Numerical Experiments of Isolated Convection under Polynya

The dense water formation process under polynya or lead is examined by numerical experiments using a three-dimensional non-hydrostatic model. Many numerical experiments on isolated convection in an initially homogeneous fluid have been performed for different sets of external parameters, in order to investigate a relationship between the convection process and the external parameters. The main focus is on the situation in which the horizontal length scale of disk-shaped buoyancy forcing (radius R) is comparable with the total water depth (H). The two dynamical regimes described in previous work—the baroclinically unstable convection and the baroclinically stable convection—are confirmed in the experiments. A horizontal shift of a convective chimney is important to a density anomaly in baroclinically stable convection. For the stable range, as R is reduced R/H < 0.7, a new regime is found, called “single-plume convection”, in which multiple convective plumes do not fully develop, and the density anomaly scale has nearly no dependency on R. This change of dependency on R is consistent with that derived by scaling analysis. The non-hydrostatic component is more significant than the hydrostatic one in the single-plume convection. The information obtained is useful for parameterizing dense water formation under ice cover in a numerical model with a large grid size; i.e., the newly formed water has a density anomaly independent of the polynya size smaller than the water depth, while the anomaly increases as the size exceeds the depth.     

南极威德尔海冰间湖形成机制的研究进展

针对南极威德尔海冰间湖的特点及影响，本文介绍了有关威德尔海冰间湖形成研究的进展情况，并对存在的问题以及未来的发展进行了分析和阐述。结果说明：动力因素、热力因素和热动力因素都只是威德尔海冰间湖形成和维持的某一方面的控制和影响因子。对威德尔海冰间湖更深入的研究，应该充分考虑较大尺度的海洋-海冰-大气相互作用。     

Dynamics and Variability of Terra Nova Bay Polynya

Abstract. We present a process study on the dynamics and variability of the Terra Nova Bay polynya in the western sector of the Ross Sea. The air-sea heat exchange is known to be particularly large in polynya during the winter, when differences between air and sea temperatures are large. We apply a 1-D model (Pease, 1987; Van Woert, 1999a, 1999b), which is modified in the latent heat parameterisation in order to account for time-dependent relative humidity and cloud coverage. Furthermore, the Ice Collection Depth is correlated linearly with a variable wind speed. The model is forced with two different meteorological data sets: the operational analysis of the European Center for Medium Range Weather Forecasts atmospheric data set and the meteorological parameters measured by an Automatic Weather Station located on the coast of Terra Nova Bay. The results are compared in terms of polynya extension, ice, and High Salinity Shelf Water production. According to the two different wind velocities, the results obtained from the different data sets clearly differ. Qualitatively, however, the results are in good agreement.     

Trophic structure and pathways of biogenic carbon flow in the eastern North Water Polynya

In the eastern North Water, most of the estimated annual new and net production of carbon (C) occurred during the main diatom bloom in 1998. During the bloom, at least 30% of total and new phytoplankton production occurred as dissolved organic carbon (DOC) and was unavailable for short-term assimilation into the herbivorous food web or sinking export. Based on particle interceptor traps and ²³⁴Th deficits, 27% of the particulate primary production (PP) sank out of the upper 50 m, with only 7% and 1% of PP reaching the benthos at shallow (≈200 m) and deep (≈500 m) sites, respectively. Mass balance calculations and grazing estimates agree that ≈79% of PP was ingested by pelagic consumers between April and July. During this period, the vertical flux of biogenic silica (BioSi) at 50 m was equivalent to the total BioSi produced, indicating that all of the diatom production was removed from the euphotic zone as intact cells (direct sinking) or empty frustules (grazing or lysis). The estimated flux of empty frustules was consistent with rates of herbivory by the large, dominant copepods and appendicularians during incubations. Since the carbon demand of the dominant planktivorous bird, Alle alle, amounted to ≈2% of the biomass synthesized by its main prey, the large copepod Calanus hyperboreus, most of the secondary carbon production was available to pelagic carnivores. Stable isotopes indicated that the biomass of predatory amphipods, polar cod and marine mammals was derived from these herbivores, but corresponding carbon fluxes were not quantified. Our analysis shows that a large fraction of PP in the eastern North Water was ingested by consumers in the upper 50 m, leading to substantial carbon respiration and DOC accumulation in surface waters. An increasingly early and prolonged opening of the Artic Ocean is likely to promote the productivity of the herbivorous food web, but not the short-term efficiency of the particulate, biological CO₂ pump.     

Formation mechanism of the Weddell Sea Polynya and the impact on the global abyssal ocean

An experiment using a global ocean–ice model with an interannual forcing data set was conducted to understand the variability in the Southern Ocean. A winter-persisting polynya in the Weddell Sea (the Weddell Polynya, WP) was simulated. The process of WP breaking out after no-WP years was explored using the successive WPs found in the late 1950s. The results suggested that the anomalously warm deep water, saline surface layer, and a cyclonic wind stress over the Maud polynya region in early winter are essential for the surface layer to be dense enough to trigger deep convections which maintain a winter-persisting polynya; also, the reanalyzed surface air temperature (SAT) over the observed polynya region is too high for an ocean–ice model’s bulk formula to yield sufficient upward heat fluxes to induce WP formation. Therefore the Weddell Polynya, a series of WPs observed from satellite in the mid-1970s, is reproduced by replacing the SAT with a climatological one. Subsequent to the successive WP events, density anomalies excited in the Weddell Sea propagate northward in the Atlantic deep basins. The Antarctic Circumpolar Current (ACC) is enhanced through the increased meridional density gradient. The enhanced ACC and its meandering over the abyssal ridges excite buoyancy anomalies near the bottom at the southwestern end of the South Pacific basin. The buoyancy signals propagate northward and eventually arrive in the northern North Pacific.     

Trapped bottom topographic waves over the inclined bottom

In the Boussinesq approximation, we study baroclinic topographic waves trapped by the flat meridional slope. The existence of these waves is explained by stratification, inclined bottom, and Earth's rotation. We deduce the evolutionary equation for the square of the envelope of a narrow-band wave packet of trapped waves. In the second order of smallness relative to the wave amplitude, we find the mean fields of velocity and density induced by the packet. It is shown that, in the limiting case of weakly nonlinear plane waves, the induced current is zonal. In the Northern hemisphere, depending on the slope of the bottom γ₁, the sign of the phase velocity σ/k (k is the zonal wave number) is either always positive (for γ₁>γ_1cr) or always negative (for γ₁<γ_1cr). If we neglect the vertical component of the Coriolis acceleration, then γ_1cr=0. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

南极麦肯齐湾冰间湖的时空变化及主要影响因素分析

利用2003—2009年AMSR-E日平均海冰密集度数据,对南极普里兹湾埃默里冰架前缘中西部的麦肯齐湾冰间湖进行了分析。针对冰架前缘冰间湖的特点,本文在阈值法和连通域法的基础上,提出了生长点法作为识别此类冰间湖的方法。研究发现,该冰间湖的开始时间为每年的3月中下旬,结束时间为每年的10月末到11月初,平均出现天数为226d。冰间湖的面积每天都发生变化,表现出天气尺度的变化特征。全年累计的冰间湖面积平均为(8.33±1.55)×105 km2。冰间湖最大面积为1.69×104 km2,出现在2004年。结合NCEP再分析数据中的日平均风速资料的分析发现,在6～8月,冰间湖的天气尺度变化主要是受风场的影响,冰间湖面积与离岸风速有很好的相关性。     

Acoustic inversion of bottom reflectivity and bottom sound-speedprofile

This paper applies a full-field technique to invert bottom sound profile and bottom reflectivity from simulated acoustic data in a shallow water environment. Bottom sound-speed profile and bottom reflectivity have been traditionally estimated using seismic reflection/refraction techniques when acoustic ray paths and travel time can be identified and measured from the data. However, in shallow water, the many multipaths due to bottom reflection/refraction make such identification and measurement rather difficult. A full-field inversion technique is presented here that uses a broad-band source and a vertical array for bottom sound-speed and reflectivity inversion. The technique is a modified matched field inversion technique referred to as matched beam processing. Matched beam processing uses conventional beamforming processing to transform the field data into the beam domain and correlate that with the replica field also in the beam domain. This allows the analysis to track the acoustic field as a function of incident/reflected angle and minimize contamination or mismatch due to sidelobe leakage     

Coupling of ocean bottom seismometers to soft bottom

Unlike response of seismometers resting on hard rock where the seismometer case moves with the rock to high frequencies, the response of ocean bottom seismometers (OBS) can be strongly affected by the low mechanical strength of ocean sediments. The motion as measured by the seismometer will not follow the expected relationships between pressure and particle motion for different wave types. Cross coupling between horizontal and vertical motions can occur, especially when there is differential motion between water and sediment. Resonant amplification and attenuation of higher frequencies also occur. Secondary seismic arrivals are especially subject to distortion. Overall response is strongly dependent upon the mass and configuration of the OBS and the rigidity and density of the bottom material. Tests at Lopez Island, Puget Sound using both directly applied mechanical transients and seismic signals with various instrument configurations demonstrate the above effects and provide some guidance for improved designs.Hawaii Institute of Geophysics Contribution No. 1172.     

Turbid bottom water layer and bottom sediment in the Seto Inland Sea

Measurement of the vertical distribution of total suspended matter (TSM) was carried out during summer throughout the Seto Inland Sea. TSM concentration near the bottom is influenced significantly by water movement and turbid bottom water is observed in all areas where median grain size (Md) of the bottom sediment is more than 47gf. The high concentration of TSM near the bottom may be due to resuspension of the surface layer of bottom sediments. Comparison of the organic content of the resuspended matter with that of the bottom sediment shows that the resuspended matter contains more organic matter with a lower C : N ratio than the bottom sediment. The C : N ratio of the resuspended matter is similar to that of TSM in the surface layer of the water column. It is thought that TSM in surface waters sinks and settles on the surface of the bottom sediment. This deposited material is then easily resuspended in the water column by tidal currents before becoming permanently incorporated into the bottom sediment.     

Acoustic stereo images of the bottom surface

In order to enhance the efficiency of the interpretation of surface images obtained with a side scanning sonar, it is proposed to supplement the standard processing software with a program for obtaining acoustic stereo images. Examples of such images synthesized with this program using the data of a bottom sonar survey with a side scanning sonar and an echo sounder are presented. The cases are considered when the information on the bottom relief contained in sonar images obtained with a standard side scanning sonar or its modifications can be used instead of the data of an echo sounding survey.     

The IO RAN lander: A bottom station for studying the chemical exchange at the sea bottom

The design and application of a lander (an apparatus developed at the Institute of Oceanology of the Russian Academy of Sciences (IO RAN) to study the chemical processes in the near-bottom layer) are described. The methodological foundations and design peculiarities of the lander were developed in the course of chamber experiments carried out at shallow depths with the assistance of divers. The transition to greater depths required development of a multifunctional automatic device capable of independently performing long-term surveys in the near-bottom layer, including those of the chemical exchange at the water-bottom interface.     

Polarography of the bottom sediments in the Sevastopol Bay

By the method of polarographic profiling performed with the help of an Au-Hg glass microelectrode, we obtained the first high-resolution vertical profiles of the distributions of oxygen, sulfides, oxidized and reduced forms of iron, reduced manganese, and iron monosulfide in pore waters of the bottom sediments in the Sevastopol Bay. It is shown that the regional features of the vertical distributions of the main polarographically active compounds are determined by the combination of several factors: the contents of organic carbon and iron and the sizes of particles of the sediments.     

Application of stereograms to visualize the bottom relief

The paper analyzes the bottom stereoscopic imaging based on the interferometric side-scan sonar data, which provides for the more efficient identification of the bottom structures owing to the perception’s extensionality relative to the three-dimensional models. The stereopair construction enabling the three-dimensional perception of the formed images is discussed.     

Spring–summer cycling of DOC, DON and inorganic N in a highly seasonal system encompassing the Northeast Water Polynya, 1993

Dissolved organic nitrogen (DON), dissolved organic carbon (DOC) and inorganic nutrient concentrations were determined in samples from an area encompassing the Northeast Water Polynya from June to August 1993. In June, still ice-covered polynya area surface waters (PySW) had significantly higher (p<0.05) DOC concentrations (110 μM, n=68) than surface water outside the polynya area (96 μM, n=6). Melting ice and ice algae are suggested as DOC sources. DOC concentrations found in this study are consistent with other studies showing higher DOC concentrations in the Arctic than in other ocean areas. As the productive season progressed, DOC concentrations in Polynya surface water (PySW) decreased (p<0.05) from 110 to 105 μM, while DON concentrations increased (p<0.05) from 5.6 to 6.1 μM, causing a significant decrease (p<0.05) in the C : N ratios of DOM from spring (C : N ratio 20) to summer (C : N ratio 17). We found a significant (p<0.05) decrease in the DOM C : N ratio in all water masses within the polynya area as the productive season progressed. DON was the largest fraction of total dissolved nitrogen (TDN) in PySW and surface waters outside the polynya area. TDN was calculated as the sum of DON, nitrate, nitrite and ammonium concentrations. DON increased (p<0.05) from 62% to 73% of TDN in PySW from spring to summer, a result of increasing DON concentrations and decreasing inorganic nitrogen concentrations over the productive season. The seasonal accumulation of DON and the corresponding decrease in nitrate concentrations in waters with primary production indicate that it is important to take the DON pool into account when estimating export production from nitrate concentration decreases in surface waters. PySW TDN concentrations decreased (p<0.05) from 9.1 (n=61) to 8.6 μM (n=60) from spring (May 25 through June 19) to summer (July 1 through July 27). The seasonal decrease in surface water TDN concentrations corresponded to increases in TDN concentrations in deeper water masses within the Polynya. Most of the TDN increase in deep water was in the form of DON. A possible explanation is that PON was dissolved (partially remineralized) in the water column at mid depths, causing increases in the DON concentration. Transfer of N from PySW (with a short residence time in the polynya area) to Polynya Intermediate Water and deep waters of the Norske and Westwind Trough with multi-year residence times keeps N from leaving the polynya area. In spring, nutrients from degradation of OM in PyIW could support primary production. The role of PyIW as an OM trap could be important in supporting primary production in the polynya area.     

On the role of bottom roughness in overflows

Overflows play an important role in the downwelling limb of the oceanic thermohaline circulation. In light of the recognition that some overflows are not homogenous but exhibit a vertical density structure, and details of topography influence the pathways of some overflows, the impact of topographic roughness on the product property distribution is explored using the 3D non-hydrostatic spectral element model Nek5000. Numerical experiments are carried out by varying bottom roughness amplitude and ambient stratification parameters, in a regime where equilibrated product water masses are formed in a non-rotating environment.Our main finding is that bottom roughness can influence the overflow product distribution such that the highest salinity classes are removed and neutral buoyancy level is attained higher up in the stratified ambient water column. It is also shown that the form drag coefficients in overflows over rough bottom can be much larger than the skin drag coefficient over smooth bottom. To our knowledge, form drag has never been measured in oceanic overflows. As such, these numerical experiments imply that such measurements would be useful for a better understanding of overflow dynamics. It is also found that the ratio of source and product overflow mass transports is robust to changes in bottom roughness. This appears to happen because the distribution of entrainment is totally different in the case of rough bottom. Entrainment tends to initiate earlier (due to vertical motion induced by topography) and terminate earlier (due to development of form drag) than that over smooth topography.     

Wave-induced bottom currents on the outer shelf

Estimates of the wave-induced oscillatory bottom currents available to move sediment on the continental shelf have been made using the Pierson—Moskowitz wave spectrum; the oscillations are substantially stronger than had previously been thought. Near a shelf edge with canyons the wave-induced currents will be locally enhanced by wave refraction by about four times under certain conditions for idealized topography. Local sediment-transport rates will be greater and will occur more frequently than had previously been supposed.     

Spring distribution of dissolved organic matter in a system encompassing the Northeast Water Polynya: Implications for early-season sources and sinks

This study addresses sources and diagenetic state of early-season dissolved organic matter (DOM) in the Northeast Water Polynya (NEWP) area northeast of Greenland from distributions of humic substance fluorescence (HSfl), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) in the water column inside and outside the NEWP area. The water masses of the polynya area had acquired their spring/summer temperature–salinity characteristics at the time of sampling, and also had individual, different DOM signatures. DOC concentrations were variable within and among water masses in the polynya area, indicating patchy local sources and sinks of DOC. PySW and polynya intermediate water (PyIW) had higher average DON concentrations and average lower C:N ratios than polynya bottom water (PyBW), indicating a larger fraction of fresh DOM in PySW and PyIW than in PyBW. Ice-covered, polynya area surface waters (PySW) had higher DOC concentrations (113±14 μM, n=68) than surface water (SW) outside the polynya area (96±18 μM, n=6). The DOM C:N ratios in a low-salinity, ice-melt subgroup of PySW samples indicate labile material, and these low-salinity surface waters appeared to have a local DOC and DON source. In contrast, HSfl was significantly lower inside than outside the NEWP area. Despite the lower HSfl values within the NEWP area, the PySW values were high when compared to open-ocean water. There were no local terrestrial sources for HSfl to the NEWP area and the East Greenland Current is therefore proposed as a likely source of allochtonous HSfl. When HSfl was used as a conservative tracer, up to 70% of the water in PySW and PyIW was found to be derived from SW, which contains a high fraction of water from the East Greenland Current. Similarly, a mixing model based on HSfl indicated that 80% of early-season DOC and 90–100% of early-season DON in PySW and PyIW were derived from SW, indicating a potentially high fraction of terrestrially-derived, relatively refractory DOM in the early-season NEWP area.     

Observation of the bottom boundary layer on the continental shelf

Current meter data from various depths near the sea bottom collected for 31 days at time intervals of 10 minutes using a subsurface buoy system at a depth at 38 m on the continental shelf off Akita, Japan have been analyzed. The results show the existence of a stationary Ekman layer. The typical range of the characteristic parameters are estimated as follows; friction velocity: 0.38 cm s^–1; Ekman layer thickness: 16 m; logarithmic layer thickness: 4 m–6 m; constant flux layer thickness: 0.4–0.6 m; Ekman veering: 28.7°; drag coefficient: 0.24×10^–2–0.53×10^–2. Veering was also observed in the logarithmic layer.     

Thickness of the mixed bottom layer in the Northern Atlantic

The thickness of the mixed bottom boundary layer (BBL) has been analyzed based on the CTD data at transoceanic sections in abyssal waters of the Northern Atlantic. The measurements were carried out at two transoceanic sections approximately along 48° N (ASV-99) and 5° N (AI-2000) in 1999 and 2000. These data, and the WOCE data obtained at four zonal sections (AR7E and AR12 along 57° N, AR01 along 24.5° S, and A06 along 7.5° N), were used for the calculation of the statistical characteristics of the BBL??s thickness H _B . The probability distribution function F(H _B ) was close to lognormal. The mean value ??H _B ?? at different latitudes was in the range from 30 to 60 m. The averaged BBL thickness = 46.1 m. The BBL??s thickness was about 1% of the ocean??s depth D; the ratio H _B /D was the minimum (0.8%) near the equator and increased up to 1.6% in the polar latitudes.