摘要:

A composite divided scale model for radar backscatter from the ocean surface is constructed. The primary scattering mechanism is assumed to be Bragg scattering for which the normalized radar backscattering cross section is proportional to the spectral density of the resonant Bragg water waves. The form of the high‐wave number equilibrium spectrum is derived on the assumption that the shortwave energy density reflects a balance between direct wind forcing and dissipation due to breaking and to viscosity. This theoretical equilibrium spectrum links the wave spectrum to the wind. This spectrum is then used in a two‐scale Bragg‐scattering model to link backscattering cross section to the full wave spectrum, which is this high‐wave number spectrum plus a gravity wave spectrum for fully developed seas. The effects of tilt and modulation of the Bragg resonant waves by the longer waves are included along with the contribution from specular reflection at low incidence angles. The model is tested against aircraft circle flightKuband radar backscatter measurements with encouraging results for vertical polarization. It is demonstrated that particularly at low wind speeds, scatterometry is sensitive to surface water temperature through its effect on the viscous dissipation of short waves. Also for low wind speeds and low incidence angles (20° or so) an additional source of specular backscatter needs to be considered: that due to gravity waves that may be left over from previously higher winds or that enter the area as swell. For high incidence angles and high winds, the two‐scale Bragg model yields values that are somewhat low compared with the data for vertical polarization. For horizontal polarization the model is somewhat low for a 40° incidence angle and much too low for higher incidence angles by amounts that cannot be explained by a combination of possible wind speed measurement errors and bias errors in the measurement of the backscatter. An explanation for these results is offered in terms of recent studies of backscatter from wedges and spilling breakers forKuband. The model is then exercised over a much wider wind speed range fromLband toKaband. For high wind speeds at anemometer height, except atLband, according to the model, the backscattering cross section becomes less sensitive to wind speed and at very high speeds decreases as the wind speed increases. The wind speed at which this “rollover” occurs is dependent on radar wavelength and incidence angle, being as low as 30 m s−1forKuband for vertical polarization at some incidence angles. The effect of wedges and breakers may overcome the predicted “rollover,” especially for horizontal polarization, but there are data to support a tendency toward saturation for vertical polarization at perhaps a somewhat higher wind speed. The two‐scale model does not appear to be sensitive to variations in the slopes of the tilting waves that would be present for nonfully developed seas. The number and size of wedges and spilling breakers will be a function of fetch and duration and, along with sea surface temperature effects, will need to be incorporated in models that recover wind speed and direction from scatterometer measurements. This rather complicated dependence of radar backscatter on wind speed, water temperature, and fetch and duration dependent wave properties contrasts strongly with current power law models. Some of the inconsistencies that have arisen in the analysis of scatterometer dat

ISSN:0148-0227    

DOI:10.1029/JC092iC05p04971

年代:1987

数据来源: WILEY

摘要:

This study sets forward a hypothesis which anticipates deep water formation due to ice edge upwelling. The upwelling can raise thermocline waters (the lower Arctic Intermediate Water) to the surface or near it, where the water is exposed to cooling, evaporation, mixing, and oxygenation. Thus upwelling can act as a preconditioning mechanism for deep convection. The conjecture would also explain the salinity range of the Greenland Sea Deep Water if the upper and lower Arctic Intermediate Water masses are mixed so that the latter has at least an 80% contribution. Also, the convection events induced by ice edge upwelling during winter season, we anticipate, could give rise to a new deep water annual production rate consistent with observations.

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05031

年代:1987

数据来源: WILEY

摘要:

A model is developed to calculate the upper ocean internal wave spectrum as modified by the surface boundary and mixed layer. The Garrett‐Munk spectrum is assumed to describe the deep ocean wave field. The main effect of the surface and mixed layer is to align the vertical structure of the waves forming vertically standing waves locally; this contrasts with the assumption of random alignment in the Garrett‐Munk model. Model spectra and coherences are calculated for idealized buoyancy frequency profiles and compared with the Garrett‐Munk model. Measurements in the upper 200 m from the Mixed Layer Dynamics Experiment in the northeast Pacific in 1983 are also compared with the model results. The most striking success of the model is predicting the observed high coherence and 180° phase difference across the mixed layer of horizontal ve

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05035

年代:1987

数据来源: WILEY

摘要:

The sea level records at the nine island stations located in the western and central equatorial Pacific show trends for the period 1974–1975 to 1981 which are well hindcasted by the linear numerical model of Busalacchi and O'Brien forced by the observed ship winds. The spatial pattern of these trends suggests that the western equatorial Pacific was losing water while the central equatorial Pacific was gaining water. These trends appear to be due to the significant weakening of the near‐equatorial easterly trades in the central Pacific which took place throughout the period 1974–1981. The trend reversal observed in 1977–1978 in sea level at the island stations located south of the equator (Rabaul and Honiara) appear to be due to a similar trend reversal in the strength of the large‐scale southea

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05045

年代:1987

数据来源: WILEY

摘要:

In ocean acoustic tomography, acoustic travel times along multipaths between sources and receivers are inverted to obtain estimates of sound speed and currents in the region sampled by ray paths. Since the information gathered along ray paths is concentrated near their upper and lower turning depths, inversions are associated with a strong sidelobe in the vertical that is well separated from the main lobe. Thus there are three important measures of system performance, namely, resolution, variance, and sidelobe contamination. We criticize a methodology for investigating trade‐off relations among these quantities that is based on a layered model of the medium. ForNlayers anN×Nresolution matrix specifies main lobe and sidelobe acceptance for each estimate. This matrix correctly represents resolution length and sidelobe contamination only when turning depths define layer interfaces. A resolution kernel is introduced that represents acceptance for a continuous medium. Because elements of the resolution matrix can be expressed as integrals over layers of the continuous resolution kernels, it is an intrinsic defect of the discrete approach that in forcing the resolution matrix close to the identity, it allows (and even encourages) cancellation of positive and negative acceptance within layers. This occurs for models with turning depths within layers. In such cases the resolution matrix may be virtually perfect, while true resolution properties are nevertheless pathologically degraded, and estimates do not in general approximate true layer averages. We also investigate the use in inversions of weightings derived from a priori knowledge and criticize the reporting of weighted rather than true sidelobe leve

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05052

年代:1987

数据来源: WILEY

摘要:

Continuous 2.7‐year time series of upper ocean currents and temperature at 0°, 28°W collected during the Seasonal Response of the Equatorial Atlantic Experiment are presented. The thermocline underwent distinctive annual cycles in response to forcing by the surface wind stress, and the core of the Equatorial Undercurrent tracked the thermocline. While the vertical position of the thermocline and the undercurrent appear to be simply related, the transport of the undercurrent did not show a replicating annual cycle, and the speed at the undercurrent core remained relatively constant. Near surface flow when averaged over the 2.7 years was not statistically different from zero, and a westward flowing South Equatorial Current was rarely established despite strong easterly wind stress in all 3 years. Cross‐equatorial flow was also zero in the mean at this loc

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05061

年代:1987

数据来源: WILEY

摘要:

Historical ship drifts and geostrophic surface currents obtained from hydrographic data are compared in the tropical Atlantic Ocean. The major components of the current system (North Equatorial Current, North Equatorial Countercurrent, South Equatorial Current) are clearly depicted by the two data sets. The main difference between the two fields is the weakness of the geostrophic currents except in the near‐equatorial band, where on the contrary, they are much larger than the ship drifts. The amplitudes of the annual signals differ, but the phases are in rather good agreement. The uncertainty concerning the ship drift data, and the limitation of the geostrophic assumption, can explain a part of the differences encountered in this study. A much better comparison is obtained if we add to the geostrophic currentugan Ekman drift currentuecomputed from the climatological wind stress and a mean constant vertical viscosity value. Then the total velocityu=ug+uecorresponds very closely in phase and amplitude to the ship drift velocity, except at the equato

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05076

年代:1987

数据来源: WILEY

摘要:

A subset of data from an array of eight inverted echo sounders and a meteorological station deployed and maintained for 15 months in the equatorial Atlantic as part of the Seasonal Response of the Equatorial Atlantic program are analyzed to determine the variability of forced oscillations of the basin. The analysis is done to study both low‐ and high‐frequency variability. At low frequencies the equatorial basin responds as a whole to the seasonally varying winds. The dominant signal is the annual cycle of the equatorial system as a composite of the response to the strong seasonal wind curl that is maximum in the western basin and the part of the zonal transfer to mass associated with the seasonal equatorial upwelling and downwelling system. Three bands of energy appear as dominant in the combined spectrum of the ocean and the atmosphere at higher frequencies: 20–30 days, 13–16 days, and the inertial gravity wave band. Each one of the bands is analyzed and discussed. Instability waves are detected on the echo sounder records. The waves have periods centered at 30.2 days and propagate westward with a wavelength of 1000 km and a phase speed of 0.35 m/s. The most energetic oscillation in the analyzed wind records is at periods centered around 14 days. The response of the ocean to this oscillation has a meridional distribution with maximum amplitude at 3°N. The most significant oceanic oscillations in the inertial gravity band are centered at 5.2 and 3.5 days. From the study of their meridional structure it is concluded that those oscillations correspond to forced inertial gravity waves with meridional structures corresponding to the meridional numbersn= 1 andn= 3, resp

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05089

年代:1987

数据来源: WILEY

摘要:

Daily averaged positions from about 280 satellite‐tracked drifting buoys launched during the First GARP Global Experiment are used to estimate the sea surface current velocity field south of 20°S. The time‐averaged surface velocity field based on velocity averages over 4°×4° areas is used to construct the surface stream function. The gross features of the mean circulation are generally in agreement with other representations of the time‐averaged circulation although they do differ in details from the climatological relative surface dynamic topographies in the southern hemisphere subtropical gyres. Mean and eddy horizontal momentum fluxes based on a 2°×2° grid are estimated for the region south of 40°S. The zonally averaged momentum flux at the sea surface for the region of the Antarctic Circumpolar Current, from 40° to 60°S, leads to a momentum flux divergence of 60 cm2s−2indicating a meridional flux of eastward momentum away from the current axis. Given the relatively large errors associated with the zonally averaged momentum fluxes, the sign of the flux divergence

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05101

年代:1987

数据来源: WILEY

摘要:

Measurements of turbulent kinetic energy k under surf zone waves are analyzed to show how k varies over depth, between breaker point and shoreline, and how k depends on the beach slope. It is found that the variation of k over depth is remarkably weak, large values being measured a few percent of the depth above the bottom. A simple model for the dissipation mechanism makes it possible to derive an empirical formula for the time‐averaged k that accurately describes all the data considered reliabl

ISSN:0148-0227    

DOI:10.1029/JC092iC05p05115

年代:1987

数据来源: WILEY