| Citation: | Guangbing Yang, Quanan Zheng, Xuejun Xiong. Subthermocline eddies carrying the Indonesian Throughflow water observed in the southeastern tropical Indian Ocean[J]. Acta Oceanologica Sinica, 2023, 42(5): 1-13. doi: 10.1007/s13131-022-2085-2
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The Indonesian Throughflow (ITF) is a low-latitude connection between the Pacific and the Indian Ocean (Sprintall et al., 2009; Wang et al., 2019). It transports warm and low-salinity water from the Pacific into the Indian Ocean and affects the heat and mass budgets and air–sea heat fluxes of the two oceans (Gordon et al., 2003; Zhou et al., 2008; Van Sebille et al., 2014). The ITF exits from the Pacific into the southeastern tropical Indian Ocean (SETIO) primarily through the Ombai Strait, the Lombok Strait, and the Timor Passage (Godfrey, 1996). The ITF waters at different depths follow different spreading pathways in the Indian Ocean (Song et al., 2004; Valsala and Ikeda, 2007). Previous studies have suggested that the upper-layer of the ITF waters (above ~200 m) are advected westward by the South Equator Current, whereas the deeper ITF waters mainly flow southward (Song et al., 2004). A recent study suggested that the ITF water also may flow northwestward within the South Java Current (Makarim et al., 2019). Because diffusion and mixing tend to destroy the salinity and temperature properties of water masses, previous studies on ITF water spreading mainly have been based on tracer experiments or ITF water fraction analysis (Song et al., 2004; Valsala and Ikeda, 2007; Makarim et al., 2019).
Oceanic eddies are abundant and ubiquitous in the ocean. They play an important role in water mass transport (Zhang et al., 2014), oceanic mixing (Zhang et al., 2019), and biogeochemical processes (Yang et al., 2019a) and thus draw a lot of attention. Based on their vertical structure, oceanic eddies can be categorized as surface eddies or subsurface eddies (Song et al., 2019), which have their maximum velocities at surface and at subsurface with depths ranging from tens to thousands of meters, respectively (Fig. 1). Both surface and subsurface eddies have anticyclonic and cyclonic types. The surface anticyclonic eddy (AE) has positive sea-level anomaly (SLA) and downward domed isopycnals, whereas the surface cyclonic eddy (CE) has negative SLA and upward domed isopycnals. The number of surface AEs usually is close to the number of local surface CEs (Chelton et al., 2011). The isopycnals of subsurface AEs have a double-convex lens structure, whereas the isopycnals of subsurface CEs have double-concave lens structure. Observations suggest the subsurface AEs are much more abundant than the subsurface CEs (Yang et al., 2019b).
Subthermocline eddies (STEs) are a type of subsurface eddy (Zhang et al., 2015, 2022; Hu et al., 2022). Their cores are below the thermocline and they typically have anticyclonic lenses of relatively homogenous water. STEs usually are long-lived and can move far from their origins (Bashmachnikov et al., 2015). They retain much of the characteristics of their formation area, therefore, they have an important effect on mass and heat transport in the ocean subsurface layer (Zhang et al., 2017). In addition, STEs also have an important effect on ocean mixing. Recent studies have shown that anticyclonic STE can elevate diapycnal mixing and can reflect downward propagating near inertial waves (Zhang et al., 2019). Because STEs have small or no surface signatures, they are rarely observed and are less understood. In situ observations, such as cruise and mooring observations, are the best way to collect this kind of data. Until now, STEs have been observed sporadically in the Northeast Atlantic (Mediterranean water eddies, Meddies; Bashmachnikov et al., 2015), the California coast (California Undercurrent eddies, Cuddies; Pelland et al., 2013), the Peru-Chile coast (Contreras et al., 2019), the northwest Indian Ocean (Red Sea water eddies, Reddies; Shapiro and Meschanov, 1991), and the Northwest Pacific (Kuroshio Extension intermediate-layer eddies, Kiddies; Li et al., 2017b). In the SETIO, a kind of cyclonic STE has been reported using in situ observations (Yang et al., 2019b).
In this study, we observed a kind of anticyclonic STE carrying ITF water in the SETIO. The spatial distribution of the STE was examined using Argo float data. We also discussed the origin, generation mechanism, and contribution to ITF water transport. In Section 2, we introduce the cruise observation and data. In Section 3, we illustrate the two STEs observed during cruise and captured by an Argo float. In Section 4, we discuss the distribution, origin, and generation mechanism of the STEs. Our conclusions are presented in Section 5.
We conducted the cruise observation along the transect of 10°S in the SETIO from December 3 to 4, 2017. We employed a 38-kHz ocean surveyor vessel-mounted acoustic Doppler current profiler (VADCP, Teledyne RD Instruments) to measure the current velocity with the first bin at depth of 33 m and the bin size of 16 m. The VADCP data were first averaged into 5-min intervals and then interpolated to a horizontal spacing of 2 000 m. The VADCP observation transect was zonal from 95°E to 100°E, as marked by the red line in Fig. 2. As shown in Fig. 2, the surface current at depth of 33 m measured by VADCP was in general agreement with the geostrophic currents derived from SLA data. We used a lowered SeaBird SBE 911 Conductivity-Temperature-Depth (CTD) profiler to measure the conductivity and temperature profiles at five stations, which were coded s-1 to s-5 from west to east. The temperature and conductivity sensors were carefully calibrated in the laboratory before the cruise. The uncertainties were better than 0.001℃ and 0.000 1 S/m. After quality control, the temperature and salinity data were averaged into 1-m bins.
The SLA and corresponding surface geostrophic currents with a (1/4)°×(1/4)° resolution were downloaded from the Copernicus Marine Environment Monitoring Service (
We downloaded the Argo profiles data and the annual grided Argo-climatology-data (2004–2017) with 1°×1° resolution (BOA_Argo, Li et al., 2017a) from the China Argo Real-time Data Center (
We downloaded the Mesoscale Eddy Trajectory Atlas Product (META3.1exp DT, DOI:
Figures 3a and b show the currents observed along the VADCP transect. As shown in Fig. 2, the VADCP transect crossed the northern periphery of large AE, the meridional velocity in the western half of the transect (blue dashed box in Fig. 3a) was mainly southward, whereas the meridional current in the eastern half of the transect (red dashed box in Fig. 3a) was mainly northward. The zonal velocities along the VADCP transect were mainly westward (Fig. 3b). The horizontally averaged meridional velocities on the western and eastern sides of the AE were symmetrical (Fig. 3d). The meridional velocities had the maximum magnitudes of ~0.2 m/s near the surface and generally were on the order of 0.05 m/s in the upper 1 000 m. This velocity structure was consistent with that of the composite eddies derived from the Argo data in the SETIO (Yang et al., 2015).
As shown by the black box in Fig. 3a, a feature with strong meridional velocities was located at 150–350 m. The meridional velocity directions were southward in the western part and northward in the eastern part. In contrast to the meridional velocities, no significant zonal velocity variation could be detected, as shown by the black dashed box in Fig. 3b. The locations of the maximum southward and northward meridional velocities are marked in Fig. 3a with blue and red vertical lines at 98.4°E and 99.4°E, respectively. We calculated the meridional velocity anomaly profiles at the two locations by removing the horizontally averaged meridional velocities in the eastern part of the AE, as shown in Fig. 3e. The meridional velocity anomaly profiles with the maximum magnitudes of ~0.5 m/s at depth of 240 m were symmetric. In the southern hemisphere, these VADCP-observed meridional and zonal velocity variations indicated a subsurface anticyclonic circulation (SAC) feature. The SAC-induced zonal velocity was insignificant (Fig. 3b), implying that the VADCP transect crossed near the center of the SAC feature and the maximum swirl velocity of the SAC was ~0.5 m/s.
The temperature and salinity profiles measured at CTD stations are shown in Figs 4a and b. The isothermals below the SAC were slightly depressed, whereas the isothermals above the SAC were slightly elevated. The salinity observations showed that the SAC feature had a low-salinity core (<34.55, at ~26 σ0 (26 σ0 express σ0=26 kg/m3)). Even though the CTD transect covered only the eastern part of the SAC feature, the existence of a lens-like structure with nearly homogenous salinity was obvious within the SAC feature. As the SAC feature was distributed below the thermocline (defined by 18℃ isothermal), the temperature, salinity, and velocity distribution characteristics indicated that the SAC was an STE with a lens-like structure. The temperature and salinity anomalies against σ0 are shown in Figs 4c and d, and significant temperature and salinity anomalies are located at the 26.0–26.5 σ0 surfaces. The negative temperature and salinity anomalies suggested the STE carried cold and fresh water.
As the VADCP transect crossed near the STE center, we estimated the horizontal scale from its meridional velocity variations. The speed radius of the STE, defined as the range from the eddy center to the maximum axial velocity (Chelton et al., 2011), was about 55 km, which was smaller than the first baroclinic deformation radius in the observation area (about 120 km, Chelton et al., 1998). The Rossby number
The most prominent feature of the STE observed on the cruise was the low-salinity core. With the assumption that STEs migrated along the isopycnal surfaces and the properties of water (such as salinity, potential temperature, and dissolved oxygen) carried by STEs were conserved during the migration (Zhang et al., 2015, 2017; Yang et al., 2019b), it was possible to trace the STE core water back to its source by finding an area that had the same water properties with the STE core. The STE core was at ~26 σ0 (~230 m at station s-1), and the distribution of annual mean salinity of the BOA_ARGO in the Indian Ocean on the 26 σ0 surface is shown in Fig. 5a. A westward spreading phenomenon of the low-salinity ITF water is marked by a ‘‘tongue’’ configuration centered at ~10°S in the isohalines. The 34.55 isohaline, which is located near the eastern coastal waters, was discontinuous because the annual mean Argo data were unavailable near the coast. Figure 5b shows the annual mean salinity distribution of the BOA_ARGO along the transect of 10°S. The salinity in the SETIO increased with depth and the low-salinity water (<34.55) on the density surfaces of the STE was located off the east of 115°E. We further illustrated the potential temperature and salinity (T-S) distribution of the CTD observations and the annual mean Argo data in Fig. 5c. The annual mean salinity of the water in the green dashed box in Fig. 5a was higher than 34.55 between 25.1 σ0 and 26.8 σ0 surfaces, whereas the water in the red dashed box in Fig. 5a was fresher. The STE core (station s-1, red dotted line) was characterized by the low salinity between 25.5 σ0 and 26.5 σ0 surfaces, which had distinct characteristics from the surrounding areas (blue dotted line and green shading), but was similar to that of the eastern coastal waters shown in the red dashed box in Fig. 5a (red shading). These results suggested that the only source of the low-salinity water in the SETIO was the ITF and the STE should carry ITF water.
Zhang et al. (2017) detected global STEs using Argo profiles by finding low potential vorticity (PV) inside a lens structure outlined by isopycnals. As shown in Fig. 4, however, the STE was located just below the thermocline and a surface AE, which tended to flatten the upper isopycnal of the lens structure and made it difficult to identify the lens structure. In addition, the STEs propagated westward in the SETIO because of the self-translation (Bashmachnikov et al., 2015) and the background current (Zhang et al., 2015). The self-translation velocity, estimated with
We then calculated, for each Argo float measuring successive profiles, the potential density as a function of the depth and the time. We identified the STEs carrying ITF water by finding low-salinity cores (<34.55) inside a lens, that is, the isopycnals above the “low-salinity core” were domed upward, whereas those below this core were depressed (as detailed in Zhang et al. (2017)). Only one STE was well captured by five successive profiles of the same Argo float (WMO ID 5900860). As shown in Fig. 7, the captured Argo-observed STE was located off the northwest edge of a surface eddy.
The temperature and salinity distributions of the Argo-observed STE are shown in Fig. 8. The STE was located basically between 100 m and 300 m. The temperature and salinity anomalies against σ0 showed a Gaussian-like distribution and had a peak value at the 26.0 σ0 surface, which was similar to that of the cruise-observed STE. The T-S distributions of the cruise- and Argo-observed STEs were close between 25.5 σ0 and 26.5 σ0 surfaces (Fig. 6b).
The generation mechanism of STEs has been broadly discussed (D’Asaro, 1988; Spall, 1995; Spall and Price, 1998; Zhurbas et al., 2004; Kurian et al., 2011; Pelland et al., 2013). The two most famous STE generation mechanisms are the front-induced water parcel subduction and topography-induced current instability. As shown in Fig. 5, the area occupied by the low-salinity ITF water decreased with depth. The isohaline of 34.55 extended westward to the west of 80°E at the sea surface, whereas it was limited to the coastal areas at the isopycnal surfaces where we had observed the STEs. Therefore, even though the observed STEs were characterized by low-salinity water, which should have come only from the ITF water in the SETIO, the observed STEs may have had different origins and generation mechanisms. The STEs may originate from the coastal area near the Ombai Strait, the Lombok Strait, and the Timor Passage because of the interaction of the current and abrupt topography (termed as coastal origin). Previous theoretical modeling studies have suggested that the coastal area around the Timor passage was favorable for STE generation (Nof et al., 2002). In addition, the observed STEs also may originate from the sea surface in open ocean as well because of the front-induced subduction (termed as local origin because the two observed STEs are all observed in the open ocean). As suggested by Sarkar et al. (2016), the surface-layer low-salinity water from river input and precipitation in the northern Bay of Bengal would generate a salinity front. The frontogenesis in the mixed layer produced low PV core, which then would be subducted by the ageostrophic circulation and advected beneath the main thermocline. The STEs finally formed through balanced adjustment. In addition, the surface eddies were abundant in the SETIO (Yang et al., 2015) and submesoscale instabilities generated in the mixed layer front of the surface eddies could lead to the subduction of low PV fluid (Brannigan et al., 2017). Thus, we speculated that the front generated by the surface eddies also played a role in the STE formation.
To discover more clues on the origin and generation of the STEs, we revisited the distribution of LSPs. Because the ITF water gradually diffused and mixed with surrounding high-salinity water as it was advected westward, the LSPs, except for those detected near the eastern coastal waters, should have revealed the existence of a low-salinity ITF water parcel. The surface and subsurface eddies could generate closed PV contours on an isopycnal surface and the water parcels trapped inside the closed PV contour would be protected from mixing with the surrounding water (Zhang et al., 2014). Therefore, the LSPs detected away from the eastern coastal waters should be inside either a surface eddy or a subsurface eddy.
In this study, we believed that an LSP would be located within and carried by a surface eddy when the distance between the LSP and the nearest eddy center was smaller than the eddy radius
Figure 9e shows the spatial distribution of the LSPs located inside and outside surface eddies. The LSPs were distributed in the area covering 4°–16°S, and 80°–120°E. The distribution of both LSPs located inside and outside surface eddies extended from southeast to northwest and was consistent with the distribution of the EKE.
Figure 10 shows the zonal distribution of three types of LSPs. Because eddies in the SETIO propagated westward, we examined the zonal distribution of LSPs from east to west. In the area east of 117°E, most of the LSPs were outside the surface eddies because the ITF just flowed into the Indian Ocean and the EKE was weak. From 115°E to 113°E, the ratio of the LSPs outside the surface eddies to that inside the surface eddies exhibited an inverse relationship with the EKE (Fig. 10b). More LSPs were located inside the surface eddies when the EKE became stronger. About two-thirds of the LSPs, however, were still located outside surface eddies. Even if an LSP was defined as being inside a surface eddy when the distance between the LSP and the nearest eddy center was smaller than
From 113°E to the west, the EKE began to decrease whereas the ratio of the LSPs outside the surface eddies to that inside surface eddies increased (Fig. 10b). This suggested that the STE may have a longer lifespan or that new STEs were generated in the open ocean (local origin). We therefore tried to find clues regarding to the possibility and mechanism of the local origin of the STE.
The CTD observations near the sea surface are shown in Fig. 11. The water in the upper 50 m was mixed well in the STE area (stations s-1 and s-2), whereas only the water in the upper 30 m was mixed well outside the STE area (stations s-3, s-4, and s-5).
The mixed layer depth is controlled by many air-sea dynamic processes. The diurnal restratification (Brainerd and Gregg, 1993), sea surface wind (Nagai et al., 2012; Yang et al., 2019c), and waves (Qiao et al., 2004, 2016; Babanin, 2006; Huang and Qiao, 2010; Huang et al., 2012) are the most well-known processes affecting the mixed layer depth. Because the CTD observation at stations s-2, s-3, and s-4 were conducted during the daytime, whereas those at stations s-1 and s-5 were conducted during the nighttime, the difference in mixed layer depth at those five stations should not be associated with diurnal restratification. The sea surface wind speed (~5 m/s) and the significant wave height (~1.9 m) during the observation at the five stations were small and had no significant change. The enhanced near-surface mixing in s-1 and s-2 also should not be associated with sea surface winds or waves.
A more plausible explanation would seem to be that the mixed layer depth was associated mainly with the surface eddies, because the observation transect extended from the surface CE toward the adjacent surface AE, and the surface AE (CE) can deepen (shallow) the mixed layer depth (Yang et al., 2017). The mixed layer depth may be even deeper at the surface AE. A deeper mixed layer, along with the front generated by surface eddies, is favored for the submesoscale instabilities and subduction of surface water parcels (Brannigan et al., 2017). In addition, because the outcropping isopycnals of surface AEs extended deeper into the thermocline, modeling result suggest that the subduction of low PV fluid in the surface AEs is much stronger than that in surface CEs (Brannigan et al., 2017). This theory has been used to explain the finding of STEs in surface AEs (Baird and Ridgway, 2012; Brannigan et al., 2017). In this study, the outermost enclosed SLA isolines were used to show the edge of the surface eddies (Figs 2 and 7), and both of the observed STEs were located near a surface AE. However, the symmetric and rotating background current field (Fig. 3) suggested that the cruise-observed STE may have been located within the surface AE. As outlined by the 34.55 isoline shown in Fig. 4b, the core of cruise-observed STE was ventilated with surface low-salinity water. These clues may suggest that the STEs can be generated in the open ocean because of surface water subduction generated in the mixed layer front of surface AEs and may be located within a surface AE.
Because the possible generation of STE in surface AEs suggested that the subsurface ITF water transport may be associated with both surface eddies and STEs, it was difficult to evaluate the respective contributions of surface eddies and the STEs to ITF water transport with the limited observations in this study. Further studies with intensive observation (such as rapid-sampling Argo floats and mooring) and fine resolution modeling (submesocale permitting) are needed to better evaluate the role of the STEs.
In this study, we observed an STE carrying fresh ITF water with concurrent CTD and VADCP measurements obtained during a cruise conducted in the SETIO in December 2017. The speed radius, mean Rossby number, and Burger number of the cruise-observed STE were 55 km, −0.7, and 2.4, respectively, which indicated that the STE was a submesoscale eddy. By examining Argo float data, we further identified another STE that carried ITF water and was well captured by five successive profiles of the same Argo float. Both of the STEs had a vertical scale of about 200 m and were characterized by a low-salinity core (<34.55) within a lens feature.
The LSPs were defined as the Argo profiles detecting low-salinity ITF water at the 26.0σ0 isopycnal surface. Because the low-salinity ITF water parcels should be carried only by surface eddies and the STEs in the area away from eastern coastal waters, the distribution of LSPs could be used to analyze the distribution, origin, and generation mechanism of the STEs carrying ITF water. The results suggested that the STEs may be widely distributed in the SETIO and may play a role in ITF water transport. Furthermore, topography-current interaction at the eastern coastal waters and front-induced subduction in the area away from coastal waters were possible STE generation mechanisms.
This study shed light on the structure, generation mechanism, and distribution of the STEs carrying ITF water. Because of limited data, however, further in situ observation and modeling studies are needed to present better knowledge of the detailed dynamics of STEs.
Acknowledgements: We thank all onboard crews of R/V Xiangyanghong 18 and the two anonymous reviewers. The cruise data can be obtained at this website (|
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Figures(11)
Supported by:
Beijing Renhe Information Technology Co. Ltd
Guangbing Yang, Quanan Zheng, Xuejun Xiong. Subthermocline eddies carrying the Indonesian Throughflow water observed in the southeastern tropical Indian Ocean[J]. Acta Oceanologica Sinica, 2023, 42(5): 1-13. doi: 10.1007/s13131-022-2085-2
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