Variability of the southwest Indian Ocean

2005 ◽  
Vol 363 (1826) ◽  
pp. 63-76 ◽  
Author(s):  
Wilhelmus P. M. de Ruijter ◽  
Herman Ridderinkhof ◽  
Mathijs W. Schouten
Keyword(s):  
Indian Ocean ◽  
Ocean Circulation ◽  
Global Scale ◽  
Equatorial Region ◽  
Meridional Overturning Circulation ◽  
Central Basin ◽  
Southwest Indian Ocean ◽  
Basin Scale ◽  
Meridional Overturning ◽  
The Indian Ocean

The variability in the southwest Indian Ocean is connected to the basin–scale and global–scale ocean circulation. Two bands of enhanced variability stretch across the Southern Indian Ocean east of Madagascar around 12○ S and 25○ S, respectively. They mark the preferred routes along which anomalies, generated by varying forcing over the central basin, near the eastern boundary or in the equatorial region, propagate westward as baroclinic Rossby waves. Sea–surface height anomalies pass along the northern tip of Madagascar and are observed by satellite altimetry to propagate into the central Mozambique Channel. There, eddies are subsequently formed that propagate southward into the Agulhas retroflection region. The anomalies along the southern band trigger the formation of large dipolar vortex pairs in the separation region of the East Madagascar Current at the southern tip of the island. South of Africa these eddies and dipoles trigger the shedding of Agulhas Rings that feed the Atlantic meridional overturning circulation with warm, salty, Indian Ocean water. Interannual variability of the forcing over the Indian Ocean, such as that associated with the Indian Ocean Dipole/El Niño climate modes, propagates along these pathways and leads to associated modulations of the eddy transports into the South Atlantic.

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.

Impact of the Equatorial Wind Stress on the Indian Ocean Shallow Meridional Overturning Circulation During the IOD Mature Phase

2020 ◽  
Author(s):  
Linfang Zhang ◽  
Yaokun Li ◽  
Jianping Li
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Overturning Circulation ◽  
Mature Phase ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Upper Level ◽  
The Impact

<p>            This paper investigates the impact of the equatorial wind stress on the Indian Ocean Shallow Meridional Overturning Circulation (SMOC) during the India Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper level. In normal years, the wind stress in the Indian Ocean is easterly between 30°S-0°and the westerly wind is between 0°and 30°N, which contributes to a southward Ekman transport at the upper level to form the climatological SMOC. During the years of positive IOD events, abnormal easterly wind near the equator, accompanying with the cold sea surface temperature anomaly (SSTA) along the coast of Sumatra and Java and the warm SSTA along the coast of East Africa, brings southward Ekman transport south of the equator while northward Ekman transport north of the equator. This leads the seawaters moving away from the equator and hence upwelling near the equator as a consequence, to form a pair of small circulation cell symmetric about the equator.</p>

scholarly journals Using the Helmholtz Decomposition to Define the Indian Ocean Meridional Overturning Streamfunction

2020 ◽  
Vol 50 (3) ◽  
pp. 679-694
Author(s):  
Lei Han ◽  
Rui Xin Huang
Keyword(s):  
Indian Ocean ◽  
Zonal Flow ◽  
Circulation Pattern ◽  
Ocean Basin ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Open Boundaries ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
The Antarctic

AbstractThe zonally integrated flow in a basin can be separated into the divergent/nondivergent parts, and a uniquely defined meridional overturning circulation (MOC) can be calculated. For a basin with significant volume exchange at zonal open boundaries, this method is competent in removing the components associated with the nonzero source terms due to zonal transports at open boundaries. This method was applied to the zonally integrated flow in the Indian Ocean basin extended all the way to the Antarctic by virtue of the ECCO dataset. The contributions due to two major zonal flow systems at open boundaries, the Indonesian Throughflow (ITF) and the Antarctic Circumpolar Current (ACC), were well separated from the rotational flow component, and a nondivergent overturning circulation pattern was identified. Comparisons with previous studies on the MOC of the Indian Ocean in different seasons showed overall consistency but with refinements in details to the south of the entry of the ITF, reflecting the influence of ITF on the MOC pattern in the domain. Other options of decomposition are also examined.

scholarly journals Submesoscale flows impact Agulhas leakage in ocean simulations

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
René Schubert ◽  
Jonathan Gula ◽  
Arne Biastoch
Keyword(s):  
Indian Ocean ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Ocean Water ◽  
Agulhas Current ◽  
Cape Basin ◽  
Northern Edge ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Shear Edge

AbstractAgulhas leakage, the warm and salty inflow of Indian Ocean water into the Atlantic Ocean, is of importance for the climate-relevant Atlantic Meridional Overturning Circulation. South of Africa, the eastward turning Agulhas Current sheds Agulhas rings, cyclones and filaments of order 100 km that carry the Indian Ocean water into the Cape Basin and further into the Atlantic. Here, we show that the resolution of submesoscale flows of order 10 km in an ocean model leads to 40 % more Agulhas leakage and more realistic Cape Basin water-masses compared to a parallel non-submesoscale resolving simulation. Moreover, we show that submesoscale flows strengthen shear-edge eddies and in consequence lee cyclones at the northern edge of the Agulhas Current, as well as the leakage pathway in the region of the filaments that takes place outside of mesoscale eddies. This indicates that the increase in leakage can be attributed to stronger Agulhas filaments, when submesoscale flows are resolved.

scholarly journals Wind-driven Oscillations in the Meridional Overturning Circulation near the Equator. Part I: Numerical models

2020 ◽  
Author(s):  
Adam T. Blaker ◽  
Joël J.-M. Hirschi ◽  
Michael J. Bell ◽  
Amy Bokota
Keyword(s):  
Numerical Models ◽  
Ocean Basin ◽  
Ocean Model ◽  
Equatorial Region ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Model Studies ◽  
Basin Scale ◽  
Meridional Overturning ◽  
State Of Rest

AbstractThe great ocean conveyor presents a time-mean perspective on the interconnected network of major ocean currents. Zonally integrating the meridional velocities, either globally or across basin-scale domains reduces the conveyor to a 2D projection widely known as the meridional overturning circulation (MOC). Recent model studies have shown the MOC to exhibit variability on near-inertial timescales, and also indicate a region of enhanced variability on the Equator. We present an analysis of three integrations of a global configuration of a numerical ocean model, which show very large amplitude oscillations in the MOCs in the Atlantic, Indian and Pacific oceans confined to the equatorial region. The amplitude of these oscillations is proportional to the width of the ocean basin, typically about 100 (200) Sv in the Atlantic (Pacific). We show that these oscillations are driven by surface winds within 10°N/S of the Equator, and their periods (typically 4-10 days) correspond to a small number of low mode equatorially trapped planetary waves. Furthermore, the oscillations can be well reproduced by idealised wind-driven simulations linearised about a state of rest.

scholarly journals Deep Meridional Overturning Circulation in the Indian Ocean and Its Relation to Indian Ocean Dipole

2014 ◽  
Vol 27 (12) ◽  
pp. 4508-4520 ◽  
Author(s):  
Weiqiang Wang ◽  
Xiuhua Zhu ◽  
Chunzai Wang ◽  
Armin Köhl
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Indian Ocean Dipole Mode ◽  
Residual Term ◽  
Temperature Anomalies ◽  
Meridional Overturning ◽  
The Indian Ocean

Abstract This paper uses the 42-yr German Estimating the Circulation and Climate of the Ocean (GECCO) synthesis data to analyze and examine the relationship of the Indian Ocean deep meridional overturning circulation (DMOC) with the Indian Ocean dipole mode (IOD). Contributions of various dynamical processes are assessed by decomposing the DMOC into the Ekman and geostrophic transport, the external mode, and a residual term. The first three terms successfully describe the DMOC with a marginal residual term. The following conclusions are obtained. First, the seasonal cycle of the DMOC is mainly determined by the Ekman component. The exception is during the transitional seasons (March–April and September–October) in the northern Indian Ocean Basin, where the geostrophic component dominates. Second, at the beginning phase of the IOD (May–June), the Ekman component dominates the DMOC structure; at and after the peak phase of the IOD (September–December), the DMOC structure is primarily determined by the geostrophic component in correspondence with the well-developed sea surface temperature anomalies, while the wind (and thus the Ekman component) plays a secondary role south of 10°S and contributes negatively within the zonal band of 10° on both sides of the equator. Therefore, there exists a surface to deep-ocean connection through which IOD-related surface wind and ocean temperature anomalies are transferred down to the deep ocean. Westward-propagating signals are observed even in the deep ocean, suggesting possible roles of Rossby waves in transferring the surface signal to the deep ocean.

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