Ocean mixing by active swimmers

近年来，浮游动物的日常垂直迁移引起的海水混合现象日益受到海洋学和生物物理学研究领域的关注。除了了解生物圈过程的愿望外，这种兴趣还源于一种建议，即游泳微生物对海水混合的影响，这种影响对全球海洋环流至关重要。目前的生物混合理论大多基于简单的比例定律或封闭模型，这些模型很少考虑大型群体集体迁移的影响。在这个项目中，我们将研究游泳者与周围水柱相互作用的垂直迁移。我们将根据文献中的实验观察并与一个实验组合作，开发一个游泳者垂直迁移的模型。在此基础上，高分辨率的直接数值模拟将同时进行，计算一群活跃游泳者的个人运动。第一个目标是揭示大规模流动结构的形成条件，这些结构已被观察到用于盐水虾的集体垂直迁移。此外，我们还探讨了这些游泳者群体在垂直盐度和温度梯度稳定分层的水中的混合效率。这些数值结果将为不解决游泳者个体长度尺度的垂直温度和盐度传输的简化模型提供基础。第二个目标是澄清活跃的游泳者对盐指发育的影响，盐指可能是上层水柱混合的敏感因素。盐指是一种特定类型的双扩散对流，也就是说，它们的出现关键取决于温度和盐度的相对扩散系数（由刘易斯数测量），这是由活跃游泳者的小尺度运动有效呈现的。实际上，盐指出现在比深层更咸、更热的近地表水中。这种情况将通过求解温度和盐度的输运方程以及包括游泳者运动在内的海水动量平衡来研究。这将揭示受生物混合影响的盐指的先决条件。更好地了解浮游动物运动和混合过程之间的相互作用将提高我们对大尺度海洋动力学的理解，从而可以改进大尺度大气-海洋耦合模式。

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.