Wave-Induced Turbulence, Linking Metocean and Large Scales

Abstract

Until recently, large-scale models did not explicitly take account of ocean surface waves which are a process of much smaller scales. However, it is rapidly becoming clear that many large-scale geophysical processes are essentially coupled with the surface waves, and those include ocean circulation, weather, Tropical Cyclones and polar sea ice in both Hemispheres, climate and other phenomena in the atmosphere, at air/sea, sea/ice and sea/land interface, and many issues of the upper-ocean mixing below the surface. Besides, the wind-wave climate itself experiences large-scale trends and fluctuations, and can serve as an indicator for changes in the weather climate. In the presentation, we will discuss wave influences at scales from turbulence to climate, on the atmospheric and oceanic sides. At the atmospheric side of the interface, the air-sea coupling is usually described by means of the drag coefficient Cd, which is parameterised in terms of the wind speed, but the scatter of experimental data with respect to such dependences is very significant and has not improved noticeably over some 40 years. It is argued that the scatter is due to multiple mechanisms which contribute into the sea drag, many of them are due to surface waves and cannot be accounted for unless the waves are explicitly known. The Cd concept invokes the assumption of constant-flux layer, which is also employed for vertical profiling of the wind measured at some elevation near the ocean surface. The surface waves, however, modify the balance of turbulent stresses very near the surface, and therefore such extrapolations can introduce significant biases. This is particularly essential for buoy measurements in extreme conditions, when the anemometer mast is within the Wave Boundary Layer (WBL) or even below the wave crests. In this presentation, field data and a WBL model are used to investigate such biases. It is shown that near the surface the turbulent fluxes are less than those obtained by extrapolation using the logarithmic-layer assumption, and the mean wind speeds very near the surface, based on Lake George field observations, are up to 5% larger. The dynamics is then simulated by means of a WBL model coupled with nonlinear waves, which revealed further details of complex behaviours at wind-wave boundary layer. Furthermore, we analyse the structure of WBL for strong winds (U10 > 20 m/s) based on field observations. We used vertical distribution of wind speed and momentum flux measured in Topical Cyclone Olwyn (April 2015) in the North-West shelf of Australia. A well-established layer of constant stress is observed. The values obtained for u⁎ from the logarithmic profile law against u⁎ from turbulence measurements (eddy correlation method) differ significantly as wind speed increases. Among wave-induced influences at the ocean side, the ocean mixing is most important. Until recently, turbulence produced by the orbital motion of surface waves was not accounted for, and this fact limits performance of the models for the upper-ocean circulation and ultimately large-scale air-sea interactions. While the role of breaking waves in producing turbulence is well appreciated, such turbulence is only injected under the interface at the vertical scale of wave height. The wave-orbital turbulence is depth-distributed at the scale of wavelength (∼10 times the wave height) and thus can mix through the ocean thermocline in the spring-summer seasons. Such mixing then produces feedback to the large-scale processes, from weather to climate. In order to account for the wave-turbulence effects, large-scale air-sea interaction models need to be coupled with wave models. Theory and practical applications for the wave-induced turbulence will be reviewed in the presentation. These include viscous and instability theories of wave turbulence, direct numerical simulations and laboratory experiments, field and remote sensing observations and validations, and finally implementations in ocean, Tropical Cyclone, ocean and ice models. As a specific example of a wave-coupled environment, the wave climate in the Arctic as observed by altimeters will be presented. This is an important topic for the Arctic Seas, which are opening from ice in summer time. Challenges, however, are many as their Metocean environment is more complicated and, in addition to winds and waves, requires knowledge and understanding of ice material properties and its trends. On one hand, no traditional statistical approach is possible since in the past for most of the Arctic Ocean there was limited wave activity. Extrapolations of the current trends into the future are not feasible, because ice cover and wind patterns in the Arctic are changing. On the other hand, information on the mean and extreme wave properties is of great importance for oceanographic, meteorological, climate, naval and maritime applications in the Arctic Seas.