ALBERT

Description: Previous work has shown that during early summer, the partial pressure of CO2 (pCO(2)) in surface waters north of about 45 degreesN in the Atlantic exhibits widespread undersaturation. In many areas. this follows after a "spring bloom" of phytoplankton, at which time, nutrient concentrations and pCO(2) decrease sharply from their winter surface values. As part of OMEX I, the late summer distribution of surface water pCO(2) was surveyed in the northeastern Atlantic on cruises of R/V Poseidon and R/V Belgica in 1995. The pattern of the surface distribution of the sea-air pCO(2) difference (Delta pCO(2)) measured on these ship surveys was generally iri accord with that observed in this area in early to mid-summer of 1981. The greatest CO2 undersaturation (-95 mu atm) during our surveys was observed near the west coast of Iceland, with Delta pCO(2) increasing to about -60 mu atm away from the coast. In shelf waters south of Ireland, the pCO(2) was relatively higher than in surface waters of the open ocean adjacent to the Celtic Shelf margin, but the Celtic Shelf waters were still undersaturated relative to the atmospheric CO2 concentration. Because of the variation of wind speed, the synoptic distribution of air-sea CO2 flux, derived from the transfer velocity and Delta pCO(2), does not resemble the distribution of Delta pCO(2) itself. The sharp increase in wind speed at about 53 degreesN, 20 degreesW during the R/V Poseidon survey produces an order of magnitude rise in the estimated air-sea flux of CO2, to a level of about 10-14 mol m(-2) a(-1). The overall synoptic picture appears to be one of moving centers of higher air-sea fluxes that occur where storms pass over regions of surface water pCO(2) undersaturation.

Description: Observations from cruises in the Arabian Sea and data from satellites are interpreted using different realizations of a multi-level primitive equation model and an eddy-permitting reduced-gravity shallow water model of the Indian Ocean. The focus is on the interannual circulation variability of the Arabian Sea, and especially of the meridional location of the Great Whirl (GW). The results suggest that the variability in the western Arabian Sea is not only due to the interannual variability in the wind field, but that a substantial part is caused by the chaotic nature of the ocean dynamics. Decreasing the friction coefficient from 1000 to 500m2s-1 in a 19o numerical reduced-gravity model, the variance of the GW location increases dramatically, and the mean position moves southward by one degree. In the eddy-permitting experiments analyzed, both mechanisms appear to determine the GW location at the onset of the GW dynamics in late summer.

Description: In January and February 1998, when an unprecedented fourth repetition of the zonal hydrographic transect at 24.5°N in the Atlantic was undertaken, carbon measurements were obtained for the second time in less than a decade. The field of total carbon along this section is compared to that provided by 1992 cruise which followed a similar path (albeit in a different season). Consistent with the increase in atmospheric carbon levels, an increase in anthropogenic carbon concentrations of Full-size image (〈1 K) was found in the surface layers. Using an inverse analysis to determine estimates of absolute velocity, the flux of inorganic carbon across 24.5° is estimated to be −0.74±0.91 and Full-size image (〈1 K) southward in 1998 and 1992, respectively. Estimates of total inorganic carbon flux depend strongly upon the estimated mass transport, particularly of the Deep Western Boundary Current. The 1998 estimate reduces the large regional divergence in the meridional carbon transport suggested by previous studies and brings into question the idea that the tropical Atlantic constantly outgasses carbon, while the subpolar Atlantic sequesters it. Uncertainty in the carbon transports themselves, dominated by the uncertainty in the total mass transport estimates, are a hindrance to determining the “true” picture. The flux of anthropogenic carbon (C★ANTH) across the two transects is estimated as northward at 0.20±0.08 and Full-size image (〈1 K) for the 1998 and 1992 sections, respectively. The net transport of C★ANTH across 24.5°N is strongly affected by the difference in concentrations between the northward flowing shallow Florida Current and the mass balancing, interior return flow. The net northward transport of C★ANTH is opposite the net flow of total carbon and suggests, as has been found by others, that the pre-industrial southward transport of carbon within the Atlantic was stronger than it is today. Combining these flux results with estimates of atmospheric and riverine inorganic carbon input, it is determined that today's oceanic carbon system differs from the pre-industrial system in that today there is an uptake of anthropogenic carbon to the south that is advected northward and stored within the North Atlantic basin.

Description: Assimilation experiments with data from the Bermuda Atlantic Time-series Study (BATS, 1989¯1993) were performed with a simple mixed-layer ecosystem model of dissolvedinorganic nitrogen (N), phytoplankton (P) and herbivorous zooplankton (H). Our aim is to optimize the biological model parameters, such that the misfits between model results andobservations are minimized. The utilized assimilation method is the variational adjoint technique, starting from a wide range of first-parameter guesses. A twin experiment displayedtwo kinds of solutions, when Gaussian noise was added to the model-generated data. The expected solution refers to the global minimum of the misfit model-data function, whereasthe other solution is biologically implausible and is associated with a local minimum. Experiments with real data showed either bottom-up or top-down controlled ecosystemdynamics, depending on the deep nutrient availability. To confine the solutions, an additional constraint on zooplankton biomass was added to the optimization procedure. Thisinclusion did not produce optimal model results that were consistent with observations. The modelled zooplankton biomass still exceeded the observations. From the model-datadiscrepancies systematic model errors could be determined, in particular when the chlorophyll concentration started to decline before primary production reached its maximum. Adirect comparision of measured 14C-production data with modelled phytoplankton production rates is inadequate at BATS, at least when a constant carbon to nitrogen C : N ratio isassumed for data assimilation.

Description: Sea-surface height data acquired by the TOPEX/POSEIDON satellite over the Arabian Sea from October 1992 to October 1998 are analyzed. Strong seasonal fluctuations are found between 61 and 101N, which are mainly associated with westward propagating annual Rossby waves radiated from the western side of the Indian subcontinent and that are continuously forced by the action of the wind-stress curl over the central Arabian Sea. An analysis of hydrographic data acquired during August 1993 and during January 1998 at 81N in the Arabian Sea reveals the existence of first- and second-mode annual Rossby waves. These waves, which can be traced as perturbations in the density fields, have wavelengths of 12�103 and 4.4�103km as well as phase velocities of 0.38 and 0.14 m/s, respectively. The waves are associated with a time-dependent meridional overturning cell that sloshes water northward and southward. Between 581 and 681E in the central Arabian Sea, we found a Rossby-wave induced transport in the upper 500m of about 10 Sv southward in August 1993 and northward in January 1998. Below 2000 m, there was still a northward transport of 3.2 Sv in August 1993 and a southward transport of 4.8 Sv in January 1998. A comparison of steric height differences between August 1993 and January 1998 calculated from the observed density fields as well as calculated from the reconstructed density fields using first- and second-mode annual Rossby waves agree quite well with the corresponding sea-surface height differences. Implications resulting from the reflection of annual Rossby waves, like fluctuations of the western boundary currents, are discussed.

Description: Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean are an important component of the global thermohaline circulation. Evidence exists that the variability in these exchanges, on both meso- and longer time scales, may significantly influence weather and climate patterns in the southern African region and the significance of these regional ocean–atmosphere interactions is discussed. Observations of the inter-ocean exchange are limited and it is necessary to augment these with estimates derived from models. As a first step in this direction, this study uses an eddy-permitting model to investigate the heat and volume transport in the oceanic region south of Africa and its variability on meso, seasonal and inter-annual time scales. On the annual mean, about Full-size image (〈1 K) (standard deviation Full-size image (〈1 K)) of heat flows west into the South Atlantic across 20°E (longitude of Cape Agulhas, the southernmost point of Africa), with just over Full-size image (〈1 K) (standard deviation Full-size image (〈1 K)) flowing north into the South Atlantic across 35°S. The seasonal variations in this transport are about 10% at 35°S in the South Atlantic and around 20% through 20°E; the model value of Full-size image (〈1 K) for summer (standard deviation ranging from Full-size image (〈1 K) in January to Full-size image (〈1 K) in March) appears consistent with respective estimates of 0.51 and Full-size image (〈1 K) derived from two WOCE summer cruises southwest of Cape Town to 45°S in 1990 and 1993. Volume transports of the Agulhas Current section through 35°S in the SW Indian Ocean range from 58 to Full-size image (〈1 K) in summer/autumn to 64–Full-size image (〈1 K) in winter/spring. The model results suggest that the inter-ocean exchange south of Africa is highly variable on seasonal through to interannual scales. If this variability is also the case in the real ocean (and the limited observations suggest that this is so), then there are likely to be significant implications for climate.

Description: In the framework of the Ocean Margin Exchange project, a multi-disciplinary study has been conducted at the shelf edge and slope of the Goban Spur in order to determine the spatial distribution, quantity and quality of particle flux, and delineate the transport mechanisms of the major organic and inorganic components. We present here a synthesis view of the major transport modes of both biogenic and lithogenic material being delivered to the open slope of the Goban Spur. We attempt to differentiate between the direct biogenic flux from the surface mixed layer and the advective component, both biogenic and lithogenic. Long-term moorings, instrumented with sediment traps, current meters and transmissometers have yielded samples and near-continuous recordings of hydrographic variables (current direction and speed, temperature and salinity) and light transmission for a period of 2.5 years. Numerous stations have been occupied for CTD casts with light transmission and collection of water samples. The sedimenting material has been analysed for a variety of marker compounds including phytoplankton pigments, isotopic, biomineral and trace metal composition and microscopical analyses. These samples are augmented by seasonal information on the distribution and composition of fine particles and marine snow in the water column. The slope shows well-developed bottom nepheloid layers always present and intermediate nepheloid layers intermittently present. Concentrations are mainly in the range 50–130 mg m−3 in nepheloid layers and 6–25 mg m−3 in clear water. A seasonal variability in the concentration at the clear water minimum is argued to be related to seasonal variations in vertical flux and aggregate break-up in transit during summer months. It is suggested that the winter sink for this seasonal change in particulate matter involves some re-aggregation and scavenging, and some conversion of particulate to dissolved organic matter. This may provide a slow seasonal pump of dissolved organic carbon to the deep ocean interior. Differences in trapped quantities at different water depths are interpreted as due to lateral flux from the continental margin. There is a major lateral input between 600 and 1050 m at an inner station and between 600 and 1440 m at an outer one. The transport is thought to be related to intermediate nepheloid layers, but those measured are too dilute to be able to supply the flux. Observed bottom nepheloid layers are highly concentrated very close to the bed (up to 5 g m−3), with a population of large aggregates. Some of these are capable of delivering the flux seen offshore during intermittent detachment of nepheloid layers into mid-water. Concentrated bottom nepheloid layers are also able to deliver large particles with unstable phytoplankton pigments to the deep sea floor in a few tens of days. Calculated CaCO3 fluxes are adjusted for dissolution, which is inferred from Ca/Al ratios to be occurring in the CaCO3-saturated upper water column where up to 80% of the CaCO3 resulting from primary production is dissolved.

Description: Between 1991 and 1999, carbon measurements were made on twenty-five WOCE/JGOFS/OACES cruises in the Pacific Ocean. Investigators from 15 different laboratories and four countries analyzed at least two of the four measurable ocean carbon parameters (DIC, TAlk, fCO2, and pH) on almost all cruises. The goal of this work is to assess the quality of the Pacific carbon survey data and to make recommendations for generating a unified data set that is consistent between cruises. Several different lines of evidence were used to examine the consistency, including comparison of calibration techniques, results from certified reference material analyses, precision of at-sea replicate analyses, agreement between shipboard analyses and replicate shore based analyses, comparison of deep water values at locations where two or more cruises overlapped or crossed, consistency with other hydrographic parameters, and internal consistency with multiple carbon parameter measurements. With the adjustments proposed here, the data can be combined to generate a Pacific Ocean data set, with over 36,000 unique sample locations analyzed for at least two carbon parameters in most cases. The best data coverage was for DIC, which has an estimated overall accuracy of ∼3 μmol kg−1. TAlk, the second most common carbon parameter analyzed, had an estimated overall accuracy of ∼5 μmol kg−1. To obtain additional details on this study, including detailed crossover plots and information on the availability of the compiled, adjusted data set, visit the Global Data Analysis Project web site at: http://cdiac.esd.ornl.gov/oceans/glodap.

Description: The active channel–levee system of the middle Bengal Fan was studied by a combined analysis of Parasound echosounder and Hydrosweep swathsounder data. The channel is characterized by highly variable sinuosities. Compared to other mud-rich submarine fans, an exceptionally low channel slope is found. The system can be subdivided into inner and outer zones of significantly different depositional architecture. The inner zone consists of the active channel and sharply separated vertical blocks, which are characterized by parallel, distinct reflectors and planforms of bends. These blocks are interpreted as abandoned channel segments (cut-off loops). The outer zones represent undisturbed levees, which are constructed of parallel and wedge-shaped sedimentary units. The wedge-shaped units, varying significantly in thickness and lateral extent, are found at the outer convex arcs of active and abandoned channel loops caused by overspilling of channelized turbidity currents at sharp bends. The parallel units are the deposits of turbidity currents, which spread their sediments over wide areas as their size significantly exceeds the cross-section of the channel. The complex vertical and horizontal distribution of partially small sedimentary units suggests a more complicated deposition in time and space as hitherto reported from other submarine fans. Within the inner zone, more than 20 cut-off loops were identified over a channel length of 90 km. In contrast to most other large mud-rich submarine fans, channel avulsion within the active channel–levee system is a frequent process. In particular, a temporal succession of at least 4 cut-off loops was reconstructed in the southern study area, indicating channel avulsion on average every 750 years. Channel avulsion seems to be a repetitious process caused by erosion through turbidite currents in a highly sinuous channel. Compared to other submarine fans, no morphological parameter shows a remarkable difference except the channel slope, which is significantly smaller than, for example, on Amazon, Congo and Mississippi fans. The interaction between this low channel slope and the flow parameter of the turbidity currents is most likely the reason for the instability of the active channel planform, leading to an exceptionally large number of meander loop breaches and cut-off loops.

Description: During the large-scale deep-sea programme BIGSET in situ measurements of sediment community oxygen consumption (SCOC) were carried out during three cruises between 1995–1998 at five abyssal sites (3190–4450 m water depth) in the deep Arabian Sea in order to elucidate the regional and temporal variation of benthic carbon remineralisation. SCOC ranged from 0.9–6.3 mmol O2 m−2 d−1, with highest values in the western and northern Arabian Sea and lowest values in the southern Arabian Sea. For the central Arabian Sea intermediate oxygen uptake rates were detected. This regional pattern mirrors the overall regional pattern of primary productivity in surface waters and vertical particulate organic carbon (POC) flux at 1000 mab. Primary productivity in Arabian Sea surface waters and particulate flux into the deep-sea are controlled by the monsoon system and the flux maxima during the SW and NE monsoon are among the highest particle fluxes recorded in the deep open ocean. Highest flux rates were recorded in the western and northern Arabian Sea and decreased towards the central and southern Arabian Sea. SCOC at our western, northern and eastern Arabian Sea stations WAST, NAST and EAST were considerably higher than so far detected in other abyssal areas of the global oceans, and vertical POC flux can account for only 20–50% of benthic carbon remineralisation (BCR). Possible explanations for the high rates of BCR at these stations that are situated close to the continental margins are discussed: the accelerated deposition of very labile organic matter due to eolian dust particles, enhanced rain efficiencies, and lateral advection. A significant temporal variability in SCOC only could be detected at the eastern and western Arabian Sea stations WAST and EAST.