ALBERT

Beschreibung: Towards a regional ocean forecasting system for the IBI (Iberia-Biscay-Ireland area): developments and improvements within the ECOOP project framework Ocean Science, 8, 143-159, 2012 Author(s): S. Cailleau, J. Chanut, J.-M. Lellouche, B. Levier, C. Maraldi, G. Reffray, and M. G. Sotillo The regional ocean operational system remains a key element in downscaling from large scale (global or basin scale) systems to coastal ones. It enables the transition between systems in which the resolution and the resolved physics are quite different. Indeed, coastal applications need a system to predict local high frequency events (inferior to the day) such as storm surges, while deep sea applications need a system to predict large scale lower frequency ocean features. In the framework of the ECOOP project, a regional system for the Iberia-Biscay-Ireland area has been upgraded from an existing V0 version to a V2. This paper focuses on the improvements from the V1 system, for which the physics are close to a large scale basin system, to the V2 for which the physics are more adapted to shelf and coastal issues. Strong developments such as higher regional physics resolution in the NEMO Ocean General Circulation Model for tides, non linear free surface and adapted vertical mixing schemes among others have been implemented in the V2 version. Thus, regional thermal fronts due to tidal mixing now appear in the latest version solution and are quite well positioned. Moreover, simulation of the stratification in shelf areas is also improved in the V2.

Beschreibung: Technical Note: Detection of gas bubble leakage via correlation of water column multibeam images Ocean Science, 8, 175-181, 2012 Author(s): J. Schneider von Deimling and C. Papenberg Hydroacoustic detection of natural gas release from the seafloor has been conducted in the past by using singlebeam echosounders. In contrast, modern multibeam swath mapping systems allow much wider coverage, higher resolution, and offer 3-D spatial correlation. Up to the present, the extremely high data rate hampers water column backscatter investigations and more sophisticated visualization and processing techniques are needed. Here, we present water column backscatter data acquired with a 50 kHz prototype multibeam system over a period of 75 seconds. Display types are of swath-images as well as of a "re-sorted" singlebeam presentation. Thus, individual and/or groups of gas bubbles rising from the 24 m deep seafloor clearly emerge in the acoustic images, making it possible to estimate rise velocities. A sophisticated processing scheme is introduced to identify those rising gas bubbles in the hydroacoustic data. We apply a cross-correlation technique adapted from particle imaging velocimetry (PIV) to the acoustic backscatter images. Temporal and spatial drift patterns of the bubbles are assessed and are shown to match very well to measured and theoretical rise patterns. The application of this processing to our field data gives clear results with respect to unambiguous bubble detection and remote bubble rise velocimetry. The method can identify and exclude the main source of misinterpretations, i.e. fish-mediated echoes. Although image-based cross-correlation techniques are well known in the field of fluid mechanics for high resolution and non-inversive current flow field analysis, we present the first application of this technique as an acoustic bubble detector.

Beschreibung: Black Sea coastal forecasting system Ocean Science, 8, 183-196, 2012 Author(s): A. I. Kubryakov, G. K. Korotaev, V. L. Dorofeev, Y. B. Ratner, A. Palazov, N. Valchev, V. Malciu, R. Matescu, and T. Oguz The Black Sea coastal nowcasting and forecasting system was built within the framework of EU FP6 ECOOP (European COastalshelf sea OPerational observing and forecasting system) project for five regions: the south-western basin along the coasts of Bulgaria and Turkey, the north-western shelf along the Romanian and Ukrainian coasts, coastal zone around of the Crimea peninsula, the north-eastern Russian coastal zone and the coastal zone of Georgia. The system operates in the real-time mode during the ECOOP project and afterwards. The forecasts include temperature, salinity and current velocity fields. Ecosystem model operates in the off-line mode near the Crimea coast.

Beschreibung: Sea surface temperature anomalies, seasonal cycle and trend regimes in the Eastern Pacific coast Ocean Science, 8, 81-90, 2012 Author(s): A. Ramos-Rodríguez, D. B. Lluch-Cota, S. E. Lluch-Cota, and A. Trasviña-Castro We used the extended reconstruction of sea surface temperature (ERSST) to analyze the variation of surface temperature and the seasonal cycle along the coast of the eastern Pacific (60° N–60° S, 61 pixels alongshore) from 1950 to 2010 (732 months). First, we analyzed the monthly anomalies and looked for a relationship of such anomalies with total solar irradiance (TSI) and then the Regime Shift Detector (RSD) was applied to detect possible temperature regimes in the series. Afterwards, we calculated a yearly temperature range per pixel (amplitude of seasonal cycle) and through the subtraction of a latitudinal theoretical curve of temperature based on solar irradiance, the residuals of the seasonal cycle were obtained. The results showed an almost complete spatial synchrony and dominance of negative anomalies from 1950 to mid-late 1970's, with a switch to near-zero and positive anomalies that lasted up to late 1990's when a new shift to negative values was detected. Such a shift lasted until the early 2000's when positive anomalies appeared again but there was a change to negative anomalies in the late 2000's. These results were supported by the RSD. The TSI variability shows a clear relationship with that of sea surface temperature anomalies and with the regime changes. This is probably due to a difference in the amount of energy received from the sun. Comparing the "cool regime" versus the "warm regime", the second one received 0.39% more energy (approximately 3 × 10 8 J m −2 ) from the sun. Seasonal cycles show larger ranges at northern latitudes (〉40° N), northern tropical-temperate transition zone (20°–26° N) and in the tropical-equatorial band (0°–30° S). The smallest ranges occur at 0°–16° N and 50°–60° S. The residuals (seasonal minus the theoretical curve) indicated a clear modulation due to advection by ocean currents.

Beschreibung: Microstructure observations during the spring 2011 STRATIPHYT-II cruise in the northeast Atlantic Ocean Science, 8, 945-957, 2012 Author(s): E. Jurado, H. A. Dijkstra, and H. J. van der Woerd Small-scale temperature and conductivity variations have been measured in the upper 100 m of the northeast Atlantic during the STRATIPHYT-II cruise (Las Palmas–Reykjavik, 6 April–3 May 2011). The measurements were done at midday and comprised 2 to 15 vertical profiles at each station. The derived turbulent quantities show a transition between weakly-stratified (mixed layer depth, MLD, 100), which was centered at about 48° N. The temperature eddy diffusivities, K T , range from 10 −5 to 10 0 m 2 s −1 in the weakly-stratified stations, and range from 3 × 10 −4 to 2 × 10 0 m 2 s −1 in the well-mixed stations. The turbulent kinetic energy dissipation rates, ε, range from 3 × 10 −8 to 2 × 10 −6 m 2 s −3 south of the transition zone, and from 10 −7 to 10 −5 m 2 s −3 north of the transition zone. The station-averaged K T values throughout the mixed layer increase exponentially with the wind speed. The station-averaged ε values throughout the mixed layer scale with the wind stress similarity variable with a scaling factor of about 1.8 in the wind-dominated stations (ε ≈ 1.8 u ☆ 3 /(−κ z )). The values of K T and ε are on average 10 times higher compared to the values measured at the same stations in July 2009. The results presented here constitute a unique data set giving large spatial coverage of upper ocean spring turbulence quantities.

Beschreibung: Impact of the sea surface temperature forcing on hindcasts of Madden-Julian Oscillation events using the ECMWF model Ocean Science, 8, 1071-1084, 2012 Author(s): E. de Boisséson, M. A. Balmaseda, F. Vitart, and K. Mogensen This paper explores the sensitivity of hindcasts of the Madden Julian Oscillation (MJO) to the use of different sea surface temperture (SST) products as lower boundary conditions in the European Centre for Medium-range Weather Forecasts (ECMWF) atmospheric model. Three sets of monthly hindcast experiments are conducted, starting from initial conditions from the ERA interim reanalysis. First, as a reference, the atmosphere is forced by the SST used to produce ERA interim. In the second and third experiments, the SST is switched to the OSTIA (Operational Sea Surface Temperature and Sea-Ice Analysis) and the AVHRR-only (Advanced Very High Resolution Radiometer) reanalyses, respectively. Tests on the temporal resolution of the SST show that monthly fields are not optimal, while weekly and daily resolutions provide similar MJO scores. When using either OSTIA or AVHRR, the propagation of the MJO is degraded and the resulting scores are lower than in the reference experiment. Further experiments show that this loss of skill cannot be attributed to either the difference in mean state or temporal variability between the SST products. Additional diagnostics show that the phase relationship between either OSTIA or AVHRR SST and the MJO convection is distorted with respect to satellite observations and the ERA interim reanalysis. This distortion is expected to impact the MJO hindcasts, leading to a relative loss of forecast skill. A realistic representation of ocean–atmosphere interactions is thus needed for MJO hindcasts, but not all SST products – though accurate for other purposes – fulfill this requirement.

Beschreibung: Imbalance of energy and momentum source terms of the sea wave transfer equation for fully developed seas Ocean Science, 8, 1085-1098, 2012 Author(s): G. V. Caudal In the concept of full development, the sea wave spectrum is regarded as a nearly stationary solution of the wave transfer equation, where source and sink terms should be in balance with respect to both energy and momentum. Using a two-dimensional empirical sea wave spectral model at full development, this paper performs an assessment of the compatibility of the energy and momentum budgets of sea waves over the whole spectral range. Among the various combinations of model functions for wave breaking and wind source terms tested, not one is found to fulfill simultaneously the energy and momentum balance of the transfer equation. Based on experimental and theoretical grounds, wave breaking is known to contribute to frequency downshift of a narrow-banded wave spectrum when the modulational instability is combined with wave breaking. On those grounds, it is assumed that, in addition to dissipation, wave breaking produces a spectral energy flux directed toward low wavenumbers. I show that it is then possible to remove the energy and momentum budget inconsistency, and correspondingly the required strength of this spectral flux is estimated. Introducing such a downward spectral flux permits fulfilling both energy and momentum balance conditions. Meanwhile, the consistency between the transfer equation and empirical spectra, estimated by means of a cost function K, is either improved or slightly reduced, depending upon the wave breaking and wind source terms chosen. Other tests are performed in which it is further assumed that wave breaking would also be associated with azimuthal diffusion of the spectral energy. This would correspondingly reduce the required downward spectral flux by a factor of up to 5, although it would not be able to remove it entirely.

Beschreibung: High frequency fluctuations in the heat content of an ocean general circulation model Ocean Science, 8, 813-825, 2012 Author(s): A. M. Huerta-Casas and D. J. Webb The transport and storage of heat by the ocean is of crucial importance because of its effect on ocean dynamics and its impact on the atmosphere, climate and climate change. Unfortunately, limits to the amount of data that can be collected and stored mean that many experimental and modelling studies of the heat budget have to make use of mean datasets where the effects of short term fluctuations are lost. In this paper we investigate the magnitude of the resulting errors by making use of data from OCCAM, a high resolution global ocean model. The model carries out a proper heat balance every time step so any imbalances that are found in the analysis must result from the use of mean fields. The study concentrates on two areas of the ocean affecting the El Nino. The first is the region of tropical instability waves north of the Equator. The second is in the upwelling region along the Equator. It is shown that in both cases, processes with a period of less than five days can have a significant impact on the heat budget. Thus, analyses using data averaged over five days or more are likely to have significant errors. It is also shown that if a series of instantaneous values is available, reasonable estimates can be made of the size of the errors. In model studies, such values are available in the form of the datasets used to restart the model. In experimental studies they may be in the form of individual unaveraged observations.

Beschreibung: A 20-year reanalysis experiment in the Baltic Sea using three-dimensional variational (3DVAR) method Ocean Science, 8, 827-844, 2012 Author(s): W. Fu, J. She, and M. Dobrynin A 20-year retrospective reanalysis of the ocean state in the Baltic Sea is constructed by assimilating available historical temperature and salinity profiles into an operational numerical model with three-dimensional variational (3DVAR) method. To determine the accuracy of the reanalysis, the authors present a series of comparisons to independent observations on a monthly mean basis. In the reanalysis, temperature (T) and salinity (S) fit better with independent measurements than the free run at different depths. Overall, the mean biases of temperature and salinity for the 20 year period are reduced by 0.32 °C and 0.34 psu, respectively. Similarly, the mean root mean square error (RMSE) is decreased by 0.35 °C for temperature and 0.3 psu for salinity compared to the free run. The modeled sea surface temperature, which is mainly controlled by the weather forcing, shows the least improvements due to sparse in situ observations. Deep layers, on the other hand, witness significant and stable model error improvements. In particular, the salinity related to saline water intrusions into the Baltic Proper is largely improved in the reanalysis. The major inflow events such as in 1993 and 2003 are captured more accurately as the model salinity in the bottom layer is increased by 2–3 psu. Compared to independent sea level at 14 tide gauge stations, the correlation between model and observation is increased by 2%–5%, while the RMSE is generally reduced by 10 cm. It is found that the reduction of RMSE comes mainly from the reduction of mean bias. In addition, the changes in density induced by the assimilation of T/S contribute little to the barotropic transport in the shallow Danish Transition zone. The mixed layer depth exhibits strong seasonal variations in the Baltic Sea. The basin-averaged value is about 10 m in summer and 30 m in winter. By comparison, the assimilation induces a change of 20 m to the mixed layer depth in deep waters and wintertime, whereas small changes of about 2 m occur in summer and shallow waters. It is related to the strong heating in summer and the dominant role of the surface forcing in shallow water, which largely offset the effect of the assimilation.

Beschreibung: Evaluation of Release-05 GRACE time-variable gravity coefficients over the ocean Ocean Science, 8, 859-868, 2012 Author(s): D. P. Chambers and J. A. Bonin The latest release of GRACE (Gravity Recovery and Climate Experiment) gravity field coefficients (Release-05, or RL05) are evaluated for ocean applications. Data have been processed using the current methodology for Release-04 (RL04) coefficients, and have been compared to output from two different ocean models. Results indicate that RL05 data from the three Science Data Centers – the Center for Space Research (CSR), GeoForschungsZentrum (GFZ), and Jet Propulsion Laboratory (JPL) – are more consistent among themselves than the previous RL04 data. Moreover, the variance of residuals with the output of an ocean model is 50–60% lower for RL05 data than for RL04 data. A more optimized destriping algorithm is also tested, which improves the results slightly. By comparing the GRACE maps with two different ocean models, we can better estimate the uncertainty in the RL05 maps. We find the standard error to be about 1 cm (equivalent water thickness) in the low- and mid-latitudes, and between 1.5 and 2 cm in the polar and subpolar oceans, which is comparable to estimated uncertainty for the output from the ocean models.

Beschreibung: The Mediterranean Ocean Colour Observing System – system development and product validation Ocean Science, 8, 869-883, 2012 Author(s): G. Volpe, S. Colella, V. Forneris, C. Tronconi, and R. Santoleri This paper presents the Mediterranean Ocean Colour Observing System in the framework of the growing demand of near real-time data emerging within the operational oceanography international context. The main issues related to the satellite operational oceanography are tied to the following: (1) the near real-time ability to track data flow uncertainty sources; (2) in case of failure, to provide backup solutions to end-users; and (3) to scientifically assess the product quality. We describe the major scientific and technological steps made to develop, maintain and improve the operational system and its products. A method for assessing the near real-time product quality is developed and its limitation discussed. Main results are concerned with the degradation, starting from mid-2010, of the MODIS Aqua channel at 443 nm with its successive recovery thanks to the new calibration scheme implemented in the recently released SeaDAS version 6.4. The product validation analysis highlights that SeaWiFS chlorophyll product over the Mediterranean Sea is the best performing in comparison with those of MODIS and MERIS. Despite their general good agreement with in situ observations, MODIS- and MERIS-derived chlorophyll present a slight and systematic underestimation of the in situ counter part. The most relevant implications induced by these results are discussed from an operational point of view.

Beschreibung: Net primary productivity, upwelling and coastal currents in the Gulf of Ulloa, Baja California, México Ocean Science, 8, 703-711, 2012 Author(s): E. González-Rodríguez, A. Trasviña-Castro, G. Gaxiola-Castro, L. Zamudio, and R. Cervantes-Duarte The Gulf of Ulloa, a highly productive area off the western coast of the Baja California Peninsula, is examined for five successive years (2003–2007) by using satellite data and seasonal net primary productivity (NPP) estimates obtained from a vertical generalised production model. The results identify that northwestern winds blow parallel to the coast throughout the year. However, highest NPP occurs from March to June. During this period, an equatorward coastal current transports water from neighbouring upwelling areas to the northern Gulf of Ulloa and in combination with local upwelling, which injects nutrients into the euphotic zone, produce the observed increase in NPP. The opposite situation occurs in late summer when a warm poleward current of tropical characteristics arrives and inhibits the productivity in the whole region and generates the yearly lowest NPP levels. Our findings reveal the importance of lateral advection in the modulation of the primary productivity in this subtropical upwelling region.

Beschreibung: Assessment of a physical-biogeochemical coupled model system for operational service in the Baltic Sea Ocean Science, 8, 683-701, 2012 Author(s): Z. Wan, J. She, M. Maar, L. Jonasson, and J. Baasch-Larsen Thanks to the abundant observation data, we are able to deploy the traditional point-to-point comparison and statistical measures in combination with a comprehensive model validation scheme to assess the skills of the biogeochemical model ERGOM in providing an operational service for the Baltic Sea. The model assessment concludes that the operational products can resolve the main observed seasonal features for phytoplankton biomass, dissolved inorganic nitrogen, dissolved inorganic phosphorus and dissolved oxygen in euphotic layers as well as their vertical profiles. This assessment reflects that the model errors of the operational system at the current stage are mainly caused by insufficient light penetration, excessive organic particle export downward, insufficient regional adaptation and some from improper initialization. This study highlights the importance of applying multiple schemes in order to assess model skills rigidly and identify main causes for major model errors.

Beschreibung: An operational model for the West Iberian coast: products and services Ocean Science, 8, 713-732, 2012 Author(s): M. Mateus, G. Riflet, P. Chambel, L. Fernandes, R. Fernandes, M. Juliano, F. Campuzano, H. de Pablo, and R. Neves This paper presents the structure and application of a regional scale operational modelling tool for the West Iberian coast, and discusses its potential for products and services for both scientific and coastal management activities. The forecasting suite includes nested hydrodynamic models forced with up-to-date meteorological forecast data and large-scale model results. The present status of the system and its recent upgrades are reviewed, offering a general description of the main components of the system: the forcing data, the circulation model, the model outputs and the validation methodology of model results. Seasonal differences in temperature, salinity and current velocity fields are illustrated and show satisfactory reproduction of the top and deep layer thermodynamics. The system provides boundary forcing for a number of local-scale model applications via downscaling of the solution and enables potential products and services from which civil society will benefit.

Beschreibung: On the shelf resonances of the Gulf of Carpentaria and the Arafura Sea Ocean Science, 8, 733-750, 2012 Author(s): D. J. Webb A numerical model is used to investigate the resonances of the Gulf of Carpentaria and the Arafura Sea, and the additional insights that come from extending the analysis into the complex angular velocity plane. When the model is forced at the shelf edge with physically realistic real values of the angular velocity, the response functions at points within the region show maxima and other behaviour which imply that resonances are involved but provide little additional information. The study is then extended to complex angular velocities, and the results then show a clear pattern of gravity wave and Rossby wave like resonances. The properties of the resonances are investigated and used to reinterpret the response at real values of angular velocity. It is found that in some regions the response is dominated by modes trapped between the shelf edge and the coast or between opposing coastlines. In other regions the resonances show cooperative behaviour, possibly indicating the importance of other physical processes.

Beschreibung: The analysis of large-scale turbulence characteristics in the Indonesian seas derived from a regional model based on the Princeton Ocean Model Ocean Science, 8, 615-631, 2012 Author(s): K. O'Driscoll and V. Kamenkovich Turbulence characteristics in the Indonesian seas on the horizontal scale of order of 100 km were calculated with a regional model of the Indonesian seas circulation in the area based on the Princeton Ocean Model (POM). As is well known, the POM incorporates the Mellor–Yamada turbulence closure scheme. The calculated characteristics are: twice the turbulence kinetic energy per unit mass, q 2 ; the turbulence master scale, ℓ; mixing coefficients of momentum, K M ; and temperature and salinity, K H ; etc. The analyzed turbulence has been generated essentially by the shear of large-scale ocean currents and by the large-scale wind turbulence. We focused on the analysis of turbulence around important topographic features, such as the Lifamatola Sill, the North Sangihe Ridge, the Dewakang Sill, and the North and South Halmahera Sea Sills. In general, the structure of turbulence characteristics in these regions turned out to be similar. For this reason, we have carried out a detailed analysis of the Lifamatola Sill region because dynamically this region is very important and some estimates of mixing coefficients in this area are available. Briefly, the main results are as follows. The distribution of q 2 is quite adequately reproduced by the model. To the north of the Lifamatola Sill (in the Maluku Sea) and to the south of the Sill (in the Seram Sea), large values of q 2 occur in the deep layer extending several hundred meters above the bottom. The observed increase of q 2 near the very bottom is probably due to the increase of velocity shear and the corresponding shear production of q 2 very close to the bottom. The turbulence master scale, ℓ, was found to be constant in the main depth of the ocean, while ℓ rapidly decreases close to the bottom, as one would expect. However, in deep profiles away from the sill, the effect of topography results in the ℓ structure being unreasonably complicated as one moves towards the bottom. Values of 15 to 20 × 10 −4 m 2 s −1 were obtained for K M and K H in deep water in the vicinity of the Lifamatola Sill. These estimates agree well with basin-scale averaged values of 13.3 × 10 −4 m 2 s −1 found diagnostically for K H in the deep Banda and Seram Seas (Gordon et al., 2003) and a value of 9.0 × 10 −4 m 2 s −1 found diagnostically for K H for the deep Banda Sea system (van Aken et al., 1988). The somewhat higher simulated values can be explained by the presence of steep topography around the sill.

Beschreibung: TOPAZ4: an ocean-sea ice data assimilation system for the North Atlantic and Arctic Ocean Science, 8, 633-656, 2012 Author(s): P. Sakov, F. Counillon, L. Bertino, K. A. Lisæter, P. R. Oke, and A. Korablev We present a detailed description of TOPAZ4, the latest version of TOPAZ – a coupled ocean-sea ice data assimilation system for the North Atlantic Ocean and Arctic. It is the only operational, large-scale ocean data assimilation system that uses the ensemble Kalman filter. This means that TOPAZ features a time-evolving, state-dependent estimate of the state error covariance. Based on results from the pilot MyOcean reanalysis for 2003–2008, we demonstrate that TOPAZ4 produces a realistic estimate of the ocean circulation in the North Atlantic and the sea-ice variability in the Arctic. We find that the ensemble spread for temperature and sea-level remains fairly constant throughout the reanalysis demonstrating that the data assimilation system is robust to ensemble collapse. Moreover, the ensemble spread for ice concentration is well correlated with the actual errors. This indicates that the ensemble statistics provide reliable state-dependent error estimates – a feature that is unique to ensemble-based data assimilation systems. We demonstrate that the quality of the reanalysis changes when different sea surface temperature products are assimilated, or when in-situ profiles below the ice in the Arctic Ocean are assimilated. We find that data assimilation improves the match to independent observations compared to a free model. Improvements are particularly noticeable for ice thickness, salinity in the Arctic, and temperature in the Fram Strait, but not for transport estimates or underwater temperature. At the same time, the pilot reanalysis has revealed several flaws in the system that have degraded its performance. Finally, we show that a simple bias estimation scheme can effectively detect the seasonal or constant bias in temperature and sea-level.

Beschreibung: Atlantic transport variability at 25° N in six hydrographic sections Ocean Science, 8, 497-523, 2012 Author(s): C. P. Atkinson, H. L. Bryden, S. A. Cunningham, and B. A. King In January and February 2010, a sixth transatlantic hydrographic section was completed across 25° N, extending the hydrographic record at this latitude to over half a century. In combination with continuous transport measurements made since 2004 at 26.5° N by the Rapid-WATCH project, we reassess transport variability in the 25° N hydrographic record. Past studies of transport variability at this latitude have assumed transport estimates from each hydrographic section to represent annual average conditions. In this study the uncertainty in this assumption is assessed through use of Rapid-WATCH observations to quantify sub-seasonal and seasonal transport variability. Whilst in the upper-ocean no significant interannual or decadal transport variability are identified in the hydrographic record, in the deep ocean transport variability in both depth and potential temperature classes suggests some interannual or decadal variability may have occurred. This is particularly striking in the lower North Atlantic Deep Water where southward transports prior to 1998 were greater than recent transports by several Sverdrups. Whilst a cooling and freshening of Denmark Straits Overflow Water has occurred which is coincident with these transport changes, these water mass changes appear to be density compensated. Transport changes are the result of changing velocity shear in the vicinity of the Deep Western Boundary Current.

Beschreibung: Propagation and dissipation of internal tides in the Oslofjord Ocean Science, 8, 525-543, 2012 Author(s): A. Staalstrøm, E. Aas, and B. Liljebladh Observations of velocity, pressure, temperature and salinity in the inner Oslofjord have been analysed to provide new information about the relationships between internal tides generated by tidal currents across the Drøbak Sill and dissipation and diffusivity in the fjord. The most energetic vertical displacement of density surfaces inside the sill is associated with the first internal mode that has maximum amplitude around sill depth. The amplitude of the vertical displacement around sill depth correlates with the amplitude of the surface elevation, and, at a distance of 1 km inside the sill, the ratio between the amplitudes is 38, decreasing to 11 at a distance of 10 km. The greatest vertical displacements inside the sill, however, are found at 40 m depth. These latter internal waves are not associated with a first-mode internal tide, but are rather associated with higher internal modes controlled by stratification. The energy flux of the internal wave propagating from the Drøbak Sill into the inner fjord on the east side of the Håøya Island is estimated to vary in the range 155–430 kW. This is the same order of magnitude as the estimated barotropic energy loss over the Drøbak Sill (250 kW), but only 4–10% of the total barotropic flux. Approximately 40–70% of the internal energy flux is lost within a distance of 10 km from the sill. The mean diffusivity below 90 m depth in this area (~20 cm 2 s −1 ) is more than four times higher than in the rest of the fjord (~5 cm 2 s −1 or less).

Beschreibung: Operational SAR-based sea ice drift monitoring over the Baltic Sea Ocean Science, 8, 473-483, 2012 Author(s): J. Karvonen An algorithm for computing ice drift from pairs of synthetic aperture radar (SAR) images covering a common area has been developed at FMI. The algorithm has been developed based on the C-band SAR data over the Baltic Sea. It is based on phase correlation in two scales (coarse and fine) with some additional constraints. The algorithm has been running operationally in the Baltic Sea from the beginning of 2011, using Radarsat-1 ScanSAR wide mode and Envisat ASAR wide swath mode data. The resulting ice drift fields are publicly available as part of the MyOcean EC project. The SAR-based ice drift vectors have been compared to the drift vectors from drifter buoys in the Baltic Sea during the first operational season, and also these validation results are shown in this paper. Also some navigationally useful sea ice quantities, which can be derived from ice drift vector fields, are presented.

Beschreibung: Impact of SLA assimilation in the Sicily Channel Regional Model: model skills and mesoscale features Ocean Science, 8, 485-496, 2012 Author(s): A. Olita, S. Dobricic, A. Ribotti, L. Fazioli, A. Cucco, C. Dufau, and R. Sorgente The impact of the assimilation of MyOcean sea level anomalies along-track data on the analyses of the Sicily Channel Regional Model was studied. The numerical model has a resolution of 1/32° degrees and is capable to reproduce mesoscale and sub-mesoscale features. The impact of the SLA assimilation is studied by comparing a simulation (SIM, which does not assimilate data) with an analysis (AN) assimilating SLA along-track multi-mission data produced in the framework of MyOcean project. The quality of the analysis was evaluated by computing RMSE of the misfits between analysis background and observations (sea level) before assimilation. A qualitative evaluation of the ability of the analyses to reproduce mesoscale structures is accomplished by comparing model results with ocean colour and SST satellite data, able to detect such features on the ocean surface. CTD profiles allowed to evaluate the impact of the SLA assimilation along the water column. We found a significant improvement for AN solution in terms of SLA RMSE with respect to SIM (the averaged RMSE of AN SLA misfits over 2 years is about 0.5 cm smaller than SIM). Comparison with CTD data shows a questionable improvement produced by the assimilation process in terms of vertical features: AN is better in temperature while for salinity it gets worse than SIM at the surface. This suggests that a better a-priori description of the vertical error covariances would be desirable. The qualitative comparison of simulation and analyses with synoptic satellite independent data proves that SLA assimilation allows to correctly reproduce some dynamical features (above all the circulation in the Ionian portion of the domain) and mesoscale structures otherwise misplaced or neglected by SIM. Such mesoscale changes also infer that the eddy momentum fluxes (i.e. Reynolds stresses) show major changes in the Ionian area. Changes in Reynolds stresses reflect a different pumping of eastward momentum from the eddy to the mean flow, in turn influencing transports through the channel.

Beschreibung: Three-dimensional modelling of wave-induced current from the surf zone to the inner shelf Ocean Science, 8, 657-681, 2012 Author(s): H. Michaud, P. Marsaleix, Y. Leredde, C. Estournel, F. Bourrin, F. Lyard, C. Mayet, and F. Ardhuin We develop and implement a new method to take into account the impact of waves into the 3-D circulation model SYMPHONIE (Marsaleix et al., 2008, 2009a) following the simplified equations of Bennis et al. (2011) which use glm2z-RANS theory (Ardhuin et al., 2008c). These adiabatic equations are completed by additional parameterizations of wave breaking, bottom friction and wave-enhanced vertical mixing, making the forcing valid from the surf zone through to the open ocean. The wave forcing is performed by wave generation and propagation models WAVEWATCH III® (Tolman, 2008, 2009; Ardhuin et al., 2010) and SWAN (Booij et al., 1999). The model is tested and compared with other models for a plane beach test case, previously tested by Haas and Warner (2009)and Uchiyama et al. (2010). A comparison is also made with the laboratory measurements of Haller et al. (2002) of a barred beach with channels. Results fit with previous simulations performed by other models and with available observational data. Finally, a realistic case is simulated with energetic waves travelling over a coast of the Gulf of Lion (in the northwest of the Mediterranean Sea) for which currents are available at different depths as well as an accurate bathymetric database of the 0–10 m depth range. A grid nesting approach is used to account for the different forcings acting at different spatial scales. The simulation coupling the effects of waves and currents is successful to reproduce the powerful northward littoral drift in the 0–15 m depth zone. More precisely, two distinct cases are identified: When waves have a normal angle of incidence with the coast, they are responsible for complex circulation cells and rip currents in the surf zone, and when they travel obliquely, they generate a northward littoral drift. These features are more complicated than in the test cases, due to the complex bathymetry and the consideration of wind and non-stationary processes. Wave impacts in the inner shelf are less visible since wind and regional circulation seem to be the predominant forcings. Besides, a discrepancy between model and observations is noted at that scale, possibly linked to an underestimation of the wind stress. This three-dimensional method allows a good representation of vertical current profiles and permits the calculation of the shear stress associated with waves and currents. Future work will focus on the combination with a sediment transport model.

Beschreibung: Particle aggregation at the edges of anticyclonic eddies and implications for distribution of biomass Ocean Science, 8, 389-400, 2012 Author(s): A. Samuelsen, S. S. Hjøllo, J. A. Johannessen, and R. Patel Acoustic measurements show that the biomass of zooplankton and mesopelagic fish is redistributed by mesoscale variability and that the signal extends over several hundred meters depth. The mechanisms governing this distribution are not well understood, but influences from both physical (i.e. redistribution) and biological processes (i.e. nutrient transport, primary production, active swimming, etc.) are likely. This study examines how hydrodynamic conditions and basic vertical swimming behavior act to distribute biomass in an anticyclonic eddy. Using an eddy-resolving 2.3 km-resolution physical ocean model as forcing for a particle-tracking module, particles representing passively floating organisms and organisms with vertical swimming behavior are released within an eddy and monitored for 20 to 30 days. The role of hydrodynamic conditions on the distribution of biomass is discussed in relation to the acoustic measurements. Particles released close to the surface tend, in agreement with the observations, to accumulate around the edge of the eddy, whereas particles released at depth gradually become distributed along the isopycnals. After a month they are displaced several hundreds meters in the vertical with the deepest particles found close to the eddy center and the shallowest close to the edge. There is no evidence of aggregation of particles along the eddy rim in the last simulation. The model results points towards a physical mechanism for aggregation at the surface, however biological processes cannot be ruled out using the current modeling tool.

Beschreibung: Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin Ocean Science, 8, 419-432, 2012 Author(s): K. Shimada, S. Aoki, K. I. Ohshima, and S. R. Rintoul Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γ n = 28.30 kg m −3 . Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year −1 . Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171–158° E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7% weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7% is assumed between the year 1995 and 2005) can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in AA-AABW.

Beschreibung: Support to oil spill emergencies in the Bonifacio Strait, western Mediterranean Ocean Science, 8, 443-454, 2012 Author(s): A. Cucco, A. Ribotti, A. Olita, L. Fazioli, B. Sorgente, M. Sinerchia, A. Satta, A. Perilli, M. Borghini, K. Schroeder, and R. Sorgente An innovative forecasting system of the coastal marine circulation has been implemented in the Bonifacio Strait area, between Corsica and Sardinia, using a numerical approach to facilitate the rapid planning and coordination of remedial actions for oil spill emergencies at sea by local authorities. Downscaling and nesting techniques from regional to coastal scale and a 3-D hydrodynamic numerical model, coupled with a wind wave model, are the core of the integrated Bonifacio Strait system. Such a system is capable of predicting operationally the dispersion of hydrocarbon spills in the area, both in forward and backward mode, through an easy-to-use graphical user interface. A set of applications are described and discussed including both operational applications aimed at providing rapid responses to local oil spill emergences and managing applications aimed at mitigating the risk of oil spill impacts on the coast.

Beschreibung: On the chemical dynamics of extracellular polysaccharides in the high Arctic surface microlayer Ocean Science, 8, 401-418, 2012 Author(s): Q. Gao, C. Leck, C. Rauschenberg, and P. A. Matrai The surface microlayer (SML) represents a unique system of which the physicochemical characteristics may differ from those of the underlying subsurface seawater (SSW). Within the Arctic pack ice area, the SML has been characterized as enriched in small colloids of biological origin, resulting from extracellular polymeric secretions (EPS). During the Arctic Summer Cloud Ocean Study (ASCOS) in August 2008, particulate organic matter (POM, with size range 〉 0.22 μm) and dissolved organic matter (DOM, 〈 0.22 μm, obtained after filtration) samples were collected and chemically characterized from the SML and the corresponding SSW at an open lead centered at 87.5° N and 5° E. Total organic carbon was persistently enriched in the SML with a mean enrichment factor (EF) of 1.45 ± 0.41, whereas sporadic depletions of dissolved carbohydrates and amino acids were observed. Monosaccharide compositional analysis reveals that EPS in the Arctic lead was formed mainly of distinctive heteropolysaccharides, enriched in xylose, fucose and glucose. The mean concentrations of total hydrolysable neutral sugars in SSW were 94.9 ± 37.5 nM in high molecular weight (HMW) DOM (〉 5 kDa) and 64.4 ± 14.5 nM in POM. The enrichment of polysaccharides in the SML appeared to be a common feature, with EFs ranging from 1.7 to 7.0 for particulate polysaccharides and 3.5 to 12.1 for polysaccharides in the HMW DOM fraction. A calculated monosaccharide yield suggests that polymers in the HMW DOM fraction were scavenged, without substantial degradation, into the SML. Bubble scavenging experiments showed that newly aggregated particles could be formed abiotically by coagulation of low molecular weight nanometer-sized gels. Aerosol particles, artificially generated by bubbling experiments, were enriched in polysaccharides by factors of 22–70, relative to the source seawater. We propose that bubble scavenging of surface-active polysaccharides could be one of the possible mechanisms for the enrichment of polysaccharides in the high Arctic open lead SML.

Beschreibung: On the influence of wind and waves on underwater irradiance fluctuations Ocean Science, 8, 455-471, 2012 Author(s): M. Hieronymi and A. Macke The influence of various wind and wave conditions on the variability of downwelling irradiance E d (490 nm) in water is subject of this study. The work is based on a two-dimensional Monte Carlo radiative transfer model with high spatial resolution. The model assumes conditions that are ideal for wave focusing, thus simulation results reveal the upper limit for light fluctuations. Local wind primarily determines the steepness of capillary-gravity waves which in turn dominate the irradiance variability near the surface. Down to 3 m depth, maximum irradiance peaks that exceed the mean irradiance E d by a factor of more than 7 can be observed at low wind speeds up to 5 m s −1 . The strength of irradiance fluctuations can be even amplified under the influence of higher ultra-gravity waves; thereby peaks can exceed 11 E d . Sea states influence the light field much deeper; gravity waves can cause considerable irradiance variability even at 100 m depth. The simulation results show that under realistic conditions 50% radiative enhancements compared to the mean can still occur at 30 m depth. At greater depths, the underwater light variability depends on the wave steepness of the characteristic wave of a sea state; steeper waves cause stronger light fluctuations.

Beschreibung: Transport of warm Upper Circumpolar Deep Water onto the western Antarctic Peninsula continental shelf Ocean Science, 8, 433-442, 2012 Author(s): D. G. Martinson and D. C. McKee Five thermistor moorings were placed on the continental shelf of the western Antarctic Peninsula (between 2007 and 2010) in an effort to identify the mechanism(s) responsible for delivering warm Upper Circumpolar Deep Water (UCDW) onto the broad continental shelf from the Antarctic Circumpolar Current (ACC) flowing over the adjacent continental slope. Historically, four mechanisms have been suggested: (1) eddies shed from the ACC, (2) flow into the cross-shelf-cutting canyons with overflow onto the nominal shelf, (3) general upwelling, and (4) episodic advective diversions of the ACC onto the shelf. The mooring array showed that for the years of deployment, the dominant mechanism is eddies; upwelling may also contribute but to an unknown extent. Mechanism 2 played no role, though the canyons have been shown previously to channel UCDW across the shelf into Marguerite Bay. Mechanism 4 played no role independently, though eddies may be advected within a greater intrusion of the background flow.

Beschreibung: On the outflow of dense water from the Weddell and Ross Seas in OCCAM model Ocean Science, 8, 369-388, 2012 Author(s): R. Kerr, K. J. Heywood, M. M. Mata, and C. A. E. Garcia We describe the seasonal and interannual variability of volume transports in the Weddell and Ross Seas using the 1/12° 20-yr simulation of the OCCAM global ocean general circulation model. The average simulated full-depth cumulative volume transports were 28.5 ± 2.9 Sv (1 Sv ≡ 10 6 m 3 s −1 ) and 13.4 ± 5.2 Sv, across the main export regions of the Weddell and Ross Seas, respectively. The values of mean outflow of Antarctic Bottom Water (AABW) (defined by neutral density γ n ≥ 28.27 kg m −3 ) from the Weddell and Ross Seas of 10.6 ± 3.1 Sv and 0.5 ± 0.7 Sv, respectively, agree with the range reported in historical observational studies. The export of Weddell Sea dense water in OCCAM is primarily determined by the strength of the Weddell Gyre. Variability in AABW export is predominantly at periods of ~1 yr and 2–4 yr.

Beschreibung: Transports and budgets in a 1/4 ° global ocean reanalysis 1989–2010 Ocean Science, 8, 333-344, 2012 Author(s): K. Haines, M. Valdivieso, H. Zuo, and V. N. Stepanov Large-scale ocean transports of heat and freshwater have not been well monitored, and yet the regional budgets of these quantities are important to understanding the role of the oceans in climate and climate change. In contrast, atmospheric heat and freshwater transports are commonly assessed from atmospheric reanalysis products, despite the presence of non-conserving data assimilation based on the wealth of distributed atmospheric observations as constraints. The ability to carry out ocean reanalyses globally at eddy-permitting resolutions of 1/4 ° or better, along with new global ocean observation programs, now makes a similar approach viable for the ocean. In this paper we examine the budgets and transports within a global high resolution ocean model constrained by ocean data assimilation, and compare them with independent oceanic and atmospheric estimates.

Beschreibung: 137 Cs off Fukushima Dai-ichi, Japan – model based estimates of dilution and fate Ocean Science, 8, 319-332, 2012 Author(s): H. Dietze and I. Kriest In the aftermath of an earthquake and tsunami on 11 March 2011 radioactive 137 Cs was discharged from a damaged nuclear power plant to the sea off Fukushima Dai-ichi, Japan. Here we explore its dilution and fate with a state-of-the-art global ocean general circulation model, which is eddy-resolving in the region of interest. We find apparent consistency between our simulated circulation, estimates of 137 Cs discharged ranging from 0.94 p Bq (Japanese Government, 2011) to 3.5 ± 0.7 p Bq (Tsumune et al., 2012), and measurements by Japanese authorities and the power plant operator. In contrast, our simulations are apparently inconsistent with the high 27 ± 15 p Bq discharge estimate of Bailly du Bois et al. (2012). Expressed in terms of a diffusivity we diagnose, from our simulations, an initial dilution on the shelf of 60 to 100 m 2 s −1 . The cross-shelf diffusivity is at 500 ± 300 m 2 s −1 significantly higher and variable in time as indicated by its uncertainty. Expressed as an effective residence time of surface water on the shelf, the latter estimate transfers to 43 ± 16 days. As regards the fate of 137 Cs, our simulations suggest that activities up to 4 mBq l −1 prevail in the Kuroshio-Oyashio Interfrontal Zone one year after the accident. This allows for low but detectable 0.1 to 0.3 m Bq l −1 entering the North Pacific Intermediate Water before the 137 Cs signal is flushed away. The latter estimates concern the direct release to the sea only.

Beschreibung: A vertical-mode decomposition to investigate low-frequency internal motion across the Atlantic at 26° N Ocean Science, 8, 345-367, 2012 Author(s): Z. B. Szuts, J. R. Blundell, M. P. Chidichimo, and J. Marotzke Hydrographic data from full-depth moorings maintained by the Rapid/\-MOCHA project and spanning the Atlantic at 26° N are decomposed into vertical modes in order to give a dynamical framework for interpreting the observed fluctuations. Vertical modes at each mooring are fit to pressure perturbations using a Gauss-Markov inversion. Away from boundaries, the vertical structure is almost entirely described by the first baroclinic mode, as confirmed by high correlation between the original signal and reconstructions using only the first baroclinic mode. These first baroclinic motions are also highly coherent with altimetric sea surface height (SSH). Within a Rossby radius (45 km) of the western and eastern boundaries, however, the decomposition contains significant variance at higher modes, and there is a corresponding decrease in the agreement between SSH and either the original signal or the first baroclinic mode reconstruction. Compared to the full transport signal, transport fluctuations described by the first baroclinic mode represent

Beschreibung: Wave climate in the Arkona Basin, the Baltic Sea Ocean Science, 8, 287-300, 2012 Author(s): T. Soomere, R. Weisse, and A. Behrens The basic features of the wave climate in the Southwestern Baltic Sea (such as the average and typical wave conditions, frequency of occurrence of different wave parameters, variations in wave heights from weekly to decadal scales) are established based on waverider measurements at the Darss Sill in 1991–2010. The measured climate is compared with two numerical simulations with the WAM wave model driven by downscaled reanalysis of wind fields for 1958–2002 and by adjusted geostrophic winds for 1970–2007. The wave climate in this region is typical for semi-enclosed basins of the Baltic Sea. The maximum wave heights are about half of those in the Baltic Proper. The maximum recorded significant wave height H S =4.46 m occurred on 3 November 1995. The wave height exhibits no long-term trend but reveals modest interannual (about 12 % of the long-term mean of 0.76 m) and substantial seasonal variation. The wave periods are mostly concentrated in a narrow range of 2.6–4 s. Their distribution is almost constant over decades. The role of remote swell is very small.

Beschreibung: Laminar and weakly turbulent oceanic gravity currents performing inertial oscillations Ocean Science, 8, 301-317, 2012 Author(s): A. Wirth The small scale dynamics of a weakly turbulent oceanic gravity current is determined. The gravity current considered is initially at rest and adjusts by performing inertial oscillations to a geostrophic mean flow. The dynamics is explored with a hierarchy of mathematical models. The most involved are the fully 3-D Navier-Stokes equations subject to the Boussinesq approximation. A 1-D and 0-D mathematical model of the same gravity current dynamics are systematically derived. Using this hierarchy and the numerical solutions of the mathematical models, the turbulent dynamics at the bottom and the interface is explored and their interaction investigated. Three different regimes of the small scale dynamics of the gravity current are identified, they are characterised by laminar flow, coherent roll vortices and turbulent dynamics with coherent streaks and bursts. The problem of the rectification of the turbulent fluxes, that is, how to average out the fluctuations and calculate their average influence on the flow, is considered. It is shown that two different regimes of friction are superposed, an Ekman friction applies to the average geostrophic flow and a linear friction, not influenced by rotation, to the inertial oscillations. The combination of the two makes the bulk friction non-local in time for the 0-D model. The implications of the results for parametrisations of the Ekman dynamics and the small scale turbulent fluxes in the planetary boundary layer are discussed.

Beschreibung: Modeling of wave-induced irradiance variability in the upper ocean mixed layer Ocean Science, 8, 103-120, 2012 Author(s): M. Hieronymi, A. Macke, and O. Zielinski A Monte Carlo based radiative transfer model has been developed for calculating the availability of solar radiation within the top 100 m of the ocean. The model is optimized for simulations of spatial high resolution downwelling irradiance E d fluctuations that arise from the lensing effect of waves at the water surface. In a first step the accuracy of simulation results has been verified by measurements of the oceanic underwater light field and through intercomparison with an established radiative transfer model. Secondly the potential depth-impact of nonlinear shaped single waves, from capillary to swell waves, is assessed by considering the most favorable conditions for light focusing, i.e. monochromatic light at 490 nm, very clear oceanic water with a low chlorophyll a content of 0.1 mg m −3 and high sun elevation. Finally light fields below irregular wave profiles accounting for realistic sea states were simulated. Our simulation results suggest that under open ocean conditions light flashes with 50% irradiance enhancements can appear down to 35 m depth, and light variability in the range of ±10% compared to the mean E d is still possible in 100 m depth.

Beschreibung: Towards an improved description of ocean uncertainties: effect of local anamorphic transformations on spatial correlations Ocean Science, 8, 121-142, 2012 Author(s): J.-M. Brankart, C.-E. Testut, D. Béal, M. Doron, C. Fontana, M. Meinvielle, P. Brasseur, and J. Verron The objective of this paper is to investigate if the description of ocean uncertainties can be significantly improved by applying a local anamorphic transformation to each model variable, and by making the assumption of joint Gaussianity for the transformed variables, rather than for the original variables. For that purpose, it is first argued that a significant improvement can already be obtained by deriving the local transformations from a simple histogram description of the marginal distributions. Two distinctive advantages of this solution for large size applications are the conciseness and the numerical efficiency of the description. Second, various oceanographic examples are used to evaluate the effect of the resulting piecewise linear local anamorphic transformations on the spatial correlation structure. These examples include (i) stochastic ensemble descriptions of the effect of atmospheric uncertainties on the ocean mixed layer, and of wind uncertainties or parameter uncertainties on the ecosystem, and (ii) non-stochastic ensemble descriptions of forecast uncertainties in current sea ice and ecosystem pre-operational developments. The results indicate that (i) the transformation is accurate enough to faithfully preserve the correlation structure if the joint distribution is already close to Gaussian, and (ii) the transformation has the general tendency of increasing the correlation radius as soon as the spatial dependence between random variables becomes nonlinear, with the important consequence of reducing the number of degrees of freedom in the uncertainties, and thus increasing the benefit that can be expected from a given observation network.

Beschreibung: Mesoscale variability of water masses in the Arabian Sea as revealed by ARGO floats Ocean Science, 8, 227-248, 2012 Author(s): X. Carton, P. L'Hegaret, and R. Baraille By analysing ARGO float data over the last four years, a few aspects of the mesoscale variability of water masses in the Arabian Sea are described. The Red Sea Outflow Water (RSOW) is concentrated in the Southwestern Gulf of Aden, in particular when a cyclonic gyre predominates in this region. Salinities of 36.5 and temperatures of 16 °C are found in this area at depths between 600 and 1000 m. RSOW is more dilute in the eastern part of the Gulf, where intense and relatively barotropic gyres mix it with Indian ocean Central Water. RSOW is also detected along the northeastern coast of Socotra, and fragments of RSOW are found between one and three degrees of latitude north of this island. In the whole Gulf of Aden, the correlation between the deep motions of the floats and the sea-level anomaly measured by altimetry is strong, at regional scale. The finer scale details of the float trajectories are not sampled by altimetry and are often related to the anomalous water masses that the floats encounter. The Persian Gulf Water (PGW) is found in the float profiles near Ras ash Sharbatat (near 57° E, 18° N), again with 36.5 in salinity and about 18–19 °C in temperature. These observations were achieved in winter when the southwestward monsoon currents can advect PGW along the South Arabian coast. Fragments of PGW were also observed in the Arabian Sea between 18 and 20° N and 63 and 65° E in summer, showing that this water mass can escape the Gulf of Oman southeastward, during that season. Kinetic energy distributions of floats with respect to distance or angle share common features between the two regions (Gulf of Aden and Arabian Sea), in particular peaks at 30, 50 and 150 km scales and along the axis of monsoon currents. Hydrological measurements by floats are also influenced by the seasonal variations of PGW and RSOW in these regions.

Beschreibung: Variational assimilation of Lagrangian trajectories in the Mediterranean ocean Forecasting System Ocean Science, 8, 249-259, 2012 Author(s): J. A. U. Nilsson, S. Dobricic, N. Pinardi, P.-M. Poulain, and D. Pettenuzzo A novel method for three-dimensional variational assimilation of Lagrangian data with a primitive-equation ocean model is proposed. The assimilation scheme was implemented in the Mediterranean ocean Forecasting System and evaluated for a 4-month period. Four experiments were designed to assess the impact of trajectory assimilation on the model output, i.e. the sea-surface height, velocity, temperature and salinity fields. It was found from the drifter and Argo trajectory assimilation experiment that the forecast skill of surface-drifter trajectories improved by 15 %, that of intermediate-depth float trajectories by 20 %, and moreover, that the forecasted sea-surface height fields improved locally by 5 % compared to satellite data, while the quality of the temperature and salinity fields remained at previous levels. In conclusion, the addition of Lagrangian trajectory assimilation proved to reduce the uncertainties in the model fields, thus yielding a higher accuracy of the ocean forecasts.

Beschreibung: An empirical model for the statistics of sea surface diurnal warming Ocean Science, 8, 197-209, 2012 Author(s): M. J. Filipiak, C. J. Merchant, H. Kettle, and P. Le Borgne A statistical model is derived relating the diurnal variation of sea surface temperature (SST) to the net surface heat flux and surface wind speed from a numerical weather prediction (NWP) model. The model is derived using fluxes and winds from the European Centre for Medium-Range Weather Forecasting (ECMWF) NWP model and SSTs from the Spinning Enhanced Visible and Infrared Imager (SEVIRI). In the model, diurnal warming has a linear dependence on the net surface heat flux integrated since (approximately) dawn and an inverse quadratic dependence on the maximum of the surface wind speed in the same period. The model coefficients are found by matching, for a given integrated heat flux, the frequency distributions of the maximum wind speed and the observed warming. Diurnal cooling, where it occurs, is modelled as proportional to the integrated heat flux divided by the heat capacity of the seasonal mixed layer. The model reproduces the statistics (mean, standard deviation, and 95-percentile) of the diurnal variation of SST seen by SEVIRI and reproduces the geographical pattern of mean warming seen by the Advanced Microwave Scanning Radiometer (AMSR-E). We use the functional dependencies in the statistical model to test the behaviour of two physical model of diurnal warming that display contrasting systematic errors.

Beschreibung: ENSURF: multi-model sea level forecast – implementation and validation results for the IBIROOS and Western Mediterranean regions Ocean Science, 8, 211-226, 2012 Author(s): B. Pérez, R. Brouwer, J. Beckers, D. Paradis, C. Balseiro, K. Lyons, M. Cure, M. G. Sotillo, B. Hackett, M. Verlaan, and E. A. Fanjul ENSURF (Ensemble SURge Forecast) is a multi-model application for sea level forecast that makes use of several storm surge or circulation models and near-real time tide gauge data in the region, with the following main goals: 1. providing easy access to existing forecasts, as well as to its performance and model validation, by means of an adequate visualization tool; 2. generation of better forecasts of sea level, including confidence intervals, by means of the Bayesian Model Average technique (BMA). The Bayesian Model Average technique generates an overall forecast probability density function (PDF) by making a weighted average of the individual forecasts PDF's; the weights represent the Bayesian likelihood that a model will give the correct forecast and are continuously updated based on the performance of the models during a recent training period. This implies the technique needs the availability of sea level data from tide gauges in near-real time. The system was implemented for the European Atlantic facade (IBIROOS region) and Western Mediterranean coast based on the MATROOS visualization tool developed by Deltares. Results of validation of the different models and BMA implementation for the main harbours are presented for these regions where this kind of activity is performed for the first time. The system is currently operational at Puertos del Estado and has proved to be useful in the detection of calibration problems in some of the circulation models, in the identification of the systematic differences between baroclinic and barotropic models for sea level forecasts and to demonstrate the feasibility of providing an overall probabilistic forecast, based on the BMA method.

Beschreibung: An historical perspective on the development of the Thermodynamic Equation of Seawater – 2010 Ocean Science, 8, 161-174, 2012 Author(s): R. Pawlowicz, T. McDougall, R. Feistel, and R. Tailleux

Beschreibung: The impacts of physical processes on oxygen variations in the North Sea-Baltic Sea transition zone Ocean Science, 8, 37-48, 2012 Author(s): L. Jonasson, J. L. S. Hansen, Z. Wan, and J. She The bottom water of the North Sea–Baltic Sea transition zone suffers from seasonal hypoxia, usually during late summer and autumn. These hypoxic events are critical for the benthic ecosystems and the concentration of dissolved oxygen is an important measure of the water quality. However, to model the subsurface dissolved oxygen is a major challenge, especially in estuaries and coastal regions. In this study a simple oxygen consumption model is coupled to a 3-D hydrodynamical model in order to analyse oxygen variations in the transition zone. The benthic and pelagic consumption of oxygen is modelled as a function of water temperature and oxygen concentration. A quantitative assessment of the model demonstrates that the model is able to resolve both seasonal and interannual variations in dissolved oxygen. Results from several experimental simulations highlight the importance of physical processes in the regulation of dissolved oxygen. Advective oxygen transport and wind induced mixing are two key processes that control the extent of hypoxia in the transition zone.

Beschreibung: The vertical structure of oceanic Rossby waves: a comparison of high-resolution model data to theoretical vertical structures Ocean Science, 8, 19-35, 2012 Author(s): F. K. Hunt, R. Tailleux, and J. J.-M. Hirschi Tests of the new Rossby wave theories that have been developed over the past decade to account for discrepancies between theoretical wave speeds and those observed by satellite altimeters have focused primarily on the surface signature of such waves. It appears, however, that the surface signature of the waves acts only as a rather weak constraint, and that information on the vertical structure of the waves is required to better discriminate between competing theories. Due to the lack of 3-D observations, this paper uses high-resolution model data to construct realistic vertical structures of Rossby waves and compares these to structures predicted by theory. The meridional velocity of a section at 24° S in the Atlantic Ocean is pre-processed using the Radon transform to select the dominant westward signal. Normalized profiles are then constructed using three complementary methods based respectively on: (1) averaging vertical profiles of velocity, (2) diagnosing the amplitude of the Radon transform of the westward propagating signal at different depths, and (3) EOF analysis. These profiles are compared to profiles calculated using four different Rossby wave theories: standard linear theory (SLT), SLT plus mean flow, SLT plus topographic effects, and theory including mean flow and topographic effects. Our results support the classical theoretical assumption that westward propagating signals have a well-defined vertical modal structure associated with a phase speed independent of depth, in contrast with the conclusions of a recent study using the same model but for different locations in the North Atlantic. The model structures are in general surface intensified, with a sign reversal at depth in some regions, notably occurring at shallower depths in the East Atlantic. SLT provides a good fit to the model structures in the top 300 m, but grossly overestimates the sign reversal at depth. The addition of mean flow slightly improves the latter issue, but is too surface intensified. SLT plus topography rectifies the overestimation of the sign reversal, but overestimates the amplitude of the structure for much of the layer above the sign reversal. Combining the effects of mean flow and topography provided the best fit for the mean model profiles, although small errors at the surface and mid-depths are carried over from the individual effects of mean flow and topography respectively. Across the section the best fitting theory varies between SLT plus topography and topography with mean flow, with, in general, SLT plus topography performing better in the east where the sign reversal is less pronounced. None of the theories could accurately reproduce the deeper sign reversals in the west. All theories performed badly at the boundaries. The generalization of this method to other latitudes, oceans, models and baroclinic modes would provide greater insight into the variability in the ocean, while better observational data would allow verification of the model findings.

Beschreibung: Mapping turbidity layers using seismic oceanography methods Ocean Science, 8, 11-18, 2012 Author(s): E. A. Vsemirnova, R. W. Hobbs, and P. Hosegood Using a combination of seismic oceanographic and physical oceanographic data acquired across the Faroe-Shetland Channel we present evidence of a turbidity layer that transports suspended sediment along the western boundary of the Channel. We focus on reflections observed on seismic data close to the sea-bed on the Faroese side of the Channel below 900 m. Forward modelling based on independent physical oceanographic data show that thermohaline structure does not explain these near sea-bed reflections but they are consistent with optical backscatter data, dry matter concentrations from water samples and from seabed sediment traps. Hence we conclude that an impedance contrast in water column caused by turbidity layers is strong enough to be seen in seismic sections and this provides a new way to visualise this type of current and its lateral structure. By inverting the seismic data we estimate a sediment concentration in the turbidity layers, present at the time of the survey, of 45 ± 25 mg l −1 . We believe this is the first direct observation of a turbidity current using Seismic Oceanography.

Beschreibung: A simple and self-consistent geostrophic-force-balance model of the thermohaline circulation with boundary mixing Ocean Science, 8, 49-63, 2012 Author(s): J. Callies and J. Marotzke A simple model of the thermohaline circulation (THC) is formulated, with the objective to represent explicitly the geostrophic force balance of the basinwide THC. The model comprises advective-diffusive density balances in two meridional-vertical planes located at the eastern and the western walls of a hemispheric sector basin. Boundary mixing constrains vertical motion to lateral boundary layers along these walls. Interior, along-boundary, and zonally integrated meridional flows are in thermal-wind balance. Rossby waves and the absence of interior mixing render isopycnals zonally flat except near the western boundary, constraining meridional flow to the western boundary layer. The model is forced by a prescribed meridional surface density profile. This two-plane model reproduces both steady-state density and steady-state THC structures of a primitive-equation model. The solution shows narrow deep sinking at the eastern high latitudes, distributed upwelling at both boundaries, and a western boundary current with poleward surface and equatorward deep flow. The overturning strength has a 2/3-power-law dependence on vertical diffusivity and a 1/3-power-law dependence on the imposed meridional surface density difference. Convective mixing plays an essential role in the two-plane model, ensuring that deep sinking is located at high latitudes. This role of convective mixing is consistent with that in three-dimensional models and marks a sharp contrast with previous two-dimensional models. Overall, the two-plane model reproduces crucial features of the THC as simulated in simple-geometry three-dimensional models. At the same time, the model self-consistently makes quantitative a conceptual picture of the three-dimensional THC that hitherto has been expressed either purely qualitatively or not self-consistently.

Beschreibung: Impact of combining GRACE and GOCE gravity data on ocean circulation estimates Ocean Science, 8, 65-79, 2012 Author(s): T. Janjić, J. Schröter, R. Savcenko, W. Bosch, A. Albertella, R. Rummel, and O. Klatt With the focus on the Southern Ocean circulation, results of assimilation of multi-mission-altimeter data and the GRACE/GOCE gravity data into the finite element ocean model (FEOM) are investigated. We use the geodetic method to obtain the dynamical ocean topography (DOT). This method combines the multi-mission-altimeter sea surface height and the GRACE/GOCE gravity field. Using the profile approach, the spectral consistency of both fields is achieved by filtering the sea surface height and the geoid. By combining the GRACE and GOCE data, a considerably shorter filter length can be used, which results in more DOT details. We show that this increase in resolution of measured DOT carries onto the results of data assimilation for the surface data. By assimilating only absolute dynamical topography data using the ensemble Kalman filter, we were able to improve modeled fields. Results are closer to observations which were not used for assimilation and lie outside the area covered by altimetry in the Southern Ocean (e.g. temperature of surface drifters or deep temperatures in the Weddell Sea area at 800 m depth derived from Argo composite.)

Beschreibung: A multi-decadal meridional displacement of the Subpolar Front in the Newfoundland Basin Ocean Science, 8, 91-102, 2012 Author(s): I. Núñez-Riboni, M. Bersch, H. Haak, J. H. Jungclaus, and K. Lohmann Observations since the 1950s show a multi-decadal cycle of a meridional displacement of the Subpolar Front (SPF) in the Newfoundland Basin (NFB) in the North Atlantic. The SPF displacement is associated with corresponding variations in the path of the North Atlantic Current. We use the ocean general circulation model MPIOM with enhanced horizontal and vertical resolutions and forced with NCEP/NCAR reanalysis data to study the relation of the SPF displacement to atmospheric forcing, intensities of the subpolar gyre (SPG) and Meridional Overturning Circulation (MOC), and Labrador Sea Water (LSW) volume. The simulations indicate that the SPF displacement is associated with a circulation anomaly between the SPG and the subtropical gyre (STG), an inter-gyre gyre with a multi-decadal time scale. A sensitivity experiment indicates that both wind stress curl (WSC) and heat fluxes (which match LSW changes) contribute to the circulation anomalies in the frontal region and to the SPF displacement. An anticyclonic inter-gyre gyre is related to negative WSC and LSW anomalies and to a SPF north of its climatological position, indicating an expanding STG. A cyclonic inter-gyre gyre is related to positive WSC and LSW anomalies and a SPF south of its climatological position, indicating an expanding SPG. Therefore, the mean latitudinal position of the SPF in the NFB (a "SPF index") could be an indicator of the amount of LSW in the inter-gyre region. Spreading of LSW anomalies intensifies the MOC, suggesting our SPF index as predictor of the MOC intensity at multi-decadal time scales. The meridional displacement of the SPF has a pronounced influence on the meridional heat transport, both on its gyre and overturning components.

Beschreibung: High resolution 3-D temperature and salinity fields derived from in situ and satellite observations Ocean Science, 8, 845-857, 2012 Author(s): S. Guinehut, A.-L. Dhomps, G. Larnicol, and P.-Y. Le Traon This paper describes an observation-based approach that efficiently combines the main components of the global ocean observing system using statistical methods. Accurate but sparse in situ temperature and salinity profiles (mainly from Argo for the last 10 yr) are merged with the lower accuracy but high-resolution synthetic data derived from satellite altimeter and sea surface temperature observations to provide global 3-D temperature and salinity fields at high temporal and spatial resolution. The first step of the method consists in deriving synthetic temperature fields from altimeter and sea surface temperature observations, and salinity fields from altimeter observations, through multiple/simple linear regression methods. The second step of the method consists in combining the synthetic fields with in situ temperature and salinity profiles using an optimal interpolation method. Results show the revolutionary nature of the Argo observing system. Argo observations now allow a global description of the statistical relationships that exist between surface and subsurface fields needed for step 1 of the method, and can constrain the large-scale temperature and mainly salinity fields during step 2 of the method. Compared to the use of climatological estimates, results indicate that up to 50% of the variance of the temperature fields can be reconstructed from altimeter and sea surface temperature observations and a statistical method. For salinity, only about 20 to 30% of the signal can be reconstructed from altimeter observations, making the in situ observing system essential for salinity estimates. The in situ observations (step 2 of the method) further reduce the differences between the gridded products and the observations by up to 20% for the temperature field in the mixed layer, and the main contribution is for salinity and the near surface layer with an improvement up to 30%. Compared to estimates derived using in situ observations only, the merged fields provide a better reconstruction of the high resolution temperature and salinity fields. This also holds for the large-scale and low-frequency fields thanks to a better reduction of the aliasing due to the mesoscale variability. Contribution of the merged fields is then illustrated to describe qualitatively the temperature variability patterns for the period from 1993 to 2009.

Beschreibung: Modelling temperature and salinity in Liverpool Bay and the Irish Sea: sensitivity to model type and surface forcing Ocean Science, 8, 903-913, 2012 Author(s): C. K. O'Neill, J. A. Polton, J. T. Holt, and E. J. O'Dea Three shelf sea models are compared against observed surface temperature and salinity in Liverpool Bay and the Irish Sea: a 7 km NEMO (Nucleus for European Modelling of the Ocean) model, and 12 km and 1.8 km POLCOMS (Proudman Oceanographic Laboratory Coastal Ocean Modelling System) models. Each model is run with two different surface forcing datasets of different resolutions. Comparisons with a variety of observations from the Liverpool Bay Coastal Observatory show that increasing the surface forcing resolution improves the modelled surface temperature in all the models, in particular reducing the summer warm bias and winter cool bias. The response of surface salinity is more varied with improvements in some areas and deterioration in others. The 7 km NEMO model performs as well as the 1.8 km POLCOMS model when measured by overall skill scores, although the sources of error in the models are different. NEMO is too weakly stratified in Liverpool Bay, whereas POLCOMS is too strongly stratified. The horizontal salinity gradient, which is too strong in POLCOMS, is better reproduced by NEMO which uses a more diffusive horizontal advection scheme. This leads to improved semi-diurnal variability in salinity in NEMO at a mooring site located in the Liverpool Bay ROFI (region of freshwater influence) area.

Beschreibung: Using dissolved oxygen concentrations to determine mixed layer depths in the Bellingshausen Sea Ocean Science, 8, 1-10, 2012 Author(s): K. Castro-Morales and J. Kaiser Concentrations of oxygen (O 2 ) and other dissolved gases in the oceanic mixed layer are often used to calculate air-sea gas exchange fluxes. The mixed layer depth ( z mix ) may be defined using criteria based on temperature or density differences to a reference depth near the ocean surface. However, temperature criteria fail in regions with strong haloclines such as the Southern Ocean where heat, freshwater and momentum fluxes interact to establish mixed layers. Moreover, the time scales of air-sea exchange differ for gases and heat, so that z mix defined using oxygen may be different than z mix defined using temperature or density. Here, we propose to define an O 2 -based mixed layer depth, z mix (O 2 ), as the depth where the relative difference between the O 2 concentration and a reference value at a depth equivalent to 10 dbar equals 0.5 %. This definition was established by analysis of O 2 profiles from the Bellingshausen Sea (west of the Antarctic Peninsula) and corroborated by visual inspection. Comparisons of z mix (O 2 ) with z mix based on potential temperature differences, i.e., z mix (0.2 °C) and z mix (0.5 °C), and potential density differences, i.e., z mix (0.03 kg m −3 ) and z mix (0.125 kg m −3 ), showed that z mix (O 2 ) closely follows z mix (0.03 kg m −3 ). Further comparisons with published z mix climatologies and z mix derived from World Ocean Atlas 2005 data were also performed. To establish z mix for use with biological production estimates in the absence of O 2 profiles, we suggest using z mix (0.03 kg m −3 ), which is also the basis for the climatology by de Boyer Montégut et al. (2004).

Beschreibung: The 2011 marine heat wave in Cockburn Sound, southwest Australia Ocean Science, 8, 545-550, 2012 Author(s): T. H. Rose, D. A. Smale, and G. Botting Over 2000 km of Western Australian coastline experienced a significant marine heat wave in February and March 2011. Seawater temperature anomalies of +2–4 °C were recorded at a number of locations, and satellite-derived SSTs (sea surface temperatures) were the highest on record. Here, we present seawater temperatures from southwestern Australia and describe, in detail, the marine climatology of Cockburn Sound, a large, multiple-use coastal embayment. We compared temperature and dissolved oxygen levels in 2011 with data from routine monitoring conducted from 2002–2010. A significant warming event, 2–4 °C in magnitude, persisted for 〉 8 weeks, and seawater temperatures at 10 to 20 m depth were significantly higher than those recorded in the previous 9 yr. Dissolved oxygen levels were depressed at most monitoring sites, being ~ 2 mg l −1 lower than usual in early March 2011. Ecological responses to short-term extreme events are poorly understood, but evidence from elsewhere along the Western Australian coastline suggests that the heat wave was associated with high rates of coral bleaching; fish, invertebrate and macroalgae mortalities; and algal blooms. However, there is a paucity of historical information on ecologically-sensitive habitats and taxa in Cockburn Sound, so that formal examinations of biological responses to the heat wave were not possible. The 2011 heat wave provided insights into conditions that may become more prevalent in Cockburn Sound, and elsewhere, if the intensity and frequency of short-term extreme events increases as predicted.

Beschreibung: Arctic surface temperatures from Metop AVHRR compared to in situ ocean and land data Ocean Science, 8, 959-970, 2012 Author(s): G. Dybkjær, R. Tonboe, and J. L. Høyer The ice surface temperature (IST) is an important boundary condition for both atmospheric and ocean and sea ice models and for coupled systems. An operational ice surface temperature product using satellite Metop AVHRR infra-red data was developed for MyOcean. The IST can be mapped in clear sky regions using a split window algorithm specially tuned for sea ice. Clear sky conditions prevail during spring in the Arctic, while persistent cloud cover limits data coverage during summer. The cloud covered regions are detected using the EUMETSAT cloud mask. The Metop IST compares to 2 m temperature at the Greenland ice cap Summit within STD error of 3.14 °C and to Arctic drifting buoy temperature data within STD error of 3.69 °C. A case study reveals that the in situ radiometer data versus satellite IST STD error can be much lower (0.73 °C) and that the different in situ measurements complicate the validation. Differences and variability between Metop IST and in situ data are analysed and discussed. An inter-comparison of Metop IST, numerical weather prediction temperatures and in situ observation indicates large biases between the different quantities. Because of the scarcity of conventional surface temperature or surface air temperature data in the Arctic, the satellite IST data with its relatively good coverage can potentially add valuable information to model analysis for the Arctic atmosphere.

Beschreibung: Characteristics of the seasonal cycle of surface layer salinity in the global ocean Ocean Science, 8, 915-929, 2012 Author(s): F. M. Bingham, G. R. Foltz, and M. J. McPhaden The seasonal variability of surface layer salinity (SLS), evaporation ( E ), precipitation ( P ), E-P , advection and vertical entrainment over the global ocean is examined using in situ salinity data, the National Centers for Environmental Prediction's Climate System Forecast Reanalysis and a number of other ancillary data. Seasonal amplitudes and phases are calculated using harmonic analysis and presented in all areas of the open ocean between 60° S and 60° N. Areas with large amplitude SLS seasonal variations include: the intertropical convergence zone (ITCZ) in the Atlantic, Pacific and Indian Oceans; western marginal seas of the Pacific; and the Arabian Sea. The median amplitude in areas that have statistically significant seasonal cycles of SLS is 0.19. Between about 60° S and 60° N, 37% of the ocean surface has a statistically significant seasonal cycle of SLS and 75% has a seasonal cycle of E-P . Phases of SLS have a bimodal distribution, with most areas in the Northern Hemisphere peaking in SLS in March/April and in the Southern Hemisphere in September/October. The seasonal cycle is also estimated for surface freshwater forcing using a mixed-layer depth climatology. With the exception of areas near the western boundaries of the North Atlantic and North Pacific, seasonal variability is dominated by precipitation. Surface freshwater forcing also has a bimodal distribution, with peaks in January and July, 1–2 months before the peaks of SLS. Seasonal amplitudes and phases calculated for horizontal advection show it to be important in the tropical oceans. Vertical entrainment, estimated from mixed-layer heaving, is largest in mid and high latitudes, with a seasonal cycle that peaks in late winter. The amplitudes and phases of SLS and surface fluxes compare well in a qualitative sense, suggesting that much of the variability in SLS is due to E-P . However, the amplitudes of SLS are somewhat different than would be expected and the peak of SLS comes typically about one month earlier than expected. The differences of the amplitudes of the two quantities is largest in such areas as the Amazon River plume, the Arabian Sea, the ITCZ and the eastern equatorial Pacific and Atlantic.

Beschreibung: Arctic Ocean circulation and variability – advection and external forcing encounter constraints and local processes Ocean Science, 8, 261-286, 2012 Author(s): B. Rudels The first hydrographic data from the Arctic Ocean, the section from the Laptev Sea to the passage between Greenland and Svalbard obtained by Nansen on his drift with Fram 1893–1896, aptly illustrate the main features of Arctic Ocean oceanography and indicate possible processes active in transforming the water masses in the Arctic Ocean. Many, perhaps most, processes were identified already by Nansen, who put his mark on almost all subsequent research in the Arctic. Here we shall revisit some key questions and follow how our understanding has evolved from the early 20th century to present. What questions, if any, can now be regarded as solved and which remain still open? Five different but connected topics will be discussed: (1) The low salinity surface layer and the storage and export of freshwater. (2) The vertical heat transfer from the Atlantic water to sea ice and to the atmosphere. (3) The circulation and mixing of the two Atlantic inflow branches. (4) The formation and circulation of deep and bottom waters in the Arctic Ocean. (5) The exchanges through Fram Strait. Foci will be on the potential effects of increased freshwater input and reduced sea ice export on the freshwater storage and residence time in the Arctic Ocean, on the deep waters of the Makarov Basin, and on the circulation and relative importance of the two inflows, over the Barents Sea and through Fram Strait, for the distribution of heat in the intermediate layers of the Arctic Ocean.

Beschreibung: Assimilating GlobColour ocean colour data into a pre-operational physical-biogeochemical model Ocean Science, 8, 751-771, 2012 Author(s): D. A. Ford, K. P. Edwards, D. Lea, R. M. Barciela, M. J. Martin, and J. Demaria As part of the GlobColour project, daily chlorophyll a observations, derived using remotely sensed ocean colour data from the MERIS, MODIS and SeaWiFS sensors, are produced. The ability of these products to be assimilated into a pre-operational global coupled physical-biogeochemical model has been tested, on both a hindcast and near-real-time basis, and the impact on the system assessed. The assimilation was found to immediately and considerably improve the bias, root mean square error and correlation of modelled surface chlorophyll concentration compared to the GlobColour observations, an improvement which was sustained throughout the year and in every ocean basin. Errors against independent in situ chlorophyll observations were also reduced, both at and beneath the ocean surface. However, the model fit to in situ observations was not consistently better than that of climatology, due to errors in the underlying model. The assimilation scheme used is multivariate, updating all biogeochemical model state variables at all depths. The other variables were not degraded by the assimilation, with annual mean surface fields of nutrients, alkalinity and carbon variables remaining of similar quality compared to climatology. There was evidence of improved representation of zooplankton concentration, and reduced errors were seen against in situ observations of nitrate and p CO 2 , but too few observations were available to conclude about global model skill. The near-real-time GlobColour products were found to be sufficiently reliable for operational purposes, and of benefit to both operational-style systems and reanalyses.

Beschreibung: Assimilation of SLA along track observations in the Mediterranean with an oceanographic model forced by atmospheric pressure Ocean Science, 8, 787-795, 2012 Author(s): S. Dobricic, C. Dufau, P. Oddo, N. Pinardi, I. Pujol, and M.-H. Rio A large number of SLA observations at a high along track horizontal resolution are an important ingredient of the data assimilation in the Mediterranean Forecasting System (MFS). Recently, new higher-frequency SLA products have become available, and the atmospheric pressure forcing has been implemented in the numerical model used in the MFS data assimilation system. In a set of numerical experiments, we show that, in order to obtain the most accurate analyses, the ocean model should include the atmospheric pressure forcing and the observations should contain the atmospheric pressure signal. When the model is not forced by the atmospheric pressure, the high-frequency filtering of SLA observations, however, improves the quality of the SLA analyses. It is further shown by comparing the power density spectra of the model fields and observations that the model is able to extract the correct information from noisy observations even without their filtering during the pre-processing.

Beschreibung: Obstacles and benefits of the implementation of a reduced-rank smoother with a high resolution model of the tropical Atlantic Ocean Ocean Science, 8, 797-811, 2012 Author(s): N. Freychet, E. Cosme, P. Brasseur, J.-M. Brankart, and E. Kpemlie Most of oceanographic operational centers use three-dimensional data assimilation schemes to produce reanalyses. We investigate here the benefits of a smoother, i.e. a four-dimensional formulation of statistical assimilation. A square-root sequential smoother is implemented with a tropical Atlantic Ocean circulation model. A simple twin experiment is performed to investigate its benefits, compared to its corresponding filter. Despite model's non-linearities and the various approximations used for its implementation, the smoother leads to a better estimation of the ocean state, both on statistical (i.e. mean error level) and dynamical points of view, as expected from linear theory. Smoothed states are more in phase with the dynamics of the reference state, an aspect that is nicely illustrated with the chaotic dynamics of the North Brazil Current rings. We also show that the smoother efficiency is strongly related to the filter configuration. One of the main obstacles to implement the smoother is then to accurately estimate the error covariances of the filter. Considering this, benefits of the smoother are also investigated with a configuration close to situations that can be managed by operational center systems, where covariances matrices are fixed (optimal interpolation). We define here a simplified smoother scheme, called half-fixed basis smoother, that could be implemented with current reanalysis schemes. Its main assumption is to neglect the propagation of the error covariances matrix, what leads to strongly reduce the cost of assimilation. Results illustrate the ability of this smoother to provide a solution more consistent with the dynamics, compared to the filter. The smoother is also able to produce analyses independently of the observation frequency, so the smoothed solution appears more continuous in time, especially in case of a low frenquency observation network.

Beschreibung: Fate of river Tiber discharge investigated through numerical simulation and satellite monitoring Ocean Science, 8, 773-786, 2012 Author(s): R. Inghilesi, L. Ottolenghi, A. Orasi, C. Pizzi, F. Bignami, and R. Santoleri The aim of this study was to determine the dispersion of passive pollutants associated with the Tiber discharge into the Tyrrhenian Sea using numerical marine dispersion models and satellite data. Numerical results obtained in the simulation of realistic discharge episodes were compared with the corresponding evolution of the spatial distributions of MODIS diffuse light attenuation coefficient at 490 nm (K490), and the results were discussed with reference to the local climate and the seasonal sub-regional circulation regime. The numerical model used for the simulation of the sub-tidal circulation was a Mediterranean sub-regional scale implementation of the Princeton Ocean Model (POM), nested in the large-scale Mediterranean Forecasting System. The nesting method enabled the model to be applied to almost every area in the Mediterranean Sea and also to be used in seasons for which imposing climatological boundary conditions would have been questionable. Dynamical effects on coastal circulation and on water density due to the Tiber discharge were additionally accounted for in the oceanographic model by implementing the river estuary as a point source of a buoyant jet. A Lagrangian particle dispersion model fed with the POM current fields was then run in order to reproduce the effect of the turbulent transport of passive tracers mixed in the plume with the coastal flow. Two significant episodes of river discharge in both winter and summer conditions were discussed in this paper. It was found that the winter regime was characterized by the presence of a strong coastal jet flowing with the ambient current. In summer the prevailing wind regime induced coastal downwelling conditions, which tended to confine the riverine waters close to the shore. In such conditions sudden wind reversals due to local weather perturbations, causing moderate local upwelling, proved to be the only effective way to disperse the tracers offshore, moving the plume from the coast and detaching large pools of freshwater.

Beschreibung: Variability of synoptic-scale quasi-stationary thermohaline stratification patterns in the Gulf of Finland in summer 2009 Ocean Science, 8, 603-614, 2012 Author(s): T. Liblik and U. Lips We present and analyze high-resolution observational data of thermohaline structure and currents acquired in the Gulf of Finland (Baltic Sea), using an autonomous buoy profiler and bottom-mounted acoustic Doppler current profiler during July–August 2009. Vertical profiles of temperature and salinity were measured in the upper 50-m layer with a 3 h time resolution, and vertical profiles of current velocity and direction were recorded with a 10 min time resolution. Although large temporal variations of vertical temperature and salinity distributions were revealed, it was possible to define several periods with quasi-stationary vertical thermohaline structure. These quasi-stationary stratification patterns persisted for 4–15 days and were dominated by certain physical processes: upwelling, relaxation of upwelling, estuarine circulation and its wind-induced reversal, and downwelling. Vertical profiles of current velocities supported the concept of synoptic-scale, quasi-stationary periods of hydrophysical fields, characterized by distinct layered flow structures and current oscillations. To estimate the contribution of different processes to the changes in stratification, a simple conceptual model was developed. The model accounts for heat flux through the sea surface, wind mixing, wind-induced transport (parallel to the horizontal salinity gradient) in the upper layer, and estuarine circulation. It reproduced observed changes in vertical stratification reasonably well. The largest discrepancies between observations and model results were found when water motions across the Gulf and associated vertical displacements of isopycnals (upwelling or downwelling) were dominant processes.

Beschreibung: In situ determination of the remote sensing reflectance: an inter-comparison Ocean Science, 8, 567-586, 2012 Author(s): G. Zibordi, K. Ruddick, I. Ansko, G. Moore, S. Kratzer, J. Icely, and A. Reinart Inter-comparison of data products from simultaneous measurements performed with independent systems and methods is a viable approach to assess the consistency of data and additionally to investigate uncertainties. Within such a context the inter-comparison called Assessment of In Situ Radiometric Capabilities for Coastal Water Remote Sensing Applications (ARC) was carried out at the Acqua Alta Oceanographic Tower in the northern Adriatic Sea to explore the accuracy of in situ data products from various in- and above-water optical systems and methods. Measurements were performed under almost ideal conditions, including a stable deployment platform, clear sky, relatively low sun zenith angles and moderately low sea state. Additionally, all optical sensors involved in the experiment were inter-calibrated through absolute radiometric calibration performed with the same standards and methods. Inter-compared data products include spectral water-leaving radiance L w (λ), above-water downward irradiance E d (0 + ,λ) and remote sensing reflectance R rs (λ). Data products from the various measurement systems/methods were directly compared to those from a single reference system/method. Results for R rs (λ) indicate spectrally averaged values of relative differences comprised between −1 and +6%, while spectrally averaged values of absolute differences vary from approximately 6% for the above-water systems/methods to 9% for buoy-based systems/methods. The agreement between R rs (λ) spectral relative differences and estimates of combined uncertainties of the inter-compared systems/methods is noteworthy.

Beschreibung: Estimation of positive sum-to-one constrained zooplankton grazing preferences with the DEnKF: a twin experiment Ocean Science, 8, 587-602, 2012 Author(s): E. Simon, A. Samuelsen, L. Bertino, and D. Dumont We consider the estimation of the grazing preferences parameters of zooplankton in ocean ecosystem models with ensemble-based Kalman filters. These parameters are introduced to model the relative diet composition of zooplankton that consists of phytoplankton, small size-classes of zooplankton and detritus. They are positive values and their sum is equal to one. However, the sum-to-one constraint cannot be guaranteed by ensemble-based Kalman filters when parameters are bounded. Therefore, a reformulation of the parameterization is proposed. We investigate two types of variable transformations for the estimation of positive sum-to-one constrained parameters that lead to the estimation of a new set of parameters with normal or bounded distributions. These transformations are illustrated and discussed with twin experiments performed with the 1-D coupled model GOTM-NORWECOM with Gaussian anamorphosis extensions of the deterministic ensemble Kalman filter (DEnKF).

Beschreibung: Validation of FOAM near-surface ocean current forecasts using Lagrangian drifting buoys Ocean Science, 8, 551-565, 2012 Author(s): E. W. Blockley, M. J. Martin, and P. Hyder In this study, the quality of near-surface current forecasts from the FOAM ocean forecasting system is assessed using the trajectories of Lagrangian drifting buoys. A method is presented for deriving pseudo-Eulerian estimates of ocean currents from the positions of Surface Velocity Program drifters and the resulting data are compared to velocities observed by the global tropical moored buoy array. A quantitative analysis of the global FOAM velocities is performed for the period 2007 and 2008 using currents derived from over 3000 unique drifters (providing an average of 650 velocity observations per day). A potential bias is identified in the Southern Ocean which appears to be caused by wind-slip in the drifter dataset as a result of drogue loss. The drifter-derived currents are also used to show how the data assimilation scheme and a recent system upgrade impact upon the quality of FOAM current forecasts.

Beschreibung: Towards high resolution mapping of 3-D mesoscale dynamics from observations Ocean Science, 8, 885-901, 2012 Author(s): B. Buongiorno Nardelli, S. Guinehut, A. Pascual, Y. Drillet, S. Ruiz, and S. Mulet The MyOcean R&D project MESCLA (MEsoSCaLe dynamical Analysis through combined model, satellite and in situ data) was devoted to the high resolution 3-D retrieval of tracer and velocity fields in the oceans, based on the combination of in situ and satellite observations and quasi-geostrophic dynamical models. The retrieval techniques were also tested and compared with the output of a primitive equation model, with particular attention to the accuracy of the vertical velocity field as estimated through the Q vector formulation of the omega equation. The project focused on a test case, covering the region where the Gulf Stream separates from the US East Coast. This work demonstrated that innovative methods for the high resolution mapping of 3-D mesoscale dynamics from observations can be used to build the next generations of operational observation-based products.

Beschreibung: Effect of variable winds on current structure and Reynolds stresses in a tidal flow: analysis of experimental data in the eastern English Channel Ocean Science, 8, 1025-1040, 2012 Author(s): K. A. Korotenko, A. V. Sentchev, and F. G. Schmitt Wind and wave effects on tidal current structure and turbulence throughout the water column are examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument has been deployed on the seafloor of 18-m mean depth, off the north-eastern French coast in the eastern English Channel, over 12 tidal cycles, and covered the period of the transition from mean spring to neap tide, and forcing regimes varied from calm to moderate storm conditions. During storms, we observed gusty winds with magnitudes reaching 15 m s −1 and wave heights reaching up to 1.3 m. Analysis of velocity spectra revealed a noticeable contribution of wind-induced waves to spectral structure of velocity fluctuations within the subsurface layer. Near the surface, stormy winds and waves produced a significant intensification of velocity fluctuations, particularly when the sustained wind blew against the ebb tide flow. As during wavy periods, the variance-derived Reynolds stress estimates might include a wave-induced contamination, we applied the Variance Fit method to obtain unbiased stresses and other turbulent quantities. Over calm periods, the turbulent quantities usually decreased with height above the seabed. The stresses were found to vary regularly with the predominantly semidiurnal tidal flow. The along-shore stress being generally greater during the flood flow (~2.7 Pa) than during the ebb flow (~−0.6 Pa). The turbulent kinetic energy production rate, P , and eddy viscosity, A z , followed a nearly regular cycle with close to a quarter-diurnal period. As for the stresses, near the seabed, we found the maximum values of estimated quantities of P and A z to be 0.1 Wm −3 and 0.5 m 2 s −1 , respectively, during the flood flow. Over the storm periods, we found the highest unbiased stress values (~−2.6 Pa) during ebb when tidal currents were opposite to the southwesterly winds while, during the flood, the surface stresses slightly exceeded those estimated for a calm period. A comparison of obtained results gives a good agreement with those of other researchers working on direct measurements of turbulence in tidal flows.

Beschreibung: Validation of the NEMO-ERSEM operational ecosystem model for the North West European Continental Shelf Ocean Science, 8, 983-1000, 2012 Author(s): K. P. Edwards, R. Barciela, and M. Butenschön This paper details updates to the Met Office's operational coupled hydrodynamic-ecosystem model from the 7 km Medium-Resolution Continental Shelf – POLCOMS-ERSEM (MRCS-PE) system (Siddorn et al., 2007) to the 7 km Atlantic Margin Model NEMO-ERSEM (AMM7-NE) system. We also provide a validation of the ecosystem component of the new operational system. Comparisons have been made between the model variables and available in situ, satellite and climatological data. The AMM7-NE system has also been benchmarked against the MRCS-PE system. The transition to the new AMM7-NE system was successful and it has been running operationally since March 2012 and has been providing products through MyOcean ( http://www.myocean.eu.org ) since that time. The results presented herein show the AMM7-NE system performs better than the MRCS-PE system with the most improvement in the model nutrient fields. The problem of nutrient accumulation in the MRCS-PE system appears to be solved in the new AMM7-NE system with nutrient fields improved throughout the domain as discussed in Sect. 4. Improvements in model chlorophyll are also seen but are more modest.

Beschreibung: Deep Western Boundary Current transport variability in the South Atlantic: preliminary results from a pilot array at 34.5° S Ocean Science, 8, 1041-1054, 2012 Author(s): C. S. Meinen, A. R. Piola, R. C. Perez, and S. L. Garzoli The first direct estimates of the temporal variability of the absolute transport in the Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm, based on neutral density, temperature, salinity, and oxygen values, the presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation. Both data sets indicate that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8° S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed in the North Atlantic, where longer records exist, with the DWBC at 34.5° S exhibiting a transport standard deviation of 25 Sv and variations of ∼ 40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observational records are too short to be definitive. A Monte Carlo-style analysis using 27 yr of model output from the same location as the observations indicates that about 48–60 months of data will be required to fully assess the deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however its statistical significance with even 27 yr of model output is low, suggesting that seasonal variations in the model are weak.

Beschreibung: Improvement to the PhytoDOAS method for identification of coccolithophores using hyper-spectral satellite data Ocean Science, 8, 1055-1070, 2012 Author(s): A. Sadeghi, T. Dinter, M. Vountas, B. B. Taylor, M. Altenburg-Soppa, I. Peeken, and A. Bracher The goal of this study was to improve PhytoDOAS, which is a new retrieval method for quantitative identification of major phytoplankton functional types (PFTs) using hyper-spectral satellite data. PhytoDOAS is an extension of the Differential Optical Absorption Spectroscopy (DOAS, a method for detection of atmospheric trace gases), developed for remote identification of oceanic phytoplankton groups. Thus far, PhytoDOAS has been successfully exploited to identify cyanobacteria and diatoms over the global ocean from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) hyper-spectral data. This study aimed to improve PhytoDOAS for remote identification of coccolithophores, another functional group of phytoplankton. The main challenge for retrieving more PFTs by PhytoDOAS is to overcome the correlation effects between different PFT absorption spectra. Different PFTs are composed of different types and amounts of pigments, but also have pigments in common, e.g. chl a , causing correlation effects in the usual performance of the PhytoDOAS retrieval. Two ideas have been implemented to improve PhytoDOAS for the PFT retrieval of more phytoplankton groups. Firstly, using the fourth-derivative spectroscopy, the peak positions of the main pigment components in each absorption spectrum have been derived. After comparing the corresponding results of major PFTs, the optimized fit-window for the PhytoDOAS retrieval of each PFT was determined. Secondly, based on the results from derivative spectroscopy, a simultaneous fit of PhytoDOAS has been proposed and tested for a selected set of PFTs (coccolithophores, diatoms and dinoflagellates) within an optimized fit-window, proven by spectral orthogonality tests. The method was then applied to the processing of SCIAMACHY data over the year 2005. Comparisons of the PhytoDOAS coccolithophore retrievals in 2005 with other coccolithophore-related data showed similar patterns in their seasonal distributions, especially in the North Atlantic and the Arctic Sea. The seasonal patterns of the PhytoDOAS coccolithophores indicated very good agreement with the coccolithophore modeled data from the NASA Ocean Biochemical Model (NOBM), as well as with the global distributions of particulate inorganic carbon (PIC), provided by MODIS (MODerate resolution Imaging Spectroradiometer)-Aqua level-3 products. Moreover, regarding the fact that coccolithophores belong to the group of haptophytes, the PhytoDOAS seasonal coccolithophores showed good agreement with the global distribution of haptophytes, derived from synoptic pigment relationships applied to SeaWiFS chl a . As a case study, the simultaneous mode of PhytoDOAS has been applied to SCIAMACHY data for detecting a coccolithophore bloom which was consistent with the MODIS RGB image and the MODIS PIC map of the bloom, indicating the functionality of the method also in short-term retrievals.

Beschreibung: Seawater capacitance – a promising proxy for mapping and characterizing drifting hydrocarbon plumes in the deep ocean Ocean Science, 8, 1099-1104, 2012 Author(s): J. C. Wynn and J. A. Fleming Hydrocarbons released into the deep ocean are an inevitable consequence of natural seep, seafloor drilling, and leaking wellhead-to-collection-point pipelines. The Macondo 252 (Deepwater Horizon) well blowout of 2010 was even larger than the Ixtoc event in the Gulf of Campeche in 1979. History suggests it will not be the last accidental release, as deepwater drilling expands to meet an ever-growing demand. For those who must respond to this kind of disaster, the first line of action should be to know what is going on. This includes knowing where an oil plume is at any given time, where and how fast it is moving, and how it is evolving or degrading. We have experimented in the laboratory with induced polarization as a method to track hydrocarbons in the seawater column and find that finely dispersed oil in seawater gives rise to a large distributed capacitance. From previous sea trials, we infer this could potentially be used to both map and characterize oil plumes, down to a ratio of less than 0.001 oil-to-seawater, drifting and evolving in the deep ocean. A side benefit demonstrated in some earlier sea trials is that this same approach in modified form can also map certain heavy placer minerals, as well as communication cables, pipelines, and wrecks buried beneath the seafloor.

Beschreibung: Predictions for oil slicks detected from satellite images using MyOcean forecasting data Ocean Science, 8, 1105-1115, 2012 Author(s): G. Zodiatis, R. Lardner, D. Solovyov, X. Panayidou, and M. De Dominicis The pan-European capacity for the Ocean Monitoring and Forecasting (MyOcean) Marine Core Service, implementing the Global Monitoring for Environment and Security (GMES) objectives, targets the provision of ocean state observations from various platforms and analysis and forecasting products to assist, among other downscaling activities, the needs of the operational response to marine safety, particularly concerning oil spills. The MEDSLIK oil spill and trajectory prediction system makes use of the MyOcean regional and Cyprus Coastal Ocean Forecasting and Observing System (CYCOFOS) downscaled forecasting products for operational application in the Mediterranean and pre-operational use in the Black Sea. Advanced Synthetic Aperture Radar (ASAR) satellite remote sensing images from the European Space Agency (ESA) and European Maritime Safety Agency CleanSeaNet (EMSA-CSN) provide the means for routine monitoring of the southern European seas for the detection of illegal oil discharges. MEDSLIK offers various ways, to be described in this paper, of coupling the MyOcean forecasting data with ASAR images to provide both forecasts and hindcasts for such remotely observed oil slicks. The main concern will be the drift of the oil slick and also, in the case of the forecast mode, its diffusive spreading, although some attempt is also made to estimate the changes in the state of the oil. The successful link of the satellite-detected oil slicks with their operational predictions using the MyOcean products contributes to the operational response chain and the strengthening of maritime safety for accidental or illegal spills, in implementation of the Mediterranean Decision Support System for Marine Safety (MEDESS-4MS) regarding oil spills.

Beschreibung: An Overview of the Synoptic Antarctic Shelf-Slope Interactions (SASSI) project for the International Polar Year Ocean Science, 8, 1111-1116, 2012 Author(s): K. J. Heywood, R. Muench, and G. Williams

Beschreibung: A global algorithm for estimating Absolute Salinity Ocean Science, 8, 1117-1128, 2012 Author(s): T. J. McDougall, D. R. Jackett, F. J. Millero, R. Pawlowicz, and P. M. Barker The International Thermodynamic Equation of Seawater – 2010 has defined the thermodynamic properties of seawater in terms of a new salinity variable, Absolute Salinity, which takes into account the spatial variation of the composition of seawater. Absolute Salinity more accurately reflects the effects of the dissolved material in seawater on the thermodynamic properties (particularly density) than does Practical Salinity. When a seawater sample has standard composition (i.e. the ratios of the constituents of sea salt are the same as those of surface water of the North Atlantic), Practical Salinity can be used to accurately evaluate the thermodynamic properties of seawater. When seawater is not of standard composition, Practical Salinity alone is not sufficient and the Absolute Salinity Anomaly needs to be estimated; this anomaly is as large as 0.025 g kg −1 in the northernmost North Pacific. Here we provide an algorithm for estimating Absolute Salinity Anomaly for any location ( x, y, p ) in the world ocean. To develop this algorithm, we used the Absolute Salinity Anomaly that is found by comparing the density calculated from Practical Salinity to the density measured in the laboratory. These estimates of Absolute Salinity Anomaly however are limited to the number of available observations (namely 811). In order to provide a practical method that can be used at any location in the world ocean, we take advantage of approximate relationships between Absolute Salinity Anomaly and silicate concentrations (which are available globally).

Beschreibung: Marine atmospheric boundary layer over some Southern Ocean fronts during the IPY BGH 2008 cruise Ocean Science, 8, 1001-1023, 2012 Author(s): C. Messager, S. Speich, and E. Key A set of meteorological instruments was added to an oceanographic cruise crossing the Southern Ocean from Cape Town to 57°33' S during the summer of 2008. The Cape Cauldron, the Subtropical, Subantarctic, Polar and southern Antarctic Circumpolar current fronts were successively crossed. The recorded data permitted to derive the exchange of momentum, heat and water vapour at the ocean-atmosphere interface. A set of 38 radiosonde releases complemented the dataset. The marine atmospheric boundary layer characteristics and air-sea interaction when the ship crossed the fronts and eddies are discussed. The specific role of the atmospheric synoptic systems advection on the air-sea interaction over these regions is highlighted. Additionally, the Subantarctic front mesoscale variability induced an anticyclonic eddy considered as part of the Subantarctic front. The specific influence of this Agulhas ring on the aloft atmosphere is also presented.

Beschreibung: Surface signature of Mediterranean water eddies in the Northeastern Atlantic: effect of the upper ocean stratification Ocean Science, 8, 931-943, 2012 Author(s): I. Bashmachnikov and X. Carton Meddies, intra-thermocline eddies of Mediterranean water, can often be detected at the sea surface as positive sea-level anomalies. Here we study the surface signature of several meddies tracked with RAFOS floats and AVISO altimetry. While pushing its way through the water column, a meddy raises isopycnals above. As a consequence of potential vorticity conservation, negative relative vorticity is generated in the upper layer. During the initial period of meddy acceleration after meddy formation or after a stagnation stage, a cyclonic signal is also generated at the sea-surface, but mostly the anticyclonic surface signal follows the meddy. Based on geostrophy and potential vorticity balance, we present theoretical estimates of the intensity of the surface signature. It appears to be proportional to the meddy core radius and to the Coriolis parameter, and inversely proportional to the core depth and buoyancy frequency. This indicates that surface signature of a meddy may be strongly reduced by the upper ocean stratification. Using climatic distribution of the stratification intensity, we claim that the southernmost limit for detection in altimetry of small meddies (with radii on the order of 10–15 km) should lie in the subtropics (35–45° N), while large meddies (with radii of 25–30 km) could be detected as far south as the northern tropics (25–35° N). Those results agree with observations.

Beschreibung: The link between the Barents Sea and ENSO events simulated by NEMO model Ocean Science, 8, 971-982, 2012 Author(s): V. N. Stepanov, H. Zuo, and K. Haines An analysis of observational data in the Barents Sea along a meridian at 33°30' E between 70°30' and 72°30' N has reported a negative correlation between El Niño/La Niña Southern Oscillation (ENSO) events and water temperature in the top 200 m: the temperature drops about 0.5 °C during warm ENSO events while during cold ENSO events the top 200 m layer of the Barents Sea is warmer. Results from 1 and 1/4-degree global NEMO models show a similar response for the whole Barents Sea. During the strong warm ENSO event in 1997–1998 an anomalous anticyclonic atmospheric circulation over the Barents Sea enhances heat loses, as well as substantially influencing the Barents Sea inflow from the North Atlantic, via changes in ocean currents. Under normal conditions along the Scandinavian peninsula there is a warm current entering the Barents Sea from the North Atlantic, however after the 1997–1998 event this current is weakened. During 1997–1998 the model annual mean temperature in the Barents Sea is decreased by about 0.8 °C, also resulting in a higher sea ice volume. In contrast during the cold ENSO events in 1999–2000 and 2007–2008, the model shows a lower sea ice volume, and higher annual mean temperatures in the upper layer of the Barents Sea of about 0.7 °C. An analysis of model data shows that the strength of the Atlantic inflow in the Barents Sea is the main cause of heat content variability, and is forced by changing pressure and winds in the North Atlantic. However, surface heat-exchange with the atmosphere provides the means by which the Barents sea heat budget relaxes to normal in the subsequent year after the ENSO events.

Beschreibung: The seasonal variability of surface layer salinity (SLS), evaporation (E), precipitation (P), E-P, advection and vertical entrainment over the global ocean is examined using in situ salinity data, the National Centers for Environmental Prediction's Climate System Forecast Reanalysis and a number of other ancillary data. Seasonal amplitudes and phases are calculated using harmonic analysis and presented in all areas of the open ocean between 60° S and 60° N. Areas with large amplitude SLS seasonal variations include: the intertropical convergence zone (ITCZ) in the Atlantic, Pacific and Indian Oceans; western marginal seas of the Pacific; and the Arabian Sea. The median amplitude in areas that have statistically significant seasonal cycles of SLS is 0.19. Between about 60° S and 60° N, 37% of the ocean surface has a statistically significant seasonal cycle of SLS and 75% has a seasonal cycle of E-P. Phases of SLS have a bimodal distribution, with most areas in the Northern Hemisphere peaking in SLS in March/April and in the Southern Hemisphere in September/October. The seasonal cycle is also estimated for surface freshwater forcing using a mixed-layer depth climatology. With the exception of areas near the western boundaries of the North Atlantic and North Pacific, seasonal variability is dominated by precipitation. Surface freshwater forcing also has a bimodal distribution, with peaks in January and July, 1–2 months before the peaks of SLS. Seasonal amplitudes and phases calculated for horizontal advection show it to be important in the tropical oceans. Vertical entrainment, estimated from mixed-layer heaving, is largest in mid and high latitudes, with a seasonal cycle that peaks in late winter. The amplitudes and phases of SLS and surface fluxes compare well in a qualitative sense, suggesting that much of the variability in SLS is due to E-P. However, the amplitudes of SLS are somewhat different than would be expected and the peak of SLS comes typically about one month earlier than expected. The differences of the amplitudes of the two quantities is largest in such areas as the Amazon River plume, the Arabian Sea, the ITCZ and the eastern equatorial Pacific and Atlantic.

Beschreibung: The MyOcean R&D project MESCLA (MEsoSCaLe dynamical Analysis through combined model, satellite and in situ data) was devoted to the high resolution 3-D retrieval of tracer and velocity fields in the oceans, based on the combination of in situ and satellite observations and quasi-geostrophic dynamical models. The retrieval techniques were also tested and compared with the output of a primitive equation model, with particular attention to the accuracy of the vertical velocity field as estimated through the Q vector formulation of the omega equation. The project focused on a test case, covering the region where the Gulf Stream separates from the US East Coast. This work demonstrated that innovative methods for the high resolution mapping of 3-D mesoscale dynamics from observations can be used to build the next generations of operational observation-based products.

Beschreibung: The aim of this study was to determine the dispersion of passive pollutants associated with the Tiber discharge into the Tyrrhenian Sea using numerical marine dispersion models and satellite data. Numerical results obtained in the simulation of realistic discharge episodes were compared with the corresponding evolution of the spatial distributions of MODIS diffuse light attenuation coefficient at 490 nm (K490), and the results were discussed with reference to the local climate and the seasonal sub-regional circulation regime. The numerical model used for the simulation of the sub-tidal circulation was a Mediterranean sub-regional scale implementation of the Princeton Ocean Model (POM), nested in the large-scale Mediterranean Forecasting System. The nesting method enabled the model to be applied to almost every area in the Mediterranean Sea and also to be used in seasons for which imposing climatological boundary conditions would have been questionable. Dynamical effects on coastal circulation and on water density due to the Tiber discharge were additionally accounted for in the oceanographic model by implementing the river estuary as a point source of a buoyant jet. A Lagrangian particle dispersion model fed with the POM current fields was then run in order to reproduce the effect of the turbulent transport of passive tracers mixed in the plume with the coastal flow. Two significant episodes of river discharge in both winter and summer conditions were discussed in this paper. It was found that the winter regime was characterized by the presence of a strong coastal jet flowing with the ambient current. In summer the prevailing wind regime induced coastal downwelling conditions, which tended to confine the riverine waters close to the shore. In such conditions sudden wind reversals due to local weather perturbations, causing moderate local upwelling, proved to be the only effective way to disperse the tracers offshore, moving the plume from the coast and detaching large pools of freshwater.

Beschreibung: Meddies, intra-thermocline eddies of Mediterranean water, can often be detected at the sea surface as positive sea-level anomalies. Here we study the surface signature of several meddies tracked with RAFOS floats and AVISO altimetry. While pushing its way through the water column, a meddy raises isopycnals above. As a consequence of potential vorticity conservation, negative relative vorticity is generated in the upper layer. During the initial period of meddy acceleration after meddy formation or after a stagnation stage, a cyclonic signal is also generated at the sea-surface, but mostly the anticyclonic surface signal follows the meddy. Based on geostrophy and potential vorticity balance, we present theoretical estimates of the intensity of the surface signature. It appears to be proportional to the meddy core radius and to the Coriolis parameter, and inversely proportional to the core depth and buoyancy frequency. This indicates that surface signature of a meddy may be strongly reduced by the upper ocean stratification. Using climatic distribution of the stratification intensity, we claim that the southernmost limit for detection in altimetry of small meddies (with radii on the order of 10–15 km) should lie in the subtropics (35–45° N), while large meddies (with radii of 25–30 km) could be detected as far south as the northern tropics (25–35° N). Those results agree with observations.

Beschreibung: A large number of SLA observations at a high along track horizontal resolution are an important ingredient of the data assimilation in the Mediterranean Forecasting System (MFS). Recently, new higher-frequency SLA products have become available, and the atmospheric pressure forcing has been implemented in the numerical model used in the MFS data assimilation system. In a set of numerical experiments, we show that, in order to obtain the most accurate analyses, the ocean model should include the atmospheric pressure forcing and the observations should contain the atmospheric pressure signal. When the model is not forced by the atmospheric pressure, the high-frequency filtering of SLA observations, however, improves the quality of the SLA analyses. It is further shown by comparing the power density spectra of the model fields and observations that the model is able to extract the correct information from noisy observations even without their filtering during the pre-processing.

Beschreibung: We present a detailed description of TOPAZ4, the latest version of TOPAZ – a coupled ocean-sea ice data assimilation system for the North Atlantic Ocean and Arctic. It is the only operational, large-scale ocean data assimilation system that uses the ensemble Kalman filter. This means that TOPAZ features a time-evolving, state-dependent estimate of the state error covariance. Based on results from the pilot MyOcean reanalysis for 2003–2008, we demonstrate that TOPAZ4 produces a realistic estimate of the ocean circulation in the North Atlantic and the sea-ice variability in the Arctic. We find that the ensemble spread for temperature and sea-level remains fairly constant throughout the reanalysis demonstrating that the data assimilation system is robust to ensemble collapse. Moreover, the ensemble spread for ice concentration is well correlated with the actual errors. This indicates that the ensemble statistics provide reliable state-dependent error estimates – a feature that is unique to ensemble-based data assimilation systems. We demonstrate that the quality of the reanalysis changes when different sea surface temperature products are assimilated, or when in-situ profiles below the ice in the Arctic Ocean are assimilated. We find that data assimilation improves the match to independent observations compared to a free model. Improvements are particularly noticeable for ice thickness, salinity in the Arctic, and temperature in the Fram Strait, but not for transport estimates or underwater temperature. At the same time, the pilot reanalysis has revealed several flaws in the system that have degraded its performance. Finally, we show that a simple bias estimation scheme can effectively detect the seasonal or constant bias in temperature and sea-level.

Beschreibung: Inter-comparison of data products from simultaneous measurements performed with independent systems and methods is a viable approach to assess the consistency of data and additionally to investigate uncertainties. Within such a context the inter-comparison called Assessment of In Situ Radiometric Capabilities for Coastal Water Remote Sensing Applications (ARC) was carried out at the Acqua Alta Oceanographic Tower in the northern Adriatic Sea to explore the accuracy of in situ data products from various in- and above-water optical systems and methods. Measurements were performed under almost ideal conditions, including a stable deployment platform, clear sky, relatively low sun zenith angles and moderately low sea state. Additionally, all optical sensors involved in the experiment were inter-calibrated through absolute radiometric calibration performed with the same standards and methods. Inter-compared data products include spectral water-leaving radiance Lw (λ), above-water downward irradiance Ed(0+,λ) and remote sensing reflectance Rrs(λ). Data products from the various measurement systems/methods were directly compared to those from a single reference system/method. Results for Rrs(λ) indicate spectrally averaged values of relative differences comprised between −1 and +6%, while spectrally averaged values of absolute differences vary from approximately 6% for the above-water systems/methods to 9% for buoy-based systems/methods. The agreement between Rrs(λ) spectral relative differences and estimates of combined uncertainties of the inter-compared systems/methods is noteworthy.

Beschreibung: We consider the estimation of the grazing preferences parameters of zooplankton in ocean ecosystem models with ensemble-based Kalman filters. These parameters are introduced to model the relative diet composition of zooplankton that consists of phytoplankton, small size-classes of zooplankton and detritus. They are positive values and their sum is equal to one. However, the sum-to-one constraint cannot be guaranteed by ensemble-based Kalman filters when parameters are bounded. Therefore, a reformulation of the parameterization is proposed. We investigate two types of variable transformations for the estimation of positive sum-to-one constrained parameters that lead to the estimation of a new set of parameters with normal or bounded distributions. These transformations are illustrated and discussed with twin experiments performed with the 1-D coupled model GOTM-NORWECOM with Gaussian anamorphosis extensions of the deterministic ensemble Kalman filter (DEnKF).

Beschreibung: The influence of various wind and wave conditions on the variability of downwelling irradiance Ed (490 nm) in water is subject of this study. The work is based on a two-dimensional Monte Carlo radiative transfer model with high spatial resolution. The model assumes conditions that are ideal for wave focusing, thus simulation results reveal the upper limit for light fluctuations. Local wind primarily determines the steepness of capillary-gravity waves which in turn dominate the irradiance variability near the surface. Down to 3 m depth, maximum irradiance peaks that exceed the mean irradiance Ed by a factor of more than 7 can be observed at low wind speeds up to 5 m s−1. The strength of irradiance fluctuations can be even amplified under the influence of higher ultra-gravity waves; thereby peaks can exceed 11 Ed. Sea states influence the light field much deeper; gravity waves can cause considerable irradiance variability even at 100 m depth. The simulation results show that under realistic conditions 50% radiative enhancements compared to the mean can still occur at 30 m depth. At greater depths, the underwater light variability depends on the wave steepness of the characteristic wave of a sea state; steeper waves cause stronger light fluctuations.

Beschreibung: The goal of this study was to improve PhytoDOAS, which is a new retrieval method for quantitative identification of major phytoplankton functional types (PFTs) using hyper-spectral satellite data. PhytoDOAS is an extension of the Differential Optical Absorption Spectroscopy (DOAS, a method for detection of atmospheric trace gases), developed for remote identification of oceanic phytoplankton groups. Thus far, PhytoDOAS has been successfully exploited to identify cyanobacteria and diatoms over the global ocean from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) hyper-spectral data. This study aimed to improve PhytoDOAS for remote identification of coccolithophores, another functional group of phytoplankton. The main challenge for retrieving more PFTs by PhytoDOAS is to overcome the correlation effects between different PFT absorption spectra. Different PFTs are composed of different types and amounts of pigments, but also have pigments in common, e.g. chl a, causing correlation effects in the usual performance of the PhytoDOAS retrieval. Two ideas have been implemented to improve PhytoDOAS for the PFT retrieval of more phytoplankton groups. Firstly, using the fourth-derivative spectroscopy, the peak positions of the main pigment components in each absorption spectrum have been derived. After comparing the corresponding results of major PFTs, the optimized fit-window for the PhytoDOAS retrieval of each PFT was determined. Secondly, based on the results from derivative spectroscopy, a simultaneous fit of PhytoDOAS has been proposed and tested for a selected set of PFTs (coccolithophores, diatoms and dinoflagellates) within an optimized fit-window, proven by spectral orthogonality tests. The method was then applied to the processing of SCIAMACHY data over the year 2005. Comparisons of the PhytoDOAS coccolithophore retrievals in 2005 with other coccolithophore-related data showed similar patterns in their seasonal distributions, especially in the North Atlantic and the Arctic Sea. The seasonal patterns of the PhytoDOAS coccolithophores indicated very good agreement with the coccolithophore modeled data from the NASA Ocean Biochemical Model (NOBM), as well as with the global distributions of particulate inorganic carbon (PIC), provided by MODIS (MODerate resolution Imaging Spectroradiometer)-Aqua level-3 products. Moreover, regarding the fact that coccolithophores belong to the group of haptophytes, the PhytoDOAS seasonal coccolithophores showed good agreement with the global distribution of haptophytes, derived from synoptic pigment relationships applied to SeaWiFS chl a. As a case study, the simultaneous mode of PhytoDOAS has been applied to SCIAMACHY data for detecting a coccolithophore bloom which was consistent with the MODIS RGB image and the MODIS PIC map of the bloom, indicating the functionality of the method also in short-term retrievals.

Beschreibung: The pan-European capacity for the Ocean Monitoring and Forecasting (MyOcean) Marine Core Service, implementing the Global Monitoring for Environment and Security (GMES) objectives, targets the provision of ocean state observations from various platforms and analysis and forecasting products to assist, among other downscaling activities, the needs of the operational response to marine safety, particularly concerning oil spills. The MEDSLIK oil spill and trajectory prediction system makes use of the MyOcean regional and Cyprus Coastal Ocean Forecasting and Observing System (CYCOFOS) downscaled forecasting products for operational application in the Mediterranean and pre-operational use in the Black Sea. Advanced Synthetic Aperture Radar (ASAR) satellite remote sensing images from the European Space Agency (ESA) and European Maritime Safety Agency CleanSeaNet (EMSA-CSN) provide the means for routine monitoring of the southern European seas for the detection of illegal oil discharges. MEDSLIK offers various ways, to be described in this paper, of coupling the MyOcean forecasting data with ASAR images to provide both forecasts and hindcasts for such remotely observed oil slicks. The main concern will be the drift of the oil slick and also, in the case of the forecast mode, its diffusive spreading, although some attempt is also made to estimate the changes in the state of the oil. The successful link of the satellite-detected oil slicks with their operational predictions using the MyOcean products contributes to the operational response chain and the strengthening of maritime safety for accidental or illegal spills, in implementation of the Mediterranean Decision Support System for Marine Safety (MEDESS-4MS) regarding oil spills.

Beschreibung: A numerical model is used to investigate the resonances of the Gulf of Carpentaria and the Arafura Sea, and the additional insights that come from extending the analysis into the complex angular velocity plane. When the model is forced at the shelf edge with physically realistic real values of the angular velocity, the response functions at points within the region show maxima and other behaviour which imply that resonances are involved but provide little additional information. The study is then extended to complex angular velocities, and the results then show a clear pattern of gravity wave and Rossby wave like resonances. The properties of the resonances are investigated and used to reinterpret the response at real values of angular velocity. It is found that in some regions the response is dominated by modes trapped between the shelf edge and the coast or between opposing coastlines. In other regions the resonances show cooperative behaviour, possibly indicating the importance of other physical processes.

Beschreibung: Thanks to the abundant observation data, we are able to deploy the traditional point-to-point comparison and statistical measures in combination with a comprehensive model validation scheme to assess the skills of the biogeochemical model ERGOM in providing an operational service for the Baltic Sea. The model assessment concludes that the operational products can resolve the main observed seasonal features for phytoplankton biomass, dissolved inorganic nitrogen, dissolved inorganic phosphorus and dissolved oxygen in euphotic layers as well as their vertical profiles. This assessment reflects that the model errors of the operational system at the current stage are mainly caused by insufficient light penetration, excessive organic particle export downward, insufficient regional adaptation and some from improper initialization. This study highlights the importance of applying multiple schemes in order to assess model skills rigidly and identify main causes for major model errors.

Beschreibung: The latest release of GRACE (Gravity Recovery and Climate Experiment) gravity field coefficients (Release-05, or RL05) are evaluated for ocean applications. Data have been processed using the current methodology for Release-04 (RL04) coefficients, and have been compared to output from two different ocean models. Results indicate that RL05 data from the three Science Data Centers – the Center for Space Research (CSR), GeoForschungsZentrum (GFZ), and Jet Propulsion Laboratory (JPL) – are more consistent among themselves than the previous RL04 data. Moreover, the variance of residuals with the output of an ocean model is 50–60% lower for RL05 data than for RL04 data. A more optimized destriping algorithm is also tested, which improves the results slightly. By comparing the GRACE maps with two different ocean models, we can better estimate the uncertainty in the RL05 maps. We find the standard error to be about 1 cm (equivalent water thickness) in the low- and mid-latitudes, and between 1.5 and 2 cm in the polar and subpolar oceans, which is comparable to estimated uncertainty for the output from the ocean models.

Beschreibung: The Gulf of Ulloa, a highly productive area off the western coast of the Baja California Peninsula, is examined for five successive years (2003–2007) by using satellite data and seasonal net primary productivity (NPP) estimates obtained from a vertical generalised production model. The results identify that northwestern winds blow parallel to the coast throughout the year. However, highest NPP occurs from March to June. During this period, an equatorward coastal current transports water from neighbouring upwelling areas to the northern Gulf of Ulloa and in combination with local upwelling, which injects nutrients into the euphotic zone, produce the observed increase in NPP. The opposite situation occurs in late summer when a warm poleward current of tropical characteristics arrives and inhibits the productivity in the whole region and generates the yearly lowest NPP levels. Our findings reveal the importance of lateral advection in the modulation of the primary productivity in this subtropical upwelling region.

Beschreibung: The impact of the assimilation of MyOcean sea level anomalies along-track data on the analyses of the Sicily Channel Regional Model was studied. The numerical model has a resolution of 1/32° degrees and is capable to reproduce mesoscale and sub-mesoscale features. The impact of the SLA assimilation is studied by comparing a simulation (SIM, which does not assimilate data) with an analysis (AN) assimilating SLA along-track multi-mission data produced in the framework of MyOcean project. The quality of the analysis was evaluated by computing RMSE of the misfits between analysis background and observations (sea level) before assimilation. A qualitative evaluation of the ability of the analyses to reproduce mesoscale structures is accomplished by comparing model results with ocean colour and SST satellite data, able to detect such features on the ocean surface. CTD profiles allowed to evaluate the impact of the SLA assimilation along the water column. We found a significant improvement for AN solution in terms of SLA RMSE with respect to SIM (the averaged RMSE of AN SLA misfits over 2 years is about 0.5 cm smaller than SIM). Comparison with CTD data shows a questionable improvement produced by the assimilation process in terms of vertical features: AN is better in temperature while for salinity it gets worse than SIM at the surface. This suggests that a better a-priori description of the vertical error covariances would be desirable. The qualitative comparison of simulation and analyses with synoptic satellite independent data proves that SLA assimilation allows to correctly reproduce some dynamical features (above all the circulation in the Ionian portion of the domain) and mesoscale structures otherwise misplaced or neglected by SIM. Such mesoscale changes also infer that the eddy momentum fluxes (i.e. Reynolds stresses) show major changes in the Ionian area. Changes in Reynolds stresses reflect a different pumping of eastward momentum from the eddy to the mean flow, in turn influencing transports through the channel.

Beschreibung: Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γn = 28.30 kg m−3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year−1. Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171–158° E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7% weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7% is assumed between the year 1995 and 2005) can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in AA-AABW.

Beschreibung: In January and February 2010, a sixth transatlantic hydrographic section was completed across 25° N, extending the hydrographic record at this latitude to over half a century. In combination with continuous transport measurements made since 2004 at 26.5° N by the Rapid-WATCH project, we reassess transport variability in the 25° N hydrographic record. Past studies of transport variability at this latitude have assumed transport estimates from each hydrographic section to represent annual average conditions. In this study the uncertainty in this assumption is assessed through use of Rapid-WATCH observations to quantify sub-seasonal and seasonal transport variability. Whilst in the upper-ocean no significant interannual or decadal transport variability are identified in the hydrographic record, in the deep ocean transport variability in both depth and potential temperature classes suggests some interannual or decadal variability may have occurred. This is particularly striking in the lower North Atlantic Deep Water where southward transports prior to 1998 were greater than recent transports by several Sverdrups. Whilst a cooling and freshening of Denmark Straits Overflow Water has occurred which is coincident with these transport changes, these water mass changes appear to be density compensated. Transport changes are the result of changing velocity shear in the vicinity of the Deep Western Boundary Current.

Beschreibung: Acoustic measurements show that the biomass of zooplankton and mesopelagic fish is redistributed by mesoscale variability and that the signal extends over several hundred meters depth. The mechanisms governing this distribution are not well understood, but influences from both physical (i.e. redistribution) and biological processes (i.e. nutrient transport, primary production, active swimming, etc.) are likely. This study examines how hydrodynamic conditions and basic vertical swimming behavior act to distribute biomass in an anticyclonic eddy. Using an eddy-resolving 2.3 km-resolution physical ocean model as forcing for a particle-tracking module, particles representing passively floating organisms and organisms with vertical swimming behavior are released within an eddy and monitored for 20 to 30 days. The role of hydrodynamic conditions on the distribution of biomass is discussed in relation to the acoustic measurements. Particles released close to the surface tend, in agreement with the observations, to accumulate around the edge of the eddy, whereas particles released at depth gradually become distributed along the isopycnals. After a month they are displaced several hundreds meters in the vertical with the deepest particles found close to the eddy center and the shallowest close to the edge. There is no evidence of aggregation of particles along the eddy rim in the last simulation. The model results points towards a physical mechanism for aggregation at the surface, however biological processes cannot be ruled out using the current modeling tool.

Beschreibung: Large-scale ocean transports of heat and freshwater have not been well monitored, and yet the regional budgets of these quantities are important to understanding the role of the oceans in climate and climate change. In contrast, atmospheric heat and freshwater transports are commonly assessed from atmospheric reanalysis products, despite the presence of non-conserving data assimilation based on the wealth of distributed atmospheric observations as constraints. The ability to carry out ocean reanalyses globally at eddy-permitting resolutions of 1/4 ° or better, along with new global ocean observation programs, now makes a similar approach viable for the ocean. In this paper we examine the budgets and transports within a global high resolution ocean model constrained by ocean data assimilation, and compare them with independent oceanic and atmospheric estimates.

Beschreibung: We describe the seasonal and interannual variability of volume transports in the Weddell and Ross Seas using the 1/12° 20-yr simulation of the OCCAM global ocean general circulation model. The average simulated full-depth cumulative volume transports were 28.5 ± 2.9 Sv (1 Sv ≡ 106 m3 s−1) and 13.4 ± 5.2 Sv, across the main export regions of the Weddell and Ross Seas, respectively. The values of mean outflow of Antarctic Bottom Water (AABW) (defined by neutral density γn ≥ 28.27 kg m−3) from the Weddell and Ross Seas of 10.6 ± 3.1 Sv and 0.5 ± 0.7 Sv, respectively, agree with the range reported in historical observational studies. The export of Weddell Sea dense water in OCCAM is primarily determined by the strength of the Weddell Gyre. Variability in AABW export is predominantly at periods of ~1 yr and 2–4 yr.

Beschreibung: ENSURF (Ensemble SURge Forecast) is a multi-model application for sea level forecast that makes use of several storm surge or circulation models and near-real time tide gauge data in the region, with the following main goals: 1. providing easy access to existing forecasts, as well as to its performance and model validation, by means of an adequate visualization tool; 2. generation of better forecasts of sea level, including confidence intervals, by means of the Bayesian Model Average technique (BMA). The Bayesian Model Average technique generates an overall forecast probability density function (PDF) by making a weighted average of the individual forecasts PDF's; the weights represent the Bayesian likelihood that a model will give the correct forecast and are continuously updated based on the performance of the models during a recent training period. This implies the technique needs the availability of sea level data from tide gauges in near-real time. The system was implemented for the European Atlantic facade (IBIROOS region) and Western Mediterranean coast based on the MATROOS visualization tool developed by Deltares. Results of validation of the different models and BMA implementation for the main harbours are presented for these regions where this kind of activity is performed for the first time. The system is currently operational at Puertos del Estado and has proved to be useful in the detection of calibration problems in some of the circulation models, in the identification of the systematic differences between baroclinic and barotropic models for sea level forecasts and to demonstrate the feasibility of providing an overall probabilistic forecast, based on the BMA method.

Beschreibung: Hydroacoustic detection of natural gas release from the seafloor has been conducted in the past by using singlebeam echosounders. In contrast, modern multibeam swath mapping systems allow much wider coverage, higher resolution, and offer 3-D spatial correlation. Up to the present, the extremely high data rate hampers water column backscatter investigations and more sophisticated visualization and processing techniques are needed. Here, we present water column backscatter data acquired with a 50 kHz prototype multibeam system over a period of 75 seconds. Display types are of swath-images as well as of a "re-sorted" singlebeam presentation. Thus, individual and/or groups of gas bubbles rising from the 24 m deep seafloor clearly emerge in the acoustic images, making it possible to estimate rise velocities. A sophisticated processing scheme is introduced to identify those rising gas bubbles in the hydroacoustic data. We apply a cross-correlation technique adapted from particle imaging velocimetry (PIV) to the acoustic backscatter images. Temporal and spatial drift patterns of the bubbles are assessed and are shown to match very well to measured and theoretical rise patterns. The application of this processing to our field data gives clear results with respect to unambiguous bubble detection and remote bubble rise velocimetry. The method can identify and exclude the main source of misinterpretations, i.e. fish-mediated echoes. Although image-based cross-correlation techniques are well known in the field of fluid mechanics for high resolution and non-inversive current flow field analysis, we present the first application of this technique as an acoustic bubble detector.

Beschreibung: With the focus on the Southern Ocean circulation, results of assimilation of multi-mission-altimeter data and the GRACE/GOCE gravity data into the finite element ocean model (FEOM) are investigated. We use the geodetic method to obtain the dynamical ocean topography (DOT). This method combines the multi-mission-altimeter sea surface height and the GRACE/GOCE gravity field. Using the profile approach, the spectral consistency of both fields is achieved by filtering the sea surface height and the geoid. By combining the GRACE and GOCE data, a considerably shorter filter length can be used, which results in more DOT details. We show that this increase in resolution of measured DOT carries onto the results of data assimilation for the surface data. By assimilating only absolute dynamical topography data using the ensemble Kalman filter, we were able to improve modeled fields. Results are closer to observations which were not used for assimilation and lie outside the area covered by altimetry in the Southern Ocean (e.g. temperature of surface drifters or deep temperatures in the Weddell Sea area at 800 m depth derived from Argo composite.)

Beschreibung: The objective of this paper is to investigate if the description of ocean uncertainties can be significantly improved by applying a local anamorphic transformation to each model variable, and by making the assumption of joint Gaussianity for the transformed variables, rather than for the original variables. For that purpose, it is first argued that a significant improvement can already be obtained by deriving the local transformations from a simple histogram description of the marginal distributions. Two distinctive advantages of this solution for large size applications are the conciseness and the numerical efficiency of the description. Second, various oceanographic examples are used to evaluate the effect of the resulting piecewise linear local anamorphic transformations on the spatial correlation structure. These examples include (i) stochastic ensemble descriptions of the effect of atmospheric uncertainties on the ocean mixed layer, and of wind uncertainties or parameter uncertainties on the ecosystem, and (ii) non-stochastic ensemble descriptions of forecast uncertainties in current sea ice and ecosystem pre-operational developments. The results indicate that (i) the transformation is accurate enough to faithfully preserve the correlation structure if the joint distribution is already close to Gaussian, and (ii) the transformation has the general tendency of increasing the correlation radius as soon as the spatial dependence between random variables becomes nonlinear, with the important consequence of reducing the number of degrees of freedom in the uncertainties, and thus increasing the benefit that can be expected from a given observation network.

Beschreibung: A simple model of the thermohaline circulation (THC) is formulated, with the objective to represent explicitly the geostrophic force balance of the basinwide THC. The model comprises advective-diffusive density balances in two meridional-vertical planes located at the eastern and the western walls of a hemispheric sector basin. Boundary mixing constrains vertical motion to lateral boundary layers along these walls. Interior, along-boundary, and zonally integrated meridional flows are in thermal-wind balance. Rossby waves and the absence of interior mixing render isopycnals zonally flat except near the western boundary, constraining meridional flow to the western boundary layer. The model is forced by a prescribed meridional surface density profile. This two-plane model reproduces both steady-state density and steady-state THC structures of a primitive-equation model. The solution shows narrow deep sinking at the eastern high latitudes, distributed upwelling at both boundaries, and a western boundary current with poleward surface and equatorward deep flow. The overturning strength has a 2/3-power-law dependence on vertical diffusivity and a 1/3-power-law dependence on the imposed meridional surface density difference. Convective mixing plays an essential role in the two-plane model, ensuring that deep sinking is located at high latitudes. This role of convective mixing is consistent with that in three-dimensional models and marks a sharp contrast with previous two-dimensional models. Overall, the two-plane model reproduces crucial features of the THC as simulated in simple-geometry three-dimensional models. At the same time, the model self-consistently makes quantitative a conceptual picture of the three-dimensional THC that hitherto has been expressed either purely qualitatively or not self-consistently.