A study of Indian Ocean Subtropical Mode Water: subduction rate and water characteristics

Abstract. We examined Indian Ocean Subtropical Mode Water (IOSTMW) and described its characteristics using an isopycnally averaged three-dimensional hydrographic dataset. Through careful examination of the spatial distribution and water characteristics of the core in the layer of minimum vertical temperature gradient, we concluded that the IOSTMW exists as a robust structure in the western part of the Indian Ocean subtropical gyre in summer. The averaged IOSTMW properties 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 27–38° S, 25–50° E.

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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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Interannual variability of Indian Ocean subtropical mode water subduction rate

Climate Dynamics ◽

10.1007/s00382-016-3322-1 ◽

2016 ◽

Vol 48 (11-12) ◽

pp. 4093-4107

Author(s):

Jie Ma ◽

Jian Lan

Keyword(s):

Indian Ocean ◽

Interannual Variability ◽

Mode Water ◽

Subtropical Mode Water ◽

Subduction Rate

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

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0254.1 ◽

2021 ◽

Author(s):

Ying ZHANG ◽

Yan DU ◽

Tangdong QU ◽

Yu HONG ◽

Catia M. DOMINGUES ◽

...

Keyword(s):

Indian Ocean ◽

Mixed Layer ◽

Layer Structure ◽

Mixed Layer Depth ◽

Southern Indian Ocean ◽

Ekman Transport ◽

Mode Water ◽

Formation Region ◽

Subantarctic Mode Water ◽

Subduction Rate

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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Interannual Variation of Annual Subduction Rate in the North Pacific Estimated from a Gridded Argo Product

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0223.1 ◽

2015 ◽

Vol 45 (9) ◽

pp. 2276-2293 ◽

Cited By ~ 12

Author(s):

Katsuya Toyama ◽

Aiko Iwasaki ◽

Toshio Suga

Keyword(s):

Pacific Decadal Oscillation ◽

North Pacific ◽

Mode Water ◽

Subtropical Mode Water ◽

The North ◽

Central Mode Water ◽

The Pacific ◽

Central Mode ◽

Subduction Rate ◽

The North Pacific

AbstractSpatiotemporal variability of the subduction rate in the North Pacific from 2005 to 2012 is examined based on the Argo observational data. The subduction rate in the subtropical North Pacific varies significantly from year to year between 25 and 50 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), and it is well correlated with the Pacific decadal oscillation. The temporal change of the subduction rate is largely determined by that of the late winter mixed layer depth through the lateral induction term. The increase (decrease) in the subduction rate in the subtropical mode water areas accompanies densification (lightening) of the mode density class of the subducted water. The subduction rate variability in the central mode water and eastern subtropical mode water regions is anticorrelated as found in the previous study using the output from an ocean GCM. The subduction rate in the central mode water density range changes dramatically, which is very large in 2005 and 2010 but almost disappears in 2009. The subduction rate variability in the western subtropical mode water regions seems to be correlated with the Pacific decadal oscillation with a lag of a few years.

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Insensitivity of subtropical mode water characteristics to meteorological fluctuations

Deep Sea Research and Oceanographic Abstracts ◽

10.1016/0011-7471(72)90069-1 ◽

1972 ◽

Vol 19 (1) ◽

pp. 1-19 ◽

Cited By ~ 8

Author(s):

Bruce A. Warren

Keyword(s):

Mode Water ◽

Subtropical Mode Water ◽

Water Characteristics

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Sensitivity of the Ventilation Process in the North Pacific to Eddy-Induced Tracer Transport*

Journal of Physical Oceanography ◽

10.1175/jpo2941.1 ◽

2006 ◽

Vol 36 (10) ◽

pp. 1895-1911 ◽

Cited By ~ 7

Author(s):

Takahiro Endoh ◽

Yanli Jia ◽

Kelvin J. Richards

Keyword(s):

Mixed Layer ◽

North Pacific ◽

Kuroshio Extension ◽

Tracer Transport ◽

Mode Water ◽

Subtropical Mode Water ◽

The North ◽

Subduction Rate ◽

The Kuroshio ◽

The North Pacific

Abstract A coarse-resolution isopycnal model coupled with a bulk mixed layer model is used to examine the effect of isopycnal thickness diffusion, which parameterizes the subgrid-scale eddy-induced tracer transport, on ventilation of the North Pacific Ocean. Three numerical experiments with thickness diffusivities of 0 m2 s−1 and around 500 and 2000 m2 s−1 are carried out. The model successfully reproduces a deep winter mixed layer in the subarctic North Pacific, leading to well-formed mode waters and the subtropical countercurrent in the experiment with thickness diffusivity around 500 m2 s−1. The annual-mean subduction rate has peaks at densities of 25.0–25.4 and 26.4 σθ. The former peak spans the densities of North Pacific Subtropical Mode Water and North Pacific Eastern Subtropical Mode Water, whereas the latter peak is centered near the density of North Pacific Central Mode Water. The annual mean obduction rate also has the former peak and a slight enhancement corresponding to the latter peak. The Kuroshio plays a crucial role in obduction of North Pacific Subtropical Mode Water by transferring it northward from the permanent pycnocline to the seasonal pycnocline around the Kuroshio Extension, the importance of which has been overlooked in previous studies. In contrast to the simple expectation that the eddy-induced tracer transport enhances the ventilation process, stronger circulation with lower thickness diffusion increases the annual-mean subduction rate by carrying the subducted water quickly away from the seasonal pycnocline into the permanent pycnocline, as well as the annual-mean obduction rate by transferring much water from the permanent pycnocline to the seasonal pycnocline. As thickness diffusivity increases, the former peaks in the subduction and obduction rates occur at lighter densities, whereas the latter peak in the subduction rate is shifted toward higher densities.

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Poleward Shift of the Kuroshio Extension Front and its Impact on the North Pacific Subtropical Mode Water in the Recent Decades

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0088.1 ◽

2020 ◽

Author(s):

Baolan Wu ◽

Xiaopei Lin ◽

Lisan Yu

Keyword(s):

North Pacific ◽

Kuroshio Extension ◽

Mixed Layer Depth ◽

Mode Water ◽

Subtropical Mode Water ◽

The North ◽

Poleward Shift ◽

Subduction Rate ◽

The Kuroshio ◽

The North Pacific

AbstractMeridional shift of the Kuroshio Extension (KE) front and changes in the formation of the North Pacific Subtropical Mode Water (STMW) during 1979-2018 are reported. The surface-to-subsurface structure of the KE front averaged over 142°E-165°E has shifted poleward at a rate of ~ 0.23±0.16° per decade. The shift was caused mainly by the poleward shift of the downstream KE front (153°E-165°E, ~ 0.41±0.29° per decade), barely by the upstream KE front (142°E-153°E). The long-term shift trend of the KE front showed two distinct behaviors before and after 2002. Before 2002, the surface KE front moved northward with a faster rate than the subsurface. After 2002, the surface KE front showed no obvious trend, but the subsurface KE front continued to move northward. The ventilation zone of the STMW, defined by the area between 16°C and 18°C isotherms or between 25 kg m-3 and 25.5 kg m-3 isopycnals, contracted and displaced northward with a shoaling of the mixed layer depth (hm) before 2002 when the KE front moved northward. The STMW subduction rate was reduced by 0.76 Sv (63%) during 1979-2018, most of which occurred before 2002. Of the three components affecting the total subduction rate, the temporal induction ( −∂hm/∂t ) was dominant accounting for 91% of the rate reduction, while the vertical pumping (−wmb) amounted to 8% and the lateral induction (−umb · ∇hm) was insignificant. The reduced temporal induction was attributed to both the contracted ventilation zone and the shallowed hm that were incurred by the poleward shift of KE front.

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