Modelling the Evolution of Currents South of South Africa since Mid-Miocene Times

The Averaged Concentration of Middle Silt form April to May

26^oE 27^oE 28^oE 29^oE 30^oE 31^oE 32^oE 33^oE 40^oS

39^oS 38^oS 37^oS 36^oS 35^oS 34^oS 33^oS

25 cm/s

Csed [mg/l]

0 1 2

>3

The Averaged Concentration of Middle Silt for Jan.

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39^oS 38^oS 37^oS 36^oS 35^oS 34^oS 33^oS

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Csed [mg/l]

0 1 2

>3

The Averaged Concentration of Middle Silt form Feb. to Mar.

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Csed [mg/l]

0 1 2

>3

The Averaged Concentration of Fine Sand form April to May

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Csed [mg/l]

0 0.5 1 1.5 2 2.5 3 3.5

>4

The Averaged Concentration of Fine Sand form Feb. to Mar.

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39^oS 38^oS 37^oS 36^oS 35^oS 34^oS 33^oS

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Csed [mg/l]

0 0.5 1 1.5 2 2.5 3 3.5

>4

The Averaged Concentration of Fine Sand for Jan.

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39^oS 38^oS 37^oS 36^oS 35^oS 34^oS 33^oS

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Csed [mg/l]

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>4

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Depth [m]

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Latitude

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Fig. 3. The model grid used in the ADM. (a) The horizontal grid.

(b)The vertical grid at 29° E (Black line in (a)).

ADM domain: 34° S - 42° S, 20° E - 35° E ADM grid:

horizontal: 0.125° grid width

vertical: 20 topography following l ayers

(Enhanced resolution both at the surface and in the deeper layer)

Boundary condition: active open boundaries Period: one model year

Topography: ETOPO2 dataset

Forcing: wind, heat, and salinity fluxes extracted from Comprehensive Ocean-Atmosphere Data Set (COADS)

Climatology: an ocean monthly climatology, World Ocean Atlas (WOA 2001)

Modelling the Evolution of Currents South of South Africa since Mid-Miocene Times

Xin Li

¹

, Gabriele Uenzelmann-Neben

¹

, and Katrin Huhn

²

1

Alfred Wegener Institute for Polar and Marine Research, PO Box 120161, D-27515 Bremerhaven, Germany, (Xin.Li@awi.de)

2

Research Center Ocean Margins, University of Bremen, Leobenerstr., D-28359 Bremen, Germany.

10˚ 12˚ 14˚ 16˚ 18˚ 20˚ 22˚ 24˚ 26˚ 28˚ 30˚ 32˚ 34˚ 36˚ 38˚ 40˚

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-6 -5 -4 -3 -2 -1 1 [km]

3

AP

MR ABK

AR

Cape Agulhas

TR

AB

NV

TB MB

Agulhas Current NADW

Benguela Current AABW

Fig. 1. (a) Oceanographic setting south of South Africa. Water mass flow paths are schematic, and based on the literature cited in the reference. The location of the Agulhas Drift is labeled.

(b) RV Sonne SO-182 Cruise track map with seismic reflection and refraction lines, and

OBS/seismic land stations indicated. The position of the parasound profile along Line AWI- 2005009 (Fig. 2) is marked as pink line.

Agulhas Drift

Introduction

The area south of South Africa has been intensively studied since this region is a critical gateway for the exchange of water masses from Atlantic, Indian and Southern Ocean (Fig. 1(a)). During the R/V SONNE cruise SO-182 in spring 2005, extensive parasound and reflection seismic data across the Agulhas Drift has been recorded (Fig. 1 (b)). These data indicate that the Agulhas Drift was built up by east-west flowing currents (Fig. 2), which are identified as the inflows of NADW and AABW.

For a better understanding of the temporal evolution of the Agulhas Drift and the development of the palaeocirculation south of South Africa, the Regional Ocean Modeling System (ROMS) is applied. It is coupled with a sediment transport model. Preliminary simulation is based on the present topog- raphy, forcing (wind, heat and salinity) and climatology data- sets.

ROMS

ROMS it is an advanced open-source model. It is a 3D, free-surface, hydro- static, primitive equation model with stretched, terrain-following vertical coordinates and orthogonal, curvilinear horizontal coor- dinates. Furthermore, the sediment transport algo- rithms have been incorpo- rated.

Outlook

Further simulations will be developed in the following aspects:

build up the model with finer grid in Transkei Basin set the sediment sources and test more sediment spectra

simulate the regional sediment transport under LGM conditions

Reference

Dingle, R.V. Birch, et al., 1987. Deep-sea sedimentary environments around southern Africa (south-east Atlantic and south-west Indian Oceans), Annals of the south African Museum, 98, 27 pp.

Niemi, T.M. Ben-Avraham, et al., 2000. Post-Eocene seismic stratigraphy of the deep ocean basin adjacent to the southeast African continental margin: a record geostrophic bottom-current systems. Mar. Geol. 162, 237-258

0

Abbreviations:

AB = Agulhas Basin, ABK = Agulhas Bank, AP = Agulhas Plateau, AR = Agulhas Ridge, MB = Mozambique

Basin,

MR = Mozambique Ridge,

NV = Natal Valley, TB = Transkei Basin, TR = Tugela River.

(a)

(b)

Fig. 2. Parasound profile along Line AWI-20050009. The Agulhas Drift and the suggested flow direction are shown.

Model Results

Fig. 4. The averaged potential temperature and salinity at the vertical section along the profile AWI-2005009 for one model year .

2) Hydrographic results in the model are stable.

The Agulhas Current is found in the surface layers with potential temperatures of around 20° C and salinities of more than 35. NADW in this region has lower temperatures of about 2°

C, and higher salinities of about 34.8 at depth between 2000m and 3500m. AABW has tem- peratures of 0.1 to 1.1° C and salinities of 34.6 to 34.7. These results agre quite well with the reference data in the literature.

During eddy

After eddy

Fine Sand (Sd = 0.05 mm) Middle Silt (Sd = 0.005 mm) Fine Sand (Sd = 0.05 mm) Middle Silt (Sd = 0.005 mm)

Fig. 5. The sediment concentrations in the water column at the vertical section along the profile AWI-2005009 (Fig. 1a, Fig. 2 and Fig. 6).

Fig. 6. The sediment concentration and the velocity field along the bottom layer of the model (Fig. 3b). The position of the parasound profiles are labed as red lines and the Line AWI-2005009 (Fig. 2) is marked as pink line.

The direction of the currents Agulhas Drift

S N

4700 4600 4500 4400

Water Depth [m]

3) Furthermore, these simulations show that the sediment transport patterns along the Agulhas Drift are strongly influenced by mesoscale eddies . In general, sediment resuspension occurs during the eddy

events, and materials are transported away from the Transkei Basin if the eddy breaks up.

Horizontal Vertical

Distance [km]

Depth [m]

The Averaged Concentration of Fine Sand form Feb. to Mar. at section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

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Before eddy

Distance [km]

Depth [m]

The Averaged Concentration of Fine Sand for Jan at section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

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Csed [mg/l]

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Profile AWI-200509

Profile AWI-200509

Distance [km]

Depth [m]

The Averaged Concentration of Fine Sand form April to May at section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

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Csed [mg/l]

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Profile AWI-200509

Distance [km]

Depth [m]

The Averaged Concentration of Middle Silt form April to May at section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

0 50 100 150 200 250 300

−4500

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−500

Csed [mg/l]

0 1 2 3

Profile AWI-200509 Distance [km]

Depth [m]

The Averaged Concentration of Middle Silt form Feb. to Mar. at section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

0 50 100 150 200 250 300

−4500

−4000

−3500

−3000

−2500

−2000

−1500

−1000

−500

Csed [mg/l]

0 1 2 3

Profile AWI-200509 Distance [km]

Depth [m]

The Averaged Concentration of Middle Silt for Jan at section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

0 50 100 150 200 250 300

−4500

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Csed [mg/l]

0 1 2 3

Profile AWI-200509

Distance [km]

Depth [m]

The Averaged Salinity of one model year at the section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

0 50 100 150 200 250 300

−4500

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−500

PSU

34.5 34.6 34.7 34.8 34.9 35 35.1 35.2 35.3 35.4

>35.5

Distance [km]

Depth [m]

The Averaged Temperature of one model year at the section (29.81 °E, 37.75 °S ~ 29.35 °E, 34.75 °S)

Agulhas Drift

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−4500

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T C °

0 2 4 6

>8

108 km

(See also

http://www.brest.ird.fr/Roms_tools/)

Agulhas Drift Model (ADM) configuration: Sediment Transport Model (SMT) configuration:

Based on current velocities adopted from the ADM model, sediment transport pathways are calculated

SMT configuration -> see ADM

Five different sediment classes are tested:

d [mm] 0.125 0.05 0.0125 0.005 0.0025 ρ_s [kg/m³] 2650 2650 2650 2650 2650 W_s[mm/s] 9.4 1.6 0.10 0.02 0.004 E_u [kg/m²s] 2.5*10^-4 4.2*10^-5 2.6*10^-6 4.2*10^-7 1.0*10^-7

τ_cr [N/m²] 0.15 0.10 0.05 0.02 0.01

d: Diameter of grain size class W_s: Settling velocity of size class

τ_cr: Critical shear stress for sediment motion

ρ_s: Density of sediment material of size class E_u: Erosion rate of size class

Profile AWI-200509

Profile AWI-200509

As a result, we found, that the middle sands with a diameter of 0.125 mm are carried out of Transkei Basin after the first half model year. Thus, the middle sand can not be deposited in the Transkei Basin. The sediment concentrations of two grain classes in the water column, before, during and after an eddy event are compared in Fig. 5. and Fig. 6.

Initial concentration (spatially uniform): 4 mg/l Porosity p=0.4

1) The ADM model reproduces the strong Agulhas cur- rent in the vicinity of the Agulhas Bank. NADW and AABW flow eastward through the Agulhas Passage into Transkei Basin (Fig. 6). The current system in the model agrees well with the scheme of currents in the Fig. 1.

(b)

VE = 288