Principles of Sea Level Measurement
Long-term tide gauge records
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What 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 Missions
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How 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?
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”.
GRAND ISLE, LOUISIANA
VARIATIONS IN MONTHLY AND ANNUAL MEAN
SEA LEVEL
Elevation Relative to Station Datum
(meters)
2.40
Monthly Mean Sea Level
Annual Mean Sea Level
2.20
2.00
1.80
LOWER CHESAPEAKE BAY : Variations In Relative
Annual Mean Sea Level - 1927 thru 2000
Linear Trend: +4.2mm/year
.
Mean sea level has risen about 300mm (1 ft.)
relative to the land since 1927
1.60
1.40
1.20
Relative Mean Sea Level Trend is 9.85
mm/yr
1.00
1950
1960
1970
1980
TIME (YEARS)
1990
2000
2010
1.900
Elevation relative to bench
mark (meters)
1940
1.800
Annual Mean Sea Level
1.700
1.600
1.500
1.400
1.300
1.200
1920
1930
1940
1950
1960
Time (years)
1970
1980
1990
2000
Sea Level Measurement from Tide Gauges
San Francisco, CA
Gulf Coast Sentinels
Sea Level Measurement from Tide Gauges
Tide Station at The Battery,
Manhattan, NYC
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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
Sea Level Measurement from Tide Gauges
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)
Sea Level Measurement from Tide Gauges
Observed hourly heights from tide gauges are compiled for each calendar month
Sea Level Measurement from Tide Gauges
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
Sea Level Measurement from Tide Gauges
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 defined
as the accepted tidal datum of Mean Sea Level at each tide station.
Sea Level Measurement from Tide Gauges
Sea Level Measurement from Tide Gauges
Sea-level trends are constructed using a linear trend fit to the observed monthly mean
sea level data over the period of record at each tide station. The trends and the standard
errors in the trends are always displayed:
Sea Level Measurement from Tide Gauges
Sea-level trends are constructed using a linear trend fit to the observed monthly mean
sea level data over the period of record at each tide station. The trends and the standard
errors in the trends are always displayed:
Sea Level Measurement from Tide Gauges
Sea-level trends are constructed using a linear trend fit to the observed monthly mean
sea level data over the period of record at each tide station. The trends and the standard
errors in the trends are always displayed:
Sea Level Measurement from Tide Gauges
Sea level trends form tide gauges have significant regional variability due to regional
oceanographic variations and due to varying rates of local and regional vertical land motion
http://tidesandcurrents.noaa.gov/sltrends/index.shtml
Sea Level Measurement from Tide Gauges
Local vertical uplift of the land
Sea Level Measurement from Tide Gauges
Local vertical land subsidence
Sea Level Measurement from Tide Gauges
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.
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 geodetic
measurements, to estimate the global rate of sea level rise.
SOURCE: R.S.NEREM, BOWIE LECTURE FALL AGU 2005
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.
Sea Level Measurement from Satellite Altimetry
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 semiannual periodic signals are also adjusted. The MSL slope is deduced from this series using a
least squares method. (Source – AVISO)
Satellite ground track
Sea Level Measurement from Satellite Altimetry
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