Changes to Indian Ocean Subantarctic Mode Water in a Coupled Climate Model as CO2Forcing Increases

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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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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The Sensitivity of a Coupled Climate Model to Its Ocean Component

Journal of Climate ◽

10.1175/2010jcli3394.1 ◽

2010 ◽

Vol 23 (19) ◽

pp. 5126-5150 ◽

Cited By ~ 20

Author(s):

A. P. Megann ◽

A. L. New ◽

A. T. Blaker ◽

B. Sinha

Keyword(s):

North Atlantic ◽

Climate Models ◽

Climate Model ◽

Ocean Model ◽

Intermediate Water ◽

Mode Water ◽

Vertical Coordinate ◽

The North ◽

Almost Everywhere ◽

Subantarctic Mode Water

Abstract The control climates of two coupled climate models are intercompared. The first is the third climate configuration of the Met Office Unified Model (HadCM3), while the second, the Coupled Hadley–Isopycnic Model Experiment (CHIME), is identical to the first except for the replacement of its ocean component by the Hybrid-Coordinate Ocean Model (HYCOM). Both models possess realistic and similar ocean heat transports and overturning circulation. However, substantial differences in the vertical structure of the two ocean components are observed, some of which are directly attributed to their different vertical coordinate systems. In particular, the sea surface temperature (SST) in CHIME is biased warm almost everywhere, particularly in the North Atlantic subpolar gyre, in contrast to HadCM3, which is biased cold except in the Southern Ocean. Whereas the HadCM3 ocean warms from just below the surface down to 1000-m depth, a similar warming in CHIME is more pronounced but shallower and confined to the upper 400 m, with cooling below this. This is particularly apparent in the subtropical thermoclines, which become more diffuse in HadCM3, but sharper in CHIME. This is interpreted as resulting from a more rigorously controlled diapycnal mixing in the interior isopycnic ocean in CHIME. Lower interior mixing is also apparent in the better representation and maintenance of key water masses in CHIME, such as Subantarctic Mode Water, Antarctic Intermediate Water, and North Atlantic Deep Water. Finally, the North Pacific SST cold error in HadCM3 is absent in CHIME, and may be related to a difference in the separation position of the Kuroshio. Disadvantages of CHIME include a nonconservation of heat equivalent to 0.5 W m−2 globally, and a warming and salinification of the northwestern Atlantic.

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Fast and slow responses of the Subantarctic Mode Water in the South Indian Ocean to global warming in CMIP5 extended RCP4.5 simulations

Climate Dynamics ◽

10.1007/s00382-021-05635-w ◽

2021 ◽

Author(s):

Xingyue Xia ◽

Lixiao Xu ◽

Shang-Ping Xie ◽

Yu Hong ◽

Yan Du

Keyword(s):

Global Warming ◽

Indian Ocean ◽

The South ◽

South Indian Ocean ◽

Mode Water ◽

South Indian ◽

Subantarctic Mode Water

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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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