摘要:

We document temporal variability of the northernmost branch of the North Atlantic Current, known as the North Subarctic Front (NSAF), for the period 1976–1985. The front's location and temperature‐salinity (T‐S) parameters were determined from 120 repeat transects along three standard sections occupied by Russian ships: (1) ocean weather station (OWS) “C” (52.75°N, 35.5°W) to St. John's, Newfoundland; (2) along 35°W, from 52°N to 36.5°N; and (3) OWS C to Cape St. Vincent, Portugal. The NSAF exhibits significant interannual variability of its location, shifting 200 km to 300 km in a few months. Along the OWS C‐St. John's section, the front reached its southernmost location in 1976–1977, then experienced two 250‐to‐300‐km northward shifts, in 1978–1979 and 1980–1981; all of the three shifts peaked in wintertime. These events nearly coincide with events of the opposite sign along the OWS C‐Cape St. Vincent section (the northernmost location in 1976–1977 and two 200‐km southward shifts, in 1978–1979 and 1980–1981). The NSAF's behavior is thus coherent near the Charlie‐Gibbs Fracture Zone at the 500‐km scale. Frontal time series (associated with the moving front) ofTandSat the 200‐m level for the cold side of the NSAF reveal two events: anS200minimum in the winter of 1976–1977, which might be a manifestation of the “Great Salinity Anomaly” of the 1970s, and a sharp drop ofS200in early 1984, which might signal the arrival of another large salinity

ISSN:0148-0227    

DOI:10.1029/96JC02794

年代:1996

数据来源: WILEY

摘要:

The TOPEX/POSEIDON altimeter data and the Geosat altimeter data from the Exact Repeat Mission (ERM) have been analyzed to show the basin‐scale features of annual sea surface height anomalies in the Labrador Sea. A complex empirical orthogonal function (CEOF) analysis is used to extract spatial and temporal patterns of altimetric sea surface height anomalies. The analysis of TOPEX/POSEIDON data has revealed that the first eigenmode has an annual variation with amplitudes of ∼5 cm, positive in summer and negative in winter. The Geosat data analysis implies similar results but shows only the sea surface height variabilities relative to those of the subtropical gyre circulation and the North Atlantic Current due to an orbit error correction. The steric height anomalies of the climatological monthly‐mean Levitus data and the sea surface height anomalies of a wind‐driven barotropic model are also analyzed using the CEOF technique. The annual cycle in the Levitus data, dominated by thermal expansion, has an amplitude of ∼4 cm and is nearly in phase with the TOPEX/POSEIDON data. The wind‐driven annual signal is approximately in phase with the TOPEX/POSEIDON and Levitus data, but its amplitude is less than ∼1 cm. A correlation analysis suggests that the basin‐scale features deduced from TOPEX/POSEIDON data are dominated by the steric height variability of the Levitus data, supplemented to much less extent by the wind‐driven response of the barotropic model. The Geosat results are found to represent the steric effect only. It is modified by the orbit error correction so much that the phas

ISSN:0148-0227    

DOI:10.1029/96JC02778

年代:1996

数据来源: WILEY

摘要:

A current meter array of 3 months duration from the Newfoundland shelf edge to the 400‐m isobath gives a mean southward transport of the Labrador Current of 3.7 Sv with a freshwater component of 0.1 Sv. Over a 2‐month period, the volume and the freshwater flow increased by 3 and 0.09 Sv, respectively. Mean currents exceeded the variable components by a factor of 3. This estimate of the total transport of the Labrador Current is low because the flow extends eastward of the array beyond the 400‐m isobath to at least 1000 m. The total volume transport of the Labrador Current through Flemish Pass is estimated roughly to range between 6.3 and 9.8 Sv if archived current meter data are incorporated with the observations from this array. The mean freshwater transport is estimated to be 0.13 ± 0

ISSN:0148-0227    

DOI:10.1029/96JC02779

年代:1996

数据来源: WILEY

摘要:

A three‐dimensional coupled ice‐ocean model has been developed to study short‐term ice motion over the eastern Canadian continental shelf. The model consists of a Hibler ice model and a diagnostic ocean model. Ice is coupled to the ocean through a surface Ekman layer. The model is implemented for the Labrador Sea using 6‐hourly winds and atmospheric pressures as input forcings. The results show that the model is able to produce many desired features of ice motion and ocean currents including wind‐generated coastal currents, an ice velocity field reflecting the influence of permanent and transient currents, and an increased sea surface tilt and ice internal stress at the coast. The model is used to simulate ice drift trajectories from six ice beacons deployed over the Labrador and Newfoundland Shelves in 1992. In the subtidal frequency range the modeled and observed ice velocities are in excellent agreement. Model errors, measured by the rms separation between the modeled and observed positions, increase with time. The increase is 10 km d−1in the first 2 days and slower after 2 days. The model results are compared with a model run without ice‐ocean coupling and a calculation using an empirical relationship and parameters (speed ratio and turning angle). The errors are 20% (no ice‐ocean coupling) and 70% (empirical relationship) larger than the errors in the coupled model in the first 2 days and much larger aft

ISSN:0148-0227    

DOI:10.1029/96JC02661

年代:1996

数据来源: WILEY

摘要:

Observed winter transport and adjusted sea level fluctuations in Hecate Strait were investigated using empirical orthogonal function and coherence analyses. The responses to large‐scale and local wind forcings were identified by the distinctive spatial patterns of their adjusted sea level responses. At the resolved periods of 2 to 48 days, the large‐scale and the local wind forcings were of roughly equal importance in driving transport fluctuations. Comparison of observations with a conceptual model indicates that the adjusted sea level in the northeast corner of the strait gives a good measure of response to both the local and large‐scale wind forcings. This provides a physical explanation for the observed high correlation between the adjusted sea level at Prince Rupert and the winter transport fluctuations in Hecate Strait. We also show that the transport fluctuations are associated with a particular spatial pattern of the velocity field, which represents roughly 1/4 of the energy in the observed winter velocity fluctuations measured at an array of current meters. This has implications for the use of surrogate transport series to hindcast oceanographic conditions in Hecate S

ISSN:0148-0227    

DOI:10.1029/96JC02400

年代:1996

数据来源: WILEY

摘要:

We use passive tracers in a one‐dimensional numerical model of the Arctic Ocean to determine the residence time in the mixed layer and the cold halocline. When run to a steady state, the model successfully reproduces most of the observed distribution of salinity and temperature in the Arctic above the Atlantic layer. Comparison of model‐calculated tritium concentrations with observational data also indicates that the transient properties are correct. An important component of the model is the implementation of a “shelf circulation” of about 0.8 Sv that simulates the observed production and interleaving of cold, highly saline shelf waters. We use the model to derive bulk residence times, which are about 25 years in the mixed layer and about 100 years in the halocline. These values are higher than those published in the literature. We explain how the model residence time is related to different tracer ages by generating age distributions with the model. It is shown that the weighted mean of these distributions corresponds well with published tracer age data. The model dynamics that correctly reproduce Arctic mixed‐layer and cold halocline vertical structure in salinity, temperature, density, and various tracers also appear to simulate the natural processes that filter out interannual fluctuations in the freshwater influx from runoff and Bering Strait flow. Under these conditions, the period of the variation must be over 30 years to get 50% of the signal through to Fram Strait. In accord with other investigations, this would suggest that short‐term fluctuations in Arctic river runoff are not the direct cause of freshwater anomalies in the northern Nor

ISSN:0148-0227    

DOI:10.1029/96JC02641

年代:1996

数据来源: WILEY

摘要:

Accurate quantification and characterization of the Arctic summer ice cover are needed for mass balance, heat flux, and modeling studies in the region. A general assessment of the state and basic characteristics of the ice cover can best be done in summer because it is when the perennial component is fully revealed. The main source of summer ice information has been passive microwave and to a lesser degree active microwave data. However, the emissivity and backscatter of sea ice are abnormal and difficult to resolve during this time period, causing large uncertainties in the interpretation of satellite data. In this study we examined the state of the sea ice cover by using special scanning microwave imager (SSM/I), synthetic aperture radar (SAR), and advanced very high resolution radiometer (AVHRR) satellite data synergistically. The surface and radiative characteristics of the summer ice cover were evaluated in the context of three special events: onset of melt, melt ponding, and freeze‐up. These events affect the emissivity and backscatter and may alter the albedo and ice structure. Onset of melt is readily detectable and is shown to migrate rapidly to the north in June. Melt ponding is not directly observable but is postulated to be the main cause of the decreases in brightness temperatures and large discrepancies between the SSM/I and SAR ice concentration results in many areas. In these areas, SAR and AVHRR results show concentrations near 100%, while the SSM/I data were as low as 70%. During freeze‐up the ice signatures are still quite different from those of midwinter ice, but the ice concentrations from SSM/I generally agree well with those from SAR data. Our results show that, generally, the average ice concentration within the pack is usually greater than 90% during the summer, which is substantially larger than that inferred previously from passive microwave data. The use of combined SAR and SSM/I data may also provide melt‐ponding fraction and first‐order estimate of albedo in the Arctic

ISSN:0148-0227    

DOI:10.1029/96JC02816

年代:1996

数据来源: WILEY

摘要:

By definition, ice which survives the summer is classified as multiyear ice. Thus the area covered by multiyear ice during the winter should be nearly equivalent to the ice area during the previous summer's minima. This condition provides a reasonable criterion for the evaluation of ice concentration and ice type retrieval algorithms using remote‐sensing data sets. From special sensor microwave imager (SSM/I) data the NASA Team algorithm estimates the multiyear, first‐year, and total ice concentrations during the winter using combinations of the polarization and spectral gradient ratios. The Team algorithm provides only estimates of ice concentration in the summer. From ERS 1 synthetic aperture radar (SAR) data the remarkably stable contrast between multiyear ice and first‐year ice in winter provides consistent estimates of multiyear ice concentrations. In the summer, multiyear ice concentration cannot be estimated from SAR or SSM/I data because free water on the surface effectively masks the backscatter and emissivity signature of this ice type. From SAR data a technique which takes advantage of the high backscatter of wind‐roughened open water as a discrimination feature is used to estimate the total ice concentration in the summer. With a year‐long (January 1992 to January 1993) data set from the Beaufort Sea we found that the multiyear ice concentration estimates from the SAR data are stable and are nearly equivalent to the ice concentration estimated at the end of the previous summer. We contrast this with the variability of the multiyear ice concentration and ice fraction estimates obtained using SSM/I data. The Team algorithm produces ice concentration and multiyear ice estimates which are consistently lower than those from the SAR data. We discuss reasons for these discrepancies and the implications of the higher than previously noted multiyear ice conce

ISSN:0148-0227    

DOI:10.1029/96JC02455

年代:1996

数据来源: WILEY

摘要:

Data collected from a voyage of RVNathaniel B. Palmerto the Bellingshausen and Amundsen Seas during August–September 1993 are used to investigate the thickness distribution of sea ice and snow cover and the processes that influence the development of the first‐year pack ice. The data are a combination of in situ and ship‐based measurements and show that the process of floe thickening is highly dependent on ice deformation; in particular, rafting and ridging play important roles at different stages of floe development. Rafting is the major mechanism in the early stages of development, and core structure data show the mean thickness of individual layers of crystals to be only 0.12 m. Most ice0.7 m and 0.2–0.5 m, respectively, and account for 40% and 36% of the surface area of the pack ice. Approximately 8% of the pack is open water. An estimate of the effects of ridging on the distribution of ice mass within the pack suggests that between 50 and 75% of the total mass is contained within the 25% of the pack that exhibits surf

ISSN:0148-0227    

DOI:10.1029/96JC02737

年代:1996

数据来源: WILEY

摘要:

The problem of estimating boundary and initial conditions for a regional open‐ocean model is addressed here. With the objective of mimicking the Synoptic Ocean Prediction (SYNOP) experiment in the Gulf Stream system, a meandering jet is modeled by the fully nonlinear barotropic vorticity equation. Simulated velocity observations are taken using current meters and acoustic tomography; twin experiments are then performed in which the adjoint method is used to reconstruct the flow field. The estimated flow is forced to resemble the true flow by minimizing a cost function with respect to some control variables. First, the vorticity initial conditions are used as control variables, and the boundary conditions are specified. The strong flow is found to induce strong dependence of the model/data misfit upon the specified boundary conditions. Second, the boundary values of stream function and vorticity are then included among the control variables. Various choices of a priori information about the control variables are employed, using various observational strategies. The major new result obtained is the successful estimation of the complete set of initial and boundary conditions, which is necessary to integrate the vorticity equation forward in time. From a time‐invariant first guess for the boundary conditions the assimilation is able to create temporal variations at the boundaries that make the interior flow match well the velocity observati

ISSN:0148-0227    

DOI:10.1029/96JC02781

年代:1996

数据来源: WILEY