Change of Regime of Air-Sea Dynamics in Extreme Metocean Conditions

As a reference point for the extreme Metocean conditions, the hurricane-scale classification is often used: that is a tropical storm becomes a hurricane if the wind speed reaches U ∼ 33m/s. In this paper, it is argued that such classification is not arbitrary, and indeed signifies change of the physical regimes in all environments near the air-sea interface: in the atmospheric boundary layer, at the surface, and through the upper ocean. This threshold is approximately the wind speed at which the drag coefficient was found to saturate in the field observations (U10 ≈ 32–33m/s), which saturation has received a lot of attention. Less known are the observations that below the surface, change of the upper-ocean mixing mechanism and of bubble dynamics occur at U10 > 35m/s. Directly at the surface, wave dynamics also undergoes essential transformations, from wave breaking (dissipation) being driven by evolution of nonlinear waves, to the breaking being forced directly by the winds, at U10 ≈ 34 m/s. It is therefore argued that the simultaneous change of physical regime in all the three air-sea environments cannot be coincidental, and consequences of the regime change for the Metocean modelling are discussed. As an important byproduct, parameterisation of wave-breaking probability is obtained in terms of the mean symmetry of surface waves. Such parameterisation allows us to estimate frequency of breaking events, based on time series of surface elevations, without explicitly detecting the breaking waves.

One-dimensional modelling of upper ocean mixing by turbulence due to wave orbital motion

Mixing of the upper ocean affects the sea surface temperature by bringing deeper, colder water to the surface. Because even small changes in the surface temperature can have a large impact on weather and climate, accurately determining the rate of mixing is of central importance for forecasting. Although there are several mixing mechanisms, one that has until recently been overlooked is the effect of turbulence generated by non-breaking, wind-generated surface waves. Lately there has been a lot of interest in introducing this mechanism into ocean mixing models, and real gains have been made in terms of increased fidelity to observational data. However, our knowledge of the mechanism is still incomplete. We indicate areas where we believe the existing parameterisations need refinement and propose an alternative one. We use two of the parameterisations to demonstrate the effect on the mixed layer of wave-induced turbulence by applying them to a one-dimensional mixing model and a stable temperature profile. Our modelling experiment suggests a strong effect on sea surface temperature due to non-breaking wave-induced turbulent mixing.