Modelling of the Surface Stokes drift in the Agulhas current system

Abstract. The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration.

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Satellite infra-red images in the Agulhas Current System

Deep Sea Research ◽

10.1016/0146-6291(78)90642-2 ◽

1978 ◽

Vol 25 (6) ◽

pp. 543-548 ◽

Cited By ~ 66

Author(s):

T.F.W. Harris ◽

R. Legeckis ◽

D. van Forest

Keyword(s):

Current System ◽

Agulhas Current ◽

Infra Red ◽

Infra Red Images

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Latent Heat Flux in the Agulhas Current

Remote Sensing ◽

10.3390/rs11131576 ◽

2019 ◽

Vol 11 (13) ◽

pp. 1576 ◽

Cited By ~ 1

Author(s):

Arielle Stela Imbol Nkwinkwa N. ◽

Mathieu Rouault ◽

Johnny A. Johannessen

Keyword(s):

Heat Flux ◽

Wind Speed ◽

Annual Cycle ◽

Current System ◽

Surface Wind ◽

Weather Forecast ◽

Specific Humidity ◽

Medium Range Weather Forecast ◽

Agulhas Current ◽

Medium Range

In-situ observation, climate reanalyses, and satellite remote sensing are used to study the annual cycle of turbulent latent heat flux (LHF) in the Agulhas Current system. We assess if the datasets do represent the intense exchange of moisture that occurs above the Agulhas Current and the Retroflection region, especially the new reanalyses as the former, the National Centers for Environmental Prediction Reanalysis 2 (NCEP2) and the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis second-generation reanalysis (ERA-40) have lower sea and less distinct surface temperature (SST) in the Agulhas Current system due to their low spatial resolution thus do not adequately represent the Agulhas Current LHF. We use monthly fields of LHF, SST, surface wind speed, saturated specific humidity at the sea surface (Qss), and specific humidity at 10 m (Qa). The Climate Forecast System Reanalysis (CFSR), the European Centre for Medium-Range Weather Forecast fifth generation (ERA-5), and the Modern-Era Retrospective analysis for Research and Applications version-2 (MERRA-2) are similar to the air–sea turbulent fluxes (SEAFLUX) and do represent the signature of the Agulhas Current. ERA-Interim underestimates the LHF due to lower surface wind speeds than other datasets. The observation-based National Oceanography Center Southampton (NOCS) dataset is different from all other datasets. The highest LHF of 250 W/m2 is found in the Retroflection in winter. The lowest LHF (~100 W/m2) is off Port Elizabeth in summer. East of the Agulhas Current, Qss-Qa is the main driver of the amplitude of the annual cycle of LHF, while it is the wind speed in the Retroflection and both Qss-Qa and wind speed in between. The difference in LHF between product are due to differences in Qss-Qa wind speed and resolution of datasets.

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The generation of Langmuir circulations by an instability mechanism

Journal of Fluid Mechanics ◽

10.1017/s0022112077001980 ◽

1977 ◽

Vol 81 (2) ◽

pp. 209-223 ◽

Cited By ~ 101

Author(s):

A. D. D. Craik

Keyword(s):

Continuous Wave ◽

Current System ◽

Plane Waves ◽

Wave Spectrum ◽

Stokes Drift ◽

Instability Mechanism ◽

Fluid Layers ◽

The Mean ◽

Langmuir Cells ◽

Relationship Of

Equations governing the current system in the upper layers of oceans and lakes were derived by Craik & Leibovich (1976). These incorporate the dominant effects of both wind and waves. Solutions comprising the mean wind-driven current and a system of ‘Langmuir’ cells aligned parallel to the wind were found for cases in which the wave field consisted of just a pair of plane waves. However, it was not clear that such cellular motions would persist for the more realistic case of a continuous wave spectrum.The present paper shows that, in the latter case, infinitesimal spanwise periodic perturbations will grow on account of an instability mechanism. Mathematically, the instability is closely similar to the onset of thermal convection in horizontal fluid layers. Physically, the mechanism is governed by kinematical processes involving the mean (Eulerian) wind-driven current and the (Lagrangian) Stokes drift associated with the waves. The relationship of this mechanism to instability models of Garrett and Gammelsrød is clarified.

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Modulation of the Agulhas Current Retroflection and Leakage by Oceanic Current Interaction with the Atmosphere in Coupled Simulations

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0168.1 ◽

2017 ◽

Vol 47 (8) ◽

pp. 2077-2100 ◽

Cited By ~ 26

Author(s):

Lionel Renault ◽

James C. McWilliams ◽

Pierrick Penven

Keyword(s):

Energy Transfer ◽

Marine Boundary Layer ◽

Current System ◽

Surface Current ◽

Current Feedback ◽

Agulhas Current ◽

Benguela Upwelling ◽

Oceanic Eddy ◽

Ocean Surface Current ◽

Mean Circulation

AbstractCoupled ocean–atmosphere simulations are carried out for the Mozambique Channel, the Agulhas Current system, and the Benguela upwelling system to assess the ocean surface current feedback to the atmosphere and its impact on the Agulhas Current (AC) retroflection and leakage. Consistent with previous studies, the authors show that the current feedback slows down the oceanic mean circulation and acts as an oceanic eddy killer by modulating the energy transfer between the atmosphere and the ocean, reducing by 25% the mesoscale energy and inducing a pathway of energy transfer from the ocean to the atmosphere. The current feedback, by dampening the eddy kinetic energy (EKE), shifts westward the distribution of the AC retroflection location, reducing the presence of eastern retroflections in the simulations and improving the realism of the AC simulation. By modulating the EKE, the AC retroflection and the Good Hope jet intensity, the current feedback allows a larger AC leakage (by 21%), altering the water masses of the Benguela system. Additionally, the eddy shedding is shifted northward and the Agulhas rings propagate less far north in the Atlantic. The current–wind coupling coefficient sw is not spatially constant: a deeper marine boundary layer induces a weaker sw. Finally the results indicate that the submesoscale currents may also be weakened by the current feedback.

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