1. Introduction
Satellite altimeters potentially provide global maps of sea surface height (SSH) variability, revealing ubiquitous mesoscale eddies in the World Ocean (Chelton et al. 2011). By transporting and stirring momentum and tracers, such as heat, salinity, and CO2, along density surfaces, eddies strongly influence ocean circulation and climate systems (Bryden and Brady 1989; McGillicuddy et al. 2007). In addition to typical surface-intensified anticyclonic eddy (AE) and cyclonic eddy, there is also a particular type of subsurface-intensified eddy within the thermocline, termed intrathermocline eddy (ITE; Dugan et al. 1982). However, due to their weak surface signals and awkward sizes, most of the ITEs are difficult to detect by remote sensing or just identified as surface-intensified eddies, and they can only be discovered occasionally through in situ observations (Thomsen et al. 2016; McCoy et al. 2020). The observed ITEs usually have a vertical extent on an order of 100 m, and a horizontal size similar with or larger than the internal Rossby radius of deformation (Gordon et al. 2002; Nauw et al. 2006). Among the previously reported ITEs in the global ocean, most of them are anticyclones and characterized by a lens-like isopycnal structure that bounds the homogenous hydrographic properties in the thermocline (Kostianoy and Belkin 1989; McGillicuddy et al. 2007). Moreover, they have a maximum geostrophic velocity, weak stratification, and low potential vorticity (PV) within their lens-shaped cores (McWilliams 1985; Barceló-Llull et al. 2017a).
The mesoscale ITEs with lens-shaped low PVs can trap water masses with tracers (e.g., heat, salinity, oxygen, and nutrients) and carry them into the oceanic interior from the upper mixed layer (ML) upon formation (Oka and Qiu 2012). These tracers within mesoscale ITEs, reflecting the oceanic and air–sea information of the formation area, can be transported far away from their origin over a long time, and are redistributed in the oceanic interiors (Shapiro et al. 1995; L’Hégaret et al. 2014; Barboni et al. 2023). Previous studies have also suggested that the biases of low-PV water subduction in current climate models are much due to the poor representation of these subsurface eddies as well as their generation processes (e.g., Naveira Garabato et al. 2001; Lee et al. 2011; Shi et al. 2018). The climate variation depends on many critical processes related to the subsurface eddies, such as how much carbon and heat they take up and how this process responds to global warming (McGillicuddy et al. 2007; Karstensen et al. 2015; Czeschel et al. 2018). More importantly, within the 100-km mesoscale field of ITEs, in situ high-resolution surveys have also revealed that active submesoscale processes and vertical secondary circulation can intensify stirring and vertical flux in the periphery (Garreau et al. 2018; Pietri and Karstensen 2018). Thus, these ITEs have multiscale dynamical processes and significant implications for air–sea interactions. This study only focuses on the mesoscale feature of ITEs and the generation mechanism of their low-PV sources, partly due to the limitation of data resolution.
The low PV anomaly (PVa) of the ITEs is the defining feature, because the velocity and density fields can be reconstructed (assuming that the flow is primarily balanced) based on only the PVa alone using a PV inversion technique (Hoskins et al. 1985). Therefore, ITE formation relies on the generation of low-PV water masses and depends on the spatially isolated PVa. Hence, the realistic ITE generation mechanisms must satisfy two conditions: providing a low-PV source and achieving this intermittently spatially (McWilliams 1985; Thomas 2008). In the open ocean, the PV of surface waters can be modified by diabatic and frictional processes until the fluid is subducted into a nearly inviscid and adiabatic interior (Thomas 2005). Once the low PV is injected into the ocean interior, it acts as a conservative yet dynamically active tracer (Rhines 1986). The two main low-PV generation mechanisms near the sea surface proposed in previous studies are described below. Down-front wind forcing along the geostrophic flow of mesoscale eddy field can effectively drive Ekman transport across the density front and destroy PV (Thomas 2005). In this mechanism, the frictional process changes the fluid PV. Simultaneously, PV can also be modified by the diabatic process. Surface cooling can drive net buoyancy loss in the surface boundary layer and reduce PV owing to diabatic atmospheric forcing processes (Worthington 1977; Oka et al. 2011). Therefore, both down-frontal wind forcing and surface cooling can theoretically contribute to the generation of a source for the ITE low PV (Thomas 2008; D’Asaro et al. 2011).
The South China Sea (SCS) is the largest semienclosed marginal sea basin in the northwestern Pacific Ocean. Owing to complex dynamic factors, such as topography, monsoonal forcing, Kuroshio intrusion, and river runoff (Su 2004), mesoscale eddies are active in the NSCS (Wang et al. 2003, 2008). These conditions are favorable for the ITE generation. Although a few previous studies revealed the existence of ITEs in the NSCS, and suggested that they were generated by Kuroshio intrusion or different water masses interaction in the Luzon Strait (Lin et al. 2017; Sun et al. 2022; Wang et al. 2023). The source and dynamic generation mechanisms of the lens-shaped low PV within ITEs have remained elusive, partly because of the lack of in situ observations with sufficient spatiotemporal resolution.
In this study, we aimed to investigate the formation mechanism of the low PV within the mesoscale ITE in NSCS using cruise observations, satellite data, and reanalysis products. The remainder of this paper is organized as follows. Section 2 provides the data and methods used in this study. The observed characteristics of the ITE are described in section 3. In addition, a new general mechanism for generating low PV within ITEs is explained. Section 4 summarizes the results with a conclusion.
2. Data and methods
a. In situ observations
A target survey for mapping mesoscale eddy was conducted in the NSCS in May 2021. Guided by the AE signal of the satellite-derived sea level anomaly (SLA) (the black box shown in Fig. 1), six hydrographic transects across the target AE were obtained during the cruise from 21 to 28 May (16.7132°–20.1885°N, 112.7287°–115.3418°E) (Fig. 2). Considering the similar results for different transects, we only show a representative east–west transect crossing the AE in this study (Transect S01–S15 shown in green dots in Fig. 2). Temperature, salinity, and depth were measured using a SeaBird 911 Plus conductivity–temperature–depth (CTD) instrument. The horizontal interval of the CTD cast is approximately 20 km, and the effective vertical resolution is 1 m. Current velocities were measured continuously in all six transects using a 75-kHz shipboard acoustic Doppler current profiler (ADCP), which provided raw data with 5-min ensembles from 16 to 600 m and a bin size of 8 m. The raw data were quality-controlled, corrected for heading misalignment, and edited using the Common Oceanographic Data Access System (Firing et al. 1995). On average, the processed profiles provide high-quality data from depths of 20–500 m with horizontal resolution of 1.5 km and vertical resolution of 8 m. To investigate the water mass characteristics and origination of the observed ITE, we also used the climatological temperature and salinity data at a 1/4° grid resolution acquired from the World Ocean Atlas 2018 (WOA18).
Origin and propagating trajectory of the observed AE in the NSCS. The blue line shows the satellite-derived trajectory of the AE over its life cycle (9 Oct 2020–1 Jul 2021), with the yellow star representing its start. The orange lines show the sea surface boundary of the AE identified by the AVISO SLA. The black vectors represent the geostrophic velocity anomaly derived from satellite altimetry data. The black box indicates the cruise target observation area, and the gray lines show the bathymetric contours (200, 600, 1000, and 2000 m).
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
Sea surface view of part of the satellite image (above) and the vertical distributions of temperature measurements taken from CTD (below) during 21–28 May 2021. The sea surface temperature (SST; color shading) is provided by the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images during cruises, CTD stations (blue and green dots), and finite size Lyapunov exponents (FSLEs; gray dots) are shown on the surface image. For the vertical slices of the temperature data (color shading) measured by CTD, the mixed layer depth (MLD) is represented by the gray line, and the isopycnals of σθ = 22 and 24 kg m−3, contoured in black, indicate the boundary of the lens-shaped homogeneous layer of the ITE. The MLD represents the depth at which the potential temperature is 0.5°C less than that at the 10-m depth (de Boyer Montégut et al. 2004), and the σθ is the potential density anomaly at a reference pressure of 0 dbar (σθ = ρ − 1000 kg m−3).
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
b. Satellite data and reanalysis products
The gridded daily SLA and surface geostrophic velocity data are obtained from the Copernicus Marine Environment and Monitoring Service (CMEMS) with a spatial resolution of 1/4° from 1993 to 2021. The SST, with a daily 1 km × 1 km grid resolution, are derived from the Level-2 MODIS and provided by the Goddard Space Flight Center (GSFC) of NASA. The FSLEs are delivered by AVISO with a daily resolution of 1/25° × 1/25°. The daily Mesoscale Eddy Trajectory Atlas (META) 3.2 Delayed-Time dataset acquired from AVISO+ is used to track the positions of mesoscale eddies. This product provides the time, position, amplitude, rotational speed, radius, and type (cyclone/anticyclone) of mesoscale eddies. In this study, we select the AEs generated west of the Luzon Strait and then propagate westward to the eastern Hainan Island from 1993 to 2021. Additionally, to reduce the risk of eddy identification arising from noise in the SSH fields, we discard the AEs with lifetimes shorter than 4 weeks (Chelton et al. 2011).
The product of the fifth generation of the European Centre for Medium-Range Weather Forecasts (ERA5) is used to analyze atmospheric forcing. The ERA5 combines numerous observations with numerical simulations to estimate atmospheric states using assimilation (Hersbach et al. 2020); it also provides a wide range of atmospheric and oceanic parameters (including net surface heat flux, evaporation, precipitation, and sea surface wind used in this study) from 1979 to present with an hourly spatial resolution of 0.25° × 0.25°. The 1/12° ocean reanalysis datasets of the global ocean analysis and forecast system (Lellouche et al. 2018) are acquired from the CMEMS to diagnose the PV along with eddy trajectories. The CMEMS reanalysis datasets based on the Nucleus for European Modeling of the Ocean have assimilated the altimeter data, vertical profiles of in situ temperature and salinity, and satellite SST data. These reanalysis datasets [including SSH, three-dimensional velocity, and temperature/salinity (T/S) field] are archived daily during 1993–2021.
c. Diagnostic estimations of potential vorticity budget
To identify what forcing condition are favorable for PV destruction and can play an important role in the formation of low-PV ITEs in this study, we integrate Eq. (5) over a control volume that encircled the AE. The upper surface of the control volume coincides with the air–sea interface enclosed by the AE edge, where the outcropping isopycnals indicate the presence of horizontal density gradients (McWilliams 2021). Notably, the AE edge identified by AVISO is the smallest closed SSH contour. The side surfaces of the AE are vertically aligned with the edge, and the bottom surface is the isopycnal bounding the eddy where the flow, frictional forces, diabatic processes, and hence the PV fluxes are weak (Thomas 2005). Therefore, the PV in the control volume is reduced if the PV flux at the surface is outward.
3. Results
a. Observed characteristics of the intrathermocline eddy in the NSCS
First, we examine the origination and sea surface characteristics of the observed AE (Fig. 1). This AE was generated near the Luzon Strait on 9 October 2020 and developed here for almost three months. It then propagated southwestward along the continental slope and away from the local forcing in the southeastern corner of Taiwan with persistent winter monsoon forcing. Around March 2021, it grew to be strongest on northeastern Dongsha Island and then deformed when it passed by the island by March end. The AE energy gradually weakened in April 2021 and moved southwest after May. Eventually, it disappeared from eastern Hainan Island on 1 July 2021.
Showing in Fig. 2, the in situ observations on the cruise capture an AE during 21–28 May 2021. The two longest transects cross its center (17.543°N, 114.164°E), presenting a lens-shaped vertical temperature structure inside it (Fig. 2). During the cruise observations, this AE is characterized by a diameter of about 150 km and a positive SLA of about 0.3 m. Interestingly, the SST of the AE shows a cold core compared with the periphery, which might be caused by eddy–wind interaction (Gaube et al. 2015) or mesoscale-modulated vertical mixing (Moschos et al. 2022). Enhanced FSLEs are also detected along with the periphery of the AE in the altimetry-derived maps (Fig. 2), suggesting potential frontogenetic staining and frontal submesoscale instabilities. Previous studies using high-resolution simulation and in situ observation also confirmed that the multiple instabilities are possibly shown in large mesoscale eddies (L’Hégaret et al. 2016; de Marez et al. 2020), which are not the focus of this paper, partly due to the observation data not resolving the submesoscale.
The vertical distribution results of the in situ observation clearly show a lens-shaped core with homogeneous thermohaline properties inside the AE (Figs. 3a,b). The isopycnals are dome-shaped in the upper part of the thermocline (30–75 m) and bowl-shaped in the lower part (75–150 m), indicating a typical ITE with a vertical scale of about 120 m. Compared to the periphery of the ITE, the center isopycnals in the lower part deepened by approximately 80 m, and in the upper part shallowed by about 20 m during the ITE case. The ADCP observations evidently present anticyclonic velocity associated with the aforementioned ITE thermohaline pattern. The ITE core is near 75 m with a maximum velocity of about 60 cm s−1, indicating the maximum subsurface velocity inside the ITE (Fig. 3d). Since the transect crosses the ITE center, the currents distribute a good symmetry structure. Note that here, it is a typical ITE in terms of the vertical hydrological structure captured by cruise observations, whereas based on the altimetry, it is just a normal surface-intensified AE.
Vertical distributions of (a) temperature (°C), (b) salinity [practical salinity units (psu)], (c) potential density (kg m−3), and (d) ADCP-derived across-transect velocity (m s−1) along the east–west transect S01–S15. The transect is shown in green dots in Fig. 2. The black lines in each panel denote the isopycnal contours with an interval of 0.5 kg m−3.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
Figures 4a and 4b show the weakened stratification inside the ITE compared with the background fields, which is conducive to smaller-scale instabilities owing to low PV and sloping isopycnals (e.g., Garreau et al. 2018; de Marez et al. 2020; Archer et al. 2020). The vertical distribution also reveals a strong decrease in the PV wrapped by the isopycnals of σθ = 22 and 24 kg m−3 within the ITE (Fig. 4c). The PVa also presents a decrease, mostly in the subsurface layer from 30 to 150 m within the ITE (Fig. 4d). Previous studies have found that ageostrophic submesoscale circulation can destabilize the flows and create significant gradients in the PV field (Mahadevan and Tandon 2006; Thomas et al. 2013). Therefore, the PV decreases nearly to zero in ITE, which may imply the potential submesoscale instabilities (e.g., Pietri and Karstensen 2018; Peng et al. 2020; Pérez et al. 2022).
Vertical distributions of (a) square of the Brunt–Väisälä frequency and (b) its anomaly (10−4 s−2), and (c) PV and (d) PVa (10−8 s−3) along the transect S01–S15. The transect is shown in green dots in Fig. 2. The black lines in each panel denote the isopycnal contours with an interval of 0.5 kg m−3.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
To investigate the origin of this subsurface water mass characterized by low PV, climatological T/S data (WOA18) is used to compare with in situ T/S data. As shown in the colored boxes in Fig. 5, the selected potential origin areas of the observed eddy are east of Luzon Strait, related to Kuroshio intruding path (blue box), and Luzon Strait (purple box). We also select the surrounding area (black box), where the ITE was captured by cruise in May 2021, to identify the difference between the water masses inside and outside the ITE. Figure 5 shows that the T/S characteristics inside the ITE are similar to those in the Luzon Strait. Moreover, the shift in the water mass properties between the ITE (green scatters) and the Luzon Strait or Kuroshio (purple and blue lines in Fig. 5, respectively) indicates that the low-PV water inside the ITE is likely formed with the AE generation. Therefore, the main objective of the rest of this paper is to investigate the generation of the lens-shaped low PV within the ITE.
T–S scatter diagram of the CTD data. The colored lines show the characteristics of the T–S profiles of the average WOA18 data within the colored boxes in the inset panel. The blue, purple, and black lines represent water mass characteristics in the Kuroshio, Luzon Strait, and cruise area, respectively (blue, purple, and black boxes in the inset, respectively). The green and orange dots show the T/S data inside (green CTD station shown in the inset) and outside (orange CTD station shown in the inset) the ITE.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
b. Generation of low PV within the ITE
From the CTD casts that cover the whole ITE, we have identified the different T/S characteristics for the water mass trapped inside the ITE (Ti, Si) and the surrounding water (To, So, shown in Fig. 5). Therefore, to track the T/S properties of the well-conserved lens-shaped low-PV water along the AE trajectory, we introduced the water mass ratios DWM, defined as
Time series of (a) SLA (m), (b) temperature (°C), (c) salinity (psu), and (d) DWM along the AE trajectory (blue lines in Fig. 1). The T/S is provided by the CMEMS dataset. Black and gray lines represent MLD and isopycnals, respectively. To examine seasonality, the data are categorized into four seasons: spring (March–May), summer (June–August), autumn (September–November), and winter (December–February).
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
The SLA characterizes the formation, development, and damping of the AE along its trajectory (Fig. 6a). During the AE formation in the autumn of 2020, there was high salinity within the ML caused by the consistent Kuroshio intrusion near the Luzon Strait. Subsequently, the AE moved away from the strait, leading to the mixing of the Kuroshio water within it with the northeastern SCS water, with a gradual decrease in salinity from January (Figs. 1 and 6c). The temperature in ML also decreased due to the sea surface cooling (Fig. 6b). Therefore, the DWM appeared close to zero within ML from January, indicating the emergence of the source for low-PV ITE core water (Fig. 6d). With the uplift of ML in spring, these water masses were trapped in the thermocline inside the AE (Fig. 6d). Notably, there were very consistent water masses with the observed ITE lens core below the ML in autumn of 2020. It likely resulted from mixing Kuroshio water and northeastern SCS water in the winter of last year, and then subducted into the thermocline near the Luzon Strait (e.g., Qu et al. 2000, 2013; Centurioni et al. 2004). Due to the generation of the AE, it was wrapped in the eddy and has been transported southwestward into the SCS. It implies the important role of ITEs in water mass transport as well as heat and salt budget, although they are not the focus of this study.
Figure 7 shows the sea surface conditions and PVa along the AE trajectory. During the autumn of 2020, surface-intensified AE first appeared with a well-mixed surface ML characterized by low PV. With the AE propagating westward in the following winter, the low PV signals in the boundary layer are presented and keep moving with it (Fig. 7c). When the wind forcing weakens and surface heating increases with the arrival of spring, the ML is raised rapidly with restratification of the water column (Fig. 7b). This persistent surface heating in spring is conducive to reducing surface buoyancy loss, and increasing PV on the AE surface, eventually capping the low-PV water and producing a subsurface-intensified ITE (Figs. 6d and 7c). Thus, these dynamically coupled diabatic processes effectively create favorable conditions for the observed ITE formation.
Time series of (a) net surface heat flux (W m2) and SST (°C), (b) wind stress (m s−2) and curl of wind stress (s−2), and (c) PVa (×10−8 s−3) along the AE trajectory (blue lines in Fig. 1). The Qnet and sea surface wind are provided by the ERA5 dataset, and the SST (PVa) is obtained (calculated) from the CMEMS dataset. Black and gray lines represent MLD and isopycnals, respectively.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
For the boundary layer turbulence in the open ocean, there are three main nonconservative processes to induce PV flux: diabatic processes induced by atmospheric forcing, wind-driven frictional processes, and TTW circulation (Thomas 2008; Wenegrat et al. 2018). Accordingly, we examined the potential impacts of these physical processes on low-PV generation during the evolution of the observed ITE. Figure 8 presents the PVa and three essential constituents of the PV flux [Jwind,
Snapshots of (a) PVa, (b)
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
To quantify the contribution of these nonadvective PV flux constituents in generating low PV, we further examined the time series of B0, EBF, and EBFg along the AE trajectory. Figure 9 identified the generation of the low PV associated with the atmospheric-forcing surface buoyancy loss. Arising from the sea surface cooling in autumn and winter, the sea surface buoyancy flux and its induced PV flux always decrease the oceanic PV, with a peak shown in early January (Figs. 9a,b, dark blue lines). Such strong buoyancy loss forcing creates a favorable condition for low-PV generation at the AE surface, which is the defining substance of the ITE. Contrastingly, the smallest magnitude of EBF among the nonadvective terms indicates that the effect of wind forcing on low-PV generation is almost negligible (Figs. 9a,b, the green line). In late March 2021, the surface buoyancy flux gradually changed from positive to negative in response to the seasonal surface heating in spring. Consequently, the surface heating-injected positive PV and shallow ML together transform the surface-intensified AE into an ITE during spring.
Time series with an average of (a) buoyancy flux (m2 s−3), (b) PV budget (m3 s−4), and (c) a cumulative change of PV flux (m3 s−3) within the AE extent along its trajectory. The trajectory is shown as a blue curve in Fig. 1.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
At the same time, we note that the AE passed by Dongsha Island (Fig. 1). The lateral friction by the slope topography is expected to be another important dynamical process to remove PV of the eddy and conducive to the ITE generation (Pelland et al. 2013; Gula et al. 2015; Perfect et al. 2020). However, the advective-induced PV flux (JA, ∼10−4 order of magnitude) is much smaller than the atmospheric-forced PV flux (
For the time-cumulative PV budget of the eddy volume (Fig. 9c), it can be seen that there is a consistently diabatic extraction of PV out of the volume in winter, leading to Δq < 0. Although both the time integrals of the frictional and diabatic terms act to destroy the PV, the diabatic process driven by the wintertime buoyancy loss from ocean to atmosphere is the main contributor to the PV removal. This configuration also shows that the EBFg results in a consistent PV injection, whereas seasonal cooling allows the boundary layer to accumulate significant PV extraction from ocean to atmosphere and leads to a net decrease in PV. Thus, although the frictional nonadvective surface buoyancy flux also contributes to PV removal, it is the wintertime buoyancy loss rather than the down-frontal wind forcing that quantitatively dominates the low-PV generation of the observed ITE case in the NSCS.
The maps of SLA and
Maps of (a)–(c) SLA (color shading; m) with geostrophic currents (vectors; m s−1) and (d)–(f) PV flux (×10−12 m s−4) induced by atmospheric diabatic forcing at the sea surface. Vertical distributions of (g)–(i) PVa (×10−8 s−3) and (j)–(l) current velocity (m s−1) along the transect indicated by blue lines in (a)–(c) on (left) 1 Mar, (center) 1 Apr, and (right) 5 May 2021. In (a)–(f), the dark gray lines denote bathymetric contours (200, 600, 1000, and 2000 m), and the green contours represent the AE edge provided by AVISO. In (d)–(f), the light gray contours with arrows indicate the daily averaged streamlines of the flow field. In (g)–(l), the gray and black lines represent the density contours, and the green line is the MLD. The white contours in (j)–(l) show the current velocities across the transect.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
In April, AE is slightly weakened and deformed along the meridional direction due to the Dongsha Island (Fig. 10b). During this period, the nonadvective PV flux dominated by the surface buoyancy flux changes dramatically from positive to negative owing to surface heating during spring (Figs. 10d,e). Due to two orders of magnitude smaller the Dongsha Island size compared to the mesoscale eddy, the advective PV flux (Figs. 9b,c, magenta lines), mostly induced by the lateral friction of slope topography, is much smaller than the nonadvective PV flux. These results indicate a transformation from PV extraction to PV injection into the upper ocean primarily by atmospheric diabatic forcing. Simultaneously, the enhanced surface heating and negative buoyancy flux tend to restratify the boundary layer and induce an increase of PV, substantially facilitating the packing of low PV into the thermocline interior. As a result, the enhanced restratification tends to weaken the lateral buoyancy gradient by isopycnals slumping and damping the surface kinetic energy of the geostrophic eddy, according to the relationship of the thermal wind balance. Therefore, the maximum current velocity of the ITE from both in situ observations and simulations is detected in the subsurface rather than in the surface layer as a normal eddy (Figs. 3d and 10l).
When the surface high PV gradually caps the low-PV water in spring, a homogenous lens is formed within the intrathermocline, wrapping the low PV inside the ITE. The subsurface maximum of the geostrophic flows in the ITE is conducive to the formation and maintenance of dome-shaped isopycnals over the lens and to keeping the low PV in the oceanic interior (Barceló-Llull et al. 2017b). These processes, in conjunction with atmospheric diabatic forcings, produce the transformation of surface-intensified AE into ITE and propagate away from its origination. It was not until late May that this eddy maintaining this particular structure was observed by an in situ cruise, as shown in Figs. 3 and 4.
c. Proposed diabatic forcing mechanism of the ITEs
To identify whether the ITEs with lens-shaped low PV commonly occurred in the NSCS and to clarify their source and general generation mechanism, we further statistically examine the ITEs during the presence of surface-intensified AEs during 1993–2021 (29 years) based on the reanalysis datasets (Fig. 11). For each mesoscale AE, the steric dynamic height anomaly (h′) is computed using the reference level at 1000 dbar, following the definition by Gill and Niiler (1973):
Trajectories of (a) all 45 surface-intensified AEs and (b) 13 ITEs that were generated west of the Luzon Strait and then propagated westward to the east of Hainan Island detected by META dataset during 1993–2021, in which blue circles (black crosses) represent eddy start (end) points. (c),(d) Composite anomaly profiles of density (δρ; kg m−3) and steric dynamic height (h′; cm) for surface-intensified AEs (blue curves) and ITEs (red curves), respectively.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
Time series profiles of PVa (color shading; 10−8 s−3) along the ITE trajectories shown in Fig. 11b. The gray contours represent the isopycnals. The time marked by black inverted triangle at the top of each panel is the occurrence time of positive PVa near the sea surface. It also indicates the formation of the lens-shaped low PV within the ITE.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
To further investigate the generation mechanism of the low PV within the ITEs, we calculate the sea surface heat, wind stress, sum of the PVa within the lens-shaped core, and B0 for the remaining 12 ITEs (Fig. 13). The results show a good correspondence between atmospheric diabatic forcing and low PV within the ITEs (Fig. 13c). The PVa constantly decreases with sea surface buoyancy loss during eddy generation in autumn or winter. In the following spring, the high PV induced by surface heating covers the preexisting low PV, resulting in a typical surface-intensified AE transforming into a subsurface-intensified ITE. Figure 12 also confirms that the appearance of the lens-shaped structures in all ITEs is in the following spring (black inverted triangle at the top of each subplot in Fig. 12). Notably, the low PV is relatively conserved and is contained in the ITE after it is capped in late May (Fig. 13c). This indicates that the lens-shaped low PV within the ITE is not only highly stable and does not dissipate with its movement but also capable of transporting the hydrographic properties of the water mass over long distances (Davey and Killworth 1984; Early et al. 2011). In conclusion, we determine that the AE generated in autumn or winter is influenced by seasonal variations in the sea surface diabatic atmospheric forcing and is easy to transform into the ITE characterized by lens-shaped low PV in the following spring.
Time series of (a) net surface heat flux (W m2) and SST (°C), (b) wind stress (m s−2) and curl of wind stress (10−7 s−2), and (c) B0 (×10−7 m2 s−3) and PVa (×10−7 s−3) within the eddy extent along the ITE trajectories shown in Fig. 11b. PVa represents the sum of PVa between the isopycnals of σθ = 22 and 24 kg m−3 along the ITE trajectory. In (c), the red shading indicates March, and the solid green line represents late May.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
d. Potential impact of the ITEs on heat and salt transport
The subsurface-intensified ITE is suggested to contribute to water mass transport (Figs. 5 and 6). Nevertheless, whether these ITEs with conserved lens-shaped low PVs in SCS can modify the T/S properties of surroundings during their propagation and dissipation remains an open question. It is intended to investigate this question using the 13 ITEs identified in this study from 1993 to 2021 (Fig. 11b). Take the vertical structure of each ITE along its propagation trajectory, normalize it on the x axis using the ITE’s lifespan, and then combine all normalized structures. The results (Fig. 14) show that during the AE formation, the isopycnals outcrop due to the high-saline water transported by Kuroshio intrusion in autumn and winter near the Luzon Strait, which also provides a precondition for the extraction of PV within the AE. Initially, the vertical structure shows the typical surface-intensified AE characterized by well-mixed water mass properties within the deep ML. When the ITE is formed, the lens-shaped core preserves its original homogeneous heat and salt properties well during its propagation.
Mean vertical distributions of (a) temperature (°C), (b) salinity (psu), and (c) PVa (×10−8 s−3) of 13 ITEs along their trajectories based on the CMEMS reanalysis data. The ITEs’ trajectories are shown in Fig. 11b. Black and gray lines represent MLD and isopycnals, respectively. The x axis is normalized by eddy lifetime T.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
Figure 15 compared the temperature, salinity, and PVa between each ITE’s lens-shaped core and its surroundings along the trajectory. The results show that the temperature within the AE is easily modified by the surroundings, with a cold core in winter and spring (Gaube et al. 2015; Moschos et al. 2022). Upon the ITE formed, the temperature within the lens-shaped low-PV core is always about 1°C higher than its surroundings (Fig. 15a). In contrast, the salinity is more conserved. As the AE wraps high-saline water away from the Luzon Strait into the NSCS, the salinity within the eddy remains essentially constant and is always higher by about 0.2 psu than the surroundings (Fig. 15b). Therefore, these ITEs potentially carry high-saline Kuroshio water into the SCS, thus contributing to the subsurface thermohaline properties here. With the variations of the Kuroshio intrusion (e.g., Qu et al. 2000; Xue et al. 2004; Qi et al. 2022), the water properties within the ITE’s core may also change. Due to the conservation of low PV, the ITE core will preserve its original thermocline properties well and remain for a long time in the ocean interior. It can impact the subsurface water properties and regional heat and salt budgets in the SCS. More importantly, when these low-PV water masses carrying the oceanic and climate information of the formation area get ventilated, it may affect the air–sea interaction of this remote region. These results imply that the subsurface-intensified mesoscale ITEs may provide an important route for the water mass transport from the western Pacific Ocean and the SCS, contributing to the southwestward heat and salt transport across the Luzon Strait. Thus, considering the ITEs is also conducive to improving the knowledge of regional heat and salt change, although it is not the focus of this paper.
Time series of the mean of (a) temperature anomaly (°C), (b) salinity anomaly (psu), and (c) PVa (×10−8 s−3) between the isopycnals of σθ = 22 and 24 kg m−3 along each ITE’s trajectory. The ITEs’ trajectories are shown in Fig. 11b. The solid black lines in (a)–(c) represented their average values. Anomalies are calculated by subtracting the average within 50 km extended from the AE edge for each depth layer.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
4. Summary and conclusions
This study investigates the ITE observed by in situ measurements in the NSCS during May 2021 and discusses the underlying diabatic forcing mechanism for generating low PV in these ITEs using satellite-derived eddy trajectories, the reanalysis dataset, and budget analysis of PV flux. The observed mesoscale ITE is characterized by a typical lens-shaped low PV inside the thermocline. This low-PV lens occupied a depth of 30–150 m and a horizontal size of ∼150 km, with subsurface-enhanced geostrophic flows reaching 60 cm s−1. On analyzing the water mass and eddy trajectory, we find that the low-PV water is not locally generated but is wrapped inside the AE and propagated westward far away from the observation domain.
Further diagnostic results of the PV budget indicate that the generation of low PV is primarily dominated by surface buoyancy loss via atmospheric diabatic forcing rather than wind-driven frictional forcing during autumn and winter. The persistent surface buoyancy losses induce an upward buoyancy flux and are conducive to the PV destruction within the weakly stratified deep ML of the preexisting AE. As seasonal atmospheric forcing changes from surface cooling during winter to surface heating in following spring, the diabatic surface buoyancy flux tends to increase PV through isopycnals slumping and boundary layer restratification. The surface-injected high PV can accumulatively cap the low-PV water in the thermocline and isolate it from atmospheric diabatic forcing by the enhanced stratification of the boundary layer. Moreover, it results in the deceleration of the geostrophic flows of the surface layer and then forms the subsurface maximum velocity layer. With geostrophic adjustment, the subsurface maximum of the geostrophic flows in the ITE is conducive to forming and maintaining the dome-shaped isopycnals over the low PV. In the presence of preexisting AEs during autumn and winter, these processes, in conjunction with atmospheric diabatic forcing, effectively transform the surface-intensified AEs into subsurface-intensified ITEs with lens-shaped low PV in the following spring (Fig. 16).
Schematic diagram of the transformation of (a) AE into (b) ITE. The low PV generated within AE is due to the surface buoyancy loss in autumn and winter. The ML and isopycnals within the AE are deepened by homogeneous low-PV water with an apparent bowl shape. In the following spring and summer, the high PV induced by surface heating covers the low PV within AE and transforms it into an ITE. The buoyancy loss induced by atmospheric diabatic forcing will result in an upward PV flux J at the sea surface.
Citation: Journal of Physical Oceanography 54, 3; 10.1175/JPO-D-23-0149.1
Further statistical and diagnostic analyses based on the reanalysis dataset from 1993 to 2021 suggest that the observed mesoscale ITE with lens-shaped low PV is a common phenomenon in the NSCS. At least 22% of the surface-intensified AEs identified by satellite altimetry are developed into ITEs. Due to the conservation of low PV in ITEs, their original thermocline properties are well conserved even after long-distance propagation and remain for a long time in the ocean interior. Therefore, these ITEs may impact the subsurface water properties and the regional heat and salt budget. More importantly, the diagnostic results indicate that these lens-shaped low PVs are produced primarily by enhanced surface buoyancy loss in autumn and winter, thus supporting the new mechanism explanation of low-PV generation of ITEs due to diabatic forcing in the NSCS, rather than wind-driven frictional forcing. These findings provide a new understanding for the source and generation mechanism of low PVs within ITEs and highlight the importance of atmospheric diabatic forcing.
In this study, only mesoscale ITE and PV flux are examined. Submesoscale modification of PV and induced vortex by the lateral friction of Dongsha Island are not the focus, partly due to the coarse data and relatively smaller size of the island. The PV removal by the island-induced lateral friction is expected to produce submesoscale vortexes on the downstream side of Dongsha Island. In the future, more field efforts and high-resolution simulations are needed to quantitatively discuss the abundance of ITEs, their variations, and other potential nonconservative mechanism, as well as their contributions to the physical and biogeochemical budget of the oceans.
Acknowledgments.
This work is supported by the National Natural Science Foundation of China (42225602, 92058201, 92258301, 42149907, 42349907, and 42349584), and the Chinese Academy of Sciences (ZDBS-LY-DQC011, SCSIO202204 and SCSIO2023QY02). We sincerely thank Yan Du and Ruixi Zheng of SCSIO, who improved this manuscript with helpful comments and fruitful discussions. We thank the R/V Shiyan 6 crew for their help in collecting the observation data during the cruise. We are also grateful to the two anonymous referees whose suggestions and remarks greatly improved the paper.
Data availability statement.
The observational data used to generate the plots in this article are available at https://doi.org/10.57760/sciencedb.09811. The WOA18 data are from https://www.ncei.noaa.gov/access/world-ocean-atlas-2018/. The SLA and surface geostrophic velocity data are obtained from the CMEMS (http://marine.copernicus.eu/services-portfolio/access-to-products/). The SST data are provided by the MODIS (http://oceancolor.gsfc.nasa.gov/). The FSLEs data are delivered by the AVISO (http://www.aviso.altimetry.fr/en/data/products/value-added-products/fsle-finite-size-lyapunov-exponents.html). The mesoscale eddy trajectories data are available at AVISO+ (https://data.aviso.altimetry.fr/aviso-gateway/data/META3.2exp_DT/). The air–sea reanalysis products are acquired from the ERA5 (https://www.ecmwf.int/en/forecasts/datasets/archive-datasets/reanalysis-datasets/era5). The global ocean reanalysis and forecast datasets are downloaded from the CMEMS (https://data.marine.copernicus.eu/product/GLOBAL_MULTIYEAR_PHY_001_030/services).
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