Anticyclonic Eddy Sheddings from Kuroshio Loop and the Accompanying Cyclonic Eddy in the Northeastern South China Sea

Zhang, Zhiwei; Zhao, Wei; Qiu, Bo; Tian, Jiwei

doi:10.1175/jpo-d-16-0185.1

Cited by 128 publications

(175 citation statements)

References 52 publications

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“…It should be noted that the seasonality of submesoscales in region ST (i.e., stronger in winter and weaker in summer) is essentially not determined by the state of the Kuroshio but by the total available potential energy stored in the mixed layer. Although the Kuroshio loop tends to occur in winter (not always) that can indeed strengthen the MSR in region ST, it is an unstable state and is usually terminated by eddy shedding (Nan et al, ; Zhang et al, ). A good example to demonstrate the indeterministic role of Kuroshio is that even after the Kuroshio eddy shedding (Kuroshio returns to its usual gap‐leaping state), the submesoscales are still very active in region ST in winter (Figure S5).…”

Section: Summary and Discussionmentioning

confidence: 99%

Spatiotemporal Characteristics and Generation Mechanisms of Submesoscale Currents in the Northeastern South China Sea Revealed by Numerical Simulations

Zhang

Qiu

et al. 2020

JGR Oceans

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Although both geostrophic‐balanced mesoscale eddies and unbalanced small‐scale processes have been well studied in the northeastern South China Sea (NE‐SCS), less attention has been devoted to the submesoscales in between (i.e., O(1–10 km)), which is recognized as an important conduit connecting the balanced and unbalanced motions. Based on the output from a 1/30° OGCM simulation, spatiotemporal characteristics and generation mechanisms of submesoscales in the NE‐SCS are investigated in this study. Through examining the submesoscale relative vorticity and vertical velocity, the regions southwest of Taiwan (ST) and Luzon Strait (LS) are identified as two hot spots of submesoscales in the NE‐SCS. Seasonally, the submesoscales in region ST are much stronger in winter than summer, while those in LS do not show a significant seasonality. Statistical analysis suggests that the strength of submesoscales in regions ST and LS is highly correlated with the product of mixed‐layer depth and mesoscale strain rate (MSR) and with the MSR itself, respectively. By conducting theoretical scaling and energetics analysis, the authors find that the mixed‐layer instability whose strength is determined by the combination of mixed‐layer depth and MSR and the barotropic instability associated with current‐islands interactions are the dominant generation mechanisms of submesoscales in the above two regions, respectively. Further examinations of the submesoscale energy budget indicate that, to keep a balanced state, the generated submesoscales have to be dissipated by a forward energy cascade, highlighting the important role of submesoscales in the energy balance of the NE‐SCS circulation.

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Section: Summary and Discussionmentioning

confidence: 99%

Spatiotemporal Characteristics and Generation Mechanisms of Submesoscale Currents in the Northeastern South China Sea Revealed by Numerical Simulations

Zhang

Qiu

et al. 2020

JGR Oceans

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“…The unbalanced processes, such as submesoscale instabilities, strain‐induced frontogenesis, and internal waves, are responsible for the scale transition from balanced to unbalanced states traversing the mesoscale and submesoscale (Benthuysen & Thomas, ; Fox‐Kemper et al, ; Gula et al, ; McWilliams, ). In the NSCS, the KE spectrum is primarily composed of energetic vortex modes and wave components, which are representative of active eddy variability (Chavanne & Klein, ; Zhang et al, ) and internal waves (Alford et al, ; Tian et al, ). Based on the limited observations and high‐resolution model output, this paper preliminarily examines the wave‐vortex and rotational‐divergent KE spectra in the mesoscale and submesoscale ranges and provides an improved understanding of the strength of balanced versus unbalanced motions and the dominant wavelengths for the KE and SSH in the NSCS.…”

Section: Summary and Discussionmentioning

confidence: 99%

Scale Transition From Geostrophic Motions to Internal Waves in the Northern South China Sea

et al. 2019

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The eddy-abundant circulation in the northern South China Sea (NSCS) tends to be dynamically complex due to monsoon forcing, Kuroshio intrusion, and emitted internal waves from the Luzon Strait. This study uses 13-year shipboard acoustic Doppler current profiler measurements (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016), orbital altimeter data, and high-resolution model output to perform wave-vortex decompositions and investigate the scale of transition from dominantly geostrophic flows to internal wave motions in the northern South China Sea. The upper ocean kinetic energy spectra transition on scales exceeds 200 km. This large scale of transition is attributed to the energetic low-mode internal waves (e.g., internal tides and inertia-gravity waves). However, inconsistencies in the decomposition reveal that the assumptions of homogeneity and isotropy required for the 1-D decomposition (Bühler et al., 2014) are sufficiently violated at smaller scales to affect the subdominant member of the decomposition on scales below 100 km. A method for direct quantification of the consequences by degree of violation using bootstrapping of the 2-D model data is developed and illustrated. Observed and modeled sea surface height spectra flatten at scales smaller than 125 km, which is found in the model to be due to the coherent, semidiurnal internal waves. The large scale of transition between geostrophic and wave motions in the South China Sea is an irreducible uncertainty for altimeter velocities (e.g., the Surface Water and Ocean Topography mission). Plain Language SummaryThe scale transition from balanced geostrophic flows to unbalanced motions is of great importance for understanding the circulation system of the South China Sea where both eddies and internal waves are particularly active. Based on observations and high-resolution simulation, this paper investigates the wave number spectra of kinetic energy and sea surface height variance in the northern South China Sea. The large transition length scale (>200 km) from balanced geostrophic flows to unbalanced motions is due to strong low-mode internal waves in the upper ocean. The violations of the assumptions for the 1-D decomposition (stationarity, homogeneity, and horizontal isotropy) are also assessed using the 2-D model data. With the model output, the low-mode semidiurnal internal tides are found to have significant impacts on the sea surface height variance at scales of~125 km. These results are directly relevant to the efficiency of diagnosing geostrophic flows from altimeters (e.g., the upcoming Surface Water and Ocean Topography mission).

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“…Owing to the debates on the intrusion paths, there is no clear definition of the KI. To clarify the KI phenomenon in this paper, an approach similar to that used by Chern et al (2010) is adopted, and the KI is defined as the Kuroshio Loop Current (red dotted path in Figure 1), which has been widely verified by both observations (Centurioni et al, 2004;Farris & Wimbush, 1996;Li & Wu, 1989;Zhang et al, 2017) and numerical simulations (Chern et al, 2010;Metzger & Hurlburt, 2001).…”

Section: Introductionmentioning

confidence: 89%

“…The analyses in the above section suggest that triggering of the KI is attributed to the eddy‐current interaction. To understand this interaction, the eddy‐energetic analysis (Tsujino et al, ), which has been used to study the eddy shedding process caused by the KI in Zhang et al (), is employed. The related equations are written as follows:

BT = - ((), u_{g}^{'} v_{g}^{'} ((), \frac{\partial u_{g}}{\partial y} + \frac{\partial v_{g}}{\partial x}) + u_{g}^{'} u_{g}^{'} \frac{\partial u_{g}}{\partial x} + v_{g}^{'} v_{g}^{'} \frac{\partial v_{g}}{\partial y}),

BC = - \frac{g^{2}}{ρ_{0}^{2} N^{2}} ((), u_{g}^{'} ρ^{'} \frac{\partial ρ}{\partial x} + v_{g}^{'} ρ^{'} \frac{\partial ρ}{\partial y}),

WSW = \frac{τ_{w}^{u} u_{0}^{'} + τ_{w}^{v} v_{0}^{'}}{ρ_{0}} .

…”

Section: Oprs Triggering the Ki Occurrencementioning

confidence: 99%

“…Second, the stratification in the northeastern corner of the SCS is modified by the Kuroshio water through the LS (Qu et al, 2000;Shaw, 1991); then, the local propagation and generation of the internal waves are influenced (Buijsman et al, 2010;Jan et al, 2012;Park & Farmer, 2013). Third, the eddy pairs shedding from the KI move westward into the SCS and modulate the mesoscale activities there (Nan, Xue, Xiu, et al, 2011;Zhang et al, 2017). Finally, positive vorticity fed from the left flank of Kuroshio plays an important role in maintaining the structure of circulation in the northeastern SCS (Chern et al, 2010).…”

Section: Introductionmentioning

confidence: 99%

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Optimal Precursors Triggering the Kuroshio Intrusion Into the South China Sea Obtained by the Conditional Nonlinear Optimal Perturbation Approach

Liang

Wang

et al. 2019

JGR Oceans

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The Kuroshio Intrusion (KI) into the South China Sea (SCS) is a key process that greatly modulates the variability of dominant phenomena in the northern SCS. Using the Regional Ocean Modeling System, the optimal precursor (OPR) triggering the KI is obtained by the Conditional Nonlinear Optimal Perturbation (CNOP) approach. The OPRs exhibit the anticyclonic eddies (AEs) spatial pattern in the southern region (19°N–20.3°N, 119°E–122°E) of the Luzon Strait. The evolution processes of the OPRs indicate that the northwestward movement and rapid growth of AEs force the Kuroshio to bend westward continuously, which leads to the transition of the Kuroshio path from leaping state to intruding state. Eddy‐energetic analyses suggested that the barotropic instabilities are responsible for the evolution of the OPRs and thus triggering the KI, while the local wind stress plays a minor role in the triggering process. Additional examinations based on satellite altimeter data and ocean reanalysis data further confirm that the AE‐like perturbation similar to the obtained OPR indeed plays a key role in the KI triggering process.

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Anticyclonic Eddy Sheddings from Kuroshio Loop and the Accompanying Cyclonic Eddy in the Northeastern South China Sea

Cited by 128 publications

References 52 publications

Spatiotemporal Characteristics and Generation Mechanisms of Submesoscale Currents in the Northeastern South China Sea Revealed by Numerical Simulations

Spatiotemporal Characteristics and Generation Mechanisms of Submesoscale Currents in the Northeastern South China Sea Revealed by Numerical Simulations

Scale Transition From Geostrophic Motions to Internal Waves in the Northern South China Sea

Optimal Precursors Triggering the Kuroshio Intrusion Into the South China Sea Obtained by the Conditional Nonlinear Optimal Perturbation Approach

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