  Gyres are central features of large-scale ocean circulation and are involved
in transporting tracers such as heat, nutrients, and carbon-dioxide within and
across ocean basins. Traditionally, the gyre circulation is thought to be
driven by surface winds and quantified via Sverdrup balance, but it has been
proposed that surface buoyancy fluxes may also contribute to gyre forcing.
Through a series of eddy-permitting global ocean model simulations with
perturbed surface forcing, the relative contribution of wind stress and surface
heat flux forcing to the large-scale ocean circulation is investigated,
focusing on the subtropical gyres. In addition to gyre strength being linearly
proportional to wind stress, it is shown that the gyre circulation is strongly
impacted by variations in the surface heat flux (specifically, its meridional
gradient) through a rearrangement of the ocean's buoyancy structure. On shorter
timescales ($\sim$ decade), the gyre circulation anomalies are proportional to
the magnitude of the surface heat flux gradient perturbation, with up to $\sim
0.15\,\mathrm{Sv}$ anomaly induced per $\mathrm{W}\,\mathrm{m}^{-2}$ change in
the surface heat flux. On timescales longer than a decade, the gyre response to
surface buoyancy flux gradient perturbations becomes non-linear as ocean
circulation anomalies feed back onto the buoyancy structure induced by the
surface buoyancy fluxes. These interactions complicate the development of a
buoyancy-driven theory for the gyres to complement the Sverdrup relation. The
flux-forced simulations underscore the importance of surface buoyancy forcing
in steering the large-scale ocean circulation.
