Quantifying energy dissipation due to wave breaking remains an essential but elusive goal for studying and modeling air-sea fluxes of heat, gas, and momentum. Previous observations have shown that lifetimes of bubble plumes and surface foam are directly related to the dissipated energy. Specifically, the foam decay time can be used to estimate the timescale of the subsurface bubble plume and the energy dissipated in the breaking process. A mitigating factor is that the foam decay time can be significantly affected by the surfactant concentration. Here we present an experimental investigation of a new technique that exploits the thermal signature of cooling foam to infer wave breaking dynamics. The experiments were conducted in a laboratory wave tank using artificial seawater with and without the addition of a surfactant. We show that the time from the start of the breaking process to the onset of cooling scales with the bubble plume decay time and the dissipated energy, and is not significantly affected by the presence of additional surfactants. We confirm observations from the field of the spatial variability of the temperature of foam generated by an individual breaking event, which has implications for inferring the spatial variability of bubble plume depth.

This study is motivated by the observation that after a wave breaking event in the ocean, the residual surface foam left in the wake of the breaker rapidly cools down. The relationship between the cooling foam and the characteristics of the breaking wave such as bubble plume dynamics, visible surface foam, and energy dissipation is investigated experimentally. Previous studies have suggested that the decay time of the visible foam can be used to determine the dynamics of the subsurface bubble plume, and to estimate the energy dissipation by the breaking process. But the foam decay process can be greatly affected by the surfactant concentration in the ocean and this effect need to be accounted for independently. We present a new approach that utilizes the thermal signature of the cooling foam to infer the breaking characteristics and is less sensitive to surfactant concentration. The experiments are conducted in a wave flume that is equipped with a piston-type wavemaker and is filled with salt water. In order to study the effects of surfactants on the cooling of the residual foam, two sets of experiments are carried out; In the first set clean salt water is used and in the second set, Triton X-100 at a concentration of approximately 200 mg/L is added to the water. Breaking waves are generated using the focusing wavepacket technique and are designed to cover a wide range of slopes and breaking intensities. The bubble plume and the surface foam are imaged using visible cameras and the surface temperature is captured using an IR camera with an overlapping field of view with the visible foam camera. It is observed that the average temperature of the foam initially increases after the passage of a breaking wave due to the disruption of the cool skin layer, but the foam starts to cool down soon after the bubble plume has subsided, and the foam regeneration is not sustained by the bubbles anymore. It is found that the time from the start of the breaking process to the onset of cooling of the foam (τcool) scales with the decay time of the bubble plume (τplume), energy dissipation, and the wavepacket slope of the breakers. It is also observed that the foam decay time is prolonged greatly by the presence of additional surfactants (consistent with the literature), but the bubble plume decay time and the onset of cooling of the foam are not significantly affected.

Cross-shore transport of larvae, pollutants, and sediment between the surf zone and the inner shelf is important for coastal water quality and ecosystems. Rip currents are known to be a dominant pathway for exchange, but the effects of horizontal temperature and salinity gradients are not well understood. Airborne visible and infrared imagery performed on the California coast show warm and cool plumes driven by rip currents in the surf zone and extending onto the shelf, with temperature differences of approximately 1$^\circ$C. The airborne imagery and modeled temperatures and tracers indicate that warm plumes exhibit more lateral spreading and transport material in a buoyant near-surface layer, whereas cool plumes move offshore in a subsurface layer. The average cross-shore extent of warm plumes at the surface is approximately one surfzone width larger than for cool plumes. Future work may explore the sensitivity of nearshore plumes to density patterns, wave forcing, and bathymetry.

Flow in rivers and the coastal ocean is controlled by the frictional force exerted on the water by riverbed or seabed roughness. The frictional force is typically characterized by a drag coefficient Cd, which is estimated from bulk measurements and often assumed constant. Here we demonstrate a relationship between bed roughness and water surface turbulence that can be used to make remote estimates of Cd. We observe that regions with higher bed roughness result in higher turbulent kinetic energy (TKE), which is transported upward by river boils to the water surface. We present a relationship between surface TKE and Cd, and validate this relationship by comparing remotely-sensed estimates of Cd to those from in situ measurements. Thus, our results provide an approach for estimating bottom roughness and Cd based entirely on remotely sensed data, including their spatial variability, which can improve modeling of river discharge and morphodynamics in data-poor regions.