Resolution convergence and sensitivity studies with North Atlantic circulation models. Part I: The western boundary 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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Weak Thermocline Mixing in the North Pacific Low-Latitude Western Boundary Current System

Geophysical Research Letters ◽

10.1002/2017gl075210 ◽

2017 ◽

Vol 44 (20) ◽

pp. 10,530-10,539 ◽

Cited By ~ 11

Author(s):

Zhiyu Liu ◽

Qiang Lian ◽

Fangtao Zhang ◽

Lei Wang ◽

Mingming Li ◽

...

Keyword(s):

North Pacific ◽

Current System ◽

Western Boundary Current ◽

Western Boundary ◽

Boundary Current ◽

Low Latitude ◽

The North ◽

The North Pacific

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The Deep Western Boundary Current and Adjacent Interior Circulation at 24°–30°N: Mean Structure and Mesoscale Variability

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0094.1 ◽

2020 ◽

Vol 50 (9) ◽

pp. 2735-2758

Author(s):

Tiago Carrilho Biló ◽

William E. Johns

Keyword(s):

North Atlantic ◽

Western Boundary Current ◽

Western Boundary ◽

Boundary Current ◽

Deep Western Boundary Current ◽

Mean Structure ◽

The Mean ◽

Theoretical Evidence ◽

Anticyclonic Eddies ◽

Geostrophic Velocities

AbstractThe mean North Atlantic Deep Water (NADW, 1000 < z < 5000 m) circulation and deep western boundary current (DWBC) variability offshore of Abaco, Bahamas, at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-yr-long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24° and 30°N and 72° and 77°W. The southward-flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv (1 Sv ≡ 106 m3 s−1). Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5 to 16 Sv. Current meter records spanning 2008–17 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: 1) periods of 250–280 days that occur regularly throughout the time series and 2) energetic oscillations with periods between 400 and 700 days that occur sporadically every 5–6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24°–30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20° and 26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby waves reflecting from the western boundary.

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Dynamics of Deep Recirculation Cells Offshore of the Deep Western Boundary Current in the Subtropical North Atlantic (15°–30°N)

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0184.1 ◽

2021 ◽

Vol 51 (1) ◽

pp. 131-145

Author(s):

Tiago Carrilho Biló ◽

William E Johns ◽

Jian Zhao

Keyword(s):

North Atlantic ◽

Primary Source ◽

Water Layer ◽

Western Boundary Current ◽

Western Boundary ◽

Boundary Current ◽

Study Region ◽

Deep Western Boundary Current ◽

Deep Water Layer ◽

Circulation Patterns

AbstractThe dynamics of the deep recirculation offshore of the deep western boundary current (DWBC) between 15° and 30°N within the upper North Atlantic Deep Water layer (1000 ≤ z ≤ 3000 m) is investigated with two different eddy-resolving numerical simulations. Despite some differences in the recirculation cells, our assessment of the modeled deep isopycnal circulation patterns (36.77 ≤ σ2 ≤ 37.06 kg m−3) shows that both simulations predict the DWBC flowing southward along the continental slope, while the so-called Abaco Gyre and two additional cyclonic cells recirculate waters northward in the interior. These cells are a few degrees wide, located along the DWBC path, and characterized by potential vorticity (PV) changes occurring along their mean streamlines. The analysis of the mean PV budget reveals that these changes result from the action of eddy forcing that tends to erode the PV horizontal gradients. The lack of a major upper-ocean boundary current within the study region, and the fact that the strongest eddy forcing is constrained within a few hundreds of kilometers of the western boundary, suggest that the DWBC is the primary source of eddy forcing. Finally, the eddies responsible for forcing the recirculation have dominant time scales between 100 and 300 days, which correspond to the primary observed variability scales of the DWBC transport at 26.5°N.

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