Motions in the Oceans: Potential and Kinetic Energy and Turbulent Dissipation

Abstract

Kinetic energy (KE) and available potential energy (APE) in the ocean are fundamental to processes such as mesoscale eddies, tides, internal gravity waves, dissipation, and the mixing fields that drive circulation in the ocean. This dissertation examines three different sub-topics in the general realm of oceanic energetics. We examine the KE and APE in state-of-the-art global numerical ocean models and in observations, across a wide range of time scales. Lastly, we use a novel dataset to quantify the temporal, geographical, and spatial variations of turbulent dissipation.

Global maps of the mesoscale eddy available potential Energy (EAPE) field are made from a high-resolution 1/12.5 degree global ocean model. Maps made from both a free-running simulation and a data-assimilative reanalysis of the HYbrid Coordinate Ocean Model (HYCOM) are compared with maps made using Argo profiles. All maps display similar features, especially in the dominance of western boundary currents. The reanalysis maps match the Argo maps more closely, demonstrating the importance of data assimilation. Global averages of the simulation, reanalysis, and Argo EAPE all agree to within about 10 percent. The model and Argo EAPE fields are compared with EAPE computed from a dataset of “Moored Historical Observations" (MHO) in conjunction with a global climatology. At MHO locations, 15-32 percent of the EAPE in the Argo estimates is due to aliased motions having periods of 10 days or less. Spatial-averages of EAPE in HYCOM, Argo, and MHO data, agree to within 50 percent at MHO locations, with both model estimates lying within error bars of observations. Analysis of the EAPE field in an idealized model suggests that much of the scatter seen in comparisons of different EAPE estimates is expected given the chaotic nature of mesoscale eddies.

Temperature variance and KE from two simulations of HYCOM (1/12, 1/25 degree) and three simulations of the Massachusetts Institute of Technology general circulation model (MITgcm; 1/12, 1/24, and 1/48 degree) are compared with the MHO dataset. The variances are computed across frequencies ranging from the supertidal to the subtidal. Improvement of temperature variance and KE with resolution varies greatly between the models, and within each frequency band. Results suggest that model resolution is most important for the supertidal band. HYCOM generally is more correlated with the MHO, and handles supertidal, semidiurnal, and diurnal velocities in a number of specific near-shelf high-velocity locations better than MITgcm does, possibly due to wave-drag. Additionally, we compare both HYCOM 1/25 degree and MITgcm 1/48 degree geostrophic eddy kinetic energy (EKE) with EKE computed from AVISO, and find that in bulk, both models compare well.

Estimates of the turbulent kinetic energy dissipation rate are made from analysis of thermistor chains at five moored locations near Palau. Moorings are located near steep topographical features, and in the far field. Long durations, fast sampling intervals, high vertical resolution, and the horizontal spread of the five moorings provide both a spatial and temporal picture of turbulent processes. Signals in turbulent dissipation have strong associations with a wide range of dynamic processes, such as mesoscale eddies, submesoscale fronts, near-inertial oscillations, spring-neap cycles, and tidal motions. We find the time-mean turbulent kinetic energy dissipation rate to decay from 10^-7 (W/kg) close to topography, to 10^-10 (W/kg) at a distance of about 35 km. Time-mean vertical profiles show bottom-enhanced dissipation, and elevated dissipation in the upper water-column.