On the background level of Tc, Sr and Cs in the North Atlantic

Mean concentrations of the anthropogenic radioactive oceanographic tracers ⁹⁹Tc, ⁹⁰Sr and ¹³⁷Cs have been measured as 0.005, 1.6 and 2.5 Bq m⁻³ in oceanic Northeast Atlantic surface water, east and northeast of the Azores, in 1992. This is, apparently, the first published value for fallout “background” ⁹⁹Tc in oceanic Atlantic water.Comparison with older data indicates an observed half life for ⁹⁰Sr and ¹³⁷Cs in the northeast Atlantic surface water of 20 yr corresponding to a mean residence time of 80–100 yr for the stable elements.The observed ⁹⁹Tc/⁹⁰Sr ratio (3 × 10⁻³) in the Azores samples is 10 times higher than the theoretical fission yield decay corrected to 1992. This is in agreement with published data on rain water samples and may be characteristic for 1960's global fallout. Furthermore, the measured ¹³⁷Cs/⁹⁰Sr ratio is not significantly different from that observed for global fallout. There do not appear to be any additional significant sources of artificial radionuclides in this region.     

Nutrient cycling and primary production in the marine systems of the Arctic and Antarctic

Primary production events in both the Arctic and the Antarctic are highly localized. Carbon-14 incubations that did not account for this caused antarctic primary production estimates to be revised too far downwards from the historic view of high productivity. The primary production regime in the Arctic is even more heterogeneous than in the Antarctic. Arctic primary production rates are in the process of being revised upwards because of a better spatial and temporal distribution of incubation experiments and a re-awakening of interest in estimating new production from the distribution of chemical variables. Similarly, recent examination of temporal changes in nitrate concentrations and recognition of the importance of ice-edge blooms has caused antarctic primary productivity to be revised upwards. In both the Arctic and the Antarctic, the ratio of “new” to total primary production is high, and neglect of this fact can lead to an underestimation of the potential that these regions have for influencing global cycles of bioactive chemicals. Some recent data on temporal changes in nitrate from Fram Strait emphasize the poor state of our knowledge by suggesting an unexpectedly high “new” production rate of 1 g C m⁻² d⁻¹ for a 35 day experiment that encountered an early Phaeocystis bloom. Chemical distributions suggest that new production over the shelf seas that border the Polar Basin is about 50 g Cm⁻² yr⁻¹.The shelves in the Arctic Ocean's marginal and adjacent seas comprise 25% of the total global continental shelf. These extensive shallow regions have much higher rates of primary production than the Polar Basin and may be globally significant sites of denitrification. Globally significant silica deposition could occur on these shelves or on the adjacent slopes.Because of the differences in geomorphology and stratification, global warming is likely to increase primary production in the Arctic and will probably decrease antarctic primary production.In addition to sharing high ratios of “new” to total primary production, high ammonium concentrations occur in the Arctic and Antarctic. It is possible that these accumulations arise from a strong repression of nitrification at low temperatures.     

Biological effects of dilution of Dead Sea brine with seawater: implications for the planning of the Red Sea–Dead Sea “Peace Conduit”

The Dead Sea is a severely disturbed ecosystem. Its water level has been decreasing at a rate of nearly 1 m per year during the last decade due to anthropogenic intervention in its water balance. Since the peace treaty between Israel and Jordan was established in 1994, a proposal for the construction of a water carrier, the “Peace Conduit,” between the Gulf of Aqaba (Red Sea) and the Dead Sea is being investigated. This water carrier is intended to mitigate damaging processes that currently occur in the Dead Sea and its surrounding area. The difference in elevation of about 416 m could be exploited for seawater desalination by reverse osmosis. To examine the possible effects of the mixing of Dead Sea brines with seawater and seawater concentrates on the Dead Sea as an ecosystem, we have set up simulation experiments under field conditions in experimental ponds at Sedom, in which Dead Sea water was diluted with Red Sea water. The main components of the Dead Sea biota are the unicellular green alga Dunaliella and several types of red halophilic Archaea. Phosphate is the limiting inorganic nutrient. Massive Dunaliella blooms developed, accompanied by dense communities of red halophilic Archaea, in some of the experimental ponds, imparting a brown-red coloration to the brines. The extent of biological development depended on the extent of dilution and on phosphate availability. The results of the simulation experiments show that biological phenomena and their impact on the Dead Sea ecosystem should be taken into consideration during the planning of the “Peace Conduit.”     

Asymmetry of Columbia River tidal plume fronts

Columbia River tidal plume dynamics can be explained in terms of two asymmetries related to plume-front depth and internal wave generation. These asymmetries may be an important factor contributing to the observed greater primary productivity and phytoplankton standing crop on the Washington shelf. The tidal plume (the most recent ebb outflow from the estuary) is initially supercritical with respect to the frontal internal Froude number F_R on strong ebbs. It is separated from the rotating plume bulge by a front, whose properties are very different under upwelling vs. downwelling conditions. Under summer upwelling conditions, tidal plume fronts are sharp and narrow (< 20–50 m wide) on their upwind or northern side and mark a transition from supercritical to subcritical flow for up to 12 h after high water. Such sharp fronts are a source of turbulent mixing, despite the strong stratification. Because the tidal plume may overlie newly upwelled waters, these fronts can mix nutrients into the plume. Symmetry would suggest that there should be a sharp front south of the estuary mouth under summer downwelling conditions. Instead, the downwelling tidal plume front is usually diffuse on its upstream side. Mixing is weaker, and the water masses immediately below are low in nutrients. There is also an upwelling–downwelling asymmetry in internal wave generation. During upwelling and weak wind conditions, plume fronts often generate trains of non-linear internal waves as they transition from a supercritical to a subcritical state. Under downwelling conditions, internal wave release is less common and the waves are less energetic. Furthermore, regardless of wind conditions, solition formation almost always begins on the south side of the plume so that the front “unzips” from south to north. This distinction is important, because these internal waves contribute to vertical mixing in the plume bulge and transport low-salinity water across the tidal plume into the plume bulge.F_R and plume depth are key parameters in distinguishing the upwelling and downwelling situations, and these two asymmetries can be explained in terms of potential vorticity conservation. The divergence of the tidal outflow after it leaves the estuary embeds relative vorticity in the emerging tidal plume water mass. This vorticity controls the transition of the tidal plume front to a subcritical state and consequently the timing and location of internal wave generation by plume fronts.     

On the parameterization of interleaving and turbulent mixing using CTD data from the Azores Frontal Zone

CTD-data obtained in the Azores Frontal Zone using a towed undulating vehicle are analyzed to study the relationship between characteristics of intrusions and mean parameters of the thermohaline field. A self-similar dependence between intrusion intensity and hydrological parameters is obtained. The most well-founded interpretation of the empirical dependence is as follows: (a) the main source supporting intrusive layering is the salt finger convection; (b) the abrupt decrease of intrusion intensity with the reduction of geostrophic Richardson number obtained from the analysis is explained by the beginning of turbulence when salt fingers do not work any longer, so the “driving force” for intrusive motion disappears. These results are consistent with the conclusions of the paper [Kuzmina N.P., Rodionov V.B., 1992. About the influence of baroclinicity upon generation of the thermohaline intrusions in the oceanic frontal zones. Izvestiya Akad. Nauk SSSR, Atmosperic and Oceanic Physics 28 (10–11), 1077–1086]. These conclusions imply that there are three main mechanisms of intrusive layering at oceanic fronts, namely the 2D baroclinic instability of geostrophic flow, the vertical shear instability and the thermohaline instability where the driving source of intrusive motion is double diffusive convection. The baroclinic and thermohaline instabilities can generate intrusions of large vertical scale, while vertical shear instability usually gives rise to thin turbulent layers. Turbulence in these thin layers can prevent salt finger convection and thus destroy the energy source of the intrusive motion conditioned by thermoclinicity. Therefore, the baroclinicity plays two parts in the processes of the intrusive layering: (1) it prevents double-diffusion interleaving by means of turbulence, and (2) it generates intrusions due to the 2D baroclinic instability of geostrophic current. Using features of thermohaline interleaving as a specific tracer of turbulent mixing, we have estimated turbulent mixing coefficient as k_tRi^−0.8 (Ri>1), where Ri is the geostrophic Richardson number. Application of the proposed approach to other frontal zones is discussed.     

The circulation of Levantine intermediate water in the northeastern Ionian sea (late winter/early spring 1986)

CTD data, collected in the northeastern Ionian Sea during late winter/early spring 1986, are used to examine the spreading of the Levantine Intermediate Water (LIW) on selected isopycnal surfaces. The distribution of salinity on these surfaces provides the qualitative aspects of the flow field. Charts of the spreading of LIW, based on θ-S analysis, emphasize the diffusive nature of the LIW spreading over a major part of the study area. However, more than half of the overall decrease in its concentration, indicative of vertical mixing, occurs at the Strait of Otranto. Geostrophic calculations, with a reference level selected on the basis of heuristic criteria, are used to derive (preserving continuity) the quantitative flow pattern. The circulation of the “upper” water (above the reference level), in the part of the study area south of 37° 30′N was found to be dominated by a mesoscale though distorted anticyclonic gyre; with the latter a total of 2.0 Sv (1 Sv = 1 × 10⁶ m³/s) of “upper” water, including 1.0 Sv of LIW, was found to be associated, presumably re-circulating in the area of the anticyclonic gyre. North of 37°30′N the flow is northward and mildly cyclonic, transporting 0.5 Sv. A major portion of the latter (= 0.4 Sv), including 0.2 Sv of LIW is directed toward the eastern side of the Otranto Strait.     

Environmental changes in the western Black Sea related to anthropogenic and natural conditions

The Black Sea northwestern shelf (NWS) ecosystem has been subjected to the strongest anthropogenic pressure of the entire Black Sea as about 80% of the freshwater runoff is discharged there.This paper presents a review of the global environmental changes related mainly to increased eutrophication in the western Black Sea basin.A case study (CoMSBlack 92a cruise) attempts to highlight the interaction of some natural and anthropogenic factors responsible for specific chemical and biological features in the western Black Sea environment. The significance of processes located near the Danube river mouth, which, due to their relatively small space scale, have not been recognized before, is clarified. Hydrological processess of specific importance to the distribution and transformation of antropogenic inputs are river plume dynamics, coastal upwelling and mixing and downwelling over the shelf break and slope in this part of the basin. A layer of low hydrological variability (“conservative layer”) appears to be a natural feature of the area, hypothesized to precondition specific biological and chemical processes there.The results suggest that the interplay between the Danube anthropogenic nutrient load with the natural hydrological fronts and gradients provides opportunities for enhanced biological activity thus contributing to the global environmental changes in the Black Sea NWS.     

Effect of mixing on microbial communities in the Rhone River plume

The biological processes involved during mixing of a river plume with the marine underlying water were studied off the Rhone River outlet. Samples of suspended and dissolved matter were collected while tracking a drifting buoy. Three trajectories were performed, at 2-day intervals, under different hydrological and meteorological situations. A biological uptake was evidenced from ammonium (NH₄) and phosphate (PO₄) shortage, indicating an early “NH₄-dependent” functioning occurring before the well-known “NO₃-based” cycle. The different ratios between NH₄, NO₃ and PO₄, as a function of salinity, were discussed to detail the preferential use in PO₄ and NH₄. Salinity zones with enhanced bacterial production, high chlorophyll a concentration, as well as DOC, NH₄ and PO₄ consumption were evidenced from 20 to 35 in salinity. It was shown that the successive abundance of bacteria and phytoplankton during transfer reflected the competition for PO₄ of both communities. On the Rhone River plume, the role played by temperature, light conditions and suspended matter upon biological activity seems relatively minor compared to salinity distribution and its related parameter: nutrient availability. It can be concluded that biological uptake in the Rhone River plume was closely related to the dilution mechanism, controlled itself by the dynamics of the plume. In windless conditions and close to the river mouth, the density gradient between marine and river water induced limited exchanges between the nutrient-rich freshwater and the potential consumers in the underlying marine water. Consequently, little biological activity is observed close to the river mouth. Offshore, mixing is enhanced and a balance is reached between salinity tolerance and nutrient availability to form a favourable zone for marine phytoplankton development. This can be quite far from the river mouth in case of a widely spread plume, corresponding to high river discharge. Under windy and wavy conditions, the plume freshwater is early and rapidly mixed, so that the extension of the “enhanced production zone” is drastically reduced and even bacteria could not benefit from the fast mixing regime induced.     

The gonad morphology and maturation in Arctic Calanus species

Gonad morphology and maturation of the dominant Arctic copepod species Calanus glacialis and C. hyperboreus were studied using a combination of observations on whole animals (“whole-body analysis”) and histological sections to describe basic reproductive processes. The comparison with C. finmarchicus shows that the internal organisation of the gonads is the same in the Calanus species: from the dorsally laying ovary, two anterior diverticula extent into the head region and two posterior diverticula extend into the abdomen. In the diverticula, the oocytes increase in size and development stage in a dorso–ventral direction. There were no physiological differences between the oocytes in the diverticula and oviducts in any of the three species. During gonad development, the morphological modifications of the gonads mainly refer to an increase in number, developmental stage and size of the oocytes both in diverticula and oviducts. During spawning the most ventral layer of oocytes is released, and then the adjacent dorsal layer begins final maturation. Thus, the final maturation processes occur simultaneously as it is essential to produce clutches.     

Ecophysiological growth characteristics and modeling of the onset of the spring bloom in the Baltic Sea

The onset of spring bloom in temperate areas is a transition period where the low productive, winter phytoplankton community is transformed into a high productive spring community. Downwelling irradiance, mixing depth and the ability of the phytoplankton community to utilize the light, are key parameters determining the timing of the onset of the spring bloom. Knowing these parameters would thus provide tools for modeling the spring bloom and enhance our knowledge of ecophysiological processes during this period.Our main objective with this study was to provide data for the growth characteristics of some key species forming the spring bloom in the Gulf of Finland, and to apply those results in a simple dynamic model for the onset of the spring bloom, in order to test if the timing of the spring bloom predicted by the models corresponds to field observations. We investigated the photosynthetic characteristics of three diatoms and two dinoflagellates (Chaetoceros wighamii, Melosira arctica, Thalassiosira baltica, Scrippsiella hangoei and Woloszynskia halophila), at low temperatures (4–5 °C). All of these species are common during spring bloom in the Baltic Sea.Cultures of these species were acclimated to different irradiance regimes prior to measurements of photosynthesis, respiration, pigment concentration and light absorption. We did not find a positive relationship between respiration and growth rate, and we hypothesize that this relationship, which is well established at higher temperatures, is negligible or absent at low temperatures (< 10 °C). Photosynthetic maximum (P_m), and maximum light utilization coefficient (α) was lowest and respiration (R) highest in the dinoflagellates.We made a model of the onset of the spring bloom in the western part of Gulf of Finland, using the obtained data together with monitoring data of mixing depth and water transparency from this area. Model results were compared to field observations of chlorophyll-a (Chl-a) concentration. There was a good agreement between the model predictions and the observed onset of the spring bloom for the diatoms. S. hangoei, however, was not able to reach positive production in the model, and W. halophila had the similar growth characteristics as S. hangoei. Consequently, these species must have other competition strategies enabling them to exist and grow during spring bloom.     

Large eddy simulations for quasi-2D turbulence in shallow flows: A comparison between different subgrid scale models

In this study, the performance of the horizontal large eddy simulation module, developed at the University of Leuven (HLES-KULeuven module) is assessed. A comparison between different subgrid scale models has been carried out. The study is concerned with the non-rotating and unstratified flows. The results of the simulation for an oscillatory backward facing (BFS) flow are presented in case of an expanding flume based on a one-length scale approach and a two-length scale approach. Three subgrid scale (SGS) models have been tested: Smagorinsky SGS model (Smagorinsky, J., (1963). General circulation experiments with the primitive equations, I. the basic experiments. Monthly Weather Review, 91(3), 99–164), Uittenbogaard SGS model (Uittenbogaard, R.E., and van Vossen, B., (2004). Subgrid-scale model for quasi-2D turbulence in shallow water. Shallow Flows. Jirka and Uijttewaal (Eds.), Taylor & Francis Group, London, ISBN 90 5809 700 5) and a proposed two-length scale approach. The first two models are considered to be a one-length scale models. A simulation without a subgrid scale model for the horizontal mixing has also been conducted. In all simulations, a quadratic friction model parameterizes the dissipation produced by the 3D-subdepth scale turbulence. The two-length scale concept uses a newly mixing length formulation for the quasi-2D turbulence and doesn't depend on the filter width in contrast to the one-length scale approach, in which the mixing length is function of the filter width. The outputs of the HLES-KULeuven module have been compared with the experimental data taken from Stelling, G.S., and Wang, L.X., (1984). Experiments and computations on separating flow in an expanding flume. Dept. Civil Engineering, Delft University of Technology, Report 2–84.). The two-length scale approach has been validated with experimental data from SERC Flood Channel Facility at HR Wallingford. In general, there is a qualitative agreement with the experimental data. It has also been found that the two-length scale approach produces more elongated and less isotropic vortex than the one-length scale models.     

Transport of radionuclides by sea-ice and dense-water formed in western Kara Sea flaw leads

A transport assessment of particle-bound and dissolved artificial radionuclides (¹³⁷Cs and ^239,240Pu) by sea-ice and dense-water formed in western Kara Sea flaw leads close to the Novaya Zemlya dumping sites is presented in this study. We both performed a “best estimate” based on available data, and a “maximum assessment” relying on simulated constant releases of 1 TBq ¹³⁷Cs and ^239,240Pu from individual dumping bays. The estimates are based on a combination of (i) the content of particulate matter in sea-ice; (ii) analytical data and numerical simulations of radionuclide concentrations in shelf surface deposits, suspended particulate matter (SPM), and the dissolved phase; and (iii) estimates of lead-ice and dense-water formation rates as well as modeling results of local ice drift pathways. In the “best estimate” case, 2.90 GBq ¹³⁷Cs and 0.51 GBq ^239,240Pu attached to sea-ice sediments can be exported from the lead areas toward the central Arctic basin. The radionuclide burden of the annually formed dense lead water in the “best estimate” amounts to 4.68 TBq ¹³⁷Cs and 0.014 TBq ^239,240Pu. In the “maximum assessment”, potential export-rates of ice-particle bound ¹³⁷Cs and ^239,240Pu toward the central Arctic would amount to 0.64 and 0.16 TBq, respectively. As much as ≈900 TBq ¹³⁷Cs and ≈6.75 TBq ^239,240Pu could be annually taken up by 34.75 dense-water rejected in the lead area. Assuming the (unlikely) instantaneous release of the total ¹³⁷Cs and ^239,240Pu inventories (≈1 PBq and 10 TBq, respectively) from the Novaya Zemlya dumping sites into the dissolved phase, the dense lead water locally formed during one winter season could take up ≈90% of the Cs and ≈68% of the Pu released.     

Inter-annual variability of hypoxic conditions in a shallow estuary

Water quality data from two monitoring programs in the Pamlico River Estuary (PRE) were analyzed for dissolved oxygen (DO), salinity, temperature, and nutrient concentrations. Data were collected bi-weekly at 8 stations from 1997 to 2003 by East Carolina University and continuously at three stations from 1999 to 2003 by the U.S. Geological Survey. Hypoxic conditions were observed mostly in the upper to middle estuary, but the frequency of hypoxic events varied between years. During June to October in 1997–1999 (referred to as the oxic summers) bottom water hypoxia (DO < 2 mg l^− 1) was found in 8.7% of the observations. By contrast, during June to October in 2001–2003 (referred to as the hypoxic summers), 37.9% of the total measurements had DO concentrations less than 2 mg l^− 1. The more frequent and/or prolonged hypoxic conditions during the hypoxic summers were closely associated with stronger salinity stratification and greater loadings of nutrient and particulate matter.Salinity stratification appeared to be governed by patterns of freshwater discharge, and frequency of wind mixing events. The “oxic” summers were characterized by continuous low freshwater inflow (except one extremely high flow event due to hurricanes), stronger northeastward wind, and more frequent wind mixing events. In contrast, the hypoxic summers were characterized by frequent moderate freshwater inflow events, and fewer wind mixing events.The greater loadings of nutrient (nitrate, ammonium, and phosphate) and particulate matter during the hypoxic summers were primarily due to higher river discharges. At the head of the PRE, no significant differences were found in concentrations of nutrient and particulate nitrogen between the oxic and the hypoxic summers. In addition, chlorophyll a concentrations were averaged above 30 μg l^− 1 (maximum 167 μg l^− 1) during the hypoxic summers, significantly higher than those during the oxic summers (averaged around 15 μg l^− 1).     

Spatio-temporal variations in deep-sea demersal communities off the Balearic Islands (western Mediterranean)

The spatial and temporal variations of deep-sea megafaunal assemblages from the western Mediterranean are analysed in the present paper. The assemblages from two locations of the Balearic Islands situated 120 km apart were compared using data collected seasonally on a bathymetric stratum covering the 150–750 m depth range during six bottom-trawl surveys. The assemblage structure, in terms of species composition, species dominance and population sizes, was differentially affected by the spatio-temporal variables analysed (depth, location and fishing period). Although depth was the main factor determining the assemblage composition, the differences obtained between the two locations were also relevant. On the upper slope these between-location differences in the dynamics of megafaunal assemblages were found to be related to the effect of fishing exploitation. Population size-based metrics and biomass spectra were good predictors of meso-scale fishing effects, and were mainly reflected by elasmobranchs and demersal teleosts. Nevertheless, the effects of fishing depended on the species considered. Two dominant large-sized fish species found on the upper slope in both localities, Galeus melastomus and Phycis blennoides, had higher biomass values associated with lower fishing effort. Although the mean body weight (MBW) of both species and also the mean maximum body weight (MMBW) of G. melastomus agreed with this pattern, the P. blennoides MMBW did not. This last case could be indicative of natural size-trends such as the bigger–deeper phenomenon which refers to the displacement of large individuals towards the deeper limit of their bathymetric distribution, beyond the maximum depth sampled in this study for this species. By contrast, the target species of the upper slope fishery, the red shrimp Aristeus antennatus, was not negatively affected by the direct impact of fishing activity and other environmental factors, such as the presence of specific water masses could also be important.     

Turbulent mixing and internal tides in Gaoping (Kaoping) Submarine Canyon, Taiwan

Turbulent overturning on scales greater than 10 m is observed near the bottom and in mid-depth layers within the Gaoping (formerly spelled Kaoping) Submarine Canyon (KPSC) in southern Taiwan. Bursts of strong turbulence coexist with bursts of strong sediment concentrations in mid-depth layers. The turbulence kinetic energy dissipation rate in some turbulence bursts exceeds 10^− 4 W kg^− 1, and the eddy diffusivity exceeds 10^− 1 m² s^− 1. Within the canyon, the depth averaged turbulence kinetic energy dissipation rate is ~ 7 × 10^− 6 W kg^− 1, and the depth averaged eddy diffusivity is ~ 10^− 2 m² s^− 1. These are more than two orders of magnitude greater than typical values in the open ocean, and are much larger than those found in the Monterey Canyon where the strong turbulent mixing has also been. The interaction of tidal currents with the complex topography in Gaoping Submarine Canyon is presumably responsible for the observed turbulent overturning via shear instability and the breaking of internal tides and internal waves at critical frequencies. Strong 1st-mode internal tides exist in KPSC. The depth averaged internal tidal energy near the canyon mouth is ~ 0.17 m² s^− 2. The depth integrated internal tidal energy flux at the mouth of the canyon is ~ 14 kW m^− 1, propagating along the axis of the canyon toward the canyon head. The internal tidal energy flux in the canyon is 3–7 times greater than that found in Monterey Canyon, presumably due to the more than 10 times larger barotropic tide in the canyon. Simple energy budget calculations conclude that internal tides alone may provide energy sufficient to explain the turbulent mixing estimated within the canyon. Further experiments are needed in order to quantify the seasonal and geographical distributions of internal tides in Gaoping Submarine Canyon and their effects on the sediment flux in the canyon.     

Estimating salinity to complement observed temperature: 1. Gulf of Mexico

This paper and its companion [Thacker, W.C., Sindlinger, L., 2007-this issue. Estimating salinity to complement observed temperature: 2. Northwestern Atlantic. Journal of Marine Systems. doi:10.1016/j.jmarsys.2005.06.007.] document initial efforts in a project with the goal of developing capability for estimating salinity on a region-by-region basis for the world oceans. The primary motivation for this project is to provide information for correcting salinity, and thus density, when assimilating expendable-bathythermograph (XBT) data into numerical simulations of oceanic circulation, while a secondary motivation is to provide information for calibrating salinity from autonomous profiling floats. Empirical relationships between salinity and temperature, which can be identified from archived conductivity–temperature–depth (CTD) data, provide the basis for the salinity estimates.The Gulf of Mexico was chosen as the first region to explore for several reasons: (1) It's geographical separation from the Caribbean Sea and the North Atlantic Ocean makes it a “small ocean” characterised by a deep central basin surrounded by a substantial continental shelf. (2) The archives contain a relatively large number of CTD data that can be used to establish empirical relationships. (3) The sharp fronts associated with the Loop Current and its rings, which separate water with different thermal and haline characteristics, pose a challenge for estimating salinity. In spite of the shelf and the fronts, the relationship between salinity and temperature was found to be sufficiently regular that a single empirical model could be used to estimate salinity on each pressure surface for the entire Gulf for all seasons. In and below the thermocline, root-mean-square estimation errors are small — less than 0.02 psu for pressures greater than 400 dbar, corresponding to potential density errors of less than 0.015 kg/m³. Errors for estimates nearer to the surface can be an order of magnitude larger.     

Using high sampling-rate ADCP for observing vigorous processes above sloping [deep] ocean bottoms

Above sloping bottoms in the ocean mixing processes are not predominantly generated by shear-induced turbulence via bottom friction. Instead, the restratifying buoyancy forces and internal waves create a highly non-linearly varying environment including ‘stratified turbulence’. Most of the resulting vigorous mixing processes that dominate sediment resuspension occur during the passage of frontal bores or solitary boluses, ‘solibores’. Here, the observed evolution of different forms of highly non-linear strictly upslope moving ‘waves’, bores or boluses are reviewed from various NIOZ projects at deep sloping bottom sites ranging from 500 to 3000 m.Such fronts pass a fixed site within a few minutes, extending some 60 ± 30 m above the bottom and occurring over much larger periods at once per subinertial or meso-scale period or approximately, but not exactly, once per tidal harmonic period. In order to observe the details of such solibore one needs specific, high-sampling rate equipment. A suitable piece of equipment is a bottom-mounted 4-beam 300 kHz acoustic Doppler current profiler (ADCP), provided it samples at a rate of about once per second over a period of at least several weeks. Not just the three components of current velocity [u,v,w] are monitored over a range of some 80 m at 1 m intervals, but also the relative ‘echo intensity’ dI, which is a measure for suspended matter and stratified turbulence. Such ADCP-observations are combined and compared with high-resolution temperature measurements. Fine details show a turbulent inner core with more or less laminar streamlines outside it. Whether a front or a bolus, the bore is never observed as a completely closed contour, as swept up turbulent material is sucked into the core at the rear end.     

Air–sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats

Measurements of the air–sea fluxes of N₂ and O₂ were made in winds of 15–57 m s^− 1 beneath Hurricane Frances using two types of air-deployed neutrally buoyant and profiling underwater floats. Two “Lagrangian floats” measured O₂ and total gas tension (GT) in pre-storm and post-storm profiles and in the actively turbulent mixed layer during the storm. A single “EM-APEX float” profiled continuously from 30 to 200 m before, during and after the storm. All floats measured temperature and salinity. N₂ concentrations were computed from GT and O₂ after correcting for instrumental effects. Gas fluxes were computed by three methods. First, a one-dimensional mixed layer budget diagnosed the changes in mixed layer concentrations given the pre-storm profile and a time varying mixed layer depth. This model was calibrated using temperature and salinity data. The difference between the predicted mixed layer concentrations of O₂ and N₂ and those measured was attributed to air–sea gas fluxes F_BO and F_BN. Second, the covariance flux F_CO(z) = wO₂′(z) was computed, where w is the vertical motion of the water-following Lagrangian floats, O₂′ is a high-pass filtered O₂ concentration and (z) is an average over covariance pairs as a function of depth. The profile F_CO(z) was extrapolated to the surface to yield the surface O₂ flux F_CO(0). Third, a deficit of O₂ was found in the upper few meters of the ocean at the height of the storm. A flux F_SO, moving O₂ out of the ocean, was calculated by dividing this deficit by the residence time of the water in this layer, inferred from the Lagrangian floats. The three methods gave generally consistent results. At the highest winds, gas transfer is dominated by bubbles created by surface wave breaking, injected into the ocean by large-scale turbulent eddies and dissolving near 10-m depth. This conclusion is supported by observations of fluxes into the ocean despite its supersaturation; by the molar flux ratio F_BO/F_BN, which is closer to that of air rather than that appropriate for Schmidt number scaling; by O₂ increases at about 10-m depth along the water trajectories accompanied by a reduction in void fraction as measured by conductivity; and from the profile of F_CO(z), which peaks near 10 m instead of at the surface.At the highest winds O₂ and N₂ are injected into the ocean by bubbles dissolving at depth. This, plus entrainment of gas-rich water from below, supersaturates the mixed layer causing gas to flux out of the near-surface ocean. A net influx of gas results from the balance of these two competing processes. At lower speeds, the total gas fluxes, F_BO, F_BN and F_CO(0), are out of the ocean and downgradient.     

Thermal exchanges at air-water interfacies and reproduction of temperature vertical profiles in water columns

For a 3-D eutrophication-diffusion macromodel of the central part of the Venice Lagoon, air-water heat fluxes are computed interpolating, through Fourier series expansion, meteoclimatic variables averaged over a thirty years survey. Also reproduced with the same interpolation methods, is the daily fluctuation of incident light as well as the annual variation of the photoperiod.With an interative procedure temperature values, to be assigned at each grid's point and corresponding to each hour of a reference year, are computed also accounting for the thermal inertia of water columns of varying depths. By statistical examination of temperature vertical profiles, depth varying diffusivities are also estimated, which enables, without assumption of an instantaneous mixing, the reproduction of heat diffusion from the surface to the bottom water cells.Procedures, preliminary refined and verified for a one-dimensional vertical system input, are next implemented on a three-dimensional submodel of reduced size provided of “open boundaries”: this last submodel, under a continuous input of energy and of matter is seen to attain a steady states as well as to be capable of simulating regime conditions. A further validation is performed, on a submodel of 43 × 47 × 20 cells, encompassing a limited portion of the macromodel and presenting the actual lagoon bathymetry.Macromodel's seasonalisation so achieved, enables for a more correct simulation of the periodical behaviour of light and of temperature, forcing functions governing the eutrophication phenomena.