摘要:

Results of a study of the oxygen isotopic composition of coastal, pelagic, and fresh waters from the northern Gulf of Alaska region are presented. This study was undertaken to investigate whether isotopic tracers could be of use in determining the important freshwater inputs to the Alaska Coastal Current (ACC) and whether they could confirm the presence of the ACC in coastal waters west of Kodiak Island. The Alaska Coastal Current, the major coastal circulation feature of the northern Gulf of Alaska, can be distinguished from oceanic waters on the basis of its lower salinity at least as far west as Kodiak Island. This study adds significantly to the small amount of oxygen isotopic information available for the waters of this region. The isotopic results suggest that in late summer, glacial meltwater may provide a substantial portion of the total freshwater runoff into the ACC, and that the ACC does extend as far to the west as Unimak Pass.

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05177

年代:1990

数据来源: WILEY

摘要:

Long baroclinic Rossby waves are potentially important in the adjustment of the tropical Pacific pycnocline to both annual and interannual wind stress curl fluctuations. Evidence for such waves is found in variations of the depth of the 20°C isotherm in the northern tropical Pacific during 1970–1987. A total of 199,067 bathythermograph profiles have been compiled from the archives of several countries; the data coverage is dense enough that westward propagating events may be observed with a minimum of zonal interpolation. After extensive quality control, 20°C depths were gridded with a resolution of 2° latitude, 5° longitude, and bimonths; statistical parameters of the data were estimated. A simple model of low‐frequency quasi‐geostrophic pycnocline variability allows the physical processes of Ekman pumping, the radiation of long (nondispersive) Rossby waves due to such pumping in midbasin, and the radiation of long Rossby waves from the observed eastern boundary pycnocline depth fluctuations. Although the wind stress curl has very little zonal variability at the annual period in the northern tropical Pacific, an annual fluctuation of 20°C depth propagates westward as a long Rossby wave near 5°N and 14°–18°N in agreement with the model hindcast. Near the thermocline ridge at 10°N, however, the annual cycle across the basin is dominated by local Ekman pumping. The wave‐dominated variability at 5°N weakens the annual cycle of geostrophic transport of the North Equatorial Countercurrenl in the western Pacific. El Niño events are associated with westerly wind anomalies concentrated in the central equatorial Pacific; up welling wind stress curl is generated in the extraequatorial tropics by these westerlies. Long up welling Rossby waves forced by this curl pattern were observed to raise the western Pacific thermocline well outside the equatorial waveguide in the later stages of El Niños, consistent with the simple long‐wave model. The observations suggest that although simple reflection of the long Rossby waves from the western boundary is not the major process affecting subsequent development on the equator, it is likely that the extraequatorial waves play some role in setting the t

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05183

年代:1990

数据来源: WILEY

摘要:

A linear, continuously stratified model is used to study the equatorial waves that are generated at the western boundary of the ocean by wind fields oscillating at periods of 30 and 60 days. Solutions are found in ocean basins with a western boundary oriented meridionally or slanted at a 45° angle, like the one in the Indian Ocean. They are evaluated analytically when the boundary is meridional but must be found numerically when the boundary is slanted. The possible equatorial waves are Kelvin and Yanai waves, and at the 60‐day period there are a few Rossby waves. Because Yanai and Rossby waves typically have short wavelengths, they are not directly forced by the wind; however, they can be strongly excited at the western boundary and in that case are highly visible in the interior ocean. When the western boundary is slanted, zonal winds also excite antisymmetric equatorial waves, resulting in considerable meridional flow on the equator. At the 30‐day period, energy associated with Kelvin and Yanai waves propagates eastward and downward from the western boundary along ray paths, producing an equatorial shadow zone in the western ocean. At the 60‐day period the presence of Rossby waves complicates the radiation pattern. Solutions compare favorably with several aspects of the observed equatorial wave field in the western Indian

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05219

年代:1990

数据来源: WILEY

摘要:

We have composited and objectively analyzed historical hydrographic observations for the North Atlantic Ocean for two periods, 1955–1959 and 1970–1974. Difference fields of steric sea level and geopotential thickness for the North Atlantic Ocean in the 0‐ to 1500‐m depth range between the 1955–1959 and 1970–1974 pentads were computed. We find that in the central portion of the subtropical gyre, steric sea level decreased by up to 17.5 dyn cm, whereas in the western subarctic gyre (north of the Gulf Stream) steric sea level increased by an amount up to 7.5 dyn cm from the earlier to the later pentad. A decrease in steric sea level (∼5 dyn cm) occurred along the eastern boundary of the N

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05233

年代:1990

数据来源: WILEY

摘要:

The circulation of the north Atlantic as deduced from an inverse calculation is tested against the historical record of tritium. The circulation model is first used to do a “forward” calculation of the tritium transient using published estimates of atmospheric injection rates, and of plausible estimates of the tracer history at the open boundaries of the model. Major discrepancies are found between this calculation and the observations of interior distributions of tritium. An attempt is then made to remove these discrepancies by modifying the atmospheric injection rates and the initial estimates of open boundary time histories, treating these boundary conditions as control variables. The procedure requires answering difficult questions as to the accuracy with which atmospheric transfers to the ocean and the boundary conditions at the northern limits of the model are known. Although removal of the model and observational tritium estimate differences is not completely satisfactory, it appears that existing tritium observations would only weakly constrain a more realistic circulation inversion. The computation, however, does suggest a strategy for future use of transient tracers for constraining circulation mod

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05239

年代:1990

数据来源: WILEY

摘要:

The theoretical studies shown here indicate that the best bands to measure and derive the total outgoing longwave radiation (OLR), surface downward flux (SDF), and cooling rates (CRs) using linear regression are (1) the band between 800 and 1200 cm−1for OLR, (2) the band between 500 and 660 cm−1or 660 and 800 cm−1for SDF, and (3) the band between 660 and 800 cm−1for CRs. These results are obtained from scatter plots of total fluxes and cooling rates associated with the various bands. The advanced very high resolution radiometer OLR is damped compared with the Nimbus 7 Earth radiation budget (ERB) OLR, which is derived from the broadband, narrow field of view ERB instrument, owing to its use of only one narrow band (centered around the 11‐μm window region) m

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05257

年代:1990

数据来源: WILEY

摘要:

Satellite altimetry data provided by GEOS 3, Seasat and currently Geosat have been successfully used to measure sea surface variability. The determination of the ocean variability has been accomplished through either examination of differences along collinear tracks or differences at points where tracks cross. In either case it is first necessary to remove the relatively large long‐wavelength error that exists in the sea surface heights due to inaccuracies in the estimation of the satellite altitude from ground based tracking data. This long‐wavelength altitude error has generally been an order of magnitude larger than the short‐wavelength variability that is being sought. A number of techniques have been applied to eliminate the altitude error with the most common being a simple modeling of the orbit error as a linear trend over the region of interest. The intent of this study is to quantify the effectiveness of using a linear model of altitude error when attempting to estimate sea surface variability. Although there are many sources of altitude error, only the influence of gravity modeling errors will be considered here. This is justified since this is the dominant error source in the estimation of the satellite altitude from ground based data. Additionally, gravity model errors are the primary source of altitude errors with wavelengths shorter than once per revolution. Analyses are presented for the case of Geosat and TOPEX altitude errors due to the expected modeling uncertainties of the GEM‐L2 and GEM‐T1 gravity fields. Results are given in terms of the rms of the crossovers after removing linear trends along track and minimizing the crossovers. It is found that for scales below 10,000 km, both gravity models are equally effective. Also, the higher altitude of TOPEX provides for significantly better performance of the linear orbit error correction model. The results indicate that the current GEM‐T1 model induces crossover discrepancies of less than 5 cm rms at scales up t

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05273

年代:1990

数据来源: WILEY

摘要:

Sea level data from St. John's, Newfoundland, and from Nain, Labrador, are compared with results from a 1/4°×1/4° resolution numerical model of the Newfoundland and Labrador shelf and the neighboring Labrador Sea. The model is barotropic, employs dynamics linearized about a state of rest, and uses linear bottom friction. The model is driven by the seasonal part of the Hellerman and Rosenstein wind stress field and by inflow from the rest of the North Atlantic specified along the eastern boundary. The latter is taken from a 1°×1° version of the model applied to the North Atlantic between 10°S and 80°N. The results indicate that our simple model can account for that part of the annual cycle of sea level on the shelf directly attributable to wind forcing. In particular, the model can account for the difference between observed monthly mean sea level at St. John's, corrected for atmospheric pressure variations, and the seasonal steric height anomalies above 150 dbar on the neighboring shelf. We also compare our model results with the linear regression analysis carried out by Thompson et al. on monthly mean sea level data from Nain for the period 1964 to 1972. The model confirms the gains calculated for both the local wind driven signal and also that part of the signal due to wind forcing over the North Atlantic beyond the Labrador Sea. Comparing St. John's with Nain, we find that at St. John's, the model response is accounted for primarily by wind set‐up on the shelf, whereas at Nain, the influence of North Atlantic wind forcing is a larger part of the total signal. These results suggest that data from the tide gauge at Nain could be useful for verifying model‐predicted changes in the large‐scale ocean circulation. The results also seem to be confirming the presence of the offshore transport variations predicted

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05279

年代:1990

数据来源: WILEY

摘要:

A common problem associated with spaceborne scatterometry, including SASS, NSCAT and ERS‐1, is that retrieval algorithms often compute multiple wind vectors per retrieval cell. Only one wind vector per retrieval cell corresponds to the true solution while the others are aliases that arise from symmetries in the response of small scale surface waves to wind forcing. The NSCAT retrieval algorithm ranks each ambiguous wind vector within a retrieval cell according to the likelihood that the wind vector is the true solution. Although the most likely wind vectors are usually correct, in approximately 40% of the retrieval cells an alias is ranked first. Based on the mathematical properties of these likelihood assignment errors, an ambiguity removal algorithm is derived that utilizes a nonlinear circular median filter (CMF) to select the true solution in each retrieval cell. The CMF algorithm was tested by analyzing twelve simulated wind fields with an average likelihood assignment error rate of 389 per 1000 retrievals. After processing by the CMF algorithm, the error rate was significantly reduced to an average of only 36 per 1000. To better understand the performance of the CMF algorithm under operational conditions, the analysis was extended by adding spatially correlated likelihood assignment errors to one of the simulated wind fields. A re‐analysis of this wind field showed that the performance of the CMF algorithm may decrease significantly if the likelihood assignment errors are correlated over several retrieval cells. In addition to deriving an ambiguity removal algorithm, a wind field smoothing technique that utilizes a circular median filter is derived and tested on one of the resolved wind fields. The results showed that CMF smoothing technique increases the spatial coherency between the true and resolved wind fields. Based on these analyses, specific recommendations are made for monitoring the performance of the wind retrieval process and for improving the accuracy of the retrieved wind fie

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05291

年代:1990

数据来源: WILEY

摘要:

A Lagrangian analysis of a particle sinking through a random mesoscale eddy field is used to evaluate the effects of horizontal diffusion and particle sinking rates on paniculate fluxes sampled by an idealized sediment trap. The analysis indicates that the spatial region where collected particles are formed (Lx) is dependent upon the mean sinking rate of the collected particles and the mesoscale eddy advective field above the trap. This weighted spatial averaging creates difficulties in the interpretation of sediment trap data as fluxes of rapidly sinking particles (>200 m d−1) represent local processes (Lx<20 km) while fluxes of slower sinking particles (200 km) for a trap deployed at 1000 m. Several examples of the potential effects that this spatial averaging may have upon the ecological interpretation of the sediment trap collected particle fluxes are presen

ISSN:0148-0227    

DOI:10.1029/JC095iC04p05305

年代:1990

数据来源: WILEY