Principles of Sea Level Measurement

1. Principles of Sea Level Measurement Long-term tide gauge records • 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 • 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?

2. 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”.

3. Sea Level Measurement from Tide Gauges San Francisco, CA Gulf Coast Sentinels

4. Sea Level Measurement from Tide Gauges 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

5. 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)

6. Sea Level Measurement from Tide Gauges Observed hourly heights from tide gauges are compiled for each calendar month

7. 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

8. 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.

9. Sea Level Measurement from Tide Gauges

10. 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:

11. 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:

12. 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:

13. 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

14. Sea Level Measurement from Tide Gauges Local vertical uplift of the land

15. Sea Level Measurement from Tide Gauges Local vertical land subsidence

16. 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.

17. 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

18. 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.

19. 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 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

20. 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)

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