Ocean gyres driven by buoyancy forcing
For seventy years oceanographers have assumed that atmospheric winds drive the midlatitude gyres. These gyres are intensified on the western boundary of the ocean, giving rise to rapid currents, such as the Gulf Stream, which play a major role in earth’s climate by transporting heat to high latitudes. The physical mechanism underlying wind-driven gyres is that the spatial variation of the wind stress field injects torque, or potential vorticity, to the upper ocean. However, model results have highlighted the role of surface buoyancy fluxes (principally heating and cooling of the ocean surface) in enhancing gyre variability. Here we present results from novel numerical simulations which show that ocean gyres can be driven by surface buoyancy fluxes alone. We explore this phenomenon through a combination of direct numerical simulation and high-resolution, three-dimensional ocean modeling, along with linear theory to demonstrate that ocean gyres must balance with surface buoyancy fluxes as well as wind stress. we conclude that the strength of gyres, and the corresponding heat transport, will be directly influenced by surface warming in response to climate change.