Remote Sensing of Coastal and Ocean Currents: An ... - CyberLeninka

Journal of Coastal Research

28

3

576–586

West Palm Beach, Florida

May 2012

Remote Sensing of Coastal and Ocean Currents: An Overview Victor Klemas School of Marine Science and Policy University of Delaware Newark, DE 19716, U.S.A. [email protected]

www.cerf-jcr.org

ABSTRACT KLEMAS, V., 2012. Remote sensing of coastal and ocean currents: an overview. Journal of Coastal Research, 28(3), 576– 586. West Palm Beach (Florida), ISSN 0749-0208. Ocean currents influence the global heat transport; weather and climate; larval transport; drift of water pollutants; sediment transport; and marine transportation. As a result, oceanographers, coastal managers, and ships need up-todate information on ocean and coastal currents. Arrays of current meter moorings can measure currents at local scales. Shore-based high-frequency radars are able to map coastal currents over a range of up to 200 km. Ocean drifters can be tracked to obtain circulation patterns over larger areas, but may take months to accomplish it. Only satellite remote sensors can determine currents synoptically over extensive ocean and coastal regions. Satellite altimetry is one of the essential remote-sensing techniques for monitoring dynamic ocean conditions, including surface currents, local wind speed, and significant wave height. Satellite altimetry measures sea surface heights, providing data on geostrophic circulation, including major ocean currents. Ocean currents can also be determined by satellite synthetic aperture radar (SAR) or tracking the movement of thermal and color features in the ocean. The flow patterns of currents like the Gulf Stream are being mapped with satellite infrared scanners. The objective of this paper is to review practical remotesensing techniques for measuring and mapping coastal and ocean currents. Coastal and ocean currents, remote sensing, SAR current mapping, satellite altimetry, feature tracking, HF radar, ocean drifters.

ADDITIONAL INDEX WORDS:

INTRODUCTION On ocean basin scales, knowledge of oceanic circulation is a significant component of planetary heat budget calculations for global climate programs. One obvious example of the ocean circulation’s influence on climate is the Gulf Stream, which transports enormous amounts of heat from the Equator and the tropics to northern Europe and southern Greenland. Thus the average temperature in northern Europe is about 6 to 9uC higher than at the same latitudes in North America (Pinet, 2009; Purkis and Klemas, 2011). Ocean currents are also important to the distribution of the ocean’s sea life. Many species of fish rely on currents to move them to breeding grounds, areas with more abundant prey, and more suitable water. The Humboldt Current is an example of a current that affects the weather and fisheries productivity. When this cold current is normally present off the coasts of Peru and Chile, it helps maintain highly productive upwelling waters and keeps the coast cool and northern Chile arid. When it becomes disrupted by such events as El Nin˜o, Chile’s climate is altered and fish become scarce (Briney, 2009; Klemas, 2011; Pinet, 2009; Polovina, Kleiber, and Kobayashi, 1999; Santos, 2000; Siegel, McGillicuddy, and Fields, 1999).

DOI: 10.2112/JCOASTRES-D-11-00197.1 received 1 November 2011; accepted in revision 19 December 2011. Published Pre-print online 04 April 2012. ’ Coastal Education & Research Foundation 2012

Knowledge of the current and wave conditions is essential to ships and shipping companies to reduce shipping costs, fuel consumption, and avoid powerful storms and disasters. For instance, the Labrador Current, which flows south out of the Arctic Ocean along the coast of Newfoundland, is known for moving icebergs into shipping lanes in the North Atlantic. Debris and other pollutants get trapped and moved around the world by currents, forming trash islands in some cases. The extent and movement of these trash islands and other pollutants, such as oil spills, can be tracked by airborne and satellite sensors (Jha, Levy, and Gao, 2008; Klemas, 2010). Along the coast and offshore there are local currents generated by tides, winds, storms, and waves. These currents are important in studies and control of local problems, such as harmful algal blooms and sediment transport. Since currents influence so many marine-related activities and processes, meteorologists, oceanographers, ships, coastal and fisheries managers, and marine-related agencies need to have up-todate information on ocean and coastal currents. Arrays of current meters and shore-based radar can provide current measurements only at local scales. To map surface currents and their influence on the environment over large ocean or coastal regions it becomes necessary to use satellite remote sensors (Clemente-Colon and Pichel, 2006; Han, 2005; Ikeda and Dobson, 1995; Yan, 2011). The objective of this paper is to review practical remotesensing techniques for measuring and mapping coastal and

Remote Sensing of Coastal and Ocean Currents

ocean currents. The effectiveness of key remote-sensing approaches will be illustrated by examples.

OCEAN AND COASTAL CURRENTS There are various physical reasons for the movement of water, such as wind stress, tides, and water density. The major ocean surface currents are generated primarily by winds. On the other hand, the atmospheric circulation, including winds, is produced by convection due to variation of temperature with latitude and the Coriolis effect caused by Earth’s rotation (Pinet, 2009). Surface currents are those found in the upper 400 m of the ocean and contain about 10% of all water in the ocean. Surface currents are primarily caused by wind friction as the wind moves over the water. The speed of a current will be approximately 3 to 4% of the speed of the generating wind. Because the major surface currents travel over long distances, the Coriolis force deflects them toward the right in the Northern Hemisphere, causing these currents to move in clockwise circular patterns or gyres. In the Southern Hemisphere they spin counterclockwise. The speed of the surface currents is greatest at the surface and decreases significantly at about 100 m below the surface. Because the surface of the ocean is higher in areas that form where the water meets land, where water is warmer, or where two currents converge, gravity pulls this water downslope and creates currents that are perpendicular to the slope due to the Coriolis force (Briney, 2009; Ikeda and Dobson, 1995; Martin, 2004; Pinet, 2009). Some of the major ocean surface currents include the California, Kuroshio, and Humboldt currents in the Pacific; the Gulf Stream, Labrador, Brazil, and Agulhas currents in the Atlantic; and the Indian Monsoon Current in the Indian Ocean. The more important coastal and offshore currents include wave-driven, tidal, wind-driven currents and buoyant river plumes. These local currents can be short-term (hourly) or longterm (seasonal) currents. Coastal currents are important in studies and control of local flooding, algal blooms, oil slick drift, sediment transport, and ship navigation (Gelfenbaum, 2005; Robinson, 2004). Deepwater currents caused by thermohaline circulation are found below 400 m and make up about 90% of the oceans. Deepwater currents are caused by the effect of gravity on density differences in the water. Density differences are a function of temperature and salinity. Warm water is less dense and rises to the surface, whereas colder, salt-laden water sinks. Thermohaline circulation is the mechanism responsible for the Global Conveyor Belt because its circulation of warm and cold water acts as a submarine river and moves water through all three major oceans (Briney, 2009; Ikeda and Dobson, 1995; Pinet, 2009). However, deep ocean currents cannot be observed by satellite or airborne remote sensors and will not be covered in this paper.

EULERIAN TECHNIQUES In situ current sensors are used in coastal areas, to calibrate and validate remote-sensing measurements of currents, and to obtain current data at depths beneath the sea surface.

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Oceanographers and coastal engineers distinguish been two basic approaches to coastal and offshore current measurements, the Eulerian and the Lagrangian methods (Morang and Gorman, 2005). Eulerian techniques measure the velocity of water flow past a point in the ocean. Lagrangian techniques measure the movement of a parcel of water in the ocean by tracking the position of surface and subsurface drifters or chemical tracers. The Eulerian method usually involves current meters on buoy moorings that are fixed to the ocean floor and measure currents at various depths, yet only at one specific site. Arrays of such buoy moorings with current meters at various depths are deployed for days up to months in coastal waters to measure currents at specific sites, such as in tidal inlets or harbor entrance channels. The impeller (propeller) current meters are pointed into the current by a vane, just like moving air orients a wind vane. Current speed is measured by an impeller that is rotated by the force of the current. Thus its rotational velocity is related to the current speed. These measurements of current speed and direction are recorded into a computer-chip memory. The current meter can be retrieved by a sound signal, which activates an acoustic link that releases the cable and instrument package from the anchor. The instrument system with its data then floats to the sea surface, where it can be located acoustically and retrieved (Pinet, 2009). Acoustic systems, such as acoustic Doppler current profilers (ADCP) and acoustic Doppler current meters, are more expensive, but can be used in areas where heavy ship traffic or storms might damage impeller current meter moorings. Shipboard ADCPs are widely used to profile currents within 200 to 300 m of the sea surface while the ship cruises between hydrographic stations. Acoustic Doppler current meters are simpler than ADCPs, and transmit continuous beams of sound to measure current velocity close to the meter, not as a function of distance from the meter. Doppler current sensors can be deployed on the sea bottom, attached to an in-line mooring, mounted on the keel of a ship, or in a buoy bridle. Narrow sound pulses are emitted by the current meter in different directions. These pulses are reflected from tiny particles or air bubbles suspended in the water that lie within several meters of the current meter. Particles drifting away or toward the meter with the current cause a characteristic Doppler shift in the reflected sound echo that allows the direction and speed of the current to be calculated (Bourgerie, Garner, and Shih, 2002; Pinet, 2009). The current direction is found by taking the measurements along two orthogonal axes, x and y. These measurements can be compensated for tilt by use of tilt sensors and referred to magnetic north by means of an internal Hall-effect compass. A microprocessor computes vector-averaged current speed and direction over the last sampling interval. A low-frequency acoustic transducer for data transmission is usually attached to the system (Aanderaa, 2004; Davidson-Arnott, 2005). Electromagnetic current meters measure the voltage resulting from the motion of a conductor (water flow velocity) through a magnetic field according to Faraday’s law of electromagnetic induction. Faraday’s law defines the voltage produced in a conductor as the product of the speed of the conductor (water flow velocity) times the magnitude of the magnetic field times

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the length of the conductor. In this case, the conductor length is the path between the sensing electrodes. The use of an alternating magnetic field and synchronous detection techniques to measure the voltage at the sensing electrodes provides a very stable, low-noise current measurement. Two orthogonal pairs of electrodes and an internal flux gate compass provide the current vector. Data obtained by such meters are usually stored internally in solid-state, nonvolatile memories (InterOcean, 2007). Many of the Eulerian current meters are part of a ‘‘package’’ that includes sensors for measuring current speed, direction, wave, tide, conductivity–temperature–depth, turbidity, and water-quality parameters. A major disadvantage of Eulerian current meters is that they can cover relatively small areas with measurements only at specific points. Also, moorings tend not to last long enough to give accurate estimates of mean velocity or interannual variability of the velocity. Fouling of the sensors can require frequent maintenance.

LAGRANGIAN TECHNIQUES Lagrangian techniques involve the release of ocean drifters that are subsequently tracked acoustically, visually, or by radio waves. The drifters are designed to float with the moving water, allowing researchers to determine the speed and direction of currents. Lagrangian current measurement techniques are typically used in sediment transport studies, in pollution monitoring, for tracking ice drift, and in global ocean current studies. Ocean drifters are specifically designed to track the movement of water (currents) at various depths. A typical design of such Lagrangian drifters includes a float or surface buoy connected by a cable to a current drogue. The drogue, set for a specific depth, acts like an underwater sail as it is pushed by the ocean current. The drogue surface facing the current is relatively large to ensure that the drifter tracks the water movement, rather than being blown by the wind. The surface float provides buoyancy, contains the electronics, and transmits data to satellites. The satellites determine the drifter’s position from the transmission signal and relay the data to ground stations, where drift is calculated from the positions. Drifting subsurface buoys can also be tracked acoustically by surface floats that communicate with global positioning system (GPS) satellites for determining exact positions (Fratantoni, 2001; Jenkins, 1992; Richardson, 1991; Uchida and Imawaki, 2003). One of the important global drifter programs has been the Argo program, a collaboration between 50 research and operational agencies from 23 countries and a component of the Global Climate Observing System and the Global Ocean Observing System. The Argo program was originally designed to operate on the same 10-day duty cycle to match measurements of the ocean’s sea surface being conducted by the TOPEX/POSEIDON and JASON-1 satellites. Whereas the satellites measure changes in the surface topography of the ocean, Argo floats measure subsurface changes in temperature and salinity. Thus the float measurements are complementary to the satellite altimetry, from which mass redistribution and surface currents can be inferred.

By 2007, a total of 3000 Argo floats had been deployed by ships and airdrops across most ocean regions. Argo floats drift at a fixed pressure (around 1000- or 1500-m depth) for 10 days. After this period, the floats move to 2000-m depth, then rise, collecting instantaneous profiles of pressure, temperature, and salinity. At the surface, the floats transmit the collected data via satellite link back to a ground station, allowing the satellite to determine their surface drift by comparing the float’s new position with the previous one. Then the floats sink again and repeat their mission. The buoyancy mechanism works by changing the density of a liquid (e.g., oil) reservoir. When the liquid is allowed to expand, it becomes less dense and the float rises, and when it is compressed, the float sinks. There was no large-scale collection of temperature and salinity profiles before Argo started in 2000, since ships could only cover small areas for short times. The real-time data provided by the Argo observation system is now used in climate, weather, oceanographic, and fisheries modeling and research. For instance, the Argo system is providing a deeper understanding of ocean dynamics and allowing much better short- and long-term predictions of weather and climate (Feder, 2000; NIWA, 2010; NOAA/PMEL, 2011). A nearshore example of the application of inexpensive ocean drifters is a study of the drift and dispersion of ocean-dumped industrial acid waste 64 km off the Delaware coast, conducted in the late 1970s. The drifters were inserted from boats and their current drogues adjusted to track the current flow near the surface, just above the summer thermocline, and well below the thermocline. The drifters radiated a 6-MHz signal that was tracked from three shore stations using loop antennas. The time sequence of drifter locations was established by triangulation. The drifter data provided information on the local currents at several depths and valuable insights into the rapid drift of waste plumes above the thermocline and their slow dispersion toward the bottom below the thermocline (Klemas and Philpot, 1981). Ocean drifters may also contain various instruments to measure water temperatures and a variety of other parameters. Ships and aircraft can drop these durable drifter buoys into the sea, where they normally have a survival rate of several hundred days (Breaker et al., 1994; Davis, 1985). For instance, drifters designed to transmit for 2 to 4 years were released from ships off Cape Blanco, Oregon. The great distances travelled by the drifters showed how water parcels and the material they contain (e.g., plankton) disperse from Oregon’s coastal ocean. The detected seasonally varying ocean circulation was found to have profound implications for the biology off the Oregon coast. Data from this drifter project were used in conjunction with other physical and biological measurements as part of the Global Ocean Ecosystem Dynamics (GLOBEC) program to help scientists to better understand the northeast Pacific Ocean ecosystem (GLOBEC, 2006). Drifters do not always stay in a parcel of water. External forces, such as winds, can cause them to drift relative to the water and fail to follow a parcel of water. Before GPS, there could be errors in determining the exact positions of the drifters. Drifters can also be captured in convergence zones of fronts and gyres, instead of following the water flow.

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Figure 1. New York Bight surface currents as measured with shore-based HF radar (courtesy of Coastal Ocean Observation Laboratory, Rutgers University).

SHORE-BASED RADAR Currents and waves strongly affect coastal ecosystems, especially in the nearshore, which is an extremely dynamic environment. Currents influence the drift and dispersion of various pollutants and, together with breaking waves, mobilize and transport sediments, resulting in erosion and morphological evolution of natural beaches. Changes in the underlying bathymetry, in turn, affect the wave and current patterns, resulting in a feedback mechanism between the hydrodynamics and morphology. The ability to monitor these processes is necessary to understand and predict the changes that occur in the nearshore region. Arrays of current meters are not always practical for determining surface currents over larger, dynamic coastal areas, since these instruments measure currents at a point and are expensive, especially when large numbers of sensors have to be deployed at considerable risk of losing them (Morang and Gorman, 2005). Over the past three decades shore-based high-frequency (HF) and microwave Doppler radar systems have been deployed to map currents and determine swell–wave parameters along the world’s coasts with considerable accuracy. High-frequency radars operate in the 3–30-MHz frequency range and use a ground-wave propagation mode of the electromagnetic waves (Barrick, Evans, and Weber, 1977; Bathgate, Heron, and Prytz, 2006; Essen, Gurgel and Schlick, 2000; Graber et al., 1997; Gurgel, Essen and Schlick, 2003; Haus, Graber and Shay, 1997; Paduan and Cook, 1997; Schofield, Kohut and Glenn, 2008). The radar surface current measurements use the concept of Bragg scattering from a

slightly rough sea surface, modulated by Doppler velocities of the surface currents. When a radar signal hits an ocean wave it usually scatters in many directions. However, when the radar signal scatters off a wave that has a wavelength half of the transmitted signal wavelength, Bragg scattering will return the signal directly to its source, resulting in a very strong received signal. Extraction of swell direction, height, period, and current velocity from HF radar data is based on the modulation imposed upon the short Bragg wavelets by the longer faster-moving swell (Paduan and Graber, 1997; Plant and Keller, 1990; Teague, Vesecky, and Fernandez, 1997). Surface currents in the New York Bight, as measured with shore-based HF radar for upwelling and downwelling favourable conditions, are shown in Figure 1. This figure also demonstrates how integration of HF data with satellite imagery provides a more complete picture of sea surface dynamics. Depending on the operating frequency selected, HF radars can attain working ranges of up to 200 km and spatial resolutions between 300 and 1000 m. Since they can perform continuous measurements, e.g., at 10-minute intervals, HF radars satisfy the high temporal resolution requirements for tracking tidal and wind-driven currents required for pollution monitoring, ship guidance, rescue operations, and coastal management (Georges et al., 1998; Gurgel, Essen and Kingsley, 1999; Paduan and Graber, 1997; Shearman and Moorhead, 1988). Surface current fields have been mapped successfully not only with shore-based but also with shipborne HF radars (Gurgel, 1997; Skop and Peters, 1997). Rapid-response HF radar units, such as the SeaSonde, are even being deployed

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Figure 2. Terra-MODIS brightness temperature image for the eastern seaboard of the United States acquired May 2, 2001. The warm waters of the Gulf Stream, with its swirls and gyres, are depicted in reds and yellows. Courtesy: NASA/Goddard Space Flight Center.

from helicopters to map currents along coastlines to improve the modeling and operational prediction of oil slick drift (Kjelaas and Whelan, 2011; Paduan and Rosenfeld, 1996). High-frequency radars have also been applied in ecological research of larval transport in upwelling areas (Bjorkstedt and Roughgarden, 1997). Although HF radar provides accurate maps of surface currents and wave information for large coastal areas, their spatial resolution, from hundreds of meters to kilometers, is more suitable for measuring mesoscale features than smallscale currents (Kosro, Barth, and Strub, 2008; Romeiser, 2007). On the other hand, microwave X-band and S-band radars have resolutions of the order of 10 m, yet have a range of only a few kilometers (Braun et al., 2008; Helzel et al., 2011; Wu, Kao, and Yang, 2010). Microwave radars are being used to remotely sense ocean waves and currents close to shore. A new X-band marine radar family (coherent and noncoherent) operating at 10-m scales can measure ocean currents and wave spectra at distances out to a few kilometers (Trizna, 2007). Marine radars are also used for tracking coastlines and detecting fixed harbor objects and other ships, especially in low-visibility coastal fog, to avoid collisions. All remote-sensing radars, including satellite-borne systems, have the advantage of no instrumentation moored in the open sea, where the instruments may be damaged or lost to storms and ships passing by (Gurgel and Schlick, 2008).

SATELLITE REMOTE SENSING Thermal Infrared Mapping of Flow Patterns Satellite measurements of ocean currents allow scientists and operational users to get current data over large expanses

of the oceans in near-real time (Dzwonkowski et al., 2010; Robinson, 2004; Wunsch, 1992). A current’s flow pattern and other features, such as Gulf Stream eddies, can be mapped using scanning thermal infrared radiometers on satellites. High-resolution satellite-derived sea surface temperature (SST) measurements are ideal for investigating western boundary currents, such as the Gulf Stream and Kuroshio, which exhibit displacements on large temporal and spatial scales. A typical example of thermal infrared mapping of ocean currents is illustrated in Figure 2, which shows the Gulf Stream as it meanders northward along the East Coast of the United States, generating warm core eddies (rings) along the way. As shown in Figure 2, the temperature gradient across the north wall of the Gulf Stream can be very strong, with changes of 4uC across several hundred meters (Joyce and Wiebe, 1983). Accurate large-scale, long-time observations of SST are also important to a wide range of global change studies. Sea surface temperatures are necessary, for example, for estimating the source of heat at the air/sea boundary. Sea surface temperature data have been used by the fish and wildlife communities to study marine habitats over many parts of the globe (Gentemann et al., 2003; Purkis and Klemas, 2011; Santos, 2000; Yang, 2009). Thermal infrared was the first method of remote sensing to gain widespread acceptance by the oceanographic and meteorological communities. Thermal infrared sensors have been deployed for over 40 years on operational meteorological satellites to provide images of cloud top temperatures, and when there are no clouds, they observe SST patterns. Thermal infrared instruments that have been used for deriving SST include the Advanced Very High Resolution Radiometer (AVHRR) on the National Oceanic and Atmospheric Administration (NOAA) Polar-orbiting Operational Environmental Satellites, along-track scanning radiometer aboard the European remote-sensing satellite (ERS-2), the geostationary operational environmental satellite imagers, and moderate resolution imaging spectroradiometer aboard National Aeronautics and Space Administration (NASA) Earth Observing System Terra and Aqua satellites (Conway, 1997; Cracknell and Hayes, 2007). Cloud cover can limit thermal infrared sensing, hindering frequent ocean current and temperature observations.

Current Velocities Derived from Visible and Thermal Feature Tracking Estimates of currents over large ocean areas, such as the continental shelves, can also be obtained by tracking the movement of natural surface features that differ detectably in color or temperature from the background waters. In satellite ‘‘feature tracking’’, sequential satellite imagery is used to determine the displacements of selected ocean features (e.g., chlorophyll plumes, patches of different water temperature, and surface slicks) over the time intervals between successive images to estimate surface flow fields. Thermal infrared imagery from the AVHRR, ocean color images from sea-viewing wide field-of-view sensor (SeaWiFS), and radar images from Synthetic Aperture Radar (SAR) have all been used to perform

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feature tracking (Kuo and Yan, 1994; Liu, Zhao, and Hsu, 2006). Satellite feature tracking has been used to estimate the surface circulation in such regions as the California Current, the Gulf Stream, the Kuroshio Current, the Gulf of Mexico, the English Channel, the west coast of Ireland, and the coast of New Zealand. Because of the requirement for accurate spatial alignment and coregistration of the imagery used in feature tracking, the technique has been more often used in coastal regions, where landmarks are available to renavigate the satellite data (Breaker et al., 1994; Romeiser, 2007; Yan and Breaker, 1993). A major disadvantage is that cloud cover frequently obscures visible and thermal infrared ocean surface features. There is also a need to improve ocean feature detection and tracking techniques and to develop more reliable operational procedures.

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milliseconds, show phase differences proportional to Doppler shifts of the backscattered signal, thus to line-of-sight target velocities. This permits a direct imaging of line-of-sight velocity fields. To obtain two SAR images with a short time lag from a moving platform, one needs two antennas separated by a corresponding distance in the flight direction. The best data quality is expected at time lags of several milliseconds at Xband frequencies and tens of milliseconds for L-band, corresponding to along-track antenna separations of tens to hundreds of meters for space-borne systems (Romeiser, 2007). The first demonstration of current measurements from space with InSAR was given by the shuttle radar topography mission in February 2000, which gave results consistent with numerical model results and provided an accuracy of 0.1 m/s. At the present time the German satellite TerraSAR-X is providing global ocean surface current measurements for many applications.

Current Mapping by SAR Although point measurements can be obtained in various ways, satellite radar techniques permit synoptic measurements of surface current fields over hundreds of square kilometers. This was first demonstrated by SEASAT SAR in 1982. Synthetic aperture radar instruments, such as the advanced SAR, record microwave backscatter patterns to identify roughness patterns, which are linked to varying surface winds, waves, and currents on the ocean surface. By using the Doppler shift information embedded in the radar return signal, it is possible to determine how surface winds and currents contribute to the Doppler shift. The Doppler shift is introduced by the relative motion between the satellite platform, the rotation of the earth, and particular facets of the sea surface from which the SAR signal scatters back to the satellite. The first two values are known for most satellites with stable orbits and altitudes and can be subtracted to extract the useful sea surface velocity information. However, more work needs to be done on modeling and interpretation of SAR signatures of ocean features associated with spatially varying surface currents and development of retrieval algorithms for future SAR missions (Robinson, 2004; Romeiser, 2007; Romeiser et al., 2005; Rufenach, Shuchman, and Lyzenga, 1983). Spatial and temporal current variations measured with SARs have been applied to the study of ocean fronts, eddies, internal waves, risk management for coastal structures, and ship operations. These measurements have been very useful for advancing the understanding of surface current dynamics and mesoscale variability, as well as for determining surface drift, important for oil dispersion, pollution transport, and for wave– current interaction capable of creating dangerous rogue waves (Robinson, 2004; Romeiser, 2007). A more recent technique, using along-track interferometric SAR (InSAR), offers a higher spatial resolution and the flexibility to acquire data anywhere in the world from satellites. It permits imaging of line-of-sight surface velocity fields with the spatial resolution of SARs, which is of the order of meters within a swath width of tens to hundreds of kilometers for satellites. Along-track interferometry exploits the fact that two complex SAR images of the same scene, acquired from the same antenna location with a short time lag of the order of

Altimetry of Geostrophic Currents A major category of ocean circulation that is discernible by satellite altimetry is geostrophic circulation, wherein the pressure gradient can be depicted by a difference in the ocean’s dynamic height. A satellite altimeter is a radar that precisely measures the range from the radar antenna to the ocean surface. Satellite altimetry produces unique global measurements of instantaneous sea surface heights relative to a reference surface, and is one of the essential tools for monitoring ocean surface conditions, including surface currents, local wind speed, and significant wave height. An overall accuracy of a few centimeters is required for observations of the dynamic sea surface elevation to be useful (Andersen, 1995; Canton-Garbin, 2008; Ducet, Le Traon, and Reverdin, 2000; Le Provost and Bennett, 1995; Ray and Cartwright, 2001; Schrama and Ray, 1994; Shaw, Chao, and Fu, 1999). As shown in Figure 3, a radar altimeter aboard a satellite is a nadir-looking active microwave sensor. Its signal pulse, transmitted vertically downward, reflects from the ocean surface back to an altimeter antenna. The round-trip time and the propagation speed of the electromagnetic waves are used to compute the range between the antenna and the ocean surface. From the altimeter-measured range, the instantaneous sea surface relative to a reference surface, such as a reference ellipsoid, can be determined if the satellite orbit relative to the reference surface is precisely known. With the knowledge of the oceanic geoid, the displacement of the sea surface from the geoid due to ocean dynamic circulation can be mapped. Repeated altimetric observations can provide a measurement of the temporal variability of the sea surface height since the geoid can be treated as time invariant for oceanographic applications. The measurements permit estimation of variable geostrophic surface currents and changes in their mass transport when combined with conventional data of the internal density structure (Robinson, 2004). A simplified explanation of geostrophic currents is that seawater naturally wants to move from a region of high pressure (or high sea level) to a region of low pressure (or low sea level). The force pushing the water toward the low-pressure region is called the pressure gradient force. In a geostrophic

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Figure 4. Mean tidal energy fluxes of the M2 barotropic tide, determined from TOPEX/POSEIDON altimeter measurements (Ray and Cartwright, 2001). Figure 3. Configuration for JASON-2 altimetry operation. Credits: The COMET Program, EUMETSAT. A color version of this figure is available in the online journal.

flow, instead of water moving from a region of high pressure to a region of low pressure, it moves along the lines of equal pressure (isobars). This occurs because the earth’s rotation results in a force being felt by the water moving from the high to the low, known as the Coriolis force. The Coriolis force acts at right angles to the flow, and when it balances the pressure gradient force, the resulting flow is known as geostrophic. The direction of flow is with the high pressure to the right of the flow in the Northern Hemisphere, and the high pressure to the left in the Southern Hemisphere (Robinson, 2004; Wikipedia, 2011). The major currents of the world’s oceans, such as the Gulf Stream, the Kuroshio Current, the Agulhas Current, and the Antarctic Circumpolar Current, are all approximately in geostrophic balance and are good examples of geostrophic currents. With a precision of a few centimeters, altimetric measurements have improved tidal charts when coupled with tidegauge data and hydrodynamic models. Figure 4 shows global mean tidal energy fluxes determined from TOPEX/Poseidon altimeter measurements (Ray and Cartwright, 2001; Ray and Mitchum, 1997). The information in Figure 4 was obtained by analyzing altimeter-derived cotidal charts and it shows that most tidal energy is dissipated in shallow seas. Only about 25 to 30% of the energy dissipation occurs in the open ocean (Egbert and Ray, 2000). Some of the better-known altimetry missions were conducted by satellites such as SEASAT, ERS-1 and 2, TOPEX/POSEIDON, ENVISAT, and JASON-1 (Elachi and van Ziel, 2006; Fu and Chelton, 2001; Han, 2005; Ikeda and Dobson 1995; Robinson, 2004; Wunsch and Stammer, 1998). Scientific results from altimetric data have significantly improved our knowledge of global ocean tides and mesoscale circulation variability in the ocean. Transport vessels and fishing fleets use altimetry products to identify wave heights and wind speeds over large areas in near-real time and determine which regions of the ocean have strong currents. Having this information improves ship routing, saves fuel, and

alerts ocean vessels about dangerous conditions. Furthermore, strong eddies that can disrupt offshore oil platform and other operations can also be detected by altimetric measurements (Yan et al., 2006). Integrating altimetry measurements into ocean forecast models helps with search and rescue operations by predicting where ships in trouble are drifting. The threedimensional hydrodynamic models using satellite altimetry data seem to be effective in reproducing the circulation features as observed in situ, but there is a need for better spatial and temporal resolution and improved ocean dynamics and tide models, especially for shallow seas and nearshore areas (Arnault, Morliere, and Merle, 1992; Dadou et al., 1996; Desai and Wahr, 1995; Mellor and Ezer, 1991; Pereira-Cardenal et al., 2011; Ray and Cartwright, 2001; Ray and Mitchum, 1997). Altimetry data are also used to model the drift of oil slicks and determine where spills are likely to come ashore (Han, 2005; Kelly, 1991; Morimoto, Yanagi, and Kaneko, 2000; Yan et al., 1994). At distances of about 50 km or closer from the coast, satellite altimetry is of limited value because of land contamination in the altimetry, large radiometric footprints of 10 km to 50 km, and the fact that coastal currents are usually not geostrophic. The radar echoes reflected off water, and off a combination of water and land, will not be identical, and only the former undergo processing. Also the computation of some required corrections is more difficult, e.g., tides are much more complex near the shore than in the open ocean, and require a very precise knowledge of the coastal geography. Wet tropospheric corrections are also less precise near the coast (ESA, 2011). Nonetheless, there are many studies that are trying to enhance the quality of altimetry data close to the coast. New processing methods and applications are being developed for littoral and shallow-water regions, which contain some of the most fragile ecosystems in the world (Deng et al., 2002; Kouraev et al., 2004).

SUMMARY AND CONCLUSIONS On ocean basin scales, knowledge of oceanic circulation is a significant component of planetary heat budget calculations for global climate programs. Global currents also influence the

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circulation of the earth’s moisture, global heat transport, weather and storm conditions, marine transportation, drift of water pollutants, larval transport, and fish aggregation. Along the coast and offshore there are short-term (hourly) and longterm (seasonal) local currents generated by tides, winds, storms, and waves. These currents are important in studies and control of local flooding, algal blooms, oil slick drift, sediment transport, ship navigation, etc. As a result, oceanographers, meteorologists, ships, coastal managers, and marinerelated agencies need to have up-to-date information on ocean and coastal currents. Surface currents are those found in the upper 400 m of the ocean and affect about 10% of all water in the ocean. Surface currents are primarily caused by wind friction as the wind moves over the water. The speed of a current will be approximately 3 to 4% of the speed of the generating wind. Because the major surface currents travel over long distances, the Coriolis force caused by Earth’s rotation deflects them toward the right in the Northern Hemisphere, forcing these currents to move in clockwise circular patterns or gyres. Deepwater currents caused by thermohaline circulation are found below 400 m and contain about 90% of the ocean water. Deepwater currents are caused by the effect of gravity on density differences in the water. Density differences are a function of temperature and salinity. Deep ocean currents cannot be observed by electromagnetic remote-sensing techniques. Coastal engineers and oceanographers distinguish between two approaches to coastal current measurement, the Eulerian and the Lagrangian methods. Eulerian current meters are usually mounted on buoy moorings that are attached to cables anchored to the sea bottom. Arrays of such moorings with current meters at various depths are deployed for days up to months in coastal waters to measure currents at specific sites. The current meters can be of a mechanical, acoustic Doppler, or electromagnetic type. Lagrangian techniques involve the release of ocean drifters that are subsequently tracked acoustically, visually, or by radio waves. The drifters are designed to float with the moving water, allowing researchers to determine the speed and direction of currents. Ocean drifters may also contain various instruments to measure water temperatures and a variety of other parameters. Shore-based HF and microwave Doppler radar systems are being used to map currents and determine swell–wave parameters along the world’s coasts with high temporal resolution. The surface current measurements use the concept of Bragg scattering from a slightly rough sea surface, modulated by Doppler velocities of the surface currents. Depending on the operating frequency selected, HF radars can attain working ranges of up to 200 km and spatial resolutions between 300 and 1000 m. In contrast, microwave X-band and S-band marine radars have resolutions of the order of 10 m, yet have a range of only a few kilometers. Since they can perform continuous measurements, e.g., at 10-minute intervals, shorebased radars satisfy the high temporal resolution requirements for tracking tidal and wind-driven currents required for pollution monitoring, ship guidance, rescue operations, and coastal management.

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Synthetic aperture radars can record microwave backscatter from the ocean surface to identify roughness patterns, which are linked to varying surface winds, waves, and currents. Spatial and temporal current variations measured with SARs have been applied to the study of ocean fronts, eddies, internal waves, risk management for coastal structures, and ship operations. These measurements have advanced the understanding of surface current dynamics and mesoscale variability, as well as surface drift (important for oil dispersion), pollution transport, and wave–current interaction (Young, Rosenthal, and Ziemer, 1985). Direct measurement of large-scale ocean currents with current meter arrays is difficult and costly on an ocean basin scale. Since large-scale currents are very nearly in geostrophic balance, their velocity can be calculated from the pressure gradient on an equigeopotential surface. The surface geostrophic current can therefore be calculated from the deviation of sea level from the equigeopotential at the ocean surface (marine geoid). Measuring sea level from space by satellite altimetry thus allows one to determine the global surface geostrophic ocean circulation and its variability. Coupled with knowledge of the geoid and the ocean density field, satellite altimetry provides a feasible approach for determining major geostrophic currents in the open ocean. Scientific results from altimetric data with a precision of a few centimeters, when coupled with tide-gauge data and hydrodynamic models, have significantly improved our knowledge of global ocean tides and mesoscale circulation variability in the deep ocean. Some of the better-known altimetry missions were conducted by satellites such as SEASAT, ERS-1 and 2, TOPEX/POSEIDON, ENVISAT, and JASON-1. The flow patterns of surface currents and other features, such as Gulf Stream eddies, can be mapped using scanning thermal infrared radiometers on satellites. Thermal infrared sensors have been deployed for over 40 years on operational meteorological satellites to provide cloud temperatures and observe SST patterns. Ocean current velocities can also be obtained by tracking the movement of natural surface features that differ detectably in color or temperature from the background waters. In satellite feature tracking sequential satellite imagery is used to determine the displacements of selected ocean features (e.g., chlorophyll plumes, patches of different temperature, etc.) over the time intervals between successive images to estimate surface flow fields. Thermal infrared imagery from the AVHRR, ocean color images from SeaWiFS, and radar images from SARs have been used to conduct feature tracking. To summarize, in coastal and offshore waters, shore-based HF radar meets most user requirements for up to 200 km from shore with a high temporal resolution. High-frequency radar should be further developed to increase its range and spatial resolution. The derived surface currents can be compared and validated with in situ sensors in coastal areas and with satellite SAR results. More work needs to be done on modeling and interpretation of SAR signatures of ocean features associated with spatially varying surface currents and development of retrieval algorithms. Feature tracking with visible and thermal infrared sensors is limited by cloud cover and needs improvements in ocean feature tracking techniques and

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development of more reliable operational procedures. On large ocean scales, significant progress has been made measuring geostrophic currents with satellite altimeters. However, satellite altimetry needs better spatial and temporal resolution and could use improved ocean dynamics and tide models, especially for shallow seas and nearshore applications.

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