The Southern Ocean plays a major role in the global overturning circulation, providing an important route for the return flow of deep water subducted in the North Atlantic. The World Ocean Circulation Experiment (WOCE) provided an unprecedented picture of the state of the world’s oceans and set new standards for high quality in-situ hydrographic data. This study combines the existing WOCE data set with new hydrographic sections, and output from global and regional ocean models to examine the mean state of the Southern Ocean circulation and the balance of fluxes around the Antarctic Circumpolar current. A historical data set in the region of Drake Passage is examined to study the large-scale water mass variability between 1926-2005. The water mass properties of the Lower Circumpolar Deep Water is constant within error bounds throughout the data set. A warming and freshening signal in the surface waters from 1997-2005 to the north of the Sub-Antarctic Front along SR01b is also presented. The major part of this work is based around an inverse study of the Southern Ocean that combines the WOCE data-set with contemporary sections, and other forcing fields to examine the balance of fluxes throughout the Southern Ocean. The study examines the effect of different parameterisations of the dianeutral mixing in the Southern Ocean, in light of the differing views of localised deep turbulent mixing from observations, and an adiabatic ocean interior from residual mean studies, The freshwater balance in the model is presented and its implications on the water mass formation and transformation of the upper and lower cells of the overturning circulation is discussed in detail.

Iron represents an important control on primary production in high nutrient low chlorophyll (HNLC) regimes and has received considerably attention during the last two decades. This work has focussed on the biogeochemistry of iron in two oceanic environments; the high latitude North Atlantic and the Scotia Sea in the Southern Ocean. The mechanisms of iron supply and the biological response of resident phytoplankton communities to iron were addressed in both study areas. Two cruises to the high latitude North Atlantic Ocean (>55 °N) during late July-early September 2007 indicated that nitrate concentrations of 2 to 5 M persisted in the surface waters. The concentration of dissolved iron (dFe) in the surface waters was very low, with an average of 0.093 (<0.010-0.218, n=43) nM, and in situ chlorophyll concentrations were < 0.5 mg m-3. In vitro iron addition experiments demonstrated that the addition of iron increased photosynthetic efficiencies (Fv/Fm) and resulted in enhanced chlorophyll in treatments amended with iron when compared to controls. A number of phytoplankton taxa, including the coccolithophore Emiliania huxleyi, were observed to increase their net growth rates following iron addition. These results provide strong evidence that iron limitation within the post spring bloom phytoplankton community contributes to the observed residual macronutrient pool during summer. Low atmospheric iron supply and suboptimal Fe:N ratios in winter overturned deep water are suggested as proximal causes for this seasonal High Nutrient Low Chlorophyll (HNLC) condition, which represents an inefficiency of the biological (soft tissue) carbon pump. Large areas of the Southern Ocean are characterised as HNLC. Satellite chlorophyll data indicate that phytoplankton blooms occur in vicinity to Southern Ocean Island systems. The bloom associated with South Georgia has the largest spatial extent and duration (16-20 weeks). Detailed measurements were made on austral spring and summer cruises to the Scotia Sea during November – early December 2006 and January – February 2008. This work presents the first comprehensive study of seasonal variations in phytoplankton biomass and iron availability in the Scotia Sea. The drawdown of nitrate between the two seasons in the South Georgia bloom was 16 M indicative of substantial new production. Surface water concentrations of dissolved iron (dFe) were slightly higher during summer than spring (0.31 nM compared to 0.20 nM, with P>0.05). We suggest that the South Georgia bloom is sustained by a continuous benthic supply of iron from the South Georgia shelf. In addition, enhanced dFe (0.34 nM) was observed in a cryptophyte dominated bloom in the southern Scotia Sea in the vicinity of South Orkney Islands. The difference in the community composition between the two natural occurring blooms highlight that Southern Ocean island systems have individual characteristics and should be viewed independently.

This study addresses the decadal variability and predictability of the Atlantic Meridional Overturning Circulation (AMOC), and associated key variables, in two IPCC-class climate models. The AMOC variability is analyzed in a new climate model CHIME, which features a novel (largely isopycnic) ocean component. Power Spectral analysis reveals enhanced variability for periods in the range 15-30 years. The primary mode of variability is associated with decadal changes in the Labrador and the Greenland-Iceland-Norwegian (GIN) seas, in both cases linked to the tropical activity about 15 years earlier. These decadal changes are controlled by the low-frequency North Atlantic Oscillation (NAO), associated with a tropical-extratropical teleconnection. Poleward advection of salinity anomalies in the mixed layer also leads to AMOC changes that are linked to convective processes in the Labrador Sea. A secondary mode of variability is associated with interannual changes in the Labrador and GIN Seas, through the impact of the NAO on local surface density. The decadal potential predictability of the AMOC and climate as represented in the non-isopycnic IPSL-CM5A model and CHIME is explored using prognostic and diagnostic approaches. The modelled AMOC has an average predictive skill of 8 and 6 years, respectively. Over the ocean, surface temperature has the highest skill up to 2 decades in the far north of the North Atlantic, in both models. Additional oceanic areas of predictability are identified in IPSL-CM5A in the tropics and subtropics. The spatio-temporal predictability of both surface temperature over land and precipitation differs somewhat between the two models, but is of limited extent compared to that of ocean variables. Predictability of climate arises from the mechanisms controlling the decadal AMOC fluctuations. Predictive skills of AMOC and climate are favoured by extreme AMOC events but the role of minimum versus maximum states remains to be clarified. The expected better predictive skills of CHIME over non-isopycnic models (due to its better preservation of water masses and more coherent internal structure to the anomalies) are not borne out.

Coccolithophores are a unique group of phytoplankton that produce calcium carbonate coccoliths. The production of coccoliths can potentially reduce the carbon sink effect associated with phytoplankton photosynthesis. However, this may be offset by a coccolith ‘ballast’ effect that increases particulate organic carbon export efficiency. During a coccolithophore bloom, large quantities of coccoliths are shed into the water column. The unique light scattering properties of calcium carbonate particles of ~2 μm in size (the size of an Emiliania huxleyi coccolith) have previously been used to develop an algorithm that enables estimates of particulate inorganic carbon (PIC) concentration to be made from space. Here, these satellite derived PIC data have been used to generate a unique set of phonological characteristics, such as start date, peak date and bloom magnitude, for global coccolithophore blooms. A comparison of similar timings generated from chlorophyll data indicates that, contrary to conventional thinking, blooms of coccolithophores can co-­‐occur with other phytoplankton taxa. An assessment of the environmental conditions associated with coccolithophore blooms suggests that current understanding of coccolithophore bloom characteristics may be associated with the peak of the bloom and that conditions at the start of the bloom can be significantly different (i.e. deep mixed layers, moderate light levels and elevated silicic acid concentrations). A global PIC inventory of 29.0 ± 4.3 MT C was determined from satellite derived PIC and mixed layer depth data. The Great Calcite Belt and tropical Pacific were found to be regions that make a significant contribution to this budget. The data produced in this thesis provide the first multi-­‐year, global overview of coccolithophores and offer a novel means of monitoring coccolithophore populations over the long-term, global scales needed to identify the possible influences of climate change.

In this study we look at the ocean circulation of the Arctic Ocean in the high-resolution OCCAM global ocean model. The Arctic Ocean consists of deep basins surrounded by a large area of continental shelves, where cooling and ice formation play an important role in dense water formation. In the model these dense waters are transported by a circumpolar boundary current into the deep convection sites of the North Atlantic Ocean. The boundary current is thought to be a continuous feature in the real ocean, however the driving force is still unknown. We provide evidence that buoyancy fluxes that occur due to air-sea exchanges on the continental shelves are an important driving force for the boundary current in the model. The formation area of the circumpolar boundary current is found in the Barents Sea, where there is a high pressure area associated with cooling of inflowing Atlantic Water (AW). The modified water, Barents Sea Water (BSW), is then able to pass through the Arctic Front as it sinks into the Arctic Basin via the St Anna Trough in a boundary current. The high density signal of these waters can be seen all around the continental slope of the Arctic Ocean as a continuous pressure gradient. The boundary pressure gradient continues into the North Atlantic, where a low pressure region is found off Cape Hatteras. A time-dependent variant of an accurate particle tracking technique has been applied to calculate pathways of the dense waters using stored velocity fields of the OCCAM model. This technique has been extended with a representation of random motions due to diffusive effects. An expression for the random motions is derived using the theory of Brownian motion, and is chosen to match the Laplacian eddy viscosity terms in the momentum equations of the OCCAM model. The trajectories of the dense waters on the Barents Sea shelf follow the boundary current, and are guided around the slope by topographical contours. However the pathways are severely affected by large-scale wind-driven features as the Trans-Arctic drift and the Beaufort Gyre, which carry water masses out of the boundary current or trap them in the Canadian Basin. It is found that it takes approximately 30 years for the bulk of BSW to reach the North Atlantic, although the major signals complete the Arctic circumference within 10 years. The transport of the BSW through the Arctic into the North Atlantic can be accurately described by a 1D advection-diffusion model with a ”diffusion” coefficient of 1.3 · 109cm2/s and an ”advection” coefficient of 2.9cm/s. This confirms that the diffusion of particles is caused by basin-scale features rather than meso-scale eddies. More dense water is formed on the Chukchi Sea shelf, which originates from the Bering Strait Outflow. There are signs that these dense waters provide forcing for eddies seen off North Alaska. A new theory is presented for calculating the Available Potential Energy (APE) on the continental shelves for driving local currents in the Arctic Ocean, using the mean offshore density structure as a reference state. The air-sea fluxes in the Barents Sea are found to create a large amount of APE on the shelves, which is lost as the dense waters sink into the Arctic Basin. Although it is found the inflowing AW already has a large amount of APE which cannot fully be converted due to the Arctic Front in Fram Strait, therefore it appears the cooling in the Barents Sea is crucial to the forcing of the boundary current. This cooling in the prescribed model air-sea fluxes is likely to be caused by enormous heat losses to the atmosphere in large ice-free regions, which are created by the inflow of warm AW.

Ocean fronts are narrow zones of intense dynamic activity that play an important role in global ocean-atmosphere interactions. Of particular significance is the circumglobal frontal system of the Southern Ocean where intermediate water masses are formed, heat, salt, nutrients and momentum are redistributed and carbon dioxide is absorbed. The northern limit of this frontal band is marked by the Subtropical Front, where subtropical gyre water convergences with colder subantarctic water. Owing to their highly variable nature, both in space and time, ocean fronts are notoriously difficult features to adequately sample using traditional in-situ techniques. We therefore propose a new and innovative statistical modelling approach to detecting and monitoring ocean fronts from AVHRR SST images. Weighted local likelihood is used to provide a nonparametric description of spatial variations in the position and strength of individual fronts within an image. Although we use the new algorithm on AVHRR data it is suitable for other satellite data or model output. The algorithm is used to study the spatial and temporal variability of a localized section of the Subtropical Front past New Zealand, known locally as the Southland Front. Twenty-one years (January 1985 to December 2005) of estimates of the front’s position, temperature and strength are examined using cross correlation and wavelet analysis to investigate the role that remote atmospheric and oceanic forcing relating to the El Nino-Southern Oscillation may play in interannual frontal variability. Cold (warm) anomalies are observed at the Southland Front three to four months after peak El Nino (La Nina) events. The gradient of the front changes one to two seasons in advance of extreme ENSO events suggesting that it may be used as a precursor to changes in the Southern Oscillation. There are strong seasonal dependencies to the correlation between ENSO indices and frontal characteristics. In addition, the frequency and phase relationships are inconsistent indicating that no one physical mechanism or mode of climate variability is responsible for the teleconnection.

As anthropogenic activities expand into the deep sea, it is only recently that the importance of deep-sea ecosystems and processes to global biogeochemical systems has become clear. If the potential impact of human activity upon deep-sea organisms and ecosystems is to be understood and predicted, experimental studies are required to improve our knowledge of their sensitivity to contamination and disturbance. Echinoderms are integral components of deep-sea benthic communities and, by virtue of their abundance, they contribute significantly to deep-sea biogeochemical processes. As such, echinoderms can be considered relevant target organisms for deep-sea experimental studies. Three approaches to the investigation of deep-sea anthropogenic impact upon echinoderms were undertaken in this study. The first was based on contaminant exposure experiments with two species of shallow-water echinoid, the eurytopic Psammechinus miliaris and the stenotopic Brissopsis lyrifera. A range of biomarkers was used to assess the responses of the echinoids to contaminant exposure. Compared with the significant cytological and molecular (assess via qPCR) responses in P. miliaris, a reduced capacity to respond to contaminant exposure was found in B. lyrifera at these levels of biological organisation. Stenotopic species are hence recommended for future experimental studies as proxies for deep-sea echinoderms which, due to their adaptation to the stable environment of the deep sea, are also considered to have a reduced capacity for homeostasis in the face of environmental perturbation. The second experimental approach involved sediment burial experiments, simulating anthropogenic drilling disturbance, with the deep-water echinoderm species Echinus acutus. ROV technology was used to perform the burial experiments in situ at 114 m depth. The application of quantitative PCR molecular biomarker methodology revealed a significant increase in the expression of a stress-70 protein in response to sediment burial. These results demonstrate the sensitivity of the qPCR technique to assess an organism’s stress-response, and its relevance to deep-sea experimental studies. Finally, the development and successful deployment of an in situ respirometer, the benthic incubation chamber system (BICS) 2, made possible the acquisition of physiological measurements from deep-sea echinoderms at the abyssal sea floor at 3500 m. The results revealed similarities between the oxygen consumption rates of shallow-water and deep-sea echinoderms. The future performance of in situ deep-sea experimentation is dependent on the development of experimental equipment that confers the ability to perform experiments in situ with ROV technology and to obtain results without interference from recovery-related side effects.

This thesis is split into the two scientific topics studied; overpressure development in deep basins and present-day and future gas hydrate dissociation in the Arctic. Locating and quantifying overpressure is essential to understand basin evolution and hydrocarbon migration in deep basins and thickly sedimented continental margins. The first part of this thesis develops two new methods, including an inverse model, to impose seismic and geological constraints on models of overpressure generated by the disequilibrium compaction and aquathermal expansion mechanisms. The results provide greater understanding of a low velocity zone (LVZ), inferred from wide-angle seismic data, in the centre of the Eastern Black Sea Basin (EBSB). The application of both methods in the study area indicate that the LVZ located within the Maikop formation, at ~3500-6500 m depth below the seabed (mbsf), is linked to overpressure generated, mainly, by disequilibrium compaction.

Phytoplankton live almost exclusively in the sunlit waters of the euphotic zone. However, in addition to sunlight, phytoplankton require a regular supply of nutrients to grow. In the open ocean such nutrients are abundant in the dark waters below the euphotic zone. Hence, to a large extent it is the physical mechanisms driving the transfer of nutrient rich water into the euphotic zone which dictate patterns of phytoplankton growth. Using a combination of observation and high resolution computer modelling this thesis investigates whether shear associated with mesoscale features leads to locally enhanced turbulent mixing and a shear-enhanced nutrient supply. Measurements of turbulent diffusivity and nutrient concentrations have been made in a region containing an eddy dipole, a strong mesoscale feature, consisting of a cyclonic eddy and an anti-cyclonically rotating mode-water eddy. The effect of this strong mesoscale feature on vertical turbulent mixing is assessed by investigating whether variations in vertical shear associated with the mesoscale feature enhance the observed vertical turbulent mixing. Using these observations of turbulent diffusivity, augmented by further measurements from two other ocean regions, a new parametrization of shear-enhanced vertical turbulent mixing is developed. The new shear-enhanced mixing parametrization is implemented in a high-resolution computer model of a mode-water eddy. This model is then used to examine the effect of interactions between the eddy and the wind on vertical nutrient fluxes. The shear enhancement to nutrient supply by mesoscale circulation is found to be potentially of much greater significance than has previously been considered. Modelling suggests that when forced by high variability winds mode-water eddies appear to be capable of locally enhancing the vertical turbulent nutrient flux by up to an order of magnitude. The work in this thesis suggests that vertical turbulent flux may well be underestimated as a stimulus to new production.

The influence of the tropical Atlantic Ocean over the climate of Europe, Africa and America is well known today. However, several questions about high-frequency processes in this region remain open. This thesis addresses the characterisation of the diurnal and other short timescale variability of the meteo-ocean variables measured in the tropical Atlantic Ocean by the PIRATA array, as well as derived air-sea heat fluxes. By combining the complementarity and mitigating the disadvantages of using the high temporal resolution of in situ data in conjunction with the excellent spatial coverage of satellite based data, this work also aims to investigate the characteristics of the Tropical Instability Waves in the tropical Atlantic. The satellite data validation process used in this study assesses each of the buoys individually, to take into account possible regional biases. A complete picture of the mean diurnal cycle and the seasonal variability of the diurnal signal is performed for the first time for the whole tropical Atlantic basin. The SST diurnal signal presents strong characteristics during the respective summer in both hemispheres. However, through the wavelet technique used in this analysis, a significant diurnal signal at the equator could be noticed during the second half of each year, indicating a possible modulation of the diurnal signal by processes with different timescales. It is suggested that Tropical Instability Waves could be one of these processes. The results presented here show that the TIW clearly vary their position and time of activity, depending on the degree of development of the equatorial cold tongue. The most active year analysed in this study was 2001, when the spectral characteristics could be observed as far north as 4oN. The imprints of the TIW are well marked in the wind fields, showing that clearly there are coupled mechanisms associated with the TIW. Moreover, this study confirms that a coupling mechanism suggested for the Pacific Ocean is also applicable to the tropical Atlantic basin. The measurements made by the TMI sensor, in conjunction with the Qscat wind data showed that the atmospheric fields are highly correlated with the SST fields at the timescale associated with the TIW. The analysis of the cross-scale relationship suggests that the passage of instability waves might affect the diurnal amplitude of SST, skin-SST and latent heat flux. The mechanisms that interact on the eastern and western side of the equatorial Atlantic tend to be distinct, especially due to the local oceanographic and meteorological conditions, and due to the different level of TIW activity.