{n,{particle}}$ is a metric to quantify the degree of SDM nonlocality. We found that, for all the available $L^*$ values we consider, the SDM nonlocality  is prevalent in the KE region, and mostly elevated values of $L_{n,{particle}}$ occur within the KE jet. As $L^*$ decreases from $2.5^\circ$ to $0.2^\circ$, the ratio $L_{n,{particle}}/L^*$ increases from 0.8 to 8.9. This result indicates that the SDM nonlocality is more non-negligible for smaller $L^*$, which corresponds to climate models with relatively high resolution. As to the SDM nonlocality prediction, we found that compared to the conventional scaling and the curve-fitting methods, the random forest approach can better represent $L_{n,{particle}}$, especially in the coastal regions and within the intense KE jet. The area of the Eulerian momentum ellipses well capture the spatial pattern, but not the magnitude,  of $L_{n,{particle}}$. Our efforts suggest that eddy parameterization schemes for eddy-permitting models may be improved by taking into account mixing nonlocality.