OS - Latest Articles

A simple and self-consistent geostrophic-force-balance model of the thermohaline circulation with boundary mixing

Fri, 27 Jan 2012 00:00:00 +0100

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.

The impacts of physical processes on oxygen variations in the North Sea-Baltic Sea transition zone

Tue, 24 Jan 2012 00:00:00 +0100

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.

The vertical structure of oceanic Rossby waves: a comparison of high-resolution model data to theoretical vertical structures

Fri, 13 Jan 2012 00:00:00 +0100

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.

Mapping turbidity layers using seismic oceanography methods

Tue, 10 Jan 2012 00:00:00 +0100

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.

Using dissolved oxygen concentrations to determine mixed layer depths in the Bellingshausen Sea

Wed, 04 Jan 2012 00:00:00 +0100

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 (O₂) 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 O₂-based mixed layer depth, z_mix(O₂), as the depth where the relative difference between the O₂ concentration and a reference value at a depth equivalent to 10 dbar equals 0.5 %. This definition was established by analysis of O₂ profiles from the Bellingshausen Sea (west of the Antarctic Peninsula) and corroborated by visual inspection. Comparisons of z_mix(O₂) 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(O₂) 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 O₂ 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).

Extraction of spatial-temporal rules from mesoscale eddies in the South China Sea based on rough set theory

Wed, 07 Dec 2011 00:00:00 +0100

Extraction of spatial-temporal rules from mesoscale eddies in the South China Sea based on rough set theory

Ocean Science, 7, 835-849, 2011

Author(s): Y. Du, X. Fan, Z. He, F. Su, C. Zhou, H. Mao, and D. Wang

In this paper, a rough set theory is introduced to represent spatial-temporal relationships and extract the corresponding rules from typical mesoscale-eddy states in the South China Sea (SCS). Three decision attributes are adopted in this study, which make the approach flexible in retrieving spatial-temporal rules with different features. The results demonstrate that this approach is effective, and therefore provides a powerful approach to forecasts in the future studies. Spatial-temporal rules in the SCS indicate that warm eddies following the rules are generally in the southeastern and central SCS around 2000 m isobaths in winter. Their intensity and vorticity are weaker than those of cold eddies. They usually move a shorter distance. By contrast, cold eddies are in 2000 m and deeper regions of the southwestern and northeastern SCS in spring and fall. Their intensity and vorticity are strong. Usually they move a long distance. In winter, a few rules are followed by cold eddies in the northern tip of the basin and southwest of Taiwan Island rather than warm eddies, indicating cold eddies may be well-regulated in the region. Several warm-eddy rules are achieved west of Luzon Island, indicating warm eddies may be well-regulated in the region as well. Otherwise, warm and cold eddies are distributed not only in the jet flow off southern Vietnam induced by intraseasonal wind stress in summer-fall, but also in the northern shallow water, which should be a focus of a future study.

Wind forcing of salinity anomalies in the Denmark Strait overflow

Wed, 30 Nov 2011 00:00:00 +0100

Wind forcing of salinity anomalies in the Denmark Strait overflow

Ocean Science, 7, 821-834, 2011

Author(s): S. Hall, S. R. Dye, K. J. Heywood, and M. R. Wadley

The overflow of dense water from the Nordic Seas to the North Atlantic through Denmark Strait is an important part of the global thermohaline circulation. The salinity of the overflow plume has been measured by an array of current meters across the continental slope off the coast of Angmagssalik, southeast Greenland since September 1998. During 2004 the salinity of the overflow plume changed dramatically; the entire width of the array (70 km) freshened between January 2004 and July 2004, with a significant negative salinity anomaly of about 0.06 in May. The event in May represents a fresh anomaly of over 3 standard deviations from the mean since recording began in 1998. The OCCAM 1/12° Ocean General Circulation Model not only reproduces the 2004 freshening event (r=0.96, p<0.01), but also correlates well with salinity observations over a previous 6 year period (r=0.54, p<0.01), despite the inevitable limitations of a z-coordinate model in representing the mixing processes at and downstream of the Denmark Strait sill. Consequently the physical processes causing the 2004 anomaly and prior variability in salinity are investigated using the model output. Our results reject the hypotheses that the anomaly is caused by processes occurring between the overflow sill and the moorings, or by an increase in upstream net freshwater input. Instead, we show that the 2004 salinity anomaly is caused by an increase in volume flux of low salinity water, with a potential density greater than 27.60 kg m⁻³, flowing towards the Denmark Strait sill in the East Greenland Current. This is caused by an increase in southward wind stress upstream of the sill at around 75° N 20° W four and a half months earlier, and an associated strengthening of the East Greenland Current.

Usefulness of high resolution coastal models for operational oil spill forecast: the "Full City" accident

Tue, 29 Nov 2011 00:00:00 +0100

Usefulness of high resolution coastal models for operational oil spill forecast: the "Full City" accident

Ocean Science, 7, 805-820, 2011

Author(s): G. Broström, A. Carrasco, L. R. Hole, S. Dick, F. Janssen, J. Mattsson, and S. Berger

Oil spill modeling is considered to be an important part of a decision support system (DeSS) for oil spill combatment and is useful for remedial action in case of accidents, as well as for designing the environmental monitoring system that is frequently set up after major accidents. Many accidents take place in coastal areas, implying that low resolution basin scale ocean models are of limited use for predicting the trajectories of an oil spill. In this study, we target the oil spill in connection with the "Full City" accident on the Norwegian south coast and compare operational simulations from three different oil spill models for the area. The result of the analysis is that all models do a satisfactory job. The "standard" operational model for the area is shown to have severe flaws, but by applying ocean forcing data of higher resolution (1.5 km resolution), the model system shows results that compare well with observations. The study also shows that an ensemble of results from the three different models is useful when predicting/analyzing oil spill in coastal areas.

Operational forecast of hydrophysical fields in the Georgian Black Sea coastal zone within the ECOOP

Fri, 25 Nov 2011 00:00:00 +0100

Operational forecast of hydrophysical fields in the Georgian Black Sea coastal zone within the ECOOP

Ocean Science, 7, 793-803, 2011

Author(s): A. A. Kordzadze and D. I. Demetrashvili

One of the parts of the Black Sea Nowcasting/Forecasting System is the regional forecasting system for the easternmost part of the Black Sea (including the Georgian water area), which has been developed within the context of the EU International projects ARENA and ECOOP. A core of the regional system is a high-resolution baroclinic regional model of the Black Sea dynamics developed at M. Nodia Institute of Geophysics (RM-IG). This model is nested in the basin-scale model of Marine Hydrophysical Institute (MHI, Sevastopol/Ukraine). The regional area is limited to the Caucasian and Turkish coastal lines and the western liquid boundary coinciding with the meridian 39.36° E. Since June 2010 we have regularly been computing 3 days' forecasts of current, temperature and salinity for the easternmost part of the Black Sea with 1 km spacing. In this study the results of two forecasts are presented. The first forecast corresponds to summer season and covers the prognostic interval from 00:00 h, 6 August to 00:00 h, 9 August 2010. The second one corresponds to autumn season and covers the prognostic interval from 00:00 h, 26 October to 00:00 h, 29 October 2010. Data needed for the forecasts – the initial and prognostic hydrophysical fields on the open boundary, also 2-D prognostic meteorological fields at the sea surface – wind stress, heat fluxes, evaporation and precipitation rates for our regional area are being placed on the MHI server every day and we are available to use these data operatively. Prognostic hydrophysical fields are results of forecast by the basin-scale model of MHI and 2-D meteorological boundary fields represent the results of forecast by regional atmospheric model ALADIN. All these fields are given on the grid of basin-scale model with 5 km spacing and with one-hour time step frequency for the integration period. The analysis of predicted fields shows that to use the model with high resolution is very important factor for identification of nearshore eddies of small sizes. It should be noted the very different character of regional circulation in summer and autumn seasons in the easternmost part of the Black Sea.

How well can we derive Global Ocean Indicators from Argo data?

Tue, 22 Nov 2011 00:00:00 +0100

How well can we derive Global Ocean Indicators from Argo data?

Ocean Science, 7, 783-791, 2011

Author(s): K. von Schuckmann and P.-Y. Le Traon

Argo deployments began in the year 2000 and by November 2007, the array reached its initial goal of 3000 floats operating worldwide. In this study, Argo temperature and salinity measurements during the period 2005 to 2010 are used to estimate Global Ocean Indicators (GOIs) such as global ocean heat content (GOHC), global ocean freshwater content (GOFC) and global steric sea level (GSSL). We developed a method based on a simple box averaging scheme using a weighted mean. Uncertainties due to data processing methods and choice of climatology are estimated. This method is easy to implement and run and can be used to set up a routine monitoring of the global ocean. Over the six year time period, trends of GOHC and GSSL are 0.54 ± 0.1 W m⁻² and 0.75 ± 0.15 mm yr⁻¹, respectively. The trend of GOFC is barely significant. Results show that there is significant interannual variability at global scale, especially for GOFC. Annual mean GOIs from the today's Argo sampling can be derived with an accuracy of ±0.11 cm for GSSL, ±0.22 × 10⁸ J m⁻² for GOHC, and ±700 km³ for GOFC. Long-term trends (15 yr) of GOIs based on the complete Argo sampling for the upper 1500 m depth can be estimated with an accuracy of ±0.04 mm yr⁻¹ for GSSL, ±0.02 W m⁻² for GOHC and ±20 km³ yr⁻¹ for GOFC – under the assumption that no systematic errors remain in the observing system.

A pre-operational three Dimensional variational data assimilation system in the North/Baltic Sea

Tue, 22 Nov 2011 00:00:00 +0100

A pre-operational three Dimensional variational data assimilation system in the North/Baltic Sea

Ocean Science, 7, 771-781, 2011

Author(s): S. Y. Zhuang, W. W. Fu, and J. She

This paper describes the implementation and evaluation of a pre-operational three dimensional variational (3DVAR) data assimilation system for the North/Baltic Sea. Univariate analysis for both temperature and salinity is applied in a 3DVAR scheme in which the horizontal component of the background error covariance is modeled by an isotropic recursive filter (IRF) and the vertical component is represented by dominant Empirical Orthogonal Functions (EOFs). Observations of temperature and salinity (T/S) profiles in the North/Baltic Sea are assimilated in the year of 2005. Effect of the 3DVAR scheme is assessed by a comparison between data assimilation run and control run. The statistical analysis indicates that the model simulation is significantly improved with the 3DVAR scheme. On average, the root mean square errors (RMSE) of temperature and salinity are reduced by 0.2 °C and 0.25 psu in the North/Baltic Sea. In addition, the bias of temperature and salinity is also decreased by 0.1 °C and 0.2 psu, respectively. Starting from an analyzed initial state, one month simulation without assimilation is carried out with the aim of examining the persistence of the initial impact. It is shown that the assimilated initial state can impact the model simulation for nearly two weeks. The influence on salinity is more pronounced than temperature.

An empirical stochastic model of sea-surface temperatures and surface winds over the Southern Ocean

Mon, 14 Nov 2011 00:00:00 +0100

An empirical stochastic model of sea-surface temperatures and surface winds over the Southern Ocean

Ocean Science, 7, 755-770, 2011

Author(s): S. Kravtsov, D. Kondrashov, I. Kamenkovich, and M. Ghil

This study employs NASA's recent satellite measurements of sea-surface temperatures (SSTs) and sea-level winds (SLWs) with missing data filled-in by Singular Spectrum Analysis (SSA), to construct empirical models that capture both intrinsic and SST-dependent aspects of SLW variability. The model construction methodology uses a number of algorithmic innovations that are essential in providing stable estimates of the model's propagator. The best model tested herein is able to faithfully represent the time scales and spatial patterns of anomalies associated with a number of distinct processes. These processes range from the daily synoptic variability to interannual signals presumably associated with oceanic or coupled dynamics. Comparing the simulations of an SLW model forced by the observed SST anomalies with the simulations of an SLW-only model provides preliminary evidence for the ocean driving the atmosphere in the Southern Ocean region.

Mean Dynamic Topography of the Black Sea, computed from altimetry, drifter measurements and hydrology data

Tue, 08 Nov 2011 00:00:00 +0100

Mean Dynamic Topography of the Black Sea, computed from altimetry, drifter measurements and hydrology data

Ocean Science, 7, 745-753, 2011

Author(s): A. A. Kubryakov and S. V. Stanichny

Mean Dynamic Topography (MDT) is a crucial parameter for estimating dynamic topography, and, therefore, geostrophic circulation from satellite altimetry measurements. In this work we use drifting buoy measurements, hydrographic profiles and along-track Sea Level Anomalies (SLA) to reconstruct MDT of the Black Sea by the "synthetic" method. Obtained MDT shows a lot of mesoscale features, which are not present in previous MDT fields of the Black Sea, mostly based on climatic data. Moreover, gradients of sea level in the synthetic MDT are significantly higher compared to other fields, which is evidence of more intense currents in the basin.

Validation of determined MDT field with independent dynamic heights and drifter buoy velocities shows good quantitative and qualitative coincidence over all Black Sea basin and improvements compare to previous fields.

New Black Sea MDT will improve quality of altimetry-derived geostrophic velocities and lead to better understanding of the spatial and temporal features of the upper layer dynamics.

A computational method for determining XBT depths

Tue, 08 Nov 2011 00:00:00 +0100

A computational method for determining XBT depths

Ocean Science, 7, 733-743, 2011

Author(s): J. Stark, J. Gorman, M. Hennessey, F. Reseghetti, J. Willis, J. Lyman, J. Abraham, and M. Borghini

A new technique for determining the depth of expendable bathythermographs (XBTs) is developed. This new method uses a forward-stepping calculation which incorporates all of the forces on the XBT devices during their descent. Of particular note are drag forces which are calculated using a new drag coefficient expression. That expression, obtained entirely from computational fluid dynamic modeling, accounts for local variations in the ocean environment. Consequently, the method allows for accurate determination of depths for any local temperature environment. The results, which are entirely based on numerical simulation, are compared with the experiments of LM Sippican T-5 XBT probes. It is found that the calculated depths differ by less than 3% from depth estimates using the standard fall-rate equation (FRE). Furthermore, the differences decrease with depth. The computational model allows an investigation of the fluid flow patterns along the outer surface of the probe as well as in the interior channel. The simulations take account of complex flow phenomena such as laminar-turbulent transition and flow separation.

Annual cycles of chlorophyll-a, non-algal suspended particulate matter, and turbidity observed from space and in-situ in coastal waters

Mon, 31 Oct 2011 00:00:00 +0100

Annual cycles of chlorophyll-a, non-algal suspended particulate matter, and turbidity observed from space and in-situ in coastal waters

Ocean Science, 7, 705-732, 2011

Author(s): F. Gohin

Sea surface temperature, chlorophyll, and turbidity are three variables of the coastal environment commonly measured by monitoring networks. The observation networks are often based on coastal stations, which do not provide a sufficient coverage to validate the model outputs or to be used in assimilation over the continental shelf. Conversely, the products derived from satellite reflectance generally show a decreasing quality shoreward, and an assessment of the limitation of these data is required. The annual cycle, mean, and percentile 90 of the chlorophyll concentration derived from MERIS/ESA and MODIS/NASA data processed with a dedicated algorithm have been compared to in-situ observations at twenty-six selected stations from the Mediterranean Sea to the North Sea. Keeping in mind the validation, the forcing, or the assimilation in hydrological, sediment-transport, or ecological models, the non-algal Suspended Particulate Matter (SPM) is also a parameter which is expected from the satellite imagery. However, the monitoring networks measure essentially the turbidity and a consistency between chlorophyll, representative of the phytoplankton biomass, non-algal SPM, and turbidity is required. In this study, we derive the satellite turbidity from chlorophyll and non-algal SPM with a common formula applied to in-situ or satellite observations. The distribution of the satellite-derived turbidity exhibits the same main statistical characteristics as those measured in-situ, which satisfies the first condition to monitor the long-term changes or the large-scale spatial variation over the continental shelf and along the shore. For the first time, climatologies of turbidity, so useful for mapping the environment of the benthic habitats, are proposed from space on areas as different as the southern North Sea or the western Mediterranean Sea, with validation at coastal stations.

N/P ratio of nutrient uptake in the Baltic Sea

Mon, 31 Oct 2011 00:00:00 +0100

N/P ratio of nutrient uptake in the Baltic Sea

Ocean Science, 7, 693-704, 2011

Author(s): Z. Wan, L. Jonasson, and H. Bi

The N/P ratio of nutrient uptake, the change of dissolved inorganic nitrogen (DIN) relative to the change of dissolved inorganic phosphorus (DIP), is a key parameter for many ecological models. In the Baltic Sea ecosystem, the N/P ratio of nutrient uptake varies among different basins and different seasons. The N/P ratio of nutrient alteration, i.e., the ratio of DIN to DIP altered before and after spring blooms, is not the same as the N/P ratio of nutrient uptake, but the former can be regarded as an indicator of the latter in the Baltic Sea. Based on the observed N/P ratio of nutrient alteration, we hypothesize a non-Redfield N/P ratio of nutrient uptake. The 3-D-ecosystem model ERGOM coupled with the circulation model DMI-BSHcmod was used to test this hypothesis. When the Redfield ratio was used in the model, the DIP surplus after spring blooms was too high and resulted in excessive growth of cyanobacteria and too much nitrogen fixation. When the non-Redfield ratio was used in the model, these problems tended to disappear. In summary, we show that: (1) the Redfield N/P ratio of nutrient uptake in the Baltic Sea tends to be too high; (2) a N/P ratio of 10:1 appears to work better than the Redfield value; and (3) the N/P ratio of nutrient uptake in the Baltic Proper during spring blooms is around 6:1. As the model limitation using one identical value for two N/P ratios for nutrient uptake and remineralization, the quantitative conclusions are only convincing as a model parameter even though it obviously improves model predictions. Whether this model parameter is consistent with the biological nutrient uptake is worth being further verified with some laboratory investigations or simulations using a more sophisticated model with independent N/P ratios for nutrient uptake and remineralization.

ENSO-correlated fluctuations in ocean bottom pressure and wind-stress curl in the North Pacific

Tue, 25 Oct 2011 00:00:00 +0200

ENSO-correlated fluctuations in ocean bottom pressure and wind-stress curl in the North Pacific

Ocean Science, 7, 685-692, 2011

Author(s): D. P. Chambers

We examine the output of an ocean model forced by ECMWF winds to study the theoretical relationship between wind-induced changes in ocean bottom pressure in the North Pacific between 1992 until 2010 and ENSO. Our analysis indicates that while there are significant fluctuations correlated with some El Niño and La Niña events, the correlation is still relatively low. Moreover, the ENSO-correlated variability explains only 50 % of the non-seasonal, low-frequency variance. There are significant residual fluctuations in both wind-stress curl and ocean bottom pressure in the region with periods of 4-years and longer. One such fluctuation began in late 2002 and has been observed by the Gravity Recovery and Climate Experiment (GRACE). Even after accounting for possible ENSO-correlated variations, there is a significant trend in ocean bottom pressure in the region, equivalent to 0.7 ± 0.3 cm yr⁻¹ of sea level from January 2003 until December 2008, which is confirmed with steric-corrected altimetry. Although this low-frequency fluctuation does not appear in the ocean model, we show that ECMWF winds have a significantly reduced trend that is inconsistent with satellite observations over the same time period, and so it appears that the difference is due to a forcing error in the model and not an intrinsic error.

The Aegean sea marine security decision support system

Mon, 24 Oct 2011 00:00:00 +0200

The Aegean sea marine security decision support system

Ocean Science, 7, 671-683, 2011

Author(s): L. Perivoliotis, G. Krokos, K. Nittis, and G. Korres

As part of the integrated ECOOP (European Coastal Sea Operational observing and Forecasting System) project, HCMR upgraded the already existing standalone Oil Spill Forecasting System for the Aegean Sea, initially developed for the Greek Operational Oceanography System (POSEIDON), into an active element of the European Decision Support System (EuroDeSS). The system is accessible through a user friendly web interface where the case scenarios can be fed into the oil spill drift model component, while the synthetic output contains detailed information about the distribution of oil spill particles and the oil spill budget and it is provided both in text based ECOOP common output format and as a series of sequential graphics. The main development steps that were necessary for this transition were the modification of the forcing input data module in order to allow the import of other system products which are usually provided in standard formats such as NetCDF and the transformation of the model's calculation routines to allow use of current, density and diffusivities data in z instead of sigma coordinates. During the implementation of the Aegean DeSS, the system was used in operational mode in order to support the Greek marine authorities in handling a real accident that took place in North Aegean area. Furthermore, the introduction of common input and output files by all the partners of EuroDeSS extended the system's interoperability thus facilitating data exchanges and comparison experiments.

Eddy characteristics in the northern South China Sea as inferred from Lagrangian drifter data

Fri, 21 Oct 2011 00:00:00 +0200

Eddy characteristics in the northern South China Sea as inferred from Lagrangian drifter data

Ocean Science, 7, 661-669, 2011

Author(s): Jiaxun Li, Ren Zhang, and Baogang Jin

Cyclonic and anticyclonic eddies from large scale to submesoscale in the northern South China Sea (NSCS) have been statistically characterized based on the satellite-tracked Lagrangian drifters using our developed geometric eddy identification method. There are in total 2208 eddies identified, 70% of which are anticyclonic eddies. If the submesoscale eddies are eliminated, the other eddies in the NSCS will show a 1.2:1 ratio of the number of anticyclones (210) to the number of cyclones (171). The spatial distribution of the eddies is regional: in southwest of Taiwan, the number of anticyclones dominates the number of cyclones, and most of them are the submesoscale anticyclones with small radii; in contrast, the large and medium cyclonic eddies are a little more than the same scale anticyclonic eddies in northwest of Luzon. The temporal distribution of eddy number in the NSCS has a close relation with the Asian monsoon. The number of the large and medium eddies peaks during the winter monsoon, while the submesoscale eddies are apt to generate in the summer monsoon. The spatial and temporal patterns have a good agreement with the results of the sea surface height anomaly (SSHA). The maximum and mean tangential velocities of anticyclones (cyclones) are 40 (30) cm s⁻¹ and 25 (15) cm s⁻¹, respectively. The calculated normalized vorticities from drifters suggest that although the mesoscale eddies may be considered in geostrophic balance, ageostrophic dynamics and centrifugal effects may play an important role for the growth and decay of the mesoscale cores.

About uncertainties in practical salinity calculations

Mon, 17 Oct 2011 00:00:00 +0200

About uncertainties in practical salinity calculations

Ocean Science, 7, 651-659, 2011

Author(s): M. Le Menn

In the current state of the art, salinity is a quantity computed from conductivity ratio measurements, with temperature and pressure known at the time of the measurement, and using the Practical Salinity Scale algorithm of 1978 (PSS-78). This calculation gives practical salinity values S. The uncertainty expected in PSS-78 values is ±0.002, but no details have ever been given on the method used to work out this uncertainty, and the error sources to include in this calculation. Following a guide published by the Bureau International des Poids et Mesures (BIPM), using two independent methods, this paper assesses the uncertainties of salinity values obtained from a laboratory salinometer and Conductivity-Temperature-Depth (CTD) measurements after laboratory calibration of a conductivity cell. The results show that the part due to the PSS-78 relations fits is sometimes as significant as the instrument's. This is particularly the case with CTD measurements where correlations between variables contribute mainly to decreasing the uncertainty of S, even when expanded uncertainties of conductivity cell calibrations are for the most part in the order of 0.002 mS cm⁻¹. The relations given here, and obtained with the normalized GUM method, allow a real analysis of the uncertainties' sources and they can be used in a more general way, with instruments having different specifications.