Skin layer recovery of free‐surface wakes: Relationship to surface renewal and dependence on heat flux and background turbulence

The thermal signatures of free‐surface wakes observed in the open ocean show that the recovery of the cool skin layer is related to the degree of surface mixing and to ambient environmental conditions. Wakes produced by two surface‐piercing cables of O (10 −2 m) in diameter are analyzed using infrared imagery. Under low‐wind‐speed conditions when the swell and surface current were aligned, the wakes exhibited distinctive patchlike features of O (1 m) in diameter that were generated by the passage of individual waves. The time t * required by the skin layer to recover from these disturbances is compared to the surface‐renewal timescale τ used in heat and gas flux models. At low wind speeds, t * is comparable to τ, but at moderate wind speeds the agreement is poor. The spatial and temporal variations in the skin temperature of these wakes are related to a wave Reynolds number used to characterize the strength of the disturbance due to the waves. The recovery process is characterized in terms of the restoring internal energy flux J r which is proportional to both the initial thickness and the thermal recovery rate of the skin layer and was found to be directly related to the strength of the surface disruption. Comparison of the wake results with laboratory and other field measurements of breaking waves implies that J r is also a strong function of the net heat flux and background turbulence, which relate directly to the existing environmental conditions such as wind stress and sea state. Our results demonstrate that J r may vary by several orders of magnitude, depending on the environmental conditions.