GCOS Ocean Surface ECV
Sea Level

Definition: Sea Level - The level of the sea after averaging out the short-term variations due to wind waves. It is used loosely as a synonym for mean sea level. (from the AMS Glossary of Meteorology)

Introduction: Tide gauge sea-level data constitute one of the few long historical ocean climate time series, but in general, sampling and global coverage of sea-level change by the tide gauge network is inadequate. To monitor global sea-level change and to put regionally observed changes into the global context, satellite ocean surface topography altimetry is essential. Knowledge of global sea-level variability increased substantially in 1993 when the TOPEX/POSEIDON altimeters commenced operation. Monitoring of global sea level is technically feasible using complementary in situ networks and satellite measurements. 
 
Networks and systems contributing to the observation and global analysis of sea level include:

1.  Global Sea Level Observing System (GLOSS) Core Network plus additional regional and national networks and specific enhancements for detecting trends and calibrating satellites
2.  Satellite high-precision altimetry
3.  Low-precision high-resolution altimeters
4.  Sub-surface temperature and salinity network
5.  Satellite high-precision measurements of the time-mean and time-varying geoid

The GLOSS Core Network of about 300 gauges has been recommended as the desired in situ measurement network. Unfortunately, data are not available to the global community from a number of these gauges.  Ideally, all gauges in this network should become geocentrically-located, and nations should exchange data effectively and timely. Figure 10 shows the present tide gauge system, including information about which records are longer than 40 years, and which gauges are geocentrically-located at this time. Although some enhancements are in place, more regional and national enhancements of the Core Network will be needed to address regional and local impacts of sea level, including extreme events. These enhancements should ensure that high-frequency sea-level observations be taken and exchanged and that historical data from tide gauges be recovered as appropriate and provided to the International Data Centres. They should also include capacity-building efforts in developing countries for undertaking local sea-level change measurements which can benefit the global system, foster needed regional enhancement and will foster the improvement of global and regional tide models.
 
To consider possible impacts, the effect of isostatic rebound and human action on subsidence needs to be quantified which locally can be of the same order as global sea-level change. Spatial scales of drainage-induced subsidence are small requiring a substantial enhancement of the local tide gauge network or geodetic measurements in critical regions. Jointly with the tide gauge network, continued operation of high-precision satellite altimetry and sun-synchronous altimeter measurements complete the sea-level network. Together they represent an integrated strategy for monitoring of sea-level variability and change globally and on regional scales. The Arctic Ocean is an essential component of this and needs to be observed as well.
 

1. The spatially complex variability of the vertical motion of the surfaces upon which tide gauges are located.  In addition to the ongoing larger-scale tectonic post-glacial adjustment, there are often local vertical movements of consequence caused by human activities. These vertical movements are of the same magnitude as the anticipated sea-level change from global warming, and must be measured at each tide gauge in order to reveal water level changes relative to the centre of the Earth.  
2.  The existence of large-scale low frequency variability of the oceanic density field which, if not appropriately and globally sampled, can introduce uncertainty in estimates of global sea-level change.
3.  The lack of detailed knowledge of mass being exchanged between polar ice sheets and the ocean has the potential to introduce a substantial uncertainty in understanding predictions of sea-level change which can by far exceed the uncertainties arising from changes in the density field of the ocean. 
4.  The need to know variability and change in surface pressure, which affects water level.  
5.  The technical challenges of making accurate high-frequency water level measurements over long periods.
6.  The present shortcomings of international data exchange.
7.  The need for significant regional and local network enhancements in order to support impact assessment and monitoring.
8.  The need for financial support for equipment purchase and maintenance, and technical assistance projects for capacity-building for small island developing states and least-developed nations.

The following set of Actions is proposed to develop an adequate sea-level observing and analysis programme together with the capacity to apply the global products on regional and local scales:
Through the GLOSS of JCOMM, implement the Core Network with geocentrically-located high-accuracy water level gauges with real-time data reporting. Since sea-level observations should now be reported to the International Data Centres in a timely fashion in accordance with IOC Resolution XXII-6 (IOC Oceanographic Data Exchange Policy)), the GLOSS will provide regular reports to the IOC on the extent to which the data are being exchanged.  
 
One high-precision altimeter at medium inclination is required at all times with planned extensive overlaps between successive missions, as well as two medium-precision, higher-inclination altimeters to provide the needed sampling. In addition, continuous precise geoid measurements are required to provide a reference for the altimeter data, to determine mass redistribution within the ocean, and to provide estimates of mass exchange between the cryosphere and the ocean. GCOS, through its participation in the WMO Consultative Meetings on High Level Policy on Satellite Matters, CGMS, and CEOS will continue to emphasize the need for the continued operation of high-precision and sun-synchronous satellite altimeters (in accordance with the GCMP). Implementing the CEOS Constellation for Ocean Surface Topography is planned to provide a sustained, systematic capability to observe the surface topography of global oceans.  

(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

Satellite Observations: Gravity field measurements from space provide the most promising advances for improved measurement of the ‘geoid’ and its time variations. The geoid is the surface of equal gravitational potential at mean sea level, and reflects the irregularities in the Earth’s gravity field at the planet’s surface caused by the inhomogeneous mass and density distribution in the interior. Such measurements are vital for quantitative determination – in combination with satellite altimetry – of ocean currents, improved global height references, estimates of the thickness of the polar ice sheets and its variations, and estimates of the mass/volume redistribution of fresh water in order to better understand the hydrological cycle.

Gravity field measurement packages on satellites often utilise combinations of different instrument types in order to derive the necessary information: single or multiple accelerometers; precise satellite orbit determination systems; and satellite to satellite tracking systems. DLR’s CHAMP gravity package (since 2000) and the NASA/DLR twin satellite GRACE mission (since 2002) have been providing new information that has resulted in new and unique models of the Earth’s gravity field and its variability over time, and determination of the geoid to centimetre accuracy at length scales of several hundred kilometres. GRACE has demonstrated that satellites can detect groundwater variations by measuring subtle temporal variations in gravity. From 2009, this data is supplemented by ESA’s GOCE satellite, which is designed to make significant advances in our understanding of ocean circulation and the crucial role which it plays in regulating the climate, as well as sea level rise and processes occurring in the Earth’s interior. GOCE data will also find a broad range of applications in the field of geodesy and surveying. A number of Earth missions, including Australia’s Fedsat, launched in 2002, have carried sensors to study the electromagnetic environment of spacecraft. Satellite-borne magnetometers provide information on the strength and direction of the Earth’s internal and external magnetic field and its time variations. Such instruments are on board the Ørsted satellite, which is Denmark’s first satellite dedicated to the magnetic field, launched in 1999. The CHAMP mission also provides these measurements, which are of value in a range of applications, including navigation systems, resource exploration drilling, spacecraft attitude control systems and assessments of the impact of ‘space weather’. Further missions are under way or planned for more in-depth, dedicated studies of magnetic field. They include DEMETER (launched June 2004), which is investigating links between earthquakes and magnetic field variations, and Swarm (from 2010), which aims to provide the best ever survey of the geomagnetic field and its temporal evolution, providing new insights by improving our knowledge of the Earth’s interior and climate. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement)

Additional Information:

• Global Ocean Observing System (GOOS) Observation Program Growth (includes Sea Level information)
• National Activities Summaries of Operational & Planned Observation Programs (Moorings, ARGO, Sea Level, XCTD/XBT/TSG, TS Hydrography, VOS, Sea Ice, Satellites, Black Sea, BOOS, NEAR-GOOS, Bio/Chem, Carbon, Coastal)

References:

• Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523
• The Second Report on the Adequacy of the Global Observing Systems for Climate in Support of the UNFCCC - April 2003 (GCOS-82/WMO/TD Noc 1143)
• CEOS EO Handbook - Earth Observation Plans by Measurement
• NOAA/NCDC - Climate Indicators
• BAMS State of the Climate Reports

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