Presenter: Brian Arbic (University of Michigan)

Description:
Surface oceanic kinetic energy is of interest for many reasons, and a greater understanding of the vertical structure of surface kinetic energy would aid interpretation of ongoing and proposed remote sensing missions (e.g., S-MODE, WACM, SKIM) that are focused on velocity measurements.  Here, kinetic energy (KE) at the sea surface (0 m) and 15 m depth in high-resolution global simulations (HYbrid Coordinate Ocean Model; HYCOM, and Massachusetts Institute of Technology general circulation model; MITgcm) is compared with KE from undrogued and drogued drifters, which respectively represent flows at 0 and 15 m.  Global maps and zonal averages are computed for four frequency bands—low-frequency (<0.5 cpd), near-inertial, diurnal, and semi-diurnal.  In the low-frequency band, near the equator, both models exhibit KE values that are too low relative to drifters.  In the near-inertial band, MITgcm KE is too low, while HYCOM KE lies closer to observations probably due to more frequently updated atmospheric forcing.  In the semi-diurnal band, MITgcm KE is too high, while HYCOM KE lies closer to the drifters due primarily to the inclusion of a parameterized topographic internal wave drag.  Drifter semi-diurnal spectra are inherently wider than model semi-diurnal spectra, due to Lagrangian sampling of spatially varying fields.  Vertical structure is defined here by the ratio of zonally averaged KE in 0 m/15 m model results and undrogued/drogued drifter results.  Over most latitudes and most frequency bands, the model ratios track the drifter ratio to within error bars.  All of the frequency bands except for the semi-diurnal band display measurable vertical structure.  Latitudinal dependence in the vertical structure is greatest in the diurnal and low-frequency bands.  If time permits we will also show results from an analysis of high-frequency precipitation variance in high-resolution coupled ocean-atmosphere simulations.