Sea surface skin temperature

Large-scale sea surface skin temperature measurements started with the use of satellites in remote sensing. The underlying principle of this kind of measurement is to determine the surface temperature via its black body spectrum. Different measurement devices are installed where each device measures a different wavelength. Every wavelength corresponds to different sublayers in the upper 500 μm of the ocean water column. Since this layer shows a strong temperature gradient, the observed temperature depends on the wavelength used.[2] Therefore, the measurements are often indicated with their wavelength band instead of their depths.[3]

Figure 1: Mean sea surface skin temperature in December and June 2021 in °C as determined through the ERA5 reanalysis dataset.

The vertical temperature profile of the surface layer of the ocean is determined by different heat transport processes. At the very interface, the ocean is in thermal equilibrium with the atmosphere which is dominated by conductive and diffusive heat transfer. Also, evaporation takes place at the interface and thus cools the skin layer. Below the skin layer lies the subskin layer, this layer is defined as the layer where molecular and viscous heat transfer dominates. At larger scales, as the much bigger foundation layer, turbulent heat transport through eddies contributes most to the vertical heat transfer.

During the day, there is additional heating by the sun. The solar radiation entering the ocean gets heats the surface following the Beer-Lambert law. Here, approximately five percent of the incoming radiation is absorbed in the upper 1 mm of the ocean.[8] Since the heating from above leads to a stable stratification, other processes dominate the heat transport, depending on the considered scale.

Regarding the skin layer with thickness ${\displaystyle \delta }$, turbulent diffusion term ${\displaystyle K_{w}}$ is negligible. For the stationary case without external heating, the vertical temperature profile obeys the following energy budget:

${\displaystyle \rho _{w}c_{w}k_{w}{\frac {\partial T}{\partial z}}=Q=LH+SH+LW,}$

Here, ${\displaystyle \rho _{w}}$ and ${\displaystyle c_{w}}$ denote the density and heat capacity of water, ${\displaystyle k_{w}}$ the molecular thermal conductivity and ${\displaystyle {\tfrac {\partial T}{\partial z}}.}$ the vertical partial derivative of the temperature. The vertical heat difference ${\displaystyle Q}$ consists of latent heat release, sensible heat fluxes and the net longwave thermal radiation.[10] The ${\displaystyle Q}$ observed in the skin layer is positive, which corresponds to a temperature increasing with depth (Note that the z-axis points downward into the ocean). This leads to a cool skin layer as can be seen in Fig. 2. A common empiric description of the vertical temperature profile within the skin layer of depth ${\displaystyle \delta }$ is:

${\displaystyle {\overline {T}}(z)=T_{b}+(T_{s}-T_{b})*e^{-z/\delta }}$

Here, ${\displaystyle T_{s}}$ and ${\displaystyle T_{b}}$ denote the temperature of the surface ad the lower boundary.[11] When including the diurnal heating, we have to include an additional heating term, depending on the absorbed short wave radiation. Integrating over ${\displaystyle z}$, we can express the temperature at depth ${\displaystyle \delta }$ as:

${\displaystyle T_{z=\delta }=T_{s}-{\frac {\delta }{\rho _{w}c_{w}k_{w}}}(Q+R_{s}f_{s})}$

where ${\displaystyle R_{s}}$ is the net shortwave solar radiation at the ocean interface and ${\displaystyle f_{s}}$ is its fraction absorbed up to depth ${\displaystyle \delta }$.[12] As can be seen in Fig. 2, the diurnal heating reduces the cool skin effect. The maximum temperature can be found in the subskin layer, where the external heating per depth is lower than in the skin layer, but where the surface cooling has a smaller effect. With further increasing depth, the temperature declines, as the proportional heating is smaller and the layer is mixed via turbulent processes.

• Sea surface temperature
• Remote sensing
• Remote sensing (oceanography)
• Thermal radiation
• Skin temperature of an atmosphere
• Sea surface interface temperature
• Sea surface subskin temperature
• Group for High Resolution Sea Surface Temperature (GHRSST)