Understanding Sea Surface Temperature
SST is a difficult parameter to define exactly
because the upper ocean (~10 m) has a complex and variable
vertical temperature structure that is related to ocean turbulence and
the air-sea fluxes of heat, moisture and momentum. Definitions of SST provide a
necessary theoretical framework that can be used to understand the information content and relationships
between measurements of SST made by different satellite and in situ instruments.
The following SST definitions were agreed at the 2nd and 3rd GHRSST-PP workshops.
Each SST definition has been carefully considered by the GHRSST-PP Science Team
in order to achieve the closest possible coincidence between what is defined and
what can be measured operationally, bearing in mind current scientific knowledge
and understanding of how the near surface thermal structure of the ocean behaves in nature.
Definitions of SST within the GHRSST-PP
Figure 1 presents a schematic diagram that summarises the definition of SST in
the upper 10m of the ocean and provides a framework to understand the
differences between complementary SST measurements. It encapsulates the effects of
dominant heat transport processes and time scales of variability associated with
distinct vertical and volume regimes of the upper ocean water column (horizontal
and temporal variability is implicitly assumed). Each of the definitions marked in the
bottom right of the figure is explained in the following sub-sections.
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The interface SST (SSTint)
is a theoretical temperature at the precise air-sea interface. It represents the
hypothetical temperature of the topmost layer of the ocean water and could be thought of as
an even mix of water and air molecules. SSTint is of no practical use because
it cannot be measured using current technology. It is important to note that it is
the SSTint that interacts with the atmosphere.
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The skin SST (SSTskin)
SSTskin is defined as the radiometric temperature measured by an infrared radiometer
operating in the 10-12 micrometer spectral waveband. As such, it represents the actual
temperature of the water across a very small depth of approximately 20 micrometers. This definition
is chosen for consistency with the majority of infrared satellite and ship mounted radiometer
measurements. SSTskin measurements are subject to a large potential diurnal cycle
including cool skin layer effects (especially at night under clear skies and low wind speed conditions)
and warm layer effects in the daytime (although the latter are not shown in Figure 1).
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The sub-skin SST (SSTsub-skin)
represents the temperature at the base of the thermal skin layer. The difference between
SSTint and SSTsubskin is related to the net flux of heat through the thermal skin layer.
For practical purposes, SSTsubskin can be well approximated to the measurement of surface
temperature by a microwave radiometer operating in the 6-11 GHz frequency range,
but the relationship is neither direct nor invariant to changing physical conditions or to
the specific geometry of the microwave measurements. SSTsubskin is the temperature of a layer
~1mm thick at the ocean surface.
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SSTdepth or SST(z)
is the terminology adopted by GHRSST-PP to represent
an in situ measurement near the surface of the ocean that is typically reported
simply as SST or "bulk" SST. For example SST6m would refer to an SST measurement made at a
depth of 6m. Without a clear statement of the precise depth at which the SST measurement was
made, and the circumstances surrounding the measurement, such a sample lacks the information
needed for comparison with, or validation of satellite-derived estimates of SST using other data
sources. The terminology has been introduced to encourage the reporting of depth (z) along with the
temperature.
All measurements of water temperature beneath the SSTsubskin are obtained from a wide variety of sensors
such as drifting buoys having single temperature sensors attached to their hull, moored buoys that sometimes
include deep thermistor chains at depths ranging from a few meters to a few thousand meters,
thermosalinograph (TSG) systems aboard ships recording at a fixed depth while the vessel is underway,
Conductivity Temperature and Depth (CTD) systems providing detailed vertical profiles of the
thermohaline structure used during hydrographic surveys and to considerable depths of several thousand
meters and various expendable bathythermograph systems (XBT). In all cases, these temperature observations
are distinct from those obtained using remote sensing techniques and measurements at a given depth arguably
should be referred to as 'sea temperature' (ST) qualified by a depth in meters rather than sea surface
temperatures. The situation is complicated further when one considers ocean model outputs for which
the SST may be the mean SST over a layer of the ocean several tens of meters thick.
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The Foundation SST (SSTfnd)
is defined as the temperature of the water column free of diurnal temperature variability or equal to the
SSTsubskin in the absence of any diurnal signal. It is named to indicate that it is the foundation
temperature from which the growth of the diurnal thermocline develops each day. SSTfnd provides a connection
with historical "bulk" SST measurements typically used as representative of the oceanic mixed layer temperature.
This definition was adopted by GHRSST-PP at the Third GHRSST-PP Workshop (Donlon, 2003b) to provide a more precise,
well-defined quantity than previous loosely defined "bulk" temperature quantities and consequently, a better
representation of the mixed layer temperature. The SSTfnd product provides an SST that is free of any diurnal
variations (daytime warming or nocturnal cooling). In general, SSTfnd will be similar to a night time minimum or
pre-dawn value at depths of ~1-5 m, but some differences could exist. Only in situ contact thermometry is able to
measure SSTfnd. SSTfnd cannot be directly measured using either microwave or infrared satellite instruments. Analysis
procedures must be used to estimate the SSTfnd from radiometric measurements of SSTskin and SSTsubskin.
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