Ocean Gyres Driven by Wind Stress and Surface Buoyancy Forcing

Abstract

Large-scale ocean circulation is central in modulating 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 winds and heat and freshwater fluxes) and steered by processes in the ocean's interior (such as mesoscale eddies and flow-topography interactions). Climate change is expected to alter these surface forcings. Despite the central role of ocean circulation in transporting heat and regulating climate, the impact of physical processes both at the ocean's surface and in the interior on the large-scale ocean circulation -- especially in the context of climate change -- remains inadequately understood. In this thesis, we focus on two large-scale circulatory ocean features: (i) the gyres (primary focus of the thesis), and (ii) the Atlantic Meridional Overturning Circulation (AMOC).

Ocean gyres are canonically believed to be driven by wind stress, while the AMOC is thought to be controlled by surface buoyancy forcing (i.e., heat and freshwater fluxes). Using a series of ocean model simulations, we find that in addition to wind stress driving gyres, surface heat 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. A major focus of this thesis is the North Atlantic basin, where the heat carried from the tropics to the Arctic by the subtropical gyre and the AMOC is sensitive to both changes in winds and surface heat fluxes. On timescales shorter than a decade, changes in each circulation's heat transport occurs primarily through anomalies in the circulation strength. On multidecadal timescales, changes in the ocean's temperature feed back to modulate the heat transport response.

Last, we focus on a key atmospheric mode of variability, the North Atlantic Oscillation (NAO), and how it affects the ocean circulation. While the NAO impacts the gyres through changes in winds and surface buoyancy fluxes, the relative importance of processes at the ocean's surface and in the interior in shaping the gyres is not fully understood. We find that NAO-induced changes in the surface wind stress drive more than 90\% of the variability in the subtropical gyre. In contrast, in addition to wind stress, the NAO-induced variability in the subpolar gyre is governed by several processes: mesoscale eddies, flow-topography interactions and the ocean's stratification (influenced by surface heat fluxes).

Our work emphasizes the under-appreciated role of surface buoyancy fluxes in driving oceanic gyre circulations, with implications for how the gyres, and thus regional climate, may change in the future.