Changes in the Subantarctic Mode Water Properties and Spiciness in the Southern Indian Ocean based on Argo Observations

AbstractThe Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the Southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004-2018. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking at σθ=26.7-26.8 kg m−3in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004-2018. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy loss. An increase in the mixed-layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre.

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Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

Journal of Climate ◽

10.1175/jcli-d-20-0482.1 ◽

2021 ◽

Vol 34 (10) ◽

pp. 3927-3953

Author(s):

Motoki Nagura

Keyword(s):

Indian Ocean ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Surface Mixed Layer ◽

The South ◽

South Indian Ocean ◽

Mode Water ◽

Horizontal Advection ◽

South Indian ◽

Subantarctic Mode Water

AbstractThis study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

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Subduction over the Southern Indian Ocean in a High-Resolution Atmosphere–Ocean Coupled Model

Journal of Climate ◽

10.1175/2011jcli3888.1 ◽

2011 ◽

Vol 24 (15) ◽

pp. 3830-3849 ◽

Cited By ~ 17

Author(s):

Mei-Man Lee ◽

A. J. George Nurser ◽

I. Stevens ◽

Jean-Baptiste Sallée

Keyword(s):

Indian Ocean ◽

Mixed Layer ◽

Current System ◽

Mixed Layer Depth ◽

Coupled Model ◽

Horizontal Resolution ◽

Layer Depth ◽

Mode Water ◽

Coarse Resolution ◽

Winter Mixed Layer

Abstract This study examines the subduction of the Subantarctic Mode Water in the Indian Ocean in an ocean–atmosphere coupled model in which the ocean component is eddy permitting. The purpose is to assess how sensitive the simulated mode water is to the horizontal resolution in the ocean by comparing with a coarse-resolution ocean coupled model. Subduction of water mass is principally set by the depth of the winter mixed layer. It is found that the path of the Agulhas Current system in the model with an eddy-permitting ocean is different from that with a coarse-resolution ocean. This results in a greater surface heat loss over the Agulhas Return Current and a deeper winter mixed layer downstream in the eddy-permitting ocean coupled model. The winter mixed layer depth in the eddy-permitting ocean compares well to the observations, whereas the winter mixed layer depth in the coarse-resolution ocean coupled model is too shallow and has the wrong spatial structure. To quantify the impacts of different winter mixed depths on the subduction, a way to diagnose local subduction is proposed that includes eddy subduction. It shows that the subduction in the eddy-permitting model is closer to the observations in terms of the magnitudes and the locations. Eddies in the eddy-permitting ocean are found to 1) increase stratification and thus oppose the densification by northward Ekman flow and 2) increase subduction locally. These effects of eddies are not well reproduced by the eddy parameterization in the coarse-resolution ocean coupled model.

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Changes in Subantartic Mode Water Properties and its Impact on Spiciness Variation in the Southern Indian Ocean

10.5194/egusphere-egu2020-21228 ◽

2020 ◽

Author(s):

Ying Zhang ◽

Yan Du ◽

Ming Feng

Keyword(s):

Indian Ocean ◽

Potential Temperature ◽

Vital Role ◽

Sea Surface ◽

Southern Indian Ocean ◽

Wind Stress Curl ◽

Mode Water ◽

Subantarctic Mode Water ◽

The Indian Ocean ◽

Global Oceans

Subantarctic Mode Water (SAMW) is formed by deep mixing in winter in the Subantarctic Zone and transported into the adjacent subtropical gyres after subduction, which plays a vital role in heat, freshwater, carbon and nutrient budgets in the global oceans. The changes in SAMW properties and its impact on spiciness variation in the southern Indian Ocean have been investigated using the gridded Argo dataset in 2004-2018. Annual mean potential temperature and salinity of the SAMW have undergone significant variations during 2004-2018, with an increase (a decrease) trend for potential temperature (salinity). An analysis of decomposition shows that the heaving process contributes to warming and salinification while spiciness causes cooling and freshening, both of which modulate the SAMW properties. A strong deepening of the isopycnal surfaces caused by positive wind stress curl anomalies over the subtropical southern Indian Ocean leads to warming/salinification heaving contribution to the changes in SAMW. The cooling/freshening contribution from spiciness process is due to a southward shift of sea surface potential density favoring colder and fresher water into the interior ocean, which is driven by an increase in wintertime sea surface temperature and salinity&#160;in the SAMW formation region. The colder and fresher water carried with the SAMW spreads along isopycnal surfaces via the Indian Ocean subtropical gyre, which results in cooling and freshening spiciness trends over the all basin of the subtropical southern Indian Ocean.&#160;

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A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation*

Journal of Atmospheric and Oceanic Technology ◽

10.1175/2009jtecho543.1 ◽

2009 ◽

Vol 26 (9) ◽

pp. 1920-1939 ◽

Cited By ~ 147

Author(s):

James Holte ◽

Lynne Talley

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Gradient Methods ◽

Temperature Threshold ◽

Intermediate Water ◽

Mode Water ◽

Southwest Atlantic ◽

Argo Data ◽

Southeastern Pacific ◽

Subantarctic Mode Water

Abstract A new hybrid method for finding the mixed layer depth (MLD) of individual ocean profiles models the general shape of each profile, searches for physical features in the profile, and calculates threshold and gradient MLDs to assemble a suite of possible MLD values. It then analyzes the patterns in the suite to select a final MLD estimate. The new algorithm is provided in online supplemental materials. Developed using profiles from all oceans, the algorithm is compared to threshold methods that use the C. de Boyer Montégut et al. criteria and to gradient methods using 13 601 Argo profiles from the southeast Pacific and southwest Atlantic Oceans. In general, the threshold methods find deeper MLDs than the new algorithm and the gradient methods produce more anomalous MLDs than the new algorithm. When constrained to using only temperature profiles, the algorithm offers a clear improvement over the temperature threshold and gradient methods; the new temperature algorithm MLDs more closely approximate the density algorithm MLDs than the temperature threshold and gradient MLDs. The algorithm is applied to profiles from a formation region of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The density algorithm finds that the deepest MLDs in this region routinely reach 500 dbar and occur north of the A. H. Orsi et al. mean Subantarctic Front in the southeastern Pacific Ocean. The deepest MLDs typically occur in August and September and are congruent with the subsurface salinity minimum, a signature of AAIW.

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Indian Ocean Subtropical Mode Water: its water characteristics and spatial distribution

Ocean Science ◽

10.5194/os-6-41-2010 ◽

2010 ◽

Vol 6 (1) ◽

pp. 41-50 ◽

Cited By ~ 15

Author(s):

T. Tsubouchi ◽

T. Suga ◽

K. Hanawa

Keyword(s):

Spatial Distribution ◽

Indian Ocean ◽

Mixed Layer Depth ◽

Careful Examination ◽

Distribution Area ◽

Mode Water ◽

Formation Region ◽

Subtropical Mode Water ◽

Water Characteristics ◽

South Indian

Abstract. We have improved a basic description (water characteristics and spatial distribution) of the Indian Ocean Subtropical Mode Water (IOSTMW) using an isopycnally averaged three-dimensional hydrographic dataset. Two mode waters and corresponding wintertime mixed layer depth maxima were observed north of the subtropical front (STF) in the South Indian Ocean: IOSTMW (within 25.8–26.2 σθ) in the region of 28–45° E and another subtropical mode water in the subtropical gyre (within 26.4–26.7 σθ) in the 60–80° E longitudinal band. Through careful examination of the spatial distribution and water characteristics of a core in the layer of minimum vertical temperature gradient (LMVTG), we identified that a mass of LMVTG corresponds to IOSTMW. The average water characteristics of the IOSTMW during approximately 1960–2004 were 16.54 ± 0.49 °C, 35.51 ± 0.04 psu and 26.0 ± 0.1 σθ. The IOSTMW distribution area was estimated to be 25–50° E, 27–38° S. The formation region and approximate water characteristics of the second subtropical mode water were also estimated. Its probable formation region was 37–42° S, 60–80° E and north of the STF, with approximate water characteristics of 12.84 ± 0.57 °C, 35.17 ± 0.11 psu and 26.57 ± 0.04 σθ.

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Origin and mechanism of Subantarctic Mode Water formation and transformation in the Southern Indian Ocean

Ocean Dynamics ◽

10.1007/s10236-010-0276-4 ◽

2010 ◽

Vol 60 (3) ◽

pp. 563-583 ◽

Cited By ~ 23

Author(s):

Ariane Koch-Larrouy ◽

Rosemary Morrow ◽

Thierry Penduff ◽

Mélanie Juza

Keyword(s):

Indian Ocean ◽

Southern Indian Ocean ◽

Mode Water ◽

Subantarctic Mode Water ◽

Water Formation

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The Origin and Fate of Subantarctic Mode Water in the Southern Ocean

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0174.1 ◽

2021 ◽

Author(s):

Zhi Li ◽

Matthew H. England ◽

Sjoerd Groeskamp ◽

Ivana Cerovečki ◽

Yiyong Luo

Keyword(s):

Mixed Layer ◽

Formation Rate ◽

Ekman Transport ◽

The South ◽

Regional Variability ◽

Mode Water ◽

South Indian ◽

Subantarctic Mode Water ◽

Circumpolar Current ◽

Mixed Layers

AbstractSubantarctic Mode Water (SAMW) forms in deep mixed layers just north of the Antarctic Circumpolar Current in winter, playing a fundamental role in the ocean uptake of heat and carbon. Using a gridded Argo product and the ERA-Interim reanalysis for years 2004-2018, the seasonal evolution of the SAMW volume is analyzed using both a kinematic estimate of the subduction rate and a thermodynamic estimate of the air-sea formation rate. The seasonal SAMW volume changes are separately estimated within the monthly mixed layer and in the interior below it. We find that the variability of SAMW volume is dominated by changes in SAMW volume in the mixed layer. The seasonal variability of SAMW volume in the mixed layer is governed by formation due to air-sea buoyancy fluxes (45%, lasting from July to August), entrainment (35%), and northward Ekman transport across the Subantarctic Front (10%). The interior SAMW formation is entirely controlled by exchanges between the mixed layer and the interior (i.e. instantaneous subduction), which occurs mainly during August-October. The annual mean subduction estimate from a Lagrangian approach shows strong regional variability with hotspots of large SAMW subduction. The SAMW subduction hotspots are consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Hotspots in the south Indian Ocean produce strong subduction of 8 and 9 Sv for the light and southeast Indian SAMW, respectively, while SAMW subduction of 6 and 4 Sv occurs for the central and southeast Pacific SAMW, respectively.

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