Wave dissipation by breaking, or the energy transfer from the surface wave field to currents and turbulence, is one of the least understood components of air–sea interaction. It is important for a better understanding of the coupling between the surface wave field and the upper layers of the ocean and for improved surface-wave prediction schemes. Simple scaling arguments show that the wave dissipation per unit length of breaking crest, ϵl, should be proportional to ρwgc5, where ρw is the density of water, g is the acceleration due to gravity and c is the phase speed of the breaking wave. The proportionality factor, or ‘breaking parameter’ b, has been poorly constrained by experiments and field measurements, although our earlier work has suggested that it should be dependent on measures of the wave slope and spectral bandwidth. In this paper we describe inertial scaling arguments for the energy lost by plunging breakers which predict that the breaking parameter b = β(hk)5/2, where hk is a local breaking slope parameter, and β is a parameter of O(1). This prediction is tested with laboratory measurements of breaking due to dispersive focusing of wave packets in a wave channel. Good agreement is found within the scatter of the data. We also find that if an integral linear measure of the maximum slope of the wave packet, S, is used instead of hk, then b ∝ S2.77 gives better agreement with the data. During the final preparation of this paper we became aware of similar experiments by Banner & Peirson (2007) concentrating on the threshold for breaking at lower wave slopes, using a measure of the rate of focusing of wave energy to correlate measurements of b. We discuss the significance of these results in the context of recent measurements and modelling of surface wave processes.

Wave–current interaction can result in significant inhomogeneities of the ocean surface wave field, including modulation of the spectrum, wave breaking rates, and wave statistics. This study presents novel airborne observations from two experiments: 1) the High-Resolution Air–Sea Interaction (HiRes) experiment, with measurements across an upwelling jet off the coast of Northern California, and 2) an experiment in the Gulf of Mexico with measurements of waves interacting with the Loop Current and associated eddies. The significant wave height and slope varies by up to 30% because of these interactions at both sites, whereas whitecap coverage varies by more than an order of magnitude. Whitecap coverage is well correlated with spectral moments, negatively correlated with the directional spreading, and positively correlated with the saturation. Surface wave statistics measured in the Gulf of Mexico, including wave crest heights and lengths of crests per unit surface area, show good agreement with second-order nonlinear approximations, except over a focal area. Similarly, distributions of wave heights are generally bounded by the generalized Boccotti distribution, except at focal regions where the wave height distribution reaches the Rayleigh distribution with a maximum wave height of 2.55 times the significant wave height, which is much larger than the standard classification for extreme waves. However, theoretical distributions of spatial statistics that account for second-order nonlinearities approximately bound the observed statistics of extreme wave elevations. The results are discussed in the context of improved models of breaking and related air–sea fluxes.

It is now well accepted that to better understand the coupling between the atmosphere and the ocean, and improve coupled ocean–atmosphere models, surface wave processes need to be taken into account. Here, properties of the directional distributions of the surface wave field across the equilibrium and saturation ranges are investigated from airborne lidar data collected during the ONR Southern California 2013 (SoCal2013) experiment, conducted off the coast of Southern California in November 2013. During the field effort, detailed characterization of the marine atmospheric boundary layer was performed from Research Platform (R/P) Floating Instrument Platform (FLIP), moored at the center of the aircraft operational domain. The wind speed ranged from approximately 1–2 to up to 11 m s−1, while the significant wave height varied from 0.8 to 2.5 m during the 10 days of data collection considered in the analysis. The directional wavenumber spectrum exhibits a clear, bimodal distribution that extends well beyond what was reported in previous studies, with the azimuthal separation between the lobes reaching ≈π for the highest wavenumbers that could be resolved: approximately 10–12 rad m−1. The results demonstrate that opposing wave components can be found in one storm system rather than requiring waves from opposing storms, with implications for ocean acoustics. With the broad wavenumber range of the directional spectra obtained from the lidar, the transition from the equilibrium to saturation ranges over a range of wind forcing conditions is found to occur for ≈ 1–2 × 10−3, where kn is the wavenumber at the upper limit of the equilibrium range, u* the friction velocity, and g the gravitational acceleration. The results are discussed in the context of Phillips’ model of the equilibrium range of wind-generated gravity waves.