Report on the Status of the Global Sea Level Observing System (GLOSS) to the IOC Assembly June-July 1999

Dr. Philip L.Woodworth
Chairman, GLOSS Group of Experts
Proudman Oceanographic Laboratory
Bidston Observatory, Birkenhead, Merseyside
L43 7RA, U.K.

Dr. Thorkild Aarup
GLOSS Technical Secretary
Intergovernmental Oceanographic
1 Rue Miollis, 75732 Paris Cedex 15, France

This report summarises the status of the Global Sea Level Observing System (GLOSS) following the sixth meeting of the Group of Experts (GE) in Toulouse 12-14 May. The list of achievements during the past two years, following the acceptance of the GLOSS Implementation Plan (GIP) by the IOC Assembly in 1997, is an impressive one, although much remains to be done. The reader will recall that the GIP called for the development of a GLOSS Core Network (GCN) of 287 stations, a network of several 10s of sites for ongoing altimeter calibration (GLOSS-ALT), a programme of investment in gauges with geodetic equipment (especially GPS) at sites with long records (the -LTT, or long term trends, set), and the use of gauges at straits and other strategic locations for ocean circulation monitoring (the -OC set).

GCN Status from a PSMSL Viewpoint (October 1988)

For the last few years, the PSMSL has provided a summary of the status of the GCN from its viewpoint. An 'operational' station from a PSMSL viewpoint means that recent Mean Sea Level (MSL) monthly and annual values have been received at Bidston, have been checked as far as possible, and have been included in the databank. For each of the GLOSS stations, we have used the year of the last data entered into the databank, if any, to place the station into one of four categories:

Category 1: 'Operational' stations for which the latest data is 1994 or later;
Category 2: 'Probably operational' stations for which the latest data is within the period 1984-1993;
Category 3: 'Historical' stations for which the latest data is earlier than 1984;
Category 4: For which no PSMSL data exist.

The numbers in Category 1, 2, 3 and 4 as of October 1998 were 183, 48, 20 and 36 respectively, representing a slow but steady improvement over previous years. Most further investment is required in remote areas (e.g. Antarctica), Africa and South America

GCN Operational and Non-Operational Stations Survey (December 1998)

In December, a detailed survey was conducted of the 287 stations of the GCN to determine which gauges were operational or not as of the previous February, the deadline for receipts of questionnaire replies from national authorities as part of 'GLOSS Handbook' updating. The replies were supplemented by 'PSMSL general knowledge' in the case of non-replies.

Of the 287 sites in the GCN (defined by 'GLOSS97'), only 42 are claimed to be non-operational. It is clear that action must be taken by national authorities to instrument the sites in order to complete the network. If national resources are limited, the authorities should be endeavouring to install equipment through bi-lateral links or possibly by making requests for second-hand equipment through IOC. Alternatively, if a site cannot feasibly be instrumented, perhaps owing to environmental conditions, then IOC should be notified so that it can be reviewed for removal from the definition of the GCN at the next revision of the network.

Reconciliation of the Statistics of the Previous Two Sections

It is clear that the December 1998 survey presents a more optimistic view of GLOSS status than do the statistics compiled from the PSMSL, a situation which requires investigation. One reason is that at some 'operational' locations (e.g. Tristan da Cunha and some Antarctic sites), the gauges take the form of simple pressure transducers which provide useful information for oceanography (e.g. for the World Ocean Circulation Experiment) but which do not supply MSL data, as conventionally defined, which can subsequently be submitted to the PSMSL. This situation is understandable and tolerable if there are good environmental or technical reasons for such a choice of technology.

However, a second reason is that while an 'operational' gauge might exist and be providing data of some kind, the expertise or facilities or manpower do not exist in order to process those data routinely and deliver them to the international community. This situation is not an acceptable one, as it clearly requires some kind of investment in hardware, software or training. The job of IOC/GLOSS is to remedy such situations as far as possible.

Higher Frequency Data (HFD) Availability and Real Time HFD

A survey by Dr. Lesley Rickards on the availability of HFD (typically hourly values) from the GCN, available either at international centres or on national servers as required by Chapter 7 of the GIP, was presented to the GE6 meeting. Altogether, there are approximately 200 tide gauge sites which have some form of HFD on-line. For some of these, real-time plots of the data are available, but not the data values themselves. Most of the remainder have hourly values available on the web from one or both of the WOCE Sea Level Data Assembly Centres (at UHSLC and BODC/PSMSL), but obviously some of these are duplicated at national sites. Eighty-five GLOSS stations report to the WOCE 'Fast Delivery' DAC at the UHSLC (see below), with data usually available within one to two months of data collection.


During the GE6 meeting, a review was given of the use of tide gauges equipped with GPS or DORIS geodetic equipment as 'tide poles' for the ongoing calibration of altimeters (GLOSS-ALT). This work has been led in particular by Dr. Gary Mitchum with contributions from several other groups. The developments are so significant that to a great extent we may call GLOSS-ALT operational. A journal paper is in preparation.

GPS at Tide Gauges

Two of the three workshops held in Toulouse prior to the GE6 meeting were concerned with the development of GPS at gauge sites for use by GLOSS-ALT and -LTT. This important topic has been led by Dr. Mike Bevis, following the PSMSL/IGS GPS Workshop in Pasadena in 1997. Much has been learned over the past two years on how to operate GPS at gauges, and a 'Manual' on how to conduct such operations will be produced before the end of the year, including a number of case studies.

GLOSS Fast Delivery

The GE6 discussed in some detail the work of the two World Ocean Circulation Expeiment (WOCE) Sea Level Centres (Fast Delivery and Delayed Mode), and congratulated them on their work during the WOCE programme. It recognised that the model of two Centres working together had been a successful one, and the meeting recommended that the Fast Centre at the University of Hawaii continue and extend its work as the 'GLOSS Fast Delivery Centre'. The Group further urged the two WOCE Centres, and other international and national sea level centres, to work towards as far as possible a 'seamless' GLOSS scientific sea level data set in line with Chapter 7 of the GIP.

The need for real time data, for science programmes (e.g. assimilation or altimeter calibration) but also for more efficient data gathering and quality control, was a special theme at GE6. Experiments in South Africa and the UK have established the utility of relatively inexpensive shaft encoders attached to modems for the digitisation of information from conventional float gauges, and a special effort will be made to arrange all authorities to go 'real time' in the near future.

IOC/GLOSS Training Courses

Two IOC/GLOSS training courses have been held in the last two years. The first was at the Proudman Oceanographic Laboratory (POL) during June 1997 for sea level scientists from Mediterranean and Black Sea countries. The second larger GLOSS training course was held at the University of Cape Town (UCT) during November 1998. The opportunity was taken to also discuss plans for action in east, south and west Africa as part of GLOSS and of the Global Ocean Observing System (GOOS) for Africa. This topic formed a large part of the discussion during GE6. A further training course is planned at the University of Sao Paulo, Brazil in September 1999.

GLOSS Training Materials

Several sets of tidal analysis software continue to be widely distributed and play a major role in improving data quality and timely delivery. The IOC Manuals and Guides No.14 on methods for operating gauges will be re-written and updated during 1999. Several GLOSS-related CDROMs have been produced over the last few years and data products continue to be made available via the web.

Newsletters and Brochures

There have been six issues of the GLOSS Bulletin on the web and a seventh is in preparation. The Afro-American GLOSS News (AAGN) also continues to be produced regularly. This newsletter has articles mostly in Spanish and Portuguese, and has been produced by the University of Sao Paulo on paper and on the web. At GE6, it was recommended that the AAGN be co-produced with the UCT so as to represent African interests the more. An updated two page brochure advertising GLOSS has been produced by the PSMSL with 2000 copies printed for circulation in the UK. We hope that others will be printed by UNESCO and that GLOSS National and Regional Contacts will arrange for printing in their own countries.

Sea Level Requirements of the GOOS Coastal Module

Dr. E. Marone (GOOS Coastal Module Vice Chairman) reviewed during the GE6 meeting the status of C-GOOS and in particular the concept of a GOOS network being developed by Prof. K. Thompson which would have major sea level requirements for hardware and training. Joint training was suggested throughout GLOSS/C-GOOS with representation on each other group's panel. The possibility was suggested into the use of data loggers at gauges to record other parameters of interest.

GLOSS GE and Sub-Group Developments

Also during the GE6 meeting, the Group endorsed the 'ex officio' right to membership of the GE by the Directors PSMSL, UHSLC, NTF, WOCE Centres, IAPSO/CMSLT, IGS and other future appropriate bodies, although it was emphasised that this list was not exclusive or exhaustive. This extension of the GE could have the benefit of increasing the number of people well-briefed about GLOSS who will be able to represent the programme at international meetings.

In addition, discussion took place on a proposal that a sub-group of the GE be formed as a source of scientific advice, especially for climate, with the sub-group potentially a joint committee with (at present) OOPC, CLIVAR/UOP and IAPSO/CMSLT. The Group endorsed the concept, and recommended that a second sub-group be considered in consultation with C-GOOS with regard to coastal sea level aspects.

More Information?

The reader is referred to the meeting report of the GE6 meeting in May 1999, which provides a good overview of current GLOSS activities. Copies should be available at the Assembly. The report contains an excellent set of recommendations and intended actions (including overlaps with C-GOOS and the search for more resources for GLOSS) for the next two years which should carry GLOSS forward into the next decade.

Recommendations of the Sixth Session of the IOC Group of Experts on GLOSS

Assembly Recommendations

. The Group recommended the establishment of a Scientific Steering Group for Climate as a joint sub-committee of the GLOSS Group of Experts and related bodies (at present OOPC, CLIVAR/UOP, IAPSO/CMSLT). Further, the Group recommended that discussions with C- GOOS be conducted with regard to a joint sub-committee for coastal sea level studies.

. The Group recommended the formation of an African GLOSS Network to coordinate all aspects of sea level activities in Africa. These activities would include a tide gauge network, GPS monitoring at specific sites, altimetric coverage, data acquisition and exchange, professional training and sea level research.

. Further, the Group supported the need to take every opportunity to upgrade the sea level instrumentation in Africa. In particular, the non-operational sites with long historical records should be revived.

. In addition, the Group supported the establishment of a network of National Oceanographic Data Centres in West Africa. This will ensure that oceanographic data, including GLOSS data, collected in the region would be immediately available in that region.

. Realizing the importance of the Arctic Ocean for studies of climate variability and the early detection of climate change, and taking into account that the presently available satellite altimetry observations do not cover the Arctic ocean sufficiently, the GLOSS group of experts:

recommends that in each country bordering the Arctic Ocean efforts are made to maintain a network of tide gauges conforming to GLOSS standards;

in particular, strongly recommends that the Canadian GLOSS tide gauges are re-established; urges the international funding agencies to support projects that will help to reverse the current downward trend in the maintenance of Russian Arctic tide gauges;

urges Denmark to secure the long-term operation of the GLOSS tide gauges in Greenland; urges Norway to establish and operate tide gauges corresponding to GLOSS standards on Jan Mayen and Bjornoya;

recommends that international support is given for the continued operation and maintenance of the tide gauge in Barentsburg;

recommends that efforts are made to co-locate an approximately equidistant subset of the Arctic tide gauges with space-geodetic techniques (GPS) and to carry out absolute gravity measurements at these gauges.

. The Group endorsed efforts to expand sea level monitoring in the Northern Indian Ocean via tide gauge network developments and associated storm surge warning systems. Further, the Group noted the current lack of data flow from the region.

. The Group stressed the importance of access of historical sea level time series from GLOSS sites for, amongst many other purposes as described in the GLOSS Implementation Plan, improved tidal constants. The Group also requested the International Hydrographic Organisation to make use of the good offices of the Permanent Service for Mean Sea Level to provide tidal constants from the IHO data bank to relevant researchers.

. The Group requested the IOC to investigate the use of GOOS TEMA programme for GLOSS-related Fellowships.

. The Group recommended that the IOC be requested to make funds available for sea level data archaeology.

Internal Recommendations/Endorsements

. The Group endorsed a proposal that the Directors of PSMSL, UHSLC, NTF, WOCE Centres, IAPSO/CMSLT, IGS etc. be recognized as ex officio members of the GLOSS GE.

. The Group congratulated the two WOCE Centres (Fast Delivery and Delayed Mode) on their work during the WOCE programme, and recognized that the model of two Centres working together had been a successful one. The Group recommended that the UHSLC continue and extend its work as the 'GLOSS Fast Delivery Centre'. The Group further urged the two WOCE Centres, and other international and national sea level centres, to work towards as far as possible a 'seamless' GLOSS scientific sea level data set in line with Chapter 7 of the GLOSS Implementation Plan.

. The Group recommended that all geodetic ties between gauges and other geodetic devices (e.g. GPS, DORIS) be accepted first as the responsibility of the gauge authorities to implement and document, with all relevant information conveyed to data centres.

. The Group urged all authorities to take steps to convert all gauges to real time reporting.

. The Group recommended that regular missions be conducted to all regions to assess facilities and requirements for tide gauge and geodetic equipment and training, and for the assessment of historic non-digital data for data archaeology, with Central and South America being one such priority region.

. The Group endorsed the aims of the EOSS activity in Europe, in particular the establishment of a European Sea Level Service which, amongst other functions, will improve the general availability of hourly sea level data from the region, and which will provide a test case for regional implementation of GLOSS. The Group urged EOSS to collaborate closely with programmes such as EuroGOOS.

. The Group recommended that countries of the Indian Ocean with gauges provided by the University of Hawaii make efforts to assume responsibility for all aspects of maintenance and data flow.

. The Group recommended that the Uruguay station at Montevideo be re-established as soon as possible with a second GLOSS site at La Paloma to be investigated.

. The Group endorses the concept of GLOUP bottom pressure measurements towards the understanding of ocean tides and dynamics of ocean circulation and the need for a GLOUP data bank of bottom pressure data alongside sea level information.