Brian Arbic
University of Texas, Austin

Global energy dissipation rate of oceanic low-frequency flows by quadratic bottom drag: Results from observations and 1/32 degree models

The energy budget of the deep ocean is a subject of great current interest, because of the potential implications for the strength of the overturning circulation. An important energy source is the 1 TW wind power input into geostrophic (low-frequency) flows. How these flows dissipate is largely unknown.
Here we discuss one potential dissipation mechanism, namely bottom boundary layer drag. First, we show that eddy kinetic energy in idealized geostrophic turbulence models has realistic length scales, vertical structure, and amplitudes only if the nondimensional bottom friction strength is of order one. This is true for both linear and quadratic bottom drag (Arbic and Scott, in press). Next, we estimate the global energy dissipation rate of oceanic low-frequency flows by quadratic bottom drag (Sen et al., in review). We use data from 290 moored near-bottom current meters. We also utilize satellite altimetry data, 1) to estimate the bias in the global integral due to the poor sampling of the ocean by current meters, and 2) to estimate global maps of bottom velocity, given relationships computed at the mooring locations between surface and bottom flows. Finally, the dissipation rate is estimated from the output of a global ocean model run at 1/32 degree resolution (Arbic et al., in review). The model is validated by comparison to the current meter dataset. Our estimates of the dissipation rate range from 0.14 to 0.83 TW, with both the low and high ends likely being unrealistic. While uncertainty remains, we can say that bottom boundary layers very likely dissipate a substantial fraction of the 1 TW wind-power input into geostrophic flows.