摘要:

On September 10, 1987, an oceanic profiling package consisting of a Guildline conductivity‐temperature‐depth probe (CTD), SeaTec transmissometer and RD Instruments 150‐kHz acoustic Doppler current profiler (ADCP) was used to investigate the hydrothermal plume emanating from vent sites located at 2200‐m depth on the Endeavour segment of Juan de Fuca Ridge in the northeast Pacific Ocean (47°58′N; 129°06′W). The CTD‐transmissometer profiles indicate that the plume immediately downstream of the vent site consisted of two 50‐m‐thick particle‐laden effluent layers separated by a comparably thick zone of relatively particle‐free ambient fluid. The lower 50 m of the water column appeared to be dominated by smaller concentrations of resuspended particles within a benthic boundary layer. Maximum temperature, salinity, and light attenuation anomalies within the main core of the plume at 1970‐m depth were approximately +0.04°C, +0.01 ppt, and +0.05 m−1, respectively. The ADCP measurements represent the first attempt to acoustically map the current shear and backscatter amplitude associated with a mid‐ocean ridge hydrothermal plume. Repeated profiles by the ADCP revealed that the top of the plume was a region of enhanced current shear with vertical changes in speed of the order of 0.05 m s−1over depths of approximately 100 m. The velocity measurements also indicate that the current reversed from northwest to southwest over the 1‐hour observation period, consistent with the predicted flow derived from moored current meters recovered immediately prior to the profiler survey. Contrary to expectation, acoustic backscatter signals within the particle‐laden layers of the plume were lower than background reference levels, while echo return amplitudes from the particle‐free layer were close to background levels. Time series plots of the amplitude data indicate that the basic layered structure of the plume is disrupted by billowlike features with vertical and horizontal wavelengths of 40 to 60 m and 20 to 40 m, respectively. We suggest that the weakened acoustic backscatter signals from the effluent layers is linked to a reduction in the numbers of zoop

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04709

年代:1989

数据来源: WILEY

摘要:

The circulation of the western Indian Ocean is examined using a reduced‐gravity model with one active layer and realistic basin geometry for the entire Indian Ocean north of 30°S. The Hellerman and Rosenstein monthly mean wind stress climatology is used to force the model. The numerical simulations reproduce the observed (Luyten and Roemmich, 1982) 26‐day waves along the equator and the 50‐day oscillations (Mysak and Mertz, 1984; Schott et al. 1988) between the equator and Madagascar. The 25‐ to 28‐day oscillations of the model meridional velocity component agree with observed values of period, amplitude, wavelength, group velocity, and phase of the seasonal modulation. The model oscillations, which are excited in August and persist into February–March, are shown to be the result of Yanai waves generated between the western boundary and 50°E. During the southwest monsoon, the Yanai waves are initiated by a complex barotropic instability associated with the southern gyre. During the early stages of the northeast monsoon, the 26‐day Yanai waves are generated by resonant forcing due to the intrusion into the equatorial waveguide of a standing, 800‐ to 900‐km‐wavelength meander of the eastward flow fed by the East African Coastal Current. Hence the simulation reveals that the 26‐day oscillations in the equatorial Indian Ocean are excited by mechanisms significantly different than that believed to be responsible for the 20‐ to 30‐day oscillations in the equatorial Atlantic and Pacific oceans. The numerical simulation also shows a 50‐day oscillation between the equator and Madagascar west of 50°E. This periodicity is due to Rossby waves generated by a barotropic instability associated with the East African Coastal Current beginning about April each year. No evidence of the 50‐day period oscillation is found in a corresponding linear simulation. Hence the barotropic instability of the oceanic currents in this region is offered as an alternative to direct wind forcing as the generating mechanism for the observed 40‐ to 60‐day osci

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04721

年代:1989

数据来源: WILEY

摘要:

Repeated Geosat altimeter measurements of sea level at ground track intersections (crossovers) and along collinear nests of profiles were used to evaluate Geosat data by (1) construction of a regional, precise surface made up of intersecting sea level profiles, (2) determination of the magnitude of the electromagnetic (EM) bias, and (3) generation of sea level time series and anomaly maps. The first procedure, after elimination of radial ephemeris error and other long‐wavelength signals, established the data's internal precision at a few centimeters. With regard to the EM bias, comparing repeated sea level profiles for different values of significant wave height (SWH) gave a value of about 1% of SWH. The final process yielded a record of sea level whose accuracy was tested by comparison with in situ measurements and wind‐driven model results. On the basis of these analyses we conclude that Geosat data can determine sea level changes with an rms accuracy of approximately 4 cm for time scales of a month or longer. For determining sea level variability, Geosat geophysical data records are a high‐quality, long‐term data set of significant value for the study of ocean d

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04737

年代:1989

数据来源: WILEY

摘要:

Low frequency (<1000 Hz) sound absorption in the ocean depends exponentially on oceanpH. While the absolute amplitudes measured in long‐range (order of 1000 km) acoustic transmission experiments depend in a complex manner on ocean structure and are difficult to interpret without ambiguity, measurements of differential absorption as a function of frequency contain significant information. For surface‐reflected rays (RSR) there is the additional factor of frequency‐dependent reflectivity. In principle, a comparison of transmitted and received acoustic spectra can be inverted to obtain oceanpH and surface roughness. Calculations performed with preliminary data from a 750‐km path in the North Pacific give qualitative agreement with theory (higher frequencies are attenuated more than lower frequencies) but are inadequate for quantitative comparisons (the experiment was not optimized for differential attenuation measurements). Our conclusion is that it is possible, but not easy, to obtain quantitative information on oceanpH and surface roughness from measurements of differential attenuation with frequency along resolved ray paths. To obtain an accuracy of 0.05 inpH with a 750‐km transmission at 550±100 Hz would require about 100 independent samples for the North Pacific (pH ≈ 7.7) and 60 independent samples for the Atlantic (pH ≈ 8.0) in order to achieve adequate averaging of amplitude fluctuations induced by internal waves and ambient

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04749

年代:1989

数据来源: WILEY

摘要:

A 4‐year expendable bathythermograph data set (1984–1987) from the area between southern Brazil and the Antarctic Peninsula provides information on the interannual variability of front locations. Two boundaries of subtropical water at different depths are identified north and south of the Brazil Current‐Falkland (Malvinas) Current confluence zone. The northern Subtropical Front is displaced over a large part of the Argentine Basin from one observational period to the other. The shallow southern Subtropical Front appears fixed to the Falkland Escarpment. The Polar Front and Subantarctic Front locations do not vary much, except for one case where a cold core eddy in the Polar Frontal Zone causes a large northward displacement of the Subantarctic

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04757

年代:1989

数据来源: WILEY

摘要:

The coastal response to local and remote wind forcing is examined using observations from the Coastal Dynamics Experiment (CODE) which took place on the northern California coast during 1981 and 1982. The responses to local and remote forcing are determined from multivariate statistical cross‐spectral analysis covering periods from 12 to 2.5 days. Remote forcing is deduced from coastal sea level. This analysis leads to discovery of a mode of sea level variability, apparently a barotropic Kelvin wave, with much greater alongshore and across‐shelf scales than those of coastally trapped waves; there is no measurable response to this mode in coastal temperature or velocity. Coastally trapped waves are also evident and have responses over the shelf much as predicted by theories which account for bottom friction. The local response to wind differs markedly from coastally trapped waves, showing the across‐shelf flow responsible for upwelling, surface‐intensified temperature fluctuations caused by advection and mixing and even the alongshore pressure gradient partly responsible for the across‐shelf return flow which, on the largest scale, balances Ekman transport. Unfortunately, even the extensive CODE observations are inadequate to diagnose the dynamics of the across‐shelf flow or accurately determine the cross‐shelf heat flux supported by eddies and the fluctuating responses to local

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04763

年代:1989

数据来源: WILEY

摘要:

Random waves normally incident on a dissipative beach induce a variety of cross‐shore flows, such as asymmetric oscillatory flow, wave grouping‐induced long‐wave flow, breaking‐induced turbulent flow, and momentum decay‐induced undertow. These flows are identified, analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two‐dimensional case of bar generation

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04785

年代:1989

数据来源: WILEY

摘要:

The dynamical regimes governing various regions of a nonlinear stratified mean equatorial undercurrent are investigated by computing zonal momentum balances in Philander and Pacanowski's (1984) numerical simulation of the equatorial Atlantic. Only in a narrow region of the simulation next to the western boundary does a subsurface regime exist where the positive zonal pressure gradient force overcomes the negative zonal frictional forces, resulting in a net inertial acceleration to the east and in the formation of the eastward undercurrent below westward surface flow. Elsewhere, the net force is westward and the undercurrent, experiencing a net loss of eastward momentum, weakens from west to east. For the parameterizations of frictional processes adopted in the model, zonal momentum is dissipated mostly by vertical friction in the upper thermocline and above, and by lateral friction below. Because the net retarding effect of vertical friction on the upper layers of the undercurrent is larger than that of lateral friction on the lower layers, there is on average an apparent migration of the location of the undercurrent core velocity from above the thermocline in the west of the Atlantic basin to below in the east. The bulk of the mean midbasin model undercurrent can be described as terminating in the overlying westward flow; only a small fraction (the deeper more inertial layers) terminates at the eastern coast. It is argued that the mechanisms at work in the model are plausible ones to explain the zonal evolution of the observed undercurrent, which should motivate a reexamination of existing data. Better agreement between simulated and observed eastern undercurrent speeds might be obtained by reducing the eddy coefficient of lateral mixing used in the simulation.

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04801

年代:1989

数据来源: WILEY

摘要:

SEQUAL/FOCAL (Seasonal equatorial Atlantic/Programme Francais Ocean et Climat dans l'Atlantique Equatorial) data are analyzed to investigate the question of the relative importance of local and remote wind forcing in the equatorial Gulf of Guinea. The difference in the timing and rate of intensification of the winds in the eastern and western equatorial Atlantic Ocean for 1983 and 1984 is used to distinguish local and remote forcing efforts. It is found that, although asymmetry of the mixed layer thermal structure in the Gulf of Guinea can be attributed to the shallow circulation driven by the local meridional wind, the rapid cooling and the formation of a sharp thermal front at the equator and a local sea surface temperature (SST) minimum just south of the equator are correlated with the shoaling of the thermocline and this in turn is attributed to the remote forcing effect of the zonal wind intensification to the west. These results provide credence for previous conclusions by Servain et al. (1982) that the Gulf of Guinea SST is more strongly correlated with wind stress in the western equatorial Atlantic Ocean.

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04816

年代:1989

数据来源: WILEY

摘要:

Seasat A Satellite Scatterometer measurements of surface wind components were made under normal weather conditions with unsurpassed space and time resolutions during August and September 1978. Longitudinal distributions of the monthly mean zonal component were markedly different in each ocean: in the Pacific the zonal profile resembled a semicircle; a linear change occurred in the Atlantic, and quasi‐uniform values prevailed in the Indian Ocean. Only in the Atlantic and Pacific was the prevailing direction of the zonal component westward. In the Pacific the monthly mean standard deviations increased towards the west. This indicated that the larger day‐to‐day wind variability observed at the western islands compared to moored buoy measurements in the eastern region was a natural phenomenon and not caused by islands. The average monthly mean slope of the wave number spectra throughout the 550‐ to 2200‐km wavelength band was −1.7, which was approximately equal to the −5/3 power law associated with turbulent motions. That the spectra levels of the zonal wind, but not the meridional component, were substantially different in each equatorial ocean represents an enigma. Largest spectral values occurred in the Atlantic where variances were nearly 10 times greater than in the Pacific, which contained the sm

ISSN:0148-0227    

DOI:10.1029/JC094iC04p04829

年代:1989

数据来源: WILEY