Ocean gyres and surface buoyancy forcing

Large-scale ocean circulation modulates weather and climate patterns by distributing heat, nutrients, and carbon dioxide within and across ocean basins. The large-scale circulation is driven by processes at the ocean's surface (such as wind stress and heat/freshwater fluxes) and steered by processes in the ocean's interior (such as mesoscale eddies and flow-topography interactions).

Ocean gyres are generally thought to be driven by wind stress at the ocean's surface, however recent results have suggested that surface buoyancy fluxes may also contribute to, or at least modulate, the strength of the gyres. In this work, we present results from a series of ocean model simulations in which we independently estimate the effects of wind stress and surface buoyancy fluxes on gyre transport. We find that surface buoyancy fluxes control the near-surface density gradients, which in turn affect the gyre circulation. The relationship between surface heat flux gradients and the gyre circulation is linear for timescales shorter than a decade, after which the relationship becomes non-linear due to density advection by the circulation. The relative importance of wind and buoyancy forcing is different for subtropical and subpolar gyres, with the subpolar region exhibiting a more complex range of flow-topography interactions and stratification feedbacks.

Our work emphasizes the under-appreciated role of surface buoyancy fluxes in steering the circulation of large-scale oceanic gyres, with implications for how these gyres, and thus regional climate, may change in the future.