Air-Sea Interactions in Light of New Understanding of Air-Land Interactions

Air-sea interactions were investigated using the data collected from the Coupled Boundary Layers/Air-Sea Transfer Experiment under low wind (CBLAST-Low) and the Surface Wave Dynamics Experiment (SWADE) over sea in comparison with the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES-99) over land. Based on the concept of the HOckey-Stick Transition (HOST) hypothesis, which emphasizes contributions of large coherent eddies in the atmospheric turbulent mixing that are not fully captured by Monin-Obukhov similarity theory, relationships between the atmospheric momentum transfer and the sea-surface roughness, the role of the sea surface temperature and oceanic waves in the turbulent transfer of atmospheric momentum, heat, and moisture, and variations of drag coefficient, Cd(z), over sea and land with wind speed are studied.

In general the atmospheric turbulence transfers over sea and land are similar. The variation of the atmospheric momentum transfer from the stable to the nearly neutral regimes is through strong shear-generated turbulent mixing over land due to the strong radiative cooling at night, and through episodic warming in formation of the weakly stable atmosphere in the nearly neutral marine atmosphere with the relatively steady sea temperature. The atmospheric momentum transfer over sea leads to high-frequency oceanic waves, and is affected by low-frequency wave-induced wind only under weak winds. The observed approximate relationship between the air-sea temperature difference and the turbulent heat transfer over sea is dominated by the fast varying air temperature as the sea surface temperature variation is relatively small. Physically, Cd(z) consists of the surface skin drag and the drag associated with the turbulent momentum transfer between z and the surface; the increase of the latter drag with decreasing wind leads to the commonly observed minimum Cd(z) over sea, which is not unique for over sea.