1. Introduction
As an important marginal sea in the northwestern Pacific, the South China Sea (SCS) is characterized by a unique basin-scale vertical three-layer circulation system (Yuan 2002; Gan et al. 2016; D. Wang et al. 2019; Zhu et al. 2019; Cai et al. 2020). The deep SCS is nearly enclosed and connects to surrounding oceans only through the Luzon Strait. Owing to the persistent pressure gradient between the two sides of the strait, North Pacific Deep Water (NPDW) penetrates the deep SCS (Qu 2002), which further drives the cyclonic circulation in the deep SCS (Yuan 2002; Wang et al. 2011). SCS abyssal circulation considerably contributes to abyssal water renewal, sedimentary processes, and the energy budget.
Cyclonic circulation in the deep SCS was predicted by the Stommel–Arons abyssal circulation theory (Stommel 1958; Stommel and Arons 1960a,b) and was further confirmed by historical water properties (Li and Qu 2006; Qu et al. 2006), historical sediment distributions (Lüdmann et al. 2005; Zheng and Yan 2012; Chen et al. 2013; Xu et al. 2021), temperature and salinity from reanalysis data (Wang et al. 2011; Zhu et al. 2017), and in situ current observations (Zhou et al. 2017; Zhou et al. 2020; Zheng et al. 2021, 2022a). Modeling studies have indicated that the penetration of NPDW through the Luzon Strait controls the formation and seasonal variation in abyssal circulation in the SCS (Lan et al. 2013, 2015).
Potential vorticity (PV) budget analysis based on numerical simulations (Lan et al. 2013; Gan et al. 2016; Cai and Gan 2019; Gan et al. 2022) and reanalysis data (Zhu et al. 2017) has been used to elucidate the dynamics of abyssal circulation formation. As the Luzon Strait is located in the northeast part of the SCS, the intrusion of NPDW through the strait carries positive PV into the SCS. Lan et al. (2013) indicated that the positive PV from the penetration of the NPDW is balanced by the negative PV caused by the friction of cyclonic abyssal circulation along the boundary of the SCS. Gan et al. (2016) clarified that the PV inputs from the deep Luzon Strait are offset by the bottom pressure torque in the internal SCS. Zhu et al. (2017) indicated that the net PV inflow in the deep SCS yields vortex stretching and cyclonic circulation based on the U.S. Navy Generalized Digital Environment Model. Using an idealized model, Cai and Gan (2019) found that vertical volume transport is critical for linking the vorticity among layers, and the vorticity related to vertical transport is the major response to external deep PV inputs. Regions off the Luzon Strait, along the north slope, near the Mindoro Strait, and in the southwest basin contribute most of the vorticity budget that is associated with the abyssal circulation in the SCS (Gan et al. 2022).
The external and internal dynamics of SCS abyssal circulation have been attributed to the positive PV penetrating from the Luzon Strait and the bottom pressure torque in the interior SCS, respectively (Cai and Gan 2019). However, previous analyses were mostly based on numerical simulations, and the dynamics have never been confirmed by in situ observations because basin-scale measurements with high spatiotemporal resolution were scarce. In addition, previous numerical simulations focused on basin-scale averaged circulations; the regional dynamic characteristics in key areas remained unclear, and the temporal variabilities and spatial distributions of the abyssal circulation maintaining dynamics have never been analyzed in detail. To obtain detailed measurements of the abyssal circulation and multiscale dynamics, 28 current- and pressure-recording inverted echo sounders (CPIESs) were distributed west of the Luzon Strait, which is one of the most important areas for the SCS vorticity budget, and the circulation dynamics were observed for over 400 days (Zheng et al. 2022a,b, 2023; Zhao et al. 2023; Wang et al. 2023a,b). The present study builds on the unprecedented data obtained from these recently published studies, with a focus on the dynamics maintaining the SCS abyssal circulation and their spatiotemporal distributions.
The remainder of the paper is organized as follows: the data and methods are described in section 2, vorticity balance in the deep northeast SCS and dynamics of abyssal circulation are described in section 3, and a summary is presented in section 4.
2. Data and methods
a. Abyssal circulation
The array consisted of 28 CPIESs distributed west of the Luzon Strait from June 2018 to July 2019; they collected near-bottom current, bottom pressure, and round-trip acoustic travel time (τ) data (Fig. 1). Preprocessing of observed records has been performed, and abyssal currents were mapped and analyzed by Zheng et al. (2022a). However, a slightly different mapping process was applied in this study following Firing et al. (2014), which was expected to show higher accuracy for streamfunction and its derivatives. Geostrophic currents and their derivatives at reference levels (i.e., 3700, 3300, and 2800 dbar) were obtained from near-bottom currents and bottom pressure data using optimal interpolation with a correlation length of 120 km. Geopotential anomaly (Φ) at each station was obtained from τ based on the gravest empirical mode method. Geostrophic streamfunction ψ, geostrophic shears, and their derivatives above reference levels were further obtained from Φ using optimal interpolation with a correlation length of 105 km. A 72-h low-pass filtered three-dimensional abyssal geostrophic current with half-day temporal intervals and 0.02° × 0.02° spatial intervals was used in this study. The patterns of abyssal geostrophic currents recorded in this study showed little difference from those of Zheng et al. (2022a). However, compared with derivative terms of velocities calculated from current patterns in Zheng et al. (2022a), these terms were improved using the method of Firing et al. (2014), in which they were directly calculated from CPIES observations based on the covariance function among variables. That is to say, the simplification of calculation steps following Firing et al. (2014) might reduce the error. More detailed processes for obtaining the geostrophic currents have been reported by Zheng et al. (2022a) and Firing et al. (2014).
Map of the study region. Positions of CPIESs (labeled dots) and year-long-averaged abyssal geostrophic circulation from July 2018 to June 2019 at 2500 m (arrows) are indicated. The arrows are 0.2° × 0.2° subsampled.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
b. Vorticity budget
The year-long averaged and seasonal vorticity terms in the SCS were calculated for further analysis. The year-long-averaged value is defined as the averaged value from July 2018 to June 2019. The season in the SCS is defined by Fang et al. (2002) as spring (April–May), summer (June–September), autumn (October–early November), and winter (mid-November–March).
3. Results and discussion
a. Vorticity balance
The vorticity terms in Eq. (4) were calculated to identify the dynamics maintaining the SCS abyssal circulation. The standard deviation of each term was comparable to its mean value, indicating a large degree of temporal variance in all terms during the observation period. A first-order balance was found between the spatially averaged COR and BPT, which showed completely opposite variabilities, whereas the others were at least one order of magnitude smaller (Fig. 2). A small value was obtained for ADV for most of the study period. The contributions of the nonlinear advection and viscosity terms to the deep vorticity budget were limited, and the acceleration term of vorticity in the deep SCS was negligible compared with COR and BPT. The correlation coefficient between COR and BPT reached −0.96, indicating that the pressure gradient forcing in the slope topography was the major response to the planetary vorticity flux induced by volume transport. COR and BPT showed lower values from August to December and higher values from January to July, which may represent seasonal variation.
Time series of spatially averaged vorticity terms in the region of observation.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
Previous studies have indicated that positive PV penetrating from the deep Luzon Strait causes a positive layer-average vorticity in the deep SCS, which yields vortex stretching and cyclonic circulation (Lan et al. 2013; Zhu et al. 2017). Vertical transport plays critical roles in linking the vorticity among layers, which develop and sustain the three-layer circulation in the SCS. Vorticity caused by vertical transport associated with slope current–topography interaction is the major response to the external vorticity flux through the Luzon Strait (Gan et al. 2016; Cai and Gan 2019).
The observations covered only the northeastern region of the SCS rather than the entire SCS basin. Although large-scale cyclonic circulation was identified from the average abyssal circulation, regional anticyclonic current structures were present in the northern and southwestern observational area. A regional anticyclonic current may result from the topography, since the current follows the isobaths which show regional anticyclonic characteristics near the western boundary of the study region (Fig. 1). In the region of observation, the spatially averaged COR showed negative values through most of the study period, especially from January to July (Fig. 2), which was caused by a regional anticyclonic current near the western boundary. Anticyclonic currents indicate a negative relative vorticity, which further cause a vorticity shrinking of the deep layer (i.e., downward transport), resulting in a horizontal divergence of abyssal currents (i.e., negative COR). The regional balances are examined below.
Hence, the spatial distribution of vorticity terms may relate to the current pattern. In Fig. 3, the patterns of year-long average values of ACCE, ADV, COR, and BPT are shown. VISC is not shown in Fig. 3 because it was notably smaller than the other terms (Fig. 2). Although vorticity terms are derived from mesoscale abyssal currents, small-scale characteristics resulting from bottom topography interacting with mesoscale currents are presented.
Spatial pattern of year-long-averaged (a) ACCE, (b) ADV, (c) COR, and (d) BPT. (e) Spatiotemporally averaged vorticity terms in the observational region. Topography above 2500 m is masked by the blue shadow. Note that the color bars indicate different ranges: (a) ±4 × 10−10, (b) ±2 × 10−8, and (c),(d) ±1 × 10−7.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
ACCE and ADV were considerably smaller than COR and BPT in the observational area. Although ACCE showed considerable temporal variance in Fig. 2, its temporally averaged spatial pattern was at least two orders smaller than that of the other terms, except for VISC, indicating that the trend in abyssal circulation over a year was limited. COR and BPT showed nearly opposite spatial patterns, indicating that they maintained the first-order balance of the vorticity equation in most regions. COR and BPT were considerably large along the eastern boundary and in the area south of 20.5°N, with a strong abyssal current, but were small in the northwestern part of the observational area, where the current was weak (Figs. 1 and 3). Along the eastern boundary, where the abyssal current showed a clear cyclonic structure (Fig. 1), the values of COR were mostly positive and those of BPT were mostly negative (Fig. 3). Although a southwestward current appeared along the western boundary south of 20.5°N, it showed a regional anticyclonic structure (Fig. 1), and a negative COR and positive BPT dominated the region (Fig. 3). Year-long average COR and BPT values were −1.16 × 10−9 and 1.33 × 10−9 m s−2, respectively, maintaining the vorticity balance in the deep SCS (Fig. 3e).
COR was further divided into CORDIV and CORBETA to quantify the contribution of the vertical and horizontal motions of the water (Fig. 4). The spatially averaged CORBETA mostly showed a positive value during the observational period (Fig. 4a) because the northward transport near the eastern boundary was smaller than the southwestward transport near the western boundary (Zheng et al. 2022a). According to
(a) Time series of spatially averaged COR, CORDIV, and CORBETA in the observational region. Spatial pattern of year-long-averaged (b) CORDIV and (c) CORBETA. Topography above 2500 m is masked by the blue shadow.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
(a) Time series of spatial averaged CORDIV, term 1, and term 2 in the observational region. Spatial pattern of year-long-averaged (b) term 1 and (c) term 2. Topography above 2500 m is masked by the blue shadow.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
Although cyclonic circulation appeared in the observation area, its detailed structure was complex (Fig. 6a). A positive relative vorticity dominated the region along the eastern boundary, whereas the region along the western boundary exhibited a negative relative vorticity. Two areas with strong abyssal currents along their eastern and western boundaries, where COR was considerably stronger than elsewhere (Fig. 6b), were selected to demonstrate the regional vorticity budgets. The current near the eastern boundary (red box in Fig. 6) exhibited considerable cyclonic characteristics (positive relative vorticity), whereas anticyclonic characteristics (negative relative vorticity) appeared near the western boundary (blue box in Fig. 6).
(a) Current (arrows) and relative vorticity (RV; shading) at 2500 m. (b) Temporally averaged COR. (c),(d) Time series of spatially averaged vorticity terms and RV from red and blue boxes in (a) and (b). Topography above 2500 m is masked by the blue shadow.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
ACCE, ADV, and VISC were negligible, and COR and BPT dominated the first-order balance in both the red and blue boxes; however, the characteristics of these terms differed. In the red box near the eastern boundary, COR was positive and BPT was negative during most of the observation period (Fig. 6c), which was consistent with the results from basin-scale abyssal cyclonic circulation in a numerical model (Cai and Gan 2019). In the blue box, COR and BPT turned negative and positive, respectively, over most of the observational period (Fig. 6d). As COR is dominated by CORDIV, a negative COR represents a horizontal divergence of abyssal currents, and therefore, indicates a downward transport from the middle layer to the deep layer. The relative vorticity was positive in the red box and negative in the blue box through most of the study period, revealing a correlation with the temporal variance in COR, especially in spring and summer. The correlation coefficient between COR and relative vorticity was 0.46 (0.37) with a time lag of 6.5 (7.0) days in the red (blue) box and increased to 0.55 (0.50) with a time lag of 7.5 (10.5) days between January 2019 and July 2019. This indicated that the regional vorticity budget in the deep SCS is related to the relative vorticity distributions, that is, the abyssal current structures. A positive vertical planetary vorticity flux occurred in the eastern (red box) and a corresponding negative flux in the western (blue box), accompanied by cyclonic and anticyclonic characteristics, respectively. The time lag possibly indicated the time for the response of the spatially averaged COR to abyssal currents.
b. Seasonal variability
As shown in Fig. 2, COR and BPT in the deep SCS showed seasonal variation. Similarly, to the year-long-averaged vorticity balance (Fig. 3e), COR and BPT maintained a first-order vorticity balance in the deep SCS in all seasons (Figs. 7a–d), whereas the values for the other terms were considerably smaller and could be ignored. COR showed minimum values of −1.44 × 10−9 m s−2 (spring) and −1.44 × 10−9 m s−2 (winter) and BPT showed maximum values of 1.53 × 10−9 m s−2 (spring) and 1.66 × 10−9 m s−2 (winter) (Figs. 7a,d). In autumn, the maximum value of COR was −0.92 × 10−9 m s−2 and the minimum value of BPT was 1.10 × 10−9 m s−2 (Fig. 7c). ACCE showed small but negative values in spring and summer and small but positive values in autumn (Figs. 7a–d), indicating the increase and decrease in anticyclonic characteristics in spring/summer and autumn, respectively. As the vorticity balance in the study area is dominated by regional anticyclonic currents near the western boundary, the abyssal currents were large in summer but weak in winter, generally following the seasonal pattern of horizontal volume transport near the western boundary (Zheng et al. 2022a).
Averaged vorticity terms in (a) spring, (b) summer, (c) autumn, and (d) winter. Abyssal circulation at 2500 m (arrows) and spatial pattern of COR (shading) in (e) spring, (f) summer, (g) autumn, and (h) winter. Topography above 2500 m is masked by the blue shadow.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
As COR and BPT showed nearly opposite temporal and spatial variabilities, only the seasonal spatial patterns of COR are displayed in Figs. 7e–h. COR showed a similar pattern across different seasons; however, the negative COR in the southwest of the observational area with abundant seamounts (blue box in Fig. 6a) was more substantial in spring than in the other seasons. The abyssal currents around the seamounts showed clear anticyclonic characteristics in spring (Fig. 7e), which corresponded to the notably negative COR. The anticyclonic characteristics decreased in summer and disappeared in autumn (Figs. 7f,g), corresponding to the weakest COR in Fig. 7c and a weak southwest current near the western boundary in autumn (with the minimum in October).
The monthly averaged values of COR and BPT in the whole observational region, along the eastern boundary (red box in Fig. 6) and near the western boundary (blue box in Fig. 6) are shown in Fig. 8. The values of COR were negative and those of BPT were positive in most months in the whole observational region and COR was dominated by CORDIV in all months (Fig. 8a). However, positive COR and negative BPT appeared along the eastern boundary (Fig. 8b), and considerable negative COR and positive BPT appeared near the western boundary (Fig. 8c). The averaged values for the eastern and western boundaries (Figs. 8b,c) were, respectively, 2 and 4 times greater than those for the study region as a whole (Fig. 8a). Positive COR values were greater in the autumn (September and October) and spring (March–May) along the eastern boundary (Fig. 8b), whereas COR values were considerably negative in spring (March–May) but to a lesser degree in the autumn near the western boundary (Fig. 8c). The seasonal variation in the whole observational region generally followed that near the western boundary (blue box in Fig. 6), as COR near the eastern and western boundary cancelled each other in autumn, while in spring, the magnitude of COR near the western boundary was much greater than the corresponding values along the eastern boundary.
Monthly averaged COR, BPT, and CORDIV in (a) the whole region of observation, as well as in (b) the red box and (c) the blue box in Fig. 6. Note that the vertical axes have different scales.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
The monthly variability in COR is directly related to the seasonal variability in abyssal currents, which is dominated by deep water overflow through the Luzon Strait (Lan et al. 2015). The abyssal currents were strong in the autumn near the eastern boundary (Zheng et al. 2022a), leading to the correspondingly large COR (Fig. 8b). However, the seasonal pattern of abyssal currents near the western boundary lags behind that near the eastern boundary, owing to the propagation of seasonal variability (Zheng et al. 2022a), which caused the large COR in spring (Fig. 8c). The red box covers parts of the southward currents along the western boundary, indicating the enhanced COR in spring (Fig. 8b).
c. Intraseasonal variability
Because COR and BPT exhibited considerable temporal variance, wavelet analysis was applied to identify the dominant periods. COR and BPT had nearly opposite variabilities, and only the spatially averaged COR was analyzed, as shown in Fig. 9a. Small-period variability (<8 days) was detected most of the time, possibly related to the variance from the penetration of NPDW in the deep Luzon Strait (Fig. 8 in Zhou et al. 2014). Intraseasonal variability was dominated by fluctuations of approximately 9–16 and 18–28 days. ACCE was only substantial in the balance at periods of less than 8 days (not shown), which indicates its minimal contribution to intraseasonal variability.
(a) Wavelet analysis of spatially averaged COR. Color indicates log2-scaled variance of wavelet transform of normalized value. The 95% confidence level is indicated by black contours. Red boxes indicate periods of 9–16 and 18–28 days. (b),(c) Standard deviation ratios of the bandpass-filtered COR to the raw COR.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
The standard deviation ratios of the bandpass-filtered COR to the raw COR were applied to quantify the contribution of intraseasonal variability to the COR variance. The 9–16-day variability was significant in summer and winter but insignificant in spring and autumn (Fig. 9a) and contributed over 30% to the total variance near the continental shelf (Fig. 9b). Such 9–16-day fluctuations in the deep SCS can be induced by topographic Rossby waves (Shu et al. 2016, 2022; Q. Wang et al. 2019) or breaking internal tides (Xie et al. 2018; J. Wang et al. 2023). The contribution of 18–28-day variability reached over 30% in the east and north of the study region but was insignificant in the southwest, corresponding to the energetic, ∼21-day topographic Rossby waves observed in the Manila Trench (Wang et al. 2021; Zheng et al. 2022b). Hence, the vorticity budget in the deep SCS reflects abyssal multiscale dynamic processes, and the two aforementioned intraseasonal variabilities contribute to over half of the abyssal COR variance. Intraseasonal variabilities in the deep SCS are mostly attributed to topographic–planetary Rossby waves (Quan et al. 2022). Topographic Rossby waves in the deep SCS are generally induced by mesoscale perturbations in the upper layer, including eddies and Kuroshio intrusions, indicating that upper-layer mesoscale perturbations considerably contribute to the vorticity balance in the deep SCS.
A strong cyclonic eddy dominated the upper ocean of the study region in August and reached the deep SCS, which was accompanied by an anticyclonic eddy induced by the Kuroshio intrusion through the Luzon Strait in the upper layer (not shown). The eddy possibly contributed to the extremely weak abyssal volume transport near the western boundary through pressure work (Zheng et al. 2022a). At the same time, a considerably positive COR appeared in August, while COR showed a negative value most of the time in the observational region (Figs. 2 and 8a). The cyclonic eddy caused an upward vertical transport and a positive CORDIV. The weak COR in August near the western boundary (Fig. 8c) was caused by the cyclonic eddy which counteracted the anticyclonic characteristics there, and the weak BPT showed the response of positive vorticity carried by the eddy. A similar phenomenon also occurred in February 2019, but did not last as long and occurred in the north, outside of the box delimitation.
The cyclonic eddy appeared in the deep SCS on 16 August 2018, accompanied by considerable positive relative vorticity (Fig. 10b). After generation, the eddy moved westward from 16 August to 5 September, and finally disappeared at the western boundary (Figs. 10b–g). COR in the blue box became positive from 17 to 31 August (Fig. 6d), which caused a positive spatially averaged COR in the observation area from 18 to 28 August (Fig. 2), corresponding to the period in which the eddy covered the region. COR showed a negative value in the blue box on 12 August before the generation of the eddy (Fig. 10i); the magnitude of the COR then decreased following the appearance of the eddy. On 28 August, when the cyclonic eddy covered the blue box, the positive and negative COR were comparable in the region (Figs. 10k–n). Following the leaving of the eddy, COR values reversed to negative again, from 1 to 9 September (Figs. 10n–p). The spatial patterns of BPT were same as those of COR with opposite values, showing the synchronous response to the eddy. This indicated that deep eddies play important roles in planetary vorticity variance and substantially contribute to the regional vorticity balance in the deep SCS.
(a)–(h) Daily maps of relative vorticity (shading) and abyssal current (arrows) at 2500 m. (i)–(p) Daily maps of COR patterns. The blue box delineates the same area shown by the blue box in Fig. 6. The magenta box delineates the extent of the cyclonic eddy. Topography above 2500 m is masked by the blue shadow.
Citation: Journal of Physical Oceanography 54, 1; 10.1175/JPO-D-23-0060.1
As discussed above, mesoscale eddies in the upper layer could reach the deep ocean and directly contribute to abyssal vorticity balance or influence the abyssal vorticity balance by stimulating topographic Rossby waves. In the region west of the Luzon Strait, Kuroshio intrusion frequently occurs in winter, which is expected to induce deep eddies (Zhang et al. 2016; Sun et al. 2020) and topographic Rossby waves (Quan et al. 2021; Zheng et al. 2022b). Therefore, upper perturbations might be expected to play a more important role in the vorticity variabilities in the deep SCS in winter. However, the observation may not be long enough to definitively determine the average impact of eddies, and a longer record would be needed to quantify their impact.
4. Summary
The vorticity balance in the deep northeastern SCS was clarified based on observations from 28 CPIESs west of the Luzon Strait for over 1 year. The first-order balance of vorticity was dominated by the planetary vorticity flux and bottom pressure torque in time and space, which reflect external and internal dynamics of SCS abyssal circulation, respectively. The planetary vorticity flux was dominated by the vertical motion, whereas the contribution of the horizontal motion was negligible.
Although the input of planetary vorticity flux from the deep Luzon Strait is expected to lead to a positive layer-average planetary vorticity flux in the deep SCS, a considerable negative planetary vorticity flux dominated the region west of the Luzon Strait. Positive (negative) planetary vorticity flux dominated the area near the eastern (western) boundary where regional current pattern showed cyclonic (anticyclonic) characteristics, owing to regional topography as the abyssal current follows the boundary. However, the magnitude of the planetary vorticity flux along the western boundary was considerably larger than that along the eastern boundary, which caused a negative spatially averaged value in the region of observation during most of the study period. Therefore, the strong observed regional patterns differed from those expected for the deep SCS as a whole.
The planetary vorticity flux along the eastern boundary was significant in spring and autumn, whereas along the western boundary, it was enhanced only in spring. The regionally averaged seasonal pattern showed the largest value in spring, following the variability along the western boundary, presumably due to the larger magnitude there forced by regional anticyclonic currents along the topography. Deep eddies significantly contributed to the planetary vorticity flux and regional vorticity balance. An extremely weak planetary vorticity flux appeared along the western boundary in August owing to the effect of the cyclonic deep eddy and caused a positive value in the observational area.
Although previous studies have clarified the external and internal dynamics of abyssal circulation in the deep SCS, they focused on the climatological layer-averaged vorticity budget and were based on numerical simulations or reanalysis data (Gan et al. 2016; Zhu et al. 2017; Cai and Gan 2019). In the current study, the dynamics maintaining the SCS abyssal circulation were determined using observations from the CPIES array deployed from June 2018 to July 2019. In addition, the spatiotemporal distributions of the vorticity terms were documented based on our unprecedented observations. Overall, this study provides an important observational confirmation of both the broad outlines and some mesoscale details of abyssal vorticity balance, which contribute to a more detailed knowledge of how vorticity balance is consistent with different sources of local forcing and how the abyssal circulation in the SCS varies spatiotemporally.
Acknowledgments.
This study was sponsored by the National Natural Science Foundation of China (Grants 41920104006 and 41906024), the Scientific Research Fund of Second Institute of Oceanography, MNR (Grant JZ2001), the Project of State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography (SOEDZZ2106 and SOEDZZ2207), the Oceanic Interdisciplinary Program of Shanghai Jiao Tong University (project number SL2021MS021), the Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (311020004), and the Global Climate Changes and Air-sea Interaction Program (GASI-02-PAC-ST-Wwin). In addition, Feng Nan was supported by the Taishan Scholars Program.
Data availability statement.
For accessing the abyssal circulation observations, please contact the corresponding author.
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