bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal variability of the Caspian Sea three-dimensional circulation, sea level and air-sea interaction
Authors: Ibrayev R. A. , Özsoy E. , Schrum C. , Sur H. İ.
Published: April 2010

Abstract. A three-dimensional primitive equation model including sea ice thermodynamics and air-sea interaction is used to study seasonal circulation and water mass variability in the Caspian Sea under the influence of realistic mass, momentum and heat fluxes. River discharges, precipitation, radiation and wind stress are seasonally specified in the model, based on available data sets. The evaporation rate, sensible and latent heat fluxes at the sea surface are computed interactively through an atmospheric boundary layer sub-model, using the ECMWF-ERA15 re-analysis atmospheric data and model generated sea surface temperature. The model successfully simulates sea-level changes and baroclinic circulation/mixing features with forcing specified for a selected year. The results suggest that the seasonal cycle of wind stress is crucial in producing basin circulation. Seasonal cycle of sea surface currents presents three types: cyclonic gyres in December–January; Eckman south-, south-westward drift in February–July embedded by western and eastern southward coastal currents and transition type in August–November. Western and eastern northward sub-surface coastal currents being a result of coastal local dynamics at the same time play an important role in meridional redistribution of water masses. An important part of the work is the simulation of sea surface topography, yielding verifiable results in terms of sea level. The model successfully reproduces sea level variability for four coastal points, where the observed data are available. Analyses of heat and water budgets confirm climatologic estimates of heat and moisture fluxes at the sea surface. Experiments performed with variations in external forcing suggest a sensitive response of the circulation and the water budget to atmospheric and river forcing.
Malvinas-slope water intrusions on the northern Patagonia continental shelf
Authors: Piola A. R. , Martínez Avellaneda N. , Guerrero R. A. , Jardón F. P. , Palma E. D. , Romero S. I.
Published: April 2010

Abstract. The Patagonia continental shelf located off southeastern South America is bounded offshore by the Malvinas Current, which extends northward from northern Drake Passage (~55° S) to nearly 38° S. The transition between relatively warm-fresh shelf waters and Subantarctic Waters from the western boundary current is characterized by a thermohaline front extending nearly 2500 km. We use satellite derived sea surface temperature, and chlorophyll-a data combined with hydrographic and surface drifter data to document the intrusions of slope waters onto the continental shelf near 41° S. These intrusions create vertically coherent localized negative temperature and positive salinity anomalies extending onshore about 150 km from the shelf break. The region is associated with a center of action of the first mode of non-seasonal sea surface temperature variability and also relatively high chlorophyll-a variability, suggesting that the intrusions are important in promoting the local development of phytoplankton. The generation of slope water penetrations at this location may be triggered by the inshore excursion of the 100 m isobath, which appears to steer the Malvinas Current waters over the outer shelf.
A model for predicting changes in the electrical conductivity, practical salinity, and absolute salinity of seawater due to variations in relative chemical composition
Authors: Pawlowicz R.
Published: April 2010

Abstract. Salinity determination in seawater has been carried out for almost 30 years using the Practical Salinity Scale 1978. However, the numerical value of so-called practical salinity, computed from electrical conductivity, differs slightly from the true or absolute salinity, defined as the mass of dissolved solids per unit mass of seawater. The difference arises because more recent knowledge about the composition of seawater is not reflected in the definition of practical salinity, which was chosen to maintain historical continuity with previous measures, and because of spatial and temporal variations in the relative composition of seawater. Accounting for these spatial variations in density calculations requires the calculation of a correction factor δSA, which is known to range from 0 to 0.03 g kg−1 in the world oceans. Here a mathematical model relating compositional perturbations to δSA is developed, by combining a chemical model for the composition of seawater with a mathematical model for predicting the conductivity of multi-component aqueous solutions. Model calculations for this estimate of δSA, denoted δSRsoln, generally agree with estimates of δSA based on fits to direct density measurements, denoted δSRdens, and show that biogeochemical perturbations affect conductivity only weakly. However, small systematic differences between model and density-based estimates remain. These may arise for several reasons, including uncertainty about the biogeochemical processes involved in the increase in Total Alkalinity in the North Pacific, uncertainty in the carbon content of IAPSO standard seawater, and uncertainty about the haline contraction coefficient for the constituents involved in biogeochemical processes. This model may then be important in constraining these processes, as well as in future efforts to improve parameterizations for δSA.
Sediment 231Pa/230Th as a recorder of the rate of the Atlantic meridional overturning circulation: insights from a 2-D model
Authors: Luo Y. , Francois R. , Allen S. E.
Published: April 2010

Abstract. A two dimensional scavenging model is used to investigate the patterns of sediment 231Pa/230Th generated by the Atlantic Meridional Overturning Circulation (AMOC) and further advance the application of this proxy for ocean paleocirculation studies. The scavenging parameters and the geometry of the overturning circulation cell have been chosen so that the model generates meridional sections of dissolved 230Th and 231Pa consistent with published water column profiles and an additional 12 previously unpublished profiles measured in the North and Equatorial Atlantic. The processes that generate the meridional sections of dissolved and particulate 230Th, dissolved and particulate 231Pa, dissolved and particulate 231Pa/230Th, and sediment 231Pa/230Th are discussed in detail. The results indicate that the relationship between sediment 231Pa/230Th at any given site and the overturning circulation is very complex. They clearly show that constraining past changes in the strength and geometry of the AMOC requires an extensive data set and they suggest strategies to maximize information from a limited number of samples.
Structure and forcing of the overflow at the Storfjorden sill and its connection to the Arctic coastal polynya in Storfjorden
Authors: Geyer F. , Fer I. , Smedsrud L. H.
Published: April 2010

Abstract. Storfjorden (Svalbard) is a sill-fjord with an active polynya and exemplifies the dense water formation process over the Arctic shelves. Here we report on our simulations of Storfjorden covering the freezing season of 1999–2000 using an eddy-permitting 3-D ocean circulation model with a fully coupled dynamic and thermodynamic sea-ice model. The model results in the polynya region and of the dense water plume flowing over the sill crest are compared to observations. The connections of the overflow at the sill to the dense water production at the polynya and to the local wind forcing are investigated. Both the overflow and the polynya dynamics are found to be sensitive to wind forcing. In response to freezing and brine rejection over the polynya, the buoyancy forcing initiates an abrupt positive density anomaly. While the ocean integrates the buoyancy forcing over several polynya events (about 25 days), the wind forcing dominates the overflow response at the sill at weather scale. In the model, the density excess is diluted in the basin and leads to a gradual build-up of dense water behind the sill. The overflow transport is typically inferred from observations using a single current profiler at the sill crest. Despite the significant variability of the plume width, we show that a constant overflow width of 15 km produces realistic estimates of the overflow volume transport. Another difficulty in monitoring the overflow is measuring the plume thickness in the absence of hydrographic profiles. Volume flux estimates assuming a constant plume width and the thickness inferred from velocity profiles explain 58% of the modelled overflow volume flux variance and agrees to within 10% when averaged over the overflow season.
Modeling the effects of size on patch dynamics of an inert tracer
Authors: Xiu P. , Chai F.
Published: April 2010

Abstract. Mesoscale iron enrichment experiments have revealed that additional iron affects the phytoplankton productivity and carbon cycle. However, the role of initial size of fertilized patch in determining the patch evolution is poorly quantified due to the limited observational capability and complex of physical processes. Using a three-dimensional ocean circulation model, we simulated different sizes of inert tracer patches that were only regulated by physical circulation and diffusion. Model results showed that during the first few days since release of inert tracer, the calculated dilution rate was found to be a linear function with time, which was sensitive to the initial patch size with steeper slope for smaller size patch. After the initial phase of rapid decay, the relationship between dilution rate and time became an exponential function, which was also size dependent. Therefore, larger initial size patches can usually last longer and ultimately affect biogeochemical processes much stronger than smaller patches.
Stochastic Heterogeneity Mapping around a Mediterranean salt lens
Authors: Buffett G. G. , Hurich C. A. , Vsemirnova E. A. , Hobbs R. W. , Sallarès V. , Carbonell R. , Klaeschen D. , Biescas B.
Published: April 2010

Abstract. We present the first application of Stochastic Heterogeneity Mapping based on the band-limited von Kármán function to a seismic reflection stack of a Mediterranean water eddy (meddy), a large salt lens of Mediterranean water. This process extracts two stochastic parameters directly from the reflectivity field of the seismic data: the Hurst number, which ranges from 0 to 1, and the correlation length (scale length). Lower Hurst numbers represent a richer range of high wavenumbers and correspond to a broader range of heterogeneity in reflection events. The Hurst number estimate for the top of the meddy (0.39) compares well with recent theoretical work, which required values between 0.25 and 0.5 to model internal wave surfaces in open ocean conditions based on simulating a Garrett-Munk spectrum (GM76) slope of −2. The scale lengths obtained do not fit as well to seismic reflection events as those used in other studies to model internal waves. We suggest two explanations for this discrepancy: (1) due to the fact that the stochastic parameters are derived from the reflectivity field rather than the impedance field the estimated scale lengths may be underestimated, as has been reported; and (2) because the meddy seismic image is a two-dimensional slice of a complex and dynamic three-dimensional object, the derived scale lengths are biased to the direction of flow. Nonetheless, varying stochastic parameters, which correspond to different spectral slopes in the Garrett-Munk spectrum (horizontal wavenumber spectrum), can provide an estimate of different internal wave scales from seismic data alone. We hence introduce Stochastic Heterogeneity Mapping as a novel tool in physical oceanography.
Sensitivity of oxygen dynamics in the water column of the Baltic Sea to external forcing
Authors: Miladinova S. , Stips A.
Published: April 2010

Abstract. A 1-D biogeochemical/physical model of marine systems has been applied to study the oxygen cycle in four stations of different sub-basins of the Baltic Sea, namely, in the Gotland Deep, Bornholm, Arkona and Fladen. The model consists of the biogeochemical model of Neumann et al. (2002) coupled with the 1-D General Ocean Turbulence Model (GOTM). The model has been forced with meteorological data from the ECMWF reanalysis project for the period 1998–2003, producing a six year hindcast which is validated with datasets from the Baltic Environmental Database (BED) for the same period. The vertical profiles of temperature and salinity are relaxed towards both profiles provided by 3-D simulations of General Estuarine Transport Model (GETM) and observed profiles from BED. Modifications in the parameterisation of the air-sea oxygen fluxes have led to a significant improvement of the model results in the surface and intermediate water layers. The largest mismatch with observations is found in simulating the oxygen dynamics in the Baltic Sea bottom waters. The model results demonstrate the good capability of the model to predict the time-evolution of the physical and biogeochemical variables at all different stations. Comparative analysis of the modelled oxygen concentrations with respect to observation data is performed to distinguish the relative importance of several factors on the seasonal, interannual and long-term variations of oxygen. It is found that natural physical factors, like the magnitude of the vertical turbulent mixing, wind speed and the variation of temperature and salinity fields are the major factors controlling the oxygen dynamics in the Baltic Sea. The influence of limiting nutrients is less pronounced, at least under the nutrient flux parameterisation assumed in the model.
Interannual-to-decadal variability of North Atlantic air-sea CO2 fluxes
Authors: Raynaud S. , Aumont O. , Rodgers K. B. , Yiou P. , Orr J. C.
Published: April 2010

Abstract. The magnitude of the interannual variability of North Atlantic air-sea CO2 fluxes remains uncertain. Fluxes inferred from atmospheric inversions have large variability, whereas those simulated by ocean models have small variability. Part of the difference is that unlike typical atmospheric inversions, ocean models come with spatial resolution at the sub-basin scale. Here we explore sub-basin-scale spatiotemporal variability in the North Atlantic in one ocean model in order to better understand why the the North Atlantic basin may well contribute very little to the global variability of air-sea CO2 flux. We made two simulations with a biogeochemical model coupled to a global ocean general circulation model (OGCM), which itself was forced by 55-year NCEP reanalysis fields. In the first simulation, atmospheric CO2 was maintained at the preindustrial level (278 ppmv); in the second simulation, atmospheric CO2 followed the observed increase. Simulated air-sea CO2 fluxes and associated variables were analysed with a statistical tool known as multichannel singular spectrum analysis (MSSA). We found that the subtropical gyre is not the largest contributor to the overall, basin-wide variability, in contrast to previous suggestions. The subpolar gyre and the inter-gyre region (the transition area between subpolar and subtropical gyres) also contribute with multipolar anomalies at multiple frequencies: these tend to cancel one another in terms of the basin-wide air-sea CO2 flux. We found a strong correlation between the air-sea CO2 fluxes and the North Atlantic Oscillation (NAO), but only if one takes into account time lags as does MSSA (maximum r=0.64 for lags between 1 and 3 years). The contribution of anthropogenic CO2 to total variability was negligible at interannual time scales, whereas at the decadal (13-year) time scale, it increased variability by 30%.
The wind-driven overturning circulation of the World Ocean
Authors: Döös K.
Published: April 2010

Abstract. The wind driven aspects of the meridional overturning circulation of the world ocean and the Conveyor Belt is studied making use of a simple analytical model. The model consists of three reduced gravity layers with an inviscid Sverdrupian interior and a western boundary layer. The net north-south exchange is made possible by setting appropriate western boundary conditions, so that most of the transport is confined to the western boundary layer, while the interior is the Sverdrupian solution to the wind stress. The flow across the equator is made possible by the change of potential vorticity by the Rayleigh friction in the western boundary layer, which is sufficient to permit water and the Conveyor Belt to cross the equator. The cross-equatorial flow is driven by a weak meridional pressure gradient in opposite direction in the two layers on the equator at the western boundary. The model is applied to the World Ocean with a realistic wind stress. The amplitude of the Conveyor Belt is set by the northward Ekman transport in the Southern Ocean and the outcropping latitude of the NADW. It is in this way possible to set the amount of NADW that is pumped up from the deep ocean and driven northward by the wind and converted in the surface layer into less dense water by choosing the outcropping latitude and the depth of the layers at the western boundary. The model has proved to be able to simulate many of the key features of the Conveyor Belt and the meridional overturning cells of the World Ocean. This despite that there is no deep ocan mixing and that the water mass conversions in the this model are made at the surface.
Mechanisms controlling primary and new production in a global ecosystem model – Part II: the role of the upper ocean short-term periodic and episodic mixing events
Authors: Popova E. E. , Coward A. C. , Nurser G. A. , de Cuevas B. , Anderson T. R.
Published: April 2010

Abstract. The use of 6 h, daily, weekly and monthly atmospheric forcing resulted in dramatically different predictions of plankton productivity in a global 3-D coupled physical-biogeochemical model. Resolving the diurnal cycle of atmospheric variability by use of 6 h forcing, and hence also diurnal variability in UML depth, produced the largest difference, reducing predicted global primary and new production by 25% and 10% respectively relative to that predicted with daily and weekly forcing. This decrease varied regionally, being a 30% reduction in equatorial areas and 25% at moderate and high latitudes. A 10% increase in the primary production was seen in the peripheries of the oligotrophic gyres. By resolving the diurnal cycle, model performance was significantly improved with respect to several common problems: underestimated primary production in the oligotrophic gyres; overestimated primary production in the Southern Ocean; overestimated magnitude of the spring bloom in the subarctic Pacific Ocean, and overestimated primary production in equatorial areas. The result of using 6 h forcing on predicted ecosystem dynamics was profound, the effects persisting far beyond the hourly timescale, and having major consequences for predicted global and new production on an annual basis.
Modelling of the circulation in the Northwestern Mediterranean Sea with the Princeton Ocean Model
Authors: Ahumada M. A. , Cruzado A.
Published: April 2010

Abstract. The Princeton Ocean Model – POM (Blumberg and Mellor, 1987) has been implemented in the Northwestern Mediterranean nested (in one-way off-line mode) to a general circulation model of the Mediterranean Sea – OGCM (Pinardi and Masetti, 2000; Demirov and Pinardi, 2002) in order to investigate if this model configuration is capable of reproducing the major features of the circulation as known from observations and to improve what has been made by previous numerical modeling works. According to the model results, the large-scale cyclonic circulation in the northern part of the Northwestern Mediterranean is, at least in the upper layers, less coherent in winter and spring than in summer and autumn. Furthermore, there is evidence that the mesoscale structure (eddies and meanders) is, during all year, a significant dynamic characteristic in this region of the Mediterranean Sea. Finally, concerning the circulation in the lower layers has been confirmed that the Levantine Intermediate Water and the Western Mediterranean Deep Water follow essentially a cyclonic path during all year.
Eastern Mediterranean biogeochemical flux model: simulations of the pelagic ecosystem
Authors: Petihakis G. , Triantafyllou G. , Korres G. , Pollani A. , Hoteit I.
Published: April 2010

Abstract. During the second phase (2003–2006) of the Mediterranean ocean Forecasting System Project (MFS) named Toward Environmental Predictions (MFSTEP) one of the three major aims was the development of numerical forecasting systems. In this context a generic Biochemical Flux Model (BFM) was developed and coupled with hydrodynamic models already operating at basin scale as well as at regional areas. In the Eastern Mediterranean basin the BFM was coupled with the Aegean Levantine Eddy Resolving MOdel (ALERMO). The BFM is a generic highly complex model based on ERSEM and although a detailed description of the model and its sub models is beyond the scope of this work a short presentation of the main processes, paying emphasis on the parameter values used is presented. Additionally the performance of the model is evaluated with some preliminary results being qualitatively compared against field observations. The model at its present form is rather promising reproducing all major important features even though there are inefficiencies mostly related to primary and bacterial productivity rates.
Forecasting circulation in the Cilician Basin of the Levantine Sea
Authors: Özsoy E. , Sözer A.
Published: April 2010

Abstract. The Cilician Basin/Shelf Model is adapted for studying the shelf circulation in the Cilician Basin – Gulf of İskenderun region of the Levantine Basin of the Eastern Mediterranean between the Turkish Mediterranean coast, Syria and the island of Cyprus. The model initial conditions and open boundary conditions are supplied by the ALERMO regional model of the Levantine Sea, while interactive surface flux boundary conditions are specified by an atmospheric boundary layer sub-model using calculated water properties and surface atmospheric variables supplied by the Skiron atmospheric model, within the nested modelling approach of the MFSTEP (Mediterranean Forecasting System: Towards Environmental Predictions) project. Sensitivity tests are performed for alternative surface boundary conditions. Model performance for shelf/meso-scale forecasts is demonstrated.
Assimilation of ocean colour data into a Biochemical Flux Model of the Eastern Mediterranean Sea
Authors: Triantafyllou G. , Korres G. , Hoteit I. , Petihakis G. , Banks A. C.
Published: April 2010

Abstract. Within the framework of the European MFSTEP project, an advanced multivariate sequential data assimilation system has been implemented to assimilate real chlorophyll data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) into a three-dimensional biochemical model of the Eastern Mediterranean. The physical ocean is described through the Princeton Ocean Model (POM) while the biochemistry of the ecosystem is tackled with the Biochemical Flux Model (BFM). The assimilation scheme is based on the Singular Evolutive Extended Kalman (SEEK) filter, in which the error statistics were parameterized by means of a suitable set of Empirical Orthogonal Functions (EOFs). A radius of influence was further selected around every data point to limit the range of the EOFs spatial correlations. The assimilation experiment was performed for one year over 1999 and forced with ECMWF 6 hour atmospheric fields. The accuracy of the ecological state identification by the assimilation system is assessed by the relevance of the system in fitting the data, and through the impact of the assimilation on non-observed biochemical processes. Assimilation of SeaWiFS data significantly improves the forecasting capability of the BFM model. Results, however, indicate the necessity of subsurface data to enhance the controllability of the ecosystem model in the deep layers.
Assessment of the importance of the current-wave coupling in the shelf ocean forecasts
Authors: Jordà G. , Bolaños R. , Espino M. , Sánchez-Arcilla A.
Published: April 2010

Abstract. The effects of wave-current interactions on shelf ocean forecasts is investigated in the framework of the MFSTEP (Mediterranean Forecasting System Project Towards Enviromental Predictions) project. A one way sequential coupling approach is adopted to link the wave model (WAM) to the circulation model (SYMPHONIE). The coupling of waves and currents has been done considering four main processes: wave refraction due to currents, surface wind drag and bo€ttom drag modifications due to waves, and the wave induced mass flux. The coupled modelling system is implemented in the southern Catalan shelf (NW Mediterranean), a region with characteristics similar to most of the Mediterranean shelves. The sensitivity experiments are run in a typical operational configuration. The wave refraction by currents seems to be not very relevant in a microtidal context such as the western Mediterranean. The main effect of waves on current forecasts is through the modification of the wind drag. The Stokes drift also plays a significant role due to its spatial and temporal characteristics. Finally, the enhanced bottom friction is just noticeable in the inner shelf.
Structure of phytoplankton (Continuous Plankton Recorder and SeaWiFS) and impact of climate in the Northwest Atlantic Shelves
Authors: Leterme S. C. , Pingree R. D.
Published: April 2010

Abstract. All marine organisms are affected to some extent by the movement and thermal properties of oceanic currents. However phytoplankton, because of its small size is most directly coupled to the physical environment. The intense hydrodynamic activity observed in the Northwest Atlantic Shelves Province makes this region especially intriguing from the point of view of physical-biological interactions. In the present work, remote sensed data of Sea Surface Height (SSH) anomalies, Sea-surface chlorophyll a concentrations (SeaWiFS), and Sea Surface Temperature (SST) are used to complement the Continuous Plankton Recorder (CPR) survey that continuously sampled a route between Norfolk (Virginia, USA; 39° N, 71° W) and Argentia (Newfoundland; 47° N, 54° W) over the period 1995–1998. Over this period, we examined physical structures (i.e. SST and SSH) and climatic forcing associated with space-time phytoplankton structure. Along this route, the phytoplankton structures were mainly impacted by the changes in surface flow along the Scotian Shelf rather than significantly influenced by the mesoscale features of the Gulf Stream. These changes in water mass circulation caused a drop in temperature and salinity along the Scotian Shelf that induced changes in phytoplankton and zooplankton abundance.
MEDARGO: A drifting profiler program in the Mediterranean Sea
Authors: Poulain P.-M. , Barbanti R. , Font J. , Cruzado A. , Millot C. , Gertman I. , Griffa A. , Molcard A. , Rupolo V. , Le Bras S. , Petit de la Villeon L.
Published: April 2010

Abstract. In the framework of the EU-funded MFSTEP project, autonomous drifting profilers were deployed throughout the Mediterranean Sea to collect temperature and salinity profile data and to measure subsurface currents. The realization of this profiler program in the Mediterranean, referred to as MEDARGO, is described and assessed using data collected between June 2004 and March 2006 (including more than 1500 profiles). Recommendations are provided for the permanent future implementation of MEDARGO in support of operational oceanography in the Mediterranean Sea. More than twenty drifting profilers were deployed from research vessels and ships-of-opportunity in most areas of the Mediterranean. They were all programmed to execute 5-day cycles with drift at a neutral parking depth of 350 m and CTD profiles from either 700 or 2000 m up to the surface. They stayed at the sea surface for about 6 h to be localised by, and transmit the data to, the Argos satellite system. The temperature and salinity data obtained with pumped Sea-Bird CTD instruments were processed and made available to the scientific community and to operational users in near-real time using standard ARGO protocols, and were assimilated into Mediterranean numerical forecasting models. In general, the cycling and sampling characteristics chosen for the MEDARGO profilers were found to be adequate for the Mediterranean. However, it is strongly advised to use GPS and global cellular phone telemetry or the future Argos bi-directional satellite system in order to avoid data compression and losses, for the continuation of the Mediterranean drifting profiler program.
Daily oceanographic analyses by the Mediterranean basin scale assimilation system
Authors: Dobricic S. , Pinardi N. , Adani M. , Tonani M. , Fratianni C. , Bonazzi A. , Fernandez V.
Published: April 2010

Abstract. This study presents the upgrade of the Optimal Interpolation scheme used in the basin scale assimilation scheme of the Mediterranean Forecasting System. The modifications include a daily analysis cycle, the assimilation of ARGO float profiles, the implementation of the geostrophic balance in the background error covariance matrix and the initialisation of the analyses. A series of numerical experiments showed that each modification had a positive impact on the accuracy of the analyses: The daily cycle improved the representation of the processes with a relatively high temporal variability, the assimilation of ARGO floats profiles significantly improved the salinity analyses quality, the geostrophically balanced background error covariances improved the accuracy of the surface elevation analyses, and the initialisation removed the barotropic adjustment in the forecast first time steps starting from the analysis.
Implementation of position assimilation for ARGO floats in a realistic Mediterranean Sea OPA model and twin experiment testing
Authors: Taillandier V. , Griffa A.
Published: April 2010

Abstract. In this paper, a Lagrangian assimilation method is presented and implemented in a realistic OPA OGCM with the goal of providing an assessment of the assimilation of realistic Argo float position data. We focus on an application in the Mediterranean Sea, where in the framework of the MFSTEP project an array of Argo floats have been deployed with parking depth at 350 m and sampling interval of 5 days. In order to quantitatively test the method, the ''twin experiment'' approach is followed and synthetic trajectories are considered. The method is first tested using ''perfect'' data, i.e. without shear drift errors and with relatively high coverage. Results show that the assimilation is effective, correcting the velocity field at the parking depth, as well as the velocity profiles and the geostrophically adjusted mass field. We then consider the impact of realistic datasets, which are spatially sparse and characterized by shear drift errors. Such data provide a limited global correction of the model state, but they efficiently act on the location, intensity and shape of the described mesoscale structures of the intermediate circulation.
Study of the combined effects of data assimilation and grid nesting in ocean models – application to the Gulf of Lions
Authors: Vandenbulcke L. , Barth A. , Rixen M. , Alvera-Azcárate A. , Ben Bouallegue Z. , Beckers J. M.
Published: April 2010

Abstract. Modern operational ocean forecasting systems routinely use data assimilation techniques in order to take observations into account in the hydrodynamic model. Moreover, as end users require higher and higher resolution predictions, especially in coastal zones, it is now common to run nested models, where the coastal model gets its open-sea boundary conditions from a low-resolution global model. This configuration is used in the ''Mediterranean Forecasting System: Towards environmental predictions'' (MFSTEP) project. A global model covering the whole Mediterranean Sea is run weekly, performing 1 week of hindcast and a 10-day forecast. Regional models, using different codes and covering different areas, then use this forecast to implement boundary conditions. Local models in turn use the regional model forecasts for their own boundary conditions. This nested system has proven to be a viable and efficient system to achieve high-resolution weekly forecasts. However, when observations are available in some coastal zone, it remains unclear whether it is better to assimilate them in the global or local model. We perform twin experiments and assimilate observations in the global or in the local model, or in both of them together. We show that, when interested in the local models forecast and provided the global model fields are approximately correct, the best results are obtained when assimilating observations in the local model.
Operational coastal ocean forecasting in the Eastern Mediterranean: implementation and evaluation
Authors: Zodiatis G. , Lardner R. , Hayes D. R. , Georgiou G. , Sofianos S. , Skliris N. , Lascaratos A.
Published: April 2010

Abstract. The Cyprus Coastal Ocean Forecasting and Observing System (CYCOFOS) has been producing operational flow forecasts of the northeastern Levantine Basin since 2002 and has been substantially improved in 2005. It is the first system in the Mediterranean to produce a forecast every day at the coastal scale. CYCOFOS uses a the POM (Princeton Ocean Model) flow model, and recently, within the frame of the MFSTEP project (Mediterranean Forecasting System, Toward Environmental Prediction), the flow model was upgraded to use the hourly SKIRON atmospheric forcing, and its resolution was increased from 2.5 km to 1.8 km. The CYCOFOS model is now nested in the ALERMO (Aegean Levantine Eddy Resolving Model) regional model from the University of Athens, which is nested within the MFS (Mediterranean Forecasting System) basin model. The Variational Initialization and FOrcing Platform (VIFOP) has been implemented to reduce the numerical transient processes following initialization. Moreover, a five-day forecast is repeated every day, providing more detailed and more accurate information, of particular value to coastal end users. Forecast results are posted on the web page http://www.ucy.ac.cy/cyocean. The new, daily, high-resolution forecasts agree exceptionally well with the ALERMO regional model. The agreement is better and results more reasonable when VIFOP is used. Active and slave experiments suggest that a four-week active period produces realistic results with more small-scale features. Bias with respect to the slave mode is negligible and there is no detectable bias with remote sensing images (for September, 2004). In situ observed hydrographic data from south of Cyprus are similar in many ways to the corresponding forecast fields. Plans for further model improvement include assimilation of observed temperature profiles (XBT), conductivity-temperature-depth (CTD) profiles from drifters or research cruises, and CT data from the CYCOFOS ocean observatory.
Reduced-order optimal interpolation for biomass assimilation
Authors: Crispi G. , Pacciaroni M. , Viezzoli D.
Published: April 2010

Abstract. The analysis of the evolution of the chemical and biological characteristics of the Mediterranean marine ecosystem requires an integrated approach. Consistently, an ecosystem description, embedded in the MFSTEP one eighth degree three-dimensional general circulation model, is tested and used following a twin experiment approach. The ecosystem model is based on the NPZD trophic chain: inorganic nitrogen, N, phytoplankton, P, zooplankton, Z and detritus, D. Assimilation of synthetic biomass data is performed by means of the reduced-order optimal interpolation system SOFA. The synthetic "sea-truth" data are daily averages obtained from a sixty-nine days reference run (RR). The twin experiments consist in performing two runs: a free run (FR) with wrong summer-depleted phytoplankton initial conditions and an assimilated run (AR), in which, starting from the same FR wrong phytoplankton concentrations, weekly averaged surface biomasses extracted from the RR results are assimilated. The comparison of the FR results with the AR ones shows a good convergence, on a basin Mediterranean scale, confirming improvements of the forecasting in each of the four ecological compartments. Regional trophic regimes are analysed and interpreted in the western and eastern Mediterranean subbasins, for explaining the deteriorating behaviour of the total nitrogen.
Transformation of Levantine Intermediate Water tracked by MedArgo floats in Western Mediterranean
Authors: Emelianov M. , Font J. , Turiel A. , Solé J. , Poulain P. , Julià A. , Vitrià M. R.
Published: April 2010

Abstract. A clustering methodology is applied to investigate the thermohaline structure of Levantine Intermediate Water (LIW) in the western Mediterranean basin. 16 free-drifting hydrographic profilers were deployed in the framework of the MFSTEP project (MedArgo component) from September 2003. A total of 925 CTD profiles data collected until the beginning of February 2006 have been used in the analysis. The results are in good agreement with the general circulation scheme for intermediate waters in the basin and confirm the hypothesis about a "discrete-continuous" thermohaline structure of LIW.
Assessment of the impact of TS assimilation from ARGO floats in the Mediterranean Sea
Authors: Griffa A. , Molcard A. , Raicich F. , Rupolo V.
Published: April 2010

Abstract. In this paper, the impact of assimilating Temperature (T) and Salinity (S) profiles from Argo floats in the Mediterranean Sea (MEDARGO) is quantitatively investigated using the Observing System Simulation Experiments (OSSE) approach. The impact of varying the number of floats and their launch positions is considered, using numerical simulations with a MOM model and a reduced-order multivariate Optimal Interpolation scheme (SOFA) for assimilation. Realistic launch positions used during the first MFSTEP phase are considered, as well as ''ideal'' positions that can be envisioned for the future, along the VOS tracks. The most effective float trajectories are identified, showing that frontal regions play a major role, and that it is crucial to maintain a sufficient coverage of them. In addition to this, also a qualitative comparison is performed between the results obtained from MEDARGO floats in ideal conditions and results from ''ideal'' profiles taken along the VOS (Volunteer Observing Ships) tracks, as for the XBT (Expandable Baththermograph) data.
15N enrichment in the surface Particulate Organic Nitrogen of the north-eastern Arabian Sea from the middle to the waning phase of the winter monsoon: possible causes
Authors: Kumar S. , Ramesh R.
Published: April 2010

Abstract. A temporal increase of ~5‰ in the average nitrogen isotopic composition (δ15NPON) of surface particulate organic nitrogen was observed in the open north-eastern Arabian Sea during January to late February-early March 2003, despite the presence of T. erithraeum (up to ~11%), a diazotroph that fixes atmospheric N2, in the latter period. Hydrographic conditions and residence time of nitrate in the water column suggested that this increase could be a combined effect of denitrification in the subsurface layer and inefficient utilization of nitrate entrained in the water column during January.
Unpredictability of internal M2
Authors: van Haren H.
Published: April 2010

Abstract. Current observations from a shelf sea, continental slopes and the abyssal North-East Atlantic Ocean are all dominated by the semidiurnal lunar (M2) tide. It is shown that motions at M2 vary at usually large barotropic and coherent baroclinic scales, >50 km horizontally and >0.5 H vertically. H represents the waterdepth. Such M2-scales are observed even close to topography, the potential source of baroclinic, "internal" tidal waves. In contrast, incoherent small-scale, ~10 km horizontally and ~0.1 H vertically, baroclinic motions are dominated around f, the local inertial frequency, and/or near 2Ω≈S2, the semidiurnal solar tidal frequency. Ω represents the Earth's rotational vector. This confirms earlier suggestions that small-scale baroclinic M2-motions generally do not exist in the ocean in any predictable manner, except in beams very near (<10 km horizontally) to their source. As a result, M2-motions are not directly important for generating shear and internal wave induced mixing in the ocean. Indirectly however, they may contribute to ocean mixing if transfer to small-scale motions at f and/or S2 can be proven. Also far from topography, small-scale motions are found at either or both of the latter frequencies. Different suggestions for the scales at these particular frequencies are discussed, ranging from the variability of "background" density gradients and associated divergence and focusing of internal wave rays to the removal of the internal tidal energy by non-linear interactions. It is noted that near f and S2 the short-wave inertio-gravity wave bounds are found in the limit of very weak stratification, which are often observed in small-scale near-homogeneous layers.
Comparing the steric height in the Northern Atlantic with satellite altimetry
Authors: Ivchenko V. O. , Danilov S. D. , Sidorenko D. V. , Schroeter J. , Wenzel M. , Aleynik D. L.
Published: April 2010

Abstract. Anomalies of dynamic height derived from an analysis of Argo profiling buoys data are analysed to assess the relative roles of contributions from temperature and salinity over the North Atlantic for the period of 1999–2004. They are compared with dynamic topography anomalies based on TOPEX/Poseidon and Jason altimetry. It is shown that the halosteric contribution to the anomalies of dynamic height is comparable in magnitude to the thermosteric one over the period analyzed. Taking both salinity and temperature into account improves the agreement between zonally averaged trends in the satellite dynamic topography and dynamic height increasing the correlation between them to 0.73 from 0.63 when only temperature variability is taken into account. The implication of this result is that the salinity contribution cannot be neglected in the North Atlantic and that one cannot rely on estimating the thermosteric part by anomalies in the sea surface dynamic topography derived from the satellite altimetry.
Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current
Authors: Treguier A. M. , England M. , Rintoul S. R. , Madec G. , Le Sommer J. , Molines J.-M.
Published: April 2010

Abstract. An eddying global model is used to study the characteristics of the Antarctic Circumpolar Current (ACC) in a streamline-following framework. In the upper layers, the meridional circulation across streamlines agrees with the theoretical view: an equatorward mean flow partially cancelled by a poleward eddy mass flux. The same calculation in a zonal average gives a completely different view and underestimates the eddy effects. Two model simulations, in which the buoyancy forcing above the ACC changes from positive to negative, suggest that the relationship between the residual meridional circulation and the surface buoyancy flux is not as straightforward as assumed by some recent theoretical studies: even the sign of the residual circulation cannot be inferred from the buoyancy forcing. Heat and salt transports by the time-mean flow are important even in the streamline framework. Streamline-averaged, two-dimensional models cannot account quantitatively for the complex three-dimensional structure of the ACC. Heat and salt are balanced in the ACC, the model drift being small, but the nonlinearity of the equation of state cannot be ignored in the density balance.
Observations of new western Mediterranean deep water formation using ARGO floats 2004–2006
Authors: Smith R. O. , Bryden H. L.
Published: April 2010

Abstract. The deep convection that occurs in the western basin of the Mediterranean Sea was investigated using ARGO float data over two consecutive winters in 2004–2005 and 2005–2006. The results showed deep mixed layers reaching 2000 m in surprising locations, namely the eastern Catalan subbasin (39.785° N, 4.845° E) and the western Ligurian subbasin (43.392° N, 7.765° E). Subsequently, new deep water was formed in March of 2005 and 2006 with θ=12.89–12.92°C, S=38.48–38.49 and σθ=29.113 kg m−3. The deep water produced in the Ligurian subbasin during 2006 was more saline, warmer and denser than any historical observations of Western Mediterranean Deep Water. The results show S, θ and σθ in the Western Mediterranean Deep Water are higher than 1990s values, with a salinity increase of 1.5×10−3 yr−1, a temperature increase of 3.6×10−3°C yr−1 and a density increase of 4.0×10−4 kg m−3 yr−1 apparent from a dataset of WMDW properties spanning 1955–2006.
On available energy in the ocean and its application to the Barents Sea
Authors: Levine R. C. , Webb D. J.
Published: April 2010

Abstract. Following meteorological practice the definition of available potential energy in the ocean is conventionally defined in terms of the properties of the global ocean. However there is also a requirement for a localised definition, for example the energy released when shelf water cascades down a continental shelf in the Arctic and enters a boundary current. In this note we start from first principals to obtain an exact expression for the available energy (AE) in such a situation. We show that the available energy depends on enstrophy and gravity. We also show that it is exactly equal to the work done by the pressure gradient and by buoyancy. The results are used to investigate the distribution of AE in the Barents Sea and surrounding regions relative to the interior of the Arctic Ocean. We find that water entering the Barents Sea from the Atlantic already has a high AE, that it is increased by cooling but that much of the increase is lost overcoming turbulence during the passage through the region to the Arctic Ocean. However on entering the Arctic enough available energy remains to drive a significant current around the margin of the ocean. The core of raised available energy also acts as a tracer which can be followed along the continental slope beyond the dateline.
Retroflection from slanted coastlines – circumventing the "vorticity paradox"
Authors: Zharkov V. , Nof D.
Published: April 2010

Abstract. The balance of long-shore momentum flux requires that the solution of retroflecting currents involves ring shedding on the western side. An important aspect of the ring dynamics is the ring intensity α (analogous to the Rossby number), which reaches its maximum value of unity when the upstream potential vorticity (PV) is zero. Friction leads to a slow-down and a decrease in α. The main difficulty is that the solution of the system of equations for conservation of mass and momentum of zonal currents leads to the conclusion that the ratio (Φ) of the mass flux going into the rings and the total incoming mass flux is approximately 4α/(1+2α). This yields the "vorticity paradox" – only relatively weak rings (α≤1/2) could satisfy the necessary condition Φ≤1. Physically, this means, for example, that the momentum-flux of zero PV currents upstream is so high that, no matter how many rings are produced and no matter what size they are, they cannot compensate for it. To avoid this paradox, we develop a nonlinear analytical model of retroflection from a slanted non-zonal coastline. We show that when the slant of coastline (γ) exceeds merely 15°, Φ does not reach unity regardless of the value of α. Namely, the paradox disappears even for small slants. Our slowly varying nonlinear solution does not only let us circumvent the paradox. It also gives a detailed description of the rings growth rate and the mass flux going into the rings as a function of time. For example, in the case of zero PV and zero thickness of the upper layer along the coastline, the maximal values of Φ can be approximately expressed as, 1.012+0.32 exp (-γ/3.41)-γ/225. Interestingly, for significant slants (γ≥30°), the rings reach a terminal size corresponding to a balance between the β-force and the upstream momentum flux. This terminal size is unrelated to the ultimate detachment and westward drift due to β. Our analytical solutions are in very good agreement with the results of a numerical model which we run.