Sampling Energy Cascade Processes in Highly Forced Frontal Zones
Abstract
We propose a focused study on the energy cascade physics of a highly-forced frontal zone rich with sub-mesoscale processes. Specifically, we wish to examine the interplay between the sub-mesoscale cascade and near-inertial band process excited by winds and convective processes. We will conduct this study as a contribution to NISKINE, utilizing the region of the North Atlantic Current Extension south of Iceland. Our effort will be primarily conducted using gliders as autonomous sampling platforms capable of surveying the ocean during periods of high-amplitude forcing when sampling by research vessels is not possible. The oceanic response to winds has long been a topic of study. Particular interest has focused on the nearinertial band response of the ocean, which provides both excited motions in the surface layer, and at depth, through the transmission of near-inertial waves (D???Asaro 1985; Simmons and Alford 2012). Concentrated studies on such physics include those as part of the ocean storms experiment (D???Asaro 1995), where many aspects of the transfer of energy between the wind-forced surface layer and the nearinertial band were documented. What was not studied was the dissipative range of the energy cascade, where sub-mesoscale processes degrade into turbulence. This cascade is likely enhanced in regions of strong frontal zones, as the contrasting water masses give rise to elevated gradients of buoyancy and velocity which give rise to mixed-layer instabilities (e.g., Boccaletti et al. 2007). Despite years of community interest, the extent to which such sub-mesoscale instabilities leak energy into turbulence at the dissipation level is unknown. This will be the specific focus of our research effort. The North Atlantic Current Extension region is an ideal study site for these interests. Winter forcing provides both high winds and convective events that lead to enhanced forcing of the near-inertial band. In the region where the North Atlantic Current Extension, strong gradients of water mass properties, currents, and vorticity lead to a rich sub-mesoscale cascade (Figure 1). This region represents the breakdown of the organized extension of the western boundary current, and as such is the site of an active cascade of variance to smaller scales. To our knowledge, there have been no previous attempts to study the turbulence levels in this region of the N. Atlantic. A key focus of our study will be not only to quantify the energy transfer rates, but also to contrast them between the radically different seasonalforcing characteristics. This will include the investigation of factors controlling near-inertial energy input,including subtleties of the mixed layer physics, the sub-mesoscale cascade, and the aspects of frontal physics. We will attempt to assess the importance of non-wind-source near inertial waves both in termsof energy and shear. We will also explore the near-surface decay and propagation of near-inertial wave energy, including any interaction with mesoscale and sub-mesoscale features and other internal waves.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 26, 2018
- Source ID
- N000141812603
Entities
People
- Louis St. Laurent
Organizations
- Office of Naval Research
- United States Navy
- Woods Hole Oceanographic Institution