Open Water Regions Research Articles

A Model For Polar Continental Shelf-Water Mass Modification

The process by which water flows from the surface ocean to the deep ocean is crucial in determining the structure and composition of the world ocean (Killworth, 1983). An important element in this process is the mechanism by which surface water on the Antarctic continental shelves becomes dense enough to sink to the abyssal ocean. These shelves are active sites of surface water modification because of the intense cooling and salt rejection due to sea-ice formation which occurs there during the winter. Prior models of water mass modification on the Antarctic continental shelves have focused on either the wind-forced circulation (Killworth, 1973), or the density-forced circulation (Solomon, unpublished; Killworth, 1974). Solomon (unpublished) showed that the density-forced circulation alone is too weak to produce a flux of saline water to the deep sea which was consistent with observations. Killworth (1973) showed that the wind-forced circulation alone in the Weddell Sea was incapable of producing a large enough flux. In a complementary model of only the density-forced circulation, Killworth (1974) showed that the change in salinity of water parcels caused by regional density forcing was reasonable; but as in Solomon's (unpublished) model, the flux of modified water was too small. The result of these model studies was that either circulation alone, wind-forced or density-forced, is insufficient to produce the observed flux of saline shelf water. The conjecture based on the results of these studies was that, when combined, the density forcing determined the change of salinity in the shelf water and the wind-forced circulation determined the flux of the modified water. To determine whether the combination of wind stress and density forcing is sufficient to produce a flux and salinity change consistent with observations, a simple model which responds to both forcings was developed. The model is essentially that used by Killworth (1974) with the addition of the flow forced by wind stress. A different density forcing is used because observations of the sea-ice cover (Zwally and others, 1983) suggest that the regions of coastal open water, and hence freezing, are much more localized than used in prior density-forced models. The localization of the density forcing was parameterized as being a Gaussian function of distance from and along the coast. The model (Grumbine, unpublished) predicts the velocity and salinity in two layers of equal depth in a square region with a flat bottom, subject to wind and density forcing. The east, south, and west boundaries represent coasts of an embayment analogous to the Weddell Sea continental shelf, with the continental shelf break in the middle, north-south, of the region, and an open northern boundary. The salinity is initially constant in each layer. The velocity is initialized to the equilibrium value of the wind-forced circulation. Then the salinity forcing is started and the evolution of the salinity fields is computed using the wind-forced velocity field and a diagnostically computed density-forced velocity field. The model was run for three cases: wind stress forcing only, density forcing only, and wind stress and density forcing combined. The wind-forced circulation alone was incapable of producing a sufficient degree of water mass modification. With only density forcing, the change in salinity was consistent with observations, but the flux was too small. When the two forcings are combined, both the change in salinity and the flux of modified water are consistent with observations. The model is verified against the flux and composition values from observations by Foster and Carmack (1976) in the deep Weddell Sea. The observed flux value of 1 to 2.5 × 106 m3 s−1 corresponds to water with a mean salinity of 34.7‰. This is the salinity of the water which was able to reach the abyssal ocean off the Weddell Sea coast. Figure 1 gives the modelled seasonal cycle and annual mean of the equatorward flux of water which corresponds to a mean salinity of 34.7‰. The figure shows the flux for the last three years of a five- year run computed across the position of the continental slope. The mean annual flux of this modified water is 2.0 × 106 m3 s−1, compared to the observation of 1 to 2.5 × 106 m3 s−1. These results confirm the conjecture that the combination of wind and density forcing is sufficient to produce a degree of modification and flux of modified water consistent with observations.

Distinctive markings of sharks: Ethological considerations of visual function

Behavioral reports on free-ranging sharks or those held under semifield conditions almost invariably contain a section on social activities. The frequent inclusion of such information allows the cautious suggestion that most, if not all, sharks possess the means by which requisite information is transferred between individuals so that social functions can be achieved. Such transfer could be readily accomplished through the visual modality in conjunction with appropriate optical signals, the latter being specific motor patterns and/or specific body markings. Many sharks possess body markings that likely serve as effective camouflage, particularly among those animals dwelling on or near the bottom. Distinctive markings are found, however, also on fishes that inhabit open water (pelagic) regions and it is one group of these enigmatic markings, the distinctive, fin markings of sharks within the well-known families Carcharhinidae and Sphyrnidae that forms the subject of this report. This report summarizes various functions that such markings could subserve. It focuses, then, on the more probable functions in light of correlational data relating the intensity of such markings to certain water conditions. The report concludes by considering one of the most puzzling cases of fin markings, the large white tipped “paddles” of the oceanic whitetip shark Carcharhinus longimanus. Evidence suggests that such markings are, in fact, visual lures that aid in the capture of extremely rapid moving prey.

Satellite and Oceanographic Observations Of Large Ice-Edge Eddies In The Kuril Basin Region Of The Okhotsk Sea

We examined the behavior of the sea ice in the Okhotsk Sea which formed over the deep Kuril Basin during the period 1978–82. When ice extended over the basin, we observed the formation of large eddies with diameters of order 200 km. We determined the size and duration of these eddies through use of the 37 GHz channel on the Nimbus 7 Scanning Multichannel Microwave Radiometer, and with the visible channel on the geostationary Himawari satellite. Within the ice cover, the satellite data show that these eddies produced open-water regions which persisted for 4–6 weeks, and that the eddies recurred year after year, even though their relative position changed. Comparison of eddy positions determined from satellite data with oceanographic positions shows that the oceanography drives the eddies. An estimate of heat loss from these eddies shows that the role of the ocean eddies is to keep the region ice-free until heat loss approaches zero, so that fluxes over the eddies primarily cool the water column without adding salt. Then as the atmosphere begins to warm in spring, the eddies tend to become ice-covered, so that melt water is introduced to their surface. Examination of the oceanography shows that the early summer water-column structure depends on the heat loss from the region during the preceding ice season, the amount of ice over the basin, and the total amount of ice formation in the Okhotsk Sea. During the heavy ice year of 1979, the upper 200–300 m were cooler, less saline, and highly oxygenated. This modification appears to be a local process, driven by eddy-induced mixing, local cooling, and ice melting. At 300–1200 m depths, water modification is caused by advection of water from outside the Kuril Basin. During heavy ice years with strong cooling, this water is more saline, colder, and richer in oxygen than during lighter ice years. The water modified in the basin can be traced into the North Pacific, where it cools and dilutes the surface water, and oxygenates the upper 200–400 m.

Satellite and Oceanographic Observations Of Large Ice-Edge Eddies In The Kuril Basin Region Of The Okhotsk Sea

We examined the behavior of the sea ice in the Okhotsk Sea which formed over the deep Kuril Basin during the period 1978–82. When ice extended over the basin, we observed the formation of large eddies with diameters of order 200 km. We determined the size and duration of these eddies through use of the 37 GHz channel on the Nimbus 7 Scanning Multichannel Microwave Radiometer, and with the visible channel on the geostationary Himawari satellite. Within the ice cover, the satellite data show that these eddies produced open-water regions which persisted for 4–6 weeks, and that the eddies recurred year after year, even though their relative position changed. Comparison of eddy positions determined from satellite data with oceanographic positions shows that the oceanography drives the eddies. An estimate of heat loss from these eddies shows that the role of the ocean eddies is to keep the region ice-free until heat loss approaches zero, so that fluxes over the eddies primarily cool the water column without adding salt. Then as the atmosphere begins to warm in spring, the eddies tend to become ice-covered, so that melt water is introduced to their surface. Examination of the oceanography shows that the early summer water-column structure depends on the heat loss from the region during the preceding ice season, the amount of ice over the basin, and the total amount of ice formation in the Okhotsk Sea. During the heavy ice year of 1979, the upper 200–300 m were cooler, less saline, and highly oxygenated. This modification appears to be a local process, driven by eddy-induced mixing, local cooling, and ice melting. At 300–1200 m depths, water modification is caused by advection of water from outside the Kuril Basin. During heavy ice years with strong cooling, this water is more saline, colder, and richer in oxygen than during lighter ice years. The water modified in the basin can be traced into the North Pacific, where it cools and dilutes the surface water, and oxygenates the upper 200–400 m.

Ontogenetic diet shifts and resource partitioning among piscivorous fishes in the Venezuelan ilanos

Resource utilization by nine abundant piscivores from a diverse tropical fish assemblage was examined over the course of a year. All nine species exhibited peak reproduction during the early wet season and a similar sequence of size-dependent shifts from a diet composed primarily of microcrustacea, to aquatic insects, and finally fishes. Three piranha species specialized on fish fins, particularly at subadult size classes (SL 30–80 mm). Gradual dessication of the floodplain during the transition season was associated with fish growth, increased fish density, and decreased aquatic primary productivity and availability of invertebrate prey. Based on 118 resource categories, average pairwise diet overlap was low during all three seasons: wet, transition, and dry. Of 72 species pairings, only one pair of fin-nipping piranhas exhibited high overlap simultaneously on three niche dimensions: food type, food size, and habitat. Adults of two species, a gymnotid knifefish and pimelodid catfish, were largely nocturnal. Patterns of habitat utilization indicate that piranhas may restrict diurnal use of the open-water region by other piscivores. Collective diet overlap of individual piscivore species with the other eight feeding guild members and collective overlaps with the entire fish community each revealed two basic seasonal trends. Four species that showed an early switch to piscivory also showed a high degree of diet separation with both the guild and community at large on a year-round basis. The five remaining species showed lowest collective diet overlaps during the transition season when availability of invertebrates was reduced and fish densities were maximal. Whereas predation may play a role in habitat separation, diffuse competition for food resources during the approximately four-month transition season probably is the principal factor yielding patterns of diet specialization.

Contributions of the Siberian shelf polynyas to the Arctic Ocean intermediate and deep water

The role of the open water regions or polynyas in the winter ice pack over the Siberian continental shelves in the production of Arctic Ocean Intermediate and Deep Water is examined through use of a combination of passive microwave satellite data, coastal weather station data, and oceanographic data. The satellite data allow us to derive a time series of the open water areas of selected polynyas for the 1978–1982 winters. The polynyas which occur north of Svalbard, around Franz Josef Land, along the west coast and north end of Novaya Zemlya, and to the east of Severnaya Zemlya and the Taymyr Peninsula are the primary oceanographic contributors. The open water time series for these polynyas are used in combination with coastal weather station data to estimate the heat loss from each polynya. For the region north of Svalbard, which is historically known as Whalers Bay, the study shows that the heat loss is sufficient to provide the observed cooling of the West Spitsbergen Current. For the other polynyas the heat loss is used to compute the ice production rates and the oceanic salt flux. We then use the historic oceanographic salinities with the salt fluxes to estimate the contribution of each polynya to the Arctic Ocean water masses. If we assume that the salt contributes only to the Arctic Intermediate Water, our results show that the shelves are capable of producing about 20–60% of this water. Alternatively, if the salt contributes only to the deep water of the Eurasian Basin, then without consideration of the mixing of the bottom water with the Greenland and Norwegian Sea water, the contribution from the shelves yields a renewal time of about 100 years. Our results imply that there is insufficient water produced in the shelf polynyas to perform all of the roles that have been historically assigned to it.

Airborne SAR imagery of ocean surface waves obtained during LEWEX: some initial results

The potential and the difficulties of using airborne synthetic-aperture radar (SAR) to study ocean surface waves are presented. Airborne C-band SAR imagery obtained with the real-time SAR processor on the Canada Centre for Remote Sensing (CCRS) Convair 580 during the Labrador Extreme Waves Experiment (LEWEX) shows waves propagating through the marginal ice zone as well as through open water regions. Examples are chosen to illustrate both coherent and noncoherent scene motion effects. Coherent scene motion effects include velocity bunching. acceleration defocus, and coherence time limitations. Noncoherent scene motion effects include scanning distortion and look misregistration. The slow platform velocity of airborne SAR precludes replication of spaceborne observations, largely due to coherent motion effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Interpretation of Seasat radar-altimeter data over sea ice using near-simultaneous SAR imagery

Abstract The backscatter properties of Seasat altimeter data in the Beaufort Sea on the 3 October 1978 show distinct zones, which arc interpreted in terms of geophysical characteristics. An overlapping and near-simultaneous synthetic-aperture radar image shows regions of open water, new ice and multi-year sea ice, which correspond to the different zones. It is found that the altimeter signal is sensitive to the ocean-ice boundary and that it indicates the ice type. The pulse-echo waveforms also suggest that several scattering components are present in the returned power over sea ice.

Diurnal distribution and behavioral responses of fishes to extreme hypoxia in an Amazon floodplain lake

Because of the need for surface access for aquatic surface respiration (ASR), fish density increases were demonstrated for the open water of a floodplain lake during severe hypoxia. This indicates an O2-induced diurnal pattern of horizontal migrations between the zone of macrophyte cover and open water. Supplemental experimental investigations seem to suggest that species such as characoids,Colossoma macropomum andSchizodon fasciatum, deviate from this pattern. During long periods of oxygen depletion, they return to the region of macrophyte growth and survive there without displaying the usual kind of ASR. Mortality studies in net cages exposed in natural water bodies confirmed that only these two species are able to survive severe hypoxia beneath macrophyte cover. The possibility of an O2-input through the root system of plants is discussed. The O2-concentration has a significant influence on the locomotory behavior and the frequency of opercular movement in characoids. There is significantly less locomotory activity beneath the macrophytes during periods of oxygen depletion among those species not forced to migrate than among those in the open water regions, where normal ASR behavior is possible.

Aircraft and Satellite Passive Microwave Observations of the Bering Sea Ice Cover During MIZEX West

Passive microwave measurements of the Bering Sea ice cover were made with the NASA CV-990 airborne laboratory and with the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) during February 1983 as part of the Bering Sea marginal zone experiment (MIZEX West). Microwave data obtained with the CV-990 include radiances from both imaging and dual-polarized fixed-beam radiometers spanning a range of frequencies from 10 to 183 GHz. The imagery at 19 and 92 GHz provide a description of the marginal ice zone delineating regions of open water, ice compactness, and ice-edge structure. Comparison of these images with ice concentration maps derived from the SMMR data shows that the maps provide an accurate description of the ice edge and interior pack polynyas. Analysis of the fixed-beam data shows that spectral differences increase with a decrease in ice thickness and the polarization at both 18 and 37 GHz distinguishes among new, young, and first-year sea ice types. Finally, these results are shown to agree with surface-based radiometric measurements.

Nimbus 7 SMMR observations of the Bering Sea ice cover during March 1979

A method of deriving sea ice concentrations from the 0.81‐cm wavelength radiances of the Nimbus 7 scanning multichannel microwave radiometer (SMMR) is described and used in a comparative study of the Bering Sea ice cover during March 1979. Case studies during 2 days in March demonstrate the utility of the SMMR for studying marginal ice zone processes. The first day provides an example of ice divergent conditions with winds from the northeast advecting the ice southwest toward the ice edge. Comparison of the SMMR‐derived ice concentration maps with surface, aircraft, and Tiros‐derived ice charts shows that the microwave data give the ice edge position to within 30 km, corresponding to the highest spatial resolution of SMMR. The, SMMR ice concentrations also give an accurate description of the general morphology of the ice cover including the presence of lee shore polynyas. The second day illustrates the effect of a storm passage on the ice cover. The SMMR ice map for this day correctly describes the retreat of the ice cover and gives the positions of local regions of open water and of high ice concentration within the pack. The two examples show that for the Bering Sea the SMMR algorithm provides accurate large‐scale to mesoscale data on both the ice edge position and the location of low and high ice concentration regions.

On the Development and Cause of the Weddell Polynya in a Sea Ice Simulation

Abstract Occurrence of the Weddell polynya, a large open-water area in the midst of the wintertime pack ice of the southern ocean, is examined through a sequence of numerical simulations of the sea ice cover. Using mean climatological input data, the numerical model realistically simulates the emergence of the polynya through the encircling of an open water region by ice. In addition, with only the wind fields altered, the model successfully simulates winter seasons totally lacking a Weddell polynya. Since the vertical ocean heat flux inserted into the model is spatially-invariant, these results call into question—without actually refuting—those explanations of the polynya based exclusively on oceanographic factors. On the other hand, the failure of the modeled polynya to survive the winter season without significant weakening, as the observed polynya did in each of the years 1974–76, suggests that the oceanography and/or the ocean/atmosphere feedbacks are essential to the polynya maintenance, even if not...

Aerobic mineralization of organic matter in the open-water region of Lake Vechten

Aerobic mineralization of organic matter in the open-water region of Lake Vechten

Diel Foraging Behavior and Prey Selection in the Golden Shiner (Notemigonus crysoleucas)

Diel migrations of the golden shiner (Notemigonus crysoleucas) from the littoral to limnetic zone of a small Michigan lake were documented through visual observations and gill netting. During the day golden shiners schooled in the littoral zone. Just after sunset schools broke up and the golden shiner migrated to the open water regions of the lake. Feeding by the golden shiner was largely coincident with the evening migration. The majority of the shiner population fed on Daphnia and most prey were obtained within 1 h of sunset. Feeding was minimal through the night with some feeding evident at dawn. Daphnia underwent a strong diel vertical migration, especially larger individuals, and showed a strong horizontal gradient of decreasing size and density from the limnetic into the littoral region. Size of Daphnia eaten by the shiner increased dramatically across the evening feeding period. This increase is due to the horizontal gradient in Daphnia and availability of larger Daphnia as they migrate to the surface. On only one date when prey densities were highest did the shiners actively select large Daphnia. The golden shiner faces a dynamic prey system both in time and space which influences prey "selection." We relate adaptations of planktivorous fishes that migrate to relative abundances of fishes in small lakes. Key words: coevolution, Daphnia, diel migration, fish, foraging behavior, golden shiner, Notemigonus, planktivore, predator–prey

Energy exchange over young sea ice in the central Arctic

A simple model of heat transport through young sea ice is combined with climatological data on air temperatures and incoming radiation in the central Arctic to predict how each component of the surface heat balance is affected by changes in ice thickness. Results indicate that during the cold months the net heat input to the atmosphere from ice in the 0‐ to 0.4‐m range is between 1 and 2 orders of magnitude larger than that from perennial ice. Once the ice exceeds a meter in thickness, there is little change in any of the heat fluxes as the ice thickens. Although both the amount of absorbed shortwave radiation and the emitted longwave radiation depend on ice thickness, it is the turbulent fluxes which undergo the largest changes. The rate of heat exchange over thin ice is shown to be extremely sensitive to snow depth and assumed boundary layer temperatures. It is concluded that with the present ice thickness distribution in the central Arctic, total heat input to the atmospheric boundary layer from regions of young ice is equal to or greater than that from regions of open water or thick ice.

Seasonal Change of Antarctic Sea Ice Cover

The winter expansion of the sea ice surrounding Antarctica and the subsequent retreat of the ice in summer may be linked with the wind stress acting on the Southern Ocean in conjunction with the heat exchange in open water regions within the ice fields.

POLLEN AND DIATOM ANALYSES OF LATE‐QUATERNARY DEPOSITS IN THE BLELHAM BASIN, NORTH LANCASHIRE

SummaryInvestigations have been made of Late‐Weichselian and Flandrian deposits in Blelham Tarn, Lancashire and other parts of its basin. Pollen and diatom analyses have been made of several cores and samples taken from the inflows. Pollen analysis has been used as a basis for chronology. The vegetational history of the region and past diatom flora of the tarn and ‘kettle‐hole’ are discussed. During the Late‐Weichselian, in pollen zones I and III, the vegetation was dominated by herbs of open habitats, whilst in pollen zone II birch woodland developed. The diatom flora consisted mainly of non‐planktonic forms with Fragilaria spp. dominant. In the Early‐Flandrian (pollen zones IV–VI) birch woodland developed and was succeeded by a mixed oak forest. Differences are shown in the pollen spectra from the ‘kettle‐hole’, marginal and open water regions of the tarn. The planktonic/non‐planktonic (P/NP) diatom ratio slowly increased in the tarn. However, dominance was retained by the non‐planktonic forms. This increase in P/NP ratio did not occur in the ‘kettle‐hole’ region. A new diatom variety Melosira distans var. blelhamensis appeared in the flora. At the opening of pollen zone VIIa Alnus rapidly increased, and for the first time planktonic diatoms became dominant in the open waters of the tarn. In littoral regions non‐planktonic forms, in particular Achnanthes minutissima var. cryptocephala, remained dominant. Sphagnum‐sedge peat accumulated in the ‘kettle‐hole’. Diatoms are scarce in this peat. During the Late‐Flandrian (pollen zone VIIb to the present day) the local vegetation was an older mixed oak forest. However, more open vegetational conditions gradually developed as shown by the increase in herbaceous pollen. Dominance was held in the open waters by the planktonic diatoms. The littoral regions favoured the growth of Achnanthes minutissima var. cryptocephala. Sphagnum‐sedge peat continued to accumulate in the ‘kettle‐hole’. Other limnological aspects considered include the tarn's trophic condition, productivity, alkalinity, the transparency of its waters and tarn level changes. In the past the tarn covered a much wider area. Field evidence shows that man has interfered with the drainage system.