摘要:

Historical ship drifts from the tropical Atlantic Ocean are compared with surface currents from a general circulation model forced with monthly mean climatological winds. The model accurately reproduces the spatial structure of the currents and its time dependence, which varies considerably from the eastern side of the basin, where a semiannual harmonic is prominent, to the western side, where an annual harmonic is dominant. However, the amplitude of the simulated surface currents in the western side of the basin is too large. Mixing processes in the model appear to be too weak, especially when the winds are weak in regions where the thermocline is deep. High‐frequency fluctuations of the winds need to be taken into account, and parameterization of the mixing needs to be improved, especially when the Richardson number is smal

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00715

年代:1987

数据来源: WILEY

摘要:

Records of more than a year's duration from three deep ocean pressure sensors and three island shallow pressure sensors, all close to the equator between 30°W and 7°E, are analyzed spectrally at frequencies below the diurnal tides. The records were taken as integral parts of the FOCAL‐SEQUAL experiments in the equatorial Atlantic, 1983–1984. The low‐frequency gravitational tides at 1, 2, 3 cycles per month formed an important, nearly simultaneous part of the signal and had to be removed. The seasonal variations associated with equatorial upwelling are prominent in the shallow but not the deep pressure signals. Practically all the non tidal deep pressure signals at 0.1–0.7 cycles per day (cpd) are highly coherent between 20°W and 6°E, some 2900 km, and show no sign of zonal propagation. A comparison of the coherent part of the signals between the equator and Ascension Island (8°S) suggests a southward component of propagation, at least in the region of the Mid‐Atlantic Ridge. These motions are evidently barotropic and are not trapped near the equator. A close relationship to the atmospherically forced barotropic waves at 4–6 days period in the Pacific, identified by Luther (1980), is suggested. Special consideration is given to a comparison of a 3.6‐km‐deep pressure record at 6°E with a shallow pressure record from Ilha São Tomé' (supplied by J. M. Verstraete). Their ordinary tidal components are practically identical, as befits purely barotropic motion, but the sub‐tidal continuum shows fairly low (though significant) coherence, with higher energy in the island record. This is attributed to the presence of internal wave modes of several meters internal amplitude. Theoretically, the contribution of internal waves should give a much stronger signal in the surface pressure than in the deep pressure signal. The relationships are demonstrated by calculations based on acoustic return times by two inverted echo sounder recorders placed close to deep pressure sites by E. J. Katz. Deduced coherences agree reasonably with observed values and are consistent with internal waves dominated by mode 1, uncorrelated with the barotropic motion, sensed in both deep an

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00725

年代:1987

数据来源: WILEY

摘要:

Sea surface temperature (SST) maps and imagery derived from the NOAA 6 satellite Advanced Very High Resolution Radiometer (AVHRR) for June and July 1981 in the eastern tropical Pacific portray the wavelike structure of the cool water along the equator from 93°W to 125°W. Cusped waves of approximately 1000‐km zonal wavelength and 25‐day period propagated westward with a phase speed of 40 km/day. The observed meridional extent between the crest and trough of the waves is about 300 km. Details in the imagery show cooler water at the cusps advected north and then east with the north equatorial countercurrent (NECC), consistent with the suggestion of a series of anticyclonic eddies occupying the shear zone between the NECC and the westward flowing south equatorial current. Absolute SST estimates from the AVHRR data agree to within 0.6°C with shipboard data taken along 110° W between 5°N and 5°S. The wavelike structures in the SST maps are also in agreement at the surface with a vertical expendable bathythermograph temperature section made along the equator between 93°W and 125°W, which shows the phase of the waves tilting westward with increasing depth over the upper 75 m. Such a phase shift, if it extended 100–200 km meridionally in either direction from the equator, would be associated with an equatorward flux of heat. Similar phase shifts appear in temperature time series at depths of 20 and 50 m, from a mooring at 0°33′N, 110°30′W. Near‐surface currents measured at this and a second mooring on the equator at 109°40′W indicate a regular pattern of northward advection when wave cusps pass them, followed by southwest flow during the passage of wave troughs, again consistent with an equatorward flux of heat, as well as with earlier theoretical

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00742

年代:1987

数据来源: WILEY

摘要:

The nearly continuous 3.5 years of altimeter data in the western North Atlantic Ocean from the GEOS 3 mission (April 1975 to November 1978) have been used to study the seasonal variability of the Gulf Stream. The differences between altimetric measurements of sea surface height made at satellite ground track intersections, called crossovers, are utilized to construct time series of sea level variations. The results indicate that the Gulf Stream in the region off Cape Hatteras has a pronounced seasonal variability. The peak‐to‐peak amplitude of the seasonal cycle in terms of cross‐stream sea level difference is about 15 cm, with a maximum in April and a minimum in December. The result is in good agreement with historic hydrographic observations and recent direct measurements of the Gulf Stream. The mechanisms responsible for the observed seasonal variability are disc

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00749

年代:1987

数据来源: WILEY

摘要:

Current meter records from 14 moorings in the deep‐sea basin (3000–4500 m deep) south of the Mendocino Fracture Zone are analyzed. All moorings had current meters between 200 m and 500 m above the bottom, and some extended to within 150 m of the surface. There were high vertical correlations between measurements on the same mooring within 1500 m of the bottom and within 800 m of the surface but almost no significant correlation in the horizontal. In the basin the presence of eddies appears strongest at depths below 1200 m. Several of the records exceed 3 years in length, and one extended for 5 years. Spectral analysis of these shows that most of the kinetic energy below 3000 m is in the temporal mesoscale (periods of 31 to 120 days), while the spectral estimates in the upper 1000 m are dominated by longer time scales. Only in the deep records is there a significant southward mean flow. Neither a mean California Current nor a poleward undercurrent is apparent in the shallower data (above 1250 m). The currents in the upper 500 m nearest the continental margin are influenced by the presence of cold filaments originating near Point Ar

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00755

年代:1987

数据来源: WILEY

摘要:

The distributions of Th isotopes in both suspended participate matter and sea water were measured in the western North Pacific and Japan Sea. Suspended particulate matter was collected simultaneously on four 142‐mm‐diameter Nuclepore filters (1.0 μm in pore size) by using an in situ deep‐sea pumping system. Particle associations were found to be 6–40% for228Th, 9–52% for230Th, and 9–72% for232Th with mean values of 26–32% for all three isotopes. In order to explain the observed Th isotope distributions in the Pacific, a reversible exchange model was developed that includes two classes (fine and large) of particle population in the ocean. The fine particles are defined as those collected by filtration and comprising most of the standing crop of particles in seawater with no significant settling flux. The large particles, on the other hand, are defined as those collected via sediment traps and represent a negligible standing stock in the water but play a predominant role in the vertical flux. The model calculation suggests that the transfer of Th from solution to fine particles is controlled by an adsorption/desorption process which depends upon the chemical reactivity of elements. Aggregation of fine particles to fast sinking large particles was estimated to occur throughout the water column on a time scale of one to five months. The large aggregates seem to disintegrate on a time scale of less than a few days, suggesting their extremely

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00772

年代:1987

数据来源: WILEY

摘要:

Internal gravity waves measured under the Arctic pack ice were strikingly different from measurements at lower latitudes. The total wave energy, integrated over the internal wave frequency band, was lower by a factor of 0.03–0.07, and the spectral slope at high frequency was nearly −1 in contrast to the − 2 observed at lower latitudes. This result has implications for theoretical investigations of the generation, evolution, and destruction of internal waves and is also important for other processes, such as the propagation of sound, and the wave‐induced turbulent diffusion of heat, plankton, and chemical

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00779

年代:1987

数据来源: WILEY

摘要:

Island wakes and headland eddies in coastal waters are identified from spaceborne and airborne remotely sensed imagery in the visible wavelengths. The imagery were obtained for the Bristol and English channels, United Kingdom, which are characterized by high tidal currents and levels of turbidity. Suspended matter in the surface waters is used as a passive tracer for the flow features. Scaling parameters (e.g., the Reynolds, Ekman, and Rossby numbers), obtained from the depth averaged equation of motion, are presented based upon information on oceanographic conditions at the times of the overpasses. The parameters are compared then with data obtained from laboratory model investigations, presented by other experimenters in the published literature. The remotely sensed data demonstrate that in coastal waters with nonuniform obstacles and bathymetry, for a given Ekman number the flow regimes occur at a lower Rossby number than might be expected from laboratory experiments. In the absence of a method for the accurate determination of the horizontal eddy viscosity, hence the Reynolds number, it is concluded that an “island wake parameter” (Wolanski et al., 1984a) should be adopted to describe such features in coastal wat

ISSN:0148-0227    

DOI:10.1029/JC092iC01p00783

年代:1987

数据来源: WILEY