OSD - Latest Articles

Assessment of the 3-D temperature and salinity observational networks in the Baltic Sea and North Sea

Wed, 01 Sep 2010 00:00:00 +0200

Assessment of the 3-D temperature and salinity observational networks in the Baltic Sea and North Sea

Ocean Science Discussions, 7, 1627-1668, 2010

Author(s): W. Fu, J. L. Høyer, and J. She

The spatial averaged correlations are presented in 1.5° × 1.5° bins for the North and Baltic Sea region. The averaged correlations are computed based on the proxy ocean data generated by the operational forecat model of Danish Meteorology Institute (DMI). It is shown that the spatial distribution of the averaged correlations could reflect the overall influence of the local atmospheric forcing, complex topography, coastlines, boundary and bottom effect, etc. Comparisons with the satellite SST data demonstrate that the proxy ocean data reproduces quite realistic results at the surface. Based on the spatial bin-averaged correlations, a general correlation model is assumed to approximate the spatial and temporal correlation structure. Parameters of the correlation model are obtained on the standard Levitus levels. It is found that the correlation model is not the typical Guaussian-type function. For instance, the exponents of the correlation model vary in the longitudinal direction from 0.75 at the surface to 1.33 at the depth of 250 m for temperature. For salinity, the temporal correlation can be approximated with an exponential function.

Two complementary quality-indicators, effective coverage rate and "explained" variance, are defined based on the correlation models obtained above. The two indicators are able to identify the "influence area" of the information content in a given observation network and the relative importance of observations at different locations. By these indicators, the 3-D temperature and salinty observational networks are assessed in the Baltic Sea and North Sea for the period 2004–2006. It is found that the surface level is more effectively covered with existing netwroks. In addition, the Belt Sea and the Baltic Proper also have good observation coverage for both temperature and salinity. However, more observations are required in the Norwegian Trench and Kattegat. In the vertical, the two indicators show smaller values from 50 m to 125 m in this region, indicating the need for more observations.

Absolute Salinity, "Density Salinity" and the Reference-Composition Salinity Scale: present and future use in the seawater standard TEOS-10

Tue, 31 Aug 2010 00:00:00 +0200

Absolute Salinity, "Density Salinity" and the Reference-Composition Salinity Scale: present and future use in the seawater standard TEOS-10

Ocean Science Discussions, 7, 1559-1625, 2010

Author(s): D. G. Wright, R. Pawlowicz, T. J. McDougall, R. Feistel, and G. M. Marion

Salinity plays a key role in the determination of the thermodynamic properties of seawater and the new TEOS-10¹ standard provides a consistent and effective approach to dealing with relationships between salinity and these thermodynamic properties. However, there are a number of practical issues that arise in the application of TEOS-10, both in terms of accuracy and scope, including its use in the reduction of field data and in numerical models.

First, in the TEOS-10 formulation for IAPSO Standard Seawater, the Gibbs function takes the Reference Salinity as its salinity argument, denoted S_R, which provides a measure of the mass fraction of dissolved material in solution based on the Reference Composition approximation for Standard Seawater. We discuss uncertainties in both the Reference Composition and the Reference-Composition Salinity Scale on which Reference Salinity is reported. The Reference Composition provides a much-needed fixed benchmark but modified reference states will inevitably be required to improve the representation of Standard Seawater for some studies. The Reference-Composition Salinity Scale should remain unaltered to provide a stable representation of salinity for use with the TEOS-10 Gibbs function and in climate change detection studies.

Second, when composition anomalies are present in seawater, no single salinity variable can fully represent the influence of dissolved material on the thermodynamic properties of seawater. We consider three distinct representations of salinity that have been used in previous studies and discuss the connections and distinctions between them. One of these variables provides the most accurate representation of density possible as well as improvements over Reference Salinity for the determination of other thermodynamic properties. It is referred to as "Density Salinity" and is represented by the symbol S_A^dens; it stands out as the most appropriate representation of salinity for use in dynamical physical oceanography. The other two salinity variables provide alternative measures of the mass fraction of dissolved material in seawater. "Solution Salinity", denoted S_A^soln, is the most obvious extension of Reference Salinity to allow for composition anomalies; it provides a direct estimate of the mass fraction of dissolved material in solution. "Added-Mass Salinity", denoted S_A^add, is motivated by a method used to report laboratory experiments; it represents the component of dissolved material added to Standard Seawater in terms of the mass of material before it enters solution. We also discuss a constructed conservative variable referred to as "Preformed Salinity", denoted S_*, which will be useful in process-oriented numerical modelling studies.

Finally, a conceptual framework for the incorporation of composition anomalies in numerical models is presented that builds from studies in which composition anomalies are simply ignored up to studies in which the influences of composition anomalies are accounted for using the results of biogeochemical models.

¹TEOS-10: international thermodynamic equation of seawater 2010, http://www.teos-10.org.

Tracers confirm downward mixing of Tyrrhenian Sea upper waters associated with the Eastern Mediterranean Transient

Tue, 24 Aug 2010 00:00:00 +0200

Tracers confirm downward mixing of Tyrrhenian Sea upper waters associated with the Eastern Mediterranean Transient

Ocean Science Discussions, 7, 1533-1557, 2010

Author(s): W. Roether and J. E. Lupton

Observations of tritium and ³He in the Tyrrhenian Sea, 1987–2009, confirm the enhanced convective mixing of intermediate waters into the deep waters that has been noted and associated with the Eastern Mediterranean Transient in previous studies. Our evidence for the mixing rests on increasing tracer concentrations in the Tyrrhenian deep waters, accompanied by decreases in the upper waters, which are supplied from the Eastern Mediterranean. The downward transfer is particularly evident between 1987 and 1997. Later on, information partly rests on increasing tritium-³He ages; here we correct the observed ³He for contributions released from the ocean floor. The Tyrrhenian tracer distributions are fully compatible with data upstream of the Sicily Strait and in the Western Mediterranean. The tracer data show that mixing reached to the bottom and confirm a cyclonic nature of the deep water circulation in the Tyrrhenian. They furthermore indicate that horizontal homogenization of the deep waters occurs on a time scale of several years. Various features point to a reduced impact of Western Mediterranean Deep Water (WMDW) in the Tyrrhenian during the enhanced-convection period. This is an important finding because it implies less upward mixing of WMDW, which has been named a major process to enable the WMDW to leave the Mediterranean via the Gibraltar Strait. On the other hand, the TDW outflow for several years represented a major influx of enhanced salinity and density waters into the deep-water range of the Western Mediterranean.

A statistical model for sea surface diurnal warming driven by numerical weather prediction fluxes and winds

Tue, 17 Aug 2010 00:00:00 +0200

A statistical model for sea surface diurnal warming driven by numerical weather prediction fluxes and winds

Ocean Science Discussions, 7, 1497-1532, 2010

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 deceedance and the observed warming exceedance. Diurnal cooling, where it occurs, is modelled as proportional to the integrated heat efflux 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.

The influence of temperature and salinity variability on the upper ocean density and mixed layer

Tue, 17 Aug 2010 00:00:00 +0200

The influence of temperature and salinity variability on the upper ocean density and mixed layer

Ocean Science Discussions, 7, 1469-1495, 2010

Author(s): R. W. Helber, J. G. Richman, and C. N. Barron

The relative influence of both temperature and salinity on the mixed layer depth (MLD) is evaluated using a relationship of binned regressions of MLD on vertical density compensation and isothermal layer depth (ILD) from a global set of in situ profile observations. Our approach is inspired by the observations of the difference between the MLD and the sonic layer depth (SLD) that evolve seasonally around the global ocean. In this article, we hypothesize that vertical density compensation governs SLD-MLD differences and can be used for mapping the relative influence of temperature and salinity on upper ocean structure. The Turner angle, computed between the surface and 200 m (bulk Turner angle, BTA), serves as a measure of vertical density compensation that quantifies times and areas where either temperature or salinity is destabilizing. For temperature destabilization the ocean exhibits cool/fresh overlying hot/salty water. For salinity destabilization the ocean exhibits hot/salty overlying cool/fresh water. These two classes of density compensation have seasonal variability with different geographical characteristics. Profiles with salinity controlled stable density and destabilizing temperature gradient are found most often at high latitudes. Profiles with temperature controlled stable density and destabilizing salinity gradient are found in the tropics and subtropics of all oceans. Results indicate that about half of the ocean has vertical density compensation that is a necessary condition for SLD-MLD differences. While density compensation is necessary, it is not a sufficient condition for predicting the dependence of MLD on BTA. Density compensation is the dominant factor in MLD variability in heavy river input and subduction regions that cover only ~14% of the ocean.

Flow and mixing around a glacier tongue

Wed, 11 Aug 2010 00:00:00 +0200

Flow and mixing around a glacier tongue

Ocean Science Discussions, 7, 1439-1467, 2010

Author(s): C. L. Stevens, C. L. Stewart, N. J. Robinson, M. J. M. Williams, and T. G. Haskell

A glacier tongue floating in the coastal ocean presents a significant obstacle to the local flow and influences oceanic mixing and transport processes. Here ocean shear microstructure observations at a glacier tongue side-wall show tidally-induced flow pulses and vortices as well as concomitant mixing. Flow speeds within the pulses reached around three times that of the ambient tidal flow amplitude and generated vertical velocity shear as large as 3×10⁻³ s⁻¹. During the maximum flow period turbulent energy dissipation rates reached a maximum of 10⁻⁵ m² s⁻³, around three decades greater than local background levels. This is in keeping with estimates of the gradient Richardson Number which dropped to around unity. Associated vertical diffusivities are higher that expected from parameterization, possibly reflecting the proximity of the cryotopography.

Influence of cross-shelf water transport on nutrients and phytoplankton in the East China Sea: a model study

Mon, 26 Jul 2010 00:00:00 +0200

Influence of cross-shelf water transport on nutrients and phytoplankton in the East China Sea: a model study

Ocean Science Discussions, 7, 1405-1437, 2010

Author(s): L. Zhao and X. Guo

A three dimensional coupled biophysical model was used to examine the supply of oceanic nutrients to the shelf of the East China Sea (ECS) and its role in primary production over the shelf. The model consisted of two modules: the hydrodynamic module was based on a nested model with a horizontal resolution of 1/18 degree, whereas the biological module was a low trophic level ecosystem model including two types of phytoplankton, three elements of nutrients, and biogenic organic material. Model results suggested that seasonal variation in chlorophyll-a had a strong regional dependence over the shelf of the ECS. The area with high chlorophyll-a appears firstly at the outer shelf in winter, and gradually migrates toward the inner shelf (offshore region of Changjiang estuary) from spring to summer. Vertically, chlorophyll-a was generally homogenous from the coastal zone to the inner shelf. In the middle and outer shelves, high chlorophyll-a appeared in the surface in spring but moved to the subsurface from summer to early autumn. The annual averaged onshore flux across the shelf break was estimated to be 1.53 Sv for volume, 9.4 kmol s⁻¹ for DIN, 0.7 kmol s⁻¹ for DIP, and 18.2 kmol s⁻¹ for silicate, which are supplied mainly from the northeast of Taiwan and southwest of Kyushu. From calculations that artificially increased the concentration of nutrients in the Kuroshio water, the additional oceanic nutrients were distributed in the bottom layer from the shelf break to the region offshore of Changjiang estuary from spring to summer, and appeared in the surface layer from autumn to winter. The contribution of oceanic nutrients to primary production over the shelf was found not only in the surface layer (mainly at the outer shelf and shelf break in winter and in the region offshore of Changjiang estuary in summer) but also in the subsurface layer over the shelf from spring to autumn.

Phytoplankton distribution and nitrogen dynamics in the Southwest Indian subtropical gyre and Southern Ocean Waters

Fri, 23 Jul 2010 00:00:00 +0200

Phytoplankton distribution and nitrogen dynamics in the Southwest Indian subtropical gyre and Southern Ocean Waters

Ocean Science Discussions, 7, 1347-1403, 2010

Author(s): S. J. Thomalla, H. N. Waldron, M. I. Lucas, J. F. Read, I. J. Ansorge, and E. Pakhomov

During the 1999 Marion Island Oceanographic Survey (MIOS 4) in late austral summer, a northbound and reciprocal southbound transect were taken along the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands and 31° S. The sections crossed a number of major fronts and smaller mesoscale features and covered a wide productivity spectrum from subtropical to subantarctic waters. Associated with the physical survey were measurements of size fractionated chlorophyll, nutrients and nitrogen (NO₃, NH₄ and urea) uptake rates. Subtropical waters were characterised by low concentrations (<0.27 mg m⁻³) of pico-phytoplankton cells (>81%) and very low f-ratios (<0.1), indicative of productivity based almost entirely on recycled ammonium and urea. Diatom growth was limited by the availability of NO₃ (<1 mmol m^-3) and SiO₄ (<1.5 mmol m⁻³) through vertical stratification that prevents the upward flux of nutrients into the euphotic zone. Biomass accumulation of small cells was likely controlled by microzooplankton grazing. In subantarctic waters, total chlorophyll concentrations increased (<0.74 mg m⁻³) and larger cells became more prevalent, however smaller phytoplankton cells and low f-ratios (>0.15) still dominated, despite sufficient NO₃ availability. The results from this study favour Si limitation, light-limited deep mixing and likely Fe deficiency as the dominant mechanisms controlling significant new production by micro-phytoplankton. Increased concentrations of micro-phytoplankton cells and and rates of new production did however occur at oceanic frontal regions (58.6% and 11.22%, respectively), and in the region of the Prince Edward archipelago (61.4% and 14.16%, respectively). Here water column stabilization and local Fe-enrichment are thought to stimulate phytoplankton growth rates, especially of diatoms. Open ocean regions such as these provide important areas for local but significant POC export and biological CO₂ draw-down in an overall HNLC Southern Ocean.

Metrological traceability of oceanographic salinity measurement results

Thu, 22 Jul 2010 00:00:00 +0200

Metrological traceability of oceanographic salinity measurement results

Ocean Science Discussions, 7, 1303-1346, 2010

Author(s): S. Seitz, R. Feistel, D. G. Wright, S. Weinreben, P. Spitzer, and P. De Bièvre

Consistency of observed oceanographic salinity data is discussed with respect to contemporary metrological concepts. The claimed small uncertainty of salinity measurement results traceable to the conductivity ratio of a certified IAPSO Standard Seawater reference is not metrologically justified if results are compared on climatic time scales. This applies in particular to Practical Salinity S_P, Reference Salinity S_R, and the latest estimates of Absolute Salinity using the TEOS-10 formalism. In climate time scales an additional contribution to the uncertainty that is related to unknown property changes of the reference material must be accounted for. Moreover, when any of these measured or calculated quantity values is used to estimate Absolute Salinity of a seawater sample under investigation, another uncertainty contribution is required to quantify the accuracy of the equations relating the actually measured quantity to the Absolute Salinity. Without accounting for these additional uncertainties, such results cannot be used to estimate Absolute Salinity with respect to the International System of Units (SI), i.e. to the unit chosen for the mass fraction of dissolved material in the sample, which is "g/kg". From a metrological point of view, such deficiencies in the calculations involving other quantities will produce SI-incompatible results. We outline how these problems can be overcome by linking salinity to primary SI measurement standards.

Mixed layer sub-mesoscale parameterization – Part 2: Results for coarse resolution OGCMs

Mon, 12 Jul 2010 00:00:00 +0200

Mixed layer sub-mesoscale parameterization – Part 2: Results for coarse resolution OGCMs

Ocean Science Discussions, 7, 1289-1302, 2010

Author(s): V. M. Canuto and M. S. Dubovikov

Recent studies have shown that sub-mesoscales (SM, ~1 km) play an important role in mixed layer dynamics and have concluded that it has become necessary to include them in ocean global circulation models, OGCMs. In part A, we developed and assessed a parameterization of the vertical SM tracer flux for OGCMs that resolve mesoscales M but not SM. In the present paper, we derive a parameterization of the vertical SM tracer flux for OGCMs that do not resolve either M or SM, as those used in climate studies.

Stability and forcing of the Iceland-Faroe inflow of water, heat, and salt to the Arctic

Fri, 09 Jul 2010 00:00:00 +0200

Stability and forcing of the Iceland-Faroe inflow of water, heat, and salt to the Arctic

Ocean Science Discussions, 7, 1245-1287, 2010

Author(s): B. Hansen, H. Hátún, R. Kristiansen, S. M. Olsen, and S. Østerhus

The flow of Atlantic water across the Greenland-Scotland Ridge (Atlantic inflow) is critical for conditions in the Nordic Seas and Arctic Ocean by importing heat and salt. Here, we present a decade-long series of measurements from the Iceland-Faroe inflow branch (IF-inflow), which carries almost half the total Atlantic inflow. The observations show no significant trend in volume transport of Atlantic water, but temperature and salinity increased during the observational period. On shorter time scales, the observations show considerable variations but no statistically significant seasonal variation is observed and even weekly averaged transport values were consistently uni-directional from the Atlantic into the Nordic Seas. Combining transport time-series with sea level height from satellite altimetry and wind stress reveals that the force driving the IF-inflow across the topographic barrier of the Ridge is mainly generated by a pressure gradient that is due to a continuously maintained low sea level in the Southern Nordic Seas. This links the IF-inflow to the estuarine and thermohaline processes that generate outflow from the Nordic Seas and lower its sea level. The IF-inflow is an important component of the system coupling the Arctic region to the North Atlantic through the thermohaline circulation, which has been predicted to weaken in the 21st century. Our observations show no indication of weakening, as yet.

Retroflection from a double slanted coastline – a model for the Agulhas leakage variability

Mon, 05 Jul 2010 00:00:00 +0200

Retroflection from a double slanted coastline – a model for the Agulhas leakage variability

Ocean Science Discussions, 7, 1209-1244, 2010

Author(s): V. Zharkov, D. Nof, and W. Weijer

The Agulhas leakage to the South Atlantic (SA) exhibits strong anti-correlations with the mass flux of the Agulhas Current. This is accompanied by the migration of the Agulhas retroflection whose normal position (NPR) is near Cape Agulhas, where the slant of the South African coast is very small. During periods of strong incoming flux (SIF), the retroflection shifts upstream to Port Elizabeth or East London, where the coastline shape has a "kink", i.e., the slant changes abruptly from small on the west side, to large (about 55°) on the east side. Here, we show that the variability of rings shedding maybe attributed to this kink.

To do so, we develop a nonlinear analytical model for retroflection near a coastline that consists of two sections, one strongly slanted (corresponding to the east side) and the other zonal (corresponding to the west side). The principal difference between this and the model of a single straight slanted coast discussed in our earlier papers is that a free purely westward propagation of eddies along the zonal coastline section is allowed in the kinked case. This introduces the interesting situation where the strong slant of the coast east of the kink prohibits the formation and shedding of rings whereas the coast west of the kink encourages such shedding. Therefore, the kink model "locks" the position of the retroflection forcing it to occur just downstream of the kink. That is, rings are necessarily shed from the retroflection area in our kinked model, regardless of the eastern coast slant. By contrast, the application of "no-kink" model for an "averaged" slant (at the same point as the kink) leads to the conclusion that shedding is almost completely arrested by the slant.

We suggest that the difference between the intensities of rings shedding during NPR and SIF is due to the shift in the zero curl line in respect to the kink. When the zero curl intersects the coast north of the kink the transport is small but it is large when the zero curl is situated south of the kink. Simple process-oriented numerical simulations are in fair agreement with our results.

Thermophysical property anomalies of Baltic seawater

Wed, 30 Jun 2010 00:00:00 +0200

Thermophysical property anomalies of Baltic seawater

Ocean Science Discussions, 7, 1103-1208, 2010

Author(s): R. Feistel, G. M. Marion, R. Pawlowicz, and D. G. Wright

While the thermodynamic properties of Standard Seawater are very well known, the quantitative effect of sea salt composition anomalies on various properties is difficult to estimate since comprehensive lab experiments with the various natural waters are scarce. Coastal and estuarine waters exhibit significant anomalies which also influence to an unknown amount the routine salinity calculation from conductivity measurements.

Recent numerical models of multi-component aqueous electrolytes permit the simulation of physical chemical properties of seawater with variable solute composition. In this paper, the FREZCHEM model is used to derive a Gibbs function for Baltic seawater, and the LSEA_DELS model to provide estimates for the conductivity anomaly relative to Standard Seawater. From additional information such as direct density measurements or empirical salinity anomaly parameterisation, the quantitative deviations of properties between Baltic and Standard Seawater are calculated as functions of salinity and temperature. While several quantities show anomalies that are comparable with their measurement uncertainties and do not demand special improvement, others exhibit more significant deviations from Standard Seawater properties. In particular density and sound speed turn out to be significantly sensitive to the presence of anomalous solute. Suitable general correction methods are suggested to be applied to Baltic Sea samples with known Practical Salinity and, optionally, directly determined density.

Estimates of radiance reflected towards the zenith at the surface of the sea

Mon, 14 Jun 2010 00:00:00 +0200

Estimates of radiance reflected towards the zenith at the surface of the sea

Ocean Science Discussions, 7, 1059-1102, 2010

Author(s): E. Aas

Remote sensing of water colour by ship-mounted sensors represents an important tool for the validation of satellite products and the monitoring of water quality. The recorded radiance from the sea has to be corrected for the surface-reflected radiance from sun and sky in order to obtain the water-leaving radiance. Here the simple case of radiance reflected towards the zenith is studied. A set of observed sky radiance and solar irradiance data from Oslo has been used together with a Gaussian slope distribution for the sea surface in order to estimate the reflected radiance. The spectral range studied is 405–650 nm, the solar zenith angles are in the range 37°–76°, and the wind speeds are up to 10 m s⁻¹. The analysis of the results show that the reflected radiance has to be separated into three contributions: sky radiance and sun rays reflected at the foam-free surface and irradiance reflected by whitecaps and foam. It is then demonstrated that by using four input values, namely the downward irradiance, the sky radiance from the zenith, the solar zenith angle and the wind speed, it is possible to obtain by simple expressions estimates of the reflected radiance that only differ from the former calculated values by relative errors of 4% or less. The analysis also indicates that for the spectral range studied neither the water-leaving radiance nor the surface-reflected radiance can be disregarded relative to the other one in the Case 2 waters of the Oslofjord-Skagerrak area. The results form a first step towards the study of reflected radiance in viewing angles differing from the nadir direction.

Suspended particles in the Canada Basin from optical and bottle data, 2003–2008

Wed, 26 May 2010 00:00:00 +0200

Suspended particles in the Canada Basin from optical and bottle data, 2003–2008

Ocean Science Discussions, 7, 1017-1057, 2010

Author(s): J. M. Jackson, S. E. Allen, E. C. Carmack, and F. A. McLaughlin

It is expected that coastal erosion, upwelling and increased river runoff from Arctic warming will increase the concentration of suspended particles in the Arctic Ocean. Here we analyze in situ transmissometer and fluorometer data from the summers of 2003 through 2008 and bottle-derived particulate organic carbon (POC) and total suspended solids (TSS) measurements sampled in the summers of 2006 and 2007 from the Canada Basin and surrounding shelves. By coupling these data sets, we explored the correlation of POC with beam attenuation coefficients to assess the viability of estimating POC concentrations from archived transmissometer data. We divided our study area into five regions to account for the significant spatial variability and found that POC (but not TSS) and attenuation were well-correlated over the Northwind Ridge, in the Canada Basin interior, and along the eastern shelf of the Canada Basin. We then estimated POC from attenuation for these regions and found that the average POC ranged from 16 to 37 μg C kg⁻¹ within the upper 50 m and from 14 to 23 μg C kg⁻¹ from 50–100 m. The strength of the chlorophyll maximum appeared to dominate the average POC values. In general, the eastern shelf was the least productive region in our study area. Neither TSS nor POC were well-correlated along the entire Beaufort shelf. Our interannual comparison from the summers of 2003 through 2008 found no evidence of increased particle concentrations over the Northwind Ridge, in the Canada Basin interior, or along the eastern shelf, however, this work provides a baseline of suspended POC concentrations.

A global comparison of Argo and satellite altimetry observations

Wed, 12 May 2010 00:00:00 +0200

A global comparison of Argo and satellite altimetry observations

Ocean Science Discussions, 7, 995-1015, 2010

Author(s): A.-L. Dhomps, S. Guinehut, P.-Y. Le Traon, and G. Larnicol

Differences and complementarities between Sea Level Anomalies (SLA) deduced from altimeter measurements and dynamic height anomalies (DHA) calculated from Argo in situ temperature (T) and salinity (S) profiles are globally analyzed. Compared to previous studies, Argo data allows a much better spatial coverage of all oceans and particularly the Southern Ocean, the use of salinity measurements and the use of a deeper reference level. The use of time series along the Argo float trajectories also provides a means to describe the vertical structure of the ocean both for the low frequency and the mesoscale part of the circulation. The comparison shows the very good consistency between Argo and altimeter observations. Correlations range from 0.9 in low latitudes to 0.3 in high latitudes where the contributions of deep baroclinic and barotropic signals are the largest. The study underlines the large influence of salinity observations on the consistency between altimetry and hydrographic observations. SLA/DHA consistency is thus improved by 35% (relative to the SLA minus DHA signal) by using measured S profiles instead of climatology data. The use of a deep reference level also significantly improves the correlation at mid and high latitudes. The role of seasonal signals on the correlation and regression analysis between altimeter and Argo observations is also analyzed. As they are mainly associated with the heating/cooling of surface layers, removing these large scale signals significantly reduces the correlation and impacts the geographical structure of the Argo/altimetry regression coefficients. These results emphasize the need to separate the different time and space scales in order to improve the merging of the two data sets. The study of seasonal to interannual SLA minus DHA signals finally reveals interesting signals related to deep ocean circulation variations. Future work is, however, needed to understand the observed differences and relate them to different forcing mechanisms.

Modeling the Nd isotopic composition in the North Atlantic basin using an eddy-permitting model

Wed, 05 May 2010 00:00:00 +0200

Modeling the Nd isotopic composition in the North Atlantic basin using an eddy-permitting model

Ocean Science Discussions, 7, 973-993, 2010

Author(s): T. Arsouze, A. M. Treguier, S. Peronne, J.-C. Dutay, F. Lacan, and C. Jeandel

Boundary Exchange (BE – exchange of elements between continental margins and the open ocean) has recently been emphasized as a key process in the oceanic cycle of neodymium (Nd). We here use a regional eddy-permitting resolution Ocean General Circulation Model (1/4°) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental marings. These exchange times are significantly lower than the previous evaluations using a low resolution model (6 months to 10 years), but however in agreement with the available seawater Nd isotope data, highlighting the importance of the model dynamics in simulating the BE process.

On the variability of Florida Straits and wind driven transports at 26° N in the Atlantic Ocean

Thu, 29 Apr 2010 00:00:00 +0200

On the variability of Florida Straits and wind driven transports at 26° N in the Atlantic Ocean

Ocean Science Discussions, 7, 919-971, 2010

Author(s): C. P. Atkinson, H. L. Bryden, J. J.-M. Hirschi, and T. Kanzow

Since April 2004 the RAPID array has made continuous measurements of the Atlantic Meridional Overturning Circulation (AMOC) at 26° N. Two key components of this system are Ekman transport zonally integrated across 26° N and western boundary current transport in the Florida Straits. Whilst measurements of the AMOC as a whole are somewhat in their infancy, this study investigates what useful information can be extracted on the variability of the Ekman and Florida Straits transports using the decadal timeseries already available. Analysis is also presented for Sverdrup transports zonally integrated across 26° N.

The seasonal cycles of Florida Straits, Ekman and Sverdrup transports are quantified at 26° N using harmonic analysis of annual and semi-annual constituents. Whilst Sverdrup transport shows clear semi-annual periodicity, calculations of seasonal Florida Straits and Ekman transports show substantial interannual variability due to variability at non-seasonal frequencies; the mean seasonal cycle for these transports only emerges from decadal length observations. The Florida Straits and Ekman mean seasonal cycles project on the AMOC with a combined peak-to-peak seasonal range of 3.5 Sv. The combined seasonal range for heat transport is 0.40 PW.

The Florida Straits seasonal cycle possesses a smooth annual periodicity in contrast with previous studies suggesting a more asymmetric structure. No clear evidence is found to support significant changes in the Florida Straits seasonal cycle at sub-decadal periods. Whilst evidence of wind driven Florida Straits transport variability is seen at sub-seasonal and annual periods, model runs from the 1/4° eddy-permitting ocean model NEMO are used to identify an important contribution from internal oceanic variability at sub-annual and interannual periods. The Ekman transport seasonal cycle possesses less symmetric structure, due in part to different seasonal transport regimes east and west of 50 to 60° W. Around 60% of non-seasonal Ekman transport variability occurs in phase section-wide at 26° N and is related to the NAO, whilst Sverdrup transport variability is more difficult to decompose.

Mixed layer mesoscales: a parameterization for OGCMs

Thu, 29 Apr 2010 00:00:00 +0200

Mixed layer mesoscales: a parameterization for OGCMs

Ocean Science Discussions, 7, 873-917, 2010

Author(s): V. M. Canuto, M. S. Dubovikov, M. Luneva, C. A. Clayson, and A. Leboissetier

We derive and assess a parameterization of the mixed layer vertical and horizontal mesoscale fluxes of an arbitrary tracer. The results, which are obtained by solving the mesoscale dynamic equations and contain no adjustable parameters, are expressed in terms of the large scale fields resolved by coarse resolution OGCMs (ocean global circulation models).

The new model can be put in the right perspective by considering the following. Thus far, the lack of a mixed layer mesoscale model that naturally satisfies the required boundary condition (the vertical flux must vanish at the surface), was remedied by extending the stream function modeled for the adiabatic deep ocean into the mixed layer using an arbitrary tapering function chosen to enforce the required boundary condition. The present model renders the tapering schemes unnecessary for the vertical flux automatically vanishes at the ocean surface. The expressions we derive for the vertical and horizontal mesoscale fluxes are algebraic and should be used in conjunction with any of the available mesoscale models valid in the adiabatic deep ocean.

We also discuss a new feature representing the effect of sub-mesoscales on mesoscales. It is shown that in the case of strong wind, one must add to the mean Eulerian velocity that enters the parameterization of the mesoscale fluxes a new term due to sub-mesoscales whose explicit form we work out.

The assessment of the model results is as follows. First, previous eddy resolving results indicated a robust re-stratification effect by mesoscales; we show that the model result for the mesoscale vertical flux leads to re-stratification (its second z-derivative is negative) and that it is of the same order of magnitude but opposite sign of the vertical flux by small scale turbulence, leading to a large cancellation. Second, since mesoscales act as a source of the eddy kinetic energy, we compare the predicted surface values vs. the Topex-Poseidon. Third, we carry out an eddy resolving simulation and assess both z-profile and magnitude of the model vertical flux against the simulation data. The tests yield positive results.

A more stratified mixed layer has implication for the oceanic absorption of heat and CO₂, a feature whose implications on climate predictions we hope to explore in the future.

The influence of the Brazil and Malvinas Currents on the southwestern Atlantic shelf circulation

Tue, 20 Apr 2010 00:00:00 +0200

The influence of the Brazil and Malvinas Currents on the southwestern Atlantic shelf circulation

Ocean Science Discussions, 7, 837-871, 2010

Author(s): R. P. Matano, E. D. Palma, and A. R. Piola

The oceanic circulation over the southwestern Atlantic shelf is influenced by large tidal amplitudes, substantial freshwater discharges, high wind speeds and – most importantly – by its proximity to two of the largest western boundary currents of the world ocean: the Brazil and Malvinas currents. This review article aims to describe the dynamical processes controlling the interaction between the shelf and the deep-ocean. The discussion is focused on two broad regions: the South Brazil Bight to the north, and Patagonia to the south. The exchanges between the Brazil Current and the South Brazil Bight are characterized by the intermittent development of eddies and meanders of the Brazil Current at the shelfbreak. However, it is argued that this is not the only – nor the most important – influence of the Brazil Current on the shelf. Numerical simulations show that the thermohaline structure of the South Brazil Bight can be entirely ascribed to steady state, bottom boundary layer interactions between the shelf and the Brazil Current. The Malvinas Current does not show the development of eddies and meanders, but its influence on the Patagonian shelf is no less important. Models and observations indicate that the Malvinas Current not only controls the shelfbreak dynamics and cross-shelf exchanges but also the circulation in the shelf's interior.