Predicting the Sea in the Future

Figure 1  Long term observation of wind conditions, sea current, waves, and water quality is being conducted jointly with Kyoto University and Kobe University using the Shirahama Oceanographic Observation Tower in Wakayama Prefecture (top). Bubble mixing distribution under breaking waves is observed using an underwater camera (bottom).

Greenhouse gas emission from human activities changes various meteorological and oceanographic equilibrium; that is, the frequency, intensity and pattern of typhoons, rainfall, storm surges, and long-term fluctuations of ocean currents. In addition, an increase in carbon dioxide gas in the atmosphere accelerates the dissolution of carbon dioxide in seawater, and the resulting acidification of seawater has a serious impact on the ecology of oceanic living things.

Ocean responses to the past meteorological impacts can be empirically estimated by ocean-wave models on the basis of oceanic observations in the past. However, there is no guarantee that the conventional models will be still available for unprecedented meteorological phenomena and modified oceanic environment beyond our experiences. New research approaches, for finding physically rational descriptions of the ocean surface processes, beyond the conventional framework are required for predicting potential oceanographic impacts may occur in the future.

 

Physics of Boundary between Atmosphere and Ocean

On the ocean surface, momentum, heat, moisture and gas are transferred mutually between the atmosphere and the ocean. Evaporation of sea water provides origin to meteorological phenomena; atmospheric current, clouds, wind, rainfall, etc. Meanwhile, wind in the atmosphere drives and develops sea waves, and finally break the crests of high waves (breaking wave) especially in storms, resulting in  large amounts of sea sprays released into the air while allowing air bubbles to be mixed into the ocean (Figure 1 and 4). During the storm events, heavy rain simultaneously forms a fresh water layer on the ocean surface and changes the heat supply from the ocean to the atmosphere. In this way, a thick mixed layer of fresh water, sea water and air develops and grows, while the conventional models assume the distinct boundary between the air and sea water, which is disrupted in the storms. Some significant discrepancies between the observed ocean responses and the model results have recently reported, indicating uncertain meteorological predictions by the current models, and thus further improvements of the models for ocean surface dynamics are required.
The next section provides our research on the mechanism to form the liquid-gas two phase flows on the ocean waves and dynamic responses of aerated ocean surfaces to meteorological field.

 

Research on Breaking Waves

The response of the marine surface exposed to a storm is highly affected by the breaking waves that provide complex aerated turbulent flows. A computational analysis has revealed that the breaking waves release the crest of a wave frontward (making a plunging breaker) and then the resulting jets splash on the forward water surface while generating sea sprays and bubbles creating the aerated layer between the atmosphere and ocean (Figure 4). The dissolution of gas and evaporation across gas-liquid interfaces are enhanced in the layer, which achieves the efficient exchange between the atmosphere and ocean. Breaking waves produce vortices beneath the surface during the splashing process, and organize a regular structure composed of counter-rotating vortices longitudinally stretched in a rib-like arrangement (Figure 2 bottom). As the surfaces of breaking-wave jets are wrapped by the counter-rotating vortices adjacent to the surfaces, the jets are divided into finger-like shapes (so-called finger jet), and then split into droplets sequentially from the tip (Figure 2 top). We also found the sub-surface vortices contribute to enhance heat exchanges across the sea surfaces via the rotational entrainment of the surfaces heated by solar radiation into depths (Figure 3). As the breaking-induced bubbles are captured within the vortices and remain there for a long time without rising to the surface (Figure 4), the bubbles efficiently supply oxygen, carbon dioxide, etc. to the sea water, which contributes to maintain the oceanic ecosystem (Figure 1).

We are conducting research on the ocean surface dynamics in terms of the exchange of force, heat, gas and moisture via mechanical interactions between the atmosphere and ocean by using numerical computations, image measurements in a wind tunnel and water tank and field observations, in order to try and predict ocean environment in the future.

Figure 2  Computed results of the free surface shape after wave breaking (top) and the structure of the counter-rotating vortices (bottom). The vortices contained in the jet entrain the water surface into the water and then divide the jet longitudinally to form finger jet. Figure 3  Measured result of water surface temperature of breaking waves. Heat is transferred into the sea water along longitudinal vortices under wave breaking (top). Computed result of water temperature in breaking waves (bottom).  Figure 4  We are conducting a parameter study on size distribution of droplets released (left) and bubbles entrained into the water (right) after wave breaking in an