The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations

Abstract

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.

A New Improved Estimation of Agulhas Leakage Using Observations and Simulations of Lagrangian Floats and Drifters

“…It is forced by the COREv2 (Large & Yeager, ) winds and fluxes that build on the NCEP reanalysis data. Schwarzkopf et al () find that INALT20 compares well to satellite altimeter data, specifically in terms of SSH variance, and although they find a broader, shallower Agulhas Current, its transport (76 Sv) is comparable to the observed transport (77 Sv) found in Beal et al (). We use the 5‐day outputs of the mean velocity fields to run our Lagrangian experiments.…”

“…The second model we use is INALT20, a nested ocean only model that covers the domain 63°S–10°N and 70°W–70°E at 1/20° horizontal resolution embedded in a 1/4° global grid (Schwarzkopf et al, ). Similarly to CCSM4, INALT20 is on a C‐grid and has 46 layers with a vertical resolution of 6 to 20 m layer thickness in the first 100 m, increasing to 250 m at the deepest layers.…”

Submesoscale Impacts on Mesoscale Agulhas Dynamics

“…The free‐slip condition was nevertheless used for the coarse‐resolution host grid as else the simulated boundary current transports are too weak. For the high‐resolution nest of a comparable (1/20)° configuration, Schwarzkopf et al () found only minor changes of the AC and AUC transports when the sidewall boundary condition is switched from free to no slip. Thus, for this study, the no‐slip lateral boundary is used for the nests.…”

“…In this study, we investigate the impact of submesoscale flows on the dynamics of mesoscale eddies in the Agulhas region via model comparison. We use a nested modeling approach to achieve a submesoscale‐permitting horizontal resolution: the implementation of two‐way interacting grid refinements (“nests”) for the greater Agulhas region into global ocean general circulation models with coarser resolution (“hosts”) (Biastoch et al, ; Holton et al, ; Schwarzkopf et al, ). Several of such configurations have been developed within the INALT family (Schwarzkopf et al, ) with horizontal grid refinements of down to (1/10)° and (1/20)° (i.a., INALT20r), as well as a double‐nested configuration with (1/60)° horizontal resolution (INALT60).…”

“…We use a nested modeling approach to achieve a submesoscale‐permitting horizontal resolution: the implementation of two‐way interacting grid refinements (“nests”) for the greater Agulhas region into global ocean general circulation models with coarser resolution (“hosts”) (Biastoch et al, ; Holton et al, ; Schwarzkopf et al, ). Several of such configurations have been developed within the INALT family (Schwarzkopf et al, ) with horizontal grid refinements of down to (1/10)° and (1/20)° (i.a., INALT20r), as well as a double‐nested configuration with (1/60)° horizontal resolution (INALT60). In this study, we compare a nonsubmesoscale‐permitting INALT20r simulation to parallel experiments where we gradually increase the vertical resolution, decrease the diffusion and the dissipation of the model, and increase the horizontal resolution to resolve more and more small‐scale and submesoscale flows.…”