摘要:

The ventilation of the lower equatorial Atlantic thermocline (100 m to 400 m) is investigated from the distributions of salinity, oxygen, the chlorofluoromethanes F‐11 and F‐12, tritium, and the excess helium 3; 1983 data suggest a correlation on isopycnal surfaces between oxygen, chlorofluoromethanes and to a lesser extent, with the apparent ages derived from F‐ll/F‐12 and helium 3/tritium. The apparent ages increase with depth from 10 years on σΘ=26.5 to over 15 years near 400 m. The tracers are largely undersaturated with respect to the surface concentrations corresponding to the apparent ages. We investigate the ventilation processes responsible for these distributions with simple circulation models, where water from the subtropical gyres reaches isopycnally the equatorial thermocline. The few model parameters are determined by fitting the simulations to the chlorofluoromethane distributions in 1983. The sensitivity of these parameters to expected errors on the past surface concentrations and to vertical mixing is comparable and can result in an error of more than 20% in the model parameters. In 1983, model [3H] and [3He]* are too large, in particular when no diapycnal mixing is modeled, but the differences could have arisen from errors on past surface concentrations. During the 1980s, the model overestimates the increase in chlorofluoromethanes, in particular F‐12. The fitted model inflow from the southern subtropical gyre between 110 m and 400 m is 14×106m3s−1, in the range of expected transports by the western boundary current near Brazil. The model oxygen utilization rate is on the order of 12 mmol kg−1yr−1at 100 m and 5 mmol kg−1yr−1at

ISSN:0148-0227    

DOI:10.1029/93JC00976

年代:1993

数据来源: WILEY

摘要:

Satellite altimetry was used to examine annual and interannual variability in the Gulf of Alaska region. Crossover data from the Geosat Geodetic Mission (GM) and collinear data from the Exact Repeat Mission (ERM) were processed separately to form sea surface height anomalies at grid points. The time series from the GM and ERM were then combined to produce a 3.75‐year data set. The time series from the ERM data set agreed fairly well with hydrographic dynamic heights at several locations, with an average correlation of 0.70 between the two data sets. The combined and ERM altimetric data sets were analyzed using empirical orthogonal functions (EOFs). These revealed variability that occurs primarily on annual and interannual time scales. A comparison with EOF analysis of the atmospheric pressure field during the same time periods showed that the annual variation in pressure seemed to be reflected in both the combined and ERM altimetric data sets. The amplitude time series of the first mode in the combined data set was very similar to the North Pacific pressure index during the 1985–1989 time frame. The maximum correlation was at a lag of 250 days. Finally, an interannual mode was present in all three data sets which was closely linked to the baroclinic variations measured by the hydrographic d

ISSN:0148-0227    

DOI:10.1029/93JC00935

年代:1993

数据来源: WILEY

摘要:

Analysis of successive satellite thermal images in Tosa Bay, Japan, along the Kuroshio, showed that a cyclonic frontal eddy is formed due to the topographic effect of Cape Ashizuri when the Kuroshio axis moves offshore. Unlike those found in the South Atlantic Bight (SAB) along the Gulf Stream, the frontal shape induced by the eddy is asymmetric. Comparison with the previous description on the eddy in the SAB shows a possibility that cold, nutrient‐rich deep water is not easily pumped up by the upwelling induced by the eddy in Tosa Bay and the surface temperature distribution is not affected much. Time series of water temperatures recorded on a ferryboat show that this phenomenon occurs about three times in a month but is not periodi

ISSN:0148-0227    

DOI:10.1029/93JC01027

年代:1993

数据来源: WILEY

摘要:

Time series of sea surface height anomaly as measured by inverted echo sounders with pressure gauges (IES/PGs) and Geosat altimetry are compared at six locations in the northwest Atlantic near 40°N, 55°W. Three IES/PG sites lay along each of two adjacent Geosat‐Exact Repeat Mission ground tracks. Better agreement was found along the westernmost ground track; pressure sensor noise is the probable source of the larger errors along the easternmost ground track. The overall correlation is 0.78; along the westernmost ground track it is 0.91. Decorrelation scales are about 100 km along track and about 5 days. Uncorrelated errors dominate, but along the westernmost ground track, IES/PG‐derived sea surface height variability is systematically smaller than that derived from Geosat data; this factor, determined by regression analysis, is about 0.81. However, a consistent adjustment of IES calibration removes the systematic discre

ISSN:0148-0227    

DOI:10.1029/93JC01220

年代:1993

数据来源: WILEY

摘要:

Natural oil seepage in the Gulf of Mexico causes persistent surface slicks that are visible from space in predictable locations. A photograph of the sun glint pattern offshore from Louisiana taken from the space shuttleAtlantison May 5, 1989, shows at least 124 slicks in an area of about 15,000 km2; a thematic mapper (TM) image collected by the Landsat orbiter on July 31, 1991, shows at least 66 slicks in a cloud‐free area of 8200 km2that overlaps the area of the photograph. Samples and descriptions made from a surface ship, from aircraft, and from a submarine confirmed the presence of crude oil in floating slicks. The imagery data show surface slicks near eight locations where chemosynthetic communities dependent upon seeping hydrocarbons are known to occur on the seafloor. Additionally, a large surface slick above the location of an active mud volcano was evident in the TM image. In one location the combined set of observations confirmed the presence of a flourishing chemosynthetic community, active seafloor oil and gas seepage, crude oil on the sea surface, and slick features that were visible in both images. We derived an analytical expression for the formation of floating slicks based on a parameterization of seafloor flow rate, downstream movement on the surface, half‐life of floating oil, and threshold thickness for detection. Applying this equation to the lengths of observed slicks suggested that the slicks in the Atlantis photograph and in the TM image represent seepage rates of 2.2–30 m31000 km−2d−1and 1.4–18 m31000 km−2d−1, respectively. Generalizing to an annual rate suggests that total natural seepage in this region is of the order of at least 20,000 m3yr−1(120,

ISSN:0148-0227    

DOI:10.1029/93JC01289

年代:1993

数据来源: WILEY

摘要:

Broadband acoustic signals transmitted from a moored 250‐Hz source to a 3‐km‐long vertical line array of hydrophones 1000 km distant in the north central Pacific Ocean were used to determine the amount of information available from tomographic techniques used in the vertical plane connecting a source‐receiver pair. A range‐independent, pure acoustic inverse to obtain the sound speed field using travel time data from the array is shown to be possible by iterating from climatological data without using any information from concurrent environmental measurements. Range‐dependent inversions indicate resolution of components of oceanic variability with horizontal wavelengths shorter than 50 km, although the limited spatial resolution of concurrent direct measurements does not provide a strong cross‐validation, since the typical cast spacing of 20–25 km gives a Nyquist wavelength of 40–50 km. The small travel time signals associated with high‐wavenumber ocean variability place stringent but achievable requirements on travel time measurement precision. The forward problem for the high‐wavenumber components of the model is found to be subject to relatively large linearization errors, however, unless the sound speed field at wavelengths greater than about 50 km is known from other measurements or from a two‐dimensional tomographic array. The high‐ocean‐wavenumber resolution that is in principle available from tomographic measurements is therefore achievable only

ISSN:0148-0227    

DOI:10.1029/93JC01246

年代:1993

数据来源: WILEY

摘要:

In February‐April 1991, episodes of 2 to 8 m s−1westerly winds of 3 to 11 days' duration occurred in the western Pacific warm pool. Resulting modifications of the upper ocean in current and hydrology are quantified using data from an equatorial mooring at 165°E and from three cruises within 30 days of one another along 165°E. During westerly wind bursts (WWB) stronger than 4 m s−1, the upper 50 m becomes isothermal to within 0.1°C and sea surface temperature (SST) drops by 0.3–0.4°C between 5°S and 2.5°N. Conversely, SST starts warming and the upper 50 m restratifies in 4–5 days after the end of WWB. In contrast to previous observations, salinity between 0 and 50 m appears almost unaffected by WWB; it freshens by 0.4 practical salinity unit in March within an area of 1°–2° of latitude around the equator but not necessarily in direct response to WWB. As for zonal circulation, surface equatorial flow accelerates eastward 2–3 days after the beginning of westerlies. Then, after less than 2 weeks, eastward and westward jets both develop from 2°N to 2°S in the upper and lower halves of the temperature mixed layer, respectively. Changes in zonal mass transport in this layer were as much as 30 Sv between 2.5°S and 2.5°N fr

ISSN:0148-0227    

DOI:10.1029/93JC01261

年代:1993

数据来源: WILEY

摘要:

Sea levels at the Papua New Guinea coast responded to the 1986–87 El Niño‐Southern Oscillation (ENSO) event much as would be expected for points on the inshore edge of a western boundary current driven by remote sea level disturbances. The first empirical orthogonal function of sea level in the region accounts for 60% of the variance, and shows a clear western boundary current signal. Sea level observations from tide gauges and estimates from adjacent expendable bathythermographs (XBTs) are in good agreement, so XBT data are used to supplement sea level results; specifically, we obtain an estimate of the western boundary current as a function of depth. The anomalous transport through Vitiaz Strait varied by about 15 Sv during the event, with possibly more following the eastern New Ireland coast. The western boundary current response appears to be located entirely equatorward of the inflow feeding it, as expected from linear theory. The boundary current followed the Trobriand Island ridge, rather than the main Papua New Guinea coastline, and appears to bifurcate along southern New Britain. A maximum response to the ENSO occurred south of New Ireland, where steric sea level appeared to vary by up to 45 cm due to the ENSO event; this may be an inertial fea

ISSN:0148-0227    

DOI:10.1029/93JC00766

年代:1993

数据来源: WILEY

摘要:

A fully prognostic Arctic ice‐ocean model is used to study the interannual variability of the sea ice during the period 1955–1975 and to explain the large variability of the ice extent in the Greenland and Iceland seas during the late 1960's. In particular, the model is used to test the conjecture of Aagaard and Carmack (1989) that the Great Salinity Anomaly (GSA) was a consequence of the anomalously large ice export in 1968. The objective here is to explore the high‐latitude ice‐ocean circulation changes due to wind field changes. In the simulations the ice extent in the Greenland Sea increased during the 1960's, reaching a maximum in 1968, as observed, and maxima in ice extent were always preceded by large pulses of ice export through the Fram Strait. The ice export event of 1968 was the largest in the simulation, being about twice as large as the average and corresponding to 1600 km3of excess fresh water. The simulated upper water column in the Greenland Sea has a salinity minimum in the fall of 1968, followed by very low winter salinities. The simulations suggest that, besides the above average ice export to the Greenland Sea, there was also fresh water export to support the larger than average ice cover. Three low‐salinity anomalies, which are created by the variability in ice production/melt, exited through the Fram Strait in 1963–1965, 1966, and 1967–1969, the later two events being associated with a net freshwater export of about 900 km3.The total simulated freshwater input of 2500 km3to the Greenland Sea compares well with the estimated total freshwater excess of the GSA of about 2200 km3as it passed through the Labrador Sea (Dickson et al., 1988). Considering the uncertainties in the model, it is possible that the ice export could account for even larger portion of the freshwater excess. However, the main conclusion is that these model results show the origin of the GSA to be in the Arctic, as suggested by Aagaard and Carmack (1989) and support the view that the Arctic may play an active role in c

ISSN:0148-0227    

DOI:10.1029/93JC01504

年代:1993

数据来源: WILEY

摘要:

The Canada Centre for Remote Sensing CV‐580 aircraft collected C‐band synthetic aperture radar (SAR) data over the marginal ice zone off the east coast of Newfoundland during the Labrador Ice Margin Experiment (LIMEX) in March 1989. One component of the LIMEX'89 program was the study of ocean waves penetrating the marginal ice zone. In this paper, we consider nearly coincidental observations of waves in ice by airborne SAR and wave‐induced ice motion measurements. We explain the wave patterns observed in the SAR imagery, and the corresponding SAR image spectra, in terms of SAR wave imaging models. These include the well‐known tilt cross‐section modulation, linear, quasi‐linear, and nonlinear velocity bunching forward mapping models (FMMs), and the assertion that the concept of coherence time limitation applies differently to the cases of waves in ice and open water. We modify the concept of the scene coherence time to include two parts: first, a decorrelation time deduced from the inherent azimuth cutoff in the nonlinear velocity bunching FMM; and second, the intrinsic scene coherence time which is a measure of the time scale over which an open water Bragg scattering patch retains its phase structure. Either of these coherence time scales could dominate the SAR image formation process, depending upon the environmental conditions (the wave spectrum and the wind speed, for example). These two coherence time scales are independently estimated based upon a quasi‐linear velocity bunching FMM applied to some of the LIMEX'89 observations. Observed SAR image spectra and forward mapped ice motion package spectra are favor

ISSN:0148-0227    

DOI:10.1029/93JC00914

年代:1993

数据来源: WILEY