For example, the circulation in the coastal region of the Gulf of Maine and Georges Bank (GOMGB) is associated with features such as the buoyancy-driven Maine Coastal Current (MCC), the Georges Bank anticyclonic frontal circulation system, the basin-scale cyclonic gyres, the deep inflow through the Northeast Channel, and the shallow outflow via the Great Southern Channel (Gangopadhyay and Robinson, 2002). Annulation is one of the important processes of mesoscale activity in the Kuroshio Extension.
INFLUENCE OF EDDIES AND FRONTS ON FISHERY AND WEATHER
SST images derived from satellite infrared radiometers consistently show folded wave patterns in the western boundary of the Gulf Stream from Cape Canaveral to Cape Hatteras (see Lee et al., 1981). Frontal flooding of this type manifests in surface waters as warm, tongue-like Gulf Stream outflows oriented southward around upwelling cold cores.
MAJOR CURRENT SYSTEMS IN THE WORLD OCEANS
- Antarctic Circumpolar Current
- Western Boundary Currents in the Atlantic Ocean
- Western Boundary Current in the Pacific Ocean: The Kuroshio Current
- Western Boundary Currents in the Indian Ocean
- Equatorial Undercurrents
This is also one of the strongest western boundary currents in the world's oceans (Lutjeharms, 2007). Measurements in the Indian Ocean provided an important test of the theories developed to explain the EUC.
CURRENTS OF DIFFERENT ORIGINS
- Wind-Driven Current
- Inertia Current
- Tidal Currents in Open Seas, Estuaries, and Ridge Valleys
- Rip Currents
Equations 1.1 and 1.2 predict a spiral structure for the currents in the surface layers of the sea. This is related to the pulsatile and unstable nature of the rip current (Haller and Dalrymple, 2001).
IMPLICATIONS OF OCEAN CURRENTS
One of the critical needs of the offshore industry is real-time information about water currents during events such as severe storms. In other areas there is strong evidence of erosion on the deep-sea floor, indicating the important role of currents in the distribution of oceanic sediments.
SURFACE CURRENT MEASUREMENTS
- Measurements Based on Motion of Drifting Surface Bodies
- Imaging of Surface Water Motion Trajectories and Patterns
- Vector Mapping Based on Current- Driven Sea Surface Wave Transport
Such methods shed much light on the nature of sea surface currents in various regions of the world's oceans. Some of the merits and limitations of photography and radiometry were shown in the previous sections.
SUBSURFACE AND ABYSSAL CURRENT MEASUREMENTS
- Early Mariners’ Contributions
- Spatially Integrated Measurements Based on Earth’s Magnetism and Oceanic
- Measurements Based on Motion of Drifting Subsurface Floats
- Measurements from Fixed Locations at Predetermined Depths
The other lever, attached to the meter, always faces the direction of water flow. The flow rate is directly related to the temperature difference of the two thermistors.
SEAFLOOR BOUNDARY LAYER CURRENT MEASUREMENTS
- Mechanical Devices
- Nonmechanical Devices
In the early history of the BASS it suffered from "zero point" uncertainty, and it had to. The full value of ultrasonic acoustic current meters for measurements of weak currents and turbulence in the lower boundary layer can only be achieved if flow obstruction is stopped by the body of the current meter.
VERTICAL PROFILING OF HORIZONTAL CURRENTS
Measuring turbulence in the ocean floor boundary layer with an array of acoustic current meters. Design of an acoustic tomographic system for measuring speed with an application in the coastal sea.
BIBLIOGRAPHY
RADIO BUOYS
- Drifter-Following Radar Transponder
- Drifter-Borne Doppler Transponder
- Radio Buoys Tracked by Polar-Orbiting Satellites
- GPS-Tracked Drifters
- Telephonically Tracked Drifters
The Doppler shift is a function of the relative velocity between the satellite and the PTT. The Doppler shift is related to the semi-angle of the apex (A) of the cone, the velocity of the satellite (v) relative to the PTT, and the velocity (c) of the cone.
LIMITATIONS OF SURFACE DRIFTERS
The basic mechanism underlying the use of radio wave (HF/VHF) Doppler radar systems to map the surface current of the sea is the phase velocity of the sea. If there is some relative motion between the radar and the target (in this case the sea surface), the return signal changes in frequency (ie the frequency of the received signal is different from the frequency of the transmitted signal). .
CROMBIE’S DISCOVERY
These Doppler shifts correspond to the phase velocity of radially advancing and receding sea surface waves (i.e., those waves whose wavefronts point in the direction of the radar) of half the radar wavelength. Any shift in the measured phase velocity of the surface sea wave relative to its theoretical value in still water in deep water can therefore be directly related to the vector component of the surface sea current moving towards or away from the radar site.
PECULIARITIES OF PULSE DOPPLER-RADAR ECHO SPECTRA
In shallow water, the phase speed of sea surface gravity waves is reduced compared to that of deep water waves. This results in the Bragg lines of the Doppler spectrum being at less than normal separation.
ESTIMATION OF SEA SURFACE CURRENT USING THE BRAGG
If the sea surface is free of disturbances (i.e. gravity waves are absent), the frequency of the backscattered signal received by the radar will be equal to the frequency of the transmitted signal, and consequently the Bragg resonance will be absent. Some of the complications of the received signals and various aspects of signal detection schemes have been discussed by Leise (1984). Shay et al.
DEPTH EXTENT OF DOPPLER
Some complications of the received signals and various aspects of signal detection schemes have been discussed by Leise (1984). Shay et al. 1995) have observed that even in a clearly wave-laden sea, normally only one Bragg peak is available in the return signal spectra from extreme distances (i.e. close to the theoretical limit of the radar range).
RADAR-BASED SEA SURFACE CURRENT MEASUREMENTS
TECHNOLOGICAL ASPECTS OF DOPPLER RADAR-BASED
In addition to the radar wavelength, the radar polarization is another important specification of the system. Polarization is the orientation (i.e. direction) of the electric vector in the electromagnetic radiation related to the terrain.
EXPERIMENTAL DEVELOPMENTS
The phase difference (fdegree) of the plane wave front received at the two antenna elements is related to the corresponding path difference l. Because it is known for a certain arrangement of the receiving antenna elements and measured, the direction of arrival q of the echo with respect to the receiving antenna elements can be estimated.
INSTRUMENTATION ASPECTS
The essence of data processing is the detection of the centroid of the first-order spectrum (first-order moment). The frequency corresponding to the center line of the spectrum is taken as the frequency of the Bragg line.
RADIAL AND TOTAL VECTOR CURRENTS
DEVELOPMENTS ON OPERATIONAL SCALES
- CODAR
- OSCR
- SeaSonde
- WERA
- Systems for Special Applications
During the chirp period T, the frequency (ie, the derivative of the phase with respect to time) increases linearly from vvo to (vo þb). The frequency resolutionv ¼ bs=T is determined by the chirp length so that Dv ¼ 1=T.
INTERCOMPARISON CONSIDERATIONS
Eight of the moorings had vector-measuring current meters (VMCMs) on board, and the ninth had an upward-facing acoustic Doppler current profiler (ADCP). This finding was confirmed by examining the S/N ratio of both measurements (Gurgel et al., 1997).
ADVANTAGES OF RADIO-WAVE DOPPLER RADAR MEASUREMENTS
Another noteworthy feature of Doppler radar technology is its inherent ability to measure currents without distinguishing Stokes drift (Barrick, 1986). An outstanding feature of the Doppler radar system is its ability to measure the sea surface shear flow at the uppermost layer of the sea surface.
ROUND-THE-CLOCK COAST- AND SHELF-OBSERVING ROLE OF DOPPLER
A fairly wide area coverage (more than 500 km2), high temporal resolution (entire observation area surveyed within a few minutes), immunity to weather conditions and suspended sediments, and land-based long-term monitoring are some of the key features of the Doppler radar systems. HF Doppler radar systems have also begun to be a convenient tool for ocean current modellers.
DETECTION AND MONITORING OF TSUNAMI-INDUCED SEA SURFACE-
Remotely sensed surface currents in Monterey Bay from land-based HF radar (Coastal Ocean Dynamics Application Radar). The coastal jet: Observations of surface currents over the Oregon continental shelf from HF radar.
AERIAL PHOTOGRAPHY IN THE VISIBLE AND INFRARED BANDS
Because of the wave-current interaction, the path of the water flow will be associated with a sharp contrast in the appearance of the surface wave pattern. Thus, aerial photography of the sea surface makes it possible to image patterns resulting from some dynamic processes in the upper ocean.
REMOTE DETECTION BY RADIOMETERS
- Passive Radiometry in the Visible-Wavelength Band
- Active Radiometry in the Visible-Wavelength Band
- Passive Radiometry in the Thermal Infrared Band
- Microwave Radiometers
The visible-wavelength passive radiometer measures the color of the sea surface under natural light. This allows the radiometer to scan a strip of sea surface perpendicular to the flight path.
ACTIVE MICROWAVE RADAR IMAGING OF SEA SURFACE
A surface of intermediate roughness reflects a portion of the incident energy and diffusely scatters the rest. In the latter situation, the group velocity of the resonant wave train decreases and the resonant waves tend to pile up as they are blocked by the adverse current, reaching saturation if the blocking is strong enough.
ACTIVE MICROWAVE RADAR IMAGING TECHNOLOGIES
- Active Microwave Imaging by the RAR Systems
- Active Microwave Imaging by SAR Systems
Distortion in the image due to the movement of the antenna-carrying vehicle is another serious handicap. In the stripmap mode configuration, the integration time is given by the azimuth extent of the antenna pattern.
ADVANCES IN THE DEVELOPMENT OF SAR TECHNOLOGY
- Interpretation of Image Data
High gain, very narrow beam width in azimuth and light weight are the special features of ERS-1 SAR. Built-in state-of-the-art tape recorders store Radarsat data, making it possible to retrieve information worldwide.
DETECTION OF SEAWATER CIRCULATION FEATURES USING
Tidal currents in estuaries often have a strong current gradient during certain phases of the tide. Breaking of the shoal waves is strong in the high-speed areas of the surf zone.
MEASUREMENT OF SEA SURFACE CURRENTS USING IMAGING
Radar imaging of a 2 by 5 km area of the sea surface showed narrow bands of high radar backscatter (indicating locally rough water) intersecting in the form of a narrow wedge connecting to a single band and thus the appearance of aY has. 1998) suggested that features such as these represent crossing ocean fronts, which become visible in the images through the breaking and steepening of surface waves by across-front current convergence and shear. According to them, aYforms form because the relatively buoyant water behind one front will always overwhelm the water in front of it and erase that part of the frontal expression from the surface.
OF ICE FLOES
SURFACE-TRACKABLE SUBSURFACE DRIFTERS
The drogue depth is determined by the length of the wire connected to the supporting surface raft/buoy and the ballast weight attached to the parachute. This requires that the resistance exerted by the drogue is significantly greater than that exerted by the surface float, the suspension cable and other parts of the system.
SATELLITE-RECOVERED POP-UP DRIFTERS
Teflon seal (d) conforms to the shape of the glass end surface to provide a watertight seal. The radio transmission format is structured to meet the RF requirements of the ARGOS system.
SWALLOW FLOATS TRACKED BY SHIP-BORNE HYDROPHONES
As the ship's head falls away from the wind direction, the time difference is observed as a function of the bearing of the line connecting the hydrophones. The depth of the float can be determined when the horizontal distance between the float and the ship is known.
SUBSURFACE FLOATS TRANSMITTING TO MOORED
If the travel time of the pulse is to be precisely determined, the transmission time of the acoustic pulse from the swim must be known precisely. Experiments showed that the outer Blake-Bahama ridge had significant influence on the organization of the eddy field in the MODE region.
SUBSURFACE FLOATS LISTENING TO MOORED ACOUSTIC SOURCES
Average swimming speed (ie, distance traveled divided by elapsed time) is 55 cm/s. The limitations of the single-mission RAFOS floats (acoustic data are only downloaded when the float surfaced at the end of its life) were eased by the development of a multi-cycle float, the MARVOR (named after the Breton word for a seahorse).
ALACE: HORIZONTALLY DISPLACED AND VERTICALLY CYCLING
This process is repeated until the absolute sum of the differences is less than 1 km. Launched successively into the Gulf Stream, Rossby and co-researchers used these rafts to study the space-time evolution of the meandering current.
SUBSURFACE FLOAT
DRIFTING PROFILING FLOATS (ARGO FLOATS)
- Profiling Observations from Polar Regions
All parts of the system are tested to make sure the float is working properly. Estimation of the surface and mid-depth currents from Argo drifts in the Pacific Ocean and error analysis.
ONE-WAY TOMOGRAPHY
The first test, held in 1981 in areas north and west of Bermuda, involved the production of a three-dimensional, time-evolving map of the area. Thus, a cold vortex in the observation area will delay the arrival of any transmission through the vortex, and a warm vortex will cause faster arrival of the acoustic transmission at the receiver.