French review of the 13thsession of the GLOSS Group of Experts and workshop commemorating the 80th anniversary of PSMSL

The study and monitoring of recent and current climatic variations in sea level is organized at the worldwide level around the Global Sea Level Observing System (GLOSS) program of UNESCO’s Intergovernmental Oceanographic Commission. Every two years, the program’s group of experts meets to assess the current situation concerning national tide gauge observatories. This year (28 October to 1 November 2013), the event was held in Liverpool, the headquarters of PSMSL. Prior to the GLOSS meeting, PSMSL (Permanent Service for Mean Sea Level) hosted a workshop on major research topics in sea level science in order to commemorate its 80th anniversary. France was represented by the University of La Rochelle and SHOM.

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Report on the workshop organized by PSMSL

The Workshop hosted by PSMSL helped to address different scientific approaches to calculating global and regional sea level trends from altimetry and tide gauge data. The importance of having a record of tidal measurements over at least 40 years was pointed out. In this respect, the complementarity of tide gauge records with the 20 years of accumulated altimetry data today was recognized as fundamental.

The need for each country to have an inventory of tide gauge data available in analogue format (paper records, marigrams) was raised to identify the priority volumes, which, once scanned, will constitute tide gauge records covering more than a century. In this respect, the French inventory was taken as a model. Though far from being exhaustive, it does already include 2200 years of analog measurements.

In addition, emphasis was placed on the importance of correcting tidal measurements for vertical movements of the land base on which the tide gauge is installed to take into account the effects of post-glacial rebound (very marked in the Polar Regions) and the tectonic movements associated with volcanic and seismic areas. In addition to the modeling of these phenomena, GNSS antennas (GPS, Galileo, GLONASS) collocated with the tide gauges are used to measure the vertical movements and reference the tidal observations to the IGS spatial data.

Finally, many presentations highlighted the complexity of the processes involved in the climatic study of changes in mean sea level: Thermal expansion due to warming of the water mass, melting ice (Greenland, Antarctica, continental glaciers), management of inland freshwater resources (dams, consumption, waste). Efforts are being made to educate the authorities about the importance of anticipating and adapting to a gradual rise in sea level estimated, for 2100, at between 26 and 82 cm, depending on the climate scenarios (IPCC 2013).

 

Report on the GLOSS sessions

During the GLOSS meeting that followed, the national representatives heard the activity reports of the GLOSS institutions and data centers and engaged in further technical discussions on the applications of sea level measurements.

Several GLOSS data centers reported on their progress: PSMSL on mean levels, UHSLC on qualified hourly data, VLIZ on raw real time data, and the French Sonel web portal on GNSS observations collocated with the tide gauges.

An inventory of the digitization techniques for analogue tide gauge measurements was also presented. It appears that only the NUNIEAU software developed by CETE Méditerranée includes specific tools for scanning marigrams, and could therefore meet the needs of the international community. Moreover, for scanning records and charts, a specific project could be based on the methods of the  ACRE project for scanning by the general public of the meteorological observations archived in the logbooks of the Royal Navy.

In keeping with the topics discussed at the workshop, special attention was paid to the levelling of data from tide gauges, tide benchmarks and the GNSS antenna. These measurements ensure that the movements measured by the antenna are identical to those of the tide gauge. For this reason, the installation of the GNSS antenna within 1km of the tide gauge station is highly recommended.


As regards measurement accuracy, it was unanimously agreed that radar sensors are better than acoustic sensors due to better vertical accuracy and virtually zero drift. However, regular verifications are still necessary (air drafts and/or tide staffs) to detect errors due to the wear of the system (delamination, shocks) and to properly control the vertical reference of the station. France, a pioneer in the use of radar tide gauge sensors, promoted its expertise, in particular with regards to stilling wells.

Finally, France, Japan, Korea, Mexico, Pakistan, Philippines, Saudi Arabia and Sweden presented their national progress reports reflecting the global scope of the GLOSS recommendations. The presentation materials are online on the portal of the IOC-GOOS.

 

References

 

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