Abyssal Circulation

abyssal_circulationSection across a fracture zone canyon in the abyssal Brazil Basin showing turbulent dissipation levels (in color) from measurements. The shaded topography follows the deepest bathymetry of the fracture zone, while the upper contour shows the shallowest level of ridge topography. An inverse estimate of the circulation streamlines (dashed contours) is shown relative to the density field (white contours). Turbulent mixing in the canyon drives upwelling toward the Mid-Atlantic Ridge axis (St. Laurent, Schmitt and Toole 2001).

The densest waters found in the ocean interior are formed at several high-latitude sites where atmospheric forcing causes surface water temperature and salinity conditions to match those of the deepest waters. These waters spill into the abyss and are carried equatorward by a system of deep boundary currents. Being far removed from the wind driven Ekman layer, processes such as tides and buoyancy forcing by turbulent mixing often contribute significantly to the flow dynamics. Flow interaction with topography, such as ridges and seamounts, is also an important factor in abyssal circulation physics.

 Internal Tides

internal_tidesEnergy flux of the internal tides as estimated from a parameterization of internal wave drag in a forward model for the barotropic tides. Globally, deep ocean internal tides extract roughly 1 Terrawatt of energy from the surface tide (Jayne and St. Laurent 2001).

Internal waves are an ubiquitous class of oceanic phenomena, where wave motion transfers energy and momentum in the deep interior of the sea. Internal tides are generated at regions where the barotropic tidal current encounters variations in bottom topography. The resulting waves arise at the dominant tidal bands. The principle lunar semidiurnal (M2) and the lunar-solar diurnal (K1) are generally the most significant constituents in many locations. Since the diurnal and Coriolis frequencies are equal at roughly 30°, freely radiating diurnal internal tides are possible only equatorward of this latitude. This accounts for roughly half the ocean area, and semidiurnal internal tides are freely radiating over nearly all of the oceans, to 75° latitude.  The internal wave energy spectrum is generally “red,” with most energy tied to low baroclinic modes of the internal tide with horizontal wavelengths between 10 and 100 km. At these scales, waves are very stable. A considerable succession of physical processes must be implicated to cascade low-mode energy to the scales where instability can act to produce turbulence.

Model Parameterizations
parameterizationEstimates of turbulent diffusivity across the South Atlantic based on a parameterization for mixing by internal tide energy. The section spans from west to east across the Brazil and Angola Basins. The shaded topography follows the deepest bathymetry of the Mid Atlantic Ridge system, while the upper contour shows the shallowest level of Ridge topography. The apexes of the Mid-Atlantic and Walvis Ridges are apparent at 15°W and 5°W, respectively (St. Laurent, Simmons, and Jayne 2002).

Models of the ocean circulation, and of the Earth’s climate, involve numerical solutions to the dynamical equations for momentum, heat and energy. Due to the computational complexity of this task, these equations are typically limited to describing only a subset of the full problem. In particular, temporal resolution is often limited to time-scales longer than about a day, and spatial-scales of about 10 km. Processes occurring on shorter time or spatial scales, such as internals waves, tidal processes, or turbulent mixing, must be incorporated into these models using a method of representation known as parameterization. The unresolved processes are represented by formulas involving parameters that govern the resolved flow. These formulas are derived using results from measurement, theory, and modeling.

Ocean Energetics

OceanenergeticsThis schematic shows various avenues of energy flow between large-scale sources and dissipation. Some kinetic energy (KE) is dissipated in boundary layers, but some energy cascades to the scale of oceanic fine structure where it is used by turbulent processes to producing mixing and buoyancy flux. Potential energy (PE) also moves in an energy cascade, in some cases dissipating through convective process, but in other cases generating kinetic energy through the generation of mesoscale eddies (St. Laurent and Simmons 2006 ).

Ocean energetics relates to the flow of energy between different scales of motion down to the scale of turbulent dissipation and mixing. The ocean is a vast thermodynamic machine, powered by the sun, the winds, and the tides. There is no concept more fundamental to the ocean than energy; it characterizes the flow of waters and the transfer of heat between warm and cold regions. The flow of energy in the ocean system, together with the flow of energy in the atmosphere, controls the climate state of the Earth.  Studies focus on the transfer of energy from large and mesoscale oceanic flows to smaller dissipative scales. Internal waves are central to this process, as they are perhaps the only mechanism that acts in the range of scales between the mesoscale (eddies) and the turbulent processes that lead mixing and diffusion.


TyphoonObserving the Evolution of Typhoon Wakes, the transfer of heat from the ocean to the atmosphere is the primary driver of tropical cyclones. Viewed as a heat engine, a typhoon extracts heat energy from the warm ocean surface and exhausts it into the upper atmosphere where excess energy is radiated to space and transported toward higher latitudes.

Typhoons require high sea surface temperatures (SST) to form, but once formed a typhoon’s maximum strength is governed largely by the amount of energy it can extract from the underlying ocean. In general, the warmer the SST, the more intense the typhoon. However, the tropical ocean is strongly stratified with relatively warm surface waters, usually several tens of meters thick, overlaying colder subsurface waters. As a typhoon travels over the ocean it generates a cold-water wake produced both by strong vertical mixing of the warm surface water with colder water below and by extraction of heat through evaporation that fuels the typhoon. This cold water wake ultimately limits the typhoon’s strength by reducing its energy supply and similarly limits the intensity of succeeding typhoons that might follow in its path. Hurricanes have been identified as significant forcing events for driving ocean mixing in the upper ocean. Boos et al. (2004) used numerical simulations to study how the vertical mixing rate represented by sub-gridscale parameterizations in ocean models should be enhanced to account for the strong mixing associated with occasional hurricanes over the open ocean. They found that several strong hurricanes per year were enough to significantly elevate mixing levels, causing changes in the thermodynamic state of the upper ocean temperature-salinity relation that were enough to change the climatological state of the model. Sriver and Huber (2007) also have recently suggested that tropical cyclones are drivers of significant vertical mixing, and may be responsible for approximately 15% of the poleward ocean heat transport. Direct in situ measurements of the turbulent mixing in typhoons would useful in backing up the model results. The restratification of the cold wake left behind the typhoon resets the conditions in the upper ocean, and determines how much energy is available to subsequent typhoons that pass through the region. Some of this restratification is accomplished by air-sea heat flux, however, Fox-Kemper et al. (2007) have argued that strong baroclinic instability should occur after the mixing event and have proposed a parameterization for the restratification process by the baroclinic eddies. Observations of the relaxation of the typhoon’s wake would provide evidence of this process and a quantifiable test of this parameterization. While these recent studies have indicated the importance of mixing in the upper ocean driven by tropical cyclones, observations of mixing processes before, during, and after such storm events in the open ocean are nonexistent due to the difficulty in making observations at sea during foul weather. The few observations of the impact of hurricanes on turbulent properties are “before and after” snapshots of data, collected when summer cruises on the Mid- and North Atlantic Bight regions of the continental shelf were interrupted by tropical storms or hurricanes moving up the US coast (MacKinnon and Gregg, 2003).