Covariability of Subantarctic Mode Water and the Southern Branch of the Subtropical Indian Ocean Countercurrent in Argo Observations

LU Yiqun, LIU Qinyu, *, and XIE Shang-Ping

Covariability of Subantarctic Mode Water and the Southern Branch of the Subtropical Indian Ocean Countercurrent in Argo Observations

LU Yiqun1), LIU Qinyu1), *, and XIE Shang-Ping2)

1) Physical Oceanography Laboratory, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, Qingdao 266100, China 2) Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093, USA

The Subantarctic Mode Water (SAMW) forms in the deep mixed layer north of the Antarctic Circumpolar Current and spreads northward into the subtropical gyre. The subtropical South Indian Countercurrent (SICC) flows eastward on the north flank of the thick SAMW layer within 22˚–32˚S from south of Madagascar at around 25˚S, 50˚E toward western Australia. The dynamical relation of the SAMW and the southern branch of the SICC (30˚–32˚S) is investigated in this work based on the monthly mean Argo data from 2004 to 2019. The physical properties of the SAMW and its pathway from the formation region are described. Most of the SAMW in the Indian Ocean sector originates from the deep mixed layers of the southeastern Indian Ocean (about 40˚S, 85˚–105˚E) and moves along the subtropical gyre. It takes around ten years to arrive east of Madagascar Island preserving its low potential vorticity characteristics. As a thick layer with homogeneous vertical properties, the SAMW forces the upper pycnocline to shoal, and the associated eastward shear results in the surface-intensified SICC. The SAMW forms a tongue-shaped thickness pattern, which influences the southern branch of the SICC above the northern flank of the thickest SAMW layer between 24˚S and 32˚S. The seasonal, interannual, and decade variations of the southern branch of the SICC are closely related to the meridional gradient of the underlying SAMW thickness. The SAMW thickened and strengthened from 2005 to 2015, thereby anchoring a strengthened SICC. The interannual covariability of the SAMW and SICC further supports the SAMW’s role in driving SICC variability.

subantarctic mode water; South Indian Ocean Countercurrent; Argo observations; long-term change; interannual variability

1 Introduction

The Subantarctic Mode Water (SAMW) is a thick layer of water located in the thermocline with nearly vertically homogeneous temperature and density properties; it covers a basin-wide area and has a relatively large amount vol-ume compared to other water types (McCartney, 1977; Ha- nawa and Talley, 2001; Sallée., 2006). The SAMW serves as the upper branch of the Southern Ocean meridional overturning circulation (McCartney, 1982; Sloyan and Rintoul, 2001; Talley, 2013) and is key for the global ocean uptake of heat, nutrients, freshwater, and carbon (Sa- bine, 2004; Marshall and Speer, 2012; Sallée, 2012).

Strong vertical convection due to ocean surface cooling in the austral winter and northward Ekman transport of cold water by the westerlies above the Antarctic Circumpolar Current results in the formation of deep mixed layers just north of the Subantarctic Front (SAF) (Dong., 2008) and provides the water source of the SAMW (Mc- Cartney, 1977; Holte and Talley, 2009; Sallée., 2010; Cerovečki., 2013). During winter thermocline ventilation, the SAMW is subducted into the thermocline through dynamic and thermodynamic processes, including current advection, buoyancy forcing, Ekman pumping, and eddy mixing (McCartney, 1977; Liu and Huang, 2012; Holte., 2012; Cerovečki., 2013; Xu., 2016). After subduction, the SAMW spreads northward along the geo- strophic circulation in the subtropical gyre (Jones., 2016). The SAMW is identified as low potential vorticity (PV) water in the density range of 26.50–27.10kgm−3inall three ocean sectors of the Southern Ocean (Hanawa and Talley, 2001; Cerovečki., 2013). The local maximal thickness of the SAMW appears in the South Indian Ocean(Hanawa andTalley,2001).TheSouthIndianSAMWtakes nine years to leave the Southern Ocean domain and enter the subtropical thermocline (Jones., 2016). On the basis of the Argo observations from 2005 to 2015, Gao. (2018) found that the SAMW south of 30˚S thickened, deepened, and warmed primarily because of the changes in the enhanced downward wind stress curl over the SA- MW formation region that caused an increase in the upper ocean heat storage.

As a large volume of low PV water within the thermocline, mode water could influence the density structure of the upper thermocline and force it to shoal northward, and it plays a key role in the formation and maintenance of the subtropical countercurrent (STCC) and subtropical front in the North Pacific Ocean, as comprehensively documented by theoretical (Kubokawa, 1997, 1999; Xie., 2011), model (Kubokawa and Inui, 1999; Yamanaka., 2008; Xu., 2012), and observational (Aoki, 2002,2007; Kobashi and Kubokawa, 2012; Menezes., 2014) studies. As reviewed by Kobashi and Kubokawa (2012), the Subtropical Mode Water (STMW) in the North Pacific is formed in the intersections of outcrops and the mixed layer depth (MLD) front and produces a thick layer of low PV water, which pushes the upper pycnocline to rise and increases baroclinic instability. The associated shear forms a northeastward surface current to the north of the slope above the southwestward current. The northeastward mean and anomaly STCC both advection heat and increase sea surface temperature and local precipitation (Xie., 2011). In North Pacific, the decade-long and long-term covariation between the STMW and STCC is obvious, but such relation is not clear in the interannual timescale (Xie., 2011; Xu., 2012).

Although the study on the North Pacific STCC has undergone significant development, only recently has serious attention been directed toward the STCC in the South Indian Ocean (SICC). The SICC was firstly described by Siedler. (2006) based on altimeter data as an eastward near-surface current through the Indian Ocean within 22˚–26˚S. They found that the SICC is stable between Madagascar and 80˚E and is less intensified between 90˚Eand 100˚E. Palastanga. (2007) also explored the SICCin ocean observations and found that it extends to the northeast above the westward flowing South Equatorial Current. Relative to the STCC in the North Pacific Ocean, the SICC features three branches, namely, the northern, central, and southern branches, as described by Menezes. (2014) based on Argo-based datasets and satellite data. According to the authors, the southern SICC around 26˚S is associated with an SAMW-induced thermal front at a depth of 100–200m, while the northern and central SICC branches are related to the meridional PV staircases in the South Indian Ocean. Note that the STMW in the South Indian Ocean is too far west (25˚–50˚E, 30˚–39˚S) to exert an influence on the SICC. Menezes(2016) found that the interannual variation of the SICC is dominated in the quasi-biennial timescale. Qu. (2020) found a quasi-biennial variability in the SAMW’s subduction rate, which indicates a possible relation between the interannual variations of the SAMW and SICC. Although the patterns of the SAMW and SICC have been established, the covariation between them remains unknown. In addition, studies on the way in which the SAMW originating from the southeast of the South Indian Ocean affects the SICC in the subtropical region are generally lacking.

Therefore, several questions arise: Does a similar covariation exist between the SAMW and SICC in the South Indian Ocean and the STMW and STCC in the North Pacific Ocean? What is the pathway through which the SAMW reaches the SICC region?

To answer these scientific questions, we refer to the Argo data collected from the Indian Ocean from 2004 to 2019 and propose a mechanistic explanation for the seasonal and interannual variations of the eastward-flowing STCC in the South Indian Ocean. The rest of the paper is organized as follows. Section 2 introduces the data, methods, and the theory of mode water influencing the SICC. Section 3 presents the climatological physical properties of the SAMW and its pathway after formation to establish the pattern of how the SAMW extends to the SICC. Section 4 shows the structure of the SICC and the associated SAMW underneath, as well as their covariability in different timescales. Section 5 provides the conclusions and discussions.

2 Data and Methods

2.1 Datasets

Argo floats have continuously been sampled in global upper oceans since 2000, and such application has allowed the study of the interannual variability of the SAMW and SICC (Aoki., 2002). Each Argo float descends to a preprogrammed parking depth (typically 1000m), drifts freely, and then ascends to the sea surface at a predeter- mined interval (10d) after descending to a depth of 2000m. The gridded monthly potential temperature and salinity data used in this study are obtained from Roemmich-Gil- son mapped Argo product for January 2005 to April 2019 (Roemmich and Gilson, 2009). We also use 10m winds from the ECMWF ERA-Interim monthly mean data to cal- culate the Sverdrup streamfunction, which indicates the pathway of wind-driven circulation. The Roemmich-Gil- sonArgoproductsandtheECMWFERA-Interimmonthly mean data can be downloaded for free from http://sio-argo.ucsd.edu/RG_Climatology.html and https://doi.org/10.5065/D68050NT, respectively.

2.2 Definitions and Calculations

Potential density is calculated from potential temperature and salinity according to Fofonoff and Millard Jr. (1983).Potential temperature and potential density are rel- ative to the ocean surface. MLD is defined as the depth where the potential density is denser than the sea surface by 0.03σ, as reported by Sallée. (2006). In the Roem- mich-Gilson Argo products, the near-surface level is 5m. The geostrophic current velocity is relative to 1975m under the assumption that the ocean is motionless.

According to the thermal wind relationship, the vertical shear of the geostrophic current is related to the horizontal density gradient. That is,

where0is the reference depth. Herein, we set0=1975mwhich is the lowest depth of the Argo data and assume that the velocity (0and0) is zero. We can calculate the geo- strophic velocity above0by integrating the horizontal gradient of the potential density at each depth.

To track the pathway of the SAMW particles after subduction, we use a simple Lagrangian method in calculating the water particles’ depth and position. Due to the conservative properties of mode water, we assume that particles move on the isopycnal surfaces, where they are subducted at the base of the winter mixed layers. In this study, we only show the main process because of the lack of mesoscale and smaller-scale observation data. The isopycnal depth is equal to particle depth. After one time step ∆=1 month from position (0,0), the new position can be determined as (1,1)=(0+0∆,0+0∆). When the new position is not located at the grid point, we use distance-weighted interpolations for the nearest four pointsto obtain the new velocity and depth at (1,1). This meth- od generates a rough trajectory of SAMW particles. The result is consistent with that of Jones. (2016), who investigated the spatial structure and timescales of the SAMW export by using an eddy-permitting southern ocean model, which indicates that the South Indian Ocean SA- MW takes nearly nine years to leave the subantarctic zone. Once a particle hits the ocean boundary or is entrained back to the mixed layer, the trajectory ends.

2.3 Covariation Mechanism Between SICC and SAMW Thickness

According to the theory relating mode waters and STCCs by Kobashi. (2006) and Xu. (2012), the thickness of SAMW layersis defined as

whereis the isopycnal depth (negative under the sea surface);ρ, ρare the potential density of the upper and lower isopycnal boundary of the SAMW, respectively; and0(ρ) is the depth of the base of the SAMW. In this study, we setρ=26.50 andρ=26.80. After taking the meridional partial derivative on a constantsurface, we rewrite Eq. (3) as

3 Feature and Movement Pathway of SAMW

The SAMW is a thick layer of vertically homogeneous properties. Its formation is related to the wintertime ther- mocline ventilation and subduction of low PV water from a deep mixed layer. The deepest mixed layer is found just north of the SAF in September after the sea surface cools throughout the austral winter (Dong., 2008). Fig.1a shows the distribution of the September MLD and SAMW thickness based on the monthly Argo data for 2004–2019. The thickest SAMW (nearly 700m) is found in and north of the deepest mixed layer (deeper than 400m) in the southeast Indian Ocean around 60˚–160˚E. The low PV water in the mixed layer is sheltered from the surface as the seasonal thermocline develops after spring and is trans- ported northwestward by the background circulation. The SAMW thicker than 100m occupies almost the whole South Indian Ocean from east of Madagascar Island to west of Australia and south of 20˚S where the SICC is located; the SAMW thickness then decreases northwestward. The majority of the South Indian SAMW is between 26.70–26.85σisopycnals, the salinity ranges from 34.40to 35.20, and the temperature ranges from 7.0℃ to 13.0℃ (Figs.1b, 1c).

Fig.2 shows a 10-year trajectory of the SAMW initiated from its formation zone from September 2005, along with the corresponding depth and PV. This figure illustrates how the SAMW spreads in the subtropical Southern Indian Ocean. The water particles formed as mode water were released from the intersection of the ventilated isopycnal surface and the bottom of the deep mixed layer in September 2005. The trajectories on the 26.60σ, 26.70σ, and 26.8σisopycnal surfaces are shown in Fig.2. Only one-third of the total particles are drawn in Fig.2 to clearly present the trajectories. The broad structure of the trajectories is influenced by the mean circulation of the South Indian Ocean subtropical gyre. The SAMW initiated between 85˚E and 115˚E moved northwestward after subduction. It then took around ten years to move along the subtropical circulation (contours in Fig.3a) to the east of Madagascar Island (Fig.2a). It went down approximately to deeper than 500m after six years at 80˚E and obducted to the sea surface at the east coast of Madagascar Island (Fig.2b). The SAMW that subducted from 116˚E to 140˚E moved relatively slow and reachedthe sea surface at the South Australian coast after ten years. Only two particles ran out of the Great Australian Bight and ended near 105˚E, 40˚–42˚S. The SAMW initiated west of 80˚E returned to the mixed layer within 1–2 years. The SAMW that subducted on isopycnal 26.70σin the central Indian Ocean took ten years to escape the Antarctic zone and move along the subtropical gyre to 20˚S. The same plots on isopycnal 26.80σalso show that the denser SAMW was easily trapped in the Great Australian Bight but that a small proportion of it was still able to escape. In sum, the accumulation of the SAMW shaped the subtropical distribution pattern shown in Fig.1a.

Fig.1 SAMW properties derived from RG_Argo monthly data for 2004 to 2019. (a) September mean SAMW thickness (color, in meters), MLD (black contours at 200m intervals), and surface SICC (vectors), where the SAMW is defined as low PV (PV≤5.0×10−11m−1s−1) water within a density range of 26.50–26.85σθ. South New Zealand separates the South Indian SAMW and the South Pacific SAMW at about 167˚E. The thick black solid line is the SAF given by Orsi et al. (1995) and marks the southern boundary of the austral winter deep MLD. (b) SAMW’s temperature-salinity diagram. The color represents the particle’s occurrence frequency, and the black contours are presented at 0.20 σθintervals. The SAMW is characterized by a salinity of 34.20–35.35, potential temperature of 6–13℃, and potential density of 26.50–26.85σθat 60˚–160˚E, 20˚S to the south of the outcrop line (near the SAF). (c) SAMW layer thickness as a function of potential density, with a standard deviation bar on top. The purple and gray bars represent the South Indian SAMW and South Pacific SAMW, respectively.

Fig.2 Lagrangian trajectories of water particles released from the bottom mixed layer on 26.60σθ, 26.70σθ, and 26.80σθ. The colors in the upper panels denote the time after release, the middle panels denote the depth of the water particle, and the lower panels show the PVs along the trajectories. The black solid line indicates the location where the isopycnal ventilates to the bottom mixed layer during austral winter. The black dots along the black line mark the initial positions of the water particles. The trajectories along isopycnals are obtained from geostrophic velocities.

Fig.3 September mean surface geostrophic currents and related SAMW. (a) Map of climatological 5m-deep currents (vectors and colors) derived from RG_Argo monthly data and the Sverdrup streamfunction derived from ERMWF IRA-Interim 10m winds (contours at 10Sv intervals). The middle panels denote the meridional section of the geostrophic currents (the red solid and blue dashed contours represent the eastward and westward currents at 0.02ms−1 intervals, respectively; the blue solid contour represents those at 0ms−1) and the related SAMW (PV≤0.5×10−10m−1s−1 in blue shading) averaged zonally between (b) 55˚–60˚E, (c) 80˚–85˚E, and (d) 105˚–110˚E. Potential density layers are drawn in blue contours at 0.1σθintervals. The black bold line is the austral winter MLD.

Three meridional sections of low PV water from the west (Fig.3b;55.5˚–60.5˚E),central(Fig.3c;80.5˚–85.5˚E),and eastern Indian Ocean (Fig.3d; 105.5˚–110.5˚E) show that the subtropical region has locally formed and lighter low PV water; the denser low PV water advects from the south- east. In the southeastern Indian Ocean, the SAMW is mostly formed on 26.75–26.90σ(Fig.3d). This heaviest low PV water is transported to the central and western Indian Ocean along the subtropical gyre (Fig.2) and lay under the lighter low PV water that formed in the central and western Indian Ocean (Figs.3b, 3c). The SICC is located just north of the maximum meridional gradient of the SAMW thickness, thereby implying a close relationship between the SICC and the SAMW.

Although previous studies have implied that the SICC might be influenced by mode water underneath, the covariability of the SICC and SAMW has not been investigated. The next section details how the SAMW anchors the SICC and its variability.

4 Covariability Between the Southern Branch of the SICC and Local SAMW

4.1 Climatic Mean and Seasonal Variation

The surface geostrophic current is shown in Fig.3a. The SICC originates from 24˚–31˚S south of Madagascar Islandand flows eastward against the wind-driven circulation, which is denoted by the Sverdrup streamfunction. The maximum speed of 0.14ms−1is found at around 55˚E. The SICC splits into two branches at the Central Indian Ridge at around 70˚E. A robust southern branch (southern SICC) flows to the west of Australia around 32˚S with a slight southward orientation. The northern branch with a northwardslantsplitsintotwobranchesat90˚–95˚E.Thesouthern one (central SICC) flows southeastward to the northwest of Australia near 24˚S. The northern one (northernSICC) continues to flow northeastward and possibly merges with the tropical Eastern Gyral Current; its northern limb seems to recirculate into the South Equatorial Current, as described by Schott. (2009). The zonally averaged meridional sections in Figs.3b–d also show the three branches of the SICC. From west to east, one core (55.5˚–60.5˚E) is shown in Fig.3b, two (80.5˚–85.5˚E) are shown in Fig.3c, and three (105.5˚–110.5˚E) are shown in Fig.3d. The depth of the SICC is above 200m in the central and western parts and 400m downstream. The three-branch structure is consistent with that of Menezes. (2014), who used three Argo-based atlases and data from six hydrographic cruises, except that the split positions herein are different possibly because of the different time average analysis.

Fig.4 Zonal velocities (ms−1) on isopycnal surfaces that cover the SAMW layer. The left panels are the climatological mean velocity for 2004–2019, the middle panels are the April mean, and the right panels are the October mean. (a)–(c) Surface zonal velocity. (d)–(f) Zonal velocity on 25.50σθ. (g)–(f) Zonal velocity difference between 26.50σθand 26.90σθ, where the SAMW core exists. (j)–(l) Zonal velocity on 27.0σθ, where the isopycnal is just below the SAMW layer. All velocities are zonally smoothed by 9 degrees of longitude to remove the eddies’ effects. The geostrophic velocity is relative to 1975m, which is the bottom layer of the Argo data.

Fig.5 Seasonal variation of the zonal velocity averaged in the upper 105m (shadings, ms−1) and in the SAMW layer (contours, ms−1) along the three meridional sections shown in Figs.3b–d.

4.2 Covariability in 2004–2019

Fig.6 shows the long temporal variability of the southern SICC zonal velocity and the meridional gradient of the SAMW thickness∂/∂averaged in the SICC domain based on Argo observations from 2004 to 2019. The zonal velocity of the SICC south branch is averaged in 50.5˚–115.5˚E, 32.5˚–24.5˚S, 5–150m depth; the SAMW thick- ness (26.50–26.80σ) gradient ∂/∂is averaged from 33.5˚S to 25.5˚S (1 degree south of the SICC box to calculate the gradient) and 50.5˚–115.5˚E. The correlation coefficient of∂/∂and the southern SICC velocity is 0.51 (=3.61×10−13), passing the 95% confidence level in a-test fluctuation. The correlation coefficient is 0.32, passing the 95%-test significance level after the linear trend is removed. This result reveals that the SICC zonal velocity is closely related to the meridional gradient of the SAMW thickness.

Fig.6 Standardized time series of (red) SICC mean eastward velocity averaged in the upper 105m over 50.5˚–112.5˚E, 31.5˚–23.5˚S; and (black) the absolute value of the meridional gradient of the SAMW mean thickness averaged in 50.5˚–115.5˚E and 33.5˚–25.5˚S. The dashed lines are based on monthly data, and the solid lines are the 13-month running mean values.

Given a 13-month running mean, the southern SICC eastward velocity and the meridional gradient of the SA- MW thickness showed an increasing trend from 2005 to 2015, and such an increase is likely related to the thickened SAMW at the same time period (Gao., 2018). Gao. (2018) found that the SAMW south of 30˚S thickened in the period of 2005–2015. As more mode water became newly formed in the south, a greater gradient was likely in the north, where the SAMW was well-preserved in the main thermocline in the subtropical region. Both terms showed significant interannual variation after 2015. Given the lack of eddy and longer timescale activities in the Argo data, we do not investigate other factors that may also affect the SICC velocity. Thus, according to the recent observation data, we show that the SICC zonal velocity is closely related to the meridional gradient of the SAMW thickness underneath it and that they both showed an increasing trend in 2005–2015.

5 Conclusions and Discussion

This study describes the properties and variations of the South Indian SAMW and its dynamic role in changing the southern branch of the SICC on the basis of monthly Argo data from 2004 to 2019. The SAMW located at 45˚–169˚E, 20˚–60˚S, with a density of 26.70–26.85σ, is largely subducted/formed in the deepest mixed layer in the Southeast Indian Ocean. In its formation region, the shallower SAMW formed in the west of 80˚E is entrained back into the mixed layer in the next winter, while deeper water formed from 116˚E to 140˚E is easily trapped in the Great Australian Bight South of Australia. Low PV water that subducted west of 115˚E takes ten years to reach its end, that is, the east of Madagascar Island in 20˚S, along the subtropical gyre from the bottom of the mixed layer to depths beyond 550m. The SICC originating from 24˚–31˚S southeast of Madagascar Island flows eastward in the upper 200m and against the westward flow along the Sverdrup streamfunction. It splits into three branches at the Central Indian Ridge around 70˚E, while the southern SICC branches of its three branches are flanked by the SAMW on its poleward sides. The thick SAMW layer of homogeneous vertical properties within the thermocline affects the vertical density structure in the subtropical ocean and thus reshapes the upper ocean circulation because of thermal wind relation. Specifically, the thick SAMW layer causes the upper isopycnal to shoal northward, anchoring on the eastward surface current above it.

The southern SICC branch is anchored by the northern flank of the SAMW, and its velocity anomaly is highly re- lated to the meridional gradient of the SAWM thickness beneath it. The meridional gradients of the SAMW thickness and SICC showed an increasing trend from 2005 to 2015 and strong interannual variations after 2015. Gao. (2018) mentioned that the increasing wind stress curl in the SAMW formation region formed more SAMW in 2005–2015. In the present study, we reveal that the greatermeridional gradient of the SAMW thickness drives a faster eastward SICC in the interannual and long-term timescales.

Acknowledgements

This work is supported by the National Key R&D Program of China (Nos. 2018YFA0605700, 2016YFA0601800) and the National Natural Science Foundation of China (No. 41876006). The Roemmich-Gilson Gridded Monthly Argo Data are available at http://sio-argo.ucsd.edu/RG_Climatology.html. The Argo data are collected and made freely available by the International Argo Program and the national programs that contribute to it. The Argo Program is part of the Global Ocean Observing System. The ECMWFERA-Interimmonthlymeandataisfreelydownloadedfrom https://doi.org/10.5065/D68050NT.

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August 4, 2020;

July 6, 2021

© Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

. E-mail: liuqy@ouc.edu.cn

(Edited by Xie Jun)