Inferring Maximum Geostrophic Current Velocities in the Norwegian-Greenland Sea from Settling-Velocity Measurements of Sediment Surface Samples: Methods, Application, and Results
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ABSTRACT Critical shear velocities and geostrophic current velocities were calculated from measured grain sizes and settling velocities using a Shields-type threshold curve of particle movement and assuming bedload transport for particles that were found to be transported by currents. Because this reconstruction of the current velocities is based on the coarsest transported component that could be recognized in the sediment, the resulting current velocities represent a maximum flow speed that was effective during deposition of sediment. The shape of the settling-velocity distributions identifies the dominant type of deposition that controlled the accumulation of sediment: residual, well-sorted, or low-energy depositional. The resulting pattern of reconstructed maximum geostrophic current velocities in the Norwegian-Greenland Sea reflects the known features of present-day thermohaline circulation, which is influenced by the bathymetry at many places. Reconstructed current velocities are higher in areas where the cross section of meridional flows is restricted by topographic barriers. The distribution of the depositional characters shows that residual and well-sorted sediment types are generally associated with higher current velocities in areas of strong geostrophic currents (e.g., Fram Strait and East Greenland continental slope). The low-energy depositional sediment type is widespread in the basins of the eastern Norwegian-Greenland Sea, where lower reconstructed maximum current velocities occur. The high reconstructed velocities on the Spitsbergen Bank can be attributed to the disturbance from storm waves in shallower water depths.Keywords:
Settling
Norwegian
Geostrophic current
Warm water of the Tsushima Current converges southwest of the Tsugaru Strait and flows relatively stable northward with most narrow cross section in the Japan Sea. Therefore, many Japanese oceanographers have evaluated the volume transport in this region to monitor the interannual or seasonal variation of the Tsushima Current system. However, the description on the water movements by the past studies have been only inferred from the dynamic caluclation on the basis of hydrographic data. From April 1992 to March 1993, we carried out the direct current measurement southwest of the Tsugaru Strait by using of a moored instrument at the depth of 130m (Apr.∼Oct.1992) and 180m (Oct.1992∼Mar.1993) to investigate the dynamical variability of the Tsushima Current. The large baroclinic events appear to be dominated mainly by passing through the seaward meander of the coastal current in May 1992 and by merging a warm eddy with the spatial scale of several hundred kilometers within the coastal curent in January 1993. Moreover, about one month-periodical fluctuations observed in the temparature and the on-offshore current corresponded with a Tsushima Current frontal eddy event that could be identified in the NOAA/AVHRR images. From a comparison between the current velocities observed by the current meter and the geostrophic ones, it is suggested that the Tsushima Current includes the evidence as the barotoropic northward flow with about 10 cm s-1 tobe unrelated with the seasonal changes of stratification. This implies that the actual Tsushima Current transports are likely to be larger than those estimated by the past.
Current meter
Geostrophic current
Meander (mathematics)
Halocline
Stratification (seeds)
Outflow
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Geostrophic current
Physical oceanography
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The physical oceanography of the shelf and slope waters of the New York Bight (Block Island to Cape May) in August of 1974 is described. Temperature, salinity, and density data, presented in surface contours and section profiles, showed the shelf/slope front, a cold core on the shelf, and a salinity core on the slope. Geostrophic currents in the slope water were inferred from the density structure, and showed two anticyclonic eddies with maximum geostrophic velocities of approximately 40 cm. Temperature and salinity profiles indicated shelf/slope mixing related to the eddies.
Geostrophic current
Temperature salinity diagrams
Eddy
Anticyclone
Physical oceanography
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In order to determine the validity of applying a geostrophic relationship to the circular currents in Lake Biwa, a series of quasi-simultaneous observations of currents and water temperature were carried out. The current measurements were made using the drifters with cross-board placed at several depths. The vertical profiles of water temperature were measured by a bathythermograph at four points about 500 m away from the current station. The comparisons between the directly measured current and estimated geostrophic current show that the geostrophic approximation is not so valid for the current observed at a specified time and space. On the other hand, the current averaged over a time long enough to cancel the effect of the slowest internal wave, can serve as the baroclinic geostrophic current including the centrifugal force.
Geostrophic current
Bathythermograph
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To plan and design ocean outfall for sewage, cooling water or other wastes it is necessary to have a knowledge of the nearshore current velocities and directions. An overview is presented of available nearshore current measurement of the Pacific Northwest Coast along with some theoretical methods for calculating velocities. Current contributions are considered from tides, winds, waves and upwelling. Available current data indicate that in nearshore areas effects due to bottom and shore configurations overshadow geostrophic and Ekman layer effects. Generally, theoretical methods for calculation of current velocities in coastal waters (within 5 nautical miles of the coastline) can give approximate values. However, for specific information it is usually necessary to perform local measurements.
Geostrophic current
Longshore drift
Nautical mile
Outfall
Ekman layer
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Geostrophic current
Hydrographic survey
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We have investigated sea level and surface currents features over the Newfoundland Shelf and Slope using the tandem TOPEX/Poseidon (T/P) and Jason altimetry data (2002–2003). The consistency and error characteristics of T/P and Jason measurements are examined not only in terms of sea level and cross‐track current anomalies but also with respect to current anomalies at crossovers and the Labrador Current transport. Nominal absolute currents are constructed by adding the altimetric geostrophic current anomalies to a climatological‐mean model circulation field. The comparison of the sea level and cross‐track current anomalies from January to July 2002 shows overall good agreement between T/P and Jason, with correlation coefficients different from zero at the 95% confidence level at almost all locations for sea level and at most locations for currents. Errors are estimated to be 2.5 cm for sea level and 10 cm/s for cross‐track current anomalies. Analyses of the current variability at crossovers indicate approximate agreement of T/P and Jason measurements, except for the Northeastern Newfoundland Shelf and Slope probably due to the ice presence during the period. Model‐altimetry combined absolute currents are used to estimate near‐surface transport associated with the shelf‐edge Labrador Current, showing good correlation between T/P and Jason estimates and strong seasonal changes. The cross‐track geostrophic current anomalies from September 2002 to December 2003 are used to calculate the root‐mean‐square (rms) current variability at crossovers and to derive the shelf‐edge Labrador Current. The interleaved T/P and Jason observations can better capture the spatial distribution of shelf and slope circulation variability.
Geostrophic current
Anomaly (physics)
Sea-surface height
Dynamic height
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Current meter
Geostrophic current
Eddy
Anticyclone
Forcing (mathematics)
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The seasonal and interannual variations of surface geostrophic current in the Southeast Pacific(5°~55°S,70°~120°W) have been analyzed using Topex/Poseidon altimeter data during 1992~2001.The results show that the South Equatorial Current is characterized with seasonal variation on the magititude of velocity,while the Peru Current and West Wind Drift reflect the shift of the current axes.The interannual variation of the current field is mainly caused by El Nino events.During the El Nino events,both South Equatorial Current and Peru Current at the low latitude region turn their directions to be eastward,and the current field at the higher latitude region is strongly weakened,with smaller current speed and random direction.
Geostrophic current
Seasonality
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Altimeter data from Jason-1 satellite are very useful in providing general and continuous information about the ocean, including sea surface currents. The main objective of study is to identify the most appropriate mathematical equation for determining sea surface current in the South China Sea. The seasonal changes of surface current during different monsoon periods in 2004 and 2005 were also identified. The equations used to derive sea surface current are geostrophic current equations, wind-driven current equations and tidal current equations. The methodology of this study involves the use of sea level height and sea surface wind speed data from Jason-1 satellite altimeter to derive geostrophic current and wind-driven current. Tidal amplitudes from co-tidal charts were used to derive tidal current. The derived surface currents were used to produce combined geostrophic and wind-driven current. Combined geostrophic and tidal current as well as total surface current which is the combination of geostrophic current, wind-driven current and tidal current were also derived. Maps of total surface current circulation pattern were produced during four monsoon periods in 2004 and 2005. Regression analysis and comparison of mean and standard deviation values with sea truth data were carried out to identify the most appropriate equation of surface current for the South China Sea. Results of the analysis indicate that total surface current speed and direction have good correlation with the sea truth data, that is 0.68 and 0.70 respectively. The analysis by comparing the mean values indicate that there are no significant difference between the means of total surface current and the means of sea truth data. The standard deviations of total surface current are smaller compared to the sea truth data values. In conclusion, altimeter data from Jason-1 satellite combined with tidal data to derive the total surface current is appropriate to determine sea surface current circulation pattern in the South China Sea.
Geostrophic current
Sea-surface height
Ocean surface topography
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