摘要:

A reduced gravity model that incorporates the geometry of western North America has been used to study the dynamics of the California Current system. Three experiments were performed: first the model was run using 19 years of wind stress from the Comprehensive Ocean‐Atmosphere Data Set (local model); a second experiment (remote model) consisted of forcing the model through its southern boundary using the results of a similar reduced gravity equatorial model; in a third experiment, both forcings were used simultaneously (local plus remote model). The main objective of this work was to analyze the low‐frequency variability on the California Current system in terms of its contributions from remote and local forcing. Away from the coast, the basic (steady) state of the model is determined by the predominantly negative wind curl through a Sverdrup balance. The general seasonal cycle is in agreement with what has been described by other authors. Through cross‐correlation and cross‐spectral analysis between the model results and observed sea level data, it was established that most of the interannual variability in sea level height at the coast is due to disturbances of equatorial origin that propagate into the region in the form of coastally trapped Kelvin waves. For the annual frequency variability, on the other hand, it was found that both local and remotely forced variability contribute to the total v

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03159

年代:1989

数据来源: WILEY

摘要:

The regions containing the two zonal currents of the subtropical gyre in the eastern North Atlantic, the Azores Current and the North Equatorial Current (NEC), have quite different physical characteristics. Associated with the Azores Current are strong horizontal thermohaline gradients that can be located easily both at the surface and at depth with temperature data alone, thus making satellite IR imagery and expendable bathythermograph profiles suitable for observing it. During winter, the surface expression of the Azores Current is often found to the north of the strongest subsurface gradients. In contrast to the Azores Current and to the central water mass boundary just to the south, the NEC has relatively weak horizontal temperature and salinity gradients, requiring density information in order to identify it. There is no clear surface manifestation found with the NEC. Common to both currents, though, is that each transportsO(8 Sv) in the upper 800 m of the ocean near 27°W, with the largest velocities being in the upper 400 m

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03181

年代:1989

数据来源: WILEY

摘要:

On the basis of extensive global data sets the total amounts of internal, potential, latent, and kinetic energy in the world oceans and atmosphere are estimated and compared for annual mean and seasonal conditions. Next, an expression for the available gravitational potential energy in the oceans is derived. The computed amounts of zonal mean and transient eddy available potential energy and the amount of kinetic energy present in the oceans are found to be much smaller (at least 1 order of magnitude) than those in the atmosphere. Because of the sparseness of directly measured subsurface values, the estimates of the kinetic energy and the transient eddy available potential energy were obtained by extrapolation from the corresponding values measured at the ocean surface. The fact that relatively large density variations are found only in the upper few hundred meters of the oceans and the greater stability of the oceans are the basic reasons for the generally small values of the potential and kinetic energy in the oceans compared with those in the atmosphere.

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03187

年代:1989

数据来源: WILEY

摘要:

Two buoy types have been tested with respect to their drift performance under drogued and undrogued conditions. Additionally, forces acting on the buoys were measured directly. Quadratic drag laws have been confirmed for the drag in water and the combined drag of wind and waves. Stokes drift contributes about one half to the wind factor of 0.023, which is obtained for undrogued buoys in the Atlantic. The forces on a windowshade drogue are given by a linear relation between force and water velocity for speeds exceeding 10 cm/s. They have been extrapolated to speeds of less than 10 cm/s by both a linear and a quadratic relationship. Correlations between drift and wind speed in the Atlantic suggest that the linear law is a better approximation under realistic conditions. According to these measurements in the Atlantic the described buoy‐drogue system with a windowshade drogue in 100‐m depth is a good current‐measuring device. Slippage is negligible for wind speeds of less than 15 m/s and is less than 2 cm/s under gale conditions. Undrogued buoys are strongly affected by wind and cannot be used for the analysis of currents without correction, even under light

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03201

年代:1989

数据来源: WILEY

摘要:

July In 1982 two detailed sections of density and absolute velocity were taken across the Gulf Stream northeast of Cape Hatteras to conduct an accurate test of geostrophy in a strong current. The sections, which were taken about 4 days apart, were each completed within 48 hours, using one ship to make closely spaced (12 km) conductivity‐temperature‐depth measurements to 2000 m, and a second ship to simultaneously take Pegasus absolute velocity profiles to the ocean bottom. The Gulf Stream path and curvature were also surveyed. The dynamically inferred velocity profiles were made absolute by matching their velocities to the Pegasus profiles at 2000 m. The geostrophic method (properly referenced) underestimated the observed velocities by 10–25 cm s−1in the core of the current above 500 m where speeds exceeded 150 cm s−1. The difference is a factor of 2 larger than the sampling and measurement errors in corresponding parts of the current, estimated to be 5–10 cm s−1in the strong current and 2–5 cm s−1elsewhere. We can account for the supergeostrophic currents quite effectively by including the centripetal acceleration (from the path curvature) in the momentum equation. In this case the differences in the current core decrease to less than 5–10 cm s−1, well within the uncertainties of the method. The transport above 2000 m is less sensitive to curvature effects; it agrees to within 3 Sv before and 1 Sv after correcting for curvature, or approximately 2% of the total 93‐Sv transport. The deep velocity field below 1000 m had significant changes (10–20 cm s−1) in structure, and even reversals, in the 4 days between the sections, with cross‐stream scales of 50–100 km. This deep variability, attributed to topographic Rossby waves, introduces more than 10 Sv uncertainty in defining the total volu

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03211

年代:1989

数据来源: WILEY

摘要:

During February 1988 a layer of almost uniform temperature and salinity between 100 and 1250 m depth was observed in the central Greenland Sea. This feature, indicative of previous deep convection, is explained in terms of a conceptual penetrative convection model.

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03223

年代:1989

数据来源: WILEY

摘要:

Moored thermistor chains at 2°N and 2°S and current‐temperature moorings at 0° are used to examine the accuracy of geostrophically estimated zonal velocity on the equator in the eastern (110°W) and western (165°E) Pacific. The meridionally differentiated form of the geostrophic balance is used to eliminate large errors due to wind‐balanced cross‐equatorial pressure gradients. Statistical analyses indicate that for time scales longer than 30–50 days, the observed and geostrophically estimated zonal velocities are similar (correlation coefficients of 0.6–0.9 and comparable amplitudes). Thus low‐frequency equatorial current oscillations are reasonably well represented by the geostrophic approximation. However, the mean currents are poorly resolved with the available array. In the eastern Pacific the mean zonal speed difference over the 10‐month comparison period is 25 cm s−1at 25 m and increases to 60 cm s−1at 125 m. At 165°E mean differences in the upper 250 m are typically 50 cm s−1over a 4‐month record. The principal reason for these large mean differences is that the meridional scale of the mean currents is smaller than the spacing of the moorings. Comparison of observed and geostrophic velocity profiles obtained from shipboard sampling indicates that meridional spacing of about 1° latitude would be optimum for e

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03228

年代:1989

数据来源: WILEY

摘要:

A unique data set of simultaneous observations of the ocean current and absolute sea surface slope in the Tsugaru Strait, Japan, has been analyzed in order to examine the effect of Coriolis force on an ocean current. First, the mean surface slope across the strait (an average over the period 1972–1980) was compared dynamically with the mean ocean current along the strait (an average over the period 1953–1978). It was found that both are in good geostrophic balance. Next, time variation of the ocean current was compared with that of sea surface inclination. In the subtidal frequency range, the variation in current velocity was found to be followed by the variation in sea surface inclination such that both are generally in geostrophic balance. Although some imbalances are seen, most of them can be explained by the sea level changes caused by the storm surge or wave setup. In the tidal frequency range, the dynamical relation between the current velocity and inclination shows good agreement with the theory. The downstream component of the balance is maintained mostly by the local acceleration and pressure gradient terms, and the cross‐stream component is maintained by the Coriolis force and pressure gradient

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03237

年代:1989

数据来源: WILEY

摘要:

Radiosonde data and surface meteorological data collected from moorings and ships on both the warm and cold sides of the North Atlantic subtropical front were analyzed for the January to June 1986 time period. These data were used to investigate the magnitude and the spatial‐temporal scales associated with the variation of the wet tropospheric range correction for the U.S. Navy's Geosat radar altimeter. Analysis consisted of both numerical integration of high‐resolution radiosonde data and use of an atmospheric model to calculate the wet tropospheric range correction for the study region during the January‐June 1986 period. Results showed that the wet tropospheric range correction varied by about 15 cm overall, i.e., ranging from approximately 10 to 25 cm. This variation however, occurred over length scales which were large and time scales which were short compared with those from the subtropical front. Based on these results, the wet tropospheric range correction represented a synoptic scale variation in the lower atmosphere which would have little effect on the mesoscale dynamic signal associated with the subtropical front or the mesoscale eddy field as measured by the Geosat alti

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03247

年代:1989

数据来源: WILEY

摘要:

Numerous data on the spectra of underwater noise due to natural as well as artificial rain show a prominent and characteristic peak at a frequency around 14 kHz. It is argued that this acoustic emission is due to bubbles entrained in the liquid by the impact of raindrops. The mechanics of the entrainment is such that only drops in a narrowly defined size range have a high probability of entraining bubbles. The narrowness of this size range may explain why the 14‐kHz peak is so ubiquitous and well define

ISSN:0148-0227    

DOI:10.1029/JC094iC03p03255

年代:1989

数据来源: WILEY