How is sea surface temperature defined? They are in fact many definitions, often related to a particular type of sensing (and consequently depth of measurement). GHRSST-PP project has reached - after many years and many debates! - some kind of consensus on this topic that can be summarized as described here.
Variations of the sea surface temperature
The thermal structure of the top few metres below the sea surface is quite complex, as shown in figure 1; Because of this, different methods for measuring SST may record different values. This has important consequences for the accuracy and the precise calibration of satellite SST datasets. In order to help to clarify the issues, the science team of GHRSST-PP has distinguished between a number of different representations of SST, set out as follow:
SSTint is the hypothetical concept of the temperature of the interfacial layer of water and air molecules at the sea surface. It is not measurable and not used in GHRSST-PP.
SSTskin is defined within GHRSST-PP as the radiometric temperature of the surface measured by an infrared radiometer operating in the 10 - 12 µm waveband. Physically it represents the temperature of the water at a depth of approximately 10 - 20 µm.
SSTdepth is the generic term used to represent the temperature measured by a contact thermometer within the upper few metres of the water column and generally referred to as the 'bulk' SST. If the water column below the skin layer is uniform (typically the case at night and when wind mixing is strong) then SSTdepth is the same as SSTsubskin, irrespective of the actual depth of the measurement (see figure 1a). However, when daytime solar shortwave radiation penetrates to heat the water below the skin layer, and if wind mixing is weak, stable stratification develops in the upper few metres of the water column, in which the temperature increases towards the sea surface (apart from the cool skin layer which lies right at the surface - see figure 1b). This phenomenon is called a diurnal thermocline. When it occurs a measurement of SSTdepth, made from a buoy or ship-mounted thermometer which does not precisely specify the sampling depth, is of limited value. A diurnal thermocline almost always collapses to a uniform temperature some time after sunset when surface cooling removes the excess heat. Under calm conditions, however, it may take several hours for the diurnal thermocline to decay entirely.
SSTsubskin represents the temperature at the base of the thermal skin layer. The thermal skin layer is a region less than 1 mm deep in which the convective exchange of heat by turbulent mixing is inhibited by the proximity of the sea surface, so that the net outward flow of heat through the surface creates a steep reduction of temperature towards the surface. Below the skin layer, turbulent processes ensure that the temperature is nearly uniform over a depth of at least a few centimetres. By definition within GHRSST-PP this corresponds to the SSTsubskin temperature. In practice SSTsubskin is assumed to be approximately equal to the radiometric temperature measured by a microwave radiometer operating in the 6-11 GHz frequency band, although the relationship is not exact or fully known.
SSTfnd is defined within GHRSST-PP as the temperature at the base of the diurnal thermocline. It is so named because it represents the foundation temperature on which the diurnal thermocline develops during the day. SSTfnd changes only gradually along with the upper layer of the ocean, and by definition it is independent of skin SST fluctuations due to wind- and radiation-dependent diurnal stratification or skin layer response. It is therefore updated at intervals of 24 hrs. SSTfnd corresponds to the temperature of the upper mixed layer which is the part of the ocean represented by the top-most layer of grid cells in most numerical ocean models. It is never observed directly by satellites, but it comes closest to being detected by a microwave radiometer which penetrates the skin, at dawn when the previous day's diurnal stratification can be assumed to have decayed and SSTsubskin, SSTdepth and SSTfnd are equal.
Diurnal Variability of SST
During calm conditions, shortwave radiation from direct insolation can penetrate the sea surface and heat the upper layer of water by as much as 5K. Once the source of heating is removed (ie the sun goes down) the upper layer loses heat, convective overturning starts, and the stratification built up during the day is rapidly eroded. Daytime measurements may not accurately represent the upper mixed layer temperature, depending on the strength of the diurnal stratification.
In situ measurements, depending on their depth, may not detect some of the diurnal stratification. It is therefore important to sample SST often enough to accurately resolve diurnal variability.