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

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NERC
NERC
NERC
NERC
NERC

£2,847,515

Sept. 30, 2019

Sept. 30, 2023

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.

Potential Impact:
As an island nation with a strong maritime/marine connection, the UK benefits enormously from its surrounding seas. These seas provide an abundant food source helping to drive the fishing industry; the strong tidal flows and exposed coastal regions provide natural locations to harness the renewable energy of ocean tides and to build ever-growing offshore wind turbine farms; and the cleanliness of the seas provide recreational benefits to the population as a whole. Along with these easy-to-see benefits of the seas around our coastlines, the seas also help moderate our weather and climate in ways that aren't immediately obvious. For example, when ocean waves break they can trap air beneath the water surface forming thousands of bubbles ranging in size from the width of a human hair up to the size of 5 pence coin. These bubbles when they are below the water surface help mix gases from the air into the sea, and when they rise up to the surface and burst, they can send tiny droplets into the atmosphere that affect cloud formation and the Earth's radiative balance. The primary focus of this research project is to study breaking waves in the sea and in the laboratory, and to use this data to improve how we model the evolution of the wave field and how we estimate the roles breaking waves and bubbles play in regulating climate and weather.

The balance between wind energy input to the ocean and wave energy dissipation at the ocean surface helps govern how the wave field evolves at any given time. This dynamical energy balance controls the wave field which is made up of many waves of different heights, lengths and frequencies. Sophisticated computer models are able to predict how this vast spectrum of waves evolves when given certain input parameters such as (i) the wind energy input, (ii) the distribution of this energy across the wave spectrum, and (iii) how much energy is lost through wave breaking. Of these three terms, it is the energy loss driven by wave breaking that is the least understood. Providing better estimates of the energy dissipation of wave breaking will help the numerical modellers improve how dissipation is incorporated in wave models. The outputs from this project will provide more detailed information on the energy lost by waves when they break, and this in turn can help us develop a better understanding of how the wave field evolves which can deliver more accurate wave forecasts. A better understanding of the evolution of ocean waves and more accurate wave forecasts can have benefits for the fishing, shipping, energy and tourist industries, as well as being important for weather forecasters and climate modellers to provide better predictions of weather and climate variation.

There is increasing awareness of the critical role physical exchange processes occurring at the air-sea interface affects our climate in a profound way by altering the composition of our atmosphere and regulating the Earth's radiative balance. Air-entrainment and bubble formation by breaking waves is a key contributor to some of these physical air-sea exchange processes. Bubble-mediated gas exchange is a vital pathway by which anthropogenic carbon dioxide emitted from fossil fuel burning is absorbed into our oceans and removed from the atmosphere. As breaking wave energy increases, more air is entrained below the water surface enhancing the potential for greenhouse gas removal from the atmosphere. The proposed research will provide more detail on how much energy is dissipated during wave breaking, on a wave-by-wave basis, which will lead to improved estimates of air-entrainment and associated gas exchange across the air-sea interface. Ultimately such data has the potential to enhance our knowledge of how the carbon cycle is affected by marine processes and predict how this might change as mean global wind speed, and hence wave breaking, continues to change in response to climate change.