Characterization of mixing errors in a coupled physical biogeochemical model of the North Atlantic: implications for nonlinear estimation using Gaussian anamorphosis

In biogeochemical models coupled to ocean circulation models, vertical mixing is an important physical process which governs the nutrient supply and the plankton residence in the euphotic layer. However, vertical mixing is often poorly represented in numerical simulations because of approximate parameterizations of sub-grid scale turbulence, wind forcing errors and other mis-represented processes such as restratification by mesoscale eddies. Getting a sufficient knowledge of the nature and structure of these errors is necessary to implement appropriate data assimilation methods and to evaluate if they can be controlled by a given observation system. <br><br> In this paper, Monte Carlo simulations are conducted to study mixing errors induced by approximate wind forcings in a three-dimensional coupled physical-biogeochemical model of the North Atlantic with a 1/4° horizontal resolution. An ensemble forecast involving 200 members is performed during the 1998 spring bloom, by prescribing perturbations of the wind forcing to generate mixing errors. The biogeochemical response is shown to be rather complex because of nonlinearities and threshold effects in the coupled model. The response of the surface phytoplankton depends on the region of interest and is particularly sensitive to the local stratification. In addition, the statistical relationships computed between the various physical and biogeochemical variables reflect the signature of the non-Gaussian behaviour of the system. It is shown that significant information on the ecosystem can be retrieved from observations of chlorophyll concentration or sea surface temperature if a simple nonlinear change of variables (anamorphosis) is performed by mapping separately and locally the ensemble percentiles of the distributions of each state variable on the Gaussian percentiles. The results of idealized observational updates (performed with perfect observations and neglecting horizontal correlations) indicate that the implementation of this anamorphosis method into sequential assimilation schemes can substantially improve the accuracy of the estimation with respect to classical computations based on the Gaussian assumption

Assimilation of HF radar surface currents to optimize forcing in the northwestern Mediterranean Sea

HF radar measurements are used to optimize surface wind forcing and baroclinic open boundary condition forcing in order to constrain model coastal surface currents. This method is applied to a northwestern Mediterranean (NWM) regional primitive equation model configuration. A new radar data set, provided by two radars deployed in the Toulon area (France), is used. To our knowledge, this is the first time that radar measurements of the NWM Sea are assimilated into a circulation model. Special attention has been paid to the improvement of the model coastal current in terms of speed and position. The data assimilation method uses an ensemble Kalman smoother to optimize forcing in order to improve the model trajectory. Twin experiments are initially performed to evaluate the method skills. Real measurements are then fed into the circulation model and significant improvements to the modeled surface currents, when compared to observations, are obtained

Visualizations of the disturbed-laminar wave-induced flow above a rippled bed

This paper discusses visualizations of wave-induced flow over a rippled bed. Experiments were conducted in a wave tank fitted with a rigid rippled bed, and flow visualizations were carried out using a fluorescent dye filmed by a digital high speed video camera. Secondary flow regimes are classified in terms of key parameters such as the ripple slope, the ratio of the amplitude of the external flow to the ripple wavelength, and a Taylor number. For weak oscillations over gentle ripples, two-dimensional structures develop in the form of large recirculation cells, while for stronger flows over medium to steep ripples these are modified by the onset of separation and vortex shedding. Three-dimensional instabilities lead to disturbed-laminar flow structures of two different forms. The most common and stable form is a structure of rings that has a well defined transverse wavelength which is found to be inversely proportional to a Taylor number. The other form, a brick pattern, is more transient in nature but is probably also related to the development of three-dimensional ripple shapes

Closing the loop Approaches to monitoring the state of the Arctic Mediterranean during the International Polar Year 20072008

During the 4th International Polar Year 2007–2009 (IPY), it has become increasingly obvious that we need to prepare for a new era in the Arctic. IPY occurred during the time of the largest retreat of Arctic sea ice since satellite observations started in 1979. This minimum in September sea ice coverage was accompanied by other signs of a changing Arctic, including the unexpectedly rapid transpolar drift of the Tara schooner, a general thinning of Arctic sea ice and a double-dip minimum of the Arctic Oscillation at the end of 2009. Thanks to the lucky timing of the IPY, those recent phenomena are well documented as they have been scrutinized by the international research community, taking advantage of the dedicated observing systems that were deployed during IPY. However, understanding changes in the Arctic System likely requires monitoring over decades, not years. Many IPY projects have contributed to the pilot phase of a future, sustained, observing system for the Arctic. We now know that many of the technical challenges can be overcome. The Norwegian projects iAOOS-Norway, POLEWARD and MEOP were significant ocean monitoring/research contributions during the IPY. A large variety of techniques were used in these programs, ranging from oceanographic cruises to animal-borne platforms, autonomous gliders, helicopter surveys, surface drifters and current meter arrays. Our research approach was interdisciplinary from the outset, merging ocean dynamics, hydrography, biology, sea ice studies, as well as forecasting. The datasets are tremendously rich, and they will surely yield numerous findings in the years to come. Here, we present a status report at the end of the official period for IPY. Highlights of the research include: a quantification of the Meridional Overturning Circulation in the Nordic Seas (“the loop”) in thermal space, based on a set of up to 15-year-long series of current measurements; a detailed map of the surface circulation as well as characterization of eddy dispersion based on drifter data; transport monitoring of Atlantic Water using gliders; a view of the water mass exchanges in the Norwegian Atlantic Current from both Eulerian and Lagrangian data; an integrated physical–biological view of the ice-influenced ecosystem in the East Greenland Current, showing for instance nutrient-limited primary production as a consequence of decreasing ice cover for larger regions of the Arctic Ocean. Our sea ice studies show that the albedo of snow on ice is lower when snow cover is thinner and suggest that reductions in sea ice thickness, without changes in sea ice extent, will have a significant impact on the arctic atmosphere. We present up-to-date freshwater transport numbers for the East Greenland Current in the Fram Strait, as well as the first map of the annual cycle of freshwater layer thickness in the East Greenland Current along the east coast of Greenland, from data obtained by CTDs mounted on seals that traveled back and forth across the Nordic Seas. We have taken advantage of the real-time transmission of some of these platforms and demonstrate the use of ice-tethered profilers in validating satellite products of sea ice motion, as well as the use of Seagliders in validating ocean forecasts, and we present a sea ice drift product – significantly improved both in space and time – for use in operational ice-forecasting applications. We consider real-time acquisition of data from the ocean interior to be a vital component of a sustained Arctic Ocean Observing System, and we conclude by presenting an outline for an observing system for the European sector of the Arctic Ocean

Strumenti e indicazioni di governance transfrontaliera. Piano d’azione e linee guida del progetto IMPACT [Instruments et indications pour la gouvernance transfrontalière. Plan d’action et lignes directrices du projet IMPACT]

Il presente documento restituisce una sintesi delle attività del progetto IMPACT ed è diviso in due parti. La prima parte evidenzia gli strumenti completati durante la durata del progetto, in particolare la rete di radar costieri per il monitoraggio delle correnti marine e la piattaforma webGIS per la consultazione dei dati raccolti ed elaborati dai partner di progetto. La seconda parte capitalizza sui suddetti strumenti per fornire sia indicazioni metodologiche che di governance. Nello specifico, la rete di radar costieri viene utilizzata per creare mappe di potenziale contaminazione e indicare in che condizioni le attività portuali possano essere più sostenibili. Le misure demografiche e i calcoli di ritenzione indicano il livello d’efficacia delle dimensioni attuali delle AMP mentre le misure di contaminazione suggeriscono di considerare un numero maggiore di stazioni nell’ottica di migliorare i piani di monitoraggio esistenti. In quest’ottica, le indicazioni rappresentano il piano d’azione e le linee guida del progetto. L’ulteriore espansione della rete di radar costieri prevista nei progetti SICOMAR plus e SINAPSI è la prova che la conclusione delle attività del progetto IMPACT sia in realtà solo un importante punto di partenzaCe document donne un résumé des activités du projet IMPACT et est divisé en deux parties. La première partie met en évidence les instruments réalisés pendant la durée du projet, en particulier le réseau de radars côtiers pour la surveillance des courants marins et la plate-forme webGIS pour la consultation des données collectées et traitées par les partenaires du projet. La deuxième partie s’appuie sur ces outils pour fournir des orientations à la fois méthodologiques et de gouvernance. Plus précisément, le réseau de radars côtiers est utilisé pour créer des cartes de contamination potentielle et indiquer dans quelles conditions les activités portuaires peuvent être plus durables. Les mesures démographiques et les calculs de rétention indiquent le niveau d’efficacité de la taille actuelle des AMP, tandis que les mesures de contamination suggèrent que davantage de stations devraient être envisagées en vue d’améliorer les plans de surveillance existants. Dans cette perspective, les indications représentent le plan d’action et les lignes directrices du projet. La poursuite de l’extension du réseau de radars côtiers prévue dans les projets SICOMAR plus et SINAPSI est la preuve que la conclusion des activités du projet IMPACT n’est en fait qu’un point de départ important