Relating Agulhas leakage to the Agulhas Current retroflection location

Abstract. The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression.

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An observed 20-year time series of Agulhas leakage

Ocean Science ◽

10.5194/os-10-601-2014 ◽

2014 ◽

Vol 10 (4) ◽

pp. 601-609 ◽

Cited By ~ 20

Author(s):

D. Le Bars ◽

J. V. Durgadoo ◽

H. A. Dijkstra ◽

A. Biastoch ◽

W. P. M. De Ruijter

Keyword(s):

Time Series ◽

Large Scale ◽

Ocean Model ◽

Dynamic Topography ◽

Return Current ◽

Major Barrier ◽

Agulhas Current ◽

The Mean ◽

The Difference ◽

Geostrophic Velocities

Abstract. We provide a time series of Agulhas leakage anomalies over the last 20-years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between the Agulhas Current and Agulhas Return Current, which allows us to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows us to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow us to determine the mean leakage but that leakage anomalies can be accurately computed.

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An observed 20 yr time-series of Agulhas leakage

Ocean Science Discussions ◽

10.5194/osd-11-171-2014 ◽

2014 ◽

Vol 11 (1) ◽

pp. 171-196 ◽

Cited By ~ 1

Author(s):

D. Le Bars ◽

J. V. Durgadoo ◽

H. A. Dijkstra ◽

A. Biastoch ◽

W. P. M. De Ruijter

Keyword(s):

Time Series ◽

Large Scale ◽

Ocean Model ◽

Dynamic Topography ◽

Return Current ◽

Major Barrier ◽

Agulhas Current ◽

The Mean ◽

The Difference ◽

Geostrophic Velocities

Abstract. We provide a time series of Agulhas leakage anomalies over the last twenty years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between Agulhas Current and Agulhas Return Current which allows to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow to determine the mean leakage but that leakage anomalies can be accurately computed.

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Implementation and Validating of the Regional Ocean Model System (ROMS) for the Sunda Strait connecting the Java Sea to the Indian Ocean

10.5194/egusphere-egu21-3457 ◽

2021 ◽

Author(s):

Subekti Mujiasih ◽

Jean-Marie Beckers ◽

Alexander Barth

Keyword(s):

Time Series ◽

Indian Ocean ◽

Vertical Profile ◽

Ocean Model ◽

Model System ◽

Satellite Sst ◽

Regional Ocean Model System ◽

Regional Ocean Model ◽

The Indian Ocean ◽

Java Sea

Regional Ocean Model System (ROMS) has been simulated for the Sunda Strait, the Java Sea, and the Indian Ocean. The simulation was undertaken for thirteen months of data period (August 2013 &#8211; August 2014). However, we only used four months period for validation, namely September &#8211; December 2013. The input data involved the HYbrid Coordinate Ocean Model (HYCOM) ocean model output by considering atmospheric forcing from the European Centre for Medium-Range Weather Forecasts (ECMWF), without and with tides forcing from TPXO and rivers. The output included vertical profile temperature and salinity, sea surface temperature (SST), seas surface height (SSH), zonal (u), and meridional (v) velocity. We compared the model SST to satellite SST in time series, SSH to tides gauges data in time series, the model u and v component velocity to High Frequency (HF) radial velocity. The vertical profile temperature and salinity were compared to Argo float data and XBT. Besides, we validated the amplitude and phase of the ROMS seas surface height to amplitude and phase of the tides-gauges, including four constituents (M2, S2, K1, O1).

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GROUND DEFORMATION EXTRACTION USING VISIBLE IMAGES AND LIDAR DATA IN MINING AREA

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences ◽

10.5194/isprsarchives-xli-b7-505-2016 ◽

2016 ◽

Vol XLI-B7 ◽

pp. 505-512 ◽

Cited By ~ 1

Author(s):

Wenmin Hu ◽

Lixin Wu

Keyword(s):

Time Series ◽

High Resolution ◽

Ground Deformation ◽

Mining Area ◽

Open Pit Mine ◽

Space Distribution ◽

Open Pit ◽

Mining Activities ◽

Study Results ◽

The Mean

Recognition and extraction of mining ground deformation can help us understand the deformation process and space distribution, and estimate the deformation laws and trends. This study focuses on the application of ground deformation detection and extraction combining with high resolution visible stereo imagery, LiDAR observation point cloud data and historical data. The DEM in large mining area is generated using high-resolution satellite stereo images, and ground deformation is obtained through time series analysis combined with historical DEM data. Ground deformation caused by mining activities are detected and analyzed to explain the link between the regional ground deformation and local deformation. A district of covering 200 km2 around the West Open Pit Mine in Fushun of Liaoning province, a city located in the Northeast China is chosen as the test area for example. Regional and local ground deformation from 2010 to 2015 time series are detected and extracted with DEMs derived from ZY-3 images and LiDAR point DEMs in the case study. Results show that the mean regional deformation is 7.1 m of rising elevation with RMS 9.6 m. Deformation of rising elevation and deformation of declining elevation couple together in local area. The area of higher elevation variation is 16.3 km2 and the mean rising value is 35.8 m with RMS 15.7 m, while the deformation area of lower elevation variation is 6.8 km2 and the mean declining value is 17.6 m with RMS 9.3 m. Moreover, local large deformation and regional slow deformation couple together, the deformation in local mining activities has expanded to the surrounding area, a large ground fracture with declining elevation has been detected and extracted in the south of West Open Pit Mine, the mean declining elevation of which is 23.1 m and covering about 2.3 km2 till 2015. The results in this paper are preliminary currently; we are making efforts to improve more precision results with invariant ground control data for validation.

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Satellite-Tracked Drifters between Africa and Antarctica

Bulletin of the American Meteorological Society ◽

10.1175/1520-0477-59.1.51 ◽

1978 ◽

Vol 59 (1) ◽

pp. 51-59 ◽

Cited By ~ 8

Author(s):

T. F. W. Harris ◽

C. C. Stavropoulos

Keyword(s):

Atlantic Ocean ◽

Current Velocity ◽

Wind Shear ◽

South Atlantic Ocean ◽

Agulhas Current ◽

Ocean Ridges ◽

Slow Drift ◽

The Mean ◽

Circumpolar Current ◽

Geostrophic Velocities

Trajectories and velocities are reported for six drifters launched in February 1976 in the South Atlantic Ocean mostly along the 10°W meridian between 35° and 57°S. These drifters had drogues centered on 7 or 57 m and the tracking durations were 33–302 days. In the Circumpolar Current the tracks were remarkably zonal, remaining generally within ±1° of latitude, except for one drifter deployed at ~42°S, which when it was south of the Cape curved equatorward in the Agulhas Current terminal region, and one at 57°S, which slanted equatorward along the Atlantic-Indian Ridge. The expected deflections occurred at the mid-ocean ridges, and generally the course of the drifters was consistent with the published streamlines of geostrophic transport. The trajectory of a drifter launched equatorward of the subtropical convergence, at 35°S, was one of complex eddying motion, accompanied by a slow drift to the northeast. In the Circumpolar Current, the latitudinal variation of zonal current velocity was qualitatively similar to the mean annual wind shear and (in the region of 0° meridian) to the geostrophic zonal surface velocities calculated by Ostapoff (1962). Drifter velocities were, however, considerably larger than the geostrophic velocities. Maximum mean velocities (~0.3 m/s) occurred along 49°50′.

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Foraminiferal tracers of Indian-Atlantic interocean exchange during the last 600 kyr

10.5194/egusphere-egu2020-3037 ◽

2020 ◽

Author(s):

José N. Pérez-Asensio ◽

Kazuyo Tachikawa ◽

Thibault de Garidel-Thoron ◽

Laurence Vidal ◽

Corinne Sonzogni ◽

...

Keyword(s):

Indian Ocean ◽

Atlantic Ocean ◽

Deep Water ◽

Water Circulation ◽

Warm Water ◽

Temperate Climate ◽

Agulhas Current ◽

Foraminiferal Species ◽

The Indian Ocean ◽

Relative Abundances

The Indian-Atlantic interocean exchange (IAIE), occurring through Agulhas current and its leakage around the southern tip of Africa, is one of the return flows of global thermohaline circulation that contributes to the temperate climate in Europe. The IAIE affects the transport of heat and salt to the zone of deep-water formation in the N Atlantic, influencing the variability of Atlantic Meridional Overturning Circulation (AMOC). During the last 600 kyr, significant climatic events took place such as the Mid-Bruhnes event (MBE) (~430 ka) that marks a transition towards more intense interglacial periods.The main objective of our study is to assess the impact of climate forcing on the strength of both surface and deep water IAIE during the last 600 kyr. For this purpose, we examined the variability of a group of warm-water planktonic foraminiferal species for tracing surface water circulation. We combined published and unpublished data from 3 cores along an Indian-Atlantic transect: two cores in the Indian Ocean, core MD96-2048 (26&#176;10&#8217;S, 34&#176;01&#8217;E, 660 m) in the source of the Agulhas current and our study core MD96-2077 (33&#186;10&#8217;S, 31&#186;14&#8217;E, 3781 m) in the middle of the Agulhas current; and one core in the Atlantic Ocean, core ODP1087 (31&#176;27&#8217;S, 15&#176;18&#8217;E 1372 m) recording the Agulhas leakage.Since Globorotalia menardii and Globorotalia tumida are frequently used to trace Agulhas leakage, their variability in Agulhas current in the Indian Ocean is of our interest. Therefore, we compared the relative abundances of the warm-water planktonic G. menardii and G. tumida species with a group of warm-water planktonic foraminiferal species to record the strength of Agulhas current in core MD96-2077. Our results show that the group of warm-water planktonic species reflects increased Agulhas current strength at glacial terminations coinciding with stronger Agulhas leakage (Atlantic core ODP1087) as observed in previous studies. However, in core MD96-2077, both G. menardii and G. tumida relative abundances increase during interglacial periods. This indicates that production of these species in the Agulhas current source region is unlikely to trace Agulhas leakage in the Atlantic Ocean. The analyses of deep-water circulation proxies (Nd isotopes, benthic O and C stable isotopes) are in progress, and they will allow us to assess the response of deep circulation to changes in Agulhas current and leakage over the last 600 kyr.

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Par@Graph – a parallel toolbox for the construction and analysis of large complex climate networks

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-319-2015 ◽

2015 ◽

Vol 8 (1) ◽

pp. 319-349 ◽

Cited By ~ 1

Author(s):

H. Ihshaish ◽

A. Tantet ◽

J. C. M. Dijkzeul ◽

H. A. Dijkstra

Keyword(s):

Time Series ◽

High Resolution ◽

Network Analysis ◽

Large Scale ◽

Ocean Model ◽

Clustering Coefficient ◽

Connected Components ◽

Eigenvector Centrality ◽

Key Innovation ◽

Climate Networks

Abstract. In this paper, we present Par@Graph, a software toolbox to reconstruct and analyze complex climate networks having a large number of nodes (up to at least O (106)) and of edges (up to at least O (1012)). The key innovation is an efficient set of parallel software tools designed to leverage the inherited hybrid parallelism in distributed-memory clusters of multi-core machines. The performance of the toolbox is illustrated through networks derived from sea surface height (SSH) data of a global high-resolution ocean model. Less than 8 min are needed on 90 Intel Xeon E5-4650 processors to construct a climate network including the preprocessing and the correlation of 3 × 105 SSH time series, resulting in a weighted graph with the same number of vertices and about 3 × 106 edges. In less than 5 min on 30 processors, the resulted graph's degree centrality, strength, connected components, eigenvector centrality, entropy and clustering coefficient metrics were obtained. These results indicate that a complete cycle to construct and analyze a large-scale climate network is available under 13 min. Par@Graph therefore facilitates the application of climate network analysis on high-resolution observations and model results, by enabling fast network construction from the calculation of statistical similarities between climate time series. It also enables network analysis at unprecedented scales on a variety of different sizes of input data sets.

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Variability and Coherence of the Agulhas Undercurrent in a High-Resolution Ocean General Circulation Model

Journal of Physical Oceanography ◽

10.1175/2009jpo4184.1 ◽

2009 ◽

Vol 39 (10) ◽

pp. 2417-2435 ◽

Cited By ~ 36

Author(s):

A. Biastoch ◽

L. M. Beal ◽

J. R. E. Lutjeharms ◽

T. G. D. Casal

Keyword(s):

High Resolution ◽

General Circulation ◽

Current System ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Ocean Model ◽

Western Boundary ◽

Boundary Current ◽

Sensitivity Experiment ◽

Agulhas Current

Abstract The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50–70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north.

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Par@Graph – a parallel toolbox for the construction and analysis of large complex climate networks

Geoscientific Model Development ◽

10.5194/gmd-8-3321-2015 ◽

2015 ◽

Vol 8 (10) ◽

pp. 3321-3331 ◽

Cited By ~ 3

Author(s):

H. Ihshaish ◽

A. Tantet ◽

J. C. M. Dijkzeul ◽

H. A. Dijkstra

Keyword(s):

Time Series ◽

High Resolution ◽

Network Analysis ◽

Large Scale ◽

Ocean Model ◽

Clustering Coefficient ◽

Connected Components ◽

Eigenvector Centrality ◽

Key Innovation ◽

Climate Networks

Abstract. In this paper, we present Par@Graph, a software toolbox to reconstruct and analyze complex climate networks having a large number of nodes (up to at least 106) and edges (up to at least 1012). The key innovation is an efficient set of parallel software tools designed to leverage the inherited hybrid parallelism in distributed-memory clusters of multi-core machines. The performance of the toolbox is illustrated through networks derived from sea surface height (SSH) data of a global high-resolution ocean model. Less than 8 min are needed on 90 Intel Xeon E5-4650 processors to reconstruct a climate network including the preprocessing and the correlation of 3 × 105 SSH time series, resulting in a weighted graph with the same number of vertices and about 3.2 × 108 edges. In less than 14 min on 30 processors, the resulted graph's degree centrality, strength, connected components, eigenvector centrality, entropy and clustering coefficient metrics were obtained. These results indicate that a complete cycle to construct and analyze a large-scale climate network is available under 22 min Par@Graph therefore facilitates the application of climate network analysis on high-resolution observations and model results, by enabling fast network reconstruct from the calculation of statistical similarities between climate time series. It also enables network analysis at unprecedented scales on a variety of different sizes of input data sets.

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