Wave-Enhanced Tracer Dispersion

Thomas, Jim und Gupta, Aman (2022): Wave-Enhanced Tracer Dispersion. In: Journal of Geophysical Research - Oceans, Bd. 127, Nr. 3, e2020JC017005

Abstract

Recent oceanic datasets indicate high gravity wave-to-balance energy ratio at submesoscales. Idealized investigations exploring such wave-dominated regimes have found that waves can break up coherent vortices and generate energetic small scale flow structures, indicating that the turbulence phenomenology is very different in wave-dominated regimes when compared to the quasi-geostrophic regime. Motivated by these recent investigations revealing significant differences in the flow dynamics in quasi-geostrophic and wave-dominated regimes and multiple oceanic observations pointing out enhanced tracer stirring at submesoscales, in this paper we compare and contrast passive tracer dispersion by barotropic flows in quasi-geostrophic and wave-dominated turbulent regimes using the reduced model explored by Thomas and Yamada (2019), https://doi.org/10.1017/jfm.2019.465. On comparing passive tracer dispersion by barotropic flows in the two different regimes, we find that wave-dominated turbulent flows stir and mix tracer fields much more rapidly than quasi-geostrophic turbulent flows that consist of well-defined coherent vortices. The presence of energetic small-scale features in wave-dominated flows increases small-scale turbulent diffusivity and results in steeper tracer variance spectrum in wave-dominated flows compared to the quasi-geostrophic flows. Our findings point out that gravity waves can play an indirect role in enhancing tracer dispersion: waves modify the flow, generating energetic small-scale structures, that makes stirring of tracers more efficient. Despite our study being idealized, we speculate the qualitative phenomenology of waves indirectly enhancing tracer dispersion to be of significance at submesoscales in the world's oceans. Plain Language Summary Recent oceanic datasets indicate high inertia-gravity wave energy levels at O (1-10) km submesoscales in the world's oceans. Inspired by these observations, we explore tracer dispersion by an idealized model that can access geostrophic turbulence regime, with large coherent vortices, and a wave-dominated regime, where high energy waves leads to the generation of energetic small-scale structures. On comparing tracer dispersion in the two regimes, we find that wave-dominated turbulent flows stir and mix tracer fields much more efficiently than geostrophic turbulent flows. Overall, wave-dominated flows facilitate faster transfer of tracer variance to small dissipative scales, resulting in steeper tracer variance spectra when compared to the tracer stirring in geostrophic turbulence regime. In the oceanographic community, inertia-gravity waves are often considered to be ineffective in stirring tracers. Our findings point out that inertia-gravity waves can play an indirect role in enhancing tracer dispersion. Waves can significantly modify the flow that stirs tracers, thereby indirectly enhancing the efficiency of tracer dispersion. Although our results were obtained using an idealized set up, we speculate that the qualitative phenomenological finding of waves indirectly enhancing tracer dispersion could be relevant at wave-dominated submesoscales in the world's oceans.

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Physik
• Themengebiete: 500 Naturwissenschaften und Mathematik > 530 Physik
• ISSN: 2169-9275
• Sprache: Englisch