QUALITY ASSESSMENT OF SEA LEVEL DATA by the UH SEA LEVEL CENTER/NATIONAL OCEANOGRAPHIC DATA CENTER JOINT ARCHIVE FOR SEA LEVEL updated: 22 Jul 2008 Station : St. Helena-A Latitude: 15 55.0S Country : United Kingdom Longitude: 005 43.0W JASL # : 292A Time Meridian: 000E (GMT ) GLOSS # : 264 TOGA #: A037 NODC #: 51005501 Contributor : National Oceanography Centre Proudman Oceanographic Laboratory 6 Brownlow Street Liverpool L69 3GP United Kingdom Originator : same Original Data: digital Instrmnt Type: Digiquartz pressure sensor and barometer Digitzd Intvl: 15 minutes Present Data : Hourly, daily, and monthly values obtained by: Hourly : average of 15-min values taken at (hour - 7.5 min) and (hour + 7.5 min) (done at TOGA SLC) Daily : 119-point convolution filter (Bloomfield, 1976) centered on noon applied to the hourly data with respective periods of the 95, 50, and 5% amplitude points at 124.0, 60.2, and 40.2 hours Monthly: Simple average of all daily values; calculated if 7 or fewer days are missing Span of data : 05 Jun 1993 - 14 Dec 2006 Gaps > 1 mon : 11 Sep 1993 - 19 Nov 1993;11 Oct 1996 - 07 Apr 1997 20 Dec 1997 - 20 Jan 1998;02 Oct 2001 - 01 Oct 2002 02 Jul 2005 - 31 Oct 2006 Time Refernce: GMT (00-23 hours) Space-filler Flag :-9999 Units : millimeters Missing Data Flag : 9999 Existing : Yes Operational : Yes Sat. Trans. : Yes Distribution : 1993-2006 Refernce Levl: Precise datum control using technique described by Woodworth et al., 1994. This series is relative to the St. Helena B-Datum June 1993. The reference level was adjusted by adding +180 mm to the original data for 1997-2006 based on information provided by the contributors. 292A is separate from 292B since they do not relate to the same datum. Comment : Phase 2 gauge is located in Jamestown Harbor by the Landing Steps. This station is part of Phase 2 of the ACCLAIM project. The ACCLAIM (Antarctic Circumpolar Current Levels by Altimetry and Island Measurements) programme in the South Atlantic and Southern Oceans consists of measurements from coastal tide gauges and bottom pressure stations, together with an ongoing research programme in satellite altimetry. ACCLAIM is the Proudman Oceanographic Laboratory's main contribution to WOCE (and, in particular, to WOCE Core 2) and one of its objectives is the study of variations of the Antarctic Circumpolar Current. ACCLAIM is managed within the Global Sea Level Change (GSLC) group at POL led by Dr.J.M.Vassie. The history of the development of ACCLAIM is described in the report by Spencer et al. (1993): Spencer, R., Foden, P.R., McGarry, C., Harrison, A.J., Vassie, J.M., Baker, T.F., Smithson, M.J., Harangozo, S.A. and Woodworth, P.L. 1993. The ACCLAIM programme in the South Atlantic and Southern Oceans. International Hydrographic Review, 70, 7-21. In that report it will be seen that, in Phase 1 of ACCLAIM from 1983, measurements at coastal tide gauge sites took the form of sub-surface pressure (SSP) measurements (units of pressure e.g. mbar) rather than sea level (units of length e.g. centimetres). SSP is here defined as the total, measured pressure recorded by a sub-surface pressure transducer, a measurement which includes the pressure load from the atmosphere as well as from the water column. It is absolutely essential that any user of ACCLAIM data realises which data type (either SSP or sea level) is being analysed. From around early 1993, the gauges at many of the sites were replaced by Phase 2 ACCLAIM 'B gauges' which record SSP, air pressure and sea level as described in: Woodworth, P.L., Vassie, J.M., Spencer, R. and Smith, D.E. 1994. Precise datum control for pressure tide gauges. To be published in Marine Geodesy. The Phase 1 coastal SSP data were acquired in different ways (e.g. with a diver-replaced Aanderaa pressure gauge at Ascension, or with a Digiquartz in the sea sensor at St.Helena, see Spencer et al. 1993 for details) and with different pressure integration periods (e.g. quarter hour, half hour, one hour). For WOCE Centre purposes the original data have generally been filtered to give one hour sampling. However, common to all records is an uncertainty connected with potential offset biases and drifts in the pressure sensors. At some sites (e.g. St.Helena) extensive tide pole data are also available and biases and long term drifts in the sensor data may eventually be rectified (this is under study at present). However, the drifts in general mean that in most cases the records should not be used, without further careful attention in particular studies, for the study of timescales seasonal or longer. The Phase 2 coastal sea level data have precise datum control (see Woodworth et al. 1994 for details) and can certainly be used for long term studies. These data have also been contributed to the Permanent Service for Mean Sea Level (PSMSL) and Global Sea Level Observing System (GLOSS). Some of the Phase 1 data and all the Phase 2 coastal data will also contain ancillary information on air pressures and sea temperatures from ACCLAIM sensors. Several of these records contain large gaps. However, the GSLC group at POL has collected extensive sets of such ancillary data from meteorological agencies for its own analysis purposes, and should be able to provide further advice. SSP measurements at deep ocean bottom pressure stations (together with inverted echo sounder and temperature measurements) have also been a feature of ACCLAIM. Measurements at Amsterdam and Kerguelen in the Indian Ocean are described in: Vassie, J.M., Harrison, A.J., Woodworth, P.L., Harangozo, S.A., Smithson, M.J. and Thompson, S.R. 1994. On the temporal variability of the transport between Amsterdam and Kerguelen Islands. Journal of Geophysical Research, 99(C1), 937-949. while most recording is now concentrated in the Scotia Sea - Drake Passage area of the Southern Ocean as part of WOCE Core 2. Tidal constituents from these recorders have been contributed to the IAPSO Pelagic Constants data base maintained by the PSMSL at Bidston on behalf of IAPSO. CI MISSING REPLACED GAPS QUESTIONABLE YEAR (%) DATA OR BAD DATA FLUCTUATIONS ---- --- -------------------- --------------- -------------------- 1993 38 254-323 none none 1994 100 none none none 1995 100 none none none 1996 77 190-192,221-222, (09)192-(19)192 none 285-366 1997 67 1-97,344-365 none none 1998 89 1-20 (16)197-(01)198 none 1999 100 none none none 2000 100 none (22)171-(13)172 none 2001 75 275-365 none none 2002 22 1-274,275-277,349- (17)312-(08)313 none 357 (02)320-(21)320 2003 94 137-153,279-285 (00)015-(16)015 none (07)026-(01)027 (07)091-(16)091 (17)105-(08)106 (08)253-(15)253 (07)254-(15)254 (07)255-(15)255 (13)299-(09)300 2004 94 122-139,145-149 none none 2005 44 145-168,183-365 none none 2006 12 1-304 none none