Energetics of Ocean Circulation

Ocean circulation is characterized by strong currents and turbulent eddies with horizontal length scales of a few 10s of km.  This motion is forced at larger scales by air-sea momentum fluxes and is dissipated at cm scales by viscous effects.  There is thus a factor of about 108 between the scale at which energy is added and that at which it is removed.  Understanding the mechanisms allowing for energy transfer over this impressive range of scales has been a focus of my research.

Energy transfer from the basin scale to the mesoscale:

This is fairly well understood in terms of geostrophic dynamics; however, our recent work has pointed to a double cascade of energy in which a ‘beta' term (related to the Coriolis parameter varying with latitude) cascades energy forward to small scales while nonlinearity provides a compensating inverse energy cascade.

Nadiga, B. and D.N. Straub, 2009, Alternating zonal jets and energy fluxes in barotropic wind-driven gyres, Ocean Modelling, 33, 258-269.

Ocean surface velocity dependence in the wind stress:

Wind stress is often parameterized as a function of the near-surface winds (e.g., via a quadratic drag law or similar).  More correctly, the stress depends on the air-sea velocity difference.  This amounts to a relatively small correction for the stress (i.e., for the air-sea momentum flux); however, our work shows that the effect on the wind power source can be much larger: accounting for this effect leads to about a 30% reduction in the wind power source.  This has subsequently been confirmed using satellite altimetry and scatterometer data. 

Duhaut, T. H. A. and D. N. Straub, 2006, Wind Stress Dependence on Ocean Surface Velocity: Implications for Mechanical Energy Input to Ocean Circulation, J. Phys. Oceanogr., Vol. 36, #2, 202-211.

Interaction between geostrophic and near-inertial motions:

It is well known that high frequency near-inertial oscillations lie super-imposed on the geostrophic flow, but is generally thought that these near-inertial motions do little to influence the geostrophic currents.  We  consider these interactions in the context of an idealized ocean wind-driven mid-latitude gyre, and show that geostrophic-to-inertial energy transfers can play a significant role in the overall energetics.  With PhD student S Taylor, we are revisiting these ideas in a more realistic setting.  Also, research listed under `loss of balance' explores related ideas.

Gertz, Aaron and David N. Straub, 2009, Effects of near-inertial motion on the midlatitude double gyre problem, J. of Phys. Oceanogr.

Reduction of the power source by rearrangement of the large scale flow:

Typically the problem of ocean circulation is posed such that the momentum input is specified, meaning that the power input is part of the solution.  Given the inverse energy cascade of two-dimensional turbulence, one expects little dissipation in the high Reynolds number limit, except via bottom drag or a loss of balance mechanism.  We considered whether wind-driven circulation in the absence of bottom friction might `run away' (develop very large velocities) or arrange itself so that the wind power input vanishes (so that no energy dissipation is required). 

Scott, R. and D. N. Straub 1998. Small viscosity behaviour of a homogeneous quasi- geostrophic ocean circulation model, J. of Marine Res.,