Quantifying Oceanic Whitecap Energy Dissipation and Bubble-Mediated Air-Sea Fluxes

点击翻译按钮获取中文摘要

The winds constantly transfer energy from the atmosphere to the global oceans and seas helping to generate surface waves, currents and tearing water droplets directly from the crests of the steepest waves. The interaction of the wind and the surface ocean is an extremely complex process that still remains to be fully understood by ocean scientists and engineers and remains an active area of research. Perhaps the most fundamental consequence of wind blowing over the surface of the oceans is the generation of waves. Our ability to forecast the generation, evolution, and decay of ocean waves is important for the way humans interact with the global oceans. For example, wave forecasts are routinely used to help shipping companies plan the transport of goods and people across the global oceans, marine engineers need to know how often large waves occur and how these waves will interact with the structures they build for use in the ocean, oceanographers need to predict the how ocean waves affect weather and climate, and recreational sailors, swimmers and surfers rely on accurate wave forecasts to safely enjoy the seas and oceans around our coastline.Of particular interest to oceanographers is the energy balance between the wind and the waves. Since the wind acts as the primary source of energy for the waves, there must be a mechanism for dissipating this energy input, otherwise the waves would continue to grow. Part of this energy dissipation occurs along our coastlines where incoming waves break as they enter shallow water, releasing their energy. This release of energy helps to entrain air into the water, to move sediment and sand, and to create chaotic turbulent water motions. However, the vast majority of wave energy is dissipated by waves breaking in the open ocean. These are easy to spot on a windy day because of the bubbles and white foam they produce, commonly called whitecaps. The importance of these whitecaps to how the Earth's climate evolves is an area of huge interest to oceanographers, atmospheric scientists and climate scientists. Within each whitecap there are thousands of bubbles ranging in size from the width of a human hair to about the width of a 5 pence piece. These bubbles are like tiny replicas of the atmosphere that exchange gas with the surrounding water. This bubble-mediated mechanism of gas transfer is very important to how much carbon dioxide is transferred from the atmosphere to the ocean. When each of these bubbles rises to the water surface and bursts it can send tiny sea spray droplets into the atmosphere, much like the fizz of a glass of soda drink that you see when you look at it from the side. When these tiny droplets are in the atmosphere they can help to form clouds over the ocean, transport bacteria from the ocean surface into the atmosphere and can scatter light from the sun. Gaining a better understanding of how much these bubbles and sea spray droplets matter to the Earth's climate is important to make accurate future projections of the Earth's climate.To tackle these difficult questions, our research will use state of the art wave making facilities to replicate breaking ocean waves in the laboratory at Imperial College, and will photograph whitecaps in the Adriatic Sea where we have access to a unique ocean observing platform that is operated by the Italian Institute of Marine Science. We will use a combination of wave height gauges, digital cameras and stereovision image processing techniques, to measure wave energy, photograph the breaking wave foam, and count the number and measure the size of bubbles generated by the breaking waves. These data will be used to improve computer models of ocean waves, and predictions of the exchange of gas between the atmosphere and the oceans for use in computer models of Earth's climate.