Red tide graph1/12/2024 The current rate of VLM measured by a GPS may not be the rate of land motion over the entire tide gauge record. This should be done with caution, however, because GPS captures time-dependent processes (unlike GIA, which is approximately constant for time-scales on the order of 100 years). It may be possible to correct for other sources of VLM (seismic motion, subsidence, etc.) using Global Positioning System (GPS) measurements. This process manifests as long-term linear trends in tide gauge observations, and GIA models can be used to remove the GIA trends from the tide gauge data. One of the most important sources of VLM is glacial isostatic adjustment (GIA), which is the ongoing slow rebound of the Earth’s crust after removing the weight of ancient ice that existed during the last glacial maximum. Considering the effect of vertical land motion (VLM) is particularly important when calculating long-term sea level trends from tide gauges.įor some applications, it is necessary to correct for vertical land motion in tide gauge observations. The local tide gauge measures the land subsidence as additional long-term relative sea level rise on top of the increase due to climate change. ![]() For example, extraction of hydrocarbons in the ground near Galveston, TX is causing the land to subside in the region. This means that tide gauge sea level observations reflect vertical motion of both the sea surface and the coastline. Tide gauges measure relative sea level, which is the height of the water relative to the height of the land. At the time of writing, radar gauges are the preferred instrumentation for new installations.Ĭonsiderations when interpreting tide gauge data Acoustic and radar gauges measure the travel time or phase change of acoustic and radar pulses sent from the sensor and reflected vertically from the sea surface. Pressure sensors rest on the ocean floor and measure the weight of the water above them, which can be converted to height using the water density. Other types of tide gauges include pressure sensors, acoustic gauges, and radar gauges. The well prevents turbulent motion of the ocean surface from disturbing the float. A stilling well is a vertical cylinder with a small hole in its side that acts as a mechanical filter as it fills and drains with changes in surrounding water level. In the past, the recorder was a pen that moved with the height of the float across long scrolls of paper modern recorders are digital. The most common type of tide gauge is mechanical in nature and consists of a float in a stilling well attached to a data recorder. The height of the tide gauge instrumentation is leveled to the benchmark with high-precision survey techniques, and the primary benchmark is generally tied to other benchmarks in the area to monitor the stability of the land and gauge. ![]() The fixed point is referred to as a benchmark, and it is a crucial component of any tide gauge installation. Ī tide gauge is an instrument that measures water level relative to a fixed point on land. In addition to process-oriented research, tide gauge data are assimilated into and provide validation for ocean models, used to detect errors and drifts in satellite altimetry, and are a key component of tsunami warning systems. Examples include tides, decadal climate variability (e.g., the Pacific Decadal Oscillation), planetary waves, storm surge, tsunamis, and glacial isostatic adjustment. Tide gauges also capture a variety of oceanic and geodetic phenomena at many time and space scales. Historical tide gauge data allow scientists to estimate global sea level rise since the industrial revolution, which is a powerful indicator of both the trajectory and impact of anthropogenic climate change. Tide gauge sea level observations are an essential component of the global ocean observing system. Phil Thompson at the University of Hawaii, December, 2015: Benjamin Hamlington at Old Dominion University, and Dr.
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