ALBERT

Description: Five hormone-treated female Japanese silver eels Anguilla japonica were tagged with ultrasonic transmitters and released by submersible in the West Pacific at seamounts of the West Mariana Ridge, their supposed spawning grounds. Four eels were tracked for 60 to 423 min in the vicinity of the seamounts. They did not settle at the seamounts but swam at a mean speed of 0.37 m s-1 into open water above deep ground. Their mean swimming depth ranged from 81 to 172 m. Experiments suggest that pre-matured A. japonica migrate to their spawning grounds in temperate warm water and at shallow depths.

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: Phosphatase (P-ase) activity was determined together with other extracellular enzyme activities, bacterial abundance and production rates during the 2 SW Monsoon process studies of the German JGOFS Arabian Sea Program. Water samples were collected along the cruise tracks from the equator to the upwelling region at the shelf edge off Oman. Depth profiles of P-ase activity were strikingly different from those of the other enzymes. While values of aminopeptidase and β-glucosidase generally decreased below the euphotic zone, P-ase increased by factors of 1 to 7. The relation between peptidase- and P-ase activity was from 4 to 21 at the surface and from 3 to 5 at 800 m depth. Because P-ase production (dissolved and cell-bound) in deep waters is mainly dependent on bacteria, P-ase activities per bacterial cell were calculated: these were, on average, 37 times higher at 800 m than at the surface. We also observed a positive correlation of P-ase activity with phosphate concentrations in the depth profiles below the euphotic zone, while this relationship was much more variable in the mixed surface layer. These observations suggest that C-limited bacteria in the deep strata did not primarily focus on the phosphate generated by their P-ase activity but on the organic C compounds which were simultaneously produced and which could probably not be taken up prior to the hydrolytic detachment of phosphate. It is hypothesised that a considerable part of the measured P-ase activity was dissolved (though it might have originated from bacteria). These enzymes may be important for the slow, but steady regeneration of phosphate and organic C in mesopelagic waters.

Description: Stomach contents of 17 sperm whales Physeter macrocephalus stranded in Scotland and Denmark during 1990-96 were analysed. All were sub-adult or adult males and stranded between November and March. They had presumably entered the North Sea during their southward migration from feeding grounds in Arctic waters. Other studies indicate that the majority of the whales were apparently healthy. The diet of these whales was found to consist almost entirely of cephalopods, principally squid of the genus Gonatus (hereafter 'Gonatus', but probably G. fabricii, an oceanic species characteristic of Arctic waters). The other prey species identified were also mostly oceanic cephalopods: the squids Histioteuthis bonnellii, Teuthowenia megalops and Todarodes sagittatus and the octopus Haliphron atlanticus. Although these results are consistent with other recent studies in the area based on single stranded whales, they differ from results of work on whales caught during commercial whaling operations in Icelandic waters (1960s to 1980s) in that little evidence of predation on fish was found in the present study. Remains of single individuals of the veined squid Loligo forbesi, the northern octopus Eledone cirrhosa and the saithe Pollachius virens provided the only possible evidence of feeding in the North Sea. We infer that sperm whales do not enter the North Sea to feed. The timing, and large and uniform sizes of the Gonatus species eaten (most had mantle lengths in the range 195 to 245 mm), as estimated from measurements of the lower beaks, and the seasonality of the strandings is consistent with the whales having fed on mature squid, possibly spawning concentrations--as has recently been reported for bottlenose whales. Assuming that the diet recorded in this study was representative of sperm whales during the feeding season, as much as 500000 t of Gonatus could be removed by sperm whales in Norwegian waters each year and up to 3 times that figure from the eastern North Atlantic as a whole. Evidence from other studies indicates that Gonatus is an important food resource for a wide range of marine predators in Arctic waters.

Description: Interpretation of diving profiles of aquatic animals would be considerably enhanced by additional behavioural information. A new sensor is presented here which records animal movements. This sensor was tested on a captive loggerhead turtle Caretta caretta which showed similar activity patterns to free-living green turtles Chelonia mydas. A computer program with user-selectable options was developed to analyse the data consistently and rapidly. Using our sensor we calculated the total resting time, which differed by less than 5% from the real resting time when the sampling interval was 2 s. The method was additionally tested for different sampling intervals to find out its applicability for field studies.

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: Nematodes of the family Stilbonematinae are known for their highly specific association with ectosymbiotic bacteria. These worms are members of the meiofauna in marine, sulfide-rich sediments, where they migrate around the redox boundary layer. In this study, bacterial ectosymbionts of 2 species of marine nematodes, Stilbonema sp. and Laxus oneistus, were shown to be capable of the respiratory reduction of nitrate and nitrite (denitrification). The use of these alternative electron acceptors to oxygen by the bacteria allows the animals to migrate into the deeper, anoxic sediments, where they can exploit the sulfide-rich patches of the deeper sediment layers. The accumulation of thiols (sulfide, thiosulfate, sulfate and glutathione) in body tissues of the worms was determined following incubation in the presence of various electron donors (sulfide, thiosulfate) and acceptors (nitrate). In their chemoautotrophic metabolic potential, the ectosymbionts of the 2 nematode species were found to resemble the phylogenetically related, intracellular symbionts of macrofaunal hosts of deep-sea hydrothermal vents and other sulfide-rich habitats.

Description: The role of tetrathionate in the sulfur cycle of Baltic Sea sediments was investigated in different habitats and under a variety of environmental conditions. Sediment profiles were recorded with regard to numbers of thiosulfate oxidizing bacteria, concentrations of sulfur compounds, and potential rates of thiosulfate oxidation. Products of thiosulfate oxidation were quantified in incubated sediment samples and in pure cultures. Evidence was found that tetrathionate is formed within these sediments, that sulfur oxidizing bacteria are present in considerable numbers, that these bacteria are of major importance in the oxidation of reduced sulfur compounds in their habitat, and that tetrathionate is an important oxidation product of these bacteria. Thiosulfate is oxidized by bacteria isolated from these sediments to varying proportions of tetrathionate, sulfate, and also elemental sulfur. In highly sulfidic sediments and in the presence of large amounts of organic matter, tetrathionate was present in sediment horizons in which thiosulfate and elemental sulfur also accumulated. A tetrathionate cycle is proposed to be active in natural marine and brackish water sediments in which, due to combined bacterial action and chemical reactions, a net oxidation of sulfide to elemental sulfur occurs in the presence of catalytic amounts of thiosulfate and tetrathionate.

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: Seabirds, like all marine endotherms, have to compensate for the extensive cooling effect of water when diving. Alone among them, cormorants (Phalacrocoracidae) have a wettable plumage and are predicted to require disproportionately large amounts of food to balance heat losses. These piscivorous birds are thus thought to have a detrimental impact on fish stocks. However, we show here that even in great cormorants from Greenland, which dive in water at 3 to 7°C, daily food intake is lower than for well-insulated European seabirds. Despite their wettable plumage, cormorants thus appear to manage their energy budgets in a remarkably efficient way. Nevertheless, the specific foraging strategies which enable this performance make cormorants dependent on high prey density areas, a feature that should be taken into account by future management plans.

Description: In Eckernförde Bay (western Baltic Sea) pockmark structures are induced by groundwater seeping out of the sediment. On 3 occasions in winter and spring 1993-94 we investigated the influence of groundwater on the reduction of salinity, on porewater chemistry, and on bacterial activities (methane oxidation and sulphate reduction). In 2 out of 3 sampling campaigns groundwater discharge could be detected. The concentration gradients of Cl- and SO4= are moved towards the sediment surface by the vertical advection of groundwater during seep times. Without groundwater discharge the porewater chemistry resembled the control site. Compared to the control site, the methane oxidation and sulphate reduction rates were elevated at the pockmark site, reaching maximum values of 49 and 269 µmol l-1 d-1 respectively. The groundwater venting from the pockmark had an end member composition of 80 mM Na+, 1.0 mM Ca++ and was depleted in Mg++. Due to mixing of these major cations along the groundwater/seawater interface, no CaCO3 precipitation was found around the pockmark site.

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: Sinking velocities of more than 300 Nitzschia closterium aggregates were determined during roller table incubation using digital image analysis. To examine the influence of transparent exopolymer particles (TEP) on aggregate settling speed, 3 experiments with different ratios of TEP to cell volume concentration were conducted. The results showed that, for N. closterium aggregates without TEP, sinking velocity (U) was significantly related to the equivalent spherical diameter (ESD) of the aggregates, yielding U (cm s-1) = 1.89 (ESD, cm)0.55. The higher was the specific TEP content of an aggregate, the lower was the sinking velocity and the less pronounced was the size versus velocity relationship. Excess densities (Δρ) of aggregates were derived from velocity measurements and 3-dimensional fractal dimensions (D3) of aggregates were calculated from scaling properties of Δρ. Values for D3 never exceeded 2 and fit well to values of the 2-dimensional fractal dimension (D2) attained from image analysis.

Description: Stable carbon isotope fractionation (ε p) of 7 marine phytoplankton species grown in different irradiance cycles was measured under nutrient-replete conditions at a high light intensity in batch cultures. Compared to experiments under continuous light, all species exhibited a significantly higher instantaneous growth rate (μi), defined as the rate of carbon fixation during the photoperiod, when cultivated at 12:12 h, 16:8 h, or 18:6 h light:dark (L/D) cycles. Isotopic fractionation by the diatoms Skeletonema costatum, Asterionella glacialis, Thalassiosira punctigera, and Coscinodiscus wailesii (Group I) was 4 to 6o/oo lower in a 16:8 h L/D cycle than under continuous light, which we attribute to differences in μi. In contrast, ε p in Phaeodactylum tricornutum, Thalassiosira weissflogii, and in the dinoflagellate Scrippsiella trochoidea (Group II) was largely insensitive to daylength-related differences in instantaneous growth rate. Since other studies have reported growth-rate dependent fractionation under N-limited conditions in P. tricornutum, μi-related effects on fractionation apparently depend on the factor controlling growth rate. We suggest that a general relationship between εi and μi/[CO2,aq] may not exist. For 1 species of each group we tested the effect of variable CO2 concentration, [CO2,aq], on isotopic fractionation. A decrease in [CO2,aq] from ca 26 to 3 µmol kg-1 caused a decrease in ε p by less than 3o/oo. This indicates that variation in μi in response to changes in daylength has a similar or even greater effect on isotopic fractionation than [CO2,aq] in some of the species tested. In both groups ε p tended to be higher in smaller species at comparable growth rates. In 24 and 48 h time series the algal cells became progressively enriched in 13C during the day and the first hours of the dark period, followed by 13C depletion in the 2 h before beginning of the following light period. The daily amplitude of the algal isotopic composition (δ13C), however, was 〈=1.5o/oo, which demonstrates that diurnal variation in δ13C is relatively small.

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: The El Niño/Southern Oscillation (ENSO) phenomenon is the strongest natural interannual climate fluctuation1. ENSO originates in the tropical Pacific Ocean and has large effects on the ecology of the region, but it also influences the entire global climate system and affects the societies and economies of manycountries2. ENSO can be understood as an irregular low-frequency oscillation between a warm (El Niño) and a cold (La Niña) state. The strong El Niños of 1982/1983 and 1997/1998, along with the more frequent occurrences of El Niños during the past few decades, raise the question of whether human-induced 'greenhouse' warming affects, or will affect, ENSO3. Several global climate models have been applied to transient greenhouse-gas-induced warming simulations to address this question4, 6, but the results have been debated owing to the inability of the models to fully simulate ENSO (because of their coarse equatorial resolution)7. Here we present results from a global climate model with sufficient resolution in the tropics to adequately represent the narrow equatorial upwelling and low-frequency waves. When the model is forced by a realistic future scenario of increasing greenhouse-gas concentrations, more frequent El-Niño-like conditions and stronger cold events in the tropical Pacific Ocean result

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: Diverse coastal seaweed communities dominated by perennial fucoids become replaced by species-poor turfs of annual algae throughout the Baltic Sea. A large-scale field survey and factorial field experiments indicated that grazers maintain the fucoid community through selective consumption of annual algae. Interactive effects between grazers and dormant propagules of annual algae, stored in a 'marine seed bank', determine the response of this system to anthropogenic nutrient loading. Nutrients override grazer control and accelerate the loss of algal diversity in the presence but not in the absence of a propagule bank. This implies a novel role of propagule banks for community regulation and ecosystem response to marine eutrophication.

Description: An experimental laboratory set-up was used to study the influence of different grain size compositions and temperatures on the growth of benthic cyanobacteria and diatoms, and on the competition between these 2 groups. Monospecific cultures of 3 species of cyanobacteria (Merismopedia punctata, Microcoleus chthonoplastes, Oscillatoria limosa), and of 2 species of benthic diatoms (Phaeodactylum tricornutum and Nitzschia sp.) were used. The organisms were cultured in 100 ml flasks filled with medium and 3 different kinds of sediment: (1) Sand (fine sand, 63 to 200 µm), (2) Mud-I (mixed fine sand and mud 〈63 µm in the ratio 80:20 wt %), (3) Mud-II (mixed fine sand and mud in the ratio 50:50 wt %). Experimental temperatures were 10, 15 and 25°C. At 10°C and 15°C, both diatom species achieved the highest biomass on the sediments of the finest grain size (50 wt % 〈 63 µm) while cyanobacteria achieved low biomass levels. Coarsening of sediments at the same temperature levels revealed a gradually lower biomass of the diatoms. Particularly on sand, the diatoms never reached the same concentrations of chlorophyll a as on mud. The cyanobacteria, on the other hand, had the highest biomass on sand at 15°C. In the competition experiments the benthic diatom species Nitzschia sp. dominated all types of sediments at 10°C and 15°C. The experiments at 25°C were dominated by the filamentous cyanobacterium M. chthonoplastes. This indicates the importance of abiotic conditions for the distribution and abundance of benthic phototrophic micro-organisms.

Description: The multivariate patterns resulting from analyses of macrobenthic abundance data at different taxonomic levels are compared to the pattern derived from various measurements obtained through sediment profiling imagery (SPI). A time-series data set from 1 station in Kiel Bay (Western Baltic) at 22 m depth including macrobenthic and SPI replicates covering 8 yr (1989 to 1996) was analyzed by means of multidimensional scaling (MDS) ordination. The macrobenthos data showed similar patterns, and there was little information loss, with decreasing taxonomic resolution from species to phylum level. The multivariate pattern in the SPI data was not significantly correlated to any of the macrofaunal patterns. However, macrofaunal and SPI patterns seemed to be complementary since they emphasized different aspects of the long-term succession in the Southern Baltic Sea. While macrofaunal patterns were sensitive to anoxia events, changes of SPI-recorded seabed characteristics were primarily related to physical disturbances possibly due to variations in fishing intensity.

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: Diving seabirds should evolve a variety of foraging characteristics which enable them to minimize energy expenditure and to maximize net energy gain while searching for prey underwater. In order to assess the related ecological adaptations in a marine predator, we studied the at-sea distribution and the diving behaviour of 23 cormorants Phalacrocorax carbo (Linnaeus) breeding at the Chausey Islands (France) using VHF-telemetry and data loggers recording hydrostatic pressure. Birds foraged within an area of approximately 1131 km2 situated north-east of the breeding colony. This zone represents only 25% of the maximal potentially available area that the birds may utilize considering their maximum foraging range of 35 km. Individual birds remained within restricted individual foraging areas (on average 18 and 10% of the total utilized area in 1994 and 1995, respectively) throughout the study period. Moreover, the cormorants studied conducted an average of 42 dives per foraging trip, lasting for an average of 40 s (maximum 152 s), and reached an average maximum dive depth of 6.1 m (maximum 32 m) with median descent and ascent angles calculated to be 18.7° and 20.3°, respectively. Overall, 64% of all dives were U-shaped dives and 36% V-shaped dives. We use these results to demonstrate how both specialization and opportunism may support the remarkably high foraging efficiency of this marine predator.

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: Cephalopods play an important role in the trophic web of the Southern Ocean, but little information is available on their biology. The 2 largest sub-Antarctic seabirds, the king penguin Aptenodytes patagonicus and the wandering albatross Diomedea exulans, feed primarily on squids during the austral winter at the Crozet Islands. We examined a large number of accumulated cephalopod beaks in the stomach of these birds together with some undigested items; first, to understand how these 2 predators share the squid resource during winter, a period of supposed low food availability, and, second, to use a diving and a flying seabird as biological samplers of Southern Ocean cephalopods. Individuals of the family Onychoteuthidae formed the bulk of the squid diet, accounting for 72.6 and 57.0% of the number of lower beaks in samples from king penguins and wandering albatrosses, respectively. Seven different species were identified, the 3 main squids being Kondakovia longimana (38.8 and 28.0% by number for penguins and albatrosses, respectively), Moroteuthis ingens (13.5 and 26.2%) and M. knipovitchi (20.1 and 2.3%). Both seabirds preyed upon the same cephalopod species, but penguins primarily took small- to medium-sized juveniles (99.0% of the onychoteuthids) and albatrosses preyed on larger adult specimens (96.0%). Fresh remains indicated that adult K. longimana and M. ingens were mature individuals which, as shown by satellite tracking of albatrosses, were taken over the slope and nearby oceanic waters surrounding the archipelago. The present study indicates that mating/spawning of K. longimana and M. ingens occurs in Crozet waters during the winter months. It also extends the biogeography of K. longimana to north of the Antarctic Polar Front, in the Polar Frontal Zone, where it has not previously been recorded.

Description: The marine habitat exploited by black-browed Diomedea melanophrys and grey-headed albatrosses D. chrysostoma breeding at Campbell Island, New Zealand, was studied using satellite telemetry. Data were analysed in relation to the bathymetry and sea-surface temperature of the foraging zones. Black-browed albatrosses spent 55% of their time on the Campbell Plateau but also carried out long foraging trips to the Polar Front and Antarctic Zone at a distance of over 2000 km. They relied heavily on juvenile Micromesistius australis, a schooling fish, during foraging trips to the shelf but over oceanic waters the squid Martialia hyadesi was the main prey taken. Grey-headed albatrosses spent 71% of their time foraging over the deep waters of the Polar Frontal Zone where M. hyadesi comprised over 90% of the mass of prey taken. No satellite-tracked birds fed over the shelf, but data from the duration of foraging trips and dietary analysis suggests that shelf-feeding is important for this species. Significant inter-species differences in the time spent in neritic and oceanic zones show that black-browed albatrosses are reliant primarily on shelf resources while grey-headed albatrosses are primarily oceanic feeders. In addition, the 2 species overlapped little in the zones used over oceanic waters, with black-browed albatrosses feeding in more southerly waters than grey-headed albatrosses. However, both species feed on M. hyadesi when foraging in association with the Polar Front.

Description: The fishery for Illex argentinus in the Southwest Atlantic is subject to large inter-annual variability in recruitment strength. In this paper we attempt to build a predictive model using sea surface temperature (SST) to examine links between recruitment to the Falkland Islands fishery and environmental variability during the juvenile and adult life history stages. SST data from the National Center for Atmospheric Research (NCAR) were found to be comparable with near-surface data derived from in situ expendable bathy-thermograph (XBT) profiles in the southern Patagonian shelf. Variation in SST during the early life stages appears to be important in determining recruitment of I. argentinus. SST in the hatching grounds of the northern Patagonian shelf during the period of hatching (particularly June and July) was negatively correlated with catches in the fishery in the following season. SST anomaly data from positions in the Pacific and Southwest Atlantic were used to examine teleconnections between these areas. Links were seen at a lag of 2 yr between the Pacific and southern Patagonian shelf, and at about 5 yr between the Pacific and northern Patagonian shelf. This is consistent with SST anomalies associated with El Niño in the Pacific propagating around the globe via the Antarctic Circumpolar Wave (ACW). Predicting cold events via teleconnections between SST anomalies in the Pacific and Atlantic would appear to have the potential to predict the recruitment strength of I. argentinus in the Southwest Atlantic.

Description: Levels of genetic diversity and population differentiation were examined in temporally (1990 to 1997) and geographically separated samples of the argentine short-finned squid Illex argentinus using 7 microsatellite loci. Number of alleles (mean number of alleles per locus over all samples = 24.1) and heterozygosity (mean observed heterozygosity per sample = 0.84) were high for all samples, indicating that these loci have a greater potential utility for investigating population genetic structure than allozyme markers used in previous studies. Genetic diversity did not differ significantly between samples taken 5 yr after commencement of the fishery (1990) and those collected during a period of progressively intense fishing pressure (1994 and 1997). Several small but significant differences in between-sample genetic variation (FST) were observed, but these could not confirm the previous suggestion of cryptic species or several well-defined stocks within the fished population.

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: The overflow and descent of cold dense water from the Denmark Strait sill-a submarine passage between Greenland and Iceland-is a principal means by which the deep ocean is ventilated, and is an important element in the global thermohaline circulation. Previous investigations of its variability-in particular, direct current measurements(1,2) in the overflow core since 1986-have shown surprisingly little evidence of long-term changes in now speed. Here we report significant changes in the overflow characteristics during the winter of 1996-97, measured using two current-meter moorings and an inverted echo sounder located at different depths in the fastest part of the now. The overflow warmed to the highest monthly value yet recorded (2.4 degrees C), and showed a pronounced slowing and thinning at its lower margin. We believe that the extreme warmth of the overflow caused it to run higher on the continental slope off east Greenland, so that the lower current meters and the echo sounder were temporarily outside and deeper than the fast-flowing core; model simulations appear to confirm this interpretation, We suggest that the extreme warmth of the overflow is a lagged response to a warming upstream in the Fram Strait three years earlier (caused by an exceptional amplification of the winter North Atlantic Oscillation). If this is so, over-now characteristics may be predictable.

Description: Epibiosis is a spatially close association between 2 or more organisms belonging to the same or different species. Through direct and indirect interactions, this association has major effects on the species involved and on community dynamics. When the effects are predominantly beneficial for epibiont and basibiont, coevolution can be expected to lead to associational specificity. Circumstantial evidence, however, suggests that many epibionts are non-specific substratum-generalists. In this arti-cle, we investigate the commonness of specificity in epibiotic associations. In a first approach, we inves-tigated the in situ recruitment preferences of potential epibionts when choosing between artificial and living substrata. After exposure for 3 wk in early summer, an early successional community had estab-lished, comprising cyanobacteria, diatoms, sesslle colonial ciliates and red algae. All species recruited on almost all substrata available. However, artificial substrata were usually preferred over living sur-faces. Consequently, the species studied are class~fied as facultative epibionts. An analysis of a list of over 2000 epibiotic associations corroborated these results, the majority of described 'epibionts' are not basibiont-specific and generally occur on non-living substrata as well. Also, basibiont species usually bear more than 1 epibiont species. Relative to each other, epibionts and basibionts are characterised by a typical set of life history traits. We conclude that specific and obligate epibionts are rare. Their scarcity is discussed in view of multilevel antifouling defences and presumptive evolutionary transi-tions from epibiosis towards endoparasitism or endosymbiosis.

Description: Diving reptiles, unlike most diving birds and mammals, return infrequently to the surface to breathe. Spending the bulk of their lives underwater, they are likely to have developed a large variety of specific behavioural patterns different from those of their warm-blooded counterparts. However, for technical reasons, underwater behaviour of these aquatic reptiles remains poorly understood. In this study green turtles Chelonia mydas nesting on Cyprus (Eastern Mediterranean) were equipped with multi-channel data loggers monitoring diving behaviour and activity (via a logger-integrated 3-D compass which served as an activity sensor) during the inter-nesting interval. Data from 2 turtles for 2 consecutive inter-nesting intervals were available for detailed dive analysis. Both turtles showed highly variable dive patterns ranging from travelling subsurface dives to specific dive types such as U- (mainly resting and foraging dives), S- (a form of energy saving swimming) and V-dives. The green turtles stayed near the coast throughout the study, dived no deeper than ca 25 m, but remained underwater for up to ca 40 min. The recordings of the activity sensor revealed high activity levels (less than 20% resting d-1) during the whole inter-nesting period which was attributed to extensive foraging. The combination of both the activity data and the dive data showed that the turtles were engaged in travelling movements for 46% of the inter-nesting time spent underwater, foraged for 34% and rested for 12% of the time. We discuss the physiological, ecological and conservation implications of these results.

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: Serial dilution experiments were conducted on JGOFS-North Atlantic cruise of RV 'Meteor' M36/2 at a 20° W transect in June and July 1996 to assess the role of microzooplankton grazing and nitrogen supply in controlling phytoplankton stocks in the subtropical and temperate northeast Atlantic. Rates of microzooplankton grazing ranged from 0.08 d-1 at 54° N to 0.53 d-1 at 40° N and mean growth rates of phytoplankton ranged from 0.19 d-1 at 54° N to 0.75 d-1 at 40° N. Both rates were positively related to seawater temperature, whereas the apparent growth yield of phytoplankton declined with increasing temperature from 0.19 µg chl a dm-3 d-1 at 54° N to 0.01 µg chl a dm-3 d-1 at 33° N. Complete nitrogen saturation of phytoplankton growth indicated light or non-nitrogenous limitation at the nitracline at 47° N and in the deep chlorophyll maximum at 33° N, whereas in the mixed layer at 47° N and 54° N the ambient nitrogen supply was sub-saturated and yielded 63 and 39% of nitrogen- saturated growth. Nitrogen supply of phytoplankton growth was dominated by external and cellular sources in nitrate-rich waters of the mixed layer at 54° N and at the nitracline at 47° N, whereas nitrogen regeneration dominated at the nitrate-depleted surface waters at 47° N. However, in the deep chlorophyll maxima at 33° N and 40° N phytoplankton growth was primarily maintained by nitrogen regeneration, although external nitrogen was sufficiently available. The recycling efficiency of the microbial community was defined as the ratio of regenerated growth yield to herbivorous grazing loss. Efficiencies of ~100% under post-bloom situations indicated tight coupling of predation, nitrogen supply and phytoplankton growth. We suggest that microzooplankton grazing has a high potential for nitrogen supply and biomass control of phytoplankton communities during summer in the temperate and subtropical northeast Atlantic.

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: The only known population of coelacanths, in the Comores, western Indian Ocean, is endangered by human predation. Historical catch data from Grande Comore reveal that annual catch rates increased steadily from 1954 until the 1970s. This trend was temporarily interrupted due to an international policy introducing motorized boats and promoting offshore fishing techniques. Coelacanths are only caught from traditional unmotorized outrigger canoes as an incidental by-catch of deep water line fishing. A complete survey of all motorized and unmotorized vessels in 1995 at Grande Comore in comparison to earlier years indicated that a recent decreased use of motors and increase of unmotorized canoe fishing has led to an increase in coelacanth catches. Conservation measures and strategies for reducing the fishing pressure exerted on coelacanths are discussed. The southwest coast of Grande Comore should be designated as a nature reserve and protected area where immediate protection measures should be taken, an opinion which is supported by Comorian authorities.

Description: VHF telemetry was used in November and December 1995 on 8 Humboldt penguins Spheniscus humboldti breeding at Pan de Azúcar Island (26°S, 70°W), Northern Chile, to determine at-sea behaviour of the birds. We obtained 2710 locations, 90% of which were within a radius of 20 km around the island. Mean travelling speed of the birds was 0.92 m s-1 and speed distribution showed peaks at 1.6 and 3 m s-1. Penguins travelling between foraging areas remained submerged for an average of 8.4 s between surfacings, whereas foraging dives lasted on average 61 s. The analysis of 79 complete foraging trips showed that tracks deviated from a straight course, and range (maximum distance from island) was only 0.37 times total horizontal distance swum. Birds did not forage synchronously or in the same foraging areas. However, foraging ranges were correlated between birds, indicating similar search strategies during periods of low food availability. The results obtained here via VHF telemetry agreed well with those of previous studies employing satellite transmitters and data loggers.

Description: The stable carbon isotope composition (δ13C) of particulate organic carbon (POC) was measured in 3 size fractions (POCtotal, POC〉20 µm, POC〈20 µm) during a phytoplankton spring bloom dominated by the diatom Skeletonema costatum in Lindåspollene, a land-locked fjord in southern Norway. In addition to standard parameters for characterizing the phytoplankton bloom (chlorophyll, nutrient, and POC concentrations, and species composition), simultaneous measurements of δ13C of dissolved inorganic carbon (DIC), total alkalinity and DIC concentration were obtained to determine temporal trends in dissolved carbon dioxide concentration and in carbon isotope fractionation (ε p) of the POC size fractions. The carbon isotope composition of the 〉20 µm size fraction, which was dominated by diatoms, was ca 2o/oo heavier than that of the 〈20 µm fraction, which was mainly composed of flagellates. δ13C of both size fractions increased by about 3o/oo over the course of the bloom. A 5o/oo increase in δ13C-PO Ctotal during the bloom resulted partly from a shift in the phytoplankton community from a flagellate- to a diatom-dominated one. Carbon isotope fractionation of all fractions decreased with declining CO2(aq) concentration (14 to 〉6 µmol l-1). A positive correlation between ε p and [CO2(aq)] in the diatom size fraction was obtained for the period of exponential growth. Deviation from this correlation occurred after the peak in cell density and chlorphyll a (chl a) concentration, when POC still continued to increase, and may be related to changing phytoplankton growth rates or to possible effects of nutrient (nitrate) limitation on ε p. Comparison of these results with those of previous field studies shows that, while an inverse relationship is consistently observed between ε p and the ratio of instantaneous growth rate and CO2 concentration {µi/[CO2(aq)]}, considerable scatter exists in this relationship. While this scatter may have partly resulted from inconsistencies between the different studies in estimating phytoplankton growth rate, it could also reflect that factors other than growth rate and CO2 concentration significantly contribute to determining isotope fractionation by marine phytoplankton in the natural environment

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: A total of 354 adult European smelts Osmerus eperlanus (L.) were tested for their ability to survive the screen system of the cooling water inflow of a power plant. With increasing number of musculature parasitic third-stage larvae of Pseudoterranova decipiens, the survival rate of O. eperlanus decreased while the total number of externally visible injuries as well as the number of seriously injured specimens increased. The results indicate that even a single specimen of P. decipiens influences resistance and stamina and affects overall mortality of 7 to 20 cm long smelts. The initial effect of the parasites is to reduce swimming speed of infested fish, which leads to more frequent contact of these fish with the fine meshed screen of the cooling water inlet before they are removed by the automatic cleaning system. If the separated fishes are returned to the main stream, it becomes apparent that the cooling water inflow selectively reduces the number of living parasitised smelt in the area. Thus, the number of parasitic third-stage P. decipiens larvae in the local smelt population which are able to complete their life-cycle is also reduced. P. decipiens makes infested smelt more susceptible to negative anthropogenic influences such as cooling water intake or trawl fisheries.

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: During a transition period from oxic to anoxic conditions in the bottom water, rates of sulfate reduction and methane production, methane fluxes, as well as concentration profiles of sulfate, sulfide and methane were measured in sediments at a central site of the Gotland Deep (Stn AL 93, 241 m depth), which is regarded as representative for the deepest part of this basin. During this period from 1993 to 1996 oxic conditions in the bottom water prevailed from spring 1994 until summer 1995 with oxygen concentrations decreasing progressively with time. In the sediments methane production occurred primarily in layers below 1 m depth and flux rates of methane to the sediment surface were characterized by a steep concentration gradient from approx. 5 mM at 4 m depth to values close to 30 μM at the surface, determined by diffusion processes and anaerobic oxidation of methane. Both processes were independent of changes at the sediment surface. Differences in the flux rates of methane between the deeper part with a mean value of 259 μmol m-2 d-1 and the upper layers with a mean of 47.7 μmol m-2 d-1 indicate that a considerable proportion of the methane is oxidized within the anoxic horizon of the sediment (71 to 86% in the layer from 40 to 70 cm). Low rates of methane production found within the top 20 cm of the sediment during periods of oxic bottom water increased after depletion of oxygen and resulted in a clear maximum of the methane concentration in the top 2 cm. Sulfate concentrations declined exponentially from values of 11.5 mM in June 1994 and 8.5 mM in October 1995 at the sediment surface to values of 2.5 mM at 20 cm depth and of less than 0.5 mM at 50 to 60 cm depth. High sulfate reduction rates (150 to 250 nmol cm-3 d-1) in the upper part of the sediment (8 to 13 cm) coincided with maxima of sulfide concentrations. During the time period of this investigation an increase of maximum sulfide concentrations in the sediment from 1 to 10 mM was measured together with decreasing oxygen concentrations in the deep water. At the same time sulfate reduction established a small but distinct maximum at the top layer of the sediment (0 to 2 cm). The relative importance of sulfate reduction and methanogenesis in the carbon budget of the Gotland Deep sediments is calculated on the basis of the actual measurements.

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: In steady state, the export of photosynthetically fixed organic matter to the deep ocean has to be balanced by an upward flux of nutrients into the euphotic zone1. Indirect geochemical estimates2 of the nutrient supply to surface waters have been substantially higher than direct biological and physical measurements3, particularly in subtropical regions. A possible explanation for the apparent discrepancy is that the sampling strategy of the direct measurements has under-represented episodic nutrient injections forced by mesoscale eddy dynamics, whereas geochemical tracer budgets integrate fluxes over longer time and space scales. Here we investigate the eddy-induced nutrient supply by combining two methods potentially capable of delivering synoptic descriptions of the ocean's state on a basin scale. Remotely sensed sea-surface height data from the simultaneous TOPEX/Poseidon and ERS-1 satellite missions are assimilated into a numerical eddy-resolving coupled ecosystem–circulation model of the North Atlantic Ocean. Our results indicate that mesoscale eddy activity accounts for about one-third of the total flux of nitrate into the euphotic zone (taken to represent new production) in the subtropics and at mid-latitudes. This contribution is not sufficient to maintain the observed primary production in parts of the subtropical gyre, where alternative routes of nitrogen supply will have to be considered.

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 ability to monitor the heat content of oceans over long distances is becoming increasingly important for understanding the role of oceans in climate change, for determining the variability of the state of the oceans, for operational ocean observing systems, and for studying large-scale ocean processes such as water-mass formation. Although the properties of the upper layers of the ocean can be routinely measured on large scales by satellite remote sensing (providing altimetric and infrared data) and with expendable probes dropped from commercial vessels, the deep interior of the ocean is more difficult to monitor. Ocean acoustic tomography1 is a promising technique for such applications, as it has the potential to provide systematic, instantaneous and repeated measurements of the ocean interior over large parts of an ocean basin. Here we demonstrate the capability of this technique for measuring the heat content across an entire (albeit small) ocean basin—the western Mediterranean Sea.

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: The effect of variable CO2 concentrations on the elemental composition (C:N:P) of marine diatoms was investigated in 2 strains of Skeletonema costatum (Grev.) Cleve. Five or 6 concentrations of dissolved molecular carbon dioxide [CO2 (aq)], ranging from 0.5 to 39 µmol l-1, were applied in dilute batch cultures. In both strains, elemental ratios were clearly dependent on [CO2 (aq)]. With decreasing CO2 concentrations, a decline in C:P and N:P and an increase in C:N was observed. The close correlation between C:P or N:P and [CO2 (aq)] corresponded to a ca 45 to 65% decrease in elemental ratios from highest (〉=30 µmol l-1) to lowest (ca 1 µmol l-1) CO2 concentrations. C:N at low [CO2 (aq)] was up to 24% higher than at high [CO2 (aq)]. To date, the elemental composition of marine phytoplankton has been considered to be independent of CO2 availability. If dependency of the C:N:P ratio on [CO2 (aq)] proves to be a general phenomenon in marine phytoplankton, changes in the elemental composition may be expected in response to the currently observed increase in partial pressure of atmospheric CO2.

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.