View source for Sea level observations

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==Monitoring sea level==

As Antarctica could play a potentially large role in 21<sup>st</sup> century sea level change, it is disappointing that we have such poor knowledge of 20<sup>th</sup> century and present-day rates of change of sea level around the continent itself. Of course, this situation is due primarily to the great difficulty of acquiring extended time series of sea level measurements in environmentally hostile areas to the same standard as is possible elsewhere. Most sea level measurements during the 19<sup>th</sup> and 20<sup>th</sup> centuries were made with float and stilling well gauges, technology which presented operational problems in Antarctica. In some locations, there were also major issues to do with datum control (e.g. the establishment of adequate local benchmark networks and maintenance of tide gauge calibration with respect to those marks in local surveys conducted during brief annual visits).

[[File:Figure 2.28 - Map of the main Antarctic tide gauges.png|thumb|'''2.28''' Main Antarctic tide gauges described in the text.]]
While many short tide gauge measurements have been made around Antarctica, primarily for the determination of tidal parameters (e.g. IHB, 2002<ref name="IHB, 2002">IHB, 2002. Status of natutical charting (part A) and hydrographic surveying (part B) in Antarctica, International Hydrographic Bureau publication S-59. (see also previous editions of this publication with the same number.)</ref>), there are few records that satisfy the quality criteria required by the Permanent Service for Mean Sea Level (Woodworth and Player, 2003<ref name="Woodworth and Player, 2003">Woodworth, P.L. and Player, R. 2003. The Permanent Service for Mean Sea Level: an update to the 21<sup>st</sup> century, ''Journal of Coastal Research'', '''19''', 287-295.</ref>) and that are long enough to be of interest for long-term change studies. The outstanding record is that from Vernadsky station (formerly Faraday station) in the Argentine Islands on the western side of the Antarctic Peninsula ([[:File:Figure 2.28 - Map of the main Antarctic tide gauges.png|Figure 2.28]]). It is now operated by the National Antarctic Scientific Center of Ukraine and contains a conventional float and stilling well gauge maintained in collaboration with the Proudman Oceanographic Laboratory. Hourly sea levels are measured by means of a paper chart recorder, with datum control provided by daily comparisons of tide gauge and tide pole observations. The sea level record from this venerable gauge commenced in 1958, the equipment having been installed at the then British Antarctic Survey Faraday base during the International Geophysical Year, thereby providing the longest sea level time series in Antarctica. The gauge received a major upgrade in the early 1990s when a pressure sensor gauge was added, and a new pressure sensor gauge with satellite transmission capability was installed in 2007.

The PSMSL data catalogue (www.pol.ac.uk/psmsl) provides one list of sea level data available from Antarctica. Notable records can be found from the Japanese Syowa base from the mid-1970s and from the three Australian bases of Mawson, Davis and Casey from the early 1990s. Other long term records are known to exist that are as yet not included in international data banks. A particularly interesting one, as it is far from the other long records mentioned above, is from the New Zealand Scott base and has been acquired since 2001 with the use of a bubbler pressure gauge attached to the reverse osmosis water pipe for the base, around which there is a permanent gap in the sea ice. France has made major efforts to instrument Dumont D&rsquo;Urville. This station has been operated since 1997 with gaps and various upgrades, and is currently being updated to real time transmission as part of the Indian Ocean Tsunami Warning System. Details of this and other gauges operated by various nations in Antarctica are often included in the national reports of the Global Sea Level Observing System (GLOSS: IOC, 1997<ref name="IOC, 1997">IOC. 1997. Global Sea Level Observing System (GLOSS) implementation plan-1997. Intergovernmental Oceanographic Commission, Technical Series, No. 50, 91pp. and Annexes.</ref>; Woodworth et al., 2003<ref name="Woodworth et al, 2003">Woodworth, P.L., Aarup, T., Merrifield, M., Mitchum, G.T. and Le Provost, C. 2003. Measuring progress of the Global Sea Level Observing System, EOS, Transactions of the American Geophysical Union, 84(50), 16 December 2003, 565, 10.1029/2003EO500009.</ref>) (see www.gloss-sealevel.org). In addition, a list of Antarctic stations with tide gauges is maintained by the Scientific Committee on Antarctic Research (SCAR) (www.geoscience.scar.org/geodesy/perm_ob/tide/tide.htm). However, an important point to make about Antarctic data is that very little of it is downloadable and as readily analysable as data from elsewhere. In particular, much data have been obtained with pressure sensors, which are subject to drifts and biases. Any analyst must consider carefully the possible data problems and the impacts on the application to which they are put.

==Sea Level Data for Ocean Circulation Studies==

One application of sea level data is in ocean circulation studies, for which analysts usually require sub-surface pressure (SSP) rather than sea level itself. SSP can be obtained at a tide gauge site either with the use of a shallow-water pressure sensor, or by adding local air pressures to the data from a gauge (e.g. float or acoustic) that records true sea level. An alternative to coastal equipment in such studies is provided by bottom pressure recorders (BPRs), which have been employed in the Drake Passage and at other Antarctic locations at various times since the International Southern Ocean Studies (ISOS) programme (Whitworth and Peterson, 1985<ref name="Whitworth and Peterson, 1985">Whitworth, T. and Peterson, R.G. 1985. Volume transport of the Antarctic Circumpolar Current from bottom pressure measurements, ''Journal of Physical Oceanography'', '''15''', 810-816.</ref>), and more recently since the World Ocean Circulation Experiment of the 1990s (Spencer and Vassie, 1997<ref name="Spencer and Vassie, 1997">Spencer, R. and Vassie, J.M. 1997. The evolution of deep ocean pressure measurements in the U.K, ''Progress in Oceanography'', '''40''', 423-435.</ref>; Woodworth et al., 2002<ref name="Woodworth et al, 2002">Woodworth, P.L., Le Provost, C., Rickards, L.J., Mitchum, G.T. and Merrifield, M. 2002. A review of sea-level research from tide gauges during the World Ocean Circulation Experiment, Oceanography and Marine Biology: An Annual Review, 40, 1-35.</ref>). Many of these deployments have been by UK groups and most records are readily available for analysis via www.pol.ac.uk/ntslf/acclaimdata/bprs.

Antarctic sea level data have great importance in understanding the variability in the Antarctic Circumpolar Current (ACC), and thereby the role of the ACC in the global climate and, ultimately, in sea level change itself. A series of papers (Woodworth et al., 1996<ref name="Woodworth et al, 1996">Woodworth, P.L., Vassie, J.M., Hughes, C.W. and Meredith, M.P. 1996. A test of the ability of TOPEX/POSEIDON to monitor flows through the Drake Passage, ''Journal of Geophysical Research'', '''101'''(C5), 11935-11947.</ref>; Hughes et al., 1999<ref name="Hughes et al, 1999">Hughes, C.W., Meredith, M.P. and Heywood, K. 1999. Wind-driven transport fluctuations through Drake Passage: a Southern mode, ''Journal of Physical Oceanography'', '''29''', 1971-1992.</ref>; Aoki, 2002<ref name="Aoki, 2002">Aoki, S. 2002. Coherent sea level response to the Antarctic Oscillation. Geophysical Research Letters, 29(20), 1950, doi:10.1029/2002GL015733.</ref>; Hughes et al., 2003<ref name="Hughes et al, 2003">Hughes, C.W., Woodworth, P.L., Meredith, M.P., Stepanov, V., Whitworth, T. and Pyne A.R. 2003. Coherence of Antarctic sea levels, Southern Hemisphere Annular Mode, and flow through Drake Passage, ''Geophysical Research Letters'', '''30'''(9), 1464, doi:10.1029/2003GL017240.</ref>; Meredith et al., 2004<ref name="Meredith et al, 2004">Meredith, M.P., Woodworth, P.L., Hughes, C.W. and Stepanov, V. 2004. Changes in the ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode, Geophysical Research Letters, 31(21), L21305, 10.1029/2004GL021169.</ref>) have demonstrated that SSP fluctuates similarly around the entire Antarctic continent and that the SSP fluctuations can be related to changes in the circumpolar ocean transport around Antarctica. SSP data can be obtained either from measurements by BPRs deployed to the south of the main ACC axis or from coastal gauges as described above. The relationship between SSP, ACC transport and the SAM applies at least on intra-seasonal timescales (i.e. periods of more than a month and less than a year but excluding the quasi-regular seasonal cycle). It also applies on inter-annual timescales, despite the presence of baroclinic variability in the ocean at these longer periods (Meredith et al., 2004<ref name="Meredith et al, 2004">Meredith, M.P., Woodworth, P.L., Hughes, C.W. and Stepanov, V. 2004. Changes in the ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode, Geophysical Research Letters, 31(21), L21305, 10.1029/2004GL021169.</ref>). However, the relationship at longer (decadal) timescales remains to be tested. The importance of sea level data in monitoring the circumpolar transport around Antarctica became more apparent with the realisation that many of the other techniques commonly employed are subject to critical aliasing, resulting in unrealistically high estimates of variability (Meredith and Hughes, 2005<ref name="Meredith and Hughes, 2005">Meredith, M.P. and Hughes, C.W. 2005. On the sampling timescale required to reliably monitor interannual variability in the Antarctic circumpolar transport, Geophysical Research Letters, 32(3), L03609, 10.1029/2004GL022086.</ref>).

Major efforts have been made recently to provide sea level data from Antarctica in real-time, resulting in more rapid determination of ACC transport than has been possible to date (Woodworth et al., 2006<ref name="Woodworth et al, 2006">Woodworth, P.L., Hughes, C.W., Blackman, D.L., Stepanov, V.N., Holgate, S.J., Foden, P.R., Pugh, J.P., Mack, S., Hargreaves, G.W., Meredith, M.P., Milinevsky, G. and Fierro Contreras, J.J. 2006. Antarctic peninsula sea levels: a real time system for monitoring Drake Passage transport, ''Antarctic Science'', '''18'''(3), 429-436.</ref>). This development is also part of a general effort by GLOSS to have as many gauges as possible in the global network delivering data in real-time data, thereby enabling faults to be identified and corrected faster than would otherwise be the case. Rothera real-time data became available in 2007, while data from Vernadsky and King Edward Point, South Georgia in the South Atlantic will follow. The latter will largely replace an older installation at Signy, South Orkney Islands. All such UK data will be obtainable via www.pol.ac.uk/ntslf/acclaimdata. Data from Syowa are available in real-time from www1.kaiho.mlit.go.jp/KANKYO/KAIYO/jare/tide/tide_index.html, while data from Australian stations are available in &lsquo;fast&rsquo; rather than &lsquo;real time&rsquo; mode (i.e. with a short delay of typically 1-2 months).
==References==
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[[Category:Observations, data accuracy and tools]]
[[Category:Sea level]]