principles of sea level measurement long-term tide gauge records what is a tide station? how is...

Principles of Sea Level Measurement

Long-term tide gauge recordsWhat is a tide station?How is sea level measured relative to the land?What types of data are used to compute mean sea level?What are the significant variations in mean sea level?How are sea level trends computed from tide gauge data?What do the records show for US coasts?How are the records used to estimate Global Sea Level?

Satellite Altimeter MissionsHow does a satellite altimeter measure sea level?How long is the present data record from satellite altimeter missions?What types of data are used to measure mean sea level variations?How are mean sea level trends determined?What to the data show both globally and regionally?How do the records compare with tide gauge data?

GRAND ISLE, LOUISIANAVARIATIONS IN MONTHLY AND ANNUAL MEAN

SEA LEVEL

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Monthly Mean Sea LevelAnnual Mean Sea Level

Relative Mean Sea Level Trend is 9.85 mm/yr

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1920 1930 1940 1950 1960 1970 1980 1990 2000

Annual Mean Sea Level

Linear Trend: +4.2mm/yearMean sea level has risen about 300mm (1 ft.)

relative to the land since 1927

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LOWER CHESAPEAKE BAY : Variations In RelativeAnnual Mean Sea Level - 1927 thru 2000

Sea Level Measurement from Tide Gauges

Tide gauges measure sea level relative to the elevation of the local land; thus sea level trends determined from tide gauge records are defined as “Relative Mean Sea Level Trends”.

San Francisco, CA Gulf Coast Sentinels


Tide Station at The Battery, Manhattan, NYC

1856 - 2009

• Data and Reference Datum Continuity• Vertical Stability of Structures Maintained and Monitored• Annual Leveling from Sensors to Local Bench Marks• Annual Preventive Maintenance• Sensor Calibration Checks and Swap-outs• Backup Sensors and Alternate Data Collection Modes• Active Quality Control and Monthly and Yearly Product Generation• Overlapping Data when New Technology Sensors Installed



Tide gauges are surveyed-in relative to land elevations (bench marks) annually. Tide gauges measure local sea level change relative to the local land. Tide gauge records include local and regional vertical land motion (subsidence or uplift)


Observed hourly heights from tide gauges are compiled for each calendar month


Monthly mean sea level values are obtained by averaging the observed hourly heights for each calendar month and accumulating a time series for each station


Monthly mean sea level values are accrued over specific 19-year periods called National Tidal Datum Epochs (NTDEs). The 19-year averages of the monthly values are definedas the accepted tidal datum of Mean Sea Level at each tide station.


Sea-level trends are constructed using a linear trend fit to the observed monthly meansea level data over the period of record at each tide station. The trends and the standarderrors in the trends are always displayed:


http://tidesandcurrents.noaa.gov/sltrends/index.shtml

Sea level trends form tide gauges have significant regional variability due to regionaloceanographic variations and due to varying rates of local and regional vertical land motion

Local vertical uplift of the land


Local vertical land subsidence



Sea level trends must always be used with knowledge of their uncertainties. The uncertainty is sea level trends are highly correlated with record length. Data records less than 30 years in length have limited value as shown in the figure below because the errors become large relative to the values of the trends themselves.

SOURCE: R.S.NEREM, BOWIE LECTURE FALL AGU 2005

Estimation of Global Sea Level from Tide Gauges

Sea level trends are adjusted to account for rates of vertical land motion using Global Isostatic Adjustment (GIA) models, or most recently local continuous geodeticmeasurements, to estimate the global rate of sea level rise.

Sea Level Measurement from Satellite Altimetry

Altimetry is a technique for measuring height. Satellite altimetry measures the time taken by a radar pulse to travel from the satellite antenna to the surface and back to the satellite receiver. Combined with precise satellite location data, altimetry measurements yield sea-surface heights.

With regard to the calculation of MSL time series for each mission (Topex/Poseidon, Jason-1, Jason-2), a mean grid of sea level anomalies (SLA=SSH-MSS) of 2°x2° must first be calculated for each cycle (~10 days) in order to distribute the measurements equally across the surface of the oceans. The global or basin mean for each grid is calculated by weighting each box according to its area, in order to give less significance to boxes at high latitudes which cover a smaller area. This then gives the time series per cycle, which is then filtered with a low-pass filter in order to remove signals of less than 2 months or 6 months, and the annual and semi-annual periodic signals are also adjusted. The MSL slope is deduced from this series using a least squares method. (Source – AVISO)

Satellite ground track



The reference mean sea level since January 1993 (left) is calculated after removing the annual and semi-annual signals. A 2-month filter is applied to the blue points, while a 6-month filter is used on the red curve. By applying the postglacial rebound correction (-0.3 mm/year), the rise in mean sea level has thus been estimated as 3.19 mm/year (mean slope of the plotted data). Analysing the uncertainty of each altimetry correction made for calculating the GMSL, as well as a comparison with tide gauges gives an error in the GMSL slope of approximately 0.6 mm/year with a 90% confidence interval. (Credits CLS/Cnes/Legos)

Sea Level Measurement from Satellite Altimetry Regional Variability in Sea Level Trends

principles of sea level measurement long-term tide gauge records what is a tide station? how is...

Documents

global sea level

mean sea level variations

relative mean sea level

tide gauge data

types of data

tide station

present data record

significant variations