Measuring Sea Level Using Tide Gauges
Measuring sea level using gauges has a 200-year history. Today, the technology has changed but the principles are the same as before, and some gauges provide very long and reliable records of water levels that can be used to observed sea level change trends. For example, Fort Point tide gauge in San Francisco has more than 100 years of record that we will access later.
Sea level is often measured locally by tide gauges (and averaged over tidal cycles) that detect high and low points in a given period of time. Local tide gauges are especially useful for people who work or recreate in coastal areas and need to know what the water level ranges will be. These data points are also important for detecting water levels during storms and other events as well as in the long-term investigation of relative water level change (rise or fall). Tide levels are also measured by floating buoys, which may also be used to detect tsunami waves. We will use tide gauge data to investigate sea level changes in different locations in the Module 4 Lab.
Measuring Sea Level Using Satellite Altimetry
With the advent of satellite altimetry in the 1960s, measurements of the sea surface took on a whole new level of accuracy. Between 1996 and 2006, altimetry took off with multiple satellites orbiting the Earth, providing much better coverage and data resolution. These measurements utilize multi-beam methods that are very precise and can measure changes in elevation on the Earth's surface to great precision in the range of centimeters. These methods have shown that water bodies are not flat but are incredibly dynamic and have high and low spots due to factors such as gravitational variability described above. Data such as ocean circulation, sea level rise, and wave heights can be measured. These measurements have provided insight into the links between the ocean and the atmosphere and how the connections drive climate. Satellite altimetry data collection began in earnest with the launch in 1992 of the TOPEX/Poseidon joint satellite mission between NASA and CNES, the French space agency. TOPEX/Poseidon proved data previously impossible to obtain. The next generation of satellites to collect these data were the NASA Jason satellites. They have been collecting data since Jason 1 was launched in 2001. Jason 2 was launched in 2008, while Jason 3 is presently collecting altimetry data. Each mission lasts about 5 years. Meanwhile, the European Space Agency’s Sentinel 3 satellite is collecting similar data as shown in Figure 4.1.