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Abstract
Long-term measurements off southwest Nova Scotia reveal the following features of the mean circulation:
(a) a westward longshore coastal current (4–10 cm s−1,
(b) an anticyclonic gyre around Brown Bank (5–15 cm s−1, and
(c) an upwelling circulation off Cape Sable (1–2 cm s−1, at bottom).
The gyre circulation appears permanent but the coastal current and upwelling exhibit annual variations of the same order as the means. There is a distinct annual signal in the longshore transport at Cape Sable (maximum westward in winter), whereas the mean transport (0.14 × 106 m4 s−1) is consistent with both geostrophic estimates and budget requirements in the Gulf of Marine. Strong seasonal cycles are also found in the salinity and density fields at Cape Sable which appear to be controlled both by buoyancy input from the coastal current and local mixing effects.
A linear diagnostic model indicates that the primary dynamical balance for the circulation is between a longshore pressure gradient and longshore mean density and stratification gradients which have summer maxima. Lesser contributions arise from longshore wind, offshore density gradient and centrifugal upwelling. Tidal rectification, deduced from coherent modulations of the semidiurnal tidal streams and low-frequency currents, supports the Browns Bank gyre circulation and drives both westward and offshore components of the near-bottom flow off Cape Sable. Thus the “centrifugal upwelling” hypothesis fails and the main driving force for Cape Sable upwelling appears to be the longshore density variations maintained by tidal mixing.
Abstract
Long-term measurements off southwest Nova Scotia reveal the following features of the mean circulation:
(a) a westward longshore coastal current (4–10 cm s−1,
(b) an anticyclonic gyre around Brown Bank (5–15 cm s−1, and
(c) an upwelling circulation off Cape Sable (1–2 cm s−1, at bottom).
The gyre circulation appears permanent but the coastal current and upwelling exhibit annual variations of the same order as the means. There is a distinct annual signal in the longshore transport at Cape Sable (maximum westward in winter), whereas the mean transport (0.14 × 106 m4 s−1) is consistent with both geostrophic estimates and budget requirements in the Gulf of Marine. Strong seasonal cycles are also found in the salinity and density fields at Cape Sable which appear to be controlled both by buoyancy input from the coastal current and local mixing effects.
A linear diagnostic model indicates that the primary dynamical balance for the circulation is between a longshore pressure gradient and longshore mean density and stratification gradients which have summer maxima. Lesser contributions arise from longshore wind, offshore density gradient and centrifugal upwelling. Tidal rectification, deduced from coherent modulations of the semidiurnal tidal streams and low-frequency currents, supports the Browns Bank gyre circulation and drives both westward and offshore components of the near-bottom flow off Cape Sable. Thus the “centrifugal upwelling” hypothesis fails and the main driving force for Cape Sable upwelling appears to be the longshore density variations maintained by tidal mixing.
Abstract
Current meter records and hydrographic data taken in the Denmark Strait overflow during a one-month experiment in August-September 1973 are analyzed. Mean conditions indicate that a strong, cold overflow current existed throughout the experiment. The most outstanding feature of the velocity and temperature spectra is a strong peak at a period of 1.8 days. These oscillations appear to amplify in the downstream direction and are highly correlated over the entire flow at the southern end of the Strait. Phase estimates indicate that velocity components are in quadrature, while the cross-stream perturbation heat flux acts to reduce the mean potential energy associated with the sloping isotherms.
To explain the low-frequency variability, a quasi-geostrophic two-layer model for channel flow with a sloping bottom is developed. Using measured values of shear and other physical parameters, the model is found to be unstable over a limited range of wavelengths and frequencies. The most unstable wave is 80 km long and has a period of 2.1 days in close agreement with peaks in the current meter spectra. Furthermore, phase differences measured across the stream are found to be consistent with the propagation in the direction of the mean flow.
The study concludes with a discussion of finite-amplitude aspects of the instabilities.
Abstract
Current meter records and hydrographic data taken in the Denmark Strait overflow during a one-month experiment in August-September 1973 are analyzed. Mean conditions indicate that a strong, cold overflow current existed throughout the experiment. The most outstanding feature of the velocity and temperature spectra is a strong peak at a period of 1.8 days. These oscillations appear to amplify in the downstream direction and are highly correlated over the entire flow at the southern end of the Strait. Phase estimates indicate that velocity components are in quadrature, while the cross-stream perturbation heat flux acts to reduce the mean potential energy associated with the sloping isotherms.
To explain the low-frequency variability, a quasi-geostrophic two-layer model for channel flow with a sloping bottom is developed. Using measured values of shear and other physical parameters, the model is found to be unstable over a limited range of wavelengths and frequencies. The most unstable wave is 80 km long and has a period of 2.1 days in close agreement with peaks in the current meter spectra. Furthermore, phase differences measured across the stream are found to be consistent with the propagation in the direction of the mean flow.
The study concludes with a discussion of finite-amplitude aspects of the instabilities.
Abstract
Low-frequency current measurements near the shelf break south of Nova Scotia indicate that the presence of topographic waves on the continental slope, and rise is associated with large-scale shoreward excursions and formation of eddies by the Gulf Stream. Two linear, inviscid, barotropic theories are formulated to model the development of the radiation field associated with such an eddy. Examples are drawn from observations of the interaction between Eddy I and surrounding waters during July–October 1976.
The fist model represents the eddy as an impulsive vorticity disturbance on constant exponential topography. Solutions are derived for three different initial distributions corresponding to a point source, a doublet and an isolated circular vortex. Results indicate that 1) after an initial burst of ultralong waves, the radiation field is reduced to a slowly dispersing set of short (50–200 km), low-frequency (periods: 10–25 days) waves which is consistent with observations at the shelf break; 2) the measured variations in wave period can be used to identify the temporal origin of the wave field, which coincides with the formation of Eddy I by a strong meander of the Gulf Stream over the outer continental rise; 3) the observed modulation of wave amplitude at the 1000 m isobath is consistent with forcing by an isolated barotropic vortex with a diameter of 70 km; and 4) the distribution of wave kinetic energy over the slope/rise region is controlled by opposing factors of topographic amplification and radial dispersion.
The second model allows for slow variations (WKB) of the topographic parameter on a realistic parabolic slope. Refraction in the WKB field of monochromatic radiation produces the observed offshore orientations of the wavenumber vector and focusing of wave energy over the continental slope. This effect may also explain the absence of a strong reflected topographic wave component in the low-frequency data set. Using a discrete set of WKB solutions to model the full dispersive wave field gives an estimate for the average upslope kinetic-energy distribution which compares favorably with deep-water measurements from two mooring periods. The decay rate for warm-core eddy energy (∼1013 J day−1), inferred from observed energy densities and linear dispersion, is consistent with published estimates for cold-core rings in the Sargasso Sea. Bottom frictional dissipation is unimportant over the continental rise but limits the longshore propagation of energy to scales of order 100 km near the shelf break.
Abstract
Low-frequency current measurements near the shelf break south of Nova Scotia indicate that the presence of topographic waves on the continental slope, and rise is associated with large-scale shoreward excursions and formation of eddies by the Gulf Stream. Two linear, inviscid, barotropic theories are formulated to model the development of the radiation field associated with such an eddy. Examples are drawn from observations of the interaction between Eddy I and surrounding waters during July–October 1976.
The fist model represents the eddy as an impulsive vorticity disturbance on constant exponential topography. Solutions are derived for three different initial distributions corresponding to a point source, a doublet and an isolated circular vortex. Results indicate that 1) after an initial burst of ultralong waves, the radiation field is reduced to a slowly dispersing set of short (50–200 km), low-frequency (periods: 10–25 days) waves which is consistent with observations at the shelf break; 2) the measured variations in wave period can be used to identify the temporal origin of the wave field, which coincides with the formation of Eddy I by a strong meander of the Gulf Stream over the outer continental rise; 3) the observed modulation of wave amplitude at the 1000 m isobath is consistent with forcing by an isolated barotropic vortex with a diameter of 70 km; and 4) the distribution of wave kinetic energy over the slope/rise region is controlled by opposing factors of topographic amplification and radial dispersion.
The second model allows for slow variations (WKB) of the topographic parameter on a realistic parabolic slope. Refraction in the WKB field of monochromatic radiation produces the observed offshore orientations of the wavenumber vector and focusing of wave energy over the continental slope. This effect may also explain the absence of a strong reflected topographic wave component in the low-frequency data set. Using a discrete set of WKB solutions to model the full dispersive wave field gives an estimate for the average upslope kinetic-energy distribution which compares favorably with deep-water measurements from two mooring periods. The decay rate for warm-core eddy energy (∼1013 J day−1), inferred from observed energy densities and linear dispersion, is consistent with published estimates for cold-core rings in the Sargasso Sea. Bottom frictional dissipation is unimportant over the continental rise but limits the longshore propagation of energy to scales of order 100 km near the shelf break.
Abstract
Records of current, temperature and salinity from a two-year mooring program at the shelf break off Nova Scotia are examined to determine the low-frequency oceanic responses to the driving surface wind field and fluctuating offshore currents associated with the Gulf Stream. The seasonal mean and subtidal variance (at periods of 2–10 days) of the cross-shelf currents reflect the strong annual cycle in the wind field measured at Sable Island. The mean vertical shear suggests a simple Ekman response to winter increases in the longshore wind component, but this model fails quantitatively because 1) the inferred surface-layer (20 m) transport is much too large and 2) the deep (150 m) “return” flow shows no annual signal. The excessive offshore near-surface transport in winter must be reconciled with the relatively stationary position of the shelf/slope-water boundary (SSB) by invoking intense cross-frontal mixing and/or a seasonal mean alongshore pressure gradient. The seasonal mean longshore currents above the main thermocline appear to he more strongly influenced by energetic topographic Rossby waves than by wind.
Weekly sea-surface temperature analyses are used to monitor off-shore forcing reflected by fluctuations in the position of the SSB in the region from 60 to 65°W. The dominant empirical mode of its space-time variance represents a uniform on–offshore translation at very low frequencies. The longshore current variance in the ocean-forced spectral band (periods 10–90 days) is 1) correlated with the low-frequency onshore displacement of the SSB in the deep water on the continental rise and 2) comparable to that in the wind-driven band (periods 2–10 days) from 50 to 150 m at the shelf break.
On the shelf, the advective onshore transports of heat and salt exhibit annual cycles similar to those in the wind field, and generally exceed the vertically integrated eddy fluxes by factors of 2–4. However, the combination of observed moan and eddy transport supports excessive alongshore gradients of temperature and salinity in the context of a simple box model, and hence may not be representative of the entire shelf.
Abstract
Records of current, temperature and salinity from a two-year mooring program at the shelf break off Nova Scotia are examined to determine the low-frequency oceanic responses to the driving surface wind field and fluctuating offshore currents associated with the Gulf Stream. The seasonal mean and subtidal variance (at periods of 2–10 days) of the cross-shelf currents reflect the strong annual cycle in the wind field measured at Sable Island. The mean vertical shear suggests a simple Ekman response to winter increases in the longshore wind component, but this model fails quantitatively because 1) the inferred surface-layer (20 m) transport is much too large and 2) the deep (150 m) “return” flow shows no annual signal. The excessive offshore near-surface transport in winter must be reconciled with the relatively stationary position of the shelf/slope-water boundary (SSB) by invoking intense cross-frontal mixing and/or a seasonal mean alongshore pressure gradient. The seasonal mean longshore currents above the main thermocline appear to he more strongly influenced by energetic topographic Rossby waves than by wind.
Weekly sea-surface temperature analyses are used to monitor off-shore forcing reflected by fluctuations in the position of the SSB in the region from 60 to 65°W. The dominant empirical mode of its space-time variance represents a uniform on–offshore translation at very low frequencies. The longshore current variance in the ocean-forced spectral band (periods 10–90 days) is 1) correlated with the low-frequency onshore displacement of the SSB in the deep water on the continental rise and 2) comparable to that in the wind-driven band (periods 2–10 days) from 50 to 150 m at the shelf break.
On the shelf, the advective onshore transports of heat and salt exhibit annual cycles similar to those in the wind field, and generally exceed the vertically integrated eddy fluxes by factors of 2–4. However, the combination of observed moan and eddy transport supports excessive alongshore gradients of temperature and salinity in the context of a simple box model, and hence may not be representative of the entire shelf.
Abstract
The economic value of seasonal climate forecasting is assessed using a whole-of-chain analysis. The entire system, from sea surface temperature (SST) through pasture growth and animal production to economic and resource outcomes, is examined. A novel statistical forecast method is developed using the partial least squares spatial correlation technique with near-global SST. This method permits forecasts to be tailored for particular regions and industries. The method is used to forecast plant growth days rather than rainfall. Forecast skill is measured by performing a series of retrospective forecasts (hindcasts) over the previous century. The hindcasts are cross-validated to guard against the possibility of artificial skill, so there is no skill at predicting random time series. The hindcast skill is shown to be a good estimator of the true forecast skill obtained when only data from previous years are used in developing the forecast.
Forecasts of plant growth, reduced to three categories, are used in several agricultural examples in Australia. For the northeast Queensland grazing industry, the economic value of this forecast is shown to be greater than that of a Southern Oscillation index (SOI) based forecast and to match or exceed the value of a “perfect” category rainfall forecast. Reasons for the latter surprising result are given. Resource degradation, in this case measured by soil loss, is shown to remain insignificant despite increasing production from the land. Two further examples in Queensland, one for the cotton industry and one for wheat, are illustrated in less depth. The value of a forecast is again shown to match or exceed that obtained using the SOI, although further investigation of the decision-making responses to forecasts is needed to extract the maximum benefit for these industries.
Abstract
The economic value of seasonal climate forecasting is assessed using a whole-of-chain analysis. The entire system, from sea surface temperature (SST) through pasture growth and animal production to economic and resource outcomes, is examined. A novel statistical forecast method is developed using the partial least squares spatial correlation technique with near-global SST. This method permits forecasts to be tailored for particular regions and industries. The method is used to forecast plant growth days rather than rainfall. Forecast skill is measured by performing a series of retrospective forecasts (hindcasts) over the previous century. The hindcasts are cross-validated to guard against the possibility of artificial skill, so there is no skill at predicting random time series. The hindcast skill is shown to be a good estimator of the true forecast skill obtained when only data from previous years are used in developing the forecast.
Forecasts of plant growth, reduced to three categories, are used in several agricultural examples in Australia. For the northeast Queensland grazing industry, the economic value of this forecast is shown to be greater than that of a Southern Oscillation index (SOI) based forecast and to match or exceed the value of a “perfect” category rainfall forecast. Reasons for the latter surprising result are given. Resource degradation, in this case measured by soil loss, is shown to remain insignificant despite increasing production from the land. Two further examples in Queensland, one for the cotton industry and one for wheat, are illustrated in less depth. The value of a forecast is again shown to match or exceed that obtained using the SOI, although further investigation of the decision-making responses to forecasts is needed to extract the maximum benefit for these industries.
Abstract
The climatological seasonal-mean hydrography and circulation in the Gulf of St. Lawrence and on the eastern Scotian and southern Newfoundland shelves are studied by reconstructing high-resolution temperature, salinity, and density fields for four seasons and numerically computing the associated circulation fields. The current fields are obtained from a three-dimensional diagnostic model, forced by baroclinic pressure gradients, seasonal wind stresses, and additional barotropic inflows across the Strait of Belle Isle and southern Newfoundland shelf upstream boundaries. The hydrographic fields suggest strong gulf–shelf interconnections, including outflow of relatively fresh surface water from the gulf to the eastern Scotian shelf, penetration of slope water at depth onto the shelves and into the gulf, and flow into the gulf through the Strait of Belle Isle. The circulation is generally cyclonic in the gulf, reinforced by inflows of Labrador and Newfoundland shelf water through the Strait of Belle Isle and Cabot Strait, while the circulation over the Scotian shelf is dominated by the southwestward shelf-break flow of water from the gulf and the Newfoundland shelf, with weaker flows from the gulf onto the inner and midshelf. Known major flow features such as the Gaspé Current, Cabot Strait outflow and inflow, and the Nova Scotian Current are realistically reproduced, and can be attributed to a combination of baroclinic pressure field and boundary inflow forcing. The model solutions are in approximate quantitative agreement with observed elevations, currents, and transports but with differences and notable uncertainties in some areas.
Abstract
The climatological seasonal-mean hydrography and circulation in the Gulf of St. Lawrence and on the eastern Scotian and southern Newfoundland shelves are studied by reconstructing high-resolution temperature, salinity, and density fields for four seasons and numerically computing the associated circulation fields. The current fields are obtained from a three-dimensional diagnostic model, forced by baroclinic pressure gradients, seasonal wind stresses, and additional barotropic inflows across the Strait of Belle Isle and southern Newfoundland shelf upstream boundaries. The hydrographic fields suggest strong gulf–shelf interconnections, including outflow of relatively fresh surface water from the gulf to the eastern Scotian shelf, penetration of slope water at depth onto the shelves and into the gulf, and flow into the gulf through the Strait of Belle Isle. The circulation is generally cyclonic in the gulf, reinforced by inflows of Labrador and Newfoundland shelf water through the Strait of Belle Isle and Cabot Strait, while the circulation over the Scotian shelf is dominated by the southwestward shelf-break flow of water from the gulf and the Newfoundland shelf, with weaker flows from the gulf onto the inner and midshelf. Known major flow features such as the Gaspé Current, Cabot Strait outflow and inflow, and the Nova Scotian Current are realistically reproduced, and can be attributed to a combination of baroclinic pressure field and boundary inflow forcing. The model solutions are in approximate quantitative agreement with observed elevations, currents, and transports but with differences and notable uncertainties in some areas.
Abstract
Bursts of topographic-Rossby-wave energy have been observed in data recorded by an array of current meters moored on the outer continental shelf and slope off Nova Scotia north of the Gulf Stream. The waves persist for three or four cycles during which both the period (10 to 23 days) and the amplitude of the oscillations are modulated. At least four different events have been observed over approximately one year and appear to be associated with warm eddies shed by the Gulf Stream. One particularly clear event, dealt with in detail, had a period of 21 days, an offshore scale of 175 km and an offshore phase speed of 8 km day−1. The array of moorings was too small to resolve accurately the longshore scale or propagation direction. The wave appears to be barotropic (nearly uniform in amplitude and phase) at the 1000 m isobath, but increasingly baroclinic in both cross-slope directions. Deep-water kinetic energy associated with the wave appears to be uniformly distributed over the upper portion of the rise.
Abstract
Bursts of topographic-Rossby-wave energy have been observed in data recorded by an array of current meters moored on the outer continental shelf and slope off Nova Scotia north of the Gulf Stream. The waves persist for three or four cycles during which both the period (10 to 23 days) and the amplitude of the oscillations are modulated. At least four different events have been observed over approximately one year and appear to be associated with warm eddies shed by the Gulf Stream. One particularly clear event, dealt with in detail, had a period of 21 days, an offshore scale of 175 km and an offshore phase speed of 8 km day−1. The array of moorings was too small to resolve accurately the longshore scale or propagation direction. The wave appears to be barotropic (nearly uniform in amplitude and phase) at the 1000 m isobath, but increasingly baroclinic in both cross-slope directions. Deep-water kinetic energy associated with the wave appears to be uniformly distributed over the upper portion of the rise.
Abstract
The waters off Cape Sable in southwestern Nova Scotia have anomalously low temperatures and high nutrients during summer. Using a three-dimensional tidal model as well as field observations obtained between 1978 and 1985, a new mechanism is proposed to explain the origin of these anomalous properties. The numerical model predicts several areas of strong tidally induced residual upwelling and downwelling. On the submarine ridge off Cape Sable, upwelling occurs over the eastern flank and downwelling over the western flank. This upward vertical transport is very effective in supplying cold, saline, and nutrient-rich water from deep to shallow layers. The predicted upwelling and downwelling are induced by a tidal rectification process resulting from tidal currents flowing over complex bottom topography. Hence, the process is named “topographic upwelling and downwelling.”
The upwelling (downwelling) is generated by the residual currents flowing from deep to shallow (shallow to deep) waters. This process is three-dimensional so that the water parcels advected onshore from deep to shallow zones in the upwelling regime may remain in the lower layer and be carried away from the upwelling region by a longshore coastal current. Thus, the upward transfer of cold and saline water in the three-dimensional topographic upwelling may not be as efficient as the classical upwelling associated with two-dimensional (on-off shore) circulation. However, efficient vertical transfer can be accomplished in conjunction with the topographic upwelling if the cross-isobath transport associated with the upwelling carries deep water to a shallow region where there is strong vertical mixing. This combination of topographic upwelling and strong tidal mixing provides the mechanism for producing the observed cold water anomaly off Cape Sable. Topographic upwelling occurs on the eastern side of the cape, and strong tidal mixing produces a well-mixed area off the cape. These processes are verified by hydrographic, current meter, and Lagrangian drift data collected in the surrounding area. The mechanism is expected to be important in other coastal regions where strong tidal currents and large variations of bottom topography are found.
Abstract
The waters off Cape Sable in southwestern Nova Scotia have anomalously low temperatures and high nutrients during summer. Using a three-dimensional tidal model as well as field observations obtained between 1978 and 1985, a new mechanism is proposed to explain the origin of these anomalous properties. The numerical model predicts several areas of strong tidally induced residual upwelling and downwelling. On the submarine ridge off Cape Sable, upwelling occurs over the eastern flank and downwelling over the western flank. This upward vertical transport is very effective in supplying cold, saline, and nutrient-rich water from deep to shallow layers. The predicted upwelling and downwelling are induced by a tidal rectification process resulting from tidal currents flowing over complex bottom topography. Hence, the process is named “topographic upwelling and downwelling.”
The upwelling (downwelling) is generated by the residual currents flowing from deep to shallow (shallow to deep) waters. This process is three-dimensional so that the water parcels advected onshore from deep to shallow zones in the upwelling regime may remain in the lower layer and be carried away from the upwelling region by a longshore coastal current. Thus, the upward transfer of cold and saline water in the three-dimensional topographic upwelling may not be as efficient as the classical upwelling associated with two-dimensional (on-off shore) circulation. However, efficient vertical transfer can be accomplished in conjunction with the topographic upwelling if the cross-isobath transport associated with the upwelling carries deep water to a shallow region where there is strong vertical mixing. This combination of topographic upwelling and strong tidal mixing provides the mechanism for producing the observed cold water anomaly off Cape Sable. Topographic upwelling occurs on the eastern side of the cape, and strong tidal mixing produces a well-mixed area off the cape. These processes are verified by hydrographic, current meter, and Lagrangian drift data collected in the surrounding area. The mechanism is expected to be important in other coastal regions where strong tidal currents and large variations of bottom topography are found.
Abstract
Using a multiple regression technique that includes both tidal and wind forcings, the tidally-induced residual current can be estimated from current meter records. However, the estimated mean (long-term averaged) tidally induced residual current is found to be very sensitive to an uncertain coefficient, ε, associated with the tidal forcing ua ε, where ua is the semi-major axis of the tidal ellipse. Thus, the estimated mean tidally induced residual current cannot be used directly to verify model results. It is suggested that the verification be carded out on û2 = (û2/ûa )ūa where ûa and û2 are respectively the spring-neap oscillation of the semi-major axis and the tidally induced residual current, and ūa is the mean value of the semi-major axis. The suggestion is derived from the findings that 1) the value of ũ2 estimated from the current meter data is insensitive to the coefficient ε, and 2) the computed ũ2 from the numerical tidal model is insensitive to the specified spring-neap oscillation of the tidal forcing (surface elevation) at the open boundaries.
By using Tee's three-dimensional tidal model, the mean and spring–neap variation of the tidally induced residual currents in the Cape Sable area, southwest of Nova Scotia were simulated. The computed values of ε are found to vary significantly in both the horizontal and vertical directions. The numerical model reproduces all the residual currents at the shallowest station where the estimation of the observed tidally induced residual current is most statistically reliable. At the other stations, the numerical model reproduces most of those currents that have high signal-to-noise ratios.
To reduce the effect of uncertainty in ε on the estimation of the mean tidally induced residual current from current meter data, the estimation can be carried out by using the values of ε computed from a reliable tidal model. An example of the estimation using this method is shown for a Cape Sable station where the computed ũ 2 has been verified by observation. In the absence of tidal modeling or in the case where accurate values of ε cannot be computed, it is suggested that ε = 2 be used for the estimation. This suggestion is derived mainly from the finding that the estimation using ε = 2 is reliable over a wider range of true ε values in the Cape Sable area than that using ε = 1 or 3.
Abstract
Using a multiple regression technique that includes both tidal and wind forcings, the tidally-induced residual current can be estimated from current meter records. However, the estimated mean (long-term averaged) tidally induced residual current is found to be very sensitive to an uncertain coefficient, ε, associated with the tidal forcing ua ε, where ua is the semi-major axis of the tidal ellipse. Thus, the estimated mean tidally induced residual current cannot be used directly to verify model results. It is suggested that the verification be carded out on û2 = (û2/ûa )ūa where ûa and û2 are respectively the spring-neap oscillation of the semi-major axis and the tidally induced residual current, and ūa is the mean value of the semi-major axis. The suggestion is derived from the findings that 1) the value of ũ2 estimated from the current meter data is insensitive to the coefficient ε, and 2) the computed ũ2 from the numerical tidal model is insensitive to the specified spring-neap oscillation of the tidal forcing (surface elevation) at the open boundaries.
By using Tee's three-dimensional tidal model, the mean and spring–neap variation of the tidally induced residual currents in the Cape Sable area, southwest of Nova Scotia were simulated. The computed values of ε are found to vary significantly in both the horizontal and vertical directions. The numerical model reproduces all the residual currents at the shallowest station where the estimation of the observed tidally induced residual current is most statistically reliable. At the other stations, the numerical model reproduces most of those currents that have high signal-to-noise ratios.
To reduce the effect of uncertainty in ε on the estimation of the mean tidally induced residual current from current meter data, the estimation can be carried out by using the values of ε computed from a reliable tidal model. An example of the estimation using this method is shown for a Cape Sable station where the computed ũ 2 has been verified by observation. In the absence of tidal modeling or in the case where accurate values of ε cannot be computed, it is suggested that ε = 2 be used for the estimation. This suggestion is derived mainly from the finding that the estimation using ε = 2 is reliable over a wider range of true ε values in the Cape Sable area than that using ε = 1 or 3.
Abstract
The Clouds and the Earth’s Radiant Energy System (CERES) instrument is a scanning radiometer for measuring Earth-emitted and -reflected solar radiation to understand Earth’s energy balance. One CERES instrument was placed into orbit aboard the Tropical Rainfall Measuring Mission (TRMM) in 1997; two were aboard the Terra spacecraft, launched in 1999; and two were aboard the Aqua spacecraft, launched in 2002. These measurements are used together with data from higher-resolution instruments to generate a number of data products. The nominal footprint size of the pixel at Earth’s surface is 16 km in the cross-scan direction and 23 km in the scan direction for the TRMM platform and 36 km in the cross-scan direction and 46 km in the scan direction for the Terra and Aqua platforms. It is required that the location on Earth of each pixel be known to 1–2 km to use the CERES data with the higher-resolution instruments on a pixel basis. A technique has been developed to validate the computed geolocation of the measurements by use of coastlines. Scenes are chosen in which the reflected solar radiation changes abruptly from the land surface to the darker ocean surface and the Earth-emitted radiation changes from the warm land to the cool ocean, or vice versa, so that scenes can be detected both day and night. The computed coastline location is then compared with the World Bank II map. The method has been applied to data from the three spacecraft and shows that the pixel geolocations are accurate to within 10% of the pixel size and that the geolocation is adequate for current scientific investigations.
Abstract
The Clouds and the Earth’s Radiant Energy System (CERES) instrument is a scanning radiometer for measuring Earth-emitted and -reflected solar radiation to understand Earth’s energy balance. One CERES instrument was placed into orbit aboard the Tropical Rainfall Measuring Mission (TRMM) in 1997; two were aboard the Terra spacecraft, launched in 1999; and two were aboard the Aqua spacecraft, launched in 2002. These measurements are used together with data from higher-resolution instruments to generate a number of data products. The nominal footprint size of the pixel at Earth’s surface is 16 km in the cross-scan direction and 23 km in the scan direction for the TRMM platform and 36 km in the cross-scan direction and 46 km in the scan direction for the Terra and Aqua platforms. It is required that the location on Earth of each pixel be known to 1–2 km to use the CERES data with the higher-resolution instruments on a pixel basis. A technique has been developed to validate the computed geolocation of the measurements by use of coastlines. Scenes are chosen in which the reflected solar radiation changes abruptly from the land surface to the darker ocean surface and the Earth-emitted radiation changes from the warm land to the cool ocean, or vice versa, so that scenes can be detected both day and night. The computed coastline location is then compared with the World Bank II map. The method has been applied to data from the three spacecraft and shows that the pixel geolocations are accurate to within 10% of the pixel size and that the geolocation is adequate for current scientific investigations.
