Congratulations to Conrad Luecke who defended his dissertation on September 2, 2018

Advisor: Brian Arbic

Abstract:

Reservoirs of 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. In parts (1) and (2) we examine the KE and Eddy Available Potential Energy (EAPE) in state-of-the-art global numerical ocean models and in observations, across a wide range of time scales. In part (3) we use a novel dataset to quantify the temporal, geographical, and vertical variations in dissipation.

 

(1) Global maps of the mesoscale Eddy Available Potential Energy (EAPE) field at a depth of 500m are created using potential density anomalies in 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 by other researchers from density anomalies in Argo profiles. The HYCOM and Argo maps display similar features, especially in the dominance of western boundary currents. The reanalysis maps match the Argo maps more closely, demonstrating the added value of data assimilation. Global averages of the simulation, reanalysis, and Argo EAPE all agree to within about 10%.

The model and Argo EAPE fields are compared to EAPE computed from temperature anomalies in a dataset of “Moored Historical Observations" (MHO) in conjunction with buoyancy frequencies computed from a global climatology. The MHO dataset allows for an estimate of the EAPE in high-frequency motions that is aliased into the Argo EAPE values. At MHO locations, 15-32% 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% at MHO locations, with both model estimates lying within error bars of observations.

Analysis of the EAPE field in an idealized model, in conjunction with published theory, suggests that much of the scatter seen in comparisons of different EAPE estimates is to be expected given the chaotic, unpredictable nature of mesoscale eddies.

 

(2) This study aims to quantify the effect of increasing model resolution on the model energetics across a wide range of frequencies. Temperature variance and kinetic energy (KE) from two simulations of the HYbrid Coordinate Ocean Model (HYCOM) at different horizontal grid spacings

(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 temperature variance and KE from a database consisting of ~3,000 moored historic observations (MHO). The variances are computed across frequencies ranging from the super tidal to the sub tidal. In both HYCOM and MITgcm, modeled temperature variance and KE tend to improve with resolution. Improvement with frequency varies greatly between the models, and within each frequency band. For instance, both models improve with resolution in the super tidal band, but show less improvement with resolution in the near-inertial band. The energy levels of the MITgcm simulations show closer agreement with observations in the super tidal band. HYCOM generally is more correlated with the MHO on a point to point basis, and handles supertidal, semidiurnal, and diurnal velocities in a small number of specific near-shelf high-velocity locations better than MITgcm does.

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. HYCOM is better correlated with AVISO globally, while MITgcm correlates slightly higher when compared to AVISO at the MHO points. This study aims to quantify the effect of increasing model resolution on the model energetics across a wide range of frequencies.

 

(3) Estimates of the turbulent kinetic energy dissipation ε 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 ε to decay from 10-7 (W/kg) close to topography, to 10-10 (W/kg) in the far field, over a distance of about 35 km. Time-mean vertical profiles show bottom enhanced dissipation, and elevated dissipation near the surface at several of the mooring locations.