ALBERT

Description: The modification of the exchange flow in a deep southern hemisphere passage, resembling the Vema Channel, by frictionally induced secondary circulation is investigated numerically. The hydrostatic primitive equation model is a two-dimensional version of the sigma-coordinate Princeton Ocean Model. The time dependent response of a stratified along-channel flow, forced by barotropic or baroclinic pressure gradients, is examined. Near the bottom, where the along-channel now is retarded, there is cross-channel Ekman nux that is associated with downwelling on the eastern side and upwelling on the western side of the channel. In the presence of stratification the cross-channel flow rearranges the density structure, which in turn acts on the along-channel velocity via the thermal wind relation. Eventually the cross-isobath Ekman flux is shut down. In the case of baroclinically driven flow of Antarctic Bottom Water through the Vema Channel the model reproduces the observed shape of the deep temperature profiles and their cross-channel asymmetry. The model offers an explanation that is alternative or supplementary to inviscid multilayer hydraulic theory that;was proposed in earlier studies. It explains the extremely thick bottom boundary layers in the center and on the western slope of the channel. The deep thermocline is spread out in the west and sharpened in the east, and the coldest water is found on the eastern side of the deep trough; The modified density field reduces the along-channel flow near the bottom and focuses it into a narrow jet on the eastern side of the channel.

Description: We examine recent observations of water mass distribution and circulation schemes at different depths of the South Atlantic Ocean to propose a layered, qualitative representation of the mean distribution of flow in this region. This furthers the simple upper layer geostrophic flow estimates of Peterson and Stramma [1991]. In addition, we assess how well ocean general circulation models (GCMs) capture the overall structure of flow in the South Atlantic in this regard. The South Atlantic Central Water (SACW) is of South Atlantic origin in the subtropical gyre, while the SACW in the tropical region in part originates from the South Indian Ocean. The Antarctic Intermediate Water in the South Atlantic originates from a surface region of the circumpolar layer, especially in the northern Drake Passage and the Falkland Current loop, but also receives some water from the Indian Ocean. The subtropical South Atlantic above the North Atlantic Deep Water and north of the Antarctic Circumpolar Current (ACC) is dominated by the anticyclonic subtropical gyre. In the eastern tropical South Atlantic the cyclonic Angola Gyre exists, embedded in a large tropical cyclonic gyre. The equatorial part of the South Atlantic shows several depth-dependent zonal current bands besides the Angola Gyre. Ocean GCMs have difficulty capturing this detailed zonal circulation structure, even at eddy-permitting resolution. The northward extent of the subtropical gyre reduces with increasing depth, located near Brazil at 16°S in the near-surface layer and at 26°S in the Antarctic Intermediate Water layer, while the tropical cyclonic gyre progresses southward. The southward shift of the northern part of the subtropical gyre is well resolved in global ocean GCMs. However, high horizontal resolution is required to capture the South Atlantic Current north of the ACC. The North Atlantic Deep Water in the South Atlantic progresses mainly southward in the Deep Western Boundary Current, but some water also moves southward at the eastern boundary.

Description: The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally-buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 10(6)m(3)s(-1)), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.

Description: The zonal circulation south of Sri Lanka is an important link for the exchange of water between the Bay of Bengal and the Arabian Sea. Results from a first array of three moorings along 80 degrees 30'E north of 4 degrees 10'N from January .1991 to March 1992 were used to investigate the Monsoon Current regime [Schott et al., 1994]. Measurements from a second array of six current meter moorings are presented here. This array was deployed along 80 degrees 30'E between 45'S and 5 degrees N from July 1993 to September 1994 to investigate the annual cycle and interannual variability of the equatorial currents at this longitude. Both sets of moorings contribute to the Indian Ocean current meter array ICM8 of the World Ocean Circulation Experiment. The semiannual equatorial jet (EJ) was showing a large seasonal asymmetry, reaching a monthly mean eastward transport of 35 Sv (1 Sv = 1 x 10(6) m(3) s(-1)) in November 1993, but just 5 Sv in May 1994. The Equatorial Undercurrent (EUC) had a maximum transport of 17 Sv in March to April 1994. Unexpectedly, compared to previous observations and model studies, the EUC was reappearing again in August 1994 at more than 10 Sv transport and was still flowing when the moorings were recovered. In addition, monthly mean ship drifts near the equator are evaluated to support the interpretation of the moored observations. Interannual variability of the EJ in our measurements and ship drift data appears to be related to the variability of the zonal winds and Southern Oscillation Index. The output of a global numerical model (Parallel Ocean Climate Model) driven by the winds for 1993/1994 is used to connect our observations to the larger scale. The model reproduces the EJ asymmetry and shows the existence of the EUC and its reappearance during summer 1994.

Description: We review what is known about the convective process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth's rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convective and geostrophic scales are not very disparate from one another. Detailed observations of the process in the Labrador, Greenland, and Mediterranean Seas are described, which were made possible by new observing technology. When interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection, great progress in our description and understanding of the processes at work is being made.

Description: Physical influences on biological primary production in the North Atlantic are investigated by coupling a four-component pelagic ecosystem model with a high-resolution numerical circulation model. A series of sensitivity experiments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusivity implicit in upstream advection approximately doubles primary production when compared with a less diffusive, higher-order, positive-definite advection scheme.This is of particular concern in the equatorial upwelling region where upstream advection leads to a considerable increase of upper ocean nitrate concentrations. Counteracting this effect of unrealistically large implicit diffusion by changes in the biological model could easily lead to misconceptions in the interpretation of ecosystem dynamics. Subgrid-scale diapycnal diffusion strongly controls biological production in the subtropical gyre where winter mixing does not reach the nutricline. The parameterization of vertical viscosity is important mainly in the equatorial region where friction becomes an important agent in the momentum balance.

Description: Phytoplankton processes in subantarctic (SA) waters southeast of New Zealand were studied during austral autumn and spring 1997. Chlorophyll a (0.2–0.3 μg L−1) and primary production (350–650 mg C m−2 d−1) were dominated by cells 〈2 μm (cyanobacteria) in both seasons. The photochemical efficiency of photosystem II (Fυ/Fm) of cells was low (0.3), indicating physiological stress. Dissolved Fe (DFe) levels in surface waters were subnanomolar, and the molecular marker flavodoxin indicated that cells were iron stressed. In contrast, Subtropical Convergence (STC) and subtropical waters had higher algal biomass/production levels, particularly in spring. In these waters, DFe levels were 〉1 nmol kg−1, there was little evidence of Fe-stressed algal populations, and Fυ/Fm approached 0.60 at the STC. In addition to these trends, waters of SA origin were occasionally observed within the STC and north of the STC, and thus survey data were interpreted with caution. In vitro Fe enrichment incubations in SA waters resulted in a switch from flavodoxin expression to that of ferredoxin, indicating the alleviation of Fe stress. In another 6-day experiment, iron-mediated increases in chlorophyll a (in particular, increases in large diatoms) were of similar magnitude to those observed in a concurrent Si/Fe enrichment; ambient silicate levels were 4 μM. A concurrent in vitro Fe enrichment, at irradiance levels comparable to the calculated mean levels experienced by cells in situ, resulted in relatively small increases (approximately twofold) in chlorophyll a. Thus, in spring, irradiance and Fe may both control diatom growth. In contrast, during summer, as mean irradiance increases and silicate levels decrease, Fe limitation, Fe/Si colimitation, or silicate limitation may determine diatom growth.

Description: The evolution of the Black Sea's salinity after the opening of the Bosporus about 7500 years ago is investigated using a simple two-box model. The model consists of watermass and salt conservation equations, and allows for changes in halocline depth. The paleoceanographic box model is forced by present-day Mediterranean inflow and outflow, and atmospheric forcings. Analytic solutions for the evolution of the box volumes are given. Model salinities reach 90% of their the present-day values in both boxes about 2,500 years after the opening of the Bosporus. The evolution of the salinities is shown to be almost independent of the evolution of the box volumes, and the results are compared with the existing paleoceanographic proxy records.

Description: A spectrum of halogenated hydrocarbon compounds in marine air masses were surveyed over an area in the western Pacific between 43°N, 150°E and 4°N, 113°E in September 1994. The ship's track between northern Japan and Singapore traversed three climatic zones of the northern hemisphere. Recently polluted air, clean marine air derived from the central Pacific Ocean from different latitudes, and marine air from the Indonesian archipelago were collected. Tetrachloroethene and trichloroethene of anthropogenic origin, brominated halocarbons as tribromomethane, dibromochloromethane and bromodichloromethane of anthropogenic and natural sources, and other trace gases were measured in the air samples. Very sparse data on the distribution of these compounds exist for the western Pacific atmosphere. The distribution patterns of the compounds were related to synoptic-scale meteorology, spatial conditions, and origin of the air masses. Anthropogenic and natural sources for both chlorinated and brominated substances were identified. Tetrachloroethene and trichloroethene concentrations and their ratios identify anthropogenic sources. Their mixing ratios were quite low compared to previously published data. They are in agreement with expected low concentrations of photochemically active substances during autumn, with an overall decrease in concentrations toward lower latitudes, and with a decrease of emissions during recent years. Strong evidence for a natural source of trichloroethene was discovered in the tropical region. The concentrations of naturally released brominated species were high compared to other measurements over the Pacific. Gradients toward the coasts and elevated concentrations in air masses influenced by coastal emissions point to significant coastal sources of these compounds. The trace gas composition of anthropogenic and natural compounds clearly identified the air masses which were traversed during the cruise.

Description: The temporal variability of the greater Agulhas Current system has important climatological consequences. Some recent results have suggested that this variability contains a large seasonal component, due to changes in the circulation at latitudes poleward of Madagascar only. A model simulation shows that the contribution of Tropical Surface Water to Agulhas Current waters, via the Mozambique Channel, also has a distinct seasonal characteristic that is brought about by the seasonal wind stress over the tropical Indian Ocean. This simulated flow through the Channel contributes substantially to the seasonality of the Agulhas Current. This model result is shown to be not inconsistent with available hydrographic observations.

Description: Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean, The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Provencal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Provencal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600-1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10(6) m(3) s(-1)), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero-Provencal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Provencal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior

Description: In this paper we discuss two different methods of inferring characteristics of the interior ocean dynamics from radar signatures of internal solitary waves visible on synthetic aperture radar (SAR) images. The first one consists in the recognition and the interpretation of sea surface patterns of internal solitary waves; the second one consists in the analysis of the modulation depth of the normalized radar backscattering cross section (NRCS) associated with internal solitary waves. For this purpose we consider a data set composed of SAR and in situ measurements carried out from 1991 to 1997 in the region of the Strait of Messina. The recognition and the interpretation of sea surface patterns of internal solitary waves in the Strait of Messina can be used to study characteristics of the density distribution in the area: The internal wave field varies with seasonal variations in the vertical density stratification and with remotely induced variations, i.e., variations induced by the larger-scale circulation, in the horizontal density distribution. In order to inquire into the possibility of inferring parameters of the interior ocean dynamics by analyzing the modulation of the NRCS associated with internal solitary waves, several numerical simulations are carried out using a radar imaging model. These simulations are performed by assuming different wind conditions and internal wave parameters. It is shown that an accurate knowledge of wind conditions is crucial for deriving internal wave parameters and hence parameters of the interior ocean dynamics from the modulation of measured NRCS associated with internal solitary waves.

Description: A series of experiments with a quasi‐geostrophic model have been carried out to investigate the influence of topographic obstacles on the translatory movement of Agulhas rings. The rings were initialized as Gaussian‐shaped anomalies in the stream function field of a two‐layer ocean at rest. Bottom topography consisted of a meridional ridge of constant height in the middle of the quadratic model domain. The vertical ring structure, the initial ring position, and the height of the ridge were varied. The general northwestward movement of the model eddies has been shown to be modified toward a more equatorward direction by encountering the upslope of the ridge. Sufficient topographic heights and strong slopes can even block the eddies and force them toward a pure meridional movement. During their translation the eddies lose their vertical coherence. After about 150 days the eddy can only be detected by the surface signal, while the lower layer eddy is dispersed by the radiation of Rossby waves. The passage of “young” (regarding the time between their initialization and their contact with the ridge) and energetic eddies is accompanied by the observation of along‐slope currents of significant strength. These may be due to the rectification of radiated Rossby waves at the topographic slope. Only eddies with a significant dynamic signal in the lower layer are influenced by the bottom topography. Strong, shallow eddies over deep lower layers can cross the ridge without strong modification of their translatory movement.

Description: Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3 s−1). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales. Modeling studies suggest that the Indian-Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Currents' retroflection. State-of-the-art global and regional “eddy-permitting” models show a reasonably realistic representation of the mean Agulhas system; but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement. In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchanges dynamics and forcing, its representation in state-of-the-art models, and, finally, the impact of the Indian-Atlantic fluxes on regional and global scale both within the Atlantic Ocean and in interaction with the overlying atmosphere.

Description: Analysis of multiple climate simulations shows much of the midlatitude Pacific decadal variability to be composed of two simultaneously occurring elements: One is a stochastically driven, passive ocean response to the atmosphere while the other is oscillatory and represents a coupled mode of the ocean‐atmosphere system. ENSO processes are not required to explain the origins of the decadal variability. The stochastic variability is driven by random variations in wind stress and heat flux associated with internal atmospheric variability but amplified by a factor of 2 by interactions with the ocean. We also found a coupled mode of the ocean‐atmosphere system, characterized by a significant power spectral peak near 1 cycle/20 years in the region of the midlatitude North Pacific and Kuroshio Extension. Ocean dynamics appear to play a critical role in this coupled air/sea mode.

Description: The coupling on decadal time scales of the mid‐latitude and tropical Pacific via an oceanic ‘bridge’ in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model. Results indicate that in the tropics decadal anomalies of isopycnal depth are forced by Ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid‐latitudes of either hemisphere. In the coupled model, temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere, but are lag correlated between southern hemisphere mid‐ and low‐latitudes. However, anomaly magnitudes on the equator are small. These results suggest that the oceanic ‘bridge’ to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial Pacific. Coupling to the southern mid‐latitudes via temperature anomalies on isopycnals remains an intriguing possibility.

Description: Analysis of global climate model simulations and observations suggest decadal, midlatitude changes in and over the North Pacific cause decadal modulation of the El Niño‐Southern Oscillation. This coupling between the two geographic regions is via atmospheric, not oceanographic, teleconnections. In essence, large scale changes in the circulation of the atmosphere over the Pacific Basin, while largest in midlatitudes, have a significant projection onto the wind field overlying the equatorial regions. These low frequency wind changes precondition the mean state of the thermocline in the equatorial ocean to produce prolonged periods of enhanced or reduced ENSO activity. The midlatitude variability that drives equatorial impacts is of stochastic origin and, although the magnitude of the signal is enhanced by ocean processes, likely unpredictable.

Description: The advection of sea ice and associated freshwater/salt fluxes in the Weddell Sea in 1986 and 1987 are investigated with a large‐scale dynamic‐thermodynamic sea ice model. The model is validated and optimized by comparison of simulated sea ice trajectories with observed drift paths of six buoys deployed on the Weddell Sea ice. The skill of the model is quantified by an error function that measures the deviations of simulated trajectories from observed 30‐day sea ice drift. A large number of sensitivity studies show how simulated sea ice transports and associated freshwater/salt fluxes respond to variations in physical parameterizations. The model reproduces the observed ice drift well, provided ice dynamics parameters are set to appropriate values. Optimized values for the drag coefficients and for the ice strength parameter are determined by applying the error function to various sensitivity studies with different parameters. The optimized model yields a mean northward sea ice volume export out of the southern Weddell Sea of 1693 km3 in 1986 and 2339 km3 in 1987. This shows the important role of sea ice transport for the freshwater budget of the Weddell Sea and gives an indication of its high interannual variability.

Description: We compare estimates of the anthropogenic CO2 content of seawater samples from the subpolar North Atlantic Ocean calculated on the basis of a back-calculation technique with measurements of the chlorofluorocarbon CFC-11. Estimated anthropogenic CO2 concentrations are in the range 10–80 µmol kg-1, while CFC-11 concentrations cover the full range from below detection limit to 〉 5 pmol kg-1 in waters at atmospheric equilibrium. The majority of the data points show a linear correlation between anthropogenic CO2 concentrations and CFC-11 saturation, which can only be explained by the strongly advective nature of the North Atlantic Ocean. Only deep eastern basin samples deviate from this general observation in that they show still significant concentrations of anthropogenic CO2 where CFC-11 is no longer detectable. In order to remove the influence of the Revelle factor reflected in the anthropogenic CO2 concentrations we have calculated 'excess' pCO2, showing an even tighter linear correlation with atmospheric equilibrium concentrations of CFC-11.

Description: This study presents basin-wide anthropogenic CO2 inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified ΔC* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO2 and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO2 column inventories are observed south of 50°S. The total anthropogenic CO2 inventory north of 35°S was 13.6±2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1±1 Pg C for the same area. Approximately 6.7±1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ±3 Pg C for 1995. These estimates are compared to anthropogenic CO2 inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61–0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.

Description: During May - August, 1997, the distributions of dissolved methane and CCl3F (CFC11) were measured in the Atlantic between 50° and 60°N. In surface waters throughout the region, methane was observed to be close to equilibrium with the atmospheric mixing ratio, implying that surface ocean methane is tracking its atmospheric history in regions of North Atlantic Deep Water formation. Despite the different atmospheric history and ocean chemistry of CH4 and CFC11, their spatial distribution patterns in the water column are remarkably similar. One-dimensional distributions have been simulated with an advection-diffusion model forced by the atmospheric histories. The results suggest that the similar patterns result from the increasing input of CH4 and CFC11 to newly formed deep waters over time, combined with the effect of horizontal mixing and the oxidation of methane on a 50 year time scale.

Description: A research cruise has documented changes in rift tectonics, volcanism, and hydrothermalism along the least studied and most enigmatic sector of a crustal complex in the southwest Pacific Ocean. Results from the longitudinal transect are expected to provide insight into processes involving the Kermadec arc-Havre backarc (KAHB) system, a continuum from oceanic spreading to continental rifting at a convergent plate boundary KAHB forms the central sector of an active, 2000-km arc-backarc complex between Tonga and New Zealand (Figure 1). The expedition also engaged in the first comprehensive survey of submarine vents in the Taupo Volcanic Zone (TVZ) at the south end of the KAHB system. Identified in the off-shore segment of TVZ were three major hydrothermal vent areas associated with late Quaternary fault structures. Data from the expedition and from other recent research in the same area addressed questions concerning the type of hydrothermal venting, magmatic heterogeneity along and across KAHB, the style of backarc rifting, and tectonic and magmatic consequences of anomalous terranes colliding with the subduction margin.

Description: The subsurface flow within the subantarctic and subtropical regions around the Brazil-Malvinas (Falkland) Confluence Zone is studied, using daily hydrographic and kinematic data from four subsurface floats and a hydrographic section parallel to the South American shelf. The float trajectories are mapped against sea surface flow patterns as visible in concurrent satellite sea surface temperature (SST) images, with focus on the November 1994 and October/November 1995 periods. The unprecedented employment of Lagrangian θ-S diagrams enables us to trace the advection of patches of fresh Antarctic Intermediate Water (AAIW) from the Confluence Zone into the subtropical region. The fresh AAIW consists of a mixture of subtropical AAIW and Malvinas Current core water. Within the subtropical gyre, these patches are discernible for extended periods and drift over long distances, reaching north to 34°S and east to 40°W. The cross-frontal migration of quasi-isobaric floats across the Confluence Zone from the subtropical to the subantarctic environment is observed on three occasions. The reverse process, float migration from a subpolar to a subtropical environment was observed once. These events were located near 40°S, 50°W, the site of a reoccurring cold core feature. Subsurface float and SST data comparison reveals similarities with analogous observations made in the Gulf Stream [Rossby, 1996] where cross-frontal processes were observed close to meander crests. The limited number of floats of this study and the complex structure of the Brazil-Malvinas Confluence Zone, however, restricts the analysis to a description of two events.

Description: We present a compilation and analysis Of N2O data from the deep-water zone of the oceans below 2000 m. The N2O values show an increasing trend from low concentrations in the North Atlantic Ocean to high concentrations in the North Pacific Ocean, indicating an accumulation of N2O in deep waters with time. We conclude that the observed N2O accumulation is mainly caused by nitrification in the global deep-water circulation system (i.e., the “conveyor belt”). Hydrothermal and sedimentary N2O fluxes are negligible. We estimate the annual N2O deep-water production to be 0.3 ± 0.1 Tg. Despite the fact that the deep sea below 2000 m represents about 95% of the total ocean volume, it contributes only about 3–16% to the global open-ocean N2O production. A rough estimate of the oceanic N2O budget suggests that the loss to the atmosphere is not balanced by the deep-sea nitrification and pelagic denitrification. Therefore an additional source of 3.8 Tg N2O yr−1 attributed to nitrification in the upper water column (0–2000 m) might exist. With a simple model we estimated the effect of changes in the North Atlantic Deep Water (NADW) formation for deep-water N2O. The upper water N2O budget is not significantly influenced by variations in the N2O deep-water formation. However, the predicted decrease in the NADW formation rate in the near future might lead to an additional source of atmospheric N2O in the range of about 0.02-0.4 Tg yr−1. This (anthropogenically induced) source is small, and it will be difficult to detect its signal against the natural variations in the annual growth rates of tropospheric N2O.

Description: Petrologic and geochemical studies of vent solids from the Main Endeavour Field (MEF) and the High Rise Field (HRF), Juan de Fuca Ridge, demonstrate that the steep‐sided vent structures characteristic of these sites form dominantly by flange growth, combined with diffuse flow through sealed portions of structures, and incorporation of flanges into structures. Geochemical calculations suggest that the prevalence of amorphous silica and flanges in Endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia. At Endeavour, as the temperature of vent fluid decreases, ammonia‐ammonium equilibrium buffers pH and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher pH than conductively cooled ammonia‐poor fluids present at most other unsedimented mid‐ocean ridge vent sites. Deposition of silica stabilizes flanges and allows structures to attain large size. It also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones, thus decreasing entrainment of seawater. Most inactive vent samples recovered from areas peripheral to the HRF and MEF are similar to barite + silica rich samples from the Explorer Ridge and Axial Seamount and likely formed from precipitation of silica and barite on a biological substrate. Active white smoker chimneys from the Clam Bed Field, located south of the HRF, are pyrrhotite rich and likely formed from vent fluids that are depleted in Zn and Cd and enriched in Pb and Ba relative to fluids exiting trans‐Atlantic geotraverse (TAG) and Cleft Segment white smoker chimneys.

Description: During the Central Equatorial Pacific Experiment, ice crystal sizes and shapes were measured in an outflow anvil. A habit (i.e., column, bullet rosette, Koch fractal polycrystal, sphere) was assigned to each particle using a self-organized neural network based on simulations of how the maximum particle dimension and area ratio varied for random orientations of these crystals. Average ice crystal size and shape distributions were calculated for 25 km long segments at six altitudes using measurements from a two-dimensional cloud probe for crystals larger than 90 μm and a parameterization for smaller crystals based on measurements from the Video Ice Particle Sampler (VIPS). Mean-scattering properties were determined by weighting the size and shape dependent single-scattering properties computed with ray-tracing algorithms according to scattering cross-section. Reflectances at 0.664, 0.875, 1.621, and 2.142 μm were then calculated using a Monte Carlo radiative transfer routine. Although these reflectances agree reasonably with those measured by the MODIS airborne simulator (MAS) above the anvil, uncertainties in cloud base and system evolution prevent a determination of whether ray-tracing or anomalous diffraction theory better predict reflectance. The calculated reflectances are as sensitive to the numbers and shapes of crystals smaller than 90 μm as to those of larger crystals. The calculated reflectances were insensitive to the classification scheme (i.e., neural network, discriminator analysis, and previously used classification scheme) for assigning particle shape to observed crystals. However, the reflectances significantly depended on assumed particle shape.

Description: The Charlie Gibbs Fracture Zone (CGFZ), a passage of 3600 m sill depth through the Mid‐Atlantic Ridge near 52°N, is a known gateway for the passage of deep waters from the Northeast Atlantic into the western basin. During a shipboard survey of August 1997 deep current profiling yielded eastward deep flow through the passage while geostrophy calculated against an intermediate reference level resulted in westward relative deep transport. The reason was an unusual and deep‐reaching northward excursion of the North Atlantic Current (NAC). Inspection of historical data showed that such interference of the NAC with the CGFZ regime occured occasionally in the past. Relocation of surface circulation patterns by decadal ocean‐climate anomalies may thus be of significance also for the deep circulation.

Description: We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section I5 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48 degrees-55 degrees E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55 degrees E after 15-20 years. It takes ICW some 25-30 years to reach 110 degrees E.

Description: The δ13C of dissolved inorganic carbon was measured on samples collected at 49°N in the northeast Atlantic in January 1994. Deeper than 2000 m, δ13C exhibits the same negative correlation versus dissolved phosphate that is observed elsewhere in the deep Atlantic. Upward from 2000 m to about 600 m, δ13C shifts to values more negative than expected from the correlation with nutrients at depth, which is likely due to penetration of anthropogenic CO2. From these data, the profile of the anthropogenic δ13C decrease is calculated by using either dissolved phosphate or apparent oxygen utilization as a proxy for the preanthropogenic δ13C distribution. The shape of the anthropogenic anomaly profile derived from phosphate is similar to that of the increase in dissolved inorganic carbon derived by others in the same area. The reconstruction from oxygen utilization results in a lower estimate of the anthropogenic δ13C decrease in the upper water column, and the vertical anomaly profile is less similar to that of the dissolved inorganic carbon increase. A 13C budget for the atmosphere, ocean, and terrestrial biosphere indicates that within the range of probable ocean CO2 uptake the ratio of δ13C to inorganic carbon change should be mostly influenced by the 13C inventory change of the biosphere. However, the uncertainty in the ratio we derive prevents a strong contraint on the size of the exchangeable biosphere.

Description: Four World Ocean Circulation Experiment (WOCE) repeat cruises (October 1990 to March 1994) in the tropical Atlantic off Brazil are used to study the spatial and temporal evolution of the chlorofluorocarbon (CFC) (components CFC-11 and CFC-12) and tritium signal in the upper North Atlantic Deep Water (NADW). Its shallowest part, located in the tropical Atlantic around 1600-m depth, is the shallow upper North Atlantic Deep Water (SUNADW). It is characterized by a distinct tracer maximum, which is presumably received through winter time convection in the subpolar North Atlantic. Here we discuss the tracer fields and the temporal evolution of the tracer signal of the SUNADW in the tropical Atlantic along two meridional sections at 44 degrees and 35 degrees W and two zonal sections at 5 degrees and 10 degrees S off Brazil. The spatial and temporal development of the tracer field in the tropical Atlantic as well as the correlation with hydrographic parameters show that the temporal tracer change being due to the arrival of "younger" water is disturbed by other processes. In particular, the impact of variable mixing and spreading pathways on the observed tracer variability in the SUNADW is evident in the observations.

Description: The deep water of the western Mediterranean Sea is known to have become warmer and saltier since about the 1950s. The causes of these changes have, however, not yet been sactisfactorily determined. Previous studies speculated on decreasing precipitation, greenhouse warming and/or anthropogenic reduction of the freshwater flux into the eastern Mediterranean. Here we report on results from a new oceanographic database of the western Mediterranean Sea together with determinations of longterm changes of the fresh water budget. We analyzed temperature and salinity data of the past 40 years to detect deviations from the longterm average. Certain areas and depth ranges are showing increases in temperature or salinity some of which have been found earlier and some which are new. From the regional and vertical distribution we conclude that the observed increases of temperature and salinity in the western Mediterranean Sea are caused both by changes in atmospheric conditions as described by the NAO‐index and by the regulation of Spanish rivers.

Description: We examine the propagation of low-frequency electromagnetic (EM) waves in the coastal ocean produced by controlled or motional impressed sources. Four important modes are the direct, up-over-down, down-over-up, and “beach” modes. The analyses of these modes are complicated by the varying bathymetry in the coastal region. We derive criteria to determine (1) which modes are important for given parameters; (2) a “matched phase” condition describing both when the up-over-down and down-over-up modes interfere constructively in the shallow zone and when the beach mode becomes important; and (3) a low-frequency cutoff, below which the EM fields are not sensitive to the details of the coastal geometry. We verify the theoretically derived criteria with numerical examples and finally discuss the importance of our results in designing navigation and communications applications for subsurface vehicles and instruments.

Description: A joint research effort is currently focused on the oceanic region south of Africa—the gateway for the exchange of mass, heat, and salt between the Indian and Atlantic Oceans (Figure lb). The name of this collaboration, KAPEX, stands for Cape of Good Hope Experiments, Kap der guten Hoffnung Experimente, or Kaap die Goeie Hoop Eksperimente in the three languages of the participating scientists. This is the first time that scientists are using acoustically tracked floats extensively in ocean regions surrounding southern Africa to measure ocean flow patterns. At the tip of Africa, the Agulhas Current from the Indian Ocean interacts with the South Atlantic Current, contributing to the northwestward flowing Benguela Current, which transports water, heat, and salt to the subtropical and subequatorial South Atlantic (Figure la). This transport increases the heat and salinity of the North Atlantic, preconditioning it for the formation of the global thermohaline circulation cell, a driving force of the world climate [Gordon etal., 1992]. Our objective in the KAPEX is to trace the flow of intermediate water around southern Africa by the Agulhas, Benguela, and South Atlantic Current systems and to answer key questions about the inter-oceanic intermediate circulation.

Description: Stable oxygen and-carbon isotope and sedimentological-paleontological investigations supported by accelerator mass spectrometry (14)C datings were carried out on cores from north of 85 degrees N in the eastern central Arctic Ocean. Significant changes in accumulation rates, provenance of ice-rafted debris (IRD), and planktic productivity over the past 80,000 years are documented. During peak glacials, i.e., oxygen isotope stages 4 and 2, the Arctic Ocean was covered by sea ice with decreased seasonal variation, limiting planktic productivity and bulk sedimentation rates. In early stage 3 and during Termination I, major deglaciations of the circum-Arctic regions caused lowered salinities and poor oxygenation of central Arctic surface waters. A meltwater spike and an associated IRD peak dated to similar to 14-12 (14)C ka can be traced over the southern Eurasian Basin of the Arctic Ocean. This event was associated with the early and rapid deglaciation of the marine-based Barents Sea Ice Sheet. A separate Termination Ib meltwater event is most conspicuous in the central Arctic and is associated with characteristic dolomitic carbonate IRD. This lithology suggests an origin of glacial ice from northern Canada and northern Greenland where lower Paleozoic platform carbonates crop extensively out.

Description: The variability of the ice volume flux into the northeast Atlantic is investigated with an optimized dynamic-thermodynamic sea ice model using 40 years (1958–1997) of atmospheric forcing fields derived from the NCEP/NCAR reanalysis project. The simulated sea ice export from the Arctic exhibits considerable interannual to decadal variability and is primarily a linear response to sea level pressure anomalies over Greenland and over the Barents and Kara Seas. Our model results suggest that ice export anomalies such as in 1968 which supposedly caused the so-called “Great Salinity Anomaly” in the northern North Atlantic are not unique but rather frequent events as part of the variability of the Arctic climate system.

Description: Recent hydrographic sections and high-quality historical data sets are used to determine geostrophic currents at subtropical latitudes in the western basin of the South Atlantic. Levels of no motion are determined from water mass information and a mass balance constraint to obtain the transport field of North Atlantic Deep Water (NADW) in this region. The incoming NADW transport of about 20 Sv from the north at 19 degrees S appears to be balanced by only one third of this transport leaving in the south and two thirds leaving to the east or northeast at the Mid-Atlantic Ridge. A simple model is proposed to determine the cause of the NADW branching. It is shown that potential vorticity preservation in the presence of topographic changes leads to a similar flow pattern as observed, with branching near the Vitoria-Trindade-Ridge and also an eastward turning of the southward western boundary current at about 28 degrees S, the latitude where a balance of planetary vorticity change and stretching can be expected.

Description: The potential for using sclerosponges, marine organisms that secrete a hard calcerous skeleton, as paleoclimatic indicators has attracted the interest of a number of scientists. Sclerosponges are composed mainly of calcium carbonate and they are very long lived. Variations in their skeletal chemistry contain proxy information regarding their environment and that information has the potential to augment, if not supplant, data from scleractinian corals in interpreting past water temperature, salinity, and productivity over periods of 100s to 1000s of years. Sclerosponges, or calcified demosponges, contain aragonite or calcite and a small amount of siliceous material. Lang et al. [1975] report that these sponges grow within a reef framework, under coral talus in the shallower parts of a reef less than 55 m deep and on steep surfaces of the fore-reef between 55 and 145 m deep. The largest and most conspicuous of the sclerosponges described by those authors is Ceratoporella nicholsoni (Figure 1), which is reported to attain a diameter in excess of 1 m. These sponges are similar in growth habit to many massive vanities of scleractinian corals, the live sponge inhabiting the upper portion of the skeleton, while the lower portion of the skeleton is essentially dead.

Description: The meridional oceanic transports of dissolved inorganic carbon and oxygen were calculated using six transoceanic sections occupied in the South Atlantic between 11 degrees S and 30 degrees S. The total dissolved inorganic carbon (TCO2) data were interpolated onto conductivity-temperature-depth data to obtain a high-resolution data set, and Ekman, depth-dependent and depth-independent components of the transport were estimated. Uncertainties in the depth-independent velocity distribution were reduced using an inverse model. The inorganic carbon transport between 11 degrees S and 30 degrees S was southward, decreased slightly toward the south, and was -2150 +/- 200 kmol s(-1) (-0.81 +/- 0.08 Gt C yr(-1)) at 20 degrees S. This estimate includes the contribution of net mass transport required to balance the salt transport through Bering Strait. Anthropogenic CO2 concentrations were estimated for the sections. The meridional transport of anthropogenic CO2 was northward, increased toward the north, and was 430 kmol s(-1) (0.16 Gt C yr(-1)) at 20 degrees S. The calculations imply net southward inorganic carbon transport of 2580 kmol s(-1) (1 Gt C yr(-1)) during preindustrial times. The slight contemporary convergence of inorganic carbon between 10 degrees S and 30 degrees S is balanced by storage of anthropogenic CO2 and a sea-to-air flux implying little local divergence of the organic carbon transport. During the preindustrial era, there was significant regional convergence of both inorganic carbon and oxygen, consistent with a sea-to-air gas flux driven by warming. The northward transport of anthropogenic CO2 carried by the meridional overturning circulation represents an important source for anthropogenic CO2 currently being stored within the North Atlantic Ocean.

Description: Numerical experiments with a medium‐resolution primitive equation model of the South Atlantic mean circulation are described. The results from the standard model realization indicate that the model succeeds in representing the large‐scale transport and circulation features. However, a comparison with a velocity field derived from surface drifter data reveals discrepancies of the modeled velocities from the observations in magnitude as well as direction of the flow field. In order to diminish the model deviations from the data, an attempt is made to couple the model to the observations through a simple data assimilation technique. The assimilated model succeeds in improving the subtropical gyre circulation. Only a minor effect on the basin‐scale integrated quantities is observed. However, the density field may be deformed as a response to the assimilation of velocity data without simultaneously adapting a corresponding density structure. The influence of the disturbance of the density structure is most prominent at the edges of the observed data set, which does not cover the entire model domain, and is confined to the upper ocean and balanced above the thermocline. We calculated a meridional heat transport that is generally in accordance with estimates from other sources. The analysis of heat and salt fluxes suggests that the model features both the so‐called “warm water path” and “cold water path” in closing the global thermohaline circulation. While heat is mainly imported in surface and thermocline waters with the Agulhas Current around South Africa, it is the Antarctic Intermediate Water that compensates for more than 50% of the salt loss by the outflowing North Atlantic Deep Water.

Description: The penetration of anthropogenic or “excess” CO2 into the North Atlantic Ocean was studied along WOCE‐WHP section A2 from 49°N/11°W to 43°N/49°W using hydrographic data obtained during the METEOR cruise 30–2 in October/November 1994. A backcalculation technique based on measurements of temperature, salinity, oxygen, alkalinity, and total dissolved inorganic carbon was applied to identify the excess CO2. Everywhere along the transect surface water contained almost its full component of anthropogenic CO2 ( ∼62 μmol kg−1). Furthermore, anthropogenic CO2 has penetrated through the entire water column in the western basin of the North Atlantic Ocean. Even in the deepest waters (5000 m) of the western basin a mean value of 10.4 μmol kg−1 excess CO2 was calculated. The maximum penetration depth of excess CO2 in the eastern basin of the North Atlantic Ocean was ∼3500 m with values falling below 5 μmol kg−1 in greater depths. These results compare well with distributions of carbontetrachloride. They are also in agreement with the current understanding of the role of the “global ocean conveyor belt” for the uptake of anthropogenic CO2 into the deep ocean.

Description: The response of the Atlantic Ocean to North Atlantic Oscillation (NAO)-like wind forcing was investigated using an ocean-only general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the interannual to multi-decadal frequency response of the ocean to a winter wind anomaly pattern. Overall, the strength of the SST response increased slightly with longer forcing periods. In the subpolar gyre, however, the model showed a broad response maximum in the decadal band (12-16 years).

Description: We report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the Cleft and Vance segments of the Juan de Fuca Ridge. The data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers. Areas of reduced crustal electrical resistivities, with values approaching that of seawater, are seen beneath the neovolcanic zones of both active spreading centers. We interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot, young oceanic crust.

Description: Vertical gradient electromagnetic sounding (VGS) on the Endeavour segment of Juan de Fuca mid‐ocean ridge reveals the presence of a 2D ridge‐parallel, conductivity anomaly. If the anomaly is caused mainly by melt in a conventional upper mantle upwelling zone alone, then the conductivity of the zone is about 0.6 S/m. The corresponding Archie's law melt fraction exceeds 0.10. A significantly lower melt fraction requires a sheet‐like, well interconnected melt. Upwelling zone conductivity can be reduced by a third if the anomaly is broadened and a crustal conductor is added to the model.

Description: A hierarchy of El Niño-Southern Oscillation (ENSO) prediction schemes has been developed during the Tropical Ocean-Global Atmosphere (TOGA) program which includes statistical schemes and physical models. The statistical models are, in general, based on linear statistical techniques and can be classified into models which use atmospheric (sea level pressure or surface wind) or oceanic (sea surface temperature or a measure of upper ocean heat content) quantities or a combination of oceanic and atmospheric quantities as predictors. The physical models consist of coupled ocean-atmosphere models of varying degrees of complexity, ranging from simplified coupled models of the “shallow water” type to coupled general circulation models. All models, statistical and physical, perform considerably better than the persistence forecast in predicting typical indices of ENSO on lead times of 6 to 12 months. The TOGA program can be regarded as a success from this perspective. However, despite the demonstrated predictability, little is known about ENSO predictability limits and the predictability of phenomena outside the tropical Pacific. Furthermore, the predictability of anomalous features known to be associated with ENSO (e.g., Indian monsoon and Sahel rainfall, southern African drought, and off-equatorial sea surface temperature) needs to be addressed in more detail. As well, the relative importance of different physical mechanisms (in the ocean or atmosphere) has yet to be established. A seasonal dependence in predictability is seen in many models, but the processes responsible for it are not fully understood, and its meaning is still a matter of scientific discussion. Likewise, a marked decadal variation in skill is observed, and the reasons for this are still under investigation. Finally, the different prediction models yield similar skills, although they are initialized quite differently. The reasons for these differences are also unclear.

Description: A radiative transfer model to compute brightness temperatures in the microwave frequency range for polar regions including sea ice, open ocean, and atmosphere has been developed and applied to sensitivity studies and retrieval algorithm development. The radiative transfer within sea ice is incorporated according to the “many layer strong fluctuation theory” of Stogryn [1986, 1987] and T. Grenfell [Winebrenner et al., 1992]. The reflectivity of the open water is computed with the three-scale model of Schrader [1995]. Both surface models supply the bistatic scattering coefficients, which define the lower boundary for the atmospheric model. The atmospheric model computes the gaseous absorption by the Liebe et al. [1993] model. Scattering by hydrometeors is determined by Mie or Rayleigh theory. Simulated brightness temperatures have been compared with special sensor microwave imager (SSM/I) observations. The comparison exhibits shortcomings of the ice model for 37 GHz. Applying a simple ad hoc correction at this frequency gives consistent comparison results within the range of observational accuracy. The simulated brightness temperatures show the strong influence of clouds and variations of wind speed over the open ocean, which will affect the sea ice retrieval even for an ice-covered ocean. Simulated brightness temperatures have been used to train a neural network algorithm for the total sea ice concentration, which accounts for these effects. Sea ice concentrations sensed from the SSM/I data using the network and the NASA sea ice algorithm show systematic differences in dependence on cloudiness.

Description: The occurrence of CF4 and SF6 in natural fluorites (CaF2) is reported. Contents are found to be typically 200–2000 ppt-mass for CF4 and 50–100 ppt-mass for SF6. CF4 and SF6 were also detected in granites where fluorite is an accessory mineral. CF4 and SF6 thus possess a large reservoir in the continental crust that by degassing can sustain natural atmospheric background levels of 40 ppt-mole and up to 0.01 ppt-mole, respectively. The article also discusses how natural SF6 may cause interference in hydrological tracer studies.

Description: The standard geometric optics (GO) technique predicts that the phase function for large nonspherical particles with parallel plane facets (e.g., hexagonal ice crystals) should have an infinitesimally narrow δ-function transmission peak caused by rays twice transmitted (refracted) in exactly the forward scattering direction. However, exact T-matrix computations and physical considerations based on the Kirchhoff approximation suggest that this peak is an artifact of GO completely ignoring physical optics effects and must be convolved with the Fraunhofer pattern, thereby producing a phase function component with an angular profile similar to the standard diffraction component. This convolution can be performed with a simple procedure which supplements the standard ray-tracing code and makes the computation of the phase function and its Legendre expansion both more physically realistic and more accurate.

Description: Current measurements from three moored arrays on the Brazilian continental slope between 20 degrees S and 28 degrees S are investigated for the existence and strength of western boundary currents from near the surface down to the North Atlantic Deep Water. The Brazil Current is found to deepen southward from 100 m to more than 670 m and to strengthen its volume transport to 16.2 x 10(6) m(3)/s. Antarctic Intermediate Water is transported in a well-developed boundary current southward at 28 degrees S and northward north of Cabo Frio (24 degrees S). This result supports earlier suggestions derived from the analysis of hydrographic data that Antarctic Intermediate Water enters the Brazil Basin from the east and bifurcates as it meets the continental break off Brazil. North Atlantic Deep Water is transported southward in a weakly developed boundary current that leads to lower estimates of volume transport than expected from earlier hydrographic data analysis.

Description: Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over‐steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane‐ or sulfide‐dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5±0.7 L m−2 d−1 for tectonics‐induced water flow [Wallmann et al., 1997b]. A geophysical estimate of dewatering from the same area [von Huene et al., 1997] based on sediment porosity reduction shows a fluid loss of 0.02 L m−2 d−1 for a 5.5 km wide converged segment near the deformation front. Our video‐guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m−2 d−1 is estimated from the biogeochemical approach. The two estimates for tectonics‐induced water flow from the accretionary prism are in surprisingly good agreement.

Description: Global analyses of monthly sea surface temperature (SST) anomalies from 1856 to 1991 are produced using three statistically based methods: optimal smoothing (OS), the Kaiman filter (KF) and optimal interpolation (OI). Each of these is accompanied by estimates of the error covariance of the analyzed fields. The spatial covariance function these methods require is estimated from the available data; the timemarching model is a first‐order autoregressive model again estimated from data. The data input for the analyses are monthly anomalies from the United Kingdom Meteorological Office historical sea surface temperature data set (MOHSST5) [Parker et al., 1994] of the Global Ocean Surface Temperature Atlas (GOSTA) [Bottomley et al., 1990]. These analyses are compared with each other, with GOSTA, and with an analysis generated by projection (P) onto a set of empirical orthogonal functions (as in Smith et al. [1996]). In theory, the quality of the analyses should rank in the order OS, KF, OI, P, and GOSTA. It is found that the first four give comparable results in the data‐rich periods (1951–1991), but at times when data is sparse the first three differ significantly from P and GOSTA. At these times the latter two often have extreme and fluctuating values, prima facie evidence of error. The statistical schemes are also verified against data not used in any of the analyses (proxy records derived from corals and air temperature records from coastal and island stations). We also present evidence that the analysis error estimates are indeed indicative of the quality of the products. At most times the OS and KF products are close to the OI product, but at times of especially poor coverage their use of information from other times is advantageous. The methods appear to reconstruct the major features of the global SST field from very sparse data. Comparison with other indications of the El Niño‐Southern Oscillation cycle show that the analyses provide usable information on interannual variability as far back as the 1860s.

Description: A Monte Carlo model is developed to calculate the microwave emissivity of the sea surface based on the Kirchhoff approximation combined with modified Fresnel coefficients. The modified Fresnel coefficient depends on the incident angle of the electromagnetic wave and the height variance of small‐scale roughness, which is an approximation to account partly for the scattering effect from small ripples. The advantage of the Monte Carlo model is its inherent capability to treat multiple scattering events. Using a two‐dimensional Gaussian distribution for the sea surface slope variability, the model is capable of simulating the azimuthal dependency of the microwave emission caused by the alignment of waves perpendicular to the wind direction. Good agreement between model calculations and measurements is obtained.

Description: More than 2000 measurements of atmospheric and dissolved methane (CH44) were performed in the central and northwestern Arabian Sea as part of the German JGOFS Arabian Sea Process Study during three cruises in March, May/June, and June/July 1997. Mean CH4 saturations in the surface waters of the central Arabian Sea were in the range of 103–107%. Significantly enhanced saturations were observed in the coastal upwelling area at the coast of Oman (up to 156%) and in an upwelling filament (up to 145%). The CH4 surface concentrations in the upwelling area were negatively correlated to sea surface temperatures. Area‐weighted, seasonally adjusted estimates of the sea‐air fluxes of CH4 gave annual emissions from the Arabian Sea of 11–20 Gg CH4, suggesting that previously reported very high surface CH4 concentrations might be atypical owing to the interannual variability of the Arabian Sea and that the emissions derived from them are probably overestimates.

Description: The interannual variability of the sea ice transport through Fram Strait is simulated with a dynamic‐thermodynamic sea ice model. Forcing with daily varying wind fields for the 7‐year period 1986–1992 causes a high variability of sea ice drift on timescales from days to years. Annual means of simulated ice transport through Pram Strait differ up to a factor of 2. Additional sensitivity studies investigate the response of sea ice transports to variations of the prescribed atmospheric and oceanic forcing. Wind speed, ocean current speed, air temperature, and precipitation rate are systematically varied over a wide range. The model predicts an almost linear relation of ice transport with wind speed and ocean current, a strong, nonlinear relation with air temperature, and a rather small sensitivity to changes in precipitation. The results show that the interannual variability of wind forcing causes considerable variations of sea ice export through Fram Strait. The fluxes of freshwater and negative latent heat associated with the sea ice transport can significantly affect the ocean circulation in the Greenland Sea and in the North Atlantic. This shows how variations of the ocean circulation are coupled to the variability of the atmosphere by the mechanism of sea ice advection. To adequately represent these important interactions in the coupled system atmosphere‐cryosphere‐ocean, both the dynamics and the thermodynamics of sea ice must be included in climate models.

Description: Convergent and upwelling circulation within the shelfbreak front in the Middle Atlantic Eight are detected using a dye tracer injected into the bottom boundary layer at the foot of the front. From the three day displacement and dispersion of two dye injections within the front we infer Lagrangian isopycnal (diapycnal) velocities and diffusivities of 2 x 10(-2) m/s (4 x 10(-6) m/s) and 9 m(2)/s (6 x 10(-6) m(2)/s). These results substantiate model predictions of Chapman and Lentz [1994] and previous dye tracer observations by Houghton [1997].

Description: Mechanical energy terms are calculated from moored current meter data in the Cape Verde Frontal Zone (about 20°N, 25°W) and compared with those derived from a mesoscale model of this frontal region. The model is of the Bleck and Boudra [1981] type with isopycnal coordinates. An initially zonal jet, representing the Canary Current, is allowed to develop under the influence of baroclinic and barotropic instability processes. We find reasonable agreement in magnitudes, somewhat smaller in the model, and similar distributions in the vertical. This leads to the conclusion that the energy transfer terms from the model can be expected to be sufficiently close to reality. Determination of the transfer terms confirms that the energy transfer in the zone is dominated by baroclinic instability processes while barotropic instability is of minor importance. Average baroclinic instability energy transfer terms reach values of 2–3 μW m−3 in the pycnocline. Peak layer mean values are of the order 10 μW m−3. It is shown that the spatial distribution of active transfer regions is closely related to the structure of the transient eddy field in the frontal zone and that strong instability processes are restricted to the pycnocline.

Description: A simple data assimilation technique has been applied for initializing coupled ocean‐atmosphere general circulation models, which is able to generate the three‐dimensional thermal state of the low‐latitude oceans by forcing the model with observed anomalies of sea surface temperature. The scheme has been tested in a multi‐year experiment in which the vertical temperature profiles in the equatorial Pacific measured by the TOGA‐TAO array have been successfully reproduced for the period '96 to '97. In a further series of eight hindcast experiments initialized between January '96 and September '97, the predictive skill of the model was tested. All experiments starting in '97 correctly simulated the evolution of the '97 El Niño, although the amplitude was slightly underestimated. While the ocean was pre‐conditioned to create an El Niño already in '96, the model correctly stayed in the cold (La Niña) phase initially. All experiments initialized in '97 forecast a La Niña event for the middle of'98.

Description: Grain‐size‐dependent flow mechanisms tend to be favored over dislocation creep at low differential stresses and can potentially influence the rheology of low‐stress, low‐strain rate environments such as those of planetary interiors. We experimentally investigated the effect of reduced grain size on the solid‐state flow of water ice I, a principal component of the asthenospheres of many icy moons of the outer solar system, using techniques new to studies of this deformation regime. We fabricated fully dense ice samples of approximate grain size 2±1 μm by transforming “standard” ice I samples of 250±50 μm grain size to the higher‐pressure phase ice II, deforming them in the ice II field, and then rapidly releasing the pressure deep into the ice I stability field. At T≤200 K, slow growth and rapid nucleation of ice I combine to produce a fine grain size. Constant‐strain rate deformation tests conducted on these samples show that deformation rates are less stress sensitive than for standard ice and that the fine‐grained material is markedly weaker than standard ice, particularly during the transient approach to steady state deformation. Scanning electron microscope examination of the deformed fine‐grained ice samples revealed an unusual microstructure dominated by platelike grains that grew normal to the compression direction, with c axes preferentially oriented parallel to compression. In samples tested at T≥220 K the elongation of the grains is so pronounced that the samples appear finely banded, with aspect ratios of grains approaching 50:1. The anisotropic growth of these crystallographically oriented neoblasts likely contributes to progressive work hardening observed during the transient stage of deformation. We have also documented remarkably similar microstructural development and weak mechanical behavior in fine‐grained ice samples partially transformed and deformed in the ice II field.

Description: A theoretical model of CO2aq-dependent phytoplankton carbon isotope fractionation (єp) and abundance (δ13Corg) is compared to observed isotopic trends with temperature and [CO2aq] in the ocean. It is shown that the model's δ13Corg response to surface ocean temperature and to [CO2aq] can simulate observed trends when the other independent model variables (phytoplankton cell growth rate, cell size, cell membrane CO2 permeability, and enzymatic isotope fractionation) are held at realistic, constant values. The possible contribution made by each of these variables to the residual scatter in δ13Corg about its trends with temperature and [CO2aq] is quantified, thus estimating a maximum isotopic sensitivity to changes in each of these variables. The model response to growth rate and especially cell size, however, appears to be unrealistically high. This may occur because the net isotopic effects of such factors may be attenuated through dependent and isotopically offsetting variations among variables. The model's indicated sensitivity to such factors as CO2 permeability, enzymatic fractionation, cell size, and cell surface area/volume provides mechanisms whereby changes in species composition can play a significant role in affecting observed variations in oceanic δ13Corg. Overall, the model is consistent with earlier suggestions that marine δ13Corg and єp variability can be explained by carbon isotope fractionation evoked by CO2aq-dependent phytoplankton. This has important implications for interpreting carbon isotopic variability encountered in plankton and their organic constituents in the present-day ocean and in the marine sedimentary record.

Description: The partial pressure of CO2 (pCO2) was measured during the 1995 South-West Monsoon in the Arabian Sea. The Arabian Sea was characterized throughout by a moderate supersaturation of 12–30 µatm. The stable atmospheric pCO2 level was around 345 µatm. An extreme supersaturation was found in areas of coastal upwelling off the Omani coast with pCO2 peak values in surface waters of 750 µatm. Such two-fold saturation (218%) is rarely found elsewhere in open ocean environments. We also encountered cold upwelled water 300 nm off the Omani coast in the region of Ekman pumping, which was also characterized by a strongly elevated seawater pCO2 of up to 525 µatm. Due to the strong monsoonal wind forcing the Arabian Sea as a whole and the areas of upwelling in particular represent a significant source of atmospheric CO2 with flux densities from around 2 mmol m−2 d−1 in the open ocean to 119 mmol m−2 d−1 in coastal upwelling. Local air masses passing the area of coastal upwelling showed increasing CO2 concentrations, which are consistent with such strong emissions.

Description: The output of the global eddy‐resolving ¼° ocean model of Semtner/Chervin (run by the Naval Postgraduate School, Monterey, California) has been used to study the oceanic temperature and heat flux in the Indian Ocean. The meridional heat flux in the northern Indian Ocean is at the low end of the observed values. A vertical overturning cell in the upper 500 m is the main contributor to the annual mean meridional heat flux across 5°S, whereas the horizontal gyre circulation, confined to the upper 500 m, dominates north of the equator. The change of monsoon winds is manifested in a reversal of the meridional circulation throughout the whole water column. The most notable result is a strong linear relationship of the meridional temperature flux and the zonal wind stress component north of 20°S. The model's Pacific‐Indian Ocean throughflow across the section at 120°E accounts for −8.8±5.1 Sv (1 Sv≡106 m3 s−1). A strong interannual variability during the model run of 3 years shows a maximum range of 12 Sv in January/February and a minimum during March through June. The inflow from the Pacific into the Indian Ocean results in a total annual mean temperature flux of −0.9 PW (1 PW≡1015 W). In the model the temperature flux from the Pacific through the Indian Ocean to the south dominates in comparison with the input of solar heat from the northern Indian Ocean.

Description: From geostrophic calculations the exchange of deep water from the Somali into the Arabian Basin through the Owen Fracture Zone has been estimated to be about 2 Sv, with a seasonal modulation of the same magnitude. After leaving the Fracture Zone, the flow bifurcates into a northern and a southern branch, each closely following the slope of the Carlsberg Ridge. The weaker vertical gradients of the hydrographic properties in the deep Arabian Basin are consistent with enhanced vertical mixing at the rugged topography over the Carlsberg Ridge.

Description: Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) are the two major sources for the ventilation of the permanent thermocline in the Indian Ocean. ICW is formed by subduction in the region of negative wind stress curl, while SAMW is formed by convective overturning at the Subantarctic Front. SAMW contributes to the depth range of ICW, but is not easily identified, because most hydrographic properties (temperature, salinity and nutrients) of SAMW do not differ much from those of ICW. This study identifies ICW and SAMW in a zonal section near 32°S and evaluates the relative importance of convection vs. subduction for the ventilation process. Oxygen and nutrient data from the eastern part (50–114°E) of WOCE section 15 are used with temperature and salinity to determine water mass fractions of subducted ICW and of SAMW from water mass mixing analysis. The individual age fields of the two water mass components are then derived from a combination of the fractions obtained with a linear oxygen/CFC mixing model. Unlike earlier studies, which derive an uncalibrated apparent age, our results express water mass age in true units of time (years). The core of the SAMW near 114°E is about 5 years old, while the core of the subducted ICW (at 60–80°E) shows an increase of age with depth, in agreement with the subduction process. ICW moves eastward with the South Indian Current, reaching an age of 35 years at 114°E. SAMW spreads westward against the mean flow through turbulent diffusion, reaching an age of 25 years at 50°E.

Description: Recent studies have indicated that drifting Arctic sea ice plays an important role in the redistribution of sediments and contaminants. Here we present a method to reconstruct the backward trajectory of sea ice from its sampling location in the Eurasian Arctic to its possible site of origin on the shelf, based on historical drift data from the International Arctic Buoy Program. This method is verified by showing that origins derived from the backward trajectories are generally consistent with other indicators, such as comparison of the predicted backward trajectories with known buoy drifts and matching the clay mineralogy of sediments sampled from the sea ice with that of the seafloor in the predicted shelf source regions. The trajectories are then used to identify regions where sediment‐laden ice is exported to the Transpolar Drift Stream: from the New Siberian Islands and the Central Kara Plateau. Calculation of forward trajectories shows that the Kara Sea is a major contributor of ice to the Barents Sea and the southern limb of the Transpolar Drift Stream.

Description: Two seasonal hydrographic data sets, including temperature, salinity, dissolved oxygen, and nutrients, are used in a mixing model which combines cluster analysis with optimum multiparameter analysis to determine the spreading and mixing of the thermocline waters in the Indian Ocean. The mixing model comprises a system of four major source water masses, which were identified in the thermocline through cluster analysis. They are Indian Central Water (ICW), North Indian Central Water (NICW) interpreted as aged ICW, Australasian Mediterranean Water (AAMW), and Red Sea Water (RSW)/Persian Gulf Water (PGW). The mixing ratios of these water masses are quantified and mapped on four isopycnal surfaces which span the thermocline from 150 to 600 m in the northern Indian Ocean, on two meridional sections along 60°E and 90°E, and on two zonal sections along 10°S and 6°N. The mixing ratios and pathways of the thermocline water masses show large seasonal variations, particularly in the upper 400–500 m of the thermocline. The most prominent signal of seasonal variation occurs in the Somali Current, the western boundary current, which appears only during the SW (summer) monsoon. The northward spreading of ICW into the equatorial and northern Indian Ocean is by way of the Somali Current centered at 300–400 m on the σθ=26.7 isopycnal surface during the summer monsoon and of the Equatorial Countercurrent during the NE (winter) monsoon. More ICW carried into the northern Indian Ocean during the summer monsoon is seen clearly in the zonal section along 6°N. NICW spreads southward through the western Indian Ocean and is stronger during the winter monsoon. AAMW appears in both seasons but is slightly stronger during the summer in the upper thermocline. The westward flow of AAMW is by way of the South Equatorial Current and slightly bends to the north on the σθ=26.7 isopycnal surface during the summer monsoon, indicative of its contribution to the western boundary current. Outflow of RSW/PGW seems effectively blocked by the continuation of strong northward jet of the Somali Current along the western Arabian Sea during the summer, giving a rather small contribution of only up to 20% in the Arabian Sea. A schematic summer and winter thermocline circulation emerges from this study. Both hydrography and water ‐ mass mixing ratios suggest that the contribution of the water from the South Indian Ocean and from the Indo‐Pacific through flow controls the circulation and ventilation in the western boundary region during the summer. However, during the winter the water is carried into the eastern boundary by the Equatorial Countercurrent and leaks into the eastern Bay of Bengal, from where the water is advected into the northwestern Indian Ocean by the North Equatorial Current. The so‐called East Madagascar Current as a southward flow occurs only during the summer, as is suggested by both hydrography and water‐mass mixing patterns from this paper. During the winter (austral summer) the current seems reversal to a northward flow along east of Madagascar, somewhat symmetrical to the Somali Current in the north.

Description: Extensive investigations of sedimentary barium were performed in the southern South Atlantic in order to assess the reliability of the barium signal in Antarctic sediments as a proxy for paleoproductivity. Maximum accumulation rates of excess barium were calculated for the Antarctic zone south of the polar front where silica accumulates at high rates. The correspondence between barium and opal supports the applicability of barium as a proxy for productivity. Within the Antarctic zone north of today's average sea ice maximum, interglacial vertical rain rates of excess barium are high, with a maximum occurring during the last deglaciation and early Holocene and during oxygen isotope chronozone 5.5. During these periods, the maximum silica accumulation was supposedly located south of the polar front. Glacial paleoproductivity, instead, was low within the Antarctic zone. North of the polar front, significantly higher barium accumulation occurs during glacial times. The vertical rain rates, however, are as high as in the glacial Antarctic zone. Therefore there was no evidence for an increased productivity in the glacial Southern Ocean.

Description: A benthic isotope record has been measured for core SO75-26KL from the upper Portuguese margin (1099 m water depth) to monitor the response of thermohaline overturn in the North Atlantic during Heinrich events. Evaluating benthic δ18O in TS diagrams in conjunction with equilibrium δc fractionation implies that advection of Mediterranean outflow water (MOW) to the upper Portuguese margin was significantly reduced during the last glacial (〈 15% compared to 30% today). The benthic isotope record along core SO75-26KL therefore primarily monitors variability of glacial North Atlantic conveyor circulation. The 14C-accelerator mass spectrometry ages of 13.54±.07 and 20.46±.12 ka for two ice-rafted detritus (IRD) layers in the upper core section and an interpolated age of 36.1 ka for a third IRD layer deeper in the core are in the range of published 14C ages for Heinrich events H1, H2, and H4. Marked depletion of benthic δ13C by 0.7–1.1‰ during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic δ18O) at Site 609 and ventilation patterns (inferred from benthic δ13C) in core SO75-26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic δ13C starts to decrease 1.5–2.5 kyr before Heinrich events Hl and H4, fully increased values are reached 1.5–3 kyr after the events, indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor's full strength well after the events. Benthic δ13C changes in the course of the Heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation, a distinct feature of thermohaline instability in numerical models. Inferrred gradual spin-up of thermohaline circulation after Hl and H4 is in contrast to abrupt wanning in the North Atlantic region that is indicated by sudden increases in Greenland ice core δ18O and in marine faunal records from the northern North Atlantic. From this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the Greenland ice core record.

Description: Since 1996, the Norwegian government has licensed hydrocarbon exploration in seven deep water areas on the continental slope north of the Norwegian Trough. Data acquired in this region, which is of interest to both scientists and the oil industry, provide an opportunity to improve understanding of the geology and development of the area through Quaternary times. Gas hydrates, slope stability, and geohazards are especially important topics for research near the Norwegian Trough.

Description: The Theoretical and Experimental Tomography in the Sea Experiment (THETIS 1) took place in the Gulf of Lion to observe the evolution of the temperature field and the process of deep convection during the 1991–1992 winter. The temperature measurements consist of moored sensors, conductivity‐temperature‐depth and expendable bathythermograph surveys, and acoustic tomography. Because of this diverse data set and since the field evolves rather fast, the analysis uses a unified framework, based on estimation theory and implementing a Kaiman filter. The resolution and the errors associated with the model are systematically estimated. Temperature is a good tracer of water masses. The time‐evolving three‐dimensional view of the field resulting from the analysis shows the details of the three classical convection phases: preconditioning, vigourous convection, and relaxation. In all phases, there is strong spatial nonuniformity, with mesoscale activity, short timescales, and sporadic evidence of advective events (surface capping, intrusions of Levantine Intermediate Water (LIW)). Deep convection, reaching 1500 m, was observed in late February; by late April the field had not yet returned to its initial conditions (strong deficit of LIW). Comparison with available atmospheric flux data shows that advection acts to delay the occurence of convection and confirms the essential role of buoyancy fluxes. For this winter, the deep mixing results in an injection of anomalously warm water (ΔT≈0.03°) to a depth of 1500 m, compatible with the deep warming previously reported.

Description: In this study, the impact of oceanic data assimilation on ENSO simulations and predictions is investigated. The authors’ main objective is to compare the impact of the assimilation of sea level observations and three-dimensional temperature measurements relative to each other. Three experiments were performed. In a control run the ocean model was forced with observed winds only, and in two assimilation runs three-dimensional temperatures and sea levels were assimilated one by one. The root-mean-square differences between the model solution and observations were computed and heat content anomalies of the upper 275 m compared to each other. Three ensembles of ENSO forecasts were performed additionally to investigate the impact of data assimilation on ENSO predictions. In a control ensemble a hybrid coupled ocean–atmosphere model was initialized with observed winds only, while either three-dimensional temperatures or sea level data were assimilated during the initialization phase in two additional forecast ensembles. The predicted sea surface temperature anomalies were averaged over the eastern equatorial Pacific and compared to observations. Two different objective skill measures were computed to evaluate the impact of data assimilation on ENSO forecasts. The authors’ experiments indicate that sea level observations contain useful information and that this information can be inserted successfully into an oceanic general circulation model. It is inferred from the forecast ensembles that the benefit of sea level and temperature assimilation is comparable. However, the positive impact of sea level assimilation could be shown more clearly when the forecasted temperature differences rather than the temperature anomalies themselves were compared with observations.

Description: The availability of CaCO3 and ³He content results for core TT 13-72 from 4.3 km depth in the equatorial Pacific [Marcantonio et al., 1996] allows the magnitude of the excess (i.e., over ambient) CaCO3 dissolution at the onset of marine glacial stages 10, 8, and 6 to be estimated. These three events are remarkably similar; during each an integrated loss of about 28 g CaCO3 per cm² occurred. While the magnitude of this loss is consistent with that expected from the interglacial to glacial pH shifts reconstructed based on boron isotope measurements on benthic foraminifera [Sanyal et al., 1995], measurements at a number of other locations and water depths will be required before this approach can be used to evaluate the global toll of these dissolution events.

Description: Since 1996, the Norwegian government has licensed hydrocarbon exploration in seven deep water areas on the continental slope north of the Norwegian Trough. Data acquired in this region, which is of interest to both scientists and the oil industry, provide an opportunity to improve understanding of the geology and development of the area through Quaternary times. Gas hydrates, slope stability, and geohazards are especially important topics for research near the Norwegian Trough.

Description: Past analyses of tropical Atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics, across the Intertropical Convergence Zone (ITCZ). By analyzing an improved 43‐year (1950–1992) record of SST [Smith et al, 1996] and other data derived from the Comprehensive Ocean‐Atmosphere Data Set (COADS), it is shown that the regions north and south of the ITCZ are statistically independent of each other at the seasonal to interannual timescales dominating the data, confirming the conclusions of Houghton and Tourre [1992]. Some dipole behavior does develop weakly during the boreal spring season, when there is a tendency for SST anomaly west of Angola to be opposite of that in the tropical North Atlantic. It is further shown that tropical Atlantic SST variability is correlated with Pacific El Niño‐Southern Oscillation (ENSO) variability in several regions. The major region affected is the North Atlantic area of NE trades west of 40°W along 10°N–20°N and extending into the Caribbean. There, about 50–80% of the anomalous SST variability is associated with the Pacific ENSO, with Atlantic warmings occurring 4–5 months after the mature phases of Pacific warm events. An analysis of local surface flux fields derived from COADS data shows that the ENSO‐related Atlantic warmings occur as a result of reductions in the surface NE trade wind speeds, which in turn reduce latent and sensible heat losses over the region in question, as well as cooling due to entrainment. This ENSO connection is best developed during the boreal spring following the most frequent season of maximum ENSO anomalies in the Pacific. A region of secondary covariability with ENSO occurs along the northern edge of the mean ITCZ position and appears to be associated with northward migrations of the ITCZ when the North Atlantic warmings occur. Although easterly winds are intensified in the western equatorial Atlantic in response to Pacific warm events, they do not produce strong local changes in SST. Contrary to expectations from studies based on equatorial dynamics, these teleconnected wind anomalies do not give rise to significant correlations of SST in the Gulf of Guinea with the Pacific ENSO. As the teleconnection sequence matures, strong SE trades at low southern latitudes follow the development of the North Atlantic SST anomaly and precede by several months the appearance of weak negative SST anomalies off Angola and stronger positive anomalies extending eastward from southern Brazil along 15°–30°S.

Description: We present long-term observations of temperature, salinity, tritium/3He, chlorofluorocarbon-11 (CFC 11), and chlorofluorocarbon-12 (CFC 12) for the central Greenland Gyre. The time series span the periods between 1952 and 1994 (temperature), 1981 and 1994 (salinity), 1972 and 1994 (tritium/3He), and 1982 and 1994 (CFCs). The correlation between hydrographic and transient tracer data indicates that low temperatures in the deep water in the early 1950s and between 1960 and 1980 reflect periods of higher deep water formation rates whereas periods of increasing temperatures in the late 1950s and between 1980 and 1994 are related to low deep water formation rates. However, the transient tracer observations obtained in the 1980s and early 1990s indicate that even during periods of low deep water formation, some water from the upper water column contributed to deep water formation between 1980 and 1994. In 1994, the deep water reached temperatures and salinities of −1.149 °C and 34.899, respectively, and no longer fits most of the classical definitions of Greenland Sea Deep Water (−1.29°C〈 Θ 〈 −1.0°C, 34.88〈S〈34.90). The temperature increase in the water column between 200 and 2000 m depth between 1980 and 1994 corresponds to an average heating rate of about 5 Wm−2 over this period, resulting in a decrease in density. The 13-year warming could be balanced by intensive cooling in two winters. The surface salinity steadily increased from 34.50 in 1991 to 34.85 in 1994.

Description: Further details of the “warming” of bottom water flowing through the Vema Channel, first reported by Zenk and Hogg [1996], are given. Because cross‐channel gradients of temperature are large and the channel is narrow, careful analysis is required to determine whether or not the bottom water temperature minimum was adequately sampled by a particular station or cruise. During a period from 1972 to 1996, at least 14 visits with quality hydrographic measurements have been made to the region. Of these, 11 were judged successful, and their data indicate an abrupt rise in potential temperature by 0.03°C from −0.18°C to −0.15°C in the early 1990s that has remained until the latest observation in the spring of 1996. Although this observation is consistent with the report of warming of Antarctic Bottom Water in the Argentine Basin by Coles et al [1996], their associated conclusion that this water mass has also freshened by 0.008 psu (on potential density surfaces) is in contradiction with our finding of no measurable change.

Description: A total of 21 about year‐long current meter records in the depth range of the upper and middle North Atlantic Deep Water (NADW) were analyzed to determine the mean and the fluctuations of the upper Deep Western Boundary Current (DWBC) in the equatorial Atlantic. The investigation was based on moored arrays at 44°W from three different deployment periods, 1989/1990, 1990/1991 and 1992/1994, and was supplemented by current profiling along 44°W and 35°W. The approximately 100‐km‐wide DWBC at 44°W, just north of the equator, was attached to the topography with the current maximum exceeding 70 cm s−1. Currents within the DWBC core followed the topography, and the close agreement between the mean current direction and the direction of maximum variance indicated that the major contribution to the DWBC variability near the equator was due to pulsing rather than meandering. For mean transports of upper and middle NADW, the current meter records were averaged over their deployment duration yielding a best estimate of 13 Sv in the depth range 1000 to 3100 m. The mean transport appeared robust, as subsets of the data from two different years yielded about the same mean transport, namely, 12.4 and 13.6 Sv. The DWBC transport time series showed a definite seasonal cycle, ranging from less than 7 Sv during September/October to about 25 Sv during January/February. Annual and semiannual transport harmonics had similar amplitudes, at about 6 Sv each, and together they explained about two thirds of the total transport variability. After crossing the equator, the DWBC splits into two cores with the major flow along a chain of seamounts near 3.5°S, near 35°W. Magnitudes and phases of the transport variability at 35°W, south of approximately 1.5°S, were similar to that at 44°W. Further, for the flow of lower NADW which was detached from the upper DWBC core, similar periodicity and phases were observed in the deep records at 44°W.

Description: The mean state of the transport field of the subtropical gyre of the South Indian Ocean has been derived for the upper 1000 m from selected historical hydrographic data. The subtropical gyre in the southwestern Indian Ocean is stronger than the flow in the other two oceans of the southern hemisphere. Most of the water in the South Indian gyre recirculates in the western and central parts of the basin. In the upper 1000 m the eastward transport of the South Indian Ocean Current starts with 60 Sv in the region southeast of South Africa. Between the longitudes of 40° and 50°E about 20 Sv of the 60 Sv recirculates in a southwest Indian subgyre. Another major diversion northward occurs between 60° and 70°E. At 90°E the remaining 20 Sv of the eastward flow splits up, 10 Sv going north to join the westward flow and only 10 Sv continuing in a northeastward direction to move northward near Australia. Near Australia, there is indication of the poleward flowing Leeuwin Current with a transport of 5 Sv. In the central tropical Indian Ocean between 10°S and 20°S, about 15 Sv flows to the west. The western boundary current of this subtropical gyre consists of the Agulhas Current along the east coast of southern Africa. Its mean flow is composed of 25 Sv from east of Madagascar and 35 Sv from recirculation in the southwest Indian subgyre south of Madagascar, with only 5 Sv being contributed from the Mozambique Channel. A net southward transport of 10 Sv results for the upper 1000 m of the South Indian Ocean. In contrast to the triangular shape of the subtropical gyre in the South Atlantic, probably caused by the cross‐equatorial flow into the North Atlantic, the area influenced by the subtropical gyre in the South Indian Ocean is more rectangular.

Description: An ocean circulation model for process studies of the Subpolar North Atlantic is developed based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model (MOM) code. The basic model configuration is identical with that of the high-resolution model (with a grid size of 1/3° × 2/5°) of the World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME), except that the domain of integration is confined to the area from 43° to 65°N. Open boundary conditions are used for the inflows and outflows across the northern and southern boundaries. A comparison with the CME model covering the whole North Atlantic (from 15°S to 65°N) shows that the regional model, with inflow conditions at 43°N from a CME solution, is able to reproduce the CME results for the subpolar area. Thus the potential of a regional model lies in its use as an efficient tool for numerical experiments aiming at an identification of the key physical processes that determine the circulation and water mass transformations in the subpolar gyre. This study deals primarily with the representation and role of the overflow waters that enter the domain at the northern boundary. Sensitivity experiments show the effect of closed versus open boundaries, of different hydrographic conditions at inflow points, and of the representation of the narrow Faeroe Bank Channel. The representation of overflow processes in the Denmark Strait is the main controlling mechanism for the net transport of the deep boundary current along the Greenland continental slope and further downstream. Changes in the Faeroe Bank Channel throughflow conditions have a comparatively smaller effect on the deep transport in the western basin but strongly affect the water mass characteristics in the eastern North Atlantic. The deep water transport at Cape Farewell and further downstream is enhanced compared to the combined Denmark Strait and Iceland-Scotland overflows. This enhancement can be attributed to a barotropic recirculation in the Irminger Basin which is very sensitive to the outflow conditions in the Denmark Strait. The representation of both overflow regions determine the upper layer circulation in the Irminger and Iceland Basins, in particular the path of the North Atlantic Current.

Description: Preliminary results on the development of the northern Somali Current regime and Great Whirl during the summer monsoon of 1995 are reported. They are based on the water mass and current profiling observations from three shipboard surveys of R/V Meteor and on the time series from a moored current-meter and ADCP array. The monsoon response of the GW was deep-reaching, to more than 1000m. involving large deep transports. The northern Somali Current was found to be disconnected from the interior Arabian Sea in latitude range 4°N–12°N in both, water mass properties and current fields. Instead, communication dominantly occurs through the passages between Socotra and the African continent. From moored stations in the main passage a northward throughflow from the Somali Current to the Gulf of Aden of about 5 Sv was determined for the summer monsoon of 1995.

Description: In this paper the performance of the global coupled general circulation model (CGCM) ECHO-2, which was integrated for 10 years without the application of flux correction, is described. Although the integration is rather short, strong and weak points of this CGCM can be clearly identified, especially in view of the model's performance of the annual cycle in the tropical Pacific. The latter is simulated with more success relative to the earlier version, ECHO-I. A better representation of the low-level stratus clouds in the atmosphere model associated with a reduction in the shortwave radiative flux at the air-sea interface improved the coupled model's performance in the southeastern tropical oceans, with a strongly reduced warm bias in these regions. Modifications in the atmospheric convection scheme also eliminated the AGCM's tendency to simulate a double ITCZ, and this behavior is maintained in the CGCM simulation. Finally, a new numerical scheme for active tracer advection in the ocean model strongly reduced the numerical mixing, which seems to enhance considerably the level of interannual variability in the equatorial Pacific. One weak point is an overall cold bias in the Tropics and midlatitudes, which typically amounts to 1°C in open ocean regions. Another weak point is the still too strong equatorial cold tongue, which penetrates too far into the western equatorial Pacific. Although this model deficiency is not as pronounced as in ECHO-1, the too strong cold tongue reduces the level of interannual rainfall variability in the western and central equatorial Pacific. Finally, the interannual fluctuations in equatorial Pacific sea surface temperatures (SSTs) are too equatorially trapped, a problem that is also found in ocean-only simulations. Overall, however, the authors believe that the ECHO-2 CGCM has been considerably improved relative to ECHO-1.

Description: In 1995, hydrographic and chlorofluorocarbon (CFCs, components F11, F12) measurements were carried out in the Gulf of Aden, in the Gulf of Oman, and in the Arabian Sea. In the Gulf of Oman, the F12 concentrations in the Persian Gulf outflow (PGW) at about 300m depth were significantly higher than in ambient surface water with saturations reaching 270%. These high values could not be caused by air-sea gas exchange. The outflow was probably contaminated with oil, and the lipophilic character of the CFCs could then lead to the observed supersaturations. The intermediate F12 maximum decreased rapidly further east and south. At the Strait of Bab el Mandeb in the Gulf of Aden, the Red Sea outflow (RSW) was saturated with F12 to about 65% at 400m depth, and decreased to 50% while descending to 800m depth. The low saturation is not surprising, because the outflow contains deep and intermediate water masses from the Red Sea which were isolated from the surface for some time. The tracer contributions to the Arabian Sea for Indian Central Water (ICW) and PGW are about equal, while below 500m depth the RSW contribution greatly exceeds ICW. Modeling the CFC budget of the Arabian Sea, the inflow of ICW north of 12 degrees N is estimated to be 1-6 Sv, depending mainly on the strength of the flow of Red Sea Water into the Arabian Sea.

Description: The flow of the low‐salinity Antarctic Intermediate Water (AAIW) at 700–1150 m depth across the Rio Grande Rise and the lower Santos Plateau is studied under the auspices of the World Ocean Circulation Experiment (WOCE) in the context of the Deep Basin Experiment. Our data set consists of several hydrographic sections, a collection of 15 RAFOS float trajectories, and records from 14 moored current meters. The data were gathered during different intervals between 1990 and 1994. The inferred flow field strongly supports a basinwide anticyclonic recirculation cell in the subtropical South Atlantic underneath the wind‐driven gyre. Its center, which appears to be southeast of the Rio Grande Rise, separates the eastward advection of AAIW below the South Atlantic Current from the westward flowing, recirculating AAIW. The two near‐shelf limbs closing the circumference of AAIW flow are formed in the east by the deep Benguela Current, potentially modulated by salty inflow of Indian Ocean Intermediate Water, and in the west by the Brazil Current system. Further important circulation elements are the Brazil‐Falkland (Malvinas) Confluence Zone at 40°S and an unnamed divergence at 28°S close to the 1000 m isobath. The resulting broad southward flow of AAIW augments the share of modified, i.e., saltier, intermediate water in the source region of the South Atlantic Current, while the smaller northward flow marks the source of a narrow equatorward Western Intermediate Boundary Current, ultimately leaving the South Atlantic. This shelf‐trapped jet is clearly documented in hydrographic data from 19°S and in nearby current meter records. The jet contrasts a sluggish flow across this latitude east of 35°W. A continuous flow of AAIW from its subpolar region in the southwestern Argentine Basin all along the western slope toward the equator appears unlikely between 35°S and 25°S.

Description: Dissolved and atmospheric nitrous oxide (N2O) were measured on the legs 3 and 5 of the R/V Meteor cruise 32 in the Arabian Sea. A cruise track along 65°E was followed during both the intermonsoon (May 1995) and the southwest (SW) monsoon (July/August 1995) periods. During the second leg the coastal and open ocean upwelling regions off the Arabian Peninsula were also investigated. Mean N2O saturations for the oceanic regions of the Arabian Sea were in the range of 99–103% during the intermonsoon and 103–230% during the SW monsoon. Computed annual emissions of 0.8–1.5 Tg N2O for the Arabian Sea are considerably higher than previous estimates, indicating that the role of upwelling regions, such as the Arabian Sea, may be more important than previously assumed in global budgets of oceanic N2O emissions.

Description: Stable isotope and faunal records from the central Red Sea show high-amplitude oscillations for the past 380,000 years. Positive δ18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53‰ and 55‰ are inferred from pteropod and benthic foraminifera δ18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic δ18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically - driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates.

Description: Long-range side-scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large-scale view of sedimentation patterns on this glacier-influenced continental margin. High-latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross-shelf troughs and deep fjords. Where ice was slower-moving, but still extended to the shelf break, large-scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large-scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented.

Description: The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide-angle seismic reflection data, whereas offshore we have used coincident steep and wide-angle reflection data along a NE-SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km.

Description: A three-dimensional Monte Carlo transfer model for polarized radiation is developed and used to study three-dimensional (3-D) effects of raining clouds on the microwave brightness temperature. The backward method is combined with the forward method to treat polarization correctly within the cloud. In comparison with horizontally homogeneous clouds, two effects are observed: First, brightness temperatures from clouds are reduced in the 3-D case due to net leakage of radiation from the sidewalls of the cloud. Second, radiation which is emitted by the warm cloud and then reflected from the water surface increases the brightness temperatures of the cloud-free areas in the vicinity of the cloud. Both effects compete with each other, leading to either lower or higher overall brightness temperatures, depending on the geometry of the cloud, the satellite viewing angle, the coverage, and the position of the cloud within the field of view (FOV) of the satellite. At 37 GHz, for example, up to 10 K differences can occur for a cloud of 50% coverage. Finite homogeneous raining clouds matching the size of the FOV of the satellite show a similar relationship between rain rates and brightness temperatures (TB) as horizontally infinite clouds. Namely, an increase of TB with increasing rain rates at low rain rates, due to emission effects, is followed by a decrease due to temperature and scattering effects. For small horizontal cloud diameter, however, the 3-D brightness temperatures may show a second maximum due to the decrease of the leakage effect with increasing rain rates. At nadir, 3-D brightness temperatures are always lower than the 1-D values with differences up to 20 K for a cloud of 5-km vertical extent and a base of 1 × 1 km. To quantify the 3-D effects for more realistic cloud structures, we used results of a three-dimensional dynamic cloud model as input for the radiative transfer codes. The same 3-D effects are obtained, but the differences between 1-D and 3-D modeling are smaller. In general, most of the differences between the 1-D and 3-D results for off-nadir view angles are pure geometry effects, which can be accounted for in part by a modified 1-D model.

Description: A weakly nonhydrostatic, two-layer numerical model based on the Boussinesq equations is presented which is capable of describing, among others, the generation and propagation of nonlinear weakly dispersive internal waves in the Strait of Gibraltar. The model depends on one space coordinate only, but it retains several features of a fully three-dimensional model by including a realistic bottom profile, a variable channel width, and a trapezoidal channel cross section. The nonlinear primitive Boussinesq equations include horizontal diffusion, bottom friction, and friction between the two water layers. The model is driven by a height difference of the mean interface depth between the Atlantic and the Mediterranean boundaries and by semidiurnal tidal oscillations of the barotropic transport. The model presented in this paper describes (1) the mean and tidal flow in the Strait of Gibraltar, (2) the variation of the depth of the interface during a tidal cycle, (3) the generation of strong depressions of the interface at the western sides of the Spartel Sill and the Camarinal Sill, (4) the generation of strong eastward propagating internal bores, and (5) their disintegration into trains of internal solitary waves. The surface convergence patterns associated with depressions of the interface at the Camarinal Sill, internal bores, and internal solitary waves are calculated and compared with roughness patterns visible on synthetic aperture radar (SAR) images of the first European Remote Sensing Satellite ERS 1. In total, 155 ERS 1 SAR scenes from 94 satellite overflights over the Strait of Gibraltar, which were acquired in the period from January 1992 to March 1995, have been analyzed. It is shown that the proposed model is capable of explaining the observed temporal and spatial evolution of surface roughness patterns associated with eastward propagating internal waves inside the Strait of Gibraltar as well as the observed east-west asymmetry of the internal wave field.

Description: From August 11 to 22, 1993, a conductivity‐temperature‐depth/acoustic Doppler current profiler survey was carried out in the Somali‐Socotra region to investigate currents and transports associated with the Great Whirl and Socotra Gyre circulation during the height of the summer monsoon. The monsoon circulation was confined to the upper 300 m depth, with intense surface currents up to 2.2 m s−1 in the Great Whirl and up to 1.4 m s−1 in the Socotra Gyre. Deeper‐reaching flow was found in the northwestern part of the Somali Basin and in the passage between the shelf of Somalia and Abd al Kuri. The Great Whirl transport was 58 Sv, of which nearly 25% were due to ageostrophic flow components. The northern part of the Great Whirl thereby appeared as a closed circulation cell in which the offshore transport was balanced by a southward transport of the same magnitude. Upwelled water was advected from the cold wedge of the upwelling regime at the Somali coast along the edge of the gyre. The water in the center of the gyre had the characteristics of Indian Equatorial Water (IEW). The Socotra Gyre carried 23 Sv of modified Arabian Sea Water (ASW). With the transports in the two anticyclonic gyres nearly balanced, the exchange of water masses between the Somali Basin, west of the Carlsberg Ridge, and the Arabian Sea occurred in two areas; about 16 Sv of warm and saline surface water of southern offshore origin entered the northern Somali Basin within a 120‐km‐wide swift current between the Great Whirl and the Socotra Gyre. The other key region for the exchange of water masses was the passage between Somalia and Abd al Kuri. There, the total northward transport was 13 Sv, with contributions of IEW, of upwelled water close to the surface, and ASW underneath.

Description: We determined atmospheric and dissolved nitrous oxide (N2O) in the surface waters of the central North Sea, the German Bight, and the Gironde estuary. The mean saturations were 104 ± 1% (central North Sea, September 1991), 101 ± 2% (German Bight, September 1991), 99 ± 1% (German Bight September 1992), and 132% (Gironde estuary, November 1991). To evaluate the contribution of coastal areas and estuaries to the oceanic emissions we assembled a compilation of literature data. We conclude that the mean saturations in coastal regions (with the exception of estuaries and regions with upwelling phenomena) are only slightly higher than in the open ocean. However, when estuarine and coastal upwelling regions are included, a computation of the global oceanic N2O flux indicates that a considerable portion (approximately 60%) of this flux is from coastal regions, mainly due to high emissions from estuaries. We estimate, using two different parameterizations of the air-sea exchange process, an annual global sea-to-air flux of 11–17 Tg N2O. Our results suggest a serious underestimation of the flux from coastal regions in widely used previous estimates.

Description: Open‐ocean deep‐water formation involves the interplay of two dynamical processes; plumes (≤1 km wide), driven by “upright” convection, and geostrophic eddies (≥5 km wide), driven by baroclinic instability. Numerical “twin” experiments are used to address two questions about the plumes: Can they be represented by a simple mixing process in large‐scale models? If so, is it important that the mixing occurs over a finite time tmix, or would instantaneous mixing produce the same effect on large‐scale properties? In numerical simulations which resolve the geostrophic eddies, we represent the plumes with a “slow” convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions. The diffusivity κ depends on tmix, the mixing timescale. The fidelity of the plume parameterization is then evaluated by comparison with plume‐resolving simulations of open‐ocean deep convection. Integral properties of the plumes, such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region, are all accurately reproduced by the slow adjustment scheme. The importance of choosing an appropriate finite value for tmix is explored by setting tmix = 12 hours in some experiments, in accordance with scaling considerations, and tmix = 0 in others, corresponding to instantaneous adjustment, the conventional assumption. In the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend on tmix. However, in neutral conditions the slow adjustment does improve the parametric representation. Our experiments confirm the picture of plumes homogenizing the water column over a time tmix.

Description: Deep convection is important in forming the dense water masses that lie below the ocean's surface and feed the global thermohaline circulation system. But the exact role that deep convection plays in these processes is a subject of much debate. Now, for the first time, a pulse-like temperature signal, produced when water generated by convection drains into a deep boundary current, has apparently been detected in the Mediterranean. This observation provides clues to the mechanisms by which the dense water escapes the convection region and makes its journey. While up to 50% of the newly formed water could incorporated into the deep boundary current this way, no increase in its transport was observed.

Description: Submersible investigations employing heat flow measurements (12 stations), sampling and imagery of the two relict high-temperature hydrothermal zones of the TAG field, the Alvin and Mir sulfide zones, elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies. Values of heat flow in the Mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers. This observation supports the hypothesis that intrusions at volcanic centers adjacent to the high-temperature hydrothermal zones supply the heat to drive hydrothermal activity. The chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field-wide high-temperature hydrothermal events recurring at intervals of tens of thousands of years, while activity at individual zones may recur at intervals of hundreds to thousands of years. A sequence of hydrothermal deposits ranges to at least 140,000 years ago from massive sulfides forming at the active mound, to recrystallization of sulfides in the active and relict zones, to pyritization of an inactive mound in the Alvin zone; low-temperature mineral phases precipitate before, during and after the sulfides.

Description: We present a new method for assimilating observations of sea surface height (SSH) into a high‐resolution primitive equation model. The method is based on the concept of reinitialization. First, the surface velocity increments necessary to adjust the model forecast to the observed geostrophic surface currents are projected onto deep velocity increments by a linear regression method. Second, changes in the density field required to balance the changes in the velocity field geostrophically are obtained from an inversion of the thermal wind equation. A unique partition of the density increments into corresponding temperature and salinity changes is realized by conserving the local θ‐S relation of the model forecast. In contrast to pure statistical methods that infer temperature and salinity changes from correlations with SSH anomalies, our approach explicitly conserves water mass properties on isopycnals. For the assimilation experiment we use optimally interpolated maps of Geosat SSH anomalies (the mean topography is taken from the model), which are assimilated into the World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME) model of the North Atlantic Ocean at 5‐day intervals covering the year 1987. It is shown that the assimilation significantly improves the model's representation of eddy activity, with the hydrographic structure of individual eddies agreeing well with independent hydrographic observations. The importance of a careful treatment of water mass properties in the assimilation process is discussed and further illustrated by comparing different assimilation schemes.

Description: CH3I was measured in open ocean waters during two cruises to the tropical Atlantic Ocean and a late fall cruise to the Greenland and Norwegian Seas (GSNS). In warm, tropical surface waters subject to high solar irradiance, average CH3I saturation anomalies were positive (1.5–7.7 pmol kg−1), indicating a sea-to-air flux. This contrasted with negative saturation anomalies (−0.65±0.02 pmol kg−1) measured in cold surface waters of the open ocean GSNS subject to low-light. High latitude oceans may therefore be a significant sink for atmospheric CH3 during the fall and winter. The locations and/or seasons where samples were analyzed were all characterized by relatively low biological production and the CH3I saturation anomaly along 19°S decreased from 7.7±0.6 to 3.4±0.4 pmol kg−1 when entering a more productive upwelling zone. Taken together these observations suggest a chemical, as opposed to biological, production mechanism for this compound in the open ocean. Within the open ocean of the GSNS, multiple linear regression between the observed CH3I saturation anomaly and variables including light intensity, water temperature, CFC-11 saturation (indicator of gas exchange and deep mixing), and distance from the Norwegian Coastal Current (indicator of coastal or southern sources) showed that light intensity was the only significant predictor, explaining 79% of the variance. Photochemical production may therefore be dominant source of CH3I within the open ocean and this may have important implications for the large-scale, seasonal cycling of iodine between the ocean and the atmosphere.

Description: We study the temporal evolution of concentrations of the chlorofluorocarbons CFC 11 and CFC 12 in the ocean, under the assumption of circulation and mixing being invariant in time. This allows us to define a time‐invariant age distribution for a given point in the ocean, where the age is defined as time since the last contact with the atmosphere occurred. This concept is evaluated for a number of fundamental situations. We deduce a tendency for low CFC 11 and CFC 12 concentrations in advective regimes to increase exponentially in time and for concentrations near to a solubility equilibrium with atmospheric concentrations to increase rather more linearly. The apparent saturations, i.e., the ratios of interior to mixed‐layer CFC concentrations, increase monotonically in time, typical rates being 5–10% per decade. The theoretical results are compatible with time trends found in repeated CFC observations in the ocean. Diagrams on the temporal evolution for different age distributions are presented for the period 1970–2000, which can serve as a general orientation. The diagrams furthermore can provide time corrections for quasi‐synoptic evaluation of CFC observations taken over an extended period of time and assist in constructing time‐dependent CFC boundary conditions for numerical models of ocean circulation.