Abstract
Abstract. The circulation in the North Atlantic subpolar gyre is complex and strongly influenced by the topography. The gyre dynamics are traditionally understood as the result of a topographic Sverdrup balance, which corresponds to a first-order balance between the planetary vorticity advection, the bottom pressure torque, and the wind stress curl. However, these dynamics have been studied mostly with non-eddy-resolving models and a crude representation of the bottom topography. Here we revisit the barotropic vorticity balance of the North Atlantic subpolar gyre using a new eddy-resolving simulation (with a grid space of ≈2 km) with topography-following vertical coordinates to better represent the mesoscale turbulence and flow–topography interactions. Our findings highlight that, locally, there is a first-order balance between the bottom pressure torque and the nonlinear terms, albeit with a high degree of cancellation between them. However, balances integrated over different regions of the gyre – shelf, slope, and interior – still highlight the important role played by nonlinearities and bottom drag curls. In particular, the Sverdrup balance cannot describe the dynamics in the interior of the gyre. The main sources of cyclonic vorticity are nonlinear terms due to eddies generated along eastern boundary currents and time-mean nonlinear terms in the northwest corner. Our results suggest that a good representation of the mesoscale activity and a good positioning of mean currents are two important conditions for a better representation of the circulation in the North Atlantic subpolar gyre.
Highlights
The North Atlantic subpolar gyre (SPG) is a key region for the meridional overturning circulation (MOC)
The aim of this paper is to investigate the dynamics of the SPG by analyzing the barotropic vorticity balance in a truly eddy-resolving σ -level coordinate model
We have studied the dynamics of the North Atlantic subpolar gyre in a numerical model with, for the first time, terrainfollowing coordinates and a mesoscale-resolving resolution ( x ≈ 2 km)
Summary
The North Atlantic subpolar gyre (SPG) is a key region for the meridional overturning circulation (MOC). The vertical structure of the currents is improved with a more barotropic structure for the boundary currents around the SPG (Marzocchi, 2015) These changes, compared to coarser-resolution models, allow the inertial effects to become more important and modify the interactions with the topography. Sonnewald et al (2019) clustered regions dominated by different barotropic vorticity balances using a global 1◦ × 1◦ model They retrieved the results of an SPG dominated by BPT effects, and a part of the gyre dominated by nonlinear (NL) effects, despite the relatively coarse resolution of the model. Yeager (2015) compared results from a 1◦ resolution model with an eddy-permitting 1/10◦ resolution model and noticed an increase in the amplitude of the NL term by a factor of 3 in some locations It did not significantly modify the first-order equilibrium between the wind, planetary vorticity, and BPT.
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