Remote sensing measurements of ocean colour
(i.e. the detection of phytoplankton
pigments) provide the only global-scale
focus on the biology and productivity of the
ocean’s surface layer. Phytoplankton are
microscopic plants that live in the ocean
and, like terrestrial plants, they contain
the pigment chlorophyll, which gives them
their greenish colour. Different shades of
ocean colour reveal the presence of
differing concentrations of sediments,
organic materials and phytoplankton. The
ocean over regions with high concentrations
of phytoplankton is shaded from blue–green
to green, depending on the type and density
of the phytoplankton population. From space,
satellite sensors can distinguish even
slight variations in colour that cannot be
detected by the human eye.
Ocean biology is important not only for
understanding ocean productivity and
biogeochemical cycling, but also because of
its impact on oceanic CO2 and the
flux of carbon from the surface to the deep
ocean. Over time, organic carbon settles in
the deep ocean, a process referred to as the
‘biological pump’. CO2 system
measurements, integrated with routine ocean
colour and ecological/biogeochemical
observations, are critical for understanding
the interactions between oceanic physics,
biology, chemistry and climate. CO2
measurements are also important for making
climate forecasts and for satisfying the
needs of climate conventions.
At a local scale, satellite observations of
ocean colour, usually in conjunction with
sea-surface temperature measurements, may be
used as an indication of the presence of
fish stocks. Measurements may also be used
to monitor water quality and to give an
indication of the presence of pollution by
identifying algal blooms. Measurements of
ocean colour are particularly important in
coastal regions where they can be used to
identify features indicative of coastal
erosion and sediment transfer.
An Ocean Theme was set up within the former
IGOS framework in 1999 to develop a strategy
for an observing system serving research and
operational oceanographic communities and
other users.
Building on the CEOS Ocean Biology and GODAE
Projects, the Ocean Theme Team published its
final report in January 2001. This brought
together information on:
— The variety of needs for global ocean
observations;
— The existing and planned observing
systems;
— The planning commitments required to ensure
long-term continuity of the observations.
Ocean colour measurements from space are the
focus of a new virtual constellation study
team within CEOS, the OCR-VC.
In recent years there has been a steady flow
of ocean colour data at various scales from
instruments such as OCTS (on ADEOS), SeaWiFs,
OCM (on IRS), MODIS (on Terra and Aqua), and
MERIS (on Envisat), as well as POLDER (on
ADEOS and Parasol). Many of these missions
have ended and continuity is being provided by
OCM-2 on Oceansat-2 (India), the HY-2 series
(China), and – in the near future – the
Sentinel-3 series (Europe, first launch
2014/15), among others. Complementing the data
obtained from polar-orbiting satellites, the
GOCI sensor on the COMS satellite (Republic of
Korea) provides the frequent revisit
capability offered by the geostationary orbit
over a 2500 km x 2500 km region centred on
130oE, 36oN.
NOAA is flying VIIRS on Suomi NPP (launched
October 2011) and its forthcoming JPSS
operational missions.
Four actions were identified in the CEOS
response to GCOS requirements:
— ISRO will lead planning of Oceansat-2
(launched September 2009), ESA and the EC of
Sentinel-3 (2014/15), and JAXA of GCOM-C
(2015), which are new missions planned to
carry an ocean colour sensor;
— Relevant CEOS agencies will examine their
respective plans to maintain continuity of the
25-km-resolution ocean colour global
product;
— CEOS agencies will cooperate to support the
combination of all existing ocean colour data
sets into a global FCDR;
— In consultation with GCOS and the relevant
user communities, CEOS agencies will explore
the means to secure continuity of the
1-km-resolution global ocean colour product
needed to fulfil the target GCOS
requirements.
Ocean surface topography data contain
information that has significant practical
applications in such fields as the study of
worldwide weather and climate patterns, the
monitoring of shoreline evolution and the
protection of ocean fisheries. Ocean
circulation is of critical importance to
Earth’s climate system. Ocean currents
transport a significant amount of energy
from the tropics towards the poles, leading
to a moderation of the climate at high
latitudes. Thus knowledge of ocean
circulation is central to understanding the
global climate. Circulation can be deduced
from ocean surface topography, which may be
measured using satellite altimetry. However,
altimeters can only provide this geostrophic
part of ocean currents to optimal accuracy
when the geoid is independently known with
sufficient accuracy.
Using satellite altimetry, large-scale
changes in ocean topography, such as those
in the tropical Pacific, may be observed.
During an El Niño event, the westward trade
winds weaken, allowing warm, nutrient-poor
water to occupy the entire tropical Pacific
Ocean. Conversely, during a La Niña event,
the trade winds are stronger, so that cold,
nutrient-rich water occupies much of the
tropical Pacific Ocean.
On a local scale, topographic information
from satellites may be used to support
off-shore exploration for resources,
detection of oil spills and optimisation of
pipeline routing on the sea floor.
The Topex/Poseidon (1992–2005), ERS-1
(1991–2000) and ERS-2 (1995–2011) research
and pre-operational missions have
demonstrated that satellite altimetry may be
utilised in a wide range of ocean research,
such as planetary waves, tides, global
sea-level change, seasonal-to-inter-annual
climate prediction, defence, environmental
prediction and commercial applications.
Thanks to its high altitude and
non-Sun-synchronous, dedicated orbit,
Topex/Poseidon could measure the height of
the ocean surface directly under the
satellite with an accuracy of 2–3 cm. The
follow-on Jason-1 mission, launched in
December 2001, aimed to: provide a 5-year
view of global ocean topography with the
same accuracy or better: increase
understanding of ocean circulation and
seasonal changes; improve forecasting of
climate events like El Niño; measure global
sea-level change; improve open ocean tide
models; and provide estimates of significant
wave height and wind speeds over the ocean.
These goals have been achieved and the
Jason-1 satellite clocked 10 years of
successful operation in 2011. Its successor
Jason-2/OSTM (Ocean Surface Topography
Mission, launch 2008), to be followed by
Jason-3 (launch 2015), continues the same
mission, with a partnership progressively
transferred from CNES and NASA to the
operational agencies EUMETSAT and NOAA.
Plans are underway to extend the series of
high-accuracy altimetry missions with the
ESA-studied Jason-CS satellites.
Information on ocean circulation may also be
obtained indirectly from features such as
current and frontal boundaries in SAR
imagery, and by using differences in ocean
surface temperature or ocean colour as
observed by visible and infrared imagers.
In their Final Report, in early 2001, the
IGOS Ocean Theme Team identified a long-term
need for continuity of a high-precision
mission (e.g. the Jason series) and at least
one polar-orbiting altimeter (e.g. the ERS
and Envisat series) to enhance
temporal/spatial coverage of the global
ocean. The launches of Jason-2/OSTM, and the
forthcoming Jason-3 and ESA Sentinel-3
missions, will contribute to this objective.
Additional satellite missions will ensure
continuity of ocean current measurements.
They include the Chinese HY-2A mission also
carrying a DORIS precise positioning system
receiver provided by CNES (launched 2011)
and the Indian–French SARAL (launched 2012)
that will carry an innovative Ka-band
altimeter in a polar orbit.
Since early 2012, ocean measurements from
ESA’s CryoSat-2 mission (primarily dedicated
to the measurement of tiny variations of the
thickness of polar ice) are being exploited
by CNES to provide global ocean observation
products in near-realtime, as a result of
the long-standing collaboration and
partnership between ESA and CNES.
Ocean altimetry, which is a unique and
powerful tool that can determine ocean
currents, accurately measure sea level and
detect sea-level rise – a critical indicator
of global warming as well as a crucial
parameter for ecosystems, coastal cities and
other human assets – has been recognised as
a priority for future sustained
observations. This is the goal of the Ocean
Surface Topography Constellation established
by CEOS for GEO.
Two actions were identified in the CEOS
response to GCOS requirements:
— Establishment of an Ocean Surface
Topography Constellation, including a future
Jason-3 mission;
— CNES and ISRO will cooperate on a new
polar-orbiting altimeter aimed at filling a
potential data gap beyond 2008 (the SARAL
mission carrying the AltiKa altimeter,
launched 2012). ESA and the EU will lead
planning for Sentinel-3 to carry an
altimeter.
Ocean salinity measurements are important
because surface salinity and temperature
control the density and stability of the
surface water. Thus, ocean mixing (of heat
and gases) and water-mass formation
processes are intimately related to
variations of surface salinity. Ocean
modelling and analysis of water mass mixing
should be enabled by new knowledge of
surface-density fields derived from surface
salinity measurements. The importance of the
ocean in the global hydrological cycle also
cannot be overstated. Some ocean models show
that sufficient surface freshening results
in slowing down the meridional overturning
circulation, thereby affecting the oceanic
transport of heat.
Sea surface salinity is emerging as a new
research product from satellite measurements
of ocean brightness temperature at L-band
(microwave) frequencies. The monitoring of
surface salinity from space, combined with
the provision of regular surface and
subsurface salinity profiles from in situ
observing systems, such as surface ships,
buoys and the Argo array, will provide a key
constraint on the balance of freshwater
input over the ocean.
This will allow for better determination of
the marine aspects of the planetary
hydrological cycle and the possibility of
important ocean circulation changes. New
research missions must demonstrate
capabilities and pave the way to future
continuous, climate-quality data records.
The contribution from space-based
observations to this variable is underway,
with ESA and NASA/CONAE (Comisión Nacional
de Actividades Espaciales of Argentina)
respectively flying demonstrator missions
(SMOS, 2009 and Aquarius/SAC-D, 2011) for
salinity measurements.
CEOS identified two actions in response to
the GCOS IP in relation to this
measurement:
— ESA will launch SMOS in late 2009 to
demonstrate measurement of the sea surface
salinity (and soil moisture) ECV; NASA/CONAE
will fly Aquarius/SAC-D in 2011 to
demonstrate measurement of the sea surface
salinity ECV.
— CEOS agencies will cooperate in developing
future plans for an Ocean Salinity
Constellation.
Essential Climate Variables: (Atmospheric)
Surface Wind Speed and Direction
High-resolution vector wind measurements at
the sea surface are required in models of
the atmosphere, ocean surface waves and
ocean circulation. They are proving useful
in enhancing marine weather forecasting
through assimilation into NWP models and in
improving understanding of the large-scale
air–sea fluxes which are vital for climate
prediction purposes. Accurate wind vector
data affect a broad range of marine
operations, including offshore oil
operations, ship movement and routing. Such
data also aid short-term weather forecasting
and the issue of timely weather warnings.
Polar-orbiting satellites provide
information on surface wind with global
coverage, good horizontal resolution and
acceptable accuracy, though temporal
frequency is marginal for regional mesoscale
forecasts. They provide useful information
in two ways:
— Scatterometers provide dense observations
of wind direction and speed along a narrow
swath, although the most recent and planned
scatterometers provide better coverage via
broader swaths (90% global coverage daily);
scatterometers have made a positive impact
on predicting marine forecasting,
operational global NWP and climate
forecasting;
— Passive microwave imagers and altimeters
provide information on wind speed only.
The single-swath scatterometer on ERS-1/2
and the broad-swath scatterometer on
QuikSCAT long provided adequate coverage,
but these missions are now complete.
QuikSCAT, launched in 1999, carried the
SeaWinds scatterometer that measured
near-surface wind speed and direction in all
weather and cloud conditions. Global
coverage by a broad-swath scatterometer is
now provided by ASCAT on the European
MetOp-A (launched 2006) and soon MetOp-B
(launched 2012) missions. Developed by ESA
as a follow-on from the ‘wind mode’ of the
AMI on the ERS series, ASCAT is used
primarily for global measurement of
sea-surface wind vectors and provides
quasi-global coverage within 24 hours. The
SSM/I (Special Sensor Microwave/ Imager), on
the US DMSP satellites, is providing
operational surface wind data. The
cooperative NASA/JAXA AMSR-E on Aqua
(launched in 2002) also provided data on
sea-surface wind speed until it stopped
operating in October 2011. AMSR-2 now flies
on JAXA's GCOM-W mission.
In recent years, the ability to detect and
track severe storms has been dramatically
improved by the advent of weather
satellites. Data from scatterometers such as
SeaWinds or ASCAT have been proven to
augment traditional satellite images of
clouds by providing direct measurements of
surface winds, enabling better determination
of a storm’s location, direction, structure
and strength.
In its response to the GCOS IP, CEOS agreed
to review the capability of passive
microwave sensors to make
scatterometer-quality measurements and will
work to ensure AM and PM satellite coverage
of surface wind speed and direction by
2015.
Essential Climate Variables: Sea-Surface
Temperature
Ocean surface temperature (often known as
‘sea-surface temperature’ or SST) is one of
the most important boundary conditions for
the general circulation of the atmosphere.
The ocean exchanges vast amounts of heat and
energy with the atmosphere and these air/sea
interactions have a profound influence on
Earth’s weather and climate patterns. SST is
linked closely with the ocean circulation,
as demonstrated time and again by the El
Niño-Southern Oscillation (ENSO) cycle. A
major research goal is to enable seasonal
and longer time scale forecasting by
development of coupled atmosphere and ocean
models that correctly link the many
processes. Progress towards this goal
depends on a more precise and comprehensive
set of SST measurements for use in
initialising and verifying such models.
Satellite remote sensing provides the only
practical means of developing such a
dataset. In situ data, predominantly from
ships of opportunity and from networks of
moored and drifting buoys, are limited in
coverage, whereas satellites offer the
potential for surveying the complete ocean
surface in just a few days. The in situ data
have a key role to play in calibrating the
satellite data and in providing information
needed for deriving bulk temperatures.
Instruments on polar satellites provide
information for short- to medium-range NWP
with global coverage, good horizontal and
temporal resolution and accuracy, except in
areas that are persistently cloud-covered.
Accurate SST determinations, especially in
the tropics, are important for seasonal to
inter-annual forecasts. The advent of high
spectral resolution infrared sounders will
enable separation of surface emissivity and
temperature, and the accuracy of the SST
product is expected to improve to an
acceptable level.
Geostationary imagers with split window
measurements are also helping to expand the
temporal coverage by making hourly
measurements, thus creating more
opportunities for finding cloud-free areas
and characterising any diurnal variations
(known to be up to 4K in cloud-free regions
with relatively calm seas). For regional
NWP, sea-surface temperature is inferred
with acceptable horizontal resolution from
polar satellites, while geostationary
satellites complement information with
better temporal resolution.
A range of instruments with thermal bands
is being used for SST measurements.
Visible/infrared imagers such as AVHRR and
MODIS currently provide the main source of
SST data, with (AATSR, ended 2012) and MODIS
providing better accuracy (0.25–0.3K).
AVHRR, however, gives greater coverage,
enabling it to track ocean currents and
monitor ENSO phenomena through its larger
swath width. The Aqua mission, which
includes MODIS along with AIRS+ and AMSR/E
(until October 2011), provides
oceanographers with further precise
information and the ability to remove
atmospheric effects. NOAA’s VIIRS instrument
on the planned JPSS missions (and flown on
Suomi NPP, launched October 2011) will
provide capabilities to produce higher
resolution and more accurate measurements of
SST than currently available from AVHRR.
Other sources of SST data include: the
Imager on the Japanese MTSAT series, the
SEVIRI and IASI instruments on the
Meteosat-8/9 (MSG-1/2) and MetOp missions,
respectively.
The GHRSST Pilot Project provides a new
generation of global, high-resolution
(<10 km) SST products, combining
complementary satellite and in situ data
(www.ghrsst-pp.org/).
GCOS is concerned that the continuity of the
4 km-resolution global data be maintained
through adequate instruments on operational
weather satellites and its quality must be
enhanced through high-precision sensors on
other Earth observation missions. CEOS has
defined four actions in support:
— An ATSR-like instrument is planned on
ESA’s Sentinel-3, presently scheduled for
launch in 2014/15. JAXA will lead planning
for the Global Change Observation
Mission-Water (GCOM-W, launched 2012) to
maintain continuity of the sea-surface
temperature ECV;
— CEOS agencies will examine their
respective plans to maintain provision of
microwave brightness temperatures for the
sea-surface temperature ECV;
— Relevant CEOS agencies will examine their
respective plans to maintain continuity of a
10 km resolution sea surface temperature
data sets global product;
CEOS agencies will cooperate to support the
combination of all existing sea-surface
temperature datasets into a global FCDR.
CEOS has established a new SST virtual
constellation team to address these
actions.
Sea state and wind speed govern air–sea
fluxes of momentum, heat, water vapour and
gas transfer. The state of the sea and
surface pressure are two features of the
weather that are important to commercial use
of the sea (e.g. ship routing, warnings of
hazards to shipping, marine construction,
off-shore drilling installations and
fisheries). Information on surge height at
the coast is key to the protection of life
and property in coastal habitats.
These data are also important for climate
purposes because they are needed for the
correct representation of turbulent air–sea
fluxes.
Wave height is influenced by wind speed and
direction, the wind ‘fetch’ and its rate of
change. In the nowcasting context, ocean
wave models are driven by NWP predictions of
surface wind. However, errors in waves
generated at large distances can accumulate.
Improvements in forecasts, especially of
long wavelength swell, can be achieved by
assimilating observations from different
sources. These are currently available from
isolated buoys, satellite altimeter and
scatterometer data. In the absence of direct
observations, initial wave state is deduced
from the wind history. This is currently
available over the sea from isolated buoys
and from low-Earth satellite scatterometer
and microwave instruments.
For global NWP, ships and buoys provide
observations of acceptable frequency that
are acceptable to marginal accuracy, but
coverage is marginal or absent over large
areas of the ocean. Altimeters on polar satellites provide
information on significant wave height with
global coverage and good accuracy, but
horizontal/temporal coverage is marginal.
Information on the 2D wave spectrum is
provided by SAR instruments with good
accuracy, but marginal horizontal/temporal
resolution.
SAR instruments can accurately measure
changes in ocean waves and winds, including
wavelength and the direction of wave fronts,
regardless of cloud, fog or darkness. The
AMI SAR on ERS-2 operated in both wave and
image mode, and the ASAR on Envisat
continued to provide the ERS wave mode
products, but with improved quality. PALSAR
on JAXA’s ALOS mission provided data on
sea-surface wind and wave spectrum required
for oil spill analysis and for studies of
coastal topography–air–sea interaction.
The ScanSAR wave data supplied by RADARSAT
continues to be provided by RADARSAT-2.
Europe’s Sentinel-1 mission will also ensure
future provision of SAR data supply.
Information from radar altimeters, such as
that on the Jason-1 and Jason-2 missions, is
limited to data on significant wave
height.
The GCOS IP recognises that altimetry and
SAR measurements useful for sea state
measures (wave height, direction, wavelength
and time period) have been continuously
available since 1991 and will be maintained
in the future, but no consolidated data
product has ever been produced. GCOS
proposes that new altimeter (wide-swath) and
SAR technologies are needed to advance
retrieval of near-shore sea state
parameters. CEOS agencies propose to
cooperate with the user community to support
efforts aimed at building on the decade-long
satellite sea state records and making a
comprehensive use of future altimeter- and
SAR-bearing missions.
In addition to the specific ocean
measurement observations discussed in
previous sections, a number of sensors are
capable of providing a range of ocean
imagery from which useful secondary
applications can be derived.
High-resolution radiometers, such as AVHRR,
AATSR, and VIIRS, have multi-channel imaging
capabilities to support the acquisition and
generation of a variety of applied products,
including visible and infrared imaging of
hurricanes. They provide observations of
large-scale ocean features, using variations
in water colour and temperature to derive
information about circulation, currents,
river outflow and water quality. Such
observations are relevant to activities such
as ship routing, coastal zone monitoring,
toxic algal bloom detection, management of
fishing fleets and sea pollution
monitoring.
High- to medium- resolution imaging sensors,
such as MERIS and MODIS, are better suited
to observations of coastal zone areas and
can provide information on sedimentation,
bathymetry, erosion phenomena and
aquaculture activity.
In addition, SAR instruments, such as on
RADARSAT-2, provide a valuable all-weather,
day and night source of information on
oceanographic features, including fronts,
eddies and internal waves. SAR imagery is
also useful for: