Asymmetric response of the North Atlantic gyres to the North Atlantic Oscillation

TL;DR

This study examines how the North Atlantic gyres respond asymmetrically to the North Atlantic Oscillation, revealing the dominant role of wind stress anomalies and other oceanic processes in shaping long-term circulation changes.

Contribution

It provides a detailed analysis of the physical processes, including wind stress, topography, and non-linear advection, that influence gyre responses to NAO variability.

Findings

Subtropical gyre intensifies by 0.90 Sv per NAO standard deviation

Subpolar gyre intensifies by 3.41 Sv per NAO standard deviation

Wind stress anomalies account for ~90% of subtropical gyre changes

Abstract

The North Atlantic Oscillation (NAO) is a leading mode of atmospheric variability, affecting the North Atlantic Ocean on sub-seasonal to multi-decadal timescales. The NAO changes the atmospheric forcing at the ocean's surface, including winds and surface buoyancy fluxes, both of which are known to impact large-scale gyre circulation. However, the relative role of other physical processes (such as mesoscale eddies and topography) in influencing gyre circulation under NAO variability is not fully understood. Here, we analyze a series of ocean--sea ice simulations using a barotropic vorticity budget to understand the long-term response of the North Atlantic gyre circulation to NAO forcing. We find that for each standard deviation increase in the NAO index, the subtropical and subpolar gyres intensify by 0.90 Sv and 3.41 Sv (1 Sv = 10$^6$ m$^3$ s$^{-1}$) respectively. The NAO-induced wind stress anomalies drive approximately 90\% of the change in the subtropical gyre's interior flow. However, in the subpolar gyre's interior, in addition to wind stress, flow-topography interactions, stratification (influenced by surface heat fluxes), and non-linear advection significantly influence the circulation. Along the western boundary the bottom pressure torque plays a key role in steering the flow, and the vorticity input by the bottom pressure torque is partly redistributed by non-linear advection. Our study highlights the importance of both atmospheric forcing and oceanic dynamical processes in driving long-term gyre circulation responses to the NAO.