Deep Convective Mixing Research Articles

A late winter hydrographic section from Tasmania to Antarctica

A hydrographic section between Tasmania and Antarctica was occupied in late winter 1991 as part of the World Ocean Circulation Experiment (WOCE). The primary purpose of the WOCE repeat section SR3 is to measure the exchange between the Indian and Pacific Oceans south of Australia. This paper describes the fronts, water masses and transport observed on the first occupation of the repeat section. The Subantarctic Front (SAF) is located between 50°S and 51°S and is the most striking feature of the vertical sections. Two additional fronts at 53°S and 59°S are associated with the Polar Front (PF), part of which turns northward to flow along the section before turning back to the east near 53°S. Very deep (>500 m) mixed layers are found north of the SAF, confirming that Subantarctic Mode Water (SAMW) is formed in this region by deep convection in winter. Chlorofluorocarbons (CFCs) are significantly undersaturated (≈90–92% of equilibrium values) in these deep mixed layers, indicating that gas exchange rates are not rapid enough to bring these deep mixed layers to equilibrium by the end of the winter period of deep convective mixing. Northward Ekman drift of cold, fresh water across the SAF is likely to be responsible for the cooler, fresher mixed layers observed immediately north of the SAF. The Antarctic Intermediate Water (AAIW) on the SR3 section is relatively low in oxygen and CFCs (≈60–70% and 10–20% of saturation values, respectively), high in potential vorticity, and high in nutrients. These characteristics suggest that the AAIW on this section is not renewed by direct and rapid ventilation near this location. Water mass properties suggest that water from the Tasman Sea spreads south and west across the northern portion of the SR3 section between 800 and 3000 m depth. A cold, fresh, CFC-rich variety of Antarctic Bottom Water is formed along the Wilkes-Adelie coast of Antarctica. The net transport across the section relative to the deepest common depth is 160 Sv. The band of eastward flow between 50°S and 53°S including the SAF carries 137 Sv to the east and dominates the net transport. Weaker flow south of 58°S contributes an additional 70 Sv. The eastward flow is compensated in part by 37 Sv of westward flow between Tasmania and 48.5°S and 8 Sv of flow to the west over the southern flank of the mid-ocean ridge. The trajectories of six ALACE floats deployed at about 950 m confirm the sense of flow inferred from the choice of a deep reference level.

The dynamics of heat lows

A numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea. of particular interest is the evolution of the distributions of relative vorticity and potential vorticity. While the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land, the relative vorticity is strongest in the early morning hours following a prolonged period of low-level convergence. Thus the heat low is not approximately in quasi-geostrophic balance. the low-level convergence is associated with the sea-breeze and later with the nocturnal low-level jet. the effects of differing sea area, land area and Coriolis parameter on various aspects of the heat low are investigated. Although a cyclonic vortex, the heat low is characterized by an anticyclonic potential-vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well-mixed boundary layer; however, the surface temperature maximum over land corresponds with a cyclonic potential-vorticity anomaly at the surface. the reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential-temperature gradient make a non-negligible contribution and of opposite sign to the potential vorticity in certain flow regions. Two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low-level atmospheric phenomena over the arid interior of Australia: these are the deep convective mixing over land during the daytime and the development of a nocturnal low-level jet, which leads to convergence in the trough. Such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores. It is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere.

The dynamics of heat lows

AbstractA numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea. of particular interest is the evolution of the distributions of relative vorticity and potential vorticity. While the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land, the relative vorticity is strongest in the early morning hours following a prolonged period of low‐level convergence. Thus the heat low is not approximately in quasi‐geostrophic balance. the low‐level convergence is associated with the sea‐breeze and later with the nocturnal low‐level jet. the effects of differing sea area, land area and Coriolis parameter on various aspects of the heat low are investigated.Although a cyclonic vortex, the heat low is characterized by an anticyclonic potential‐vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well‐mixed boundary layer; however, the surface temperature maximum over land corresponds with a cyclonic potential‐vorticity anomaly at the surface. the reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential‐temperature gradient make a non‐negligible contribution and of opposite sign to the potential vorticity in certain flow regions.Two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low‐level atmospheric phenomena over the arid interior of Australia: these are the deep convective mixing over land during the daytime and the development of a nocturnal low‐level jet, which leads to convergence in the trough. Such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores. It is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere.

An evaluation of deep convective mixing in the Goddard Chemical Transport Model using International Satellite Cloud Climatology Project cloud parameters

The simulation of deep convective mixing in the Goddard Chemical Transport Model (GCTM) is evaluated by comparing 1990–1992 distributions of upper tropospheric convective mass flux and cloud top pressure from the Goddard Earth Observing System data assimilation system (GEOS‐1 DAS) with deep convective cloud fields from the International Satellite Cloud Climatology Project (ISCCP). Deep convective mixing in the GCTM is calculated using convective information from the GEOS‐1 DAS. Therefore errors introduced when deep convection is parameterized in the GEOS‐1 DAS affect the distribution of trace gases in the GCTM. The location of deep convective mixing in the tropics is fairly well simulated, although its north‐south extent is overestimated by >5°. The frequency of deep convective mixing also appears to be overestimated in the tropics, resulting in GCTM‐calculated upper tropospheric concentrations of carbon monoxide in the tropics that are larger and less variable than those observed. The spatial extent of deep convective mixing in the subtropics is overestimated at several locations including the Caribbean throughout the year and the South Pacific Convergence Zone during June‐August. The extent of deep convection is underestimated over midlatitude marine storm tracks. DAS‐calculated cloud top pressures differ from ISCCP cloud top pressures by less than one‐half a GCTM layer (35 hPa) at most longitudes in the tropics; however, cloud top pressures are overestimated by more than 35 hPa (i.e., the vertical extent of deep convection is underestimated) over wintertime midlatitude storm tracks and the Indian Ocean and underestimated by more than 35 hPa at locations that include the Gulf of Mexico during December–February and central South America during June–August.

Chlorofluorocarbon uptake in a World Ocean model: 2. Sensitivity to surface thermohaline forcing and subsurface mixing parameterizations

Part 1 of this study [ England et al., 1994] examined the sensitivity of simulated oceanic Chlorofluorocarbon (CFC) to changes in the way the air‐sea gas exchange rate is parameterized in a World Ocean model. In part 2 we consider more closely the role of surface thermohaline forcing and subsurface mixing parameterizations in redistributing CFC‐11 and CFC‐12 in the ocean. In particular, a series of five different model ocean experiments are forced with the same air‐sea CFC flux parameterization. The five cases include (1) a control run with a standard seasonal cycle in surface forcing and traditional Cartesian mixing, (2) a run in which the production rate and salinity of Antarctic Bottom Water (AABW) is increased, (3) a run in which the production, outflow rates, and density of North Atlantic Deep Water (NADW) is increased, (4) a run with enhanced isopycnal mixing of passive tracers, and finally (5) a run in which the effects of eddies on the mean ocean flow are parameterized following Gent and McWilliams [1990]. The simulated CFC uptake in the Southern Ocean far exceeds observations in the first four experiments. The excessive uptake is linked to the poor model simulations of Southern Ocean deep water masses, where, for example, model Circumpolar Deep Water is typically 0.2 to 0.4 kg m3 too buoyant. The insufficient density of the deep water allows for extensive penetration of convective adjustment to great depth during winter, in contrast to observations, and this results in excessive downward mixing of the CFC‐enriched surface waters. Compared with the control experiment, the Southern Ocean CFC uptake is reduced in the cases with increased AABW salinity and NADW density, as a result of slightly higher deep water density and reduced wintertime convection in those experiments. Nevertheless, CFC uptake in the Southern Ocean still substantially exceeds observed ocean CFC content in the adjusted surface forcing cases. The most extreme uptake occurs in case 4, where, in addition to deep convective mixing of CFC, there is also mixing into the ocean interior along isopycnal surfaces having an unrealistic orientation. The Southern Ocean CFC uptake in case 5, using the mixing scheme of Gent and McWilliams [1990], is dramatically reduced over that in the other runs. Only in this run do deep densities approach the observed values, and wintertime convection is largely suppressed south of the Antarctic Circumpolar Current. Deep penetration of CFC‐rich water occurs only in the western Weddell and Ross Seas. This run yields CFC sections in the Southern Ocean which compare most favourably with observations, although substantial differences still exist between observed and simulated CFC. The simulation of NADW production is problematic in all runs, with the CFC signature indicating primary source regions in the Labrador Sea and immediately to the southeast of Greenland, while the Norwegian‐Greenland Sea overflow water (which is dominant in reality) plays only a minor role. Lower NADW is insufficiently dense in all runs. Only in the run with surface forcing designed to enhance NADW production does the CFC signal penetrate down the western Atlantic boundary in a realistic manner. However, this case exhibits an unrealistic net ocean surface heat loss adjacent to Greenland and so cannot be advocated as a technique to improve model NADW production. Conventional depth sections and volumetric maps of CFC concentration indicate that on the decadal timescales resolved by CFC uptake the dominant determining factor in overall model ventilation is the choice of subsurface mixing scheme. The surface thermohaline forcing only determines more subtle aspects of the subsurface CFC content. This means that the choice of subgridscale mixing scheme plays a key role in determining ocean model ventilation over decadal to centennial timescales. This has important implications for climate model studies.

Seasonal and interannual variability of oceanic carbon dioxide species at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site

The seasonal and interannual dynamics of the oceanic carbon cycle and the strength of air-sea exchange of carbon dioxide are poorly known in the North Atlantic subtropical gyre. Between October 1988 and December 1993, a time series of oceanic measurements of total carbon dioxide (TCO 2), alkalinity (TA) and calculated pCO 2 was obtained at the Bermuda Atlantic Time-series Study (BATS) site (31°50′N, 64°10′W) in the Sargasso Sea. These measurements constitute the most extensive set of CO 2 species data collected in the oligotrophic North Atlantic. Seasonal changes in surface and water-column CO 2 species were ∼40–50 μmol kg −1 in TCO 2, ∼20 μmol kg −1 in TA, and ∼90–100 μatm in calculated pCO 2. These large changes were driven principally by deep convective winter mixing, temperature forcing and biological activity. TA was well correlated with salinity (with the exception of a 15–25 μmol kg −1 drawdown of TA on one cruise resulting from open-ocean calcification). TCO 2 and pCO 2 were well correlated with seasonal temperature changes (8–9°C). Other underlying processes, such as biological production, advection, gas exchange of CO 2 and vertical entrainment, were important modulators of the carbon cycle, and their importance varied seasonally. Each spring-to-summer, despite the absence of measurable nutrients in the euphotic zone, a 35–40 μmol kg −1 decrease in TCO 2 was attributed primarily to the biological uptake of TCO 2 (evaporation/precipitation balance, gas exchange, and advection were also important). An increase in TCO 2 during the fall months was associated primarily with entrainment of higher TCO 2 subsurface waters. These seasonal patterns require a reassessment of the modelling of the carbon cycle using nutrient tracers and Redfield stoichiometries. Overall, the region is a weak sink (0.22–0.83 mol C m −2 year −1) for atmospheric CO 2. Upper ocean TCO 2 levels increased between 1988 and 1993, at a rate of ∼ 1.7 μmol kg −1 year −1. This increase appears to be in response to the uptake of atmospheric CO 2 through gas exchange or natural variability of the subtropical gyre.

Seasonal variability of light availability and utilization in the Sargasso Sea

A 2 year time series of optical, biogeochemical, and physical parameters, taken near the island of Bermuda, is used to evaluate the sources of temporal variability in light availability and utilization in the Sargasso Sea. Integrated assessments of light availability are made by examining the depth of constant percent incident photosynthetically available radiation (%PAR) isolumes. To first order, changes in the depth of %PAR isolumes were caused by physical processes: deep convective mixing in the winter which led to the spring bloom and concurrent shallowing of %PAR depths and the occurrence of anomalous thermohaline water masses during the summer and fall seasons. Spectral light availability variations are assessed using determinations of diffuse attenuation coefficient spectra which illustrate a significant seasonal cycle in colored detrital paniculate and/or dissolved materials that is unrelated to changes in chlorophyll pigment concentrations. Temporal variations in the photosynthetic light utilization index Ψ are used to assess vertically integrated light utilization variations. Values of Ψ are highly variable and show no apparent seasonal pattern which indicates that Ψ is not simply a “biogeochemical constant” Determinations of in situ primary production rates and daily mean PAR fluxes are used to diagnose the relative role of light limitation in determining vertically integrated rates of primary production ∫PP. The mean depth of the light‐saturated zone (the vertical region where the daily mean PAR flux was greater than or equal to the saturation irradiance Ik) is only ∼40 m, although more than one half of ∫PP occurred within this zone. Production model results illustrate that accurate predictions of ∫PP are dependent upon rates of light‐saturated production rather than upon indices of light limitation. It seems unlikely that significant improvements in simple primary production models will come from the partitioning of the Earth's seas into biogeochemical provinces.

Springtime distributions and variability of biogenic particulate matter in Gulf Stream warm-core ring 82B and surrounding N.W. Atlantic waters

Distributions of particulate matter, chlorophyll and other physical/bio-optical properties were investigated during April–May 1982 in warm-core ring (WCR) 82B and surrounding Shelf, Slope, Gulf Stream and Sargasso Sea waters. Biogenic particles present in the core waters of WCR 82B were strongly influenced by the day-to-day variation in air-sea heat flux. Particles grew/accumulated in surface waters during periods of transient stratification/quiescence and were removed from the euphotic zone as deep as 400 m during periods of strong heat loss and deep convective mixing. This process enriched ring core waters below the euphotic zone compared to all other waters sampled. Outside of ring-core waters, stratification was already permanently established and surface concentrations of particles/chlorophyll increased radially outward to maximum values in entrainment zone waters at the ring periphery. Slightly lower concentrations were observed in the Slope Water and Shelf Water. Comparisons of chlorophyll and beam attenuation coefficient data suggested that the beam attenuation coefficient reflects bulk distributions of particulate mass and not pigment biomass, consistent with earlier studies. Profiles of SPM and chlorophyll at a Gulf Stream station showed subduction of high (650 μg kg −1) concentrations of biogenic particles produced at the surface near the Gulf Stream/Slope Water front. Such subduction events may have significant impact on the oxygen budget of the N. Atlantic thermocline.

Convective modifications of water masses in the Agulhas

The nature of convective modifications of water masses in the Agulhas Retroflection region is considered in relation to both the temperature and salinity ( TIS) properties, and in relation to the gas transfer across the air-sea interface. Data from the 1983 Agulhas Retroflection Cruise (ARC) are used along with satellite sea surface thermal data and a set of models to explore the evolution of water masses in this area. Air-sea interaction over warm pools of water isolated from the Retroflection in rings produces a cold mode water which is found extensively in the eastern South Atlantic. Quantities of a warmer South Indian Subtropical Mode Water are also formed in the Retroflection. A water mass mixing model with the inclusion of air-sea fluxes is used to quantify the possible heat, salt and gas fluxes over the Retroflection region. Heat fluxes between 125 and 610 W m −2 encompass the 225 W m −2 expected from the regional climatology. Evaporation minus precipitation results based on the most likely water masses available for modification are between 2 and 10 × 10 −5 kg s −1. This is comparable to the climatological annual values of around 6 × 10 −5 kg s −1. Consideration of oxygen and chlorofluoromethane (CFM) content of the converted layers found in the ARC data suggests that the convective changes in the surface layers are highly episodic. The CFMs and oxygen are less saturated in the converted layers than what might be expected from standard piston velocity arguments. The gas flux can be rationalized with a simple mixed-layer model, where the deep convective mixing tied to cooling events is interspaced with longer periods where the active mixing is restricted to a near surface wind mixed layer. The mixed layer comes to near equilibrium and is only periodically mixed into the deeper thermohaline converted layer. This greatly reduces the gas flux into these mode water layers, relative to what would occur in a steady convective case.

A comparison of parameterized vs. measured transilient mixing coefficients for a convective mixed layer

The parameterization for transilient turbulence coefficients suggested by Stull and Driedonks (SD. 1987) is tested against the large-eddy simulations (LES) of Ebert et al. (ESS, 1989) for the special case of an idealized convective boundary layer. The SD parameterization is based on a nonlocal approximation to the turbulence kinetic energy (TKE) equation, and requires turbulent exchange (i.e., the matrix of transilient mixing coefficients is assumed to be symmetric) and dominance of the smaller eddies (i.e., elements closer to the main diagonal of the matrix are greater). Measurements from the LES model, however, show that the transilient matrix is asymmetric in convective situations, with larger eddies dominating. Mean-state conditions such as the deep convective mixing and mixed-layer growth are satisfactorily described by the parameterization, but the surface layer is too deep and the entrainment zone thickness is poorly defined. Turbulence properties such as skewed vertical velocity distributions are not possible within the constraint of a symmetric matrix, and partial convective overturning is also not possible because of the restriction that small eddies dominate. Future improved parametcrizations might continue to be based on the TKE equation, but should allow transilient matrix asymmetry.

The Weddell‐Scotia confluence in midwinter

The southern central Scotia Sea, site of the Weddell‐Scotia Confluence where outflowing Weddell Sea waters converge with the eastward flowing waters of the Scotia Sea, was sampled during June‐August (austral winter) 1988 with respect to temperature and salinity. Both drogued and ice‐mounted drifters, tracked by Argos, were deployed in the region and yielded Lagrangian drift tracks of ice and water motion. The data substantiate past accounts of the region, based upon summer field research, as dominated by eastward flow upon which a complex array of mesoscale features is superimposed. Weddell‐Scotia Confluence Water, documented by past summer work in the region and characterized by decreased static stability, was not detected, and the Scotia Front was not well defined. The region was one of intense mixing activity and primarily anticyclonic mesoscale features. Two such features, one an eddy and the other either an eddy or a meander in the Scotia Front, dominated the mesoscale field. With warm cores and containing Polar Front Water, they may have been advected eastward from Drake Passage or may have formed as detached eddies from a sharp northward bend in the Polar Front which typically lies just west of the study region. Several smaller eddies, primarily anticyclonic and some having warm cores, were also detected. There was no evidence of the deep convective mixing which has been hypothesized, on the basis of past summer data, to occur in winter, and vigorous vertical mixing was limited to a 100‐m‐thick upper mixed layer. Vertical stability in the upper layers was enhanced by low‐salinity water derived from melting ice. Temperature‐salinity analyses show that winter water in the study region can be derived through isopycnal mixing between waters from the Scotia Sea and waters from the northwestern Weddell Sea. This is in apparent contrast with summer conditions, wherein conditioning of water either through vertical mixing or via lateral mixing on continental margins has been invoked to arrive at the water mass characteristics which typify the Weddell‐Scotia Confluence.

Global patterns of ocean transparency: Implications for the new production of the open ocean

Climatological mean fields of ocean transparency derived from Secchi depth measurements held by the National Oceanographic Data Center are presented. Clearly evident are the major coastal upwelling regions off Peru, northwest and south Africa as well as the coastally enriched areas globally. The central gyre regions in both North Atlantic and North Pacific waters are separated by a sharp discontinuity in water clarity from the more northern waters; the change in Secchi depth across this subtropical front is equivalent to a difference of ∼1 mg chlorophyll a m−3 over meridional distances of 1000 km. Sufficient data exist in the western Pacific to demonstrate that the front migrates approximately 1000 km seasonally from a northern extreme at ∼28° N in September–November, to a southern extreme at 18° N in March–May. A simple explanation for this feature, which is also clearly evident in recent satellite ocean color images of the the northern ocean basins, is offered in terms of the effective vertical ‘upwelling’ and resulting new production associated with seasonal variations in deep convective mixing.

The use of Cs and Sr isotopes as tracers in the Arctic Mediterranean Seas

The Arctic Mediterranean Seas constitute an oceanic region in which the thermohaline circulation has a strong advective component and deep ventilation processes are very active relative to other oceanic areas. Details of the nature of these circulation and ventilation processes have been revealed through use of Cs and Sr isotopes from bomb-fallout and nuclear-waste sources as ocean tracers. In both cases, their regional input is dominated by advective supply in the Norwegian Atlantic Current and Norwegian Coastal Current, respectively. The different temporal, spatial, and compositional input patterns of these tracers have been used to study different facets of the regional circulation. These input differences and some representative applications of the use of these tracers are reviewed. The data discussed derive from samples collected both from research vessels and from Arctic ice camps. The topics addressed include: ( a ) the role of Arctic Intermediate Water as source, supplying recent surface water to North Atlantic Deep Water via the Denmark Strait overflow; ( b ) deep convective mixing in the Greenland Sea; ( c ) circulation or recirculation of Atlantic water in the Arctic basins; and ( d ) the role of Arctic shelfwaters in the ventilation of intermediate and deep water in the Eurasian and Canadian basins.

Seasonal variability of carbon dioxide, nutrients and oxygen in the northern North Atlantic surface water: observations and a model*

The seasonal variation of various surface water properties has been monitored at a station located at about 120 miles south of the Iceland-Greenland sill during the two-year period, March 1983 through May 1985. These properties include the temperature, salinity, mixedlayer depth, partial pressure of CO 2 in seawater and the concentrations of dissolved total CO 2 , oxygen and nutrients. It was observed that during the summer, the C0 2 partial pressure and the concentrations of CO 2 and nutrients in surface water were lowest, while the oxygen concentration was highest. This situation was reversed during the winter. The seasonal variation is attributed mainly to the high photosynthetic utilization rate of carbon and nutrients in a strongly stratified and shallow surface mixed layer during the summer. The winter observations are attributed to the upward transport of deep waters rich in total CO 2 and nutrients by deep convective mixing. In order to account for the observed seasonal variation, a vertically one-dimensional, two-box Ocean model has been constructed. The vertical mixing between the surface mixed layer and deep water is characterized in terms of changes in the mixed layer thickness with time, and the biological productivity is related to the solar insolation and nutrient concentration in the mixed layer. Gas exchange of oxygen and CO 2 between the mixed layer and the atmosphere is taken into consideration. When this model is calibrated using the observed phosphate concentration in surface water, it yields seasonal variations of carbon and oxygen values consistent with the observations, with an exception of a large excursion of spring time values resulting from phytoplankton blooms. It is shown that the spring bloom effect can be simulated by a short-term reduction of the phosphate residence time and of the gas exchange rate. DOI: 10.1111/j.1600-0889.1987.tb00205.x

Utilization of nitrogen and phosphorus by primary producers in warm-core ring 82-B following deep convective mixing

Rates of utilization for NO 3 −, NH 4 +, urea, and PO 4 − by primary producers and the abundances of particulate C, N, and P were determined for the euphotic zone of warm-core ring 82-B in April and early May 1982. The ring had formed in late February, and our study took place between a period of deep convection and the formation of a stable seasonal thermocline. Deep convection enriched the surface waters with NO 3 − and PO 4 −, which remained in sufficient quantities to preclude nutrient limitation of primary production during the course of the study. Concentrations of HN 4 + and urea were always less than or equal to the limit of detection (0.03 μmol kg −1). The fraction of total nitrogen taken up as NO 3 − (ƒ ratio) was high (0.62–0.66) in the core region of the ring. During the course of this study, the concentration of NO 3 − decreased near the surface, allowing a NO 3 − gradient to develop, while the concentration of particulate N (PN) and the rate of N uptake increased. However, these processes were not in balance. Only about 16% of the NO 3 − based production remained in the euphotic zone. The PN:Chl a ratio was higher at the bottom of the euphotic zone than at the top. This indicates that detrital PN was being formed faster than it was exported by either sinking or mixing processes. The overall reduction in euphotic zone NO 3 − concentrations was about one third the measured consumption rate. Midway through the study a storm was apparently responsible for redistributing NO 3 − in the euphotic zone, and perhaps mixing NO 3 − up from greater depths. After that event, the mean NO 3 − uptake rate was about twice the rate of NO 3 − disappearance. Upward flux of NO 3 − associated with the relaxation of depressed density surfaces as the ring lost kinetic energy could have supplied NO 3 − at a rate equivalent to 40–110% the mean NO 3 − uptake rate for the period. The highest rates of NO 3 − uptake were observed at the end of the study, and were in excess of the calculated rates for upward flux. This may explain the increasing magnitude of NO 3 − gradients observed durinng the final few days of this study.

Anthropogenic Chlorofluoromethanes in the Greenland and Norwegian Seas

The concentrations of two industrially produced chlorofluoromethanes, CCl(3)F(F-11) and CCl(2)F(2)(F-12), have been measured in the water column and in the marine atmosphere of the Greenland and Norwegian seas. Measurable concentrations of these two chlorofluoromethanes have penetrated to the deep basins of both of these regions, and the general characteristics of their vertical distributions are similar to those of the bomb-produced radioisotopes injected into the atmosphere on a similar time scale. The data have been fitted to a time-dependent box model based on deep convective mixing in the Greenland Sea and lateral exchange between the deep basins. The model calculations for the two chlorofluoromethanes in the Greenland Sea give similar results, with a time scale for deep convection of about 40 years. The time scale for lateral mixing between the deep Greenland Sea and the deep Norwegian Sea is estimated to be 20 to 30 years, although the agreement between the calculations for the two chlorofluoromethanes is limited by analytical uncertainties at the low concentrations found in the deep Norwegian Sea and by uncertainties in the model assumptions.

Diffusion time scales in white dwarfs

view Abstract Citations (176) References (77) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffusion time scales in white dwarfs. Fontaine, G. ; Michaud, G. Abstract The time scales of a few representative elements diffusing in hydrogen- and helium-rich white-dwarf envelopes are presented. The dependences on depth, the stellar mass, and effective temperature are investigated. It is shown that diffusion processes are sufficiently rapid that the observed monoelemental character of white-dwarf spectra can be explained. Even deep convective mixing, present in the surface layers of cooler white dwarfs, cannot bring back to the photosphere heavy elements that have diffused downward. The present calculations lead to estimates of diffusion time scales that are so short that no trace element should be seen in the spectra of white dwarfs. Mechanisms such as accretion competing with diffusion may have to be invoked to explain the small but measurable abundances of heavy elements in the spectra of these objects. For the vast majority of white dwarfs these conclusions are not affected by our having neglected radiative forces or by inaccuracies in the transport coefficients. However, improvements in the estimates of transport coefficients in dense plasmas are needed. Publication: The Astrophysical Journal Pub Date: August 1979 DOI: 10.1086/157247 Bibcode: 1979ApJ...231..826F Keywords: Atmospheric Diffusion; Convective Flow; Stellar Atmospheres; Stellar Envelopes; Thermal Diffusion; White Dwarf Stars; Carbon; Gaseous Diffusion; Helium; Hydrogen; Time Dependence; Astrophysics; Stellar Envelopes:Diffusion; White Dwarfs:Envelopes full text sources ADS | data products SIMBAD (8)

The hydrogen profile, previous mixing, and loops in the H-R diagram during core helium burning

view Abstract Citations (19) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The hydrogen profile, previous mixing, and loops in the H-R diagram during core helium burning. Schlesinger, B. M. Abstract Static models of 5-solar-mass stars burning core helium have been calculated which consist of three zones: the core of a 5-solar-mass evolutionary model whose outer boundary is the outer edge of the hydrogen-burning shell; an envelope with chemical composition constant from the surface inward; and an intermediate zone whose composition varies with mass fraction. Models differ in the composition profile of the intermediate zone. Both the total excess of helium in the intermediate zone over what would be present if it had the envelope composition and the distribution of this excess helium can significantly influence the effective temperature of the star. Large excesses and concentration to the shell favor redder stars. The effect of changing the composition profile inside the nuclear-burning shell is not clear. The rapid transition from red to blue in pre-Cepheid models in which deep convective mixing has occurred at the giant tip is the result of the helium distribution in the intermediate zone. A summary is given of factors that affect the composition profile at different stages of evolution and consequently affect the length of the Cepheid loop. The effect on models of changing input parameters may then be deduced from how the input change affects each of these factors. Publication: The Astrophysical Journal Pub Date: March 1977 DOI: 10.1086/155070 Bibcode: 1977ApJ...212..507S Keywords: Cepheid Variables; Helium; Hydrogen; Nuclear Fusion; Stellar Evolution; Stellar Models; Abundance; Chemical Composition; Profiles; Stellar Envelopes; Stellar Mass; Astrophysics full text sources ADS |

Formation and exchange of deep water in the Greenland and Norwegian seas

The formation and exchange of deep water in the Greenland and Norwegian seas are modeled by a time-dependent box model. Using tritium and radiocarbon data, primarily from the Geochemical Sections (GEOSECS) 1972 operation, the time scale for deep convective mixing in the Greenland Sea is estimated to be about 30 years, which implies that about 100 m of surface water are mixed down annually. The time scale for exchange between the deep Greenland Sea and the deep Norwegian Sea is estimated to be at least 100 years. It appears, on the basis of tritium data, that the formation and exchange of deep water in the Greenland and Norwegian seas are essentially isolated from the formation of the dense overflow waters of the Denmark Strait and the Iceland-Faroe Passage.

Geosecs North Atlantic radiocarbon and tritium results

Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors in Δ 14C are± 4‰ , and in tritium, ± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in 14C and 3H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both 14C and 3H might be associated with the Antarctic intermediate water.