scholarly journals The Shallow Overturning Circulation in the Indian Ocean

2018 ◽  
Vol 48 (2) ◽  
pp. 413-434 ◽  
Author(s):  
Motoki Nagura ◽  
Michael J. McPhaden
Keyword(s):  
Indian Ocean ◽  
Western Boundary ◽  
Western Coast ◽  
Intermediate Water ◽  
Depth Range ◽  
Mode Water ◽  
Ocean Reanalysis ◽  
Subantarctic Mode Water ◽  
The Indian Ocean ◽  
Mean Circulation

AbstractThe number of in situ observations in the Indian Ocean has dramatically increased over the past 15 years thanks to the implementation of the Argo profiling float program. This study estimates the mean circulation in the Indian Ocean using hydrographic observations obtained from both Argo and conductivity–temperature–depth (CTD) observations. Absolute velocity at the Argo float parking depth is used so there is no need to assume a level of no motion. Results reveal previously unknown features in addition to well-known currents and water masses. Some newly identified features include the lack of an interior pathway to the equator from the southern Indian Ocean in the pycnocline, indicating that water parcels must transit through the western boundary to reach the equator. High potential vorticity (PV) intrudes from the western coast of Australia in the depth range of the Subantarctic Mode Water, which leads to a structure similar to a PV barrier. The subtropical anticyclonic gyre retreats poleward with depth, as happens in the subtropical Atlantic and Pacific. An eastward flow was found in the eastern basin along 15°S at the depth of the Antarctic Intermediate Water—a feature expected from property distributions but never before detected in velocity estimates. Meridional mass transport indicates about 10 Sv (1 Sv ≡ 106 m3 s−1) southward flow at 6°S and 18 Sv northward flow at 20°S, which results in meridional convergence of currents and thermocline depression at about 16°–20°S. These estimated absolute velocities agree well with those of an ocean reanalysis, which lends credibility to the strictly databased analysis.

scholarly journals Modeling Decadal Changes on the Indian Ocean Section I5 at 32°S

2007 ◽  
Vol 20 (13) ◽  
pp. 3106-3130 ◽  
Author(s):  
R. J. Murray ◽  
Nathaniel L. Bindoff ◽  
C. J. C. Reason
Keyword(s):  
Indian Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Late Winter ◽  
Intermediate Water ◽  
Mode Water ◽  
Density Class ◽  
The Indian Ocean ◽  
1960S And 1970S ◽  
The 1960S

Abstract A near-global ocean model with resolution enhanced in the southern Indian Ocean has been spun up to seasonal equilibrium and then driven by NCEP–NCAR reanalysis 1 monthly mean forcings and Hadley SSTs over the period 1948–2002. The aim was to simulate changes in the subsurface properties observed in hydrographic surveys at 32°S in the Indian Ocean in 1965, 1987, and 2002. These surveys showed a zonally averaged cooling on isopycnals of 0.5° and 0.3°C in mode and intermediate waters between 1965 and 1987 and a warming of the mode water coupled with a continued cooling of the intermediate water between 1987 and 2002. The major changes in isopycnal depth and temperature modeled in this study were confined to the mode water and were qualitatively similar to those observed but concentrated in a lower density class and in the eastern half of the section. The dominant changes here were multidecadal, with maximum temperatures on the σθ = 26.7 kg m−3 isopycnal being reached in 1968 and minimum temperatures in 1990. The simulations showed a propagation of interannual anomalies toward the section from a region of deep late winter mixed layers in the southeast Indian Ocean within a period of several years. Surface temperatures in this region were lowest in the 1960s and highest in the late 1980s. Temperatures on isopycnals showed the opposite variation, consistent with SST having the controlling effect on mixed layer density and depth. Isopycnal depths within the mode water were strongly correlated with temperature, implying a redistribution of mode water density classes, the greatest volume of mode water being produced in a higher density class (σθ = 26.8–27.0 kg m–3) during the period of cooler surface forcing in the 1960s and 1970s than during the warmer period following (σθ = 26.6–26.8 kg m–3).

scholarly journals Water Mass Export from Drake Passage to the Atlantic, Indian, and Pacific Oceans: A Lagrangian Model Analysis

2005 ◽  
Vol 35 (7) ◽  
pp. 1206-1222 ◽  
Author(s):  
Yann Friocourt ◽  
Sybren Drijfhout ◽  
Bruno Blanke ◽  
Sabrina Speich
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Drake Passage ◽  
General Circulation Models ◽  
Lagrangian Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Intermediate Water ◽  
Boundary Current ◽  
The Indian Ocean

Abstract The northward export of intermediate water from Drake Passage is investigated in two global ocean general circulation models (GCMs) by means of quantitative particle tracing diagnostics. This study shows that a total of about 23 Sv (Sv ≡ 106 m3 s−1) is exported from Drake Passage to the equator. The Atlantic and Pacific Oceans are the main catchment basins with 7 and 15 Sv, respectively. Only 1–2 Sv of the water exported to the Atlantic equator follow the direct cold route from Drake Passage without entering the Indian Ocean. The remainder loops first into the Indian Ocean subtropical gyre and flows eventually into the Atlantic Ocean by Agulhas leakage. The authors assess the robustness of a theory that relates the export from Drake Passage to the equator to the wind stress over the Southern Ocean. Our GCM results are in reasonable agreement with the theory that predicts the total export. However, the theory cannot be applied to individual basins because of interocean exchanges through the “supergyre” mechanism and other nonlinear processes such as the Agulhas rings. The export of water from Drake Passage starts mainly as an Ekman flow just northward of the latitude band of the Antarctic Circumpolar Current south of South America. Waters quickly subduct and are transferred to the ocean interior as they travel equatorward. They flow along the eastern boundaries in the Sverdrup interior and cross the southern basins northwestward to reach the equator within the western boundary current systems.

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

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

<p><span>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 </span>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 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.</p><p> </p>

scholarly journals Observational estimates of turbulent mixing in the southeast Indian Ocean

2021 ◽  
Author(s):  
Ajitha Cyriac ◽  
Helen E. Phillips ◽  
Nathaniel L. Bindoff ◽  
Huabin Mao ◽  
Ming Feng
Keyword(s):  
Indian Ocean ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Bottom Topography ◽  
Intermediate Water ◽  
Depth Range ◽  
Mean Flow ◽  
Mode Water ◽  
South Indian ◽  
Spatio Temporal

AbstractThis study investigates the spatio-temporal variability of turbulent mixing in the eastern South Indian Ocean using a collection of data from EM-APEX profiling floats, shipboard CTD and microstructure profilers. The floats collected 1566 profiles of temperature, salinity and horizontal velocity data down to 1200 m over a period of about four months. A fine-scale parameterization is applied to the float and CTD data to estimate turbulent mixing. Elevated mixing is observed in the upper ocean, over bottom topography and in mesoscale eddies. Mixing is enhanced in the anticyclonic eddies due to trapped near-inertial waves within the eddy. We found that cyclonic eddies contribute to turbulent mixing in the depth range of 500 – 1000 m, which is associated with downward propagating internal waves. The mean diapycnal diffusivity over 250 – 500 m depth is O(10−6) m2 s−1 and it increases to O(10−5) m2 s−1 in 500 – 1000 m in cyclonic eddies. The turbulent mixing in this region has implications for watermass transformation and large-scale circulation. Higher diffusivity (O(10−5) m2 s−1) is observed in the Antarctic Intermediate Water (AAIW) layer in cyclonic eddies whereas weak diffusivity is observed in the Subantarctic Mode Water (SAMW) layer (O(10−6) m2 s−1). Counter-intuitively, then, the SAMW watermass properties are strongly affected in cyclonic eddies whereas the AAIW layer is less affected. Comparatively high diffusivity at the location of the South Indian Countercurrent (SICC) jets suggests there are wave-mean flow interactions in addition to the wave-eddy interactions that warrant further investigation.

scholarly journals Intermediate-Depth Circulation of the Indian and South Pacific Oceans Measured by Autonomous Floats

2005 ◽  
Vol 35 (5) ◽  
pp. 683-707 ◽  
Author(s):  
Russ E. Davis
Keyword(s):  
Indian Ocean ◽  
Seasonal Variability ◽  
World Ocean ◽  
South Pacific ◽  
Western Boundary ◽  
Mean Flow ◽  
The North ◽  
The Indian Ocean ◽  
The Mean ◽  
Mean Circulation

Abstract As part of the World Ocean Circulation Experiment, 306 autonomous floats were deployed in the tropical and South Pacific Ocean and 228 were deployed in the Indian Ocean to observe the basinwide circulation near 900-m depth. Mean velocities, seasonal variability, and lateral eddy diffusivity from the resultant 2583 float-years of data are presented. Area averages, local function fits, and a novel application of objective mapping are used to estimate the mean circulation. Patterns of mean circulation resemble those at the surface in both basins. Well-developed subtropical gyres, twice as strong in the Indian Ocean as in the Pacific, feed western boundary currents. Tropical gyres are separated by eastward flow along the equator in both hemispheres of both basins, although the Indian subcontinent splits the north Indian tropical gyre. The Antarctic Circumpolar Current (ACC) and west wind drifts are prominent in both basins, generally tending slightly southward but deviating to the north behind the Del Cano, Kerguelen, and Campbell Plateaus and, of course, South America. Remarkably, the eastern boundaries of the southern subtropical gyres in all three basins apparently occur in the ocean interior, away from land. The Indian Ocean’s subtropical gyre, and perhaps part of the South Atlantic’s, reaches east to a retroflection just upstream of the Campbell Plateau south of New Zealand. Seasonal variability at 900 m is focused around the equator with weaker variability found near certain bathymetric features. There is a remarkable agreement between the observed seasonable variability and that predicted by the Jet Propulsion Laboratory (JPL)–Estimating the Circulation and Climate of the Ocean (ECCO) data-assimilating numerical model. Aside from seasonal effects, eddy variability is greatest along the equator, in tropical and subtropical western basins, and along the ACC. Integrals of velocity across regional passages (Tasman Sea, Mozambique Channel) provide useful reference for hydrographic analyses of transport. Across whole ocean basins, however, the uncertainty associated with the appropriate continuity relation for horizontal flow (e.g., geostrophy vs nondivergence) is comparable to the mean flow.

scholarly journals The sloshing and diapycnal meridional overturning circulations in the Indian Ocean

2020 ◽  
Author(s):  
Lei Han
Keyword(s):  
Indian Ocean ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Coast ◽  
Ekman Pumping ◽  
Meridional Heat Transport ◽  
Vertical Movements ◽  
Sloshing Mode ◽  
Meridional Overturning ◽  
The Indian Ocean

AbstractThe meridional overturning circulation (MOC) seasonality in the Indian Ocean is investigated with the ocean state estimate product, ECCO v4r3. The vertical movements of water parcels are predominantly due to the heaving of the isopycnals all over the basin except off the western coast. Aided by the linear propagation equation of long baroclinic Rossby waves, the driving factor determining the strength of the seasonal MOC in the Indian Ocean is identified as the zonally-integrated Ekman pumping anomaly, rather than the Ekman transport concluded in earlier studies. A new concept of sloshing MOC is proposed, and its difference with the classic Eulerian MOC leads to the so-called diapycnal MOC. The striking resemblance of the Eulerian and sloshing MOCs implies the seasonal variation of the Eulerian MOC in the Indian Ocean is a sloshing mode. The shallow overturning cells manifest themselves in the diapycnal MOC as the most remarkable structure. New perspectives on the upwelling branch of the shallow overturn in the Indian Ocean are offered based on diapycnal vertical velocity. The discrepancy among the observation-based estimates on the bottom inflow across 32°S of the basin is interpreted with the seasonal sloshing mode. Consequently, the “missing mixing” in the deep Indian Ocean is attributed to the overestimated diapycnal volume fluxes. Decomposition of meridional heat transport (MHT) into sloshing and diapycnal components clearly shows the dominant mechanism of MHT in the Indian Ocean in various seasons.

Variations of the Indian Ocean Walker circulation since the Last Glacial Maximum revealed by reconstructed and simulated zonal wind intensity

2021 ◽  
Author(s):  
Xinquan Zhou ◽  
Stéphanie Duchamp-Alphonse ◽  
Masa Kageyama ◽  
Franck Bassinot ◽  
Xiaoxu Shi ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Surface Temperature ◽  
Equatorial Indian Ocean ◽  
Walker Circulation ◽  
Sea Surface ◽  
The Indian Ocean ◽  
Sst Anomaly ◽  
Mean Circulation ◽  
The Last Glacial Maximum ◽  
The Last Glacial

<p>Today, precipitation and wind patterns over the equatorial Indian Ocean and surrounding lands are paced by monsoon and Walker circulations that are controlled by the seasonal land-sea temperature contrast and the inter-annual convection over the Indo-Pacific Warm Pool, respectively. The annual mean surface westerly winds are particularly tied to the Walker circulation, showing interannual variability coupled with the gradient of Sea Surface Temperature (SST) anomaly between the tropical western and southeastern Indian Ocean, namely, the Indian Ocean Dipole (IOD). While the Indian monsoon pattern has been widely studied in the past, few works deal with the evolution of Walker circulation despite its crucial impacts on modern and future tropical climate systems. Here, we reconstruct the long-term westerly (summer) and easterly (winter) wind dynamics of the equatorial Indian Ocean (10°S−10°N), since the Last Glacial Maximum (LGM) based on i) primary productivity (PP) records derived from coccolith analyses of sedimentary cores MD77-191 and BAR94-24, retrieved off the southern tip of India and off the northwestern tip of Sumatra, respectively and ii) the calculation of a sea surface temperature (SST) anomaly gradient off (south) western Sumatra based on published SST data. We compare these reconstructions with atmospheric circulation simulations obtained with the general coupled model AWI-ESM-1-1-LR (Alfred Wegener Institute Earth System Model).</p><p>Our results show that the Indian Ocean Walker circulation was weaker during the LGM and the early/middle Holocene than present. Model simulations suggest that this is due to anomalous easterlies over the eastern Indian Ocean. The LGM mean circulation state may have been comparable to the year 1997 with a positive IOD, when anomalously strong equatorial easterlies prevailed in winter. The early/mid Holocene mean circulation state may have been equivalent to the year 2006 with a positive IOD, when anomalously strong southeasterlies prevailed over Java-Sumatra in summer. The deglaciation can be seen as a transient period between these two positive IOD-like mean states.</p>

Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

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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