Air–sea interface heat transfer: Roles of Langmuir circulations and microscale breaking waves

We investigate the effects of Langmuir circulations (LCs) and microscale breaking waves (MBW) on heat transfer across the air–sea interface using direct numerical simulation of air–water two-phase flow that directly resolves surface waves. Wave–current interactions generate LCs for both young and old waves, accompanied by MBW for the old waves in the present simulation. The study finds that LCs reduce the thermal molecular layer thickness through upwelling motions, thereby enhancing molecular diffusion at the water surface. Below the surface layer, turbulent transport by downwellings dominates heat transfer, increasing the temperature mixing into the bulk. However, near-surface turbulence enhanced by MBW disrupts the surface layer, strengthening turbulent transport and thereby dominating heat transfer in the case of old waves. After generating LCs, MBW interacts with them, jointly modulating interfacial dynamics and weakening the water surface constraint, further increasing turbulent heat flux and promoting heat transfer. Surface renewal analysis shows that the energetic small-scale coherent vortices generated by MBW form intense surface diverged and converged regions, characterized by transverse structures, leading to the frequent renewal of surface fluids. In the young waves, the counter-rotating vortex pairs induced by LCs contribute to surface renewal, forming streamwise elongated, streaky surface-temperature structures. When considering the contribution of turbulent transport to heat transfer, the heat transfer rate obtained from the present numerical study shows good agreement with the modeled value of surface divergence theory.