Abstract's details
Characterizing submesoscale variability in the southern California Current System
CoAuthors
Event: 2016 Ocean Surface Topography Science Team Meeting
Session: Science II: From large-scale oceanography to coastal and shelf processes
Presentation type: Type Oral
Contribution: PDF file
Abstract:
The partition between geostrophically balanced and unbalanced submesoscale (10-100 km) upper ocean flows and their projection on sea surface height (SSH) have practical implications for the interpretation of new altimeter missions that will resolve these relatively unobserved scales. Here we examine the southern California Current System (CCS), a region characterized by a weak baroclinic mean flow and energetic meso- and submeso- scales. Our approach is to use multiple high-resolution altimetric, in situ observational and model datasets and to exploit their different strengths in order to obtain a more complete picture of the physical processes that contribute to kinetic energy and SSH variability at scales between 100 to 10 km.
Prior studies have focused on strong baroclinic jets, e.g., the Gulf Stream, the Antarctic Circumpolar Current and the Kuroshio Extension, where the spectral slope flattens at the transition from the meso- to the submesoscale, due to an increasing contribution from inertia-gravity waves at submesoscales. In the CCS, the submesoscale is as energetic as in the strong jets, but the mesoscale is much weaker, and as a result the transition lacks a discernible change in spectral slope. A Helmholtz and vortex-wave decomposition of the kinetic energy spectrum is used to identify balanced and unbalanced contributions, since spectral slope alone does not distinguish them. At horizontal scales greater than 70 km, we find that the kinetic energy is dominated by balanced geostrophic motions. At scales from 70 km to 10 km, unbalanced contributions such as inertia-gravity waves contribute nearly equally as balanced motions. Two high resolution (1/24 and 1/48 degree) numerical simulations using the MIT general circulation model (MITgcm) with embedded tides are analyzed. The model results suggest that surface-layer submesoscale turbulence and inertia-gravity waves undergo seasonal cycles of roughly equal amplitude but opposing phase, with inertia-gravity waves peaking in early autumn when the stratification is maximum. The phase cancellation results in kinetic energy spectra that exhibit only weak seasonality, as confirmed by in situ observations.
Prior studies have focused on strong baroclinic jets, e.g., the Gulf Stream, the Antarctic Circumpolar Current and the Kuroshio Extension, where the spectral slope flattens at the transition from the meso- to the submesoscale, due to an increasing contribution from inertia-gravity waves at submesoscales. In the CCS, the submesoscale is as energetic as in the strong jets, but the mesoscale is much weaker, and as a result the transition lacks a discernible change in spectral slope. A Helmholtz and vortex-wave decomposition of the kinetic energy spectrum is used to identify balanced and unbalanced contributions, since spectral slope alone does not distinguish them. At horizontal scales greater than 70 km, we find that the kinetic energy is dominated by balanced geostrophic motions. At scales from 70 km to 10 km, unbalanced contributions such as inertia-gravity waves contribute nearly equally as balanced motions. Two high resolution (1/24 and 1/48 degree) numerical simulations using the MIT general circulation model (MITgcm) with embedded tides are analyzed. The model results suggest that surface-layer submesoscale turbulence and inertia-gravity waves undergo seasonal cycles of roughly equal amplitude but opposing phase, with inertia-gravity waves peaking in early autumn when the stratification is maximum. The phase cancellation results in kinetic energy spectra that exhibit only weak seasonality, as confirmed by in situ observations.