摘要:

The deep waters of the central and southern Greenland Sea are examined usingBartlett1989 and 1990 summer data displayed at very high temperature and salinity resolutions. The mixing of Arctic Ocean Deep Water (AODW) with Greenland Sea Deep Water (GSDW) in proportions of about 1:1 to form “new” Norwegian Sea Deep Water (NSDW) is confirmed. However, the previously accepted route of this new NSDW through the ridge system into the Norwegian Sea, the Jan Mayen Channel, is shown to pass deep water only intermittently; a passage through Mohn's Ridge farther north, beginning near 72°06′N, 2°W, is identified as a more likely route. Part of the ultimate AODW‐GSDW mixture is shown to continue on along the Greenland continental slope into the Iceland Sea. A peculiar isothermal condition was found in the deeper parts of the GSDW. This is postulated to result from upward diffusion of temperature from a layer initially having the adiabatic temperature gradient. Above the isothermal layer is one of warming and decreasing salinity, the result of recent successive deep convections to differe

ISSN:0148-0227    

DOI:10.1029/92JC02752

年代:1993

数据来源: WILEY

摘要:

Ocean temperatures in the upper 250 m in the northern North Pacific (60°N, 149°W) increased by more than 1°C from 1972 to 1986 but are now decreasing. Subsurface temperature anomalies are well correlated (∼0.58) with the air temperature anomalies at Sitka, Alaska; hence the coastal air temperatures can be used as a proxy data set to extend the ocean temperature time series back to 1828. Up to 30% of the low‐frequency variance can be accounted for with the 18.6‐year nodal signal. Additionally, spectral analysis of these air temperature variations indicates a significant low‐frequency peak in the range of the 18.6‐year signal. Similar low‐frequency signals have been reported for Hudson Bay air temperatures since 1700, for sea surface temperatures in the North Atlantic from 1876 to 1939, and for sea level in the high‐latitude southern hemisphere. The water column temperature variations presented here are the first evidence that the upper ocean is responding to this very long period tidal forcing. An enhanced high‐latitude response to the 18.6‐year forcing is predicted by equilibrium tide theory, and it should be most evident at latitudes poleward of about 50°. These low‐frequency ocean‐atmosphere variations must be considered in high‐latitude assessments of global climate change, since they are of the same magnitude as many of t

ISSN:0148-0227    

DOI:10.1029/92JC02750

年代:1993

数据来源: WILEY

摘要:

An aircraft data set of coincident Kaband (33.6 GHz, vertical polarization) passive microwave images and aerial photographs acquired in the Chukchi‐Beaufort Sea region in March 1983 was analyzed to evaluate radiometric signatures of deformational features that occur in sea ice. A total of 115 fractures and 197 pressure ridges were examined with respect to physical appearance (relative age, snow cover, ice type, width, orientation) as observed in photographs, and radiometric character (brightness temperature, radiometric contrast with respect to adjacent ice, radiometric profile across the feature) as measured from digital passive microwave images. Of the deformational features that were observed in aerial photographs, 82% had radiometric signatures of sufficient contrast to be observed in passive microwave images. Fractures and ridges have equal chance of detection, but fractures cannot be distinguished from pressure ridges on the basis of brightness temperature, radiometric contrast, or characteristics of radiometric profiles measured across these features. However, visual analysis of passive microwave images, which takes into account contextual information, does allow fractures to be distinguished from pressure ridges in many instances. Radiometric signatures of both fractures and ridges are more likely to be radiometrically warmer (as opposed to cooler) than adjacent ice, which suggests that saline ice is a significant constituent of most deformational features. New ridges are more likely to be radiometrically warm than old ridges, probably because brine drains from the ridge as it ages (which reduces emissivity) and snow accumulates in drifts along the ridge trend (which enhances scattering). However, brightness temperatures of snow‐covered ridges extend across a range that is approximately 15 K cooler, and 10 K warmer than the range observed for snow‐free ridges. Old features show higher radiometric contrast with respect to adjacent ice than new features, which increases their probability of dete

ISSN:0148-0227    

DOI:10.1029/92JC02753

年代:1993

数据来源: WILEY

摘要:

A laboratory experiment was performed to study a case in which a snow cover introduced on an established saline ice sheet resulted in physical processes that significantly affected the microwave brightness temperature over a period of a few weeks. Saline ice was grown to a thickness of 240 mm in an outdoor pool at ambient air temperatures. Precipitation, was allowed by use of a movable roof. Brightness temperatures were measured at 10 and 85 GHz before and for several weeks after one snowfall. During the same period, the vertical temperature profile and crystallography of the snow column, as well as ice structure and salinity at the original ice surface, were monitored. The 10‐GHz brightness temperature dropped by as much as 100 K from bare ice values during the first few days after the snow fell, because of a saline slush layer which formed at the bottom of the snow. The saline water in the slush layer apparently was forced up through the unbroken ice by the added snow load. The slush layer eventually froze into an added highly emissive frazil ice layer which raised the 10‐GHz brightness temperature to above its bare ice values. The frazil ice layer was similar to superimposed frazil ice observed on freezing leads in high‐latitude ice packs. The 85‐GHz brightness temperature did not change from bare ice values soon after the snowfall but dropped by about 40 K over the following 20 days. We use a simple dielectric model to qualitatively test the dependence of 10‐GHz brightness temperature on relevant physical conditions at the bottom of the snow. At 85 GHz the snow layer was optically thick, and the brightness temperature drop was probably the result of increased volume scatter from the growing sn

ISSN:0148-0227    

DOI:10.1029/92JC02756

年代:1993

数据来源: WILEY

摘要:

Microwave brightness temperature measurements across C band during the growth of a saline ice sheet are presented. The data were collected with the Stepped Frequency Microwave Radiometer (SFMR) as part of a cooperative experiment at the U.S. Army Cold Regions Research and Engineering Laboratory, CRRELEX '88. Data at the six frequencies of the SFMR exhibited oscillatory behavior as the ice sheet developed, which indicates a coherent effect in the emission. Ice thickness and angle of incidence were monitored continuously, making it possible to solve for the dielectric constant of the sheet at ice thicknesses corresponding to local brightness temperature maxima and minima. The resulting values for dielectric constant are large for extremely thin ice but then decrease to approach the accepted first‐year ice value as the ice becomes optically thic

ISSN:0148-0227    

DOI:10.1029/92JC02755

年代:1993

数据来源: WILEY

摘要:

Nitrogen (nitrate and ammonium) uptake was measured in the Greenland Sea during April–May 1989 as part of the Coordinated Eastern Arctic Experiment (CEAREX). The region was dominated by a massive bloom ofPhaeocystis pouchetii, and particulate nitrogen concentrations reached 450 mmol m−2within the bloom. Particulate carbon:nitrogen ratios were similar to other polar assemblages, suggesting that the particulate matter composition was not dominated by carbon‐rich extracellular material. Vertically integratedfratios ranged from 0.09 to 0.90 and generally decreased with time, as did nitrate concentrations. Nitrate uptake rates showed no temporal trend, but ammonium uptake rates were low initially and increased greatly by the end of the study. New production rates were very high, with carbon‐based productivities calculated from nitrate uptake rates and observed C:N ratios up to 4.9 g C m−2d−1. Based on a simple mass balance of organic matter, much (∼60%) of the biogenic material produced in thePhaeocystisbloom apparently was removed from the surface layer. It is hypothesized that the formation of aggregates led to increased fluxes of particulate organic matter from the surface layer. As a result,Phaeocystisblooms may play important roles in the vertical transport of carbon and nitrogen in the

ISSN:0148-0227    

DOI:10.1029/92JC02754

年代:1993

数据来源: WILEY

摘要:

Regressional relations between sea surface temperature and NOAA satellite advanced very high resolution radiometer radiometric data, and between in situ temperature and nitrate concentrations in the surface and subsurface waters of the Ushant tidal front were significant. Nitrate images constructed from satellite‐derived temperature data agreed well with sea truth data. Nitrate transport into the euphotic zone in a spring‐neap tidal cycle, as deduced from satellite images, agreed with nitrate uptake rate and phytoplankton productivity. It is suggested that satellite‐derived nitrate images can be useful tools for assessing nitrate‐based productivity over large geographical areas in short time in

ISSN:0148-0227    

DOI:10.1029/92JC02445

年代:1993

数据来源: WILEY

摘要:

Particulate organic carbon flux from the shelf to oceanic waters during upwelling between 19°N and 25°N off Mauritania has been analyzed using coastal zone color scanner (CZCS) imagery. Biomass in the giant filament has been estimated using surface pigment concentration registered by CZCS during time series in December 1983, March 1984, and October 1984. Evidence for active phytoplankton growth in offshore waters is presented. Formation of the giant filament seems to be favored by the combination of a number of factors perhaps unique to the Mauritanian shelf. The large‐scale circulation patterns promote offshore advection, and the typically long period of wind forcing can move the upwelling center to the shelf break area, thus causing maximal phytoplankton growth rate to be achieved in oceanic waters. The relatively high pigment concentrations registered by CZCS in oceanic waters suggest that nutrients have not been depleted on the shelf and that in situ growth is occurring in deep waters. We have used a baroclinic, hydrodynamic model to estimate cross‐slope flux of particulate organic carbon at the 200‐m isobath and derive a mean value of about 1 × 105g C s−1during each of the CZCS t

ISSN:0148-0227    

DOI:10.1029/92JC01714

年代:1993

数据来源: WILEY

摘要:

The high quality of wave fields simulated by the third‐generation wave model WAM has already been demonstrated in various validation studies using in situ measurements as well as data from satellites as reference. However, owing to limitations of the reference data sets, the previous studies concentrated on relatively small regions or short time periods only, for which adequate measurements were available. In this paper the first global verification of the WAM model over a full 1‐year period is presented. The significant wave heights hindcast for 1988 by the WAM model as implemented at the European Centre for Medium Range Weather Forecasts are compared with measurements obtained by the Geosat radar altimeter. The wave heights from WAM and Geosat show good agreement in general. However, significant regional and seasonal differences are found. The underestimation of WAM wave heights in the southern hemisphere, which was already known from a validation study for the Seasat period, shows significant seasonal variations. The hindcast wave heights are underestimated by about 20% in large parts of the southern hemisphere and the tropical region during May‐;September. For the rest of the time, the agreement with Geosat data is fairly good. Together with the fact that also the rms variability of wave heights in the tropical region is clearly underestimated by WAM, this can possibly be attributed to simplifications like the neglect of atmospheric stratification effects when converting wind speeds to the wind stress fields driving WAM. Furthermore, the intercomparison indicates that low wave heights below ∼ 1.5 m are generally overestimated by WAM. As it is planned to use altimeter wave heights for updating wave models in future data assimilation systems, it is quite important to have efficient quality control criteria for these data. We show that some additional Geosat parameters, e.g., the off‐nadir angle of the altimeter, can be useful quality parameters. The difference between the Geosat and WAM wave heights shows a clear dependence on the additional parameters in some cases, which must be related to quality problems of the Geosat data. Some new criteria for the rejection of incorrect Geosat data points are

ISSN:0148-0227    

DOI:10.1029/92JC02258

年代:1993

数据来源: WILEY

摘要:

This paper presents a study of the temporal and spatial variability of the significant wave height (Hs) based on stationarity and correlation (spectral) analyses. A sea state is defined as a stationary state of a piecewise stationary stochastic random wave process. Rupture detection, i.e., detection of abrupt changes inHs, is used to identify the stationary components of the wave process. A sea state is characterized by its energy (i.e.,Hs) and by its duration of stationarity (spatial analysis) and length of stationarity (temporal analysis). Intensive in situ measurements ofHsand Geosat radar altimeter data are used to study the temporal and spatialHsscales at two locations, in the North Sea and in the equatorial Atlantic.The stationarity analysis leads to the following results; (1) duration and length show a great variability and are distributed according to exponential probability laws, (2)Hsis distributed according to a Gumbel probability law in both time and space, (3) energy and duration and energy and length can be considered as statistically independent, and (4) the duration and length distributions present very similar nondimensional statistical characteristics. Stationary state detection can also be used to filter the high‐frequency geophysical and/or instrumental noise from theHsvariations. A spectral analysis is performed on the rawHs, the stationary states, and the residual. The salient features of the results are summarized as follows: (1) for both locations, the spectra of the filtered data are consistent with a power law dependence on the wavenumber or frequency, (2) the spectral dependence is nearly the same for time and space, which suggests a mean linear dispersion relation forHs, (3) the slope of the spectra are close to the −5/3 turbulence cascade (−1.74 for the North Sea, −1.69 for the equatorial Atlantic), and (4) the residual spectrum is nearly a white noise spectrum indicating the quality of the stationary state detection fi

ISSN:0148-0227    

DOI:10.1029/92JC02625

年代:1993

数据来源: WILEY