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 increase towards the sea floor, where powerful internal waves break and mix the water column (see schematic to the right). Since the density of seawater increases monotonically 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 meters of the sea floor, where water can only mix upwards with lighter waters, 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 brings these abyssal waters upwards in the water column.
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 a zonal current, the Antarctic Circumpolar Current, as well as a divergence in surface waters. The surface water divergence (Ekman suction) pulls deep waters to the surface along sloping surfaces of constant density (isopycnals). Baroclinic instabilities associated with the sloping isopycnals manifest themselves as vigorous mesoscale eddies, which in turn oppose the overturning. Due to the treacherous conditions of the Southern Ocean and the difficulty of modelling dynamically important 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.
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. 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 advected in the Ocean.