Measuring Sea Level Change Measuring Sea Level Change

Measuring Sea Level Change

Measuring Sea Level Change Overview

What is Sea Level? Mean high tide Mean sea level Mean low tide Mean – Arithmetic average Mean sea level – The average sea level over a large region

Sea Level Variation Astronomical Tidal Forces Earth’s water closest to the moon is acted on more strongly by the moon’s gravity The solid Earth is pulled harder than the water on the side opposite the moon The Sun produces similar tidal forces and accounts for about 1/3 of ocean tides

Sea Level Variation Depth of Tampa Bay changes by 20 cm between winter and summer Primary cause is thermal expansion (water expands when it warms) Mangroves & coastal habitats must be able to handle the changing water line

Sea Level Variation El Niño and Multi-Year Changes Readings at Fort Point, San Francisco Depth Gauge During El Niño years, sea level along West Coast is higher than in La Niña years Higher seas caused by reduced trade winds, allows up to 8 inches of sea level rise

Measuring Sea Level Rise Traditional Stilling Well Mechanical device “draws” sea level on slowly rolling paper Tide Gauge Benchmark Pulley and Counterweight Bedrock Float Small openings near bottom of well designed to “filter out” wave motion Sea bed Tides and changes in sea level are measured relative to a permanent local benchmark

Measuring Sea Level Rise Modern Tide Gauge Station Satellite Antenna Data Collection Computers Tide Gauge Benchmark Acoustic Sensor Calibration Tube Pressure Gauge Computers enable in-depth analysis measured Data available instantly Instantaneous Water Level Individual waves

Measuring Sea Level Rise Modern Tide Gauge Station Housing keeps equipment dry Measures wind speed/direction Acoustic sensor Solar panels Satellite antenna sends data to Internet Computer to process and store data

Measuring Global Sea Level JASON-2 satellite position data GPS Altimeter Microwave radiometer measuring water vapor Range Satellite altitude Sea surface height Ellipsoid • Deep ocean couldn’t be measured until about 25 years ago • Satellites now measure elevation using altimetry • Sea level determined by comparing measured height and GPS data

Causes of Sea Level Change Terrestrial water storage, Extraction of groundwater, Building reservoirs Subsidence in river delta, Land movements, Tectonic displacements Seepage into aquifers Circulation changes, Storm surges, Water expands as it warms Exchange of ocean water with glaciers and ice sheets

Summary • Mean Sea Level • Sea Level Variation • Coastal Tide Gauge Stations • Measuring Global Sea Level • Causes of Sea Level Change