The Impact of the Ocean Thermal Skin Layer on Air-Sea Interfacial Heat Fluxes

Abstract:

The upper ocean heat content has been observed to be increasing over the past few decades much of which has been attributed to anthropogenic effects resulting in an increase in greenhouse gases thereby increasing the amounts of incoming longwave (LW_in) radiation impinging onto the ocean’s surface. However, the penetration depth of LW_in extends to micrometer scales, where the ocean's thermal skin layer (TSL) exists, and does not directly heat the upper few meters of the ocean thereby raising the conundrum of how does the upper ocean warm with increasing levels of infrared (IR) radiation. The TSL consists of a strong temperature gradient on the aqueous side of the interface that sustains the upward heat flux by molecular conduction. As such, we hypothesize the heat lost through the air-sea interface which is controlled by the TSL, modulates the amount of heat stored in the upper few meters of the ocean. An analysis of properties of the retrieved TSL profiles from a shipboard IR spectrometer with heat fluxes (specifically LW_in) and wind speeds from two cruises limited to night-time data are presented. We also show a comparison between these properties with current published viscous layer models. The results indicate that the data have an inherent wind speed dependence with net flux thereby requiring a segregation of the data into wind speed bins to acknowledge the effects of wind-driven shear in the analysis. The temperature differences derived from the models indicates that at low wind speeds (<2 m/s), where wind-driven shear effects are negligible and buoyancy effects dominate, the TSL profile’s gradient is decreasing with increased LW_in which leads to a lowered net heat flux and is in agreement with our hypothesis. However our field results show an opposite effect (higher gradient at higher LW_in) which is believed to be due to the formation of a thicker TSL at low winds. The presence of a thicker TSL suggests that more of the vertical temperature gradient lies below the depth of the deepest retrieved temperature, limited by the IR emission depth of <0.1 mm.