Large-scale impacts of the mesoscale environment on mixing from wind-driven internal waves

Fine-scale estimates of the average diffusivity
TitleLarge-scale impacts of the mesoscale environment on mixing from wind-driven internal waves
Publication TypeJournal Article
Year of Publication2018
AuthorsWhalen C.B, MacKinnon JA, Talley LD
JournalNature Geoscience
Date Published2018/11
Type of ArticleArticle
ISBN Number1752-0894
Accession NumberWOS:000448672100010
Keywordsannual cycle; eddy field; finescale; generation; Geology; gravity-waves; kinetic-energy; mixed-layer; near-inertial motions; parameterizations; quasi-geostrophic flow; turbulent dissipation

Oceanic mesoscale structures such as eddies and fronts can alter the propagation, breaking and subsequent turbulent mixing of wind-generated internal waves. However, it has been difficult to ascertain whether these processes affect the global-scale patterns, timing and magnitude of turbulent mixing, thereby powering the global oceanic overturning circulation and driving the transport of heat and dissolved gases. Here we present global evidence demonstrating that mesoscale features can significantly enhance turbulent mixing due to wind-generated internal waves. Using internal wave-driven mixing estimates calculated from Argo profiling floats between 30 degrees and 45 degrees N, we find that both the amplitude of the seasonal cycle of turbulent mixing and the response to increases in the wind energy flux are larger to a depth of at least 2,000 m in the presence of a strong and temporally uniform field of mesoscale eddy kinetic energy. Mixing is especially strong within energetic anticyclonic mesoscale features compared to cyclonic features, indicating that local modification of wind-driven internal waves is probably one mechanism contributing to the elevated mixing observed in energetic mesoscale environments.

Short TitleNat. Geosci.

We have presented comprehensive observations showing that on basin scales the oceanic mesoscale plays a role in the mixing triggered by wind-driven internal waves. These results imply that the mesoscale assists in controlling the location, both in the horizontal and vertical, and timing of wind-driven internal wave mixing on large scales. Our observations also indicate that anticyclonic vorticity slightly enhances mixing arising from wind-driven internal waves, suggesting vorticity of this sign alters wave properties and propagation. Spatial and temporal patterns of mixing control the rate at which heat, greenhouse gasses, and freshwater are transported away from the surface, amplify or diminish associated air-sea coupled interactions, and set regional density-driven circulation patterns. Therefore if the mesoscale is key in setting a share of these patterns as our results indicate, then the mesoscale’s involvement in internal wave driven mixing is crucial to parameterize in climate models and incorporate into our global view of ocean mixing.

Student Publication: