Ocean mixing by active swimmers

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The mixing of seawater caused by the daily vertical migration of zooplankton received growing attention in the field of oceanography and biophysical research in the last years. Besides the desire to understand processes in the biosphere, this interest originates from a suggested impact of swimming microorganism on the mixing of seawater on scales even critical for the global oceanic circulation. Current theories for biomixing are mostly based on simple scaling laws or closure models that poorly consider effects from a collective migration of large swarms.In this project, we will study the vertical migration of swimmers that interact with their surrounding water column. We will develop a model for the vertical migration of swimmers based on experimental observations from literature and by collaborating with an experimental group. On this basis, highly resolved direct-numerical simulations will be conducted simultaneously accounting for the individual motion of a host of active swimmers. A first goal is to reveal the conditions for the formation of large-scale flow structures that have been observed for the collective vertical migration of brine shrimp. Furthermore, we explore the mixing efficiency of theses swarms of swimmers on water that is stable stratified due to vertical salinity and temperature gradients. These numerical results will provide a basis for simplified models of vertical temperature and salinity transport that do not resolve length scales of individual swimmers.A second goal is to clarify the impact of active swimmers on the development of salt fingers, which can be a sensitive factor in the mixing of the upper water column. Salt fingers are a certain type of double-diffusive convection, i.e., their emergence crucially depends on the relative diffusivity of temperature and salinity (measured by the Lewis number), which is effectively rendered by the small-scale motion of active swimmers. Actually, salt fingers arise in near-surface water that is saltier and warmer than in deeper layers. Such a situation will be studied by solving the transport equation for temperature and salinity together with the momentum balance for seawater including swimmers motion. This will reveal the preconditions for salt fingers influenced by biomixing.A better understanding of the interplay between zooplankton motion and mixing processes will improve our understanding of oceanic dynamics on large scales and can thus improve coupled large-scale atmospheric-ocean models.