Submesoscale-enhanced filaments and frontogenetic mechanism within mesoscale eddies of the South China Sea
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Abstract: Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade, but in the eddy-active South China Sea (SCS) the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood. This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution (~500 m) simulation. The satellite images show that the elongated highly productive stripes with a typical lateral scale of ~25 km and associated filaments are frequently observed at the periphery of mesoscale eddies. The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d) due to submesoscale instabilities. The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m2. This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments. Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity. The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy, but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence. In this context, these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy, as well as enhanced vertical heat transport.
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Key words:
- submesoscale process /
- vertical exchange /
- frontogenesis /
- South China Sea
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Figure 1. Climatological map of eddy kinetic energy (shading) and the trajectory of mesoscale eddies (color lines) in winter (December, January, February) from 1993 to 2020 in the South China Sea (a); satellite-observed chlorophyll (Chl) concentrations for the AE1 on December 4, 2013 (b) and the AE2 on November 8, 2015 (c). The data in the shelf (<500 m) have been removed. The red, green, and blue bold lines in a show the eddy case trajectory of AE1, AE2 and simulated eddy SE, respectively. Purple contours denote the sea level anomaly and vectors show the geostrophic velocity anomaly. AE1, AE2: anticyclonic eddies.
Figure 2. Snapshot of surface Rossby number (
$Ro{\text{ = }}\text{ζ} {\text{/}}f$ ) in the nested South China Sea winter simulations. The resolution of ROMS1 is 1.5 km. The boundary of the second nested domain with ∆x=500 m is delineated by a black box.
Figure 3. Observed sea surface temperature (SST) field at the time of Fig. 1b (a); snapshot of chlorophyll (Chl) (shading) and SST (contours at an interval of 0.04°C) for the submesoscale filaments (b). Purple contours denote the sea level anomaly. Black spots show the distribution of high-productive filaments (>0.3 mg/m3).
Figure 4. Simulated sea surface temperature (SST) with horizontal velocity (vectors) (a) and a cross-eddy slice of salinity along the 117.5°E section (red dashed lines) (b) on December 18; cross-eddy profiles of normalized relative vorticity
$(\text{ζ} {\text{/}}f)$ and frontal sharpness (M4) averaged over the mixed layer depth (MLD) (c). The isobaths at 200 m and 1 500 m are shown by black contours. Gray contours denote the potential density at an interval of 0.1 kg/m3. The thick gray line denotes the MLD.
Figure 5. Frontal sharpness
$ {M^4} $ (a), strain rate$ St $ (b), relative vorticity$ \zeta $ normalized by$ f $ (c), and normalized divergence${\text{δ} \mathord{\left/ {\vphantom {\delta f}} \right. } f}$ (d) at the surface for the mesoscale eddy shown in Fig. 4. Black contours denote the isobaths at 200 m and 1 500 m. A segment of the strongest filament is outlined by the black box.
Figure 6. Surface heat flux Qnet (shading) with surface wind stress (vectors) (a); snapshot (b) and vertical section (c) of Ertel potential vorticity (PV)
$ q $ ; filament-averaged profiles of the Ertel PV terms and Richardson number Ri (d). The cross-filament transects (shown at an interval of 30 transects) is represented by cross-filament lines in a. The filament axis is defined by the strongest frontal sharpness. Thin gray contours show the fields of frontal sharpness >1×10−13 s−4. The isopycnal surfaces are shown by black contours (kg/m3) and the mixed layer depth is denoted by the gray line in c. The segments of$0.25 < Ri < 0.95$ and$Ri > 1$ are shown in dark green and green, respectively.
Figure 7. Submesoscale vertical velocity
$ w' $ (a) and mesoscale vertical velocity$\overline w$ (b) at a depth of 40 m; vertical section of submesoscale along-filament velocity$ {u'_{\text{c}}} $ averaged along the filament (c); submesoscale velocity profiles averaged over the mixed layer depth (MLD) in the filamentary region (d). Vectors in a and b denote the surface submesoscale and mesoscale flow, respectively. Gray contours show the potential density at an interval of 0.1 kg/m3. Black contours show the isopycnal surfaces and the gray line denotes the MLD. Vectors at the section show the cross-filament$ {v'_{\text{c}}} $ and vertical velocities$ w' $ at submesoscale.
Figure 8. Vertical heat flux
$ {Q_{\text{t}}} $ (a) and vertical buoyancy flux (VBF) (b) averaged over the mixed layer; vertical section of$ {Q_{\text{t}}} $ (shading), potential density (black contours; kg/m3) and the mixed layer depth (gray line) averaged along the filament (c); vertical profiles of$ {Q_{\text{t}}} $ and VBF averaged horizontally in the filamentary region (d). Contours in a denote the surface heat loss$ {Q_{{\text{net}}}} $ .
Figure 9. Horizontal strain rate
$ St $ (a) and frontogenetic tendency$ {F_{{\text{adv}}}} $ (b) at the surface; vertical distribution of frontogenetic tendency (shading) and potential density (black contours; kg/m3) along a cross-filament section (the green line in b) (c); instantaneous filament-averaged profiles for the parameters (d). Black lines in a show the direction of the principal strain axis$ {\theta _{\text{p}}} $ . The gray line denotes the mixed layer depth (MLD).
Figure 10. Surface horizontal strain rate associated with mesoscale flows
$ S{t_{\text{m}}} $ (a) and submesoscale flows$ S{t_{\text{s}}} $ (b); cross-filament slices of the$ S{t_{\text{m}}} $ (c) and$ S{t_{\text{s}}} $ (d). The horizontal mesoscale flows and submesoscale perturbations are shown by vectors. Black contours denote the isopycnal surfaces (kg/m3). The mixed layer depth is denoted by the thick gray line.
Figure 11. Mixed layer-averaged total frontogenetic tendency F (a) and its terms caused by geostrophic self-straining
${F_{\rm{g}}}$ (b), ageostrophic horizontal advection$ {F_u} $ (c), external straining deformation${F_{{\text{α}}}}$ (d), ageostrophic vertical advection$ {F_w} $ (e), and vertical mixing${F_{{\kappa _{\rm{v}}}}}$ (f). Vectors in b and c show the submesoscale geostrophic and ageostrophic flows, respectively. -
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