Inflow Of Deep Water Research Articles

Variability of water properties, heat and salt fluxes in the Arabian Sea, between the onset and wane of the 1995 southwest monsoon

Abstract We investigate the variability of the circulation, water masses, heat and salt fluxes in the Arabian Sea over the course of the southwest monsoon. Two zonal sections taken along 8°30′N in 1995 as part of the Indian Ocean WOCE hydrographic program are used. The first was occupied in early June at the onset of the southwest monsoon winds, the second in late September, at the wane of the monsoon. The September section was found to be generally warmer (+0.32°C) and saltier (+0.04) than in June, despite a 50 mm drop in mean sea level. Therefore, the common assumption that an increase in sea-surface height follows an increase in heat content (the hydrostatic response) does not hold. Instead, we conclude that the heat content increases due to the advection of Arabian Sea Surface Water and Red Sea Water onto the section from the north, and the drop in sea level is due to a loss of mass, rather than heat, from the water column. There are large uncertainties involved in diagnosing the heat-flux divergence across the Arabian Sea, because the seasonal variability of the water masses and circulation in the basin mean that our data are not representative of a steady state. We treat each section separately and find an oceanic heat export of −0.72 PW in June and −0.19 PW in September, implying a basin cooling rate of about −0.36 PW in June and a slight heating of 0.12 PW in September. In June the mass and heat balances are dominated by the Ekman transport and the Somali Current, with very flat density surfaces resulting in a small interior geostrophic transport. By September the Ekman transport has reduced, and it is primarily the interior transport that balances a strong Somali Current. There are two main overturning cells in June and September: A shallow one of approximate magnitude 15 Sv in June and 0 Sv in September, which reaches depths of no more than 500 m and is driven by Ekman divergence at the surface; and a deep cell of magnitude 1 Sv representing a weak inflow and subsequent upwelling of Circumpolar Deep water. The deep cell implies a basin-averaged upwelling velocity of 3.2×10−5 cm s−1 through 2200 m.

Major depositional events under the deep Pacific inflow

The main inflow of deep water to the Pacific is via a deep western boundary current. As the current passes along the continental margin off eastern New Zealand, it receives much terrigenous sediment, which, together with the biogenic pelagic supply, has been reworked into a suite of large drifts. Ocean Drilling Program Site 1123 on the North Chatham Drift and Site 1124 on the Rekohu Drift have yielded high-resolution and well- dated records of carbonate and terrigenous fluxes for the past 3 m.y. Deposition at both drifts was affected by a deep western boundary current background sediment flux and by glacial-interglacial cycles. In addition, drifts received site-specific flux pulses caused by (1) sediment remobilization under a vigorous Antarctic Circumpolar Current, (2) increased oceanic productivity, (3) overspill of sediment from the newly formed Hikurangi Channel, and (4) generally increased discharge from the Hikurangi Channel under intensified Quaternary uplift and paleoclimatic oscillations.

Modeling and Observations of Deep Water Renewal and Entrainment in a Swedish Sill Fjord

A baroclinically forced deep-water renewal event in a small Swedish sill fjord is investigated using a one-dimensional numerical model and a dataset that resolves temporal variations in salinity and oxygen. The observations indicate an almost complete renewal of the basin water within a period of 2-3 weeks. The details of the renewal process are emphasized by modeling the sill flow as well as the resulting dense bottom plume, with various rates of entrainment. It is found that sill mixing is relatively unimportant, but that entrainment increases the deep-water inflow by a factor of 2-4. Different formulas for calculating plume entrainment are compared and the model's sensitivity to variations in sill flow and bottom friction is investigated. It is shown that even weak entrainment, occurring at shallow depths where the density difference between the plume and the resident water is large, has a significant impact. Entrainment prolongs the time it takes for a complete renewal (i.e., to flush out all resident water) and, on the moderate timescales considered here, yields lower post-renewal salinity and oxygen concentrations. This implies that entrainment (and mixing) during renewal may be as important as basin water diffusion in setting the timescale for forthcoming renewal events.

Zonal circulation in the NW Atlantic and Caribbean from a meridional World Ocean Circulation Experiment hydrographic section at 66°W

A World Ocean Circulation experiment Hydrographic Program section along 66°W in the North Atlantic was made in 1997. In addition to the usual variables (hydrographic and tracer) measured in WOCE onetime sections, we made lowered acoustic doppler current profiler (LADCP) measurements at nearly all of the stations. The section closed off a portion of the western North Atlantic to the west of the line, making a closed volume for constraining the circulation. In addition, the deep portions of the Caribbean do not communicate with the rest of the basin. By combining mass, silica, and LADCP information as constraints an inverse calculation obtained reveals a strong eastward transport in the Gulf Stream bounded by westward flowing water on either side. Within these energetic flows we see evidence for recently ventilated Classical Labrador Sea Water, which has not reached the Deep Western Boundary Current north of Puerto Rico in any significant amounts. Within the Caribbean our major new finding is a deep cyclonic circulation below sill depth in excess of 100 times the inflow of deep Atlantic water through the major deep sill: the Anegada‐Jungfern Passage. The signature of the deep Atlantic source water is most prominent in CFCs and both a bottom and mid‐depth maximum are present in the Caribbean to the south of Puerto Rico. Off the coast of Venezuela, however, only the deeper CFC maximum is found. For the entire section the net overturning circulation, heat flux, and freshwater fluxes are all consistent with expectations based on water mass formation and air‐sea exchanges to the west of our section, but the annual mean air‐sea fluxes of heat and freshwater from Comprehensive Ocean‐Atmosphere Data Sets appear somewhat too small in comparison with our single‐section result.

LIVING (STAINED) BENTHIC FORAMINIFERAL RESPONSE TO PRIMARY PRODUCTION AND HYDROGRAPHY IN THE DEEPEST PART OF THE GULLMAR FJORD, SWEDISH WEST COAST, WITH COMPARISONS TO HOGLUND'S 1927 MATERIAL

Living (stained) benthic foraminifera were studied in the deepest part (116 m) of the classical marine study area, the Gullmar Fjord on the Swedish west coast. This is a silled fjord with a strongly stratified water column where stagnant conditions temporarily develop in the deeper parts. The aim of this study is to determine how the living foraminiferal assemblage develops with seasonal variations in bottom water hydrography and primary production in the surface water. From August 1993 to December 1994, monthly hydrographic and oxygen measurements were made. Parallel undisturbed sediment cores were taken using a Multiple-corer. Foraminifera from the >63 μm fraction were stained with Rose Bengal to separate living from dead specimens. Monthly values of the surface water chlorophyll a content were also available and give a measure of the primary production. During spring 1994 the dominant foraminiferal species, Stainforthia fusiformis, multiplied its population size by seven times during one month as a consequence of food input from the spring phytoplankton bloom. The abundance of this species correlates positively with the surface water chlorophyll a content. Populations of several other species also increased during the spring of 1994. A deep-water inflow in February 1994 may have triggered reproduction of several foraminiferal species, recognized as a temporary population decline. We also compared our results with Hoglund’s (1947) foraminiferal study, performed in the same area in the summer of 1927. We found that a major alteration trending towards a more opportunistic low oxygen tolerant fauna has occurred since then.

Benthic foraminifera and their response to hydrography, periodic hypoxic conditions and primary production in the Koljö fjord on the Swedish west coast

The Koljö fjord on the Swedish west coast is a silled fjord characterised by strong stratification and stagnant bottom water, with periodically occurring hypoxic or anoxic conditions. In the Koljö fjord, renewal of the deep water generally occurs during winter. This study investigates how living benthic foraminifera react to hydrographic variations, periodic oxygen deficiency and variations in primary production. A series of monthly hydrographic measurements was made from August 1993 to December 1994, combined with sediment sampling along a (12–43 m) depth transect at five different sites. Monthly values of surface chlorophyll-a were available. Two periods of hypoxia to anoxia with one intervening period of oxic conditions, together with two autumn phytoplankton blooms and a spring phytoplankton bloom, made it possible to achieve the aims of this study. Below the pycnocline, three foraminiferal species: Elphidium excavatum clavatum, Elphidium incertum and Elphidium magellanicum represented more than 95% of the fauna. When oxygen content was very low, the foraminiferal fauna decreased but did not die out completely. A deep-water inflow in January 1994 caused the oxygen content to rise, but the foraminiferal population did not start to grow until three months later when the spring phytoplankton bloom sedimented out. Under oxic conditions, food availability seemed to limit the foraminiferal population. In itself, a very high organic content in the sediments does not seem to be a suitable food source; it is more likely that fresh phytoplankton is a potent food for these foraminifera. Reproduction of E. excavatum clavatum and E. incertum seems to have been triggered by increased food supply and sudden fluctuations in hydrographic variables. These foraminifera appear to grow from juvenile to adult in less than a month.

Investigation of subsurface water flow along the continental margin of the Eurasian Basin using the transient tracers tritium, 3He, and CFCs

During August–September 1993 the R/V Polarstern expedition ARK IX/4 was made to the Eurasian continental margin of the Arctic Ocean. Sections of oceanographic stations leading from the outer shelf over the continental slopes toward the deep Eurasian Basin were occupied north of the Barents and Laptev Seas. The distributions of the transient tracers CFC‐11, CFC‐12, tritium, and 3He were measured along these sections and are discussed in relation to water mass distribution and circulation along the continental margin of the Eurasian Basin. The tracer data strongly corroborate the temperature and salinity data [Schauer et al., 1997], clearly showing the inflow of Atlantic water in two branches, the Fram Strait branch and the Barents Sea branch. High CFC and tritium concen‐trations extended down to 1300 m over the Laptev continental slope, indicating strong inflow of the Barents Sea branch between Franz Josef Land and Severnaya Zemlya. The apparent ages of water in the Fram Strait branch were estimated to be about 2 years north of the Barents Sea and 6 years at the eastern end of the Nansen Basin, which corresponds to an apparent mean current speed of about 1 cm s−1. The age of the core of the Barents Sea branch was also estimated to be about 6 years over the Laptev slope region and to increase by 0.5 years as it flowed through this region. A lower limit for the inflow of newly formed Barents Sea Branch Water was estimated to be 0.5 Sv, and an overall transport for the sum of Fram Strait branch plus Barents Sea branch flow of 4 Sv was estimated from this aging. In the deep water there was clear evidence for the inflow of Norwegian Sea Deep Water through Fram Strait, but there appeared to be no significant input of shelf‐derived water from the Santa Anna Trough. If there was any shelf‐derived input to Eurasian Basin Bottom Water, it entered at a deeper depth than was sampled on the slope sections in the Laptev Sea or its flow is intermittent and was not occurring during the ARK IX/4 expedition.

Three-Dimensional Structure of Thermohaline Circulation Steered by Bottom Topography

An investigation based on the expansion in terms of diffusive vertical modes is made on the effect of an isolated seamount on the three-dimensional structure of thermohaline circulation driven by an inflow of deep water. While the circulation has a relatively simple structure dominated by the first baroclinic mode in a flat ocean, it becomes rather complicated in an ocean with a seamount. Nevertheless its three-dimensional structure is reconstructed and understood well by analyzing the dynamics of the vertical modes. It is shown that the vertical modes respond to the topography-induced stretching in a similar way irrespective of the mode number. When synthesized in the deep layer, therefore, the vertical modes interfere constructively to yield the same pattern of circulation as a single vertical mode or the diffusive reduced-gravity model. It is thus confirmed that the abyssal circulation certainly has the features predicted by the diffusive reduced-gravity model such as cyclonic circulation over the mountain or downwelling in the geostrophic island, in which geostrophic contours close on themselves. Close to the surface, however, different modes of different vertical structures interfere destructively so that the horizontal distribution of topography-induced circulation in the deep layer decays, upward shifting its center westward.

Water property and current distributions along the WHP‐P9 section (137°–142°) in the western North Pacific

The Japan Meteorological Agency carried out full‐depth, high‐resolution hydrographic measurements in July‐August 1994 along the 137°–142°E line as a contribution to the World Ocean Circulation Experiment. The line spans the subtropical and tropical regions of the western North Pacific with a full set of water mass and current distributions. At 28°N to the south of the Kuroshio recirculation, there exists an eastward current distinct from the currents associated with the subtropical fronts. The eastward current is characterized by the meridional thermal gradient extending down to 1500‐m depth. Deep silicate has a meridional boundary at 25°N, forming two cores of the maximum to the north and south of the boundary. The northern core centered at 28°N is meridionally coincident with other property extrema, indicating an inflow from the Northwest Pacific Basin down to 2000‐m depth. The southern core centered at 16°N suggests another westward inflow from the East Mariana Basin to the West Mariana Basin. A broad isopycnal depression at middepths and bottom water properties over the Shikoku and West Mariana Basins indicate a near‐bottom circulation along which the inflow water through the Yap‐Mariana Junction at 12°N fills the West Mariana Basin and then spreads to the Shikoku Basin and Philippine Basin successively. A part of the bottom water from the South Pacific bifurcates around the Yap‐Mariana Junction and enters the West Caroline Basin via the Yap Trench. This supports another path of deep water inflow to the Philippine Basin from the southernmost channel at 4000‐m depth.

Dynamic impact of oscillating sources on the North Atlantic deep circulation: A process study

In this process study the dynamic effects of fluctuating deep water sources on the deep circulation of the North Atlantic are examined by means of a 1½‐layer model. The model is used with a realistic topography of the Atlantic Ocean between 65°N and 25°S and has an open southern boundary. According to observations, the northern inflow of deep water is subject to variability on timescales between days and decades. The same holds for the Deep Western Boundary Current (DWBC), which acts as a link between the deep water inflow and the interior circulation. The timescales of the observed variability motivate the forcing functions used in the model. They determine the nature of the models response. The flow response relative to the local mean flow can be considerably larger than the original perturbation at the source. Normalizing the perturbed flow with the local steady state flow leads to the identification of sensitive areas. Their location and shape are determined by the difference between the characteristic travel paths of the perturbation and the streamlines of the steady state circulation. In the eastern equatorial Atlantic a forcing with periods between 1 and 3 months gives rise to larger flow perturbations than a forcing with periods between half a year and 1 year. Forcing with the latter periods leads to a stronger response in the western equatorial Atlantic and in the interior ocean. Changes in the direction of the deep equatorial flow are driven by low‐frequency oscillations of the DWBC in the model. This is consistent with observations in the area. Perturbations radiate into the interior away from the eastern boundary as long Rossby waves. As this propagation is along geostrophic contours, consequently large parts of the abyssal interior are inaccessible. The results support the notion that the area where the DWBC encounters the equator is not only a key region for the “slow” interhemispheric exchange of heat and tracers but also for the transport of information by “fast” internal waves along the equator.

Deep waters of the Arctic Ocean: origins and circulation

The Oden 91 Expedition has provided a data set that makes it possible to deduce more detailed ideas regarding the origin and circulation of waters in the Arctic Ocean. The three possible sources for the deep water of the Arctic Ocean are: (1) density flows down the continental slope triggered by brine enhanced waters formed on the continental shelves but consisting primarily of waters entrained from the Atlantic and intermediate layers; (2) inflow of Atlantic Water over the Barents Sea shelf that has experienced a density increase by cooling and freezing in that sea and then sinks with little entrainment down the St Anna Trough into deep layers of the Arctic Ocean; and (3) the inflow of Norwegian Sea Deep Water through Fram Strait. Of these three sources, the first appears to contribute the most to the Arctic Ocean deep water and the third the least. The Eurasian Basin communicates with the Canadian Basin through a boundary current that enters the Canadian Basin north of Siberia and leaves it north of Greenland. The fact that both the temperature and salinity are higher in the Canadian Basin than in the Eurasian Basin at levels above as well as below the sill depth of the Lomonosov Ridge indicates that slope convection is active in the Canadian Basin. The deepest layers have constant salinity, but show a weak temperature decrease towards the bottom. This suggests that these layers of the Canadian Basin are not primarily renewed by convection down the continental slope but by a spill over of Eurasian Basin Deep Water across the central part of the Lomonosov Ridge. A model which incorporates density flows triggered by high salinity shelf water and water overflowing the Lomonosov Ridge flom the Eurasian Basin is applied to reproduce the observed profiles of the Canadian Basin and to establish the relative importance of these two sources. By incorporation of I4C profiles into the model, estimates of the exchange rates of water in these layers can be made.

Spin‐up and breakdown of source‐driven deep North Atlantic flow over realistic bottom topography

In this study we examine the time dependent behaviour of the deep North Atlantic dynamics. We use a 1½‐layer, nonlinear, shallow water model with realistic topography and high‐spatial resolution. It is driven by an inflow of deep water from a northern source and by parameterized density exchange processes at the interface. The evolving circulation pattern is complex. The Mid‐Atlantic Ridge with the Charlie Gibbs Fracture Zone has a fundamental influence on the large‐scale flow field. A reduction of the inflow causes a temporary reversal of the interior currents. The perturbation travels along the geostrophic contours. The response times of the currents for the spin‐up and breakdown experiments are of the order of years. They differ for different areas of the North Atlantic Ocean. The area which is affected last by the perturbation is the northwesternmost interior corner, which is close to the position of the source.

A circulation model of the abyssal Tasman Sea

A primitive equation shallow-water model with idealized topography has been used to investigate the dynamics of the abyssal Tasman Sea. The questions addressed relate to the influence of topography on the circulation pattern and the forcing mechanisms. Circulations driven by both inflow of Deep Antarctic Circumpolar Current water and East Australian Current eddies have been considered. The former usually results in upwelling that drives cyclonic flow throughout the basin. Strong flow is mainly restricted to regions of sharp potential vorticity gradients such as the continental rise, while flow over the abyssal plain is generally much weaker. Simulated East Australian Current eddies propagate their influence through Rossby wave generation. Over the abyssal plain, potential vorticity gradients are weak and the resulting low energy waves have little effect on the wider circulation. Eddies over the continental rise are more efficient at generating longer waves which can propagate large distance along geostrophic contours. Those generated in the west (East Australian rise) tend to be strongly scattered by blocked geostrophic contours to the north, resulting in enhanced dissipation and very limited mean flow generation. However, those generated in the east (Lord Howe Rise) propagate into the basin interior and generate strong mean flows along geostrophic contours. The model results suggest that Circumpolar Current forcing can most easily account for available observations.

Exchange of Water and Nutrients Between the Skagerrak and the Kattegat

Simultaneous CTD and current measurements have been made repeatedly at several stations in a cross-section between the Danish island Læsø and the Swedish west coast. The cross-section is situated in the northern Kattegat which forms the outer part of the Baltic estuary. Parallel nutrient and oxygen measurements were made in order to obtain a suitable set of background data for ecological modelling of the Kattegat. Here, this dataset is used to calculate fluxes of water, nutrients and oxygen, as functions of salinity. A division is made between the biologically passive winter period, November to February (when the inorganic nutrients components are nearly conservative) and a biologically active 'summer' period from March to October. A striking feature is the much stronger inflow of high salinity deep water during winter, approximately 64 000 m 3 s -1, compared to only 28 000 m 3 s -1 during summer. The inflow is concentrated to salinities higher than 33 PSU. The surface water outflow occurs in the interval 23 to 27 PSU. While the inflow of oceanic deep water to the Kattegat occurs on the deeper eastern side of Læsø, the outflow of surface water from the Baltic seems to be dominated by the flux west of Læsø.

A model of the deep water flow into the Baltic Sea

Deep water renewal in the Baltic basins is a consequence of gravitationally driven bottom currents, transporting water from the North Sea area towards the deeper parts of the estuary. The dense currents have to pass several topographic constrictions, one of them being the Bornholm Channel. In this study, a numerical two-layer, limited area model has been applied to the southern Baltic Proper, in which the Bornholm Channel is located. The mathematical problem was solved as an initial value problem with rigid boundaries. The results were used to obtain a simplified formula, determining the dense bottom water flow as a function of the stratification in the upstream Arkona Basin. This relation makes it possible to calculate the deep water inflow in detail, since hydrographic observations are frequent, whereas direct transport measurements are scarce in the area. The calculated flow characteristics in the Bornholm Channel are in accordance with observations. The volume flow was found to vary between 0.9 × 10 4 and 2.9 × 10 4 m 3 s -1 , using upstream basin values of density differences and pycnocline depths close to the observed mean magnitudes. The model indicates that inertia, Coriolis and frictional forces along with the upstream stratification and topography control the channel flow. Quasi-steady state is obtained within 24 h, whereas the time-scale for a water volume to pass the channel is approximately 2.3 days. DOI: 10.1034/j.1600-0870.1992.t01-4-00005.x

Dispersal of suspended matter in Makasar Strait and the Flores Basin

In November 1984 in Makasar and the Flores Basin water samples were collected (T, S, dissolved O 2, total CO 2), bottom samples (sediment composition) and suspended matter (particle composition, particle size). A sediment trap was moored in the Flores Basin at 4600 m depth for nearly four months, covering the dry season. In the Flores Basin there are indications for bottom flow resuspending bottom material or preventing suspended material from settling; in Makasar Strait there is probably inflow of deep water both from the south and from the north, resulting in a very slow bottom water flor. Bottom deposits in Makasar Strait and the Flores Basin are predominantly terrigenous, with an admixture of organic carbonate and silica (mostly coccoliths). Volcanic material is primarily present near to the volcanoes in the south and reaches the deeper basins by slumping. In the suspended matter no volcanic particles and little planktonic material were found, although the latter form 10 to 15% of the top sediment and of the material deposited in the sediment trap. In suspension particles with a large concentration of tin (Sn) were found associated mainly with iron. They probably come from northern Kalimantan or northern Sulawesi. Suspended matter concentrations were mainly less than 0.5 mg·dm −3, only off the Mahakam river mouth were concentrations higher than 1 mg·dm −3. Particle size was erratic because of the variable composition of the coarser particles in suspension. Organic matter concentrations in suspension (in mg·dm −3) roughly follow the distribution of total suspended matter but organic content (in %) of the suspended matter does not show any trends. All organic matter in suspension is of marine origin except in the Mahakam river and estuary. Deposition rates, as estimated from the sediment trap results, are 150 mg·cm −2·a −1 for the total sediment, 26 mg·cm −2·a −1 for carbonate and 13 mg·cm −2·a −1 for organic matter. Flocs and fibres in suspension were only found in and below the Mahakam river plume that reaches ca 400 km from the river mouth to the southeast, and in surface waters associated with plankton (diatoms). The formation of these flocs (broken-up macroflocs or marine snow) is primarily related to particle concentration, turbulence, and the presence of organisms that produce sticky material or glue particles together.

Weddell Gyre: Temperature maximum stratum

At depths below 200 m, relatively warm, salty water spreads poleward from the Antarctic Circumpolar Current (ACC). This deep water mass is cooled, through interaction with the atmosphere, feeding the formation of dense bottom water which in turn influences much of the world ocean. Significant poleward spreading is accomplished within the Weddell gyre of the Atlantic sector of the southern ocean. An extensive data set obtained in the 1980s within the Weddell gyre provides a detailed view of the subsurface temperature maximum (T‐max) stratum defining the top of the deep water mass. The seasonal sea ice growth and retreat pattern reflects the T‐max distribution. There is evidence for subdivision of the T‐max into two cold centers, which may reflect secondary circulation features within the gyre. The primary inflow of warm deep water is derived from the southern edge of the ACC near 20°–30°E. The inflow spreads to the west along 65°S and is divided into smaller “pools” of relatively warm water west of Maud Rise with a cold feature directly over the rise, in a manner shown by models of topographically generated eddies. Variability of the T‐max stratum in the vicinity of Maud Rise arising from changes in the vigor of the large‐scale circulation may influence the regional vertical heat fluxes and be related to interannual variability of the sea ice cover and to the recurring polynya feature typical of the region.

A Numerical Study of Loop Current Intrusions and Eddy Shedding

The dynamics of the eddy shedding by the Loop Current in the Gulf of Mexico have been investigated using three nonlinear numerical models: two-layer, barotropic and reduced gravity. The barotropic and reduced gravity models demonstrate the individual behavior of the external and internal modes, and provide insight into how they interact in the two-layer model. Because of the economy of the semi-implicit free surface models, it was possible to perform over 100 experiments to investigate the stability properties of the Loop Current. Typically, the models were integrated 3–5 years to statistical equilibrium on a 1600 km×900 km rectangular domain with a resolution of 20 km×18.75 km. Prescribed inflow through the model Yucatan Channel was compensated by outflow through the Florida Straits. A long-standing hypothesis is that the Loop Current sheds eddies in response to quasi-annual variations in the inflow. We find that the Loop Current can penetrate into the Gulf, bend westward, and shed realistic anticyclonic eddies at almost an annual frequency with no time variation in the inflow. In this regime, the eddy-shedding rate depends on the internal Rossby wave speed, an eddy diameter derived from conservation of potential vorticity on a β-plane, the angle of the inflow, and to a lesser extent on the Reynolds number. Eddy shedding can be prevented by reducing the Reynolds number sufficiently. However, the Loop Current still spreads far westward. The steady-state solution for a highly viscous case was found to be almost the same as the mean over an eddy cycle for a lower viscosity case which shed discrete eddies of large amplitude. Eddy shedding and westward spreading of the Loop can be prevented at higher Reynolds numbers when the beta Rossby number RB = vc/(βLp2) > 2, where the appropriate length scale Lp is one-half the port separation distance and vc is the velocity at the core of the current. Differential rotation (β) is also of great importance in determining the diameter and westward speed of the eddies and the penetration of the Loop Current into the Gulf. In a few of the two-layer experiments, baroclinic and mixed instabilities were encountered, but experiments dominated by a horizontal shear instability of the internal mode produced the most realistic results. For sufficiently high Reynolds numbers the shear instability occurred in both the barotropic and reduced gravity models. However, for realistic parameter values eddy shedding occurred in the two-layer and reduced gravity models, but not in the barotropic model. Consistent with potential vorticity conservation dynamics, the Loop Current and its eddy shedding behavior were quite insensitive to the location and width of the inflow and outflow ports, so long as the western boundary did not interfere with the shedding process and the ports were not separated by much less than 1/???? times a theoretical eddy diameter, i.e., when RB < 2. When the entire eastern boundary was opened, the outflow remained confined to a current adjacent to the southern boundary. Also, while the solution depends on the maximum velocity at inflow, it is relatively insensitive to the shape of the inflow profile. In the presence of significant deep-water inflow through the Yucatan Straits, bottom topography may prevent Loop Current penetration, westward spreading and eddy shedding. In these cases the interaction between the bottom topography and the pressure field near the Florida Shelf results in a near balance between the pressure torques and the nonlinear terms in the mass transport vorticity equation. When the Yucatan Straits deep-water inflow is reduced or the Florida Shelf is moved to the east, the eddy shedding reappears. A kinematic analysis shows that a sufficiently strong current following f/h contours of the Florida Shelf and intersecting the Loop Current at large angles can locally prevent northward penetration of the Loop Current and effectively reduce the port separation. Thus, the effect of the Florida Shelf is similar to cases in the reduced gravity model where the ports are too close for eddy shedding to occur, i.e., when RB>2. Bottom topography also inhibits development of baroclinic instability, yielding solutions more closely resembling those from the reduced gravity model than from the two-layer flat bottom model. However, movement of the shed eddies is significantly modified by the introduction of topography. In the presence of realistic time variations in the upper layer inflow, the eddy-shedding period is dominated by the natural period rather than the forcing period, although the influence of the latter is not negligible.

Deep silicate content as evidence of renewal processes in the Venezuela Basin, Caribbean Sea

Average dissolved silicate concentrations in waters deeper than 1400 m of the Venezuela Basin (Eastern Caribbean Sea) increase from 27.7 μ m in the north to greater than 29.0 μ m in the south. Standard deviations of these values (a measure of temporal heterogeneity) decline from 1.8 in the north to 1.0 in the south. These gradients result from sporadic inflow of silicate-poor North Atlantic Deep Water over the Jungfern Sill at the north end of the basin. This inflow lowers the silicate concentrations and increases temporal variability in the north. Mixing of this inflow water southward across the basin causes the observed north-to-south gradients in mean silicate concentrations and in standard deviations about these means.

Biogenous Deep-Sea Sediments: Fractionation by Deep-Sea Circulation

There are two types of marginal seas: the lagoonal type with deep water outflow, and the estuarine type with deep water inflow. Lagoonal basins are characterized by waters that have low nutrient concentrations and high salinity and are well aerated; their sediments are rich in calcium carbonate. Estuarine basins contain waters with high nutrient concentrations, low salinity, and relatively low oxygen values; their sediments are rich in silica and organic matter and tend toward being anaerobic. The Atlantic Ocean is “lagoonal” in that it has a deep water outflow and collects lime on its floor; the Pacific Ocean is “estuarine” with a deep water inflow and silica-rich sediments. The oceanic fractionation of silica from lime through basin-basin exchange of water at different depth levels is due to both supply and preservation patterns. Supply of biogenous precipitates to the ocean floor exceeds influx of solutes to the ocean, requiring dissolution on the ocean floor to maintain steady state. Supply is greatest in estuarine basins. Dissolution of lime proceeds fastest in estuarine basins, which have CO 2 -rich deep waters, whereas dissolution of silica is greatest in lagoonal basins, where the deep water is farthest from saturation because of low dissolved nutrient concentrations. Trace elements may conceivably be affected also by this fractionation mechanism. Some implications for the geological record are discussed.