摘要:

When zonal velocity profiles from the central equatorial Pacific are expanded into sums of vertical normal modes, the peak amplitudes occur consistently near modes 1, 5, 12, and 18. The first two of these peaks sum to a recognizable equatorial undercurrent profile. The two higher‐mode peaks blunt and thicken the equatorial undercurrent (EUC) and add a westward extremum near 340 m. During the 1982–1983 El Niño, the low‐mode component of the flow (modes 1–10) was more variable than the residual component, which was dominated by the modes near 12 and 18. The residuals below 120 m persisted as alternating westward and eastward current bands (the “shallow jets”) through the disappearance of the EUC in the second half of 1982 and its reappearance in 1983. The shallow jets were nearly constant in depth until late 1982 and then gradually rose through the end of the time series in June 1983. Differences in temporal behavior among the mode amplitude peaks indicate that these peaks are describing dynamically separate components of the flow which are not tightly coupled on time scales shorter than

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09213

年代:1988

数据来源: WILEY

摘要:

The Gulf of California is a long, narrow marginal sea lying between the Baja California peninsula and the mainland of Mexico. Air‐sea fluxes of heat and moisture in the gulf are enhanced because of geographical isolation from the Pacific provided by the mountainous Baja California peninsula. In the northern gulf, annual evaporation rates are about 1 m y−1. Unlike most evaporative basins, however, the gulf gains heat from the atmosphere at an annual average rate of 20 to 80 W m−2(Bray, 1988). Given the unusual air‐sea forcing of the gulf, what form or forms should water mass formation take? The annual moisture loss and heat gain require that high‐salinity surface water be transported downward to an intermediate depth and that cold, fresh inflow be transported upward to an intermediate depth. This is accomplished through several mechanisms. (1) Winter convection: this occurs only in a limited geographical region, the Wagner Basin of the far northern gulf, except in El Niño–Southern Oscillation years, when convection appears to be more widespread. (2) Dispersion of convected water in small eddylike features: this occurs within a large‐scale southward transport possibly driven by the large‐scale density gradient associated with atmospheric fluxes. (3) An anticyclonic circulation in the northern gulf: this is found south of the convection region and transports high‐salinity water off the shallow shelves and to substantial depths, where it may mix with water of central gulf origin. (4) Tidal mixing: most of the energy available for mixing in the northern gulf derives from the tides. In particular, tidal mixing over the sill in the island region is responsible for the substantial reduction in salinity of northern gulf waters as they ente

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09223

年代:1988

数据来源: WILEY

摘要:

There have been several recent studies outlining attempts to extract Arctic sea ice motion from Seasat synthetic aperture radar imagery (Hall and Rothrock, 1981; Curlander et al., 1985; Fily and Rothrock, 1986) These studies all highlighted the need to automate, or at least minimize, the amount of operator intervention involved in the procedure. This paper presents the results of a fully automated system to extract ice motion from an image pair. The problem of finding a field of displacements between two sequential images is posed as an estimation problem, and the system is shown to be an optimal solution to this problem. The performance of the system is shown to be superior to ice motion systems that use only cross correlation. The shortcomings of the system, and of area correlation in general, are outlined, and ways of overcoming them are suggested.

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09241

年代:1988

数据来源: WILEY

摘要:

Sea surface temperatures (SSTs) from ship observations are assimilated into the Mellor‐Yamada level 2 one‐dimensional model of the mixed layer using a nonlinear version of the Kalman filter, similar to the extended Kalman filter. The model is initialized from expendable bathythermograph (XBT) temperature profiles at a weather ship location and is driven using the ship's observed meteorological forcing. As the SSTs arrive, usually every 3 hours, they are blended with the model results to produce an analysis. At the end of 3 and 5 days, the analyses are compared with another XBT profile at the same location. The data assimilation improves on the model forecasts by giving greater accuracy in the 20 to 25 m near the surface, which is a large portion of the very shallow mixed layer in these data. No improvement was found below the mixed la

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09253

年代:1988

数据来源: WILEY

摘要:

Laboratory and field measurements of suspended sediment in the nearshore suggest that fluid accelerations are an important factor in sediment transport by oscillatory waves. Here, Eulerian accelerations of the cross‐shore velocity are calculated from measurements of velocity obtained by an array of bottom‐mounted electromagnetic flow meters spanning a natural surf zone. Large shoreward accelerations of brief duration are associated with the steep front faces of both near‐breaking and breaking waves. Weaker offshore accelerations of longer duration occur during passage of the more gently sloped rear faces. The acceleration field is thus strongly skewed in the shoreward direction. Power spectra and bispectra indicate, as expected, that statistics of the acceleration field are significantly influenced by high‐frequency motions but are rather insensitive to su

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09261

年代:1988

数据来源: WILEY

摘要:

A spectral model of irradiance is presented for the computation of light energy available at the surface and at various depths in the ocean for the wavelength range from 400 to 700 nm. For any latitude, irradiances are computed as a function of geographic location, date, and time. Application of the model is demonstrated through computation of the profiles of vertical attenuation coefficient and of the effective specific absorption of phytoplankton. The model results are compared with those from conventional procedures, which disregard spectral and angular distributions of the underwater light field, for calculation of the effective specific absorption. The magnitude of the errors incurred by such simplifications is estimated and is shown to be nonnegligible and variable with solar elevation, depth, and the phytoplank‐on pigment concentration in the wate

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09270

年代:1988

数据来源: WILEY

摘要:

In general, the return signal scattered from the ocean surface used to form synthetic aperture radar (SAR) images contains contributions from at least two scattering mechanisms. In addition to resonant Bragg‐type scattering, specular point scattering becomes important as the angle of incidence becomes small ( ≲ 20°). In this paper we include the specular point rough surface scattering mechanism in a model for the mean SAR image of the ocean surface and examine the effects of this scattering mechanism theoretically. We find that the complete mean SAR intensity image consists of a sum of images due to specular point scattering and Bragg‐type resonant scattering. Because surface specular points have a short coherence time and move with considerable velocities, the contribution to the mean image due to these scatterers is of low azimuthal resolution and is displaced from the actual sea surface, typically by several SAR resolution cells. The bandwidth of this image can easily exceed the bandwidth of a typical SAR processor, leading to a loss of mean image intensity. The local backscatter cross‐section modulation is strong and nonlinear in the slope of the longwave field in the SAR range direction. At small incidence angles, this causes the specular point return from wave slopes tipped toward the SAR to become much brighter than the Bragg‐scattering return. Taken together, these effects are capable of producing azimuthally oriented streaks in SAR images, such as have been observed by Seasat. We present numerical estimates of coherence time, azimuthal displacement, cross‐section modulation, etc., computed using the parameters of the Seasat and shuttle imaging radar‐B SARs as well as typical parameters for an air

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09281

年代:1988

数据来源: WILEY

摘要:

Subharmonic resonance of edge waves by incident and reflected waves has been studied thus far for the case of a plane infinite beach. The analysis will be extended here to the case of a curved coastline, with a large radius of curvature and slowly varying beach slope in the longshore direction. It will be shown that the effects of such slow beach slope changes on a propagating edge wave are similar to the familiar shoaling effects on incident waves. The case of subharmonic edge wave generation on large shallow islands will be discussed in detail. The nonlinear analysis will show that within a certain range of island sizes, the generation mechanism can produce a stable standing edge wave around the island. For larger islands the solution disintegrates into two out‐of‐phase envelopes of opposite‐going edge waves. For still larger islands, the generated progressive edge waves become unstable to sideband modula

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09295

年代:1988

数据来源: WILEY

摘要:

In order to assess the performance of current meters within and near the surf zone, data from biaxial electromagnetic current meters with spherical and open frame probe geometries were intercompared. One bottom‐mounted flow meter of each type was deployed in a mean depth of 7.0 m for 17 days, and two sensors of each type were deployed in a mean depth of 2.0 m for 5 days. Sensors in the shallow deployment were frequently in the surf zone. Hourly averaged mean flows measured by different sensor types are highly correlated, averaging above 0.98. The largest difference between measured mean flows is a constant bias, typically about 3.0 cm/s, which is roughly equal to the estimated accuracy of the sensor offset calibrations. Root‐mean‐square deviations from this constant bias are less than 2.0 cm/s, and are contributed to by errors in both gain calibration and sensor orientation. Comparisons of measured (surface gravity wave) oscillatory currents were made both between current meter types and with velocities inferred from the application of linear theory to pressure sensor data. Correlations between time series ofUTrms(the rms total oscillatory velocity for a 1‐hour record) were all above 0.99 in 7.0‐m depth and averaged 0.95 for the shallow deployment. The averageUTrmsratio (over all hour‐long records) was within 1.0 ±0.07 for all current meter pairs in both deployments, which is consistent with the estimated 5% uncertainties in the flow meter gain calibration. Typical fluctuations of theUTrmsratio of any spherical and open frame sensor pair about its mean ratio, indicative of flow meter gain distortions probably associated with variations in the hydrodynamic environment, were less than 0.04 for any one deployment. Ratios ofUTrmsfrom both deployments taken together suggest that the open frame sensor overresponds, relative to the spherical probe, by about 5% at low (about 10.0 cm/s) total (mean +UTrms) speeds, and underresponds by about 5% at higher total speeds of about 75 cm/s. Relative to pressure data and linear theory, both flow meter types overrespond at low total speeds and underrespond at high total speeds. We are, however, unable to determine whether these apparent gain deviations (of less than 10%) of the flow meters relative to pressure are associated with errors in the linear theory used to convert pressure to velocity, or with the response characteristics of the flow meters. Cross spectra between all sensors (including pressure) show high coherence and phase differences of a few degrees, and they suggest that the response of both flow meters is only slightly frequency dependent. Various practical difficulties in accurately measuring the flow‐induced microvolt potentials in electromagnetically noisy environments, with potentially interfering current meters,

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09302

年代:1988

数据来源: WILEY

摘要:

Tritium and salinity data were used in an investigation of the transport and fate of freshwater on the South Atlantic Bight (SAB) shelf. Simultaneous tritium and salinity samples were collected at selected locations as part of the April 1985 Spring Removal Experiment (SPREX). A three‐source mixing model was used with tritium and salinity as tracers to determine if fresh water from the Savannah River could be distinguished from other freshwater sources on the SAB inner shelf. Variations in tritium concentration, integrated freshwater fraction, and model results were related to hydrographic and meteorological conditions. Results suggest that salinity and tritium concentration can be used to identify Savannah River water on the shelf and that Savannah River water maintains a distinct tritium‐salinity relationship at least as far north as Charleston, South Carolina. Thus tritium may be a convenient and useful tool for studying nearshore mixing and transport processes in the

ISSN:0148-0227    

DOI:10.1029/JC093iC08p09315

年代:1988

数据来源: WILEY