Mixing-driven overturning circulation

At the high latitudes of the Arctic and Antarctic, cold, salty, and hence dense surface waters sink to fill the bottom of the ocean. Since seawater mass is conserved in the ocean, we know that these abyssal waters must return to the upper ocean elsewhere. Oceanographers originally theorized that these dense abyssal waters upwelled by mixing vigorously with lighter overlaying deep waters. This theory was subsequently challenged by observations of the vertical profile of ocean mixing. Turbulent ocean mixing was shown to be much too weak in most of the ocean but increased dramatically near the sea floor, where powerful internal waves break and mix the water column (see schematic to the right). Since the density of seawater increases with depth, deep interior ocean water mixes preferentially with denser waters and thus the mixing results in sinking, not upwelling, in the interior ocean. Only within a few dozen meters of the sea floor, where the sea floor inhibits mixing, do we expect ocean mixing to result in upwelling. In my research, I use theory and idealized models to explore how this bottom-intensified ocean turbulence interacts with sloping boundaries (ocean ridges and continental slopes) to bring these abyssal waters back towards the surface.

Ocean Mixing
A schematic of turbulent processes in the global ocean. I am particularly interested in abyssal ocean turbulence. [Source: GFDL]

Wind-driven overturning circulation

The mixing-driven upwelling described above can only upwell abyssal waters up to depths of about 2000m, above which wind-driven processes return deep waters to the surface. Easterly winds reach their maximum intensity in the land-free latitudes of the Southern Ocean, driving Ekman upwelling and the Antarctic Circumpolar Current (ACC). The winds pull deep waters to the surface along sloping surfaces of constant density (isopycnals). These sloping isopycnals (and the associated ACC) are generally unstable and spawn vigorous mesoscale eddies, act to slow the upwelling until the two strike a balance. Due to the treacherous conditions of the Southern Ocean and the difficulty of modelling mesoscale eddies, much of our current understanding of the wind-driven Southern Ocean overturning circulation comes from idealized models and theory. I use both high-resolution climate models and observations to further our understanding of the wind-driven overturning circulation.

Southern Ocean Circulation
A schematic of Southern Ocean circulation with arrows showing meridional (orange) and zonal (red) circulations. Surface processes transform upwelling Deep Waters into denser in the South and lighter Waters in the North. [Source: Lynne Talley]

Lagrangian analysis of geophysical flows

While geophysical flows are typically viewed from a fixed frame of reference (Eulerian perspective), it is often desirable to view geophysical flows from a frame of reference that follows the flow (Lagrangian perspective). The Lagrangian approach is particularly useful for determining how water (or anything carried by water) flows from one region to another or is transformed over time. Lagrangian analysis can be applied either to inherently Lagrangian observations (i.e. surface drifters or Argo floats) or to velocity field output from Ocean General Circulation Models by integrating in time. Applications of Lagrangian analysis include tracking of nutrients, larvae, microplastics, icebergs, oil droplets, etc. As a physical oceanographer, I am particularly interested in using Lagrangian analysis to track how seawater and tracers such as heat and salinity are moved around by ocean currents.

Particle trajectories of Circumpolar Deep Water (CDW)
Lagrangian trajectories of virtual water parcels in a coupled climate model show upwelling of water from the horizon at a depth of 2000 meters (purple) at 30 ° South to the surface of the ocean (yellow) around Antarctica.