Sea Surface Microlayer

The sea surface microlayer covers the upper 10 to 1000 micrometre deep boundary layer of the ocean where important physical, chemical, and biological processes take place. As turbulence is damped close to the surface molecular transport processes take over the transfer of momentum, heat and mass from the upper ocean to the sea surface. As characterictic features molecular sublayers estend from the surface to depths of about 1000 µm (viscous sublayer), 500 µm (conductive or thermal sublayer), and 50 µm (diffusive sublayer). The conductive sublayer is also referred to as the cool skin. The structures of the molecular sublayers are complex due to the variability of wind stress acting on the sea surface, due to heat, radiative and gas fluxes crossing these layers, and due to rainfall. A schematic vertical section through the ocean is shown in Figure 1. The logarithmic scale ranging from the diameter of a molecule to the maximum depth of the world ocean underlines the importance of the top millimetre of the sea.



Fig. 1: Schematic representation of the molecular sublayer in the upper ocean.



Among the main concerns in the temperature difference across the cool skin of the ocean has been the interpretation of satellite-derived sea surface temperatures as equivalent to in-situ bulk temperature measurements under clear-sky conditions, the only case where space-borne infrared imagery can be used to monitor the sea surface. While common in-situ measurements of sea surface temperature are representative for the upper decimetres or metres of the bulk water infrared radiometers receive radiation from the upper few micrometres only. Besides this trouble of the remote sensing community growing interest in the cool skin of the ocean is expressed by the possibility to parameterize the air-sea gas-transfer coefficient. While field measurements of the gas transfer have been often inconclusive the similarity of the transfer of passive properties such as gas and heat allows the extrapolation from the molecular thermal conductivity to molecular diffusivity. The latter interest is not restricted to clear-sky situations but is of interest in cloudy and precipitating situations, too. The same is true for the impact of the conductive layer on the solubility of gases. Due to the temperature dependence of the solubility any gas transfer calculated with bulk temperatures will possibly be biased to lower transfer rates. Typical temperature differences across the conductive sublayer are of the order 0.3 K. However, the actual value varies with heat, radiative, and momentum fluxes in the upper ocean and is also modified by rainfall so that a range of variability can be expected from -1 K to 1 K.


A global distribution of the monthly mean nocturnal skin effect is shown in Figure 2 for August 1987. The parameters needed to parameterize the skin effect have been derived from satellite measurements of the Advanced Very High Resolution Radiometer, flown on the NOAA polar orbiters and from the Special Sensor Microwave/Imager, flown as part of the DMSP satellites (Schlüssel, 1996). The map reveals a high spatial variability of the skin effect even after averaging over an entire month. Lowest values are found in the northern Pacific Ocean and in the Arctic Ocean where warm and moist air is advected over cold water and concurent deep cloud cover minimizes the energy flux to the atmosphere. In those areas the bulk versus skin temperature differences vanish or even revert their sign showing slightly negative values. Maximum differences are found in subtropical areas and in the storm tracks of the southern oceans due to high energy fluxes leaving the ocean.



Fig. 2: Global distribution of the temperature difference across the cool skin of the ocean during night in August 1987.

Molecular Oceanic Boundary LAyer Model (MOBLAM)


The model describes the difference across the conductive sublayer and the air-sea gas transfer coefficient by modelling surface renewals due to turbulence elements caused by breaking waves, free convection, and rainfall as well as molecular diffusion between the renewals. The model includes the impact of free convection in calm situations, forced convection at moderate wind speed and longwave breaking at gale-force winds where the transition from one to the other regime is controlled by the surface Richardson and the Keulegan numbers (Soloviev and Schlüssel, 1994). Absorption of solar radiation is included describing the heating of the surface layer and the implied damping of convective instabilities (Soloviev and Schlüssel, 1996). For cases with rainfall the additional mixing by rain and the cooling of the near-surface water by rain is described together with the creation of a haline diffusion layer (Schlüssel et al., 1997). Measurements in the NE-Atlantic Ocean (Schluessel et al., 1990) have been used to determine model constants. The model has been extensively tested with field data from the TOGA/COARE campaign and compared to other parameterizations of the temperature difference across the cool skin (Soloviev and Schlüssel, 1996; Schlüssel et al., 1997; Craeye and Schlüssel, 1998). A Fortran code of MOBLAM is available.

References


Email: "Peter Schluessel" <schluessel barney eumetsat fred de>


© Peter Schlüssel, 5 January 1999