Waves that develop on bodies of water have fascinated humans for ages. Unfortunately, the energy associated with these oscillations of the surface of a large water body such as the ocean can be hazardous to humans, ships, and seaside property. On the other hand, the presence of waves on the ocean or a large lake is indicative of various air-sea interactions and can be used to estimate the winds or as an aid to navigation. Recently, Earth scientists have been investigating the role of waves in their study of air-sea interaction, ocean circulation, and global climate change.
Whereas some waves are generated by astronomical forces (tides), others by earthquakes, landslides, or volcanic eruptions (triggering tsunamis), most waves are produced by near-surface winds. The frictional drag between local or distant wind and the water produces wind waves. Waves that are generated by local winds near the observer are sometimes called wind-sea, or simply sea, whereas waves produced by some distant weather system, often thousands of kilometers away and several days previous, are usually called swell.
Experienced sailors usually are very skilled at describing the state of the sea. The term sea state refers to the overall appearance of the sea surface, including the dimensions of wind-generated waves and the presence of such surface phenomena as white caps, foam, or spray.
Prior to the development of accurate anemometers (instruments that measure wind speed) in the 19th century, a method was developed to estimate the wind speed from the observed sea state. The scale that was used was based upon the wind force scale originally proposed in 1805 by Francis Beaufort, later to become an Admiral in the British Empire's Royal Navy. Initially he had devised a method for estimating the force of the wind using the amount of sail needed by a fully rigged sailing vessel, a British frigate of the day. His scale was divided into 13 increments, ranging from 0 (calm) to 12 (hurricane). Thus, a wind of force 1 (light air) was "just sufficient to give steerage way" for a frigate, whereas a force 12 (hurricane) was " that which no canvas could withstand." By 1838, the Beaufort scale was modified to specify the sea state, such that force 1 winds would produce "ripples with appearance of scales: no foam crests" (that, is wave heights on the order of 3 in.) whereas the sea state associated with a force 12 wind would be one where the "air filled with foam; sea completely white with driving spray; visibility greatly reduced." [Note the Milwaukee NWS Forecast Office has sample pictures of the sea state for each Beaufort force increment on their Marine Weather Page. Scroll down to the "Interesting Marine Info" section and click on any Beaufort force number under the title Guide to Sea State and Winds.]
While the state of the sea depends mainly on the wind speed, it is also influenced by swell, fetch (distance that the near-surface wind travels over the water in a particular direction) and the duration of the wind blowing in that direction. Other factors such as water depth, heavy rain, or ice can also affect the wave height. Waves described in the Beaufort wind scale are those to be expected on the open sea--waves in enclosed waters tend to be smaller and steeper. In the 20th century, the Beaufort wind scale was quantified in terms of wind speeds based upon experiments with anemometers, such that a force 1 wind has a speed between 1 and 3 mph, while a force 12 wind speed is greater than 74 mph. The scale has been extended to include the effects of the wind on land, along with dividing hurricane force winds into 4 categories, with 17 corresponding to the highest force number (winds in excess of 125 mph).
The sea also provides an indication of the wind direction, since wave crests tend to move in approximately the direction of the generating near-surface wind.
Experience indicates that waves do not have a single wave height or wavelength, but exhibit a variety of sizes within a wave spectrum. The size variation is in part due to the variability of the winds, which also exhibits large differences over a continuum of space and time scales. One of the most useful statistics for describing wave characteristics is the significant wave height. Significant wave height is defined as the average height of the highest one-third of waves observed. Since smaller waves usually are not visible against the background of larger waves, we can assume that significant wave height approximates the visually observed mean wave height. Wavelength is the distance between successive crests (or troughs) and wave period is the time interval between the passage of successive wave crests and is inversely proportional to the wavelength. The dominant wave period represents the period of waves exhibiting the maximum wave energy. Swells generated by distant winds have a longer period (or longer wavelength) than waves generated locally.
Statistical analysis of waves indicates that the largest individual wave that one might encounter in a storm would be roughly twice as high as the significant wave height under most weather conditions.
Spurred by the need for better weather and climate forecasts, the meteorological and oceanographic communities have expanded their monitoring of wind and wave conditions over the open ocean to include more observations on greater spatial and temporal scales. These expanded wind and wave observations require more sophisticated techniques than can be afforded by visual observations from ships of opportunity traversing limited regions of the ocean. The Current Marine Data link found on the DataStreme WES Website leads to the displays of the most recent Global Analysis Wave Map with significant wave height in all global ocean basins identified using a color coded analysis of heights (in meters), together with vectors depicting the mean direction in which the waves are traveling. Corresponding charts of the individual ocean basins are also available from this site. The near-surface wind and ocean wave data needed to produce these charts come from a variety of sources including ships, moored automated buoys, and orbiting satellites.
Moored buoys, deployed by various nations in their coastal waters, serve as instrumented platforms for making automated weather and oceanographic observations. The National Data Buoy Center (NDBC), an agency within the National Weather Service (NWS) of the National Oceanic and Atmospheric Administration (NOAA), operates approximately 80 moored buoys in the coastal and offshore waters of the western Atlantic Ocean, the Gulf of Mexico, the Great Lakes, and the Pacific Ocean from the Bering Sea to southern California, around the Hawaiian Islands and in the South Pacific. These buoys have accelerometers or inclinometers that measure the heave acceleration or the vertical displacement provided to the buoy by the waves during a specified data acquisition time interval. These measurements are then processed by an onboard computer that uses statistical wave models to generate wind-sea and swell data that are then transmitted to shore stations. These transmitted wave data include significant wave height, average wave period of all waves, and dominant wave period during each 20-minute sampling interval. Selected buoys also provide directional wave data, such as mean wave direction. These selected buoys also require measurements of the orientation of the buoy's hull (azimuth, pitch, and roll).
Another type of instrumented buoy that measures waves is a DART (Deep-ocean Assessment and Reporting of Tsunamis) station deployed by NOAA's Pacific Marine Environmental Laboratory (PMEL) and now operated by NDBC. Ten DART systems have been deployed in the North Pacific Ocean along the Aleutians, in the Gulf of Alaska, off the Pacific coast of the continental United States and close to the Hawaiian Islands for the early detection of tsunamis (see interactive map). Another DART instrument is located in the South Pacific off South America, as well as a comparable buoy owned and operated by the Chilean Navy off the northern coast of Chile. Recently, five DART instruments have been deployed in the western North Atlantic offshore of North Carolina and the Bahamas, in the central Gulf of Mexico south of the Mississippi River Delta and in the Caribbean southwest of Puerto Rico. Part of this DART system consists of a pressure-recording sensor that is placed on the sea floor at depths to 500 m (1600 ft). The instrument is capable of detecting water pressure changes associated with a tsunami wave as small as one centimeter. An acoustic link transmits the recorded data from the ocean floor to a nearby moored surface buoy, where the data are then relayed via orbiting satellite to land stations for dissemination by either of NOAA's two Tsunami Warning Centers.
Satellites equipped with radar scatterometers use the sea-state to estimate the near-surface wind speed and direction. A scatterometer is a sensor that measures the return reflection or scattering of a microwave (radar) signal sent to Earth's surface from a high altitude aircraft or satellite. A rough ocean surface reflects back (backscatters) to the antenna on the satellite a stronger signal than a smooth ocean surface because energy from the radar signal is reflected back more effectively when steeper waves are present than when the ocean surface is relatively smooth. Computers estimate wave height and then the wind speed from the power differences in the returned signal. If two emitted beams are spaced with a precise angular distance, slight variations in the return signals from the roughened surface could permit a determination of the wind direction. Within the last several years, the National Aeronautics and Space Administration (NASA) launched several missions that have measured near-surface winds over the ocean. These missions include the initial NSCAT (NASA Scatterometer), launched in 1996, and the interim SeaWinds on QuikSCAT (Quick Scatterometer) launched in 1999. The SeaWinds instrument was launched 14 December 2002 on the Midori 2 satellite; sample images show the available products from SeaWinds.
Wave forecasts are crucial not only for marine interests, where high waves could spell disaster for ships on the open sea, but also for residents of coastal communities where waves and swells could produce high surf, coastal flooding, beach erosion or destruction of shore structures. In recent years, improvements have been made in methods for forecasting wave heights. In order to predict the sea state and ultimately the wave height, near-surface wind speeds at future time intervals have to be determined. These wind forecasts are obtained by one of the current operational numerical weather prediction models.
Once the anticipated surface wind speeds have been calculated, forecasted wave heights and other information such as dominant wave period and dominant wave direction are produced using a wave-forecast model. The earliest operational hemispheric or global scale wave forecast models were developed approximately 30 years ago. These models incorporate information concerning the forcing provided by the near-surface local winds, along with the dissipation of waves, wave interactions and the propagation of swell from non-local sources. NOAA's Ocean Prediction Center (formerly called Marine Prediction Center or MPC), a component of the National Centers for Environmental Prediction (NCEP), and the US Navy operate various wave-forecast models. Currently, some of the models are run twice daily out to a lead-time of 5 days, utilizing a grid spacing of one degree of latitude and longitude.
The National Weather Service issues various marine-related advisories, watches and warnings to the public that pertain to a variety of severe weather conditions as well as unusual water, waves, and current conditions that could affect life and property. The area of responsibility includes coastal waters and the open waters of the Atlantic and Pacific Ocean and the Gulf of Mexico. Some of these statements cover tropical weather systems, including tropical storms and hurricanes. The Ocean Prediction Center issues marine warnings for situations not involving tropical weather systems. Their responsibility covers coastal and offshore waters as well as the high seas for latitudes poleward of 30 degrees north, while the Tropical Prediction Center is responsible for waters equatorward of this latitude. The Pacific Tsunami Warning Center (PTWC) in Honolulu, HI and the West Coast/Alaska Tsunami Warning Center (WC/ATWC) in Palmer, AK issue tsunami watches and warnings for Pacific basin. The National Hurricane Center (NHC) in Miami, FL, a part of the Tropical Prediction Center, is responsible for issuing statements concerning tropical weather systems for the North Atlantic Basin (including the Gulf of Mexico and the Caribbean Sea) and the eastern Pacific Basin (to 140 degrees West longitude). The Pacific Hurricane Center in Honolulu, HI monitors the Central Pacific to the International Date Line (at 180 degrees). The Hawaiian Islands are located within this region.
The following list includes terminology used by the National Weather Service for marine weather statements:
The following statements can be issued for several types of events, not limited to tropical weather systems:
The following list of public statements pertaining to tropical weather:
The National Hurricane Center (Tropical Prediction Center) issues Tropical Weather Outlooks. The information contained in these guidance products is used on television weathercasts. Outlooks include levels of risk.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2006, The American Meteorological Society.