Abyssal Ocean Research Articles

Renewal time and transport of unventilated Central Intermediate Water of the Weddell Sea derived from biogeochemical properties

We have investigated the intermediate water mass of the central Weddell Gyre using TCO2 and oxygen data of FS Polarstern cruises in 1992, 1996 and 1998. This water mass, designated as Central Intermediate Water (CIW), is enriched in CO2 and depleted in O2 relative to its source water due to biological degradation. CO2 enrichment and O2 depletion were quantified by calculating the difference between the concentrations in the CIW and those in the more southern source water, the Circumpolar Deep Water, which derives from the Antarctic Circumpolar Current. Inventories of enrichment and depletion were determined over the whole depth range of CIW, i.e. about 200–800 m. The O2 depletion inventory was greater than that of TCO2 enrichment which is in line with a biological origin of the signal. Spatial and interannual variation appeared to be small. Because subsurface remineralization in the central Weddell Gyre is largely restricted to the CIW, the export production estimate from previous work has been applied to compute the renewal time of CIW from these inventories. A renewal time of only three years was found. TCO2- and O2-based computations were consistent, the former showing larger variation, though. From renewal time and volume of the CIW, a transport velocity (renewal rate) of 6–7 Sv was obtained. Of this, about 1 Sv is upwelled into the surface layer. The remaining 5–6 Sv CIW must be exported to the north, which is opposite to previous views. Results of water mass age and transport rate have thus been obtained using a method based on biogeochemical parameters. As the CIW cannot be identified by temperature and salinity, nor with transient tracers because it is hardly ventilated, this is the only way to obtain such results. As part of the CIW export, a large amount of remineralized CO2 enters the abyssal oceans where it is sequestered for long periods of time. The CIW is a principal and highly efficient player in the biological pump mechanism of the Southern Ocean.

Twentieth century sea level: An enigma

Changes in sea level (relative to the moving crust) are associated with changes in ocean volume (mostly thermal expansion) and in ocean mass (melting and continental storage): zeta(t) = zeta(steric)(t) + zeta(eustatic)(t). Recent compilations of global ocean temperatures by Levitus and coworkers are in accord with coupled ocean/atmosphere modeling of greenhouse warming; they yield an increase in 20th century ocean heat content by 2 x 10(23) J (compared to 0.1 x 10(23) J of atmospheric storage), which corresponds to zeta(greenhouse)(2000) = 3 cm. The greenhouse-related rate is accelerating, with a present value zeta(greenhouse)(2000) approximately 6 cm/century. Tide records going back to the 19th century show no measurable acceleration throughout the late 19th and first half of the 20th century; we take zeta(historic) = 18 cm/century. The Intergovernmental Panel on Climate Change attributes about 6 cm/century to melting and other eustatic processes, leaving a residual of 12 cm of 20th century rise to be accounted for. The Levitus compilation has virtually foreclosed the attribution of the residual rise to ocean warming (notwithstanding our ignorance of the abyssal and Southern Oceans): the historic rise started too early, has too linear a trend, and is too large. Melting of polar ice sheets at the upper limit of the Intergovernmental Panel on Climate Change estimates could close the gap, but severe limits are imposed by the observed perturbations in Earth rotation. Among possible resolutions of the enigma are: a substantial reduction from traditional estimates (including ours) of 1.5-2 mm/y global sea level rise; a substantial increase in the estimates of 20th century ocean heat storage; and a substantial change in the interpretation of the astronomic record.

Effects of Bottom Boundary Layer Parameterization on Reproducing Deep and Bottom Waters in a World Ocean Model

Abstract A simple bottom boundary layer (BBL) model is developed to be incorporated into a z-coordinate medium resolution ocean general circulation model. In the BBL model, velocity is calculated from the pressure gradient, which is also calculated within the BBL. Preliminary experiments using an idealized basin model clearly document that, for reproducing realistic overflow/downslope flow, it is essential to adopt the horizontally distributed Rayleigh drag coefficient in the BBL model and also that, for avoiding warming of the abyssal ocean owing to unphysical strong flows created by the pressure gradient error along the steep slope, it is necessary to limit the area of the BBL. This BBL model is successfully incorporated into a World Ocean model with 1° × 1° resolution, producing the overflow/downslope flow in the northern North Atlantic and around Antarctica. The dense overflow/downslope flow water provides the nucleus of the abyssal water in the World Ocean, leading to the better representation of the...

Zonal Influences in a Modified Stommel-Arons Model of the Abyssal Ocean Circulation

Zonal Influences in a Modified Stommel-Arons Model of the Abyssal Ocean Circulation

On the export of Antarctic Bottom Water from the Weddell Sea

Abstract A survey of the current field over the South Scotia Ridge, obtained with a lowered Acoustic Doppler Current Profiler (LADCP), is presented. There is a pattern of northward (southward) flow on the western (eastern) side of each of four deep passages in the ridge, which is supported by tracer measurements. The net full-depth LADCP-referenced geostrophic transport over the ridge is 22±7 Sv (1 Sv=10 6 m 3 s −1 ) northward, with the jets on either side of the passages transporting 5–10 Sv in alternating directions. The corresponding Weddell Sea Deep Water (WSDW) transport over the ridge is 6.7±1.7 Sv. This is a factor of 4 larger than the only previous estimate in the literature, and suggests that a significant proportion of the Antarctic Bottom Water (AABW) invading the world ocean abyss escapes the Weddell Sea via the Scotia Sea. The net full-depth and WSDW transports over the ridge are modified to 7±6 and 4.7±0.7 Sv, respectively, by a box inverse model of the western Weddell Gyre. The model incorporates the WOCE A23 crossing of the central part of the gyre and a set of five constraints synthesizing our previous oceanographic knowledge of the region. It diagnoses that 9.7±3.7 Sv of AABW are formed in the Weddell Sea, and that comparable amounts are exported over the South Scotia Ridge (∼48%) and further east (∼52%) assuming that no AABW enters the Weddell Gyre from the Indian Ocean. The WSDW fraction with neutral density γ n >28.31 kg m −3 transported over the ridge upwells in the Scotia Sea at a rate of 6×10 −6 m s −1 , an order of magnitude larger than many basin-scale estimates of deep upwelling in the literature. In contrast, the Weddell Sea Bottom Water exported to the eastern Weddell Gyre entrains upward at a rate of 8×10 −7 m s −1 , more typical of other open-ocean regions. When their different ventilation histories are considered, the comparable transports and disparate upwelling rates of the AABW exported over the South Scotia Ridge and farther east may be crucial to our understanding of teleconnections between the Weddell Sea and the global ocean.

On the transport, variability and origin of dense water masses crossing the South Scotia Ridge

Abstract The deep Scotia Sea is filled with ventilated Weddell Sea Deep Water. This in turn is an essential contributor to the ventilation of the World Ocean abyss. Depending on the formation process and/or its location along the Weddell Sea periphery, deep and bottom water masses follow different routes to cross the South Scotia Ridge. A primitive equation, hydrostatic, terrain-following coordinate ocean general circulation model (BRIOS-1) is used to investigate the water mass export from the Weddell Sea. The model is circumpolar focusing on the Weddell Sea, with particularly high resolution (∼20 km) in the DOVETAIL area. Eastward Weddell Sea Deep Water transport of 24×106 m3 s−1 is found in the northern limb of the Weddell Gyre across 44°W. Export rates of Weddell Sea Deep Water through gaps in the South Scotia Ridge are estimated to be 6.4×106 m3 s−1 with a semi-annual cycle of ±0.6×106 m3 s−1, which can be correlated to atmospheric cyclone activity and Weddell Gyre strength. Sensitivity studies considering extreme sea-ice conditions in the Weddell Sea show higher (lower) exports in years of minimum (maximum) winter sea-ice extent. Lagrangian particle trajectories illustrate the pathways of water masses from the inner Weddell Sea into the Scotia Sea through various gaps in the South Scotia Ridge. They highlight the existing flow divergence on the northwestern continental shelf, with one branch entering Bransfield Strait and the other continuing eastwards subsequently filling the deep Weddell and Scotia seas. Water masses flowing through the major gaps originate from the southwestern and southeastern Weddell Sea continental shelves. However, water masses formed east of the Weddell Sea (e.g., Prydz Bay) also seem to feed the deep Scotia Sea, since a large portion of floats flowing northward through the gaps of the South Scotia Ridge have been in contact with the mixed-layer processes outside the inner Weddell Sea.

Cooling and ventilating the Abyssal Ocean

The abyssal ocean is filled with cold, dense waters that sink along the Antarctic continental slope and overflow sills that lie south of the Nordic Seas. Recent integrations of chlorofluorocarbon‐11 (CFC) measurements are similar in Antarctic Bottom Water (AABW) and in lower North Atlantic Deep Water (NADW), but Antarctic inputs are ≈ 2°C colder than their northern counterparts. This indicates comparable ventilation rates from both polar regions, and accounts for the Southern Ocean dominance over abyssal cooling. The decadal CFC‐based estimates of recent ventilation are consistent with other hydrographic observations and with longer‐term radiocarbon data, but not with hypotheses of a 20th‐century slowdown in the rate of AABW formation. Significant variability is not precluded by the available ocean measurements, however, and interannual to decadal changes are increasingly evident at high latitudes.

Computations of the energy flux to mixing processes via baroclinic wave drag on barotropic tides

The geographical distribution of barotropic to baroclinic transfer of tidal energy by baroclinic wave drag in the abyssal ocean is estimated. Using tidal velocities from a state-of-the-art numerical tidal model, the total loss of barotropic tidal energy in the deep ocean (between 70°S and 70°N and at depths greater than 1000 m) is estimated to be about 0.7 TW ( M 2) corresponding to a mean value of the energy flux ( e) of 2.4×10 −3 W/m 2. The distribution of e is however highly skewed with a median of about 10 −6 W/m 2. Only 10% of the area is responsible for more than 97% of the total energy transfer. To assess the possible influence of the relatively coarse bathymetry representation upon the present estimate, complementary calculations using better resolved sea floor topography are carried out over a control area around the Hawaiian Ridge. There are no major differences between the results achieved using the two different bathymetry databases. Fluxes of about 16 GW or 6×10 −3 W/m 2 are computed in both cases, and the main contributions to the total fluxes originate in the same range of e-values and cover equally large parts of the total area. It is not clear whether the present model is valid at flat or subcritical bottom slopes. However, for the Hawaiian region, only 2% of the total energy flux as calculated in the present study originates in areas of critical and subcritical slopes.

Sensitivity of the thermohaline circulation to modern and glacial surface boundary conditions

We analyze the results from over 20 numerical experiments with a global ocean general circulation model (OGCM) under different present and last glacial maximum (LGM) surface boundary conditions. The model exhibits a large range of circulation strengths and patterns, including strong and deep overturning of North Atlantic Deep Water (NADW) under LGM boundary conditions. Parameterizations tuned for today's ocean and fixed atmospheric temperatures contribute to this failure. The experiments show a close relation between the NADW and Antarctic Bottom Water (AABW) overturning cells in the Atlantic. High transport in the NADW cell is associated with little or no transport of AABW. The strength of the AABW overturning cell depends sensitively on the treatment of sea ice in the Southern Ocean. An atmospheric energy balance model coupled with the OGCM allows a free response of the sea ice component. Because of the close link between NADW and AABW, the northern source of deep water must be relatively weak during the LGM. An ocean circulation consistent with paleoclimatic evidence only results when the surface salinity in the northern North Atlantic is reduced. The solution then shows a southward expansion of the Arctic domain and a southward shift of the deep water formation area in the North Atlantic. The glacial NADW is less dense than modern NADW, and AABW occupies a larger part of the abyssal ocean. Compared to modern distributions, there is almost no glacial Antarctic Intermediate Water causing middepth warming in the southern and tropical Atlantic.

Transient Adjustment of Circulation in a Midlatitude Abyssal Ocean Basin with Realistic Geometry and Bathymetry

Abstract The early stages in the adjustment of the circulation in a midlatitude abyssal basin with realistic geometry and bathymetry are studied using an inverted 1½-layer model of the Eastern Mediterranean Sea as a natural test basin. The model is forced with a localized sidewall mass source and a compensating distributed uniform mass sink. The basin is spun up by topographic waves with planetary vorticity gradients playing a minor role. A shallow sill in the middle of the basin dynamically separates the abyssal Eastern Mediterranean Sea into two regions. Despite a constant mass flux into the basin, the resulting energy input is time dependent and is correlated with nonlocal dynamics. A positive feedback develops between the source region and the interior circulation during the early adjustment; for a higher interface height level in the source region, the amount of energy injected by the mass source increases. A series of simulations with filtered bathymetries show that the presence of high-wavenumber t...

Oxygen and carbon stable isotope tracers of Weddell Sea water masses: new data and some paleoceanographic implications

Stable oxygen isotopic composition of sea water and stable carbon isotopes of dissolved inorganic carbon (DIC) on the continental shelf in the southern Weddell Sea are presented. Using the stations sampled during the summer 1995 two sections can be constructed, one closely parallel to the ice shelf edge and the other perpendicular to the upper continental slope. Generally, δ 18O values clearly separate between different shelf water masses depending on the content of meteoric meltwater added during melting of glacial ice. Extrapolation of the mixing line between the cores of High Salinity Shelf Water (HSSW) and supercooled Ice Shelf Water (ISW) reveals δ 18O values of the glacial ice of −27‰, whereas extrapolation of the mixing line between the δ 18O values of the most-saline HSSW and lowest temperature ISW results in δ 18O values of −34‰ for glacial ice. These values point to an origin of meltwater from below the ice shelf, where ice is less depleted in 18O, since deep beneath the ice shelf close to the grounding line, values may reach −40‰. If values between −34 and −27‰ are used as δ 18O end member values for glacial ice, the amount of meltwater from the ice shelf that adds to the formation of ISW off the Filchner–Ronne Ice Shelf ranges from 0.2 to 0.8%, in agreement with previous studies based on δ 18O and 4He. Carbon isotopic fractionation due to gas exchange between the atmosphere and the ocean at cold temperatures results in Δ δ 13C DIC values of 0.20±0.17‰ for Weddell Sea Deep Water, the water mass that ventilates the global abyssal ocean, typically defined as Antarctic Bottom Water (AABW). This confirms the low end of the range estimated previously (0.2–0.4‰), and thus corroborates the dominance of biology in shaping the deep and bottom water δ 13C signal. It has been hypothesized that different modes of glacial/interglacial Antarctic bottom water formation may be separated by different stable isotopic compositions of deep-sea foraminiferal calcite. Here I show that differences between Δ δ 13C and δ 18O values of HSSW and ISW, both of which contribute to bottom water formation today, are too small to be resolved in deep and bottom water masses. Therefore, glacial/interglacial changes in relative proportions of these water masses in Antarctic deep and bottom water cannot be separated by stable isotopes of fossil benthic foraminiferal calcite.

On the insensitivity of the wind-driven circulation to bottom topography

An analytical model is developed for the wind-driven circulation in a continuously stratified ocean overlying variable bottom topography. In the ocean interior we adopt a linear relation between potential vorticity, density and Montgomery potential, resulting in Welander's solution for an adiabatic internal thermocline. The horizontal structure of the circulation is described by a characteristic equation, obtained by imposing a boundary condition of no-normal flow at the sea floor and a prescribed vertical velocity through the base of the surface Ekman layer. The characteristics, which determine the extent to which bottom topography steers the circulation within the upper ocean, are dominated by latitude circles at low latitudes, but are increasingly influenced by the bottom topography at higher latitudes as the thermocline widens and intersects the sea floor. A solution is evaluated for the full three-dimensional circulation in the North Pacific. We find classical Sverdrup gyres within the thermocline, increasingly zonal flows at mid-depths, and weak topographically-bounded gyres within the abyssal ocean.

Interactions between Geostrophic Eddies and the Mean Circulation over Large-Scale Bottom Topography

Abstract The presence of intense recirculations in the abyssal oceans has been revealed by both observations and modeling studies. A suggested mechanism is the interactions between geostrophic eddies and the mean circulation in the presence of variable bottom topography. Here such interactions are studied using an idealized numerical model, consisting of a single active abyssal layer overlying variable bottom topography. An initial ensemble of eddies quickly organize themselves to generate a mean anticyclonic circulation around seamounts. During this rearrangement energy is approximately conserved while potential enstrophy is dissipated, consistent with previous studies of geostrophic turbulence. A parameterization of geostrophic eddies is formulated in terms of an eddy-induced transport, U*. Based on results from the eddy-resolving experiments it is hypothesized that U* should dissipate potential enstrophy while conserving energy. Using variational methods, a solution for U* is found that dissipates pote...

Ocean mixing studied near Hawaiian Ridge

The Hawaii Ocean Mixing Experiment (HOME) is a grassroots program to study turbulent mixing processes near the Hawaiian Ridge. The HOME is motivated by the desire to understand diffusive aspects of the advective‐diffusive balance that mediates the general circulation of the oceans. HOME is focused on tidally driven mixing, given the ubiquity of the tide as a deep‐sea energy source.As the sea surface cools at high latitude, surface waters sink. Subsidence rate is sufficient to fill the worlds ocean with cold bottom water in approximately 3,000 years. Diffusive processes that transfer heat into the abyssal ocean are required to maintain a steady‐state thermal structure. An effective eddy diffusivity of order Kp=10−4 m2 s−1, 700 times the molecular diffusivity of heat, is necessary [Munk, 1966]. Such a diffusivity might be supported by either mechanical mixing (turbulent transport) or thermodynamic (so‐called doubly diffusive) processes.

The effect of upper ocean eddies on the non‐steric contribution to the barotropic mode

The non‐steric contribution to sea surface height (SSH) variability hampers the use of satellite altimeter data in mapping steric‐related variability. Here, two eddy‐resolving 1/16° world ocean simulations are used to investigate the effects of mesoscale flow instabilities on the non‐steric (or abyssal ocean) contribution to the global barotropic mode. Model results show the non‐steric component accounting for >50% of the total SSH variability over 37% of the world ocean in the model, predominantly at mid and high latitudes. Most of this is either wind‐driven and deterministic or eddy‐driven and nondeterministic. Upper ocean flow instabilities drive deep flows and generate non‐steric SSH variability maxima (5–10 cm rms or more) in many major current systems throughout the world ocean. Resulting ocean anomalies are a nondeterministic response to atmospheric forcing and an eddy‐resolving data‐assimilative ocean model that demonstrates the essential dynamics is needed to depict their evolution.

Water-mass formation and potential vorticity balance in an abyssal ocean circulation

Our goal is to develop some understanding and intuition regarding abyssal ocean circulations. To do this we investigate highly idealized, source/sink-driven flows computed by a single layer, numerical ocean model forced by a prescribed source or sink. The interior circulation is always found to be very slow and Stommel-Arons like. On the other hand, the intense boundary currents may vary considerably from case to case, depending largely upon the potential vorticity (PV) associated with the source or sink. If the source is imposed by downwelling along a northern boundary, then the associated PV flux is zero, and the resulting steady circulation can induce no net frictional torque. The result is a rather complex pattern of boundary layer flow that includes a strong recirculation along the northern boundary. If the same mass flux is injected as a laminar, horizontal inflow, then the associated PV flux is significant, and must be balanced in steady state by frictional torque. The result is a unidirectional boundary layer flow away from the source. Other experiments elucidate the effect of vortex stretching on topography. For example, a horizontal outflow over shallowing topography induces a net cyclonic frictional torque in the boundary layer circulation of the basin. An understanding of the steady state PV balance thus appears to confer some insight into the form of boundary layer flows in these abyssal circulations.

The δ15N of nitrate in the Southern Ocean: Nitrogen cycling and circulation in the ocean interior

We report analyses of the nitrogen isotopic composition of nitrate in the eastern Indian and Pacific sectors of the Southern Ocean. In this paper, we focus on the subsurface data as well as data from the deep waters of other ocean basins. Nitrate δ15N is relatively invariant in much of the abyssal ocean (i.e., below 2.5 km), with a value of 4.8±0.2‰ observed in Lower Circumpolar Deep Water, North Atlantic Deep Water, and central Pacific deep water. The isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean, the Southern Ocean surface being an important exception. In the Subantarctic Zone (north of the Polar Frontal Zone) the nitrate δ15N of Upper Circumpolar Deep Water is ∼0.7‰ greater than that of Lower Circumpolar Deep Water. This isotopic enrichment appears to result from denitrification in the low‐latitude water masses with which Upper Circumpolar Deep Water communicates. The isotopic enrichment of Upper Circumpolar Deep Water is diminished in the Antarctic, probably because of the remineralization of sinking organic N, which has a low δ15N in the Antarctic. Relative to the other water masses of the Southern Ocean, the Subantarctic thermocline has a very low nitrate δ15N for its nitrate concentration because of exchange with the low‐latitude thermocline, where this isotopic signature appears to originate. This signature of the low‐latitude thermocline has two probable causes: (1) mixing with low‐nitrate surface water and (2) the oxidation of newly fixed N.

Oceanographic tracer release experiments using sulphur hexafluoride

We review recent results of experiments using sulfur hexafluoride tracer releases to investigate ocean mixing, gas exchange, and response to iron fertilization. A release method has been devised that allows large‐scale mixing experiments to be initiated using ∼ 100 kg or more of tracer accurately targeted on a given density surface. Combined with the very low detection limit of SF6, this means that these experiments can last up to several years and cover thousands of kilometers. The experiments have revealed that in the open ocean pycnocline, rates of mixing are low (0.1–0.2 cm2 s−1 at 300 m in the subtropical North Atlantic, for example). An approximately inverse relationship between diapycnal mixing and buoyancy frequency is suggested by most (but not all) tracer investigations to date, though this may not hold in the abyssal ocean far from any boundary. Much smaller scale releases into surface waters have been used to obtain measurements of gas exchange and to enable biogeochemical studies such as iron fertilization experiments on well‐defined volumes of water. These experiments take advantage of the fact that in surface water, continuous rapid analysis of sulfur hexafluoride enables the tracer to be tracked and mapped by the observing ship. Recent gas exchange studies using the “dual‐tracer” method are summarized; we suggest that they may be reconciled to values based on global 14C exchange by appealing to the effect of organic films observed in coastal waters by Frew [1997]. The tracer technique makes possible biogeochemical “patch” studies such as the Ironex I and II experiments in the equatorial Pacific. The use of the tracer measurements to guide and normalize measurements of the effect of added iron is illustrated.

Experiments Reveal How Heat Is Mixed into Cold Dense Water in the Abyssal Ocean

If the world's oceans relied only on molecular diffusion and smooth laminar flow to spread the Sun's heat, they'd consist of a thin Sun-warmed layer atop a mass of icy water. But other transport mechanisms are at work. In the North Atlantic, for instance, wind-driven currents push warm water from the Caribbean to the Arctic, where it cools, sinks, and flows back southward. And throughout its journey, the seawater is swirled and agitated by continent-sized gyres and centimeter-sized turbulent eddies.

Evidence for enhanced mixing over rough topography in the abyssal ocean

The overturning circulation of the ocean plays an important role in modulating the Earth's climate. But whereas the mechanisms for the vertical transport of water into the deep ocean--deep water formation at high latitudes--and horizontal transport in ocean currents have been largely identified, it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished. Turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise. However, measurements of the internal wave field, the main source of energy for mixing, and of turbulent dissipation rates, have typically implied diffusivities across surfaces of equal density of only approximately 0.1 cm2 s(-1), too small to account for the return flow. Here we report measurements of tracer dispersion and turbulent energy dissipation in the Brazil basin that reveal diffusivities of 2-4 cm2 s(-1) at a depth of 500 m above abyssal hills on the flank of the Mid-Atlantic Ridge, and approximately 10 cm2 s(-1) nearer the bottom. This amount of mixing, probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry, may be large enough to close the buoyancy budget for the Brazil basin and suggests a mechanism for closing the global overturning circulation.