Process Studies on the Generation of Near-Inertial Internal Gravity Waves by Wind

• Main Author: Voelker, Georg Sebastian
• Other Authors: Walter, Maren, Sutherland, Bruce R
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Bremen 2017
• Subjects: physical oceanography; simulation; hybrid slab model; fluid dynamics; 530; 530 Physics; ddc:530; Pollard; North Atlantic; North Atlantic oscillation
• Online Access: https://media.suub.uni-bremen.de/handle/elib/1269; https://nbn-resolving.org/urn:nbn:de:gbv:46-00106033-11

The generation of internal gravity waves in the ocean associated to wind stress forcing is studied with the aid of two numerical models. In a non-linear, axisymmetric and Boussinesq model the response to an idealized stationary wind stress pulse is investigated. The major generation mechanisms for internal gravity waves, inertial pumping, excites radially outward propagating low mode wave packets. The energy radiated relative to the wind work is found to be only about 0.02%, which is 2-3 orders of magnitude lower than generally observed. In addition to the detailed axisymmetric simulation, a simplified hybrid slab model based on the classical slab model of (Pollard and Millard, 1970) is considered. In particular, the inertial pumping resulting from the divergent, horizontal, near-inertial response of the surface slab is regarded the boundary condition to the internal gravity wave field below the mixed layer. In a set of idealized experiments, laterally moving wind stress fronts are the most efficient driver for vertical motion depending on their translation speed. The application of the hybrid slab model to the North Atlantic for the years 1989 and 1996 as examples of opposite phases of the North Atlantic Oscillation shows latitude bands with characteristic ratio of energy radiated as internal gravity waves to the wind work. These are partly associated to the wind field structure in these meridional regimes. The ratio of total energies transferred equals to 9% for both years. In an application of the hybrid slab model to the axisymmetric setup it is found that the structure of the inertial oscillations of the axisymmetric simulation are well reproduced. However, the near-inertial response of the hybrid slab model has an 8 times larger amplitude and does not include non-linear effects. The ratio of radiated energy to the wind work for the stationary storm estimated with the hybrid slab model is about 1%. This ratio is increased by a factor 30 when the same wind stress structure is moving across the domain with 21 ...