Diagnostics of Coherent Eddy Transport in the South China Sea Based on Satellite Observations

Abstract

The large discrepancy between Eulerian and Lagrangian work motivates us to examine the leakage of Eulerian eddies and quantify the contribution of coherent eddy transport in the South China Sea (SCS). In this study, Lagrangian particles with a resolution of 1/32° are advected by surface geostrophic currents derived from satellite observations spanning 23 years, and two types of methods are employed to identify sea surface height (SSH) eddies and Lagrangian coherent structures. SSH eddies are proven to be highly leaky during their lifetimes, with more than 80% of the original water leaking out of the eddy interior. As a result of zonal and meridional eddy propagation, the leaked water exhibits a spatial pattern of asymmetry relative to the eddy center. The degree of eddy leakage is found to be independent of several eddy parameters including the nonlinearity parameter U/c, which has been commonly used to assess eddy coherency. Finally, the Lagrangian coherent structures in the SCS are diagnosed and the associated coherent eddy diffusivity is calculated. It is found that coherent eddies contribute to less than 5% of the total eddy material transport in both zonal and meridional directions. These findings suggest that previous studies based on the Eulerian framework significantly overestimate the contribution of coherent eddy transport in the SCS.

A global Lagrangian eddy dataset based on satellite altimetry

“…It is clear that, the thresholds of 0.25 and 0.01 (adopted by Tian et al, 2022) will greatly overestimate and underestimate, respectively, the size of the coherent eddy. This parameter combination has been employed successfully in our previous studies (Liu et al, 2019(Liu et al, , 2022a. In addition, we repeatedly conduct the test of RCLV identification in the random regions and time periods, and as shown in Figure 4, the determined parameters perform well in identifying RCLVs with lifetimes of 30 and 90 days.…”

“…where (X, Y ) is the position for the particle released initially at point (x 0 , y 0 ) and ζ ′ is the instantaneous relative vorticity deviation from the spatial average over the whole domain. The LAVD (always positive) examines the average magnitude of The algorithm employed for detecting RCLVs has been described in previous studies (Abernathey and Haller, 2018;Tarshish et al, 2018;Liu et al, 2019;Zhang et al, 2019;Liu et al, 2022a). Once a local LAVD maximum is determined, we search outward for closed LAVD curves.…”

“…However, many recent works provide clear evidence that Eulerian methods strongly overestimate the degree of material transport by mesoscale eddies. Observations and numerical simulations both suggest that Eulerian eddies are far from coherent structures because there is strong and persistent water exchange across the Eulerian eddy boundary (such as the SSH contour) during the eddy lifespan (Beron-Vera et al, 2013;Wang et al, 2016;Liu et al, 2019Liu et al, , 2022a. The contribution of coherent structures to the total eddy transport is very limited, and most eddy transport is induced by incoherent motions such as swirling and filamentation outside the eddy cores (Wang et al, 2015;Abernathey and Haller, 2018;Zhang et al, 2019;Xia et al, 2022).…”

“…The contribution of coherent structures to the total eddy transport is very limited, and most eddy transport is induced by incoherent motions such as swirling and filamentation outside the eddy cores (Wang et al, 2015;Abernathey and Haller, 2018;Zhang et al, 2019;Xia et al, 2022). In addition, U/c has been shown to be an ineffective indicator of eddy coherent transport because the leakage magnitude of initially trapped water is generally significant and does not depend on this parameter (Liu et al, 2022a). The overestimation of coherent eddy transport might be attributed to the common shortcomings of Eulerian methods (see discussion in Haller, 2015;Abernathey and Haller, 2018).…”

“…Therefore, it is necessary to develop a global Lagrangian eddy dataset based on observational data. So far, we have conducted a series of works towards this goal, including regional eddy identification (Abernathey and Haller, 2018;Liu et al, 2022a), parameter sensitivity tests (Tarshish et al, 2018), and numerical experiments (Sinha et al, 2019;Liu et al, 2019;Zhang et al, 2019). In this study, we extend the work of Abernathey and Haller (2018) to the global ocean to identify coherent eddies using the LAVD method, and we generate a Lagragnian eddy dataset based on altimetry observations.…”