QUALITY ASSESSMENT OF SEA LEVEL DATA by the UH SEA LEVEL CENTER/NATIONAL OCEANOGRAPHIC DATA CENTER JOINT ARCHIVE FOR SEA LEVEL updated: 28 Feb 2019 Station : Ascension Latitude: 07 55.0S Country : United Kingdom Longitude: 014 25.0W JASL # : 291A Time Meridian: 000E (GMT ) GLOSS # : 263 TOGA #: A036 NODC #: 50017401 Contributor : National Oceanography Centre Proudman Oceanographic Laboratory (POL) 6 Brownlow Street Liverpool L69 3GP United Kingdom Originator : same Original Data: digital Instrmnt Type: 1) 1993-5/2010 Digiquartz pressure sensor and barometer (POL B gauge); replaced Mar 1999 2) Nov 2010- KPSI differential pressure gauge Digitzd Intvl: 15 minutes Present Data : Hourly, daily, and monthly values obtained by: Hourly : 1) average of 15-min values taken at (hour-7.5 min) and (hour+7.5 min) (done at the UHSLC) 2) KPSI, spot hourly of 15-min reading on hour 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 : 23 May 1993 - 19 Jun 2013 Gaps > 1 mon : 26 Jun 1998 - 03 Apr 1999;03 Apr 2001 - 06 Sep 2005 05 Oct 2007 - 25 Nov 2007;24 Apr 2009 - 15 Jul 2009 19 Sep 2009 - 28 Oct 2009;20 Nov 2009 - 07 Jan 2010 30 May 2010 - 28 Nov 2010 Time Refernce: GMT (00-23 hours) Space-filler Flag :-9999 Units : millimeters Missing Data Flag : 9999 Existing : Yes Operational : Yes Sat. Trans. : Yes Distribution : 1993-2013 Refernce Levl: Precise datum control using technique described by Woodworth et al., 1994. This series is relative to the Ascension B-Datum May 1993. The reference level was adjusted by adding +8 mm to the original data from 8Z 03 Apr 1999 - the end of the record (11/2009), based on information provided by the contributors. The Jan-May 2010 data were corrected by -4mm. The Nov-Dec 2010 were corrected by -870mm. The Jan 2011-Jun 2013 data were corrected by -883mm. Comment : This gauge is in English Bay, Ascension and is not to be confused with Phase 1 pressure sensor which was in Clarence Bay. 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. The gap from 2001-2005 occurred because the station had no half-tide pressure datum control and large apparent drifts in the full-tide pressure sensor. Starting Sep 2005 is the 'all in one' B gauge. Data from a radar gauge were used to check datum stability since the half-tide sensor was out for Apr 2006 - Jun 2008. The overlap of Digiquartz and KPSI pressure sensors in 12/2010 suggest uncertainty of about 8 mm for the KPSI. The JASL Jan 2011 - Mar 2012 data were replaced in Feb 2019 with the "ascnn.sl" file downloaded from https://www.ntslf.org/files/acclaimdata/phase2/. See notes in the Inventory link. This series represents the real or in situ sea level signal, as opposed to sub-surface pressure gauges without venting or atmospheric pressure compensation which measure the absolute pressure (water plus air column). CI MISSING REPLACED GAPS QUESTIONABLE YEAR (%) DATA OR BAD DATA FLUCTUATIONS ---- --- -------------------- --------------- -------------------- 1993 59 327-334 none none 1994 100 none none none 1995 100 none (01)032-(20)032 none 1996 92 26-41,47-60,138-139 none none 1997 98 82-90 none none 1998 49 177-365 none none 1999 75 1-93 none none 2000 100 none (16)165-(15)166 none 2001 25 93-365 none none 2002 0 1-365 none none 2003 0 1-365 none none 2004 0 1-366 none none 2005 30 1-249 (09)325-(16)325 none (09)326-(18)326 (10)327-(18)327 2006 100 none none none 2007 86 278-329 none none 2008 98 165-171 none none 2009 50 114-196,206-212,227- none none 229,258-301,303-313 324-365 2010 48 1-7,141-143,150-332 none none 2011 100 none (12)056-(19)056 none (01)058-(16)058 (04)235-(13)235 2012 100 none (16)092-(00)093 none (23)189-(16)190 2013 46 none (10)069-(01)070 none (17)079-(01)080 (21)169-(08)170 (10)069-(01)070 (17)079-(01)080 (21)169-(08)170