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Seminar abstracts

Ocean-atmosphere interactions represent a key component of the hydrological cycle in tropical regions and their variability has profound influences on low-latitude climate. We set out to investigate the link between the parameterization of ocean-atmosphere fluxes in global circulation models and climate variability at large regional scales across the tropics, in order to set the stage for the future implementation of new surface flux schemes over the ocean. We consider the recent CMIP5 climate simulations ensemble for the historical period 1979-2005, run with the latest version of the IPSL Earth System Model in coupled (IPSLCM5A) and corresponding atmosphere stand-alone (AMIP) modes. In order to identify the major drawbacks related to processes missing from the model we develop a “toolbox” for evaluating the model representation of heat and momentum surface fluxes, which takes into consideration the large uncertainties in the observational turbulent flux products. Focusing on the link with sea surface temperature, we also make a comparison between the IPSL-CM5A and the AMIP simulation results in order to assess the effects of oceanatmosphere feedbacks on the surface fluxes. This analysis will lead to a more in-depth investigation of the relationships between processes in the atmospheric boundary and ocean mixed layers and the variability of ocean-atmosphere fluxes and sea surface temperature in the tropical oceans.

Karyn Morrissey
Valuing the Marine Resource: An Irish Perspective

The realisation that the world’s oceans play an important role in climate regulation and many territory activities, notably food production, coupled with economic changes and the rapid advancement in ocean technology have seen a shift in the perception of the importance of marine resources. This increased focus on marine resources means that governments and policy-makers require accessible and reliable information regarding the role of the marine sector. This information may then be used to formulate new environmental and economic policy measures. The aim of this paper is to define, describe and quantify the value of the marine sector in Ireland. This paper outlines a methodology that provides a robust analysis of the Irish marine sector for 2007. It was found that in 2007 the Irish marine sector contributed €1.44 billion in GVA to the wider Irish economy and employed over 17,000 individuals in full time equivalents.

Stephen Belcher
A global perspective on mixing in the ocean surface boundary layer

The ocean surface boundary layer couples the atmosphere with the deep oceans and mediates the exchange of heat momentum and trace gases. Current coupled climate models have biases when compared to recent Argo measurements leading to systematic errors in modelled sea surface temperature and trace gas concentrations. I will highlight two processes not represented in current parameterisations. I will present diagnostics based on global data sets that illustrate the importance of these processes.

Judith Wolf
Why Model Waves?

The first thing anyone observes about the sea is the surface wind waves. These can be treated in various ways: from empirical observation, to pure mathematics or as an engineering problem, and in between are often regarded as part of meteorology. I will argue their essential role in oceanography, by demonstrating the ubiquity of waves in coastal and deep ocean processes. Focussing on the shelf and coastal dynamics, we see how much energy is contained in waves and the impact of their breaking on the shore. At the shoreline, many processes can be observed by eye such as refraction, diffraction, rip currents and breakers. Less obvious, although fundamental, is the impact of waves on the air-sea interaction processes and their effects on mixing and sediment transport. The main types of wave model are introduced and their capabilities and limitations are discussed, showing some recent developments in wave modelling, especially coupling with other components of the physical system. New work on air-sea interaction and bottom friction together with the potential for harnessing wave power will be illustrated.

Hannah Hiester
Simulating gravity currents with adaptive meshes

Adaptive mesh models aim to capture transient and complex dynamics in an efficient manner by refining or coarsening the mesh as the flow evolves. The utility of unstructured adaptive meshes (adaptive meshes) for ocean modelling is assessed through simulations of gravity current flows. The prime focus is on gravity currents generated in the idealised lock-exchange set-up and simulated with the Imperial College Ocean Model (Fluidity-ICOM). The Froude number (non-dimensional front speed) and background potential energy (a measure of the mixing) are used to evaluate the performance of fixed and adaptive meshes. The adaptive mesh simulations can produce comparable values of the diagnostics to the higher resolution fixed mesh simulations whilst using at least one order of magnitude fewer nodes. The results also compare well with published values. The choice of metric, which guides the mesh adapt, is shown to be crucial to the ability of the adaptive mesh to represent the flow. This decision will remain key for ocean models, from idealised studies to scenarios of increasing complexity. The potential for good representation of the dynamics and efficiency gains with adaptive meshes demonstrated here offers a promising outlook for their use in ocean modelling.

John Hunter
A Simple Technique for Objectively Estimating an Allowance for Uncertain Sea-Level Rise

Projections of sea-level rise are inherently uncertain, leading to considerable debate over suitable vertical allowances by which future infrastructure would need to be raised. Words such as "plausible" and "high-end" abound, with little objective or statistically valid support. A simple and objective method of determining a future sea-level rise allowance is derived, based on the projected rise in mean sea level and its uncertainty, and on the variability of present tides and storm surges (storm tides). The method preserves the expected frequency of flooding events under a given projection of sea-level rise. It is assumed that the statistics of storm tides relative to mean sea level are unchanged. The method is demonstrated using the GESLA (Global Extreme Sea-Level Analysis) data set of roughly hourly sea levels, covering 198 sites over much of the globe. Two possible projections of sea-level rise are assumed for the 21st century: one based on the Third and Fourth Assessment Reports of the Intergovernmental Panel on Climate Change and a larger one based on research since the Fourth Assessment Report.

Fenwick Cooper
Estimating changes in turbulent systems due to a forcing

The fluctuation-dissipation theorem has been touted as an exciting new tool for the calculation of the forced response of turbulent systems. For example the change in the climate due to a change in the solar heating. I will give a brief conceptual introduction to this theorem and explain some of its limitations. I will present the results of our efforts to apply it to simple general circulation models and an overview of current progress in this area.

Aurélie Duchez
Control of the North Mediterranean Current in the Gulf of Lions using data assimilation

Ocean forecasting systems are now able to supply reliable forecasts of the open ocean. Operational oceanography developments in coastal areas are yet less important despite the important stakes that represent those areas. Present altimetric satellites (Jason / Envisat) are not able to provide accurate information near coastal areas. SARAL-AltiKa altimeter should provide high resolution measurements of the sea surface elevation as closed as 5km from the coasts. It will thus provide, in conjunction to in-situ data, useful information to understand those areas characterized by complex circulations. The Gulf of Lions (North West Mediterranean Sea) is a microtidal coastal area both influenced by intense atmospheric forcings (a complex wind system and important river discharges from the Rhône river) and the general circulation: the North Mediterranean Current (NMC). Numerical configurations of the Gulf of Lions presently used in operational systems have a 1/16° resolution. Those models are yet not able to represent a NMC with good enough characteristics according to real observations, despite its important role on the whole circulation. We have thus used the SEEK filter data assimilation method to combine the information provided by a regional 1/16° resolution configuration of the model NEMO and observations. To simulate SARAL-AltiKa altimetry, we have decided to do an OSSE (for Observing System Simulation Experiment) and to extract a synthetic observation database from a high resolution (1/64°) configuration of the Gulf of Lions which is in a good agreement with real observations. Sensitivity studies have been done to characterize this database as well as the main characteristics of the experiment. We have thus chosen a database made of SARAL-AltiKa altimetry and in-situ temperature / salinity profiles extracted each 20 km. Other sensibility studies have been done to work out the SEEK filter error parameterizations. To assess the quality of this experiment, statistical diagnostics and other physical criteria based on the assessment of the improvement of the ocean circulation have been developped and show a good quality of this experiment. Comparisons between the free 1/16° simulation and the experience with assimilation show that data assimilation has significantly improved the characteristics of the NMC as well as its seasonal and mesoscale variability, which has improved, too, the quantity of cross shelf exchanges. We have noticed, too, an improvement of shelf water characteristics which thus allows the formation of dense waters in winter, their convection and cascading over the shelf break. We have thus implemented a data assimilation system based on the SEEK filter data assimilation method and experimented an OSSE in the coastal area of the Gulf of Lions using AltiKa altimetry in addition to in situ data profiles. Thanks to appropriate parameterizations of this system and after being validated, we have managed to control the NMC's characteristics as well as main shelf processes. Data assimilation of new kinds of observations in operational systems should thus allow the control of more and more small scale processes closed to the coast that are not controlled with present observational systems.

Helen Phillips
Observations of the non-equivalent barotropic structure of the Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) has been traditionally considered an equivalent barotropic current, where the direction of flow does not change with depth. This paradigm for the ACCs behaviour was proposed by Killworth (1992) on observing the high correlation between eastward velocities at different depths in a 6-year mean of the FRAM simulation. High vertical coherence of horizontal velocity is also seen in direct current meter measurements. The equivalent barotropic idea motivated simplified models of the ACC, and allowed the development of Gravest Empirical Mode (GEM) climatologies of the Southern Ocean. However, if we allow no rotation of the flow with depth, then vertical motion is confined to be small or non-existent, and important dynamical interactions achieved by vertical modes higher than the barotropic and first baroclinic are ignored. The equivalent-barotropic idea has proven very insightful but there is much more to the story of the ACC. Strong rotation of horizontal velocity vectors with depth is clearly seen in new high-vertical-resolution direct measurements of ACC velocity profiles from EM-APEX floats at the northern Kerguelen Plateau. Rotation with depth is particularly enhanced in profiles around a large cyclonic meander to the northeast of Kerguelen. Vertical velocities estimated from the conservation of mass equation are large (up to 100 m/day). We describe the distribution of flow rotation and diagnosed vertical velocity across the northern Kerguelen Plateau, and examine their variability in the context of curvature of the flow, slope of the density field, and location of the ACC fronts. Recent work (Zika et al 2011) suggests that the eddy contribution to the Meridional Overturning Circulation (MOC) is achieved by transient (standing) eddies contributing the vertical (meridional) exchange. We endeavour to examine our observational snapshot of an ACC eddy in the context of this idea inspired by an eddy-resolving numerical model.
Killworth, P. D., An equivalent-barotropic mode in the Fine Resolution Antarctic Model, J. Phys. Oceanogr., 22, 1379­1387, 1992.
Zika, J. D. et al., Cancellation of the Deacon Cell by Vertical Eddy Fluxes, J. Phys. Oceanogr., submitted, 2011.

Jarle Berntsen
Subgrid scale physics in numerical ocean modelling

There is a growing literature on subgrid scale (SGS) parameterizations for ocean models. It has been demonstrated that many important subgrid scale processes are not adequately represented in todays SGS schemes, and the assumptions behind the schemes are being questioned. In the seminar, some of the hot-topics in the SGS literature will be addressed.

Agustín Sanchez Arcilla & Luigi Cavaleri
The Field_AC project

Coastal-zone oceanographic predictions seldom appraise the land discharge as a boundary condition. River fluxes are sometimes considered, but neglecting their 3D character, while the “distributed” continental run-off is not taken into consideration. Moreover, many coastal scale processes, particularly those relevant in geographically restricted domains (coasts with harbours or river mouth areas), are not well parameterized in present simulations. Because of this situation, local predictions still present significant errors and are not robust enough, even being locally wrong for sharp gradient events, such as flash flood discharges into the Mediterranean sea. This hampers decision-making in coastal zones. The FIELD_AC project aims at providing an improved operational service for coastal areas and to generate added value for shelf and regional scale predictions from GMES Marine Core Services. Local assimilation will be analysed together with advanced error metrics to provide a reliable service that can be transferred to public and private parties, using the spin-off company associated to the project. This will be achieved by the introduction of more comprehensive “land” boundary conditions, improved local parameterizations and new coupling terms/strategies for the studied field cases. They cover a representative range of meteo-oceanographic drivers for “geometrically” restricted domains (Catalan coast, Venice Gulf, Liverpool Bay, German Bight Wadden Sea). FIELD_AC will bridge the gap from shelf predictions to local (river mouth or harbour/beach scales) simulations required at the coastal zone. This will result in a wider demand for operational services and an enhanced use of in-situ and remote observations. Such improvements (services and expertise) will require the advancement of the present state of the art.

Joel Hirschi
Deterministic and chaotic variability of the meridional overturning circulation

The first 6.5 years of observations of the meridional overturning circulation (MOC) at 26.5 have shown that the MOC exhibits a large subannual to seasonal variability. The origins of this variability is only partly understood. In particular it is not clear how much of the observed MOC variability can readily be explained by the atmospheric forcing and how much might be due to unpredictable (“chaotic”) processes such as eddies (and to a lesser extent internal waves). The advent of satellite observations have shown that internal waves and eddies are ubiquitous in the ocean. However, despite the increasing wealth of ocean observations the impact of both eddies and internal waves on the variability of the ocean circulation is far from fully understood. In my talk I will show an attempt to isolate the forced from the internal (“chaotic”) MOC variability in an eddy-permitting 1/4 degree global ocean model. The results suggest than on sub- to interannual timescales the internal (“chaotic”) MOC fluctuations account for about 20 to 30% of the total MOC variability. Despite this sizable contribution from eddies and internal waves fields we find that on subannual to interannual timescales the MOC variability is largely constrained by the atmospheric forcing. The model study also reveals a previously unreported large MOC variability on daily timescales with peak-to-peak fluctuations of up to 50 Sv (1 Sv = 10^6 m-3/s). MOC anomalies originate from mid-latitudes from where they propagate equatorwards. This high-frequency variability of the MOC is caused by near inertial gravity waves that are generated by the wind variability at mid-latitudes. The near inertial gravity waves and the large MOC fluctuations both disappear when the wind stress variability is removed.

Jeff Polton
Physical and Dynamical Oceanography of Liverpool Bay

Liverpool Bay is a macro-tidal shallow sub-section of the semi-enclosed Irish Sea, where the UK National Oceanography Centre maintained a multiple platform, multidisciplinary ocean science Observatory for 10 years. Using both models and observational data we review dominant physical processes that control the Liverpool Bay oceanography, looking in particular at the freshwater plume and the residual circulation. We then synthesise combined model and observed physics and explore the residual circulation in more detail and build a methodology to predict the residual flow in places where there are either no observations or where the model performs poorly.

Georgy Shapiro
Basin wide changes in the circulation of the Celtic Sea and its potential environmental impact due to tidal energy generation

It is common sense that extraction of ocean kinetic energy in large quantities may change circulation patterns in coastal seas thus influencing both the general state of the environment and the extractable energy resource. This paper assesses the backward effect on the ocean currents by a hypothetical array of tidal turbines installed in the shallow sea. In this study, a 3D ocean circulation model POLCOMS is used to estimate (i) the actual values of extractable kinetic energy at different levels of rated power capacity of the farm, and (ii) alterations to the pattern of residual currents and pathways of passive tracers. Numerical modelling has been carried out for a hypothetical tidal farm located in the Celtic Sea north of Cornwall, an area known for its high level of tidal energy. Modelling results clearly indicate that extracted power does not grow linearly with the increase in the rated capacity of the farm. Residual currents are altered as far as a hundred kilometres. Some of the passive drifters simulated in the model,experience significant variations in the end-to-start distance due to energy extraction ranging from 13% to 238% while others are practically unaffected. This pilot study shows that the area-wide effects have to be taken into account when designing the tidal farm to minimise the negative effects on the environment and enhance the positive ones.

James Maddison
Eddy parameterisation and integral constraints: A geometric framework

Ocean climate models are unlikely routinely to have sufficient resolution to resolve the turbulent ocean eddy field. The need for the development of improved mesoscale eddy parameterisation schemes therefore remains an important task. The current dominant mesoscale eddy closure is the Gent and McWilliams scheme, which enforces the down-gradient mixing of buoyancy. While motivated by the action of baroclinic instability on the mean flow, this closure neglects the horizontal fluxes of horizontal momentum. The down-gradient mixing of potential vorticity is frequently discussed as an alternative parameterisation paradigm. However, such a scheme, without careful treatment, violates fundamental conservation principles, and in particular violates conservation of momentum. A new parameterisation framework is presented which preserves conservation of momentum by construction, and further allows for conservation of energy. The framework has one dimensional parameter, the total eddy energy, and five dimensionless and bounded geometric parameters. The popular Gent and McWilliams scheme exists as a limiting case of this framework. Hence the new framework enables for the extension of the Gent and McWilliams scheme, in a manner consistent with key physical conservations.

Mike Walkden
Predicting coastal erosion under accelerated sea level rise; recent research applied in practice

Accelerated sea level rise is expected to cause significant increases in coastal erosion rates around the world. There is great uncertainty about how much erosion we should expect, and how rates of recession will vary from coast to coast. This is partly due to a general paucity of methods for predicting the sensitivity of coasts to rates of sea level rise. In response to this, two new methods have been developed over the last decade, through research funded jointly by the Tyndall Centre for Climate Change Research, Defra/EA and the EPSRC. These methods have made various scientific contributions, and are now being applied in practice. The seminar will outline these developments and demonstrate their application to a series of coastal studies, ranging from large scale Shoreline Management Plans to detailed shore profile analysis for the nuclear industry.

Alan Davies
Sediment transport in shallow sandy estuaries

Predicting the development of estuaries, for example around the coast of Wales, is both important for managers and also challenging for researchers. This talk will review some recent work carried out at Bangor, highlighting initially future flood risk in three shallow, sandy estuaries, assessed using the Telemac modelling system. The flood or ebb dominance of the net sediment transport in such estuaries is influenced strongly by the presence of channels and sand flats, the formation and dissipative characteristics of which have been studied. The modelling outcomes depend upon the description of the bed roughness and, in the case of the Dyfi Estuary, field observation suggests that a spatially-varying dune roughness should be used.

Robert J. Nicholls
Adaptation to Sea Level Rise

Coastal areas constitute important habitats, and they contain a large and growing proportion of population and economic activity, including economic centres such as London, New York, Tokyo, Shanghai, Mumbai, and Lagos. Sea-level rise is a long-term threat to these areas. Global-mean sea levels rose 17 cm through the twentieth century due to global warming: they are likely to rise more rapidly due to the same cause through the twenty-first century when a rise of more than 1 m is possible. In some locations, local (or relative) sea-level rise may be exacerbated by subsidence, especially due to ground fluid withdrawal from, and drainage of, susceptible soils. Relative sea-level rise has a range of potential impacts, including higher extreme sea levels (and flooding), coastal erosion, salinization of surface and ground waters, and degradation of coastal habitats such as wetlands. In the worst case, large land areas could be lost and millions of people could be displaced by sea-level rise. Appropriate responses include mitigation of climate (a global response) and subsidence (a local response) and/or adaptation (also a local response). A combination of these strategies appears to be the most appropriate response to sea-level rise. Adaptation responses can be characterized as (1) protect, (2) accommodate, or (3) retreat. While these adaptation responses could reduce impacts significantly, they will need to be consistent with responses to all coastal hazards, as well as with wider societal and development objectives; hence, an integrated coastal management philosophy is required. In some developed countries, including England and the Netherlands, proactive adaptation plans are already being formulated. Coastal cities worldwide will be a major focus for adaptation efforts because of their concentrations of people and assets. Developing countries will pose adaptation challenges, especially in deltaic areas and small islands, which are the most vulnerable settings.

Victoria Harris
Modelling Community Responses in Plankton to Climate Change

Over the past few decades a “regime shift” has been reported in the behaviour of North Atlantic plankton. Since the plankton play a fundamental role in the oceanic food chain, changes in the composition of the taxa will have an impact for many marine organisms. A community analysis of the Continuous Plankton Recorder (CPR) dataset can be used to identify behaviour of functional groups and their responses to known climate indices. In order to visualise the dataset over multiple dimensions, namely space, time and species, a new approach to the statistical analysis must be taken. Sparse principal component analysis can be used to model the joint behaviour of functional groups in space and time. This approach allows the identification of regions which are particularly sensitive to climate effects. It can be shown across the Copepod subgroup that warm water species have a positive relationship with the northern hemisphere warming trend, whilst cold water species are in decline. This might be a driving force behind the so-called “regime shift”. Other species subgroups can be shown to respond to natural climate oscillations, for instance Diatom species are primarily driven by the Atlantic Multidecadal Oscillation. The influence of these climate indices on both sea surface temperature and species responses is spatially inhomogeneous, meaning that this spatio-temporal analysis can grant a deeper understanding of the complex structure.

David Webb
The actual diurnal tides to the west of Britian & The modelling of diurnal tides near the shelf edge using Arakawa C-grid type tidal models.

Continental shelf resonances are believed to be responsible for dissipating a large fraction of the tidal energy in the ocean. The talk will report on an attempt to learn more about such resonances using a numerical model of the Gulf and Carpenteria and the Arafura Sea. It is shown that the tides of the region depend in part on individual resonances and in part on the cooperative effect of groups of resonances. Preliminary results will also be shown from studies of the English Channel and Irish Sea and of Hudson Bay and Hudson Strait.

Heidi Diersen
Seagrass is always greener: Optical Remote Sensing of the Seafloor

Much effort has been directed at imaging the seafloor either with satellites, aircraft or remotely operated vehicles in order to monitor ecologically important and vulnerable marine habitats. In particular, seagrass meadows absorb and scatter light in proportion to their density and can be remotely sensed using optical methods. A physicist may want to consider a seagrass meadow as a simple, flat green reflector. However, seagrasses are more complex from a biological perspective and should be considered as 3-dimensional, dynamic and optically active organisms that influence the properties of the water above and sediment below. Here, the optical properties of various species of seagrasses measured in diverse environments are presented from clear tropical waters of the Bahamas to turbid estuarine waters of Elkhorn Slough, CA and Port Aransas TX. Methods to quantify seagrass and measure the optical properties from leaf to canopy level are presented. Canopy architecture plays a role in the projected color and bathymetry necessary for remote sensing techniques. Seagrasses are disappearing at alarming rates in most coastal areas around the globe due to eutrophication. Development of more accurate remote sensing methods based on both physics and biology is warranted to assess current and future distributions.

Kerry Marten
Measuring and Modelling Nearshore Sediment Transport Processes – A Practical Approach

Often in the application of sediment transport theory, predictors perfected in the lab do not compare well with field data. This seminar shows how a practical approach using field data can model bed roughness and suspended sediment concentration profiles in the ‘real world’, where constantly changing conditions mean that these quantities are time-dependent. The acoustic data were obtained in 7m of water, comprising tidal currents up to 1m/s and wave heights up to 2m across three storm events. Bedform data of unprecedented quality were obtained using a three-dimensional acoustic ripple profiler (3D-ARP). Key results include a wave-current dominance criterion, the contribution of current mega-ripples to suspended sediment load, and the two-layer form of diffusivity profiles above the different observed bedform types. These results and others were included in the algorithm ‘nearShore_susSed’, which takes simple inputs to predict bed roughness and concentration profiles in a time-dependent sense. The modular nature and simple to use interface of the algorithm mean that the model is an ideal tool for coastal planners and engineers who require a stand-alone but state-of-the-art calculation.

James While
Data assimilation in shelf sea models

At the present time the Met Office has two shelf sea models that include data assimilation of SST observations. These models are the AMM7 model of the north west European shelf and the PGM4 model of the Persian gulf. Two separate methods are used for assimilating data into these models: an analysis correction scheme for the AMM7, and a more advanced 3DVar scheme for the PGM4. Both schemes assimilate data from a number of satellite instruments and ships, with the AMM7 also assimilating data from drifters and moored buoys. Despite the different schemes, in both the AMM7 and PGM4 the representation of SST is significantly improved by the data assimilation. In this talk I will describe the two models and the data assimilation methodologies applied to them. Results demonstrating the positive effect of the data assimilation on model SST will be presented. The talk will also detail our current ideas for developing the data assimilation systems into the future.

David Munday
Impact of Mesoscale Eddies on Southern Ocean Circulation

Recent advances suggest that the circulation of the Southern Ocean is a balance between low latitude wind forcing, globally integrated diapycnal mixing, and deep water formation (in both hemispheres). Eddy processes play an essential role, with modelling studies demonstrating that coarse resolution ocean models, with or without parameterised eddies, and eddy-resolving ocean models respond in fundamentally different ways to changes in forcing. Such models are sensitive to a large range of parameters, such as frictional coefficients and eddy diffusivities, many of which are uncertain at best. We will discuss eddy-permitting model results aimed at understanding the response of such models to climatically-relevant forcing perturbations, in this case wind stress and diapycnal mixing. These results are aimed at testing ideas regarding the invariance of circumpolar transport (eddy saturation) and the reduced sensitivity of overturning (eddy compensation) when in the eddying regime. Our results indicate that these ideas are robust to large wind perturbations, but are less applicable to changes in diapycnal mixing. Simple biogeochemical experiments demonstrate the climate link, and suggest that changes to wind forcing may not be a way to explain past climate variability.

Andrew Meijers
Journeys in the Southern Ocean: Dynamics from satellites, dense water formation and the inverse modelling of mixing. A grab-bag of research highlights.

Over my past eight years of Southern Ocean research I've been involved in a wide variety of fascinating (I think) topics. From looking at large scale ACC dynamics using altimetry, to developing inverse models of ocean mixing, to drilling holes in the sea ice and dropping things through. This seminar takes a quick look at key findings and highlights, as well as my future research plans at the British Antarctic Survey. There should be equations and pretty pictures of sea ice in equal measure, something for everyone!

• The development of the satellite Gravest Empirical Mode (satGEM), a gridded, time-varying, quasi-observational estimate of ACC temperature, salinity and velocities. This product allows the assessment of large scale ACC properties, such as eddy response to changes in the SAM and evidence for an eddy saturated ACC, and the separation of the bulk ACC heat and freshwater trends over the last 20 years into components driven by shifts in water masses, and those driven by the gain/loss of heat and freshwater in the system.

• The formation and variability of dense shelf overflows in the Mertz Glacier Tongue region of East Antarctica. Observations and high resolution modelling show a clear seasonal pattern in both the off-shelf export and circulation of dense shelf water, as well as sensitivity to surface buoyancy flux forcing.

• East Antarctica cruises: A dense water source west of Cape Darnley, and evidence of MCDW intrusions onto the shelf near the Totten Glacier Tongue.

• The ongoing development of the Tracer Contour Inverse Method (TCIM). This inverse approach allows us to create an overdetermined system of equations from climatology that can be solved for explicit diapycnal and isopycnal eddy diffusion, potentially allowing us to estimate spatial patterns of mixing across the globe.

Ming Li
Modelling sand transport and morphology dynamics in Liverpool Bay and Yellow River Estuary.

Prediction of sediment transport in the near shore region is particularly important to the coastal management and protection in the face of rising sea level and climate change. However, the worldwide differences in geomorphology setting and sediment characteristics often mean very different sediment transport and coastal morphological dynamics, which is particularly challenging for the practical modelling. This presentation will focus on computer simulation of sand transport using Telemac package at Liverpool Bay, where the inshore sediment is dominated by macro-tides and strong offshore waves, in comparison to the Yellow River delta, NE China in which fine sediment is transported via fluvial flow into the Bohai Bay. Model results were used to reveal the influences from spatial varying sediment sizes on the overall morphological processes at these two sites. Predictions in potential impacts form sediment supply reductions on the future morphology are also compared to assist better understanding of the underlying long term morphological evolution at these two sites.

Fred Wobus
Dense water cascading in the Arctic Ocean – overview and idealised modelling results

Flows of dense water down the continental slope – cascades – are initiated in Arctic shelf regions by surface cooling or sea ice formation. In the Arctic Ocean there is growing understanding of the influence of cascading on the formation and maintenance of the halocline and on the global overturning circulation. The process is also believed to be influential in the off-shelf transport of carbon and other suspended or dissolved matter. Cascades thus present a potentially important link between shelf waters and the deep ocean, but their role in the Arctic Ocean remains largely unquantified. Shelf-ocean exchange is inhibited by a combination of steep slopes and geostrophy, so there is considerable interest in processes that break the geostrophic constraint to facilitate cross-slope flow. Some theoretical background and ‘reduced physics’ models for near-bottom gravity currents are reviewed, a classification of cascades and their life-cycle phases is introduced and some observations from the Storfjorden cascade in Svalbard are presented. Results from recent modelling experiments using POLCOMS and NEMO show that a hydrostatic ocean circulation model is capable of representing the correct boundary layer physics to successfully model cascading. The POLCOMS model is validated against laboratory experiments and then set up to simulate conditions that were not created in the laboratory to study the properties and behaviour of density-stratified cascades. A second model setup using idealised geometry in NEMO is used to investigate the flow of a dense bottom current into ambient stratification. The results indicate that knowledge of the initial conditions can be used to predict the eventual fate of the cascade, such as its final depth/density level.

Carl Wunsch
The Atlantic Meridional Overturning Circulation, Its Nature, and Implications for Observations

The so-called Atlantic meridional overturning circulation (AMOC) has been a major focus of studies of present, past, and future climate states. Because only fragmentary observations of its nature and variability exist, the discussion has been dominated by model results. Here advantage is taken of a nearly-20-year least-squares fit of a general circulation model to global oceanic data sets to describe the decadal variability structure of the AMOC and its predictability. Although the extent to which the results are applicable to century and longer variations remains obscure, the complex spatial and temporal structure of ocean circulation variability more generally, suggests that uses of integral measures of the circulation are inadequate, and that any real understanding will require more extended observations of the three-dimensional global circulation extending over many years.

Gabriele Messori
The sporadic nature of atmospheric heat transport by transient motions

My talk will focus on meridional atmospheric heat transport due to transient eddies. Probability density functions (PDFs) of eddy heat transport display a near-zero most likely value and a very large skewness, which highlights the dominant role played by extreme events. When considering zonal sections, in both the Northern and Southern Hemispheres, events in the top 5 percentiles typically contribute to over half of the net poleward transport. As a result of this sensitivity to extremes, a large fraction of the heat transport by transient eddies, at a given location and season, is realised through randomly spaced bursts (a few per season), rather than through a continuum of events. The predominance of extreme events can be explained by the favourable phase relationship between meridional velocity and moist static energy temporal anomalies. This and the spatiotemporal characteristics of the events are compatible with Eady type growing systems.