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: