Many general circulation models predict a reduction in overturning strength in the 21st century as a response to anthropogenic forcing, meaning that novel methods of monitoring individual components of the subtropical North Atlantic circulation are required. This observational study outlines efforts to monitor upper ocean transports near the 36°N latitude line and to identify possible forcing mechanisms. Specifically, an optimal interpolation scheme is employed to synthesise annual and seasonal sections of the Atlantic using T/S data from Argo floats and the Line W array from 2002 to 2007. Combining these data with an estimate of the barotropic velocity yields accurate values of the interior geostrophic transport above 1000 dbar. Close to the western boundary, where errors incurred by the scheme are larger, altimetric SSH differences are used to quantify the Gulf Stream transport above 2000 dbar at 10-day resolution. Finally, a detailed analysis of wind stress and wind stress curl fields of the subtropical North Atlantic is used to estimate both the Ekman and Sverdrup transports and to isolate the dominant time and space scales of variability. The mean zonally integrated interior transport above 1000 dbar between the eastern boundary and the Gulf Stream was -48.0 ± 3.3 Sv, where the error represents the standard error of the seasonal baroclinic transport estimates. The size of the variability between seasons was similar to the interannual variability (standard deviations of 6.6 Sv and 7.7 Sv). Most variability at interannual timescales arises from changes in the density structure of the main thermocline west of 40°W. Neither interannual nor seasonal variability in the interior transport correlate with changes in the Sverdrup transport, though the Sverdrup relation does account for the mean upper ocean transport in the eastern basin. Gulf Stream transport across Line W in the period 2002 to 2008 was estimated to be 87.6 (± 0.8 Sv standard error) in the upper 1000 dbar, with a peak transport in late summer. In line with earlier studies, the seasonal cycle of the transport appears to be correlated with local wind stress curl forcing but determining the precise mechanism requires further theoretical and modelling work.

The scales over which ocean properties vary play a critical role in the assimilation of ocean data. Previous studies do not explore these scales in great detail and pre-Argo observations limited investigations to the upper ocean (<500m). In this study Argo data have been used to develop a method of estimating the correlation scales of salinity on depth and potential temperature surfaces to depths of 1600m. Zonal and meridional scales of mean field salinity and salinity anomalies are examined in the Pacific and Atlantic. Correlation scales are calculated by the fitting of a Gaussian function to the differences in salinity between observation pairs that satisfy given spatial and temporal separation requirements. The intercept and scale that result in the best fit are taken to be the optimum values for the region. The significance of the correlation scale is tested by the calculation of the F-ratio. Comparisons of the scales show mean field scales to be larger than anomaly field scales. Scales are found to be anisotropic in all cases but anomaly field scales differ in size and pattern between the Pacific and Atlantic. Correlation scales exhibit dependence on latitude and depth with longer scales observed in the tropics and in general, scales shorten with depth down to ?800m and then increase below this. Comparing scales of S(z ) with those of S(?) indicates there is no significant difference between z and ? scales in the Pacific. However, salinity differences on ?-surfaces are lower than on z, linked to the heaving of water masses and the vertical gradient of salinity. Atlantic scales of S(?)prove to be significantly larger than those of S(z ). Recommendations are made for the use of these correlation scales in assimilation models

The aim of this thesis is to provide a more comprehensive and dynamic understanding of sea surface temperature anomaly (SSTA) re-emergence in the North Atlantic, by (a) re-evaluating the utility of re-emergence for sea surface temperature (SST) variability quantitatively and/or by analysing its temporal variability, and (b) investigating the e�ects of horizontal advection, subduction and mesoscale phenomena on the occurrence and nature of SSTA re-emergence in the ocean. Such analyses may contribute towards an improved representation of SSTA re-emergence in seasonal forecast models, which is currently unsatisfactory. Analyses are undertaken with a new observational product and ocean model hindcasts. Through analysis of observations, it is demonstrated that SSTA re-emergence links the European winter extremes of 2009/10 and early 2010/11. Reemergence contributes towards the winter-to-winter persistence of a SSTA tripole pattern, associated with a record negative phase of the North Atlantic Oscillation (NAO). Its timing coincides with a shift to record negative NAO values. This suggests an active involvement in the extreme winter weather of early-winter 2010/11. The atmospheric and oceanic conditions of 2009-11 are analogous to previous years in the late-1960s, which suggest similar physical processes are involved in the establishment of a large and signi�cant SSTA re-emergence. The dynamic influences of subduction and horizontal advection on SSTA re-emergence are investigated through numerical passive tracer experiments at 1� resolution. A dominant and time-dependent influence of horizontal advection upon the degree of re-emergence in the northeast Atlantic is revealed. The time-dependent role is linked to spatial variations in the subtropical-subpolar gyre boundary, caused by gyre-scale changes in the upper ocean velocity field. Subduction is more detrimental to SSTA re-emergence in the Sargasso Sea. Significant interannual variations in the amount of subduction are apparent. These variations are primarily driven by interannual changes in lateral induction. Passive tracer experiments, and the statistical properties of SSTA re-emergence in an eddy-permitting (1/4 �) ocean model are analysed, to ascertain the effects of mesoscale phenomena on the re-emergence process. Statistical diagnostics of SSTA re-emergence at 1/4 � resolution demonstrate better correspondence with observations than at 1� resolution. This highlights an important contribution from mesoscale processes to the shape and extent of SSTA re-emergence. These mesoscale influences involve significant reorganisations of the large-scale near-surface circulation, imposing different regional influences of horizontal advection upon the occurrence of re-emergence. This is due to the more realistic simulation of key features of the large-scale ocean circulation at 1/4 � resolution. These results support the growing call for higher resolution ocean models in future simulations of the large-scale mean climate and its variability.

The temporal relationship between meltwater pulse 1a (mwp-1a) and the climate history of the last deglaciation remains a subject of debate. By combining the GRIP δ18O ice core record on the new Greenland Ice Core Chronology 2005 (GICC05) timescale with the U/Th-dated Barbados coral record, it is conclusively derived that mwp-1a did not coincide with the sharp Bølling warming, but with the abrupt cooling of the Older Dryas. To evaluate whether there is a relationship between meltwater injections, North Atlantic Deep Water (NADW) formation and climate change (i.e., the long term change in the average weather), a high-resolution magnetic (κARM/κ) proxy record of NADW flow intensity from Eirik Drift, south of Greenland, is presented. A record of mean sortable silt grain sizes (an established proxy for near bottom current flow speed), obtained from the same samples on which the κARM/κ was measured, shows remarkable similarity to the magnetic record and validates κARM/κ as a proxy for NADW flow intensity. The record of κARM/κ indicates only a relatively minor 200-yr weakening of NADW flow, coincident with mwp-1a. This compilation of records also indicates that during Heinrich event 1 (H1) and the Younger Dryas there were no discernible sea-level rises, and yet these periods were characterised by intense NADW slowdowns. Records of planktonic foraminiferal δ18O, as well as lithic and foraminiferal counts from Eirik Drift are combined with previous studies from the Nordic seas and the ‘Ice Rafted Debris (IRD) belt’, and portray a sequence of events through the interval of H1. These events progressed from an onset of meltwater release around 19 ka BP, through the ‘conventional’ H1 phase from ~17.5 ka BP, to a final phase between 16.5 and 14.6 ka BP, characterised by a pooling of fresh waters in the Nordic Seas, which were injected hyperpycnally. This build up of fresh waters was purged from the Nordic Seas, preconditioning the Nordic Seas for convective deep-water formation. This allowed the abrupt re-start of NADW formation in the Nordic Seas at the Bølling warming. In contrast to previous estimates for the duration of H1 (i.e., 1000 years to only a century or two), the total, combined composite signal of H1 presented here had a duration of nearly 4000 yrs (~19–14.6 ka BP), now spanning the established period of NADW shutdown. Clearly, deep-water formation and climate are not simply controlled by the magnitude or rate of meltwater addition. Instead, the results presented here emphasise that the location of meltwater pulses may be more important, with NADW formation being particularly sensitive to surface freshening in the Arctic/Nordic Seas.

No description available.

Small scale roughness of the seafloor is of direct relevance to a range of interests, including boundary layer hydrodynamics, sediment transport and high-frequency acoustic scattering. Despite its importance, only few studies have quantitatively resolved seafloor height at the relevant scales. In particular, characterisation of roughness over vegetated beds is needed to better understand hydrodynamics and sediment transport in the coastal zone. A new Benthic Roughness Acoustic Device (BRAD) has been developed to define microtopographical roughness through high-resolution imagery of the seabed. BRAD, composed of a profiling sonar – the Sediment Imager Sonar (SIS) – and a motor, both mounted on a frame, enables measurements of the seabed elevation over an area of 1.7 m2. A threshold method was established to detect the seabed from the SIS raw data. Laboratory deployments were carried out in order to assess the system accuracy over known targets and its ability to discriminate sediment sizes. Field deployments at 6 sites enabled the imaging of a variety of seabed types; in particular bioturbated fine sand and mud, seagrass canopies, gravelly sand and ripple fields. Spectral analysis applied on the seabed elevations was used to characterise roughness type. Seagrasses are flowering plants that have adapted to the submerged marine environment. They develop extensive underwater meadows in coastal areas around the world, forming complex, highly productive ecosystems. The SIS was used together with a towed video camera system to survey a seagrass (Zostera marina) bed in Calshot, UK. A method was developed to assess Z. marina presence from the SIS data and its results were tested against video data. The SIS proved to be a useful tool for seagrass surveying and the use of the SIS and the video yielded a preliminary map of the seagrass bed. Seagrass canopies exert strong effects on water flow inside and around them. The influence of Zostera marina canopies on flow, turbulence, roughness and sediment movement was evaluated through laboratory experiments. Numerous runs were carried out in an annular and a straight, recirculating flume using live Z. marina and a mobile sand layer. Flow was greatly decelerated inside the canopy while turbulence was increased. The Turbulent Kinetic Energy was observed to be maximal at the canopy/water interface. This was hypothesised to be related to the canopy ‘wavy’ motion. Sediment movement was observed within the canopy as scour around the stems. Ripples formed downstream of the canopy at velocities lower than the sediment threshold of motion. Intermittent turbulence associated with the burst phenomenon is thought to be responsible for this.

Concerns regarding the climatic implications of the increase in atmospheric CO2 concentrations throughout the anthropocene have provided the impetus to obtain a mechanistic understanding of oceanic processes and their role in regulating atmospheric pCO2. One important mechanism is the functioning of the biological pump which partitions carbon between the atmosphere and ocean reservoirs over relevant time scales. Current uncertainties revolve around the accuracy of upper ocean particle flux measurements, and the effect of iron and ballast minerals on the strength and efficiency of the biological carbon pump. This study documents the design and deployment of a neutrally buoyant sediment trap (PELAGRA). In the north-east Atlantic organic carbon fluxes were measured using this new technology and compared to indirect estimates of export based on 234Th and nutrient budgets. The vertical fluxes of 234Th into the traps were less than those estimated from the 234Th water column budget, which is interpreted to be the result of previous export events removing 234Th from the water column and the lateral advection of gradients of total 234Th/238U disequilibria confounding the Eulerian budgeting approach adopted. Successful simultaneous deployments in July 2006 at different depths provided a direct measurement of the attenuation of flux with depth, which at 1.8 is substantially greater than the canonical value of 0.856. PELAGRA deployments in the Southern Ocean were conducted as part of the CROZEX project, which examined the role of iron supply on bloom dynamics and subsequent export. Using a mass balance approach to account for the seasonal depletion of dissolved silica acid in surface waters and Si fluxes from the euphotic zone, potential surface export(100m) of organic carbon from +Fe bloom area was estimated to be in the order of 11-15 g C m-2, which is higher than previous estimates obtained from artificial fertilisation experiments. The issue of temporal decoupling between production and export processes was addressed by employing retrospective estimates of production. Particle export efficiency in the +Fe region to the north of the plateau (25-70%) was higher than similar estimates in the –Fe region (11-20%). Diatom size was well correlated with a range of calculated export ratios(100m). The main diatoms involved in the export from the surface were E. Antarctica in the +Fe region and F. kerguelensis in the –Fe region. E. Antarctica fluxes also dominated deep-water (3000m) diatom fluxes in the +Fe region, and its importance is attributed to the regions proximity to the Crozet Islands, where resting spores and dissolved iron are advected into the bloom area during the winter. Deep-water carbon fluxes measured to the south of the plateau. Deep-water carbon fluxes measured south of the plateau (0.09 g C m-2 yr-1) are consistent with previous measurements in a similar environment. In the +Fe region to the north, deep water fluxes were 0.4 g C m-2 yr-1 indicating that natural iron fertilisation can increase the strength of the biological carbon pump by a factor of 4. Comparison of fluxes with satellite-derived productivity also suggests that the efficiency of the biological pump in transferring organic carbon to the deep-ocean is increased by a factor of 3 in the presence of iron. The flux and composition of amino acids, in relation to the dominant mineral phases that comprised the particulate flux in the NE Atlantic and the Southern Ocean was also examined. The fraction of carbon that could be accounted for by the total hydrolysable amino acids varied very little (20-30%) with sample composition. Protein amino acids were used to quantify the degradation state of the settling particulate material. Specific amino acids seem to infer diatomaceous rather than calcareous as the dominant organic matter source. Multiple linear regression analysis reveals that mineral fluxes can only explain a very small amount of the variability in amino acid composition, which does not support previous hypotheses that relate mineral fluxes and organic carbon fluxes through the differential protective capacity of various mineral phases.

The Order Enoplida (Phylum Nematoda) has been proposed as a divergent nematode lineage—Enoplid nematodes are thought to exhibit morphological and developmental characteristics present in the ‘ancestral nematode’. However, previous molecular phylogenies have failed to unequivocally confirm the position of this group. The Enoplida is primarily comprised of free-living marine species; if these taxa represent close relatives of the nematode ancestor, this relationship would presumably imply a marine origin for the phylum. Prior to this investigation, few publically available gene sequences existed for Enoplid nematodes, and published sequences represented only shallow water fauna from Northwest Europe. This study has aimed to improve resolution at the base of the nematode tree, using drastically increased taxon-sampling within the previously neglected Enoplid clade. Morphological identifications, nuclear gene sequences (18S and 28S rRNA), and mitochondrial gene sequences (Cox1) were obtained from marine specimens representing a variety of deep-sea and intertidal habitats. Molecular data were used to assess the phylogenetic placement of the Enoplid clade, resolve internal taxonomic relationships within this group, and investigate relationships between shallow water and deep-sea fauna. Despite rigorous empirical testing and comprehensive taxon sampling, large-scale phylogenetic analyses based on 18S rRNA sequences (using both Maximum Likelihood and Bayesian Inference methods) failed to provide added resolution at the base of the nematode tree. Molecular data from the 18S rRNA gene was unable to confirm the placement of Enoplida as a divergent lineage representing the sister taxon to all other nematodes. These findings highlight the limitations of the 18S gene for resolving the deepest evolutionary splits amongst nematode clades. Analysis of internal relationships reveals that the Enoplida is split into two main clades, with groups consisting of terrestrial and primarily marine fauna, respectively. For marine taxa, deep-sea and shallow-water specimens from the same genus consistently appear as sister taxa. Deep-sea nematode species may have arisen via several evolutionary routes; some deep-sea clades appear to represent recently derived forms, while other groups seem to have radiated much earlier. Nematodes from deep-sea sites exhibit no obvious clustering according to depth or geographic location, and specimens represent a wide taxonomic range within the Enoplida. In addition, there seems to be some molecular evidence for purportedly cosmopolitan nematode species; identical gene sequences were recorded between distant shallow water locations, as well as between deep-sea and shallow water habitats. Data from Enoplid nematodes suggests an intriguing pattern for nematode species distributions—validating these preliminary insights will require a large amount of molecular data from many additional geographic locations. Future studies will also need to incorporate data from additional genetic loci (or use phylogenomic methods) in order to build robust deep phylogenies.

Uncertainty about the causes of glacial-interglacial CO2 variations demonstrates our incomplete grasp of fundamental processes that govern our climate and thus one of the foremost problems in palaeoceanography and Earth System Science regards the mechanism(s) responsible for natural changes in atmospheric CO2 concentration. It is becoming clear that the Southern Ocean overturning circulation plays an important role in the global carbon cycle because altered communication between the atmosphere and abyss in the Southern Ocean is relatively well documented and often implicated in explanations of past and future climate changes, but the ambiguity of the paleoceanographic record defies interpretation of the mechanisms involved. Using a coarse resolution ocean general circulation model and coupled biogeochemistry code, an ensemble of idealised perturbations to external forcing and internal physics of the Southern Ocean is examined to explain the processes that link ocean circulation, nutrient distributions and biological productivity, and determine the extent to which the Southern Ocean governs the partitioning of CO2. Strengthened or northward-shifted winds result in oceanic outgassing and increased atmospheric carbon dioxide levels, while weakened or southward-shifted winds cause oceanic carbon uptake and reduced atmospheric carbon dioxide concentration. Driven by the work done on the ocean by the winds, changes in the rate or spatial pattern of the Southern Ocean residual overturning circulation lead to alteration of upper ocean stratification and the rate and depth from which carbon and nutrient-rich deep waters are upwelled to the surface. These surface waters, imprinted with the pattern of air-sea gas exchange, are subducted to intermediate depths in the ocean interior, not the abyss as previous suggested. These results are robust to significant alterations to surface heat and freshwater boundary conditions, mesoscale eddy activity and rates of air-sea gas exchange and represent a significant proportion of the change in glacial-interglacial CO2 that can be currently generated by altered ocean circulation in a variety of models, revealing that the upper limb of the Southern Ocean overturning circulation is important in determining atmospheric CO2 levels.

The aim of this study was to improve our understanding of the natural sources and processes of the dissolved manganese associated with the Southern Ocean and the Tropical North-Eastern (NE) Atlantic Ocean. A flow injection analyser (FIA) with chemiluminescence (CL) detection was first set up for the determination of dissolved manganese (DMn, ≤ 0.2 μm). Extension work was undertaken to solve problems relating to our limited level of understanding of the CL reaction, and the behaviour of the resins used to either preconcentrate the manganese (i.e. Toyopearl AF-Chelate 650M) or to remove the interfering elements (cleaning resin, i.e. NTA Superflow and 8-Hydroxyquinoline). Using the system built in our laboratory, the determination of the interference of other dissolved trace metals (e.g. Fe, Ni, Cu, Co, Zn and Cd) were possible with dissolved manganese measurement in order to obtain a good measurable CL peak for dissolved manganese. The distribution of manganese around the Crozet Islands was examined and was used to provide a conceptual framework for future studies. Dissolved manganese concentrations were measured in samples collected from nine vertical profiles taken across the Crozet Plateau (80 - ~4500 m water depth) that show evidence of a range of processes influencing the manganese distributions. Dissolved manganese varied between 0.1 and 2.44 nM, and the resulting detailed section showed evidence of an island souce is identified which suggests that the plateau and the associated sediments are a source of manganese. Waters further north also appear to be affected by this input of both coastal and shelf origin, although dissolved manganese decrease as a function of distance to the north of the plateau with a gradient of 0.096 nM/km as a result of dispersion and mixing. This gradient was then combined with short-lived Radium isotopes profiles, allowing the determination of a lateral advective flux of manganese (up to 538 nmol/m2/d). Estimates of atmosphere and vertical fluxes of manganese to surface waters were also calculated. It was then possible to estimate a pre-bloom concentration of ~0.4 nM. A set of surface samples were collected from the Tropical NE Atlantic Ocean, and were analysed for dissolved manganese. Results suggest the land-sources of manganese near to the Canary Islands, the Cape Verde Islands and the African Continent, where high dissolved manganese concentrations were determined, with the highest is ~3.90 nM. The lateral advective flux of manganese was higher (47 μmol/m2/d) than the atmospheric flux of manganese (0.17 μmol/m2/d), thus making the shelf+sediment as the most prominent sources of dissolved manganese in the seawater close to the islands. From this atmospheric flux of manganese, it was then possible to estimate the manganese enrichment around the further offshore dust event regions of 0.73 nM/yr and consistent with the dissolved manganese background concentrations. This support the low residence time calculated in dust event regions of around 1 year.