ALBERT

Beschreibung: A satellite-based global analysis of high-resolution (0.25o) ocean surface turbulent latent and sensible heat fluxes was developed by the Objectively Analyzed air-sea Fluxes (OAFlux) project. Resolving air-sea flux down to the order to 0.25o is critical for the description of the air-sea interaction on mesoscale scales. In this study, we evaluate the high-resolution product in depicting air-sea exchange in the eddy-rich Gulf Stream region. Two approaches were used for evaluation, one is point-to-point validation based on six moored buoys in the region, and another is basin-scale analysis in terms of wavenumber spectra and probability density functions (PDFs). An intercomparison is also carried out between OAFlux-0.25o, OAFlux-1o, and four atmospheric reanalyses. Results indicate that OAFlux-0.25 o is able to depict sharp oceanic fronts and has the best performance among the six participating products in comparison with buoy measurements. The mean OAFlux-0.25 o differences in latent and sensible heat flux with respect to the buoy are 7.6 Wm -2 (7.7%) with root-mean-square (RMS) difference of 44.9 Wm -2 , and 0.0 Wm -2 with RMS difference of 19.4 Wm -2 , respectively. Large differences are primarily due to mismatch in SST between gridded data and point measurements when strong spatial gradients are presented. The wavenumber spectra and decorrelation length scale analysis indicate OAFlux-0.25 o depicts eddy variability much better than OAFlux-1 o and the four reanalyses; however, its capability in detecting eddies with smaller scale still needs to be improved. Among the four reanalyses, CFSR stands out as the best in comparison with OAFlux-0.25°.

Beschreibung: Breaking waves and Langmuir circulation are two important turbulent processes in the wind-driven upper ocean. To investigate their roles in generating turbulence in the surface boundary layer of a coastal ocean, a Large Eddy Simulation (LES) model is used to simulate the turbulence measurements collected at the Marthad's Vineyard Coastal Observatoryd's Air-Sea Interaction Tower, during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) experiment in 2003. The model provides reasonable predictions for the vertical profiles of vertical velocity variance, turbulent kinetic energy (TKE), energy dissipation rates and heat flux. It shows breaking waves dominating turbulence generation near the ocean surface and turbulent large eddies characteristic of Langmuir circulation deeper in the water column. Diagnostic analysis of TKE budget in the model shows a dominant balance between turbulent transport and dissipation near the surface and a dominant balance between shear production and dissipation at deeper depths. Although the Stokes production is a significant term in the TKE budget balance near the surface, it is smaller than shear production. The turbulent transport is large in the near-surface zone and is still significant in the region affected by Langmuir circulation. These results are in agreement with a conclusion inferred from a recent analysis of the near-surface turbulence measurements at the CBLAST site.

Beschreibung: A Gulf of Mexico performance evaluation and comparison of coastal circulation and wave models was executed through harmonic analyses of tidal simulations, hindcasts of Hurricane Ike (2008) and Rita (2005), and a benchmarking study. Three unstructured coastal circulation models (ADCIRC, FVCOM, and SELFE) validated with similar skill on a new common Gulf scale mesh (ULLR) with identical frictional parameterization and forcing for the tidal validation and hurricane hindcasts. Coupled circulation and wave models, SWAN+ADCIRC and WWMII+SELFE, along with FVCOM loosely coupled with SWAN, also validated with similar skill. NOAA's official operational forecast storm surge model (SLOSH) was implemented on local and Gulf scale meshes with the same wind stress and pressure forcing used by the unstructured models for hindcasts of Ike and Rita. SLOSH's local meshes failed to capture regional processes such as Ike's forerunner and the results from the Gulf scale mesh further suggest shortcomings may be due to a combination of poor mesh resolution, missing internal physics such as tides and nonlinear advection, and SLOSH's internal frictional parameterization. In addition, these models were benchmarked to assess and compare execution speed and scalability for a prototypical operational simulation. It was apparent that a higher number of computational cores are needed for the unstructured models to meet similar operational implementation requirements to SLOSH, and that some of them could benefit from improved parallelization and faster execution speed.

Beschreibung: Pairs of moorings equipped with current profilers were deployed at each end of the Dardanelles Strait and remained in place for over 13 months. Current observations were able to resolve well the exchange flow and volume fluxes. Volume fluxes showed distinct temporal variability in upper and lower layers, especially evident on synoptic time scales. The synoptic flux variability in the upper layer was coherent with the local atmospheric forcing and the bottom pressure anomaly gradient, while the flux variations in the lower layer were related to the bottom pressure anomaly gradient. Estimated volume flux values were often two or more times larger than their respective annual means. Annual upper-layer flux means were 25.66·10 -3 and 36.68·10 -3 Sv, whereas the lower-layer averages were 14.02·10 -3 and 31.67·10 -3 Sv for the Marmara and Aegean sections, respectively. The fluxes also showed that there was a net low-salinity water outflow to the Aegean Sea, and that they varied weakly on longer time scales (monthly to seasonal). High-salinity water fluxes (≥ 39 psu) were used to calculate strait-averaged vertical eddy diffusivities which ranged between 10 -4 and 10 -2 m 2 s -1 . Additionally, microstructure observations were used to evaluate vertical eddy diffusivities. These estimates indicated that mixing in the strait varied spatially and temporarily, and it was dependent on complex strait geometry, exchange flow status, and partially on meteorological conditions. Large values of eddy diffusivities, with a depth-averaged mean of 1.3·10 -2 m 2 s -1 , and vigorous mixing were found in the Nara Pass, the narrowest section in the Dardanelles Strait.

Beschreibung: In this paper, simulated variability of the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC) and their relationship have been investigated. For the first time, climate models of the Coupled Model Intercomparison Project phase 5 (CMIP5) provided to the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC-AR5) in historical simulations have been used for this purpose. The models show the most energetic variability on the multidecadal timescale band both with respect to the AMO and AMOC, but with a large model spread in both amplitude and frequency. The relationship between the AMO and AMOC in most of the models resembles the delayed advective oscillation proposed for the AMOC on multidecadal timescales. A speed up (slow down) of the AMOC is in favor of generating a warm (cold) phase of the AMO by the anomalous northward (southward) heat transport in the upper ocean, which reversely leads to a weakening (strengthening) of the AMOC through changes in the meridional density gradient after a delayed time of ocean adjustment. This suggests that on multidecadal timescales the AMO and AMOC are related and interact with each other.

Beschreibung: Atmospheric boundary layer rolls and their impact on upper ocean circulation were investigated using a combination of two high-resolution data sources: (1) data from the Advanced Synthetic Aperture Radar (ASAR) onboard ENVISAT with a spatial sampling of approximately 500m×500m, and (2) continuous observations taken at the research platform FINO 1 with two-minutes temporal sampling at eight heights between 33 and 100m. The parallel analysis of instantaneous image data in combination with the FINO 1 time series enabled us to quantify both the spatial and temporal dynamics of mesoscale and submesoscale wind variations. The influence of these variations with different temporal and spatial scales on the hydrodynamics of the German Bight was addressed using outputs from a three-dimensional circulation model. It was demonstrated that while the coupling between wind and tidal forcing triggered substantial responses at mesoscales, the response of surface currents and sea surface temperature to the atmospheric boundary layer rolls appeared relatively weak. However, these ocean surface responses closely follow the surface footprint of the atmospheric boundary layer rolls, the signatures of which become more pronounced in the absence of strong tidal flows.

Beschreibung: State estimation techniques have been well established in open ocean systems; however they are less often used in coastal applications due to non-linearity. Using 4D-variational data assimilation in a triple one-way nested system, we investigate the processes that control coastal dynamics for a test case along the western coast of Oahu, Hawaii. All available observations are combined with the model dynamics for 13 months. Over this time, the residual error between the model and observations was improved by nearly 30% in the surface temperature and 34% in the along-shore ADCP currents. The barotropic and baroclinic tides are found dominate the local circulation; however, island and atmospheric interaction generates an island wake effect that is important to the sub-tidal dynamics of the region. The baroclinic tides exhibit well defined energy paths, and the initial condition corrections, despite altering the density waveguide, have little influence on the propagation of the baroclinic energy, which is controlled by the propagation of baroclinic tides generated outside of the domain. We find the larger-scale, advected dynamics control the local surface circulation through boundary condition adjustment, accounting for 45% of the total corrections made via data assimilation system. The initial conditions controls little of the evolution of this local, coastal flow and has a short persistence. The wind-stress control vector is important in the central region of the domain inducing flow toward the lee of the island. Our results reveal that coastal studies may not be initial value problems, rather they are forced problems that require a knowledge of the large-scale energy propagated into the region.

Beschreibung: The effects of solar radiation diurnal cycle on intraseasonal mixed layer variability in the tropical Indian Ocean during boreal wintertime Madden-Julian Oscillation (MJO) events are examined using the HYbrid Coordinate Ocean Model. Two parallel experiments, the main run and the experimental run, are performed for the period of 2005-2011 with daily atmospheric forcing except that an idealized hourly shortwave radiation diurnal cycle is included in the main run. The results show that the diurnal cycle of solar radiation generally warms the Indian Ocean sea surface temperature (SST) north of 10°S, particularly during the calm phase of the MJO when sea surface wind is weak, mixed layer is thin, and the SST diurnal cycle amplitude ( dSST ) is large. The diurnal cycle enhances the MJO-forced intraseasonal SST variability by about 20% in key regions like the Seychelles–Chagos Thermocline Ridge (SCTR; 55°-70°E, 12°-4°S) and the central equatorial Indian Ocean (CEIO; 65°-95°E, 3°S-3°N) primarily through nonlinear rectification. The model also well reproduced the upper-ocean variations monitored by the CINDY/DYNAMO field campaign between September-November 2011. During this period, dSST reaches 0.7°C in the CEIO region, and intraseasonal SST variability is significantly amplified. In the SCTR region where mean easterly winds are strong during this period, diurnal SST variation and its impact on intraseasonal ocean variability are much weaker. In both regions, the diurnal cycle also has large impact on the upward surface turbulent heat flux Q T and induces diurnal variation of Q T with a peak-to-peak difference of O (10 W m -2 ).

Beschreibung: The horizontal and vertical flux of particulate material in the nearshore of southern Lake Michigan (0 – 40 m) was estimated with the naturally occurring radionuclide 234 Th. Horizontal fluxes of 234 Th supplemented apparent vertical fluxes of 234 Th in the water column (based on local 234 Th/ 238 U disequilibria) by a factor of 7 to 14, reinforcing the importance of lateral transport in coastal environments. Calculated onshore transport of particulate material across the 40 m isobath was as high as 1.1 × 10 6 kg km -1 d -1 , and exceeds estimates of terrigenous (riverine and bluff erosion) loading. Estimates of onshore flux of organic carbon exceeded areal primary productivity by as much as ~ 300 %, and should be considered in nearshore carbon budgets. Bottom tethered sediment traps (placed 5 m above the bottom) measured sedimentation rates that were approximately 1 order of magnitude lower than 234 Th derived mass fluxes from the water column and ~ 2 orders of magnitude lower than 234 Th derived mass fluxes to the lakebed. We ascribe this difference to under-collecting by the sediment trap either because of trap hydrodynamics or flux occurring below the trap capture plane. Cross-shore fluxes showed a periodicity of ~ 4 days and correlated strongly with a topographic vorticity wave that is present throughout the year in southern Lake Michigan. The impact of this wave (as a driver of bidirectional cross-shore flux) on biogeochemical cycling and both nearshore and offshore food webs has not yet been explicitly considered.

Beschreibung: Time series (1990-2011) of sea ice thickness observed by moored sonars in the Transpolar Drift in Fram Strait are examined. Contrasting the post 2007 years against the 1990s, three remarkable changes in the monthly ice thickness distributions are highlighted: 1) The thickness of old level ice (modal thickness) is reduced by 32 percent, 2) the old ice modal peak width is reduced by 25 percent, and 3) the fraction of (ridged) ice thicker than 5 m is reduced by 50 percent. The combined effect on the mean ice thickness is a reduction from an annual average of 3.0 m during the 1990s to 2.2 m during 2008-2011. Most of the thinning took place after 2005-2006. While the old ice modal thickness and peak width show signs of recovery after 2008, the decreasing trend in fraction of ridged ice and mean ice thickness persists until the end of the record in 2011. The ice observed in Fram Strait carries an integrated signal of Arctic change due to the advection of ice from many sites in the Arctic. Based on concurrence in timing, we conclude that much of the thinning quantified here is reflecting recent change in the age composition of the Arctic ice cover towards younger ice. The old level ice remains thin, and as such the ice cover remains preconditioned for new summers of very low sea ice extent.

Beschreibung: Hydrographic observations, ocean state estimates, and ocean objective analyses are combined to investigate the decadal variations of the North Equatorial Current (NEC) in the Pacific at 137°E during the last three decades (1975-2005). Observations show that the decadal NEC transport has three maxima around 1980/1981, 1994/1995, and 2004/2005 and two minima around 1989/1990 and 1999/2000, respectively. Associated with these maxima/minima, the sea surface height (SSH) falls/rises and the subsurface isopycnals shoal/deepen in the southern part of NEC, resulting in westward/eastward zonal velocity anomalies. Results from the ocean state estimates and ocean objective analyses show good agreement with observations. Further analysis indicates that the observed zonal velocity anomalies at 137°E are part of the cyclonic/anticyclonic gyre anomalies formed in the tropical northwestern Pacific east of the Philippines, coinciding with the tropical gyre. Results from a 1½ layer reduced gravity model suggest that these oceanic variations are mainly controlled by the decadal wind forcing in the tropical western Pacific and can be attributed to both local Ekman dynamics and baroclinic Rossby wave propagation.

Beschreibung: Based on observations and ocean reanalysis, this study analyzes the variability of salinity and its related ocean dynamics in the equatorial Indian Ocean (IO). The results show significant interannual variability of salinity associated with the Indian Ocean Dipole (IOD) mode in the boreal fall. During the positive phase of IOD (pIOD), when anomalous easterly winds prevail, westward advection along the equator strengthens in summer, while the eastward advection associated with the Yoshida-Wyrtki Jet weakens in fall. Analysis of salinity budget indicates that salinity anomalies are mainly due to advection, of which zonal component is dominant. As zonal current anomalies are symmetric with respect to the equator, the equatorward large northern IO zonal salinity gradient is more important than the current anomalies in determining the asymmetric distribution of low-salinity advection. During the mature phase of pIOD, low-salinity water is advected westward, which in turn shoals the surface mixed layer, thereby providing a favorable condition for warmer sea-surface temperature in the western equatorial IO. During the decay phase of pIOD, low-salinity water is advected across the equator to the southwestern IO. When pIOD concurs along with El Niño, the strengthened off-equatorial anticyclonic circulations, which is associated with El Niño, advect low-salinity water poleward after the decay phase.

Beschreibung: Submarine canyons that cut into the continental shelf are regions of enhanced upwelling. The depth of upwelling and flux through the canyons determines their role in exchange between the shelf and the open ocean. Scaling analyses that relate these quantities to the strength of the flow, stratification, Coriolis parameter and topographic shape parameters allow their estimation in the absence of a full numerical simulation or a detailed field study. Here we add the effect of the continental shelf slope to the scaling of the depth of upwelling, upwelling flux, and deep water stretching. The scaling is then tested using a three-dimensional primitive equation model over 18 distinct geometries. The impact of the continental shelf is significant for real canyons with changes in the depth of upwelling of up to 11% and of the flux of upwelling of up to 70%. The numerical simulations clearly show three types of canyon upwelling, a symmetric time-dependent flux, the dominant advection-driven flux and a new flux that appears to be related to internal waves. They also suggest that the canyon width is more important than the upstream canyon shape in determining the strength of the flow across the canyon.

Beschreibung: The Northwestern Mediterranean Sea (NWMS) is one of the most productive areas of the Mediterranean Sea. The NWMS pelagic planktonic ecosystem is strongly influenced by hydrodynamics, in particular winter deep convection. Here we investigate the response of this ecosystem and associated carbon cycle to oceanic and atmospheric winter conditions interannual variability. For that we developed a tridimensional coupled physical-biogeochemical model, ran annual simulations forced by XXth climate conditions and performed statistical and budget analysis. Our coupled model reproduces correctly the seasonal evolution of the NWMS pelagic planktonic ecosystem. It however overestimates the contribution of nanophytoplankton to the total phytoplanktonic biomass and GPP, underestimates the bacteria biomass and represents the spring bloom with one month delay. Our results confirm that the control of phytoplanktonic development and bacteria growth by the phosphorus availability is a marked specificity of the NWMS, that is temporally reduced by deep convection. They confirm the relevance of the Behrenfeld [2010] hypothesis in explaining the bloom dynamics. The variability of the winter atmospheric conditions induces differences of vertical mixing and water temperature that propagate into the whole NWMS ecosystem through a chain of relationships. The high frequency filtering associated with averaging diagnostics explains that this variability seems weak at the NWMS scale. However for most of the variables and processes, differences induced by the winter atmospheric variability are significant at the annual scale. Net metabolism and deep carbon export are systematically positive and show larger variabilities related respectively to the water temperature and convection intensity.

Beschreibung: Frontal meanderings are generally difficult to predict. In this study, we demonstrate through an exercise with the Iceland-Faeroe Front (IFF) that satisfactory predictions may be achieved with the aid of hydrodynamic instability analysis. As discovered earlier on, underlying the IFF meandering is a convective instability in the western boundary region followed by an absolute instability in the interior; correspondingly the disturbance growth reveals a switch of pattern from spatial amplification to temporal amplification. To successfully forecast the meandering, the two instability processes must be faithfully reproduced. This sets stringent constraints for the tunable model parameters, e.g., boundary relaxation, temporal relaxation, eddy diffusivity, etc. By analyzing the instability dispersion properties, these parameters can be rather accurately set, and their respective ranges of sensitivity estimated. It is shown that too much relaxation inhibits the front from varying; on the other hand, too little relaxation may have the model completely skip the spatial growth phase, leading to a meandering way more upstream along the front. Generally speaking, dissipation/diffusion tends to stabilize the simulation, but unrealistically large dissipation/diffusion could trigger a spurious absolute instability, and hence a premature meandering intrusion. The belief that taking in more data will improve the forecast does not need to be true; it depends on whether the model setup admits the two instabilities. This study may help relieve modelers from the laborious and tedious work of parameter tuning; it also provides us criteria to distinguish a physically relevant forecast from numerical artifacts

Beschreibung: In this paper we assess the risk of future coastal flooding in the Severn Estuary, examining the contribution from low probability extreme sea level rise scenarios resulting from the possibility of increased rates of ice sheet mass loss in the coming century. A simple asymmetric probability distribution is constructed to include sea level rise scenarios of up to 1.9 m by 2100, based on recent assessments of future sea level rise in the UK. A regular sampling procedure, sampling every 1 mm, is used to increase the boundary water levels associated with a current 1:200 year event to force a two-dimensional hydrodynamic model of coastal inundation to examine the influence of sea level rise on inundation of the Somerset Levels region. From the resulting ensemble of predictions an estimation of risk (conditioned upon the hazard and the probability of occurrence) by 2100 is established. The results indicate that although the likelihood of extreme sea level rise due to rapid ice sheet mass loss is low, the resulting hazard can be large, resulting in a significant (29.7%) increase to the projected risk. These findings clearly demonstrate that uncertainty in future sea level rise, mostly associated with the rate of ice sheet mass loss, is a vital component of coastal flood risk, and therefore, needs to be accounted for by decision makers when considering mitigation policies related to coastal flooding.

Beschreibung: The in-situ data in the Deepwater Navigation Channel (DNC), Yangtze River Estuary (YRE), China, in the dry season 2009, shows spring tides associated with greater maximum velocities, more mixing, less stratification, and diffused fluid mud; whereas neap tides are associated with smaller maximum velocities, greater stratification, inhibited mixing, and stratified fluid muds. The balance of salt flux indicates the seaward salt transport is dominated by fluvial flows, and the landward salt transport is generated by compensation flows during spring tides, but shear effects during neap tidal cycles. The balance of suspended sediment flux illustrates the offshore sediment transport is dominated by fluvial flows as well, but the onshore transport is induced by tidal-pumping effects on spring tides, and shear effects on neaps. The suspended sediment transport is strongly affected by the salinity distribution and salinity-gradient-induced stratification in the DNC. The spring-neap asymmetry is generated by the estuarine gravitational circulation during low-flow conditions; while the flood-ebb asymmetric stratification within a tidal cycle is due to the semidiurnal-tidally movement of the salt front.

Beschreibung: In recent years the latitudinal position of the Subtropical Front (STF) has emerged as a key parameter in the global climate. A poleward positioned front is thought to allow a greater salt flux from the Indian to the Atlantic Ocean and so drive a stronger Atlantic Meridional Overturning Circulation. Here, the common view that the STF aligns with the zero windstress curl (WSC) is challenged. Based on the STF climatologies of Orsi [1995], Belkin and Gordon [1996], and Graham and De Boer [submitted], and on satellite scatterometry winds, we find that the zero WSC contour lies on average ~10º, ~8º, and ~5º poleward of the front for the three climatologies, respectively. The circulation in the region between the Subtropical gyres and the zero WSC contour is not forced by the WSC but rather by the strong bottom pressure torque that is a result of the interaction of the Antarctic Circumpolar Current with the ocean floor topography. The actual control of the position of the STF is crucially dependent on whether the front is regarded as simply a surface water mass boundary or a dynamical front. For the Agulhas Leakage problem the southern boundary of the so-called Super Gyre may be the most relevant property but this cannot easily be identified in observations.

Beschreibung: The M 2 tidal circulation in Algeciras Bay (Strait of Gibraltar) is analyzed using a 3-D, nonlinear, baroclinic, hydrodynamic model, in conjunction with observed data series. Results show the influence of the density-stratification on the vertical structure of the M 2 currents in Algeciras Bay, although its tidal dynamics shows major differences with respect to the Strait of Gibraltar. Whereas the M 2 currents in the Strait present mainly barotropic behaviour, the baroclinic effects prevail in Algeciras Bay. A notable finding is the presence of a tidal M 2 counter-current system between the upper Atlantic and the lower Mediterranean water layers within the Bay, with amplitudes of up to 25 cm s -1 . The interface between the two layers oscillates in anti-phase relation with respect to the free-surface elevation, with amplitudes of almost 20 m. The presence of the submarine Algeciras Canyon was found to be determinant in the three-dimensional structure of tidal currents within the Bay, strengthening the baroclinic tidal regime of currents. This situation has quantitative consequences for the flow exchange processes between Algeciras Bay and the outer Strait, with rates 20 times higher than those obtained when considering only the barotropic behaviour, as well as inflow/outflow lateral recirculation volumes during half a tidal cycle that account for more than 20% of the net accumulated volume. This flow exchange system was found to be affected by the nonlinear interaction processes between the first baroclinic period of resonance of Algeciras Bay and the M 2 tide.

Beschreibung: Impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions, 1 km and 5 km. The focus is the northwestern Gulf of Mexico, where the Loop Current eddies are surrounded by smaller vortices. By increasing the horizontal resolution, the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically, and with them the vertical velocities. Inside the coherent eddies and at their peripheries increased vertical velocities for increasing resolution are associated to near inertial motions and they are not limited to the mixed layer, but are found at all depths. Horizontal velocities, on the contrary, are similar. Lagrangian isobaric tracers are deployed close to the surface and at 100 m, and three dimensional, neutrally buoyant particles are released close to the surface, at the base of the mixed layer and at 100 m. The modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution. Close to the ocean surface, however, convergence zones, generated by submesoscale ageostrophic motions and resolved at 1 km resolution, influence the details of the tracer distributions. Vertical dispersion increases by several folds for increasing resolution at all depths explored, with the largest differences found close to the surface. Therefore submesoscales processes play a fundamental role in driving vertical transport in eddy dominated flows, both within and below the mixed layer, for times comparable to the Eulerian time scale.

Beschreibung: The 2011 Tohoku tsunami devastated the northeastern Japan coasts and caused localized damage to coastal infrastructure across the Pacific. The tsunami resulted in strong currents around the Hawaiian Islands that led to closure of harbor and marinas for up to 38 hours after its arrival. We utilize a non-hydrostatic model to reconstruct the tsunami event from the seismic source for elucidation of the physical processes and inference of coastal hazards. A number of tide gauges, bottom pressure sensors, and ADCPs provided point measurements for validation and assessment of the model results in Hawaii. Spectral analysis of the computed surface elevation and current reveals complex flow patterns due to multi-scale resonance. Standing waves with 33 to 75 min period develop along the island chains, while oscillations of 27 min or shorter are primarily confined to an island or an island group with interconnected shelves. Standing edge waves with periods 16 min or shorter, which are able to form nodes on the reefs and inside harbors, are the main driving force of the observed coastal currents. Resonance and constructive interference of the oscillation modes provide an explanation of the impacts observed in Hawaii with implications for emergency management in Pacific island communities.

Beschreibung: This study characterizes the seasonal cycle of the Catalan inner-shelf circulation using observations and complementary numerical results. The relation between seasonal circulation and forcing mechanisms is explored through the depth-averaged momentum balance, for the period between May 2010 and April 2011 when velocity observations were partially available. The monthly-mean along-shelf flow is mainly controlled by the along-shelf pressure gradient and by surface and bottom stresses. During summer, fall and winter, the along-shelf momentum balance is dominated by the barotropic pressure gradient and local winds. During spring, both wind stress and pressure gradient act in the same direction and are compensated by bottom stress. In the cross-shelf direction the dominant forces are in geostrophic balance, consistent with dynamic altimetry data.

Beschreibung: The Southern Ocean Subtropical Front (STF) is thought to play a key role in the global climate system. Theory suggests that the latitude of the STF regulates the volume of saline Agulhas Leakage into the Atlantic Ocean from the Indian. Here we use satellite sea surface temperature (SST) and height (SSH) data to study the physical characteristics of the STF water mass boundary. We find that the strong currents in this region do not align with the surface water mass boundary. Therefore we provide a new climatology for these currents which we define as the Dynamical STF (DSTF). The DSTF is the eastward extension of the western boundary current in each basin and marks the southern boundary of the subtropical gyre circulations. It is a deep water mass boundary. At the centre of each basin it merges with the Sub-Antarctic Front. The DSTF is characterised by strong SST and SSH gradients, and has no seasonal cycle. On the eastern side of basins the STF surface water mass boundary coincides with a separate region of enhanced temperature gradients which we call the Subtropical Frontal Zone (STFZ). The STFZ is comprised of multiple SST fronts and has a large seasonal cycle. There are no strong SSH gradients in the STFZ. Separating the DSTF and STFZ is a band of weak SST gradients. Given the clear separation of these features as well as stark contrast in characteristics and seasonal cycles it is counterproductive and misleading to refer to them both as the STF.

Beschreibung: A time-dependent, 1D coupled ice-ocean model is used to quantify the impact of ocean stratification on the Arctic ice cover. The model results show that the ice growth during winter equals the ice melt in summer for areas with a well-developed cold halocline layer (CHL), provided that the initial ice thickness is around 3 m, while thinner initial ice thickness results in net growth. Areas with weak salt stratification can have a negative annual thickness change irrespective of the initial ice thickness and are thus dependent on ice import in order to remain ice-covered. The model results also show that ocean stratification is mostly important for ice-thickness development during the growing season. Areas with weak stratification have an ocean heat flux up to 8 Wm -2 reaching the ice during the growing season, while areas with a CHL have an average of about 0.7 Wm -2 . In the extreme area north of Svalbard, the ocean heat fluxes are typically around 25 Wm -2 but can be up to 400 Wm -2 during the initial adjustment, when the warm Atlantic water has direct contact with the ice. A general outcome of the study is that, depending on ocean stratification, the ice cover of Arctic Ocean can be divided into one part with net ice growth (the major part) and another part with net ice melt (mainly in the Nansen Basin).

Beschreibung: : A three-dimensional wave-current-sediment coupled numerical model with wetting and drying process is developed to understand hydrodynamics and sediment transport dynamics in the Deepwater Navigation Channel (DNC), the North Passage of the Yangtze River Estuary (YRE), China. The model results are in good agreement with observed data, and statistics show good model skill scores and correlation coefficients. The model well reproduces the spring-neap variation between a well-mixed estuary and a highly-stratified estuary. Model results indicate that the estuarine gravitational circulation plays the most important role in the estuarine turbidity maximum (ETM) formation in the DNC. The upstream non-local sediment intrusion through the spill-over-mechanism is a major source of sediment trapping in the North Passage after the morphological changes. Numerical studies are conducted to show scenarios in the YRE under the effects of different forcings (river-discharges, waves, and winds). Between these study cases, surface-wave-breaking relieves the sediment trapping and bottom-wave-current-interaction aggravates the bed erosion and elevates the SSC in the ETM; the former and the latter have the least and largest influence on the suspended sediment transport in the DNC. The wind effects have a greater influence on sediment trapping than the river-discharges, and the steady-northwesterly-wind condition favours the siltation in the DNC most. The significance of density-driven turbidity current is also assessed, which can enhance the saline-water intrusion and suppress the turbulent mixing in the bottom boundary layer.

Beschreibung: The Arctic Ocean is changing rapidly but there are no longterm time series observations on the state of the phytoplankton community that could allow a link to be made from physical/chemical pressures to the impact on marine ecosystems. Here, we test the idea that space-for-time (SFT) substitution might predict temporal change in the Canada Basin premised on differences in the present state of phytoplankton in other geographic zones, specifically the ratio in the abundance of picophytoplankton to nanophytoplankton (Pico:Nano). We compared the change in Pico:Nano observed in the Canada Basin from 2004 to 2012 to the different average states of this ratio in 26 other ocean ecological regions. Our results show that as upper ocean nitrate concentration changed in the Canada Basin from year to year, the concomitant change in Pico:Nano was statistically commensurate with the difference that this ratio exhibits between Longhurst ecological provinces in relation to nitrate concentration. Lower average concentration of nitrate in the upper water column is associated with a higher value of Pico:Nano, a result consistent with resource control of phytoplankton size structure in the ocean. We suggest that SFT substitution allows an explanation of temporal progression from spatial pattern as a test of mechanism, but such statistical prediction is not necessarily a projection of future states.

Beschreibung: Particle aggregation plays an important role in many marine biogeochemical processes such as determining the vertical flux of particulate material and trace metal scavenging. Models of particle aggregation vary in complexity and in this paper I compare the behavior of a detailed size-spectrum model with that of a series of simple, two size-class models with different representations of aggregation, all of which have appeared in the literature. The simplest model uses a first-order representation of aggregation kinetics, while two other models have non-linear representations of aggregation. The simplest model is unable to reproduce the dynamic or steady-state behavior of the size-spectrum model. Results from the two non-linear size-class models show better agreement with the behavior of the size-spectrum model. I find that the mode of aggregation and the dependence of aggregation on particle size are crucial for understanding the differences between the models.

Beschreibung: This study investigates how the quality of sea surface temperature (SST) observations made by drifting buoys (drifters) and ships for 1996-2010 can be improved through retrospective quality control (QC) against a reference field. The observations used are a blend of delayed mode data taken from the International Comprehensive Ocean-Atmosphere Data Set (versions 2.0 and 2.5) and real time data obtained from the Global Telecommunication System. A comparison of drifter and ship measurements on a platform-by-platform basis to high quality SST estimates from the ATSR Reprocessing for Climate (ARC) project reveals drifter observations are generally of good quality but frequently suffer from gross errors, whilst ship observations are generally of worse quality and show a diverse range of measurement errors. QC procedures are developed which similarly assess drifter and ship SST observations through comparison with the Met Office Operational SST and Sea Ice Analysis (OSTIA). These procedures make use of seasonal background error variance estimates now available for OSTIA. Drifter observations displaying some commonly observed gross errors are flagged and ship callsigns whose observations are deemed unreliable are blacklisted. Validation of the QC outcomes against ARC and Argo demonstrates that this retrospective QC improves the quality of drifter and ship observations, though some limitations are discussed.

Beschreibung: Observations of the spatially dependent velocity field over movable bedforms subjected to slightly skewed and asymmetric regular wave forcing were collected. The dynamics between the ripple elements is dominated by coherent vortices, characterized by the swirling strength, and evidenced in the temporal and spectral characterization. Within the boundary layer, spectral energy in the second harmonic (3 f 0 ) is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients. First- and second-moment velocity statistics were used to address the spatial variability of the intra-ripple hydrodynamics. Estimates of displacement and momentum thicknesses (δ* and δ mom ), are smaller than suggested by the higher harmonics, but consistently highlight areas of adverse and favorable pressure gradients. Shear stress and roughness estimates were inferred by fitting a logarithmic model to first- and second-moment statistics of the velocity field. The maximum Shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half-wave period (onshore). First-moment roughness estimates are similar in magnitude to bedload parameterizations provided by Li et al . [1997], and about a factor of 3 larger than second-moment estimates. Assessment of the vertical transfer of horizontal momentum derived using a Reynolds decomposition, suggests that stresses inferred from the logarithmic law using first-moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half-wave period.

Beschreibung: The tidal regime in the Bohai Sea, China, is investigated using observations and an established numerical tidal model. The area has recently experienced rapid coastline changes due to natural developments of the Yellow River delta and large-scale anthropogenic land reclamation. These morphological changes are not reflected in most global bathymetric databases and are thus rarely incorporated into investigations of the Bohai Sea. It is shown that there have indeed been significant changes in the tidal regime in the Bohai Sea over the last 35 years, with M 2 amplitudes changing up to 20 cm in some parts. The model captures some of these changes when the appropriate bathymetries are used. Furthermore, the simulations show that the tides in the Bohai Sea have become more sensitive to future sea level rise and the way in which it is implemented in the model (i.e. whether or not flood defenses are included).These sensitivity changes are due to the recent coastal developments.

Beschreibung: We investigate the temporal variability and trends of pH and of the aragonite saturation state, Ω arag , in the southern California Current System on the basis of a 6 year timeseries from Santa Monica Bay, using bi-weekly observations of dissolved inorganic carbon and combined calculated and measured alkalinity. Median values of pH and Ω arag in the upper 20 m are comparable to observations from the subtropical gyres, but the temporal variability is at least a factor of 5 larger, primarily driven by short-term upwelling events and mesoscale processes. Ω arag and pH decrease rapidly with depth, such that the saturation horizon is reached already at 130 m, on average, but it occasionally shoals to as low as 30 m. No statistically significant linear trends emerge in the upper 100 m, but Ω arag and pH decrease, on average, at rates of -0.009 ± 0.006 yr -1 and -0.004 ± 0.003 yr -1 in the 100 to 250 m depth range. These are somewhat larger, but not statistically different from the expected trends based on the recent increase in atmospheric CO 2 . About half of the variability in the deseasonalized data can be explained by the El Niño Southern Oscillation (ENSO), with warm phases (El Niño) being associated with above normal pH and Ω arag . The observed variability and trend in Ω arag and pH is well captured by a multiple linear regression model on the basis of a small number of readily observable independent variables. This permits the estimation of these variables for related sites in the region.

Beschreibung: A new framework for simulating the El Niño/Southern Oscillation (ENSO) using a generalized linear model (GLM) is provided. The GLM provides a versatile and computationally inexpensive method for investigating ENSO dynamics, by conditioning an ENSO index on an arbitrary set of input variables. Here, the system state (El Niño/neutral/La Niña) at previous times is combined with the first few principal components of SST and thermocline depth. Despite having relatively few degrees of freedom, the model accurately reproduces 20th century SST time series, seasonal variance, power spectra and autocorrelation functions for both the eastern and western Pacific. The GLM also has good overall forecast skill, especially at sub-yearly lead times; performance is competitive with models currently used for operational ENSO forecasting. The model is then used to examine changes to El Niño/La Niña statistics under CO 2 increases, by using the GLM to represent simulations run with the NCAR Community Climate System Model, versions 3.5 and 4. GLM simulations of 21st century CCSM4 changes to El Niño/La Niña magnitudes show insignificant results, despite a slight increase in El Niño persistence. GLM fits conditioned on millennial stabilized CCSM3.5 simulations with varying CO 2 levels, however, show a weakening and shortening of El Niño events as CO 2 concentration increases, while La Niña events become markedly stronger and do not change significantly in length. The reduction in El Niño persistence in CCSM3.5 is consistent with previous results showing that at higher CO 2 levels, a stronger seasonal cycle creates a Southern Hemisphere ‘seasonal footprint’ leading to more efficient El Niño termination.

Beschreibung: Measurements near the edge of fast ice in Freemansundet, Svalbard, reveal mixing processes associated with tidal advection of a sharp front in salinity, including possible supercooling induced by double diffusion in a fully turbulent water column. The front translated back and forth with the semidiurnal tide between an area of mobile (drifting) ice in Storfjorden proper, and the narrow sound covered by fast ice. Water on each side of the front was near its salinity-determined freezing temperature. Instruments deployed about 400 m into the sound from the fast ice edge measured current, temperature, conductivity, and turbulence quantities through several tidal cycles. Turbulence data illustrate that as the steep horizontal salinity (density) gradient advected past the measurement site, vertical shear near the fast-ice base induced marked flood/ebb asymmetry in turbulent mixing. As fresher water entered the sound on the flood phase, inward transport of denser water near the upper boundary was retarded, leading to statically unstable conditions and enhanced turbulence. The opposite occurred during ebb tide, as denser water under-ran lighter. Transient episodes of supercooling accompanied frontal passage on both flood and ebb phases. The most likely explanation for a zone of supercooled water within the strongly mixed frontal region is that during mixing of fresher, slightly warmer (but still at freezing) water from outside with saltier, colder water in the sound, the former constituent lost heat faster than gaining salt. This interpretation (differing turbulent diffusivities for heat and salt) challenges strict application of Reynolds analogy for highly turbulent shear flow.

Beschreibung: The detection of long-term trends in geophysical time series is a key issue in climate change studies. This detection is affected by many factors: the size of the trend to be detected, the length of the available datasets, and the noise properties. Although the noise auto-correlation observed in geophysical time series does not bias the trend estimate, it affects the estimation of its uncertainty and consequently the ability to detect, or not, a significant trend. Ignoring the noise auto-correlation level typically leads to an over-detection of significant trends. Satellite time series have been providing remote observations of the sea surface for several decades. Due to satellite lifetime, usually between 5 and 10 years, these time series do not cover the same period and are acquired by different sensors with different characteristics. These differences lead to unknown level shifts (biases) between the datasets, which affect the trend detection. In this work, we develop a generic framework to detect and evaluate linear trends and level shifts in multi-sensor time series of satellite chlorophyll-a concentrations, as provided by the MERIS and SeaWiFS ocean color missions. We also discuss the optimization of the observation networks, in terms of needed time overlap between successive time series in order to reduce the uncertainty on the detection of long-term trends. For the incoming Sentinel-3 - OLCI mission that should be launched at the end of 2014, we propose a global map of the duration of this future time series necessary to actually enhance the trend detection performed with the joint SeaWiFS-MERIS analysis.

Beschreibung: The effects of exopolymers on the particle size distributions (PSD) of suspended cohesive sediments were investigated in laboratory using four abundant clay minerals, kaolinite, illite, Na-montmorillonite, and Ca-montmorillonite, and two exopolymers, xanthan and guar, at six different exopolymer to clay ratios (E/C) (i.e., 0, 1, 2, 5, 10, and 20 wt.%) to represent the compositional variability of cohesive sediments in natural waters. Results show that the clay-exopolymer suspensions possess multimodal PSD. Statistical deconvolution of the PSD curves indicates that the suspensions consist of four discrete particle groups, primary particles, flocculi, microflocs, and macroflocs, all of which exhibit a unimodal lognormal distribution. Furthermore, such deconvolution quantifies the mean size and fraction of each particle group, leading to a more quantitative understanding of PSD kinetics of these sediments. Both clay surface charges and exopolymer polarity as well as the E/C affect the PSD kinetics. While neutral guar causes flocculation for all four clay minerals, anionic xanthan only induces flocculation for kaolinite with very low surface charges, but not for the other three clay minerals with relatively high charges. The fraction of each particle group also varies with the E/C, and such complex changes depend upon the interfacial interactions between clay particles and exopolymer molecules. For each exopolymer, critical E/C exist that can lead to a maximum or minimum fraction of microflocs or macroflocs. The role of exopolymer bridging, Coulomb force, and hydrogen bond in affecting the PSD kinetics of cohesive sediments is also discussed.

Beschreibung: ABSTRACT Heat and water mass transports tagged by water type in a bay were investigated using daily outputs from a high-resolution land–sea coupled model. The modeled circulation and water property distribution were similar to those reported by observations. In this paper, the heat angle is introduced to accurately define the roles of the lateral heat flux ( LF ) into the bay and the net surface heat flux on temperature changes in the bay water. As a result, ocean phenomena in the bay can be categorized by using the heat angle in an intensive LF regime on short-period timescales and a gradual LF regime on intra-seasonal timescales. Our close examination revealed that the velocity fields can be classified into three flow patterns: a twin vortex accompanied by positive LF , a clockwise flow with negative LF , and an anticlockwise flow with both LF s. These patterns occur in both intensive and gradual LF regimes. Intensive wind-driven LF forced by atmospheric disturbances was often observed from summer to autumn in 2008 accompanying the intrusion of southern subtropical Tsugaru warm water that was colder than the deep bay water ( LF 〈 0) and subarctic Oyashio water that was warmer than the surface bay water ( LF 〉 0), but both were hardly found in 2009. This thermal contrast affects the interannual difference in the stratification inherent in the bay. Our integrated analysis method is useful for prompt and robust understanding of the thermal and dynamic states in a bay based on ocean simulation data.

Beschreibung: [1]  Basic constraints on the dense water formation rate and circulation resulting from cooling around an island are discussed. The domain under consideration consists of an island surrounded by a shelf, a continental slope, and a stratified ocean. Atmospheric cooling over the shelf forms a dense water that penetrates down the sloping bottom into the stratified basin. Strong azimuthal flows are generated over the sloping bottom as a result of thermal wind. Thermally direct and indirect mean overturning cells are also forced over the slope as a result of bands of convergent and divergence Reynolds stresses associated with the jets. The Coriolis force associated with the net mass flux into the downwelling region over the slope is balanced by these nonlinear terms, giving rise to a fundamentally different momentum budget than arises in semi-enclosed marginal seas subject to cooling. A similar momentum balance is found for cases with canyons and ridges around the island provided that the terms are considered in a coordinate system that follows the topography. Both eddy fluxes and the mean overturning cells are important for the radial heat flux, although the eddy fluxes typically dominate. The properties of the dense water formed over the shelf (temperature, diapycnal mass flux) are predicted well by application of baroclinic instability theory and simple heat and mass budgets. It is shown that each of these quantities depends only on a nondimensional number derived from environmental parameters such as the shelf depth, Coriolis parameter, offshore temperature field, and atmospheric forcing.

Beschreibung: The Pacific Decadal Oscillation (PDO) has been shown to have significant climatic and environmental impacts across the Pan-Pacific basin; however, there are no records of PDO activity from the South China Sea (SCS), the largest marginal sea in the northwest Pacific Ocean. This study suggests that a series of geochemical profiles obtained from a modern coral in the northern SCS records annual PDO activity dating back to 1853. These geochemical data are significantly correlated with the PDO index, and their patterns of variation closely match those of the PDO index over the last century. The relationship between the PDO and coral geochemistry may be related to the influence of the PDO on rainfall on Hainan Island. Rainfall patterns influence the volume of terrestrial runoff, which, in turn, is a primary determinant of δ 18 O and Δδ 18 O values in coral; however, coral δ 13 C values are also influenced by the 13 C Suess effect. The results indicate that Sr/Ca ratios in coral are affected by a combination of sea surface temperature and terrestrial runoff.

Beschreibung: [1]  The attenuation of downward planar irradiance can be quantified by , the diffuse attenuation coefficient calculated from the surface to the depth where the irradiance E d at wavelength λ falls to 10% of its surface value. Theoretical studies by Gordon [1989], Limnol. Oceanogr. 34, 1389-1409 and Lee et al . [ 2005a], J. Geophys. Res. 110, C02016 suggest that can be derived from the absorption coefficient, a ( λ ) and the backscattering coefficient, b b ( λ ), using equations incorporating either the solar zenith angle ( θ a ) or the subsurface distribution function ( D 0 ) and empirical coefficients derived by radiative transfer modelling. These results have not, however, been validated against in situ measurements. We have therefore assessed the performance of both models using measurements of a ( λ ), b b ( λ ) and for 100 stations in UK coastal waters. Best results were obtained from the Lee et al . [2005] model, for which over 90% of the predicted values in the 440 nm to 665 nm range were within +/- 0.1 m -1 of those measured in situ. A strong linear relationship ( R 2  〉 0.95, mean relative difference 5.4%) was found between at 490 nm and the reciprocal of the depth of the mid-point of the euphotic zone ( z 10% , PAR). This allowed ( z 10% , PAR) to be predicted from measured values of a (490 nm), b b (490 nm) and θ a , using the Lee et al . model as an intermediate step, with an RMS error of 1.25 m over the 2.5 m to 25.0 m range covered by our data set.

Beschreibung: [1]  The role of asymmetric tidal mixing (ATM) in subtidal estuarine dynamics is investigated using a series of generic numerical experiments that simulate narrow estuaries under different stratification and external forcing conditions. The focus is on quantifying the characteristics of ATM-induced flow and its contributions to stratification and salt transport. The flow induced by ATM has a two-layer vertical structure in periodically stratified estuaries, similar to that of the density-driven flow. It has a three-layer vertical structure in the central regime of weakly stratified estuaries, and a reverse two-layer structure in highly stratified estuaries. The changes in vertical distribution of ATM-induced flows result from the influence of stratification on the covariance of eddy viscosity and vertical shear. Such covariance represents the driving force of ATM-induced flow in the tidally averaged momentum equation. Compared to density-driven flow, ATM-induced flow dominates in periodically stratified estuaries with strong tides, has the same order of magnitude in weakly stratified estuaries with moderate tides, and is less important in highly stratified estuaries with weak tides. In contrast to density-driven flow that always increases estuarine stratification and transports salt landward, the ATM-induced flow exhibits different behaviors because of its varying vertical structure. In estuaries with strong tides, ATM-induced flow tends to enhance stratification and to transport salt landward, similar to density-driven flow. In estuaries with weak tides, ATM-induced flow tends to reduce stratification and to transport salt seaward.

Beschreibung: We examine the basin-wide trends in sea ice circulation and drift speed, and highlight the changes between 1982-2009 in connection to local winds, multiyear sea ice coverage, ice export, and the thinning of the ice cover. The polarity of the Arctic Oscillation (AO) is used as a backdrop for summarizing the variance and shifts in decadal drift patterns. The 28-year circulation fields show a net strengthening of the Beaufort Gyre and the Transpolar Drift, especially during the last decade. The imprint of the Arctic Dipole Anomaly on mean summer circulation is evident (2001-2009) and enhances summer ice area export at the Fram Strait. Between 2001 and 2009, the large spatially averaged trends in drift speeds (winter: +23.6%/decade, summer: +17.7%/decade) are not explained by the much smaller trends in wind speeds (winter: 1.46%/decade, summer: -3.42%/decade). Notably, positive trends in drift speed are found in regions with reduced multiyear sea ice coverage. Over 90% of the area of the Arctic Ocean has positive trends in drift speed and negative trends in multiyear sea ice coverage. The increased responsiveness of ice drift to geostrophic wind is consistent with a thinner and weaker seasonal ice cover and suggests large-scale changes in the air-ice-ocean momentum balance. The retrieved mean ocean current field from decadal-scale average ice motion captures a steady drift from Siberia to the Fram Strait, an inflow north of the Bering Strait, and a westward drift along coastal Alaska. This mean current is comparable to the geostrophic current from satellite-derived dynamic topography.

Beschreibung: [1]  We present a data-assimilated model of ocean's phosphorus cycle that is constrained by climatological phosphate, temperature, salinity, sea-surface height, sea-surface heat and freshwater fluxes, as well as CFC-11 and natural Δ 14 C. Export production is estimated to be 5.8 ± 2.0 × 10 12  mol P/year of which 26 ± 6% originates in the Southern Ocean (SO) south of 40°S. The biological pump efficiency, defined as the proportion of the ocean's phosphate inventory that is regenerated, is 39 ± 7%. Dividing the SO south of 40°S into a subantarctic zone (SANTZ) and an antarctic zone (ANTZ) separated by the latitude of maximum Ekman divergence, we estimate that the SANTZ and ANTZ account respectively for 23 ± 5% and 3 ± 1% of global export production, 17 ± 4% and 3 ± 1% of the regenerated nutrient inventory, and 31 ± 1% and 43 ± 5% of the preformed nutrient inventory. Idealized SO nutrient depletion experiments reveal a large-scale transfer of nutrients into circumpolar and deep waters, and from the preformed to the regenerated pool. In accord with the concept of the biogeochemical divide, we find that nutrient drawdown in the ANTZ is more effective than in the SANTZ for increasing the efficiency of the biological pump, while having a smaller impact on production in regions north of 40°S. Complete SO nutrient drawdown would allow the biological pump to operate at 94% efficiency by short circuiting the transport of nutrients in northward Ekman currents leading to a trapping of nutrients in circumpolar and deep waters that would decrease production outside the SO by approximately 44% while increasing it in the SO by more than 725%.

Beschreibung: [1]  We have developed, validated, and applied a synthetic method to monitor the off-equatorial eastward currents in the central tropical Atlantic. This method combines high-density expendable bathythermograph (XBT) temperature data along the AX08 transect with altimetric sea level anomalies (SLAs) to estimate dynamic height fields from which the mean properties of the North Equatorial Countercurrent (NECC), the North Equatorial Undercurrent (NEUC) and the South Equatorial Undercurrent (SEUC), and their variability can be estimated on seasonal to interannual timescales. On seasonal to interannual timescales, the synthetic method is well suited for reconstructions of the NECC variability, reproduces the variability of the NEUC with considerable skill, and less efficiently describes variations of the SEUC, which is located in a region of low SLA variability. A positive correlation is found between interannual variations of the NECC transport and two indices based on an interhemispheric sea surface temperature (SST) gradient and southeasterly wind stress in the central tropical Atlantic. The NEUC is correlated on interannual timescales with SSTs and meridional wind stress in the Gulf of Guinea and zonal equatorial wind stress. This study shows that both altimetry and XBT data can be effectively combined for near-real-time inference of the dynamic and thermodynamic properties of the tropical Atlantic current system.

Beschreibung: [1]  Parameterizations of near-bed sediment processes are commonly associated with the poor predictive skill of coastal sediment transport models. We implement a two-dimensional Reynolds-Averaged Navier-Stokes (RANS) model to directly assess these parameterizations by reproducing measurements obtained in large-scale wave flume experiments. A sediment transport model has been coupled to wave hydrodynamics and turbulence, and numerical experiments provide temporal and spatial variations of free surface, flow velocity, sediment concentration and turbulence quantities. Model-data comparisons enable the direct assessment of how key suspension processes are represented and of the inherent variability of the sediment transport model. We focus on the different processes occurring above rippled beds versus dynamically flat beds. Numerical results show that increasing roughness alone is not sufficient to have good predictive capability above steep ripples. Some parameterization of the vortex entrainment process is necessary and a simple modification, which leads to constant sediment diffusivity above steep-rippled beds, is sufficient to obtain good predictions of wave-averaged suspended concentrations. Model-data comparisons for the turbulent kinetic energy are also presented and highlight the need to account for the effect of vortex entrainment on near-bed turbulence and transfer of momentum.

Beschreibung: The meridional heat flux required to balance the heat lost by ocean to atmosphere at high latitudes must be accomplished by some mechanism other than mean advection and the heat flux by eddies crossing the Antarctic Circumpolar Current (ACC) may be a candidate. In this study, the positions of the main ACC fronts are determined basing on 23 XBT transects collected from 1994 to 2010 and are compared to those detected through satellite altimetry. Then cold core anomalies in XBT sections are identified and altimetry is used to follow the spatial-temporal evolution of these cold, low sea level anomalies. Mean values of main parameters, such as speed (0.35 Km/h), lifetime (79 weeks) and diameter (105 Km), are estimated. Moreover, estimations of rotational speed (0.9÷76.8 cm/s), ocean surface layer heat content along temperature sections and eddy Available Heat Anomaly (mean value -9.74*10 9 Jm -2 ) give a wider description of the detected eddies. In our study area, the spawning of eddies is found to occur downstream of the Southeast Indian Ridge and in correspondence of the Polar Front (PF) when regarding to the ACC frontal structure The contribution of eddies to the global heat budget is linked to their ability to cross the ACC fronts but also to the capacity of keeping partially unaltered the properties of water inside them. Analysis of the relation between the translation and rotational speeds shows that a typical eddy may effectively be a significant part (0.8%) of the net meridional heat transport across the PF with a mean heat content/anomaly of -7.65*10 19 J.

Beschreibung: Recent dramatic changes of freshwater systems in high latitudes will allow Submarine Fresh Groundwater Discharge (SFGD) to play a more important role in the coastal environment; especially in that SFGD will directly effect heat flux. Toyama Bay, along the central western Japan coast, is a suitable and representative case study area to estimate SFGD flux using hydrographic properties since multiple high-flow rate SFGD sites exist there. Salinities averaged over the water column (depth range 10–100 m) measured during research cruises in June 2003 and May 2005 show lower levels in the eastern than the western area in this bay. Together with monthly hydrographic properties over a 10-year period (1987-1998), the low salinity water mass in the eastern area exists consistently but distinctly and varies systematically, as does nutrient flux, affected by SFGD more than riverine input. SFGD fluxes in June 2003 (1 x 10 8 m 3 month -1 ) and May 2005 (〈1 x 10 8 m 3 month -1 ) were estimated using a box model, which is divided into a shallow box (0 – 40 m) and a deeper box (40 – 100 m). The monthly flux ratio between the SFGD and the river inputs is 13% in June, comparable to higher values reported in other global studies. Our results demonstrate that the box model analysis, based on hydrographic observations in coastal areas, is an efficient approach that can be used to estimate SFGD fluxes between the land and ocean.

Beschreibung: Time series measurements of temperature, salinity and surface meteorological parameters recorded at 8°N, 90°E in the southern central Bay of Bengal (BoB) from a Research Moored Array for African-Asian-Australian Monsoon Analysis and predication (RAMA) buoy are used to document temperature inversions and their influence on the mixed layer heat budget during the winters, defined as October to March, of 2006-07 (W67) and 2007-08 (W78). There is a marked difference in the frequency and amplitude of temperature inversion between these two winters, with variations much stronger in W78 compared to W67. The formation of temperature inversions is favored by the existence of thick barrier layers, which are also more prominent in W78 compared to W67. Inversions occur when heating in the barrier layer below the mixed layer by penetrative shortwave radiation is greater than heating of the mixed layer by net surface heat flux and horizontal advection. Our analysis further demonstrates that intraseasonal and year-to-year variability in the frequency and magnitude of temperature inversions during winter have substantial influence on mixed layer temperature through the modulation of vertical heat flux at the base of mixed layer.

Beschreibung: The Integrated Ocean Observing System Super-regional Coastal Modeling Testbed had one objective to evaluate the capabilities of three unstructured-grid fully current-wave coupled ocean models (ADCIRC/SWAN, FVCOM/SWAVE, SELFE/WWM) to simulate extratropical storm-induced inundation in the U.S. northeast coastal region. Scituate Harbor (MA) was chosen as the extratropical storm testbed site, and model simulations were made for the May 24-27 2005 and April 17-20 2007 (“Patriot's Day Storm”) nor'easters. For the same unstructured mesh, meteorological forcing and initial/boundary conditions, inter-model comparisons were made for tidal elevation, surface waves, sea surface elevation, coastal inundation, currents and volume transport. All three models showed similar accuracy in tidal simulation and consistency in dynamic responses to storm winds in experiments conducted without and with wave-current interaction. The three models also showed that wave-current interaction could 1) change the current direction from the along-shelf direction to the onshore direction over the northern shelf, enlarging the onshore water transport and 2) intensify an anti-cyclonic eddy in the harbor entrance and a cyclonic eddy in the harbor interior, which could increase the water transport towards the northern peninsula and the southern end and thus enhance flooding in those areas. The testbed inter-model comparisons suggest that major differences in the performance of the three models were caused primarily by 1) the inclusion of wave-current interaction, due to the different discrete algorithms used to solve the three wave models and compute water-current interaction, 2) the criterions used for the wet-dry point treatment of the flooding/drying process simulation, and 3) bottom friction parameterizations.

Beschreibung: ABSTRACT The network comprising 61 high-frequency radar systems along the U.S. West Coast (USWC) provides a unique, high-resolution, and broad scale view of ocean surface circulation. Subinertial alongshore surface currents show poleward propagating signals with phase speeds of O (10) and O (100 to 300) km -1 that are consistent with historical in-situ observations off the USWC and that can be possibly interpreted as coastally trapped waves (CTWs). The propagating signals in the slow mode are partly observed in southern California, which may result from scattering and reflection of higher mode CTWs due to curvature of shoreline and bathymetry near Point Conception, California. On the other hand, considering the order of the phase speed in the slow mode, the poleward propagating signals may be attributed to alongshore advection or pressure-driven flows. A statistical regression of coastal winds at National Data Buoy Center buoys on the observed surface currents partitions locally and remotely wind-forced components, isolates footprints of the equatorward propagating storm events in winter off the USWC, and shows the poleward propagating signals year-round.

Beschreibung: Over 450 Argo profiling floats equipped with oxygen sensors have been deployed, but no Quality Control (QC) protocols have been adopted by the oceanographic community for use by Argo data centers. As a consequence, the growing float oxygen dataset as a whole is not readily utilized for many types of biogeochemical studies. Here, we present a simple procedure that can be used to correct first-order errors (offset and drift) in profiling float oxygen data by comparing float data to a monthly climatology (World Ocean Atlas 2009). Float specific correction terms for the entire array were calculated. This QC procedure was evaluated by: 1) comparing the climatology-derived correction coefficients to those derived from discrete samples for 14 floats, and 2) comparing correction coefficients for 7 floats that had been calibrated twice prior to deployment (once in the factory and once in-house), with the second calibration ostensibly more accurate than the first. The corrections presented here constrain most float oxygen measurements to better than 3% at the surface.

Beschreibung: From late January to mid-February 2012 the Gulf of Trieste (North Adriatic Sea) was affected by a severe winter weather event characterized by cold air and strong northeasterly wind (Bora). The atmospheric forcing caused large surface heat fluxes which produced remarkable effects on the gulf, particularly the production of a very cold and dense water mass. Temperatures as low as 4 °C were observed in the deepest part of the gulf, similar to that which was observed in winter 1929, which was probably the most severe winter in the region over more than a century. The density anomaly attained values up to 30.58 kg m -3 , even greater than in 1929. Surface heat fluxes were estimated using bulk formulas and the meteorological and marine observations available at three stations. Mean daily heat losses exceeded 1000 W m -2 . A comparison of this event with similar past events was made using proxy heat fluxes, available since 1978, to account for the air-sea interactions, and using temperature and salinity observations, performed since 1996, to account for the effect of heat fluxes on ocean properties. The 2012 Bora episode turned out to be the most severe event of this kind in the Gulf of Trieste for at least the last 35 years, and is comparable to that which occurred in 1929. A significant linear correlation was also found between the total surface heat loss and the density increase of the waters in the part of the gulf deeper than 20 m.

Beschreibung: [1]  We investigate long-term trends in dissolved oxygen in the North Atlantic from 1960 to 2009 on the basis of a newly assembled high-quality dataset consisting of oxygen data from three different sources: CARINA, GLODAP and the World Ocean Database. Oxygen trends are determined along isopycnal surfaces for eight regions and five water masses using a general least-squares linear regression method that accounts for temporal auto-correlation. Our results show a significant decrease of oxygen in the Upper (UW), Mode (MW) and Intermediate (IW) waters in almost all regions over the last 5 decades. Over the same period, oxygen increased in the Lower Intermediate Water (LIW) and Labrador Sea Water (LSW) throughout the North Atlantic. The observed oxygen decreases in the MW and IW of the northern and eastern regions are largely driven by changes in circulation and/or ventilation, while changes in solubility are the main driver for the oxygen decrease in the UW and the increases in the LIW and LSW. From 1960 until 2009 the UW, MW, and IW horizons have lost a total of −57 ± 34 Tmol, while the LIW and LSW horizons have gained 46 ± 47 Tmol, integrating to a roughly constant oxygen inventory in the North Atlantic. Comparing our oxygen trends with those of the oceanic heat content, we find an O 2 to heat change ratio of −3.6 ± 2.8 nmol J −1 for the UW, MW and IW, and a ratio of −2.8 ± 3.4 nmol J −1 for the LIW and LSW. These ratios are substantially larger than those expected from solubility alone.

Beschreibung: [1]  The warm and saline Subtropical Water carried by the North Atlantic Current undergoes substantial transformation on its way to higher latitudes, predominantly from oceanic heat loss to the atmosphere. The geographical distribution of the surface forced water mass transformation is assessed in multicentury climate simulations from three different climate models (BCM, IPSLCM4, and MPI-M ESM), with a particular focus on the eastern subpolar North Atlantic Ocean. A diagnosis, originally introduced by Walin (1982), estimates the surface water mass transformation from buoyancy forcing. While the depth structure of the Atlantic Meridional Overturning Circulation (AMOC) is similar in all models, their climatological heat and freshwater fluxes are very different. Consistently, the models differ in their mean pathways of the North Atlantic Current, location of upper ocean low salinity waters, as well as in sea ice cover. In the two models with an excessive sea ice extent in the Labrador Sea, most of the water mass transformation in the subpolar region occurs in the eastern part (east of 35°W). The variability of the eastern water mass transformation on decadal time scales is related to the variable warm northward flow into the subpolar region, the upper branch of AMOC, where a strengthened flow leads an intensified transformation. This relationship seems to disappear with a weak connection between the Subtropical and Subpolar gyres.

Beschreibung: [1]  The composition of the Fram Strait freshwater outflow is investigated by comparing 10 sections of concurrent salinity, δ 18 O, nitrate and phosphate measurements collected between 1997 and 2011. The largest inventories of net sea ice meltwater are found in 2009, 2010 and 2011. The 2009–2011 sections are also the first to show positive fractions of sea ice meltwater at the surface near the core of the EGC. Sections from September 2009–2011 show an increased input of sea ice meltwater at the surface relative to older September sections. This suggests that more sea ice now melts back into the surface in late summer than previously. Comparison of April, July and September sections reveals seasonal variations in the inventory of positive sea ice meltwater, with maximum inventories in September sections. The time series of sections reveals a strong anti-correlation between meteoric water and net sea ice meltwater inventories, suggesting that meteoric water and brine may be delivered to Fram Strait together from a common source. We find that the freshwater outflow at Fram Strait exhibits a similar meteoric water to net sea ice meltwater ratio as the central Arctic Ocean and Siberian shelves, suggesting that much of the sea ice meltwater and meteoric water at Fram Strait may originate from these regions. However, we also find that the ratio of meteoric water to sea ice meltwater inventories at Fram Strait is decreasing with time, due to an increased surface input of sea ice meltwater in recent sections.

Beschreibung: [1]  The variation of El Niño Southern Oscillation (ENSO) events from the mid-nineteenth century until the beginning of the twenty-first century is explored using an ocean reanalysis. A comparison of the reanalysis with three sea surface temperature reconstructions shows that the timing of events is similar in all four products, however there are important differences in the strength and location of events. The difference between the reconstructions is sometimes larger than the difference between the reanalysis and a given reconstruction. These differences are larger in the first half of the record, a period for which there are relatively sparse observations. The reanalysis is used to explore decadal variability and trends in the frequency, duration, and propagation direction of ENSO events. There is considerable decadal variability of these ENSO characteristics with the time between events ranging from several months to ten years and the duration of ENSO varying from 5 to 27 months. As has been previously shown for the strength and location of ENSO there is little overall trend in the characteristics. Having a three dimensional representation of the ocean from the reanalysis allows exploration of subsurface changes during ENSO. An analysis of subsurface anomalies shows that during ENSO events the subsurface anomalies are highly correlated with the strength of surface anomalies over the 140 year period. Overall, there is no evidence that there are changes in the strength, frequency, duration, location or direction of propagation of El Niño and La Niña anomalies caused by global warming during the period from 1871 to 2008.

Beschreibung: [1]  A hydrodynamic model is used to investigate convergent alongshore flows over the Texas-Louisiana shelf, characterized by down-coast flows over the northern shelf encountering up-coast flows over the southern shelf. The model's ability to reproduce realistic current, salinity, and surface elevation fields is demonstrated through positive model skill scores when comparing model simulations to observational data. The convergent flows are explored on both weather band and seasonal time scales. For weather band scales, this study focuses on wintertime convergent events. The model-predicted locations of convergent flows are supported by current measurements. We find that the formation of convergent flows is primarily caused by along-coast variation in the alongshore component of wind forcing, which in turn is due to the curvature of the Texas-Louisiana coastline. In general, the alongshore currents are well correlated with alongshore winds. However, the convergence points of currents and winds are not colocated, but rather, convergence points in ocean currents typically occur down coast of convergence points in the wind. This offset is demonstrated to be mainly caused by buoyancy forcing that can drive the convergence location in the currents farther down coast. No specific temporal pattern is found for the weather band convergence locations, whereas at seasonal time scales, the monthly mean convergence exhibits a prominent seasonal pattern, with up-coast migration of convergence locations in spring and summer and down-coast migration in fall and winter.

Beschreibung: [1]  Tsunamis generated by deformable granular landslides are physically modeled in a three-dimensional tsunami wave basin based on the generalized Froude similarity. The dynamic landslide impact characteristics were controlled by means of a novel pneumatic landslide generator. The wave amplitudes, periods, and wavelengths are related to the landslide parameters at impact with the landslide Froude number being a dominant parameter. Between 1 and 15% of the landslide kinetic energy at impact is converted into the wave train energy. The wave amplitudes decay in radial and angular directions from the landslide axis. The first wave crest mostly travels with speeds close to the theoretical approximation of the solitary wave speed. The measured tsunami wave profiles were either of the nonlinear oscillatory or nonlinear transition type depending primarily on the landslide Froude number and relative slide thickness at impact. The generated waves range from shallow to deep water depth regimes, with the majority being in the intermediate water depth regime. Wave characteristics are compared with other two- and three-dimensional landslide tsunami studies and the results are discussed.

Beschreibung: [1]  Numerical modeling was used to provide a new estimate of the amount of 137 Cs released directly into the ocean from the Fukushima Daiichi nuclear power plant (NPP) after the accident in March 2011 and to gain insights into the physical processes that led to its dispersion in the marine environment during the months following the accident. An inverse method was used to determine the time-dependent 137 Cs input responsible for the concentrations observed at the NPP's two liquid discharge outlets. The method was then validated through comparisons of the simulated concentrations with concentrations measured in seawater at different points in the neighborhood of the plant. An underestimation was noticed for stations located 30 km offshore. The resulting bias in the release inventory was estimated. Finally, the maximum 137 Cs activity released directly to the ocean was estimated to lie between 5.1 and 5.5 PBq (Peta Becquerel = 10 15  Bq) but uncertainties remain on the amount of radionuclides released during the first few days after the accident. This estimate was compared to previous ones and differences were analyzed further. The temporal and spatial variations of the 137 Cs concentration present in the coastal waters were shown to be strongly related to the wind intensity and direction. During the first month after the accident, winds blowing toward the south confined the radionuclides directly released into the ocean to a narrow coastal band. Afterwards, frequent northward wind events increased the dispersion over the whole continental shelf, leading to strongly reduced concentrations.

Beschreibung: [1]  The study used 126 buoy time series as a benchmark to evaluate a satellite-based daily, 0.25-degree gridded global ocean surface vector wind analysis developed by the Objectively Analyzed airs-sea Fluxes (OAFlux) project. The OAFlux winds were produced from synthesizing wind speed and direction retrievals from 12 sensors acquired during the satellite era from July 1987 onward. The 12 sensors included scatterometers (QuikSCAT and ASCAT), passive microwave radiometers (AMSRE, SSMI and SSMIS series), and the passive polarimetric microwave radiometer from WindSat. Accuracy and consistency of the OAFlux time series are the key issues examined here. A total of 168,836 daily buoy measurements were assembled from 126 buoys, including both active and archive sites deployed during 1988–2010. With 106 buoys from the tropical array network, the buoy winds are a good reference for wind speeds in low and mid-range. The buoy comparison shows that OAFlux wind speed has a mean difference of −0.13 ms −1 and an RMS difference of 0.71 ms −1 , and wind direction has a mean difference of −0.55 degree and an RMS difference of 17 degrees. Vector correlation of OAFlux and buoy winds is of 0.9 and higher over almost all the sites. Influence of surface currents on the OAFlux/buoy mean difference pattern is displayed in the tropical Pacific, with higher (lower) OAFlux wind speed in regions where wind and current have the opposite (same) sign. Improved representation of daily wind variability by the OAFlux synthesis is suggested, and a decadal signal in global wind speed is evident.

Beschreibung: [1]  Previous studies that analyzed ocean color satellite data have suggested that the primary mechanism of surface chlorophyll (Chl) response to the MJO is wind-induced turbulent mixing and the corresponding mixed layer entrainment. In this study, this notion is examined with an ocean biophysical model in an ensemble framework, focusing on upper ocean processes (z 〈 200 m). As a whole, the model's mean Chl state is lower than observations except in the tropical Pacific basin, but its seasonal variation is acceptable, particularly in the tropical Indian Ocean. In this basin, the model can simulate surface Chl responses to the MJO consistent with the observations in terms of the phase-by-phase anomaly evolution patterns. These Chl responses are mostly induced by surface wind forcing, which is consistent with previous studies. Further investigation of subsurface variations is performed at select grid points, and it is revealed that (1) entrained nutrients are the primary source of enhancement for surface Chl concentration and detrainment blooms are relatively less common; (2) in limited regions, Ekman pumping can effectively reduce Chl concentration; and (3) both entrainment/detrainment and Ekman pumping mechanisms rely on background states of nutrient availability, so the same forcing can result in completely different Chl responses depending on the background state.

Beschreibung: [1]  Over a range of 132.5 m, 54 temperature sensors (1 mK relative accuracy) were moored yearlong while sampling at 1 Hz around 1455 m in the open Canary Basin. Coherence between individual records shows a weak but significant peak above the local buoyancy frequency N for all vertical separations Δ z 〈 100 m, including at sensor interval Δ z = 2.5 m. Instead of a dominant zero-phase difference over the range of sensors, as observed for internal waves at frequencies f 〈 σ 〈 N , with f denoting the inertial frequency, this superbuoyant coherence shows π -phase difference. The transition from zero-phase difference, for internal waves, to π -phase difference is abrupt and increases in frequency for decreasing Δ z 〈 10 m. For Δ z 〉 10 m, the transition is fixed at N t ≃ 1.6 N , which is also the maximum value of the small-scale buoyancy frequency, and limits the internal wave band on its high-frequency side. In the time domain it is observed that this high-frequency coherence mainly occurs when nonlinearities in the temperature gradient, such as steps in the temperature profile, are advected past the sensors. A simple kinematic model of fine-structure contamination is proposed to reproduce this observation. The canonical −2 slope of the temperature spectrum above N is not observed in the in situ data, which rather slope as −8/3. The −8/3 slope can, however, be reproduced in our model, provided the jumps in the temperature profile are not infinitely thin.

Beschreibung: [1]  A high-resolution coastal model is used to investigate the transport, filling and flushing times of the freshwater introduced from the Mississippi and Atchafalaya Rivers on the Texas-Louisiana Shelf. The model is forced with realistic forcing, and is nested within hindcasts from the HYCOM operational model. The Mississippi and Atchafalaya discharges are each tagged with dye so that they can be identified and treated separately. The seasonal patterns of freshwater transport are consistent with that expected for the prevailing seasonal winds, but with significant interannual variability. In non-summer months, the major freshwater transport is downcoast and mainly occurs in a narrow band inside of 20-m isobath. In summer, the transport decreases dramatically near the coast due to the competing effects of downcoast buoyancy driven flow and upcoast wind-driven flow. The freshwater transport is upcoast over the mid shelf, in summer, with an offshore component consistent with Ekman transport. We define the shelf domain as the region enclosed by the 100-m isobath, and the along-shore limit of the entire model domain, approximately from the Louisiana-Mississippi border to the Texas-Mexico border. Filling times, based on the river discharge, range from ∼3 months (non-summer) to ∼6 months (summer) for Mississippi, while for Atchafalaya from ∼3–4 months to ∼1 year. Flushing times, based on the fresh water flux out of the shelf domain, are more variable, ranging from several months to several years.

Beschreibung: [1]  Using a three-dimensional ocean reanalysis data product, the influence of two types of the El Niños on the North Equatorial Countercurrent (NECC) across the Pacific basin is examined in this study. Eastern-Pacific El Niños (EP-El Niños) exert a significant impact on the interannual changes of the NECC, whereas Central-Pacific El Niños (CP-El Niños) have little influence. From the developing to mature phase of the EP-El Niños that occur s frequently in the latter half of a calendar year, the NECC tends to intensify and shift southward. While the EP-El Niño-related position changes of the NECC is out of phase with its seasonal position changes, the EP-El Niño-related intensity variability of the NECC is in phase with its seasonal modulation. During both types of El Niños, the El Niño-modulated wind stress curl forcing changes the upper ocean thickness, modifying the NECC intensity and central position. The weaker and shorter fluctuations of wind stress curl forcing during the CP-El Niño events induce irregular and insignificant changes of the NECC.

Beschreibung: [1]  The spreading of two clusters of satellite-tracked surface drifters was quantified by ‘relative dispersion’ and ‘relative diffusivity’ (the time derivative of relative dispersion) in two regions of the southern Great Barrier Reef (GBR): a low-density reef matrix of complex topography and a shelf lagoon over flat bathymetry. Although based on a small data set, averaged values of these parameters over a 12 day period with concurrent measurements for the two regions indicated that relative dispersion and diffusivity of drifter pairs were 182 and 52 times greater, respectively, along the reef matrix than found in the lagoon. Relative diffusivities in the southern GBR were anisotropic and varied largely in space and time within the spatial scales of 1–100 km and temporal scales of hours to 16–20 days; they were considerably enhanced by high-frequency tidal processes. Submesoscale processes (〈20 km) were important in the southern GBR, particularly in areas with complex topography where secondary circulations around the reefs and regions of steep bathymetry caused abrupt increase in dispersion. Although the dispersion was much higher along the reef matrix, the presence of ring circulations around the islands and wake eddies in the lee of islands can act as trapping mechanisms to keep particles within the reef matrix, retarding the loss to open water areas. This provides an efficient physical mechanism for both enhancing larval connectivity between reefs and, at the same time, trapping larvae when in the vicinity of reefs.

Beschreibung: [1]  This paper discusses the use of Jason-2 radar altimeter measurements to estimate the Ganga-Brahmaputra surface freshwater flux into the Bay of Bengal for the period mid-2008 to December 2011. A previous estimate was generated for 1993–2008 using TOPEX-Poseidon, ERS-2 and ENVISAT, and is now extended using Jason-2. To take full advantages of the new availability of in situ rating curves, the processing scheme is adapted and the adjustments of the methodology are discussed here. First, using a large sample of in situ river height measurements, we estimate the standard error of Jason-2–derived water levels over the Ganga and the Brahmaputra to be respectively of 0.28 m and 0.19 m, or less than ∼4% of the annual peak-to-peak variations of these two rivers. Using the in situ rating curves between water levels and river discharges, we show that Jason-2 accurately infers Ganga and Brahmaputra instantaneous discharges for 2008–2011 with mean errors ranging from ∼2180 m 3 /s (6.5%) over the Brahmaputra to ∼1458 m 3 /s (13%) over the Ganga. The combined Ganga-Brahmaputra monthly discharges meet the requirements of acceptable accuracy (15–20%) with a mean error of ∼16% for 2009–2011 and ∼17% for 1993–2011. The Ganga-Brahmaputra monthly discharge at the river mouths is then presented, showing a marked interannual variability with a standard deviation of ∼12500 m 3 /s, much larger than the data set uncertainty. Finally, using in situ sea surface salinity observations, we illustrate the possible impact of extreme continental freshwater discharge event on the northern Bay of Bengal as observed in 2008.

Beschreibung: [1]  Intraseasonal signals with periods of 2 to 3 weeks in near-surface alongshore current measurements are detected from four moorings (K1–K4) deployed from 2000 to 2004 at the 11°S section close to the Brazilian coast as part of the German CLIVAR Tropical Atlantic Variability Project. This section crosses the path of the North Brazil Undercurrent, the most powerful western boundary current in the South Atlantic Ocean. We investigate the origin of this intraseasonal variability of the North Brazil Undercurrent by relating the oceanic oscillation of the alongshore currents to its atmospheric counterpart, the meridional wind stress. On average, the results indicate a well-defined lagged (10 days) correlation (∼0.6) structure between meridional wind stress and alongshore currents. The oceanic region with the highest cross-correlations is identified as a relatively narrow band along the Brazilian coast, from 22°–36°S and 40°–50°W, bounded in the north by an eastward change in coastline orientation. The cross-wavelet transform establishes the common power between the time series of meridional wind stress and alongshore currents, predominantly during austral winter and spring. These signals propagate equatorward with an alongshore speed of 285 ± 63 km day −1 , consistent with Coastal Trapped Wave theory.

Beschreibung: [1]  From the Surface Velocity Program (SVP) drifter current and QuikSCAT wind data, the relationship between the observed near-surface current vectors and surface wind vectors for the northwestern Pacific Ocean under high winds (20–50 m s −1 ) are obtained with quantitative estimations of near-surface drift ratio (current speed versus wind speed) r (∼2%) and near-surface drift angle α (∼0°–10° to the right of the winds). These estimations keep unchanged after removing the surface geostrophic component. From the SVP drifter current and daily WindSat wind data, the estimated r is still approximately 2%. Three linear regression equations are obtained between the observed near-surface current speeds and the surface wind stress for the high wind range.

Beschreibung: [1]  Carbon, nitrogen, and chlorophyll are three commonly used variables to quantify phytoplankton productivity. However, owing to the complex effect of light, temperature, and nutrient, these three variables are usually decoupled from each other. By implementing a biogeochemical model that treats phytoplankton carbon, nitrogen, and chlorophyll separately in a Pacific basin-scale circulation model, we have investigated the physical and biogeochemical controls on phytoplankton C:Chl and C:N ratios at different scales. The model reproduces the general features of phytoplankton dynamics in the Pacific basin. Model results indicate a region dependent pattern of driving mechanisms for C:Chl ratio, where C:Chl is largely controlled by light condition in high latitude, by light and nutrient levels in midlatitude, and by nutrient and temperature in the equatorial Pacific, while C:N ratio seems to be region independent and is mostly determined by nutrient and temperature. Pacific basin-scale empirical orthogonal functions (EOF) decompositions suggest both C:Chl and C:N are regulated by the El Niño/Southern Oscillation (ENSO) events to the first-order variability. Owing to the variation of C:Chl, the decoupling between phytoplankton carbon and chlorophyll exists, especially at seasonal and long-term temporal scales. Climate change can affect phytoplankton pigment, biomass, and physiological state. As a consequence, using chlorophyll as a proxy for phytoplankton biomass to interpret decadal variation or long-term trend might be with bias.

Beschreibung: [1]  Diurnal temperature variability in the top 50 m of the ocean is assessed by pairing Argo temperature profiles with geographically colocated Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperatures (SSTs) collected within ±24 h of each other. Data pairs with time separations of up to ±3 h are used to evaluate systematic differences between the two data sets. Daytime SSTs are warmer than Argo 5 m temperatures in low-wind conditions, as expected due to diurnal surface warming effects. SSTs also tend to be warmer than Argo 5 m temperatures when columnar water vapor is less than ∼7 mm. These effects are removed empirically. For Argo data collected within ±24 h of satellite overpass times, temperature differences between Argo and AMSR-E show evidence of a diurnal cycle detectable at 5 m depth and below. The diurnal amplitude decreases with increasing latitude and increasing depth to the base of the mixed layer and is stronger in summer than in winter. At 5 m depth, the amplitude of the summer diurnal cycle ranges from about 0.1°C at the equator to 0.05°C near 60° latitude. At latitudes where the diurnal amplitude exceeds about 0.04°C, maximum temperatures occur at about 16:50 ±0:40 local time, and minimum temperatures occur at about 07:50 ±0:40 local time. Above the base of the mixed layer, the time of the diurnal maximum increases with depth, consistent with downward propagation of the diurnal signal, while the time of the minimum implies an upward propagation.

Beschreibung: [1]  In situ measurements are used, together with sea surface height data, to study the development and variability of the Great Whirl (GW), a large quasi-stationary anticyclone that appears off the coast of Somalia during the southwest monsoon season. We find that anticyclonic circulation indicative of the GW appears on average in April, almost two months before the onset of the southwest monsoon winds. This early initiation is coincident with the arrival of annual Rossby waves at the western boundary. With the onset of the monsoon winds in early June, the GW-proper intensifies quickly, remaining at its peak throughout July, August, and September, and dissipating about one month after the winds have died. The GW is present for an average 166 ± 30 days per year and the position of its northern flank, close to 9°N, coincides with the latitude of zero wind stress curl. The intraseasonal variability of the GW is intense as a result of mutual advection with one to three flanking cyclones, which accompany the GW 70% of the time and tend to circulate clockwise around it. There is no consistent seasonal pattern for the development or dissipation of the GW, although movement to the southwest is common toward the end of the season. The GW of 1995 deepened from 200 m in June to over 1000 m in September, and strengthened from a swirl transport of 10 to 60 Sv. Cool waters in its core resulted from advection via the Somali Current and some local vertical mixing.

Beschreibung: [1]  Laboratory experiments are conducted to investigate the interactions of self-propagating barotropic cyclones and baroclinic anticyclones with an island. Results are interpreted in the context of observations around Okinawa Island, Japan, where ubiquitous arrivals of cyclones and anticyclones on the southeastern side of the island influence the flow around it, thereby impacting both the Ryukyu Current's and the Kuroshio's transport. In the laboratory, baroclinic anticyclones generate a buoyant current that flows clockwise around an island whereas barotropic cyclones generate a counterclockwise current. In both cases, the interaction is governed by conservation of circulation Γ around the island, which establishes a balance between the dissipation along the island in contact with the eddy and the dissipation along the island in contact with the generated current. Laboratory results and scaling analysis suggest that the interaction between an anticyclone (cyclone) and Okinawa Island should result in an instantaneous increase (decrease) of the Ryukyu Current transport and a delayed increase (decrease) of the Kuroshio transport. The estimated delays are in good agreement with those obtained with field measurements suggesting that the dynamics at play in the laboratory may be relevant for the flow around Okinawa Island.

Beschreibung: [1]  The late austral summer (February–April) phytoplankton bloom that occurs east of Madagascar exhibits significant interannual variability and at its largest extent covers ~1% of the world's ocean surface area. The bloom raises many intriguing questions about how it begins, is sustained, propagates to the east, exports carbon, and ends. It has been observed and studied using satellite ocean color observations, but the lack of in situ data makes it difficult to address these questions. Here we describe observations that were made serendipitously on a cruise in February 2005. These show clearly for the first time the simultaneous existence of a deep chlorophyll maximum at ~70–110 m depths (seen in SeaSoar fluorimeter data) and a surface chlorophyll signature [seen in Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color data]. The observations also show the modulation of the biological signature at the surface by the eddy field but not of the deep chlorophyll maximum. Trichodesmium dominates the bloom nearer to Madagascar, while the diatom Rhizosolenia clevei (and its symbiont Richelia intracellularis ) dominates further from the island. The surface bloom seen in the SeaWiFS data is confined to the shallow (~30 m) mixed layer. It is hypothesized that the interannual variability in bloom intensity may be due to variations in coastal upwelling and thus the supply of iron, which is a micronutrient that can limit diazotroph growth.

Beschreibung: During freeze-up and consolidation, sea ice rejects to its surface brine of marine origin that is incorporated into overlying snow. To evaluate transport of biological components in brines from ice to snow, vertical profiles of temperature, salinity, bacterial abundance and extracellular polysaccharide substances (EPS) were obtained through snow and first-year sea ice (Barrow, Alaska) in consecutive winters (2010, 2011). Snow profiles showed strong interannual variation, with 2010 presenting higher values and wider ranges in salinity (0.3–30.9, practical salinity), bacterial abundance (2.8 × 10 2 –1.5 × 10 4 cells mL −1 ) and particulate EPS (pEPS, 0.04–0.23 glu-eq mg L −1 ) than 2011 (0–11.9, 2.7 × 10 3 –4.2 × 10 3 cells mL −1 and 0.04–0.09 glu-eq mg L −1 , respectively). Surface ice also differed interannually, with 2010 presenting again higher salinity (19.4, n = 1), bacterial abundance (5.4 × 10 4 –9.6 × 10 4 cells mL −1 ) and pEPS (0.13–0.51 glu-eq mg L −1 ) than 2011 (7.7–11.9, 1.7 × 10 4 –2.2 × 10 4 cells mL −1 and 0.01–0.09 glu-eq mg L −1 , respectively). Transport of bacteria and pEPS from sea-ice brines into snow was evident in 2010 but not 2011, a year with more extreme winter conditions of colder temperature, thinner snow and stronger wind. By size fraction, the smallest EPS (〈0.1  μ m) dominated (〉80%) total EPS in both ice and snow; the 〉3  μ m fraction of EPS in snow appeared to have an atmospheric source. Evaluation of membrane integrity by Live/Dead stain revealed a high percentage (85%) of live bacteria in saline snow, identifying this vast environment as a previously unrecognized microbial habitat.

Beschreibung: [1]  Hydrographic and stable isotope ( 18 O) data from 4 summer surveys in the Laptev Sea are used to derive fractions of sea-ice meltwater and river water. Sea-ice meltwater fractions are found to be correlated to river water fractions. While initial heat of river discharge is too small to melt the observed 0–158 km 3 of sea-ice meltwater, arctic rivers contain suspended particles (SPM) and colored dissolved organic material (CDOM) that preferentially absorb solar radiation. Accordingly heat content in surface waters is correlated to river water fractions. But in years when river water is largely absent within the surface layer absolute heat content values increase to considerably higher values with extended exposure time to solar radiation and sensible heat. Nevertheless no net sea-ice melting is observed on the shelf in years when river water is largely absent within the surface layer. [2]  The total freshwater volume of the central-eastern Laptev Sea (72–76°N, 122–140°E) varies between ~1000–1500 km 3 (34.92 reference salinity). It is dominated by varying river water volumes (~1300–1800 km 3 ) reduced by an about constant freshwater deficit (~350–400 km 3 ) related to sea-ice formation. Net sea-ice melt (~109–158 km 3 ) is only present in years with high river water budgets. Intermediate to bottom layer (〉25 salinities) contain ~60% and 30% of the river budget in years with low and high river budgets, respectively. The average mean residence time of shelf waters was ~2–3 years during 2007–2009.

Beschreibung: [1]  Intense phytoplankton blooms were observed along the Patagonian shelf-break with satellite ocean color data, but few in situ optical observations were made in that region. We examine the variability of phytoplankton absorption and particulate scattering coefficients during such blooms on the basis of field data. The chlorophyll- a concentration, [Chla], ranged from 0.1 to 22.3 mg m -3 in surface waters. The size fractionation of [Chla] showed that 80% of samples were dominated by nanophytoplankton (N-group) and 20% by microphytoplankton (M-group). Chlorophyll-specific phytoplankton absorption coefficients at 440 and 676 nm, a * ph (440) and a * ph (676), and particulate scattering coefficient at 660 nm, b * p (660), ranged from 0.018 to 0.173, 0.009 to 0.046, and 0.031 to 2.37 m 2 (mg Chl a ) -1 , respectively. Both a * ph (440) and a * ph (676) were statistically higher for the N-group than M-group and also considerably higher than expected from global trends as a function of [Chla]. This result suggests that size of phytoplankton cells in Patagonian waters tends to be smaller than in other regions at similar [Chla]. The phytoplankton cell size parameter, S f , derived from phytoplankton absorption spectra, proved to be useful for interpreting the variability in the data around the general inverse dependence of a * ph (440), a * ph (676), and b * p (660) on [Chla]. S f also showed a pattern along the increasing trend of a * ph (440) and a * ph (676) as a function of the ratios of some accessory pigments to [Chla] . Our results suggest that the variability in phytoplankton absorption and scattering coefficients in Patagonian waters is caused primarily by changes in the dominant phytoplankton cell size accompanied by covariation in the concentrations of accessory pigments.

Beschreibung: [1]  In the Eastern Bering Sea, changes in sea ice have been implicated in recent major upper-trophic level shifts. However, the underlying relationships between sea ice and primary producers have not been well tested. Here, we combine data from multiple satellite platforms, reanalysis model results and biophysical moorings to explore the dynamics of spring and summer primary production in relation to sea ice conditions. In the northern Bering Sea, sea ice consistently retreated in late spring, leading to ice-edge phytoplankton blooms in cold (0–1 °C) waters. However, in the southeastern Bering Sea, sea ice retreat was far more irregular. Although this did not significantly alter bloom timing, late retreat led to blooms at the ice-edge while early retreat led to blooms in open waters that were warmer (≤5.4 °C) and 〉70% more productive. Early sea ice retreat also led to higher productivity in summer, likely due to weaker thermal stratification. Overall, annual net primary production during warm years of early sea ice retreat was enhanced by 40–50% compared to years with late sea ice retreat in the southeastern Bering Sea. These findings suggest the potential for future sea ice loss to enhance overall carrying capacity of the southeastern Bering Sea ecosystem. Consistently warm blooms in the future may also channel more energy flow toward the pelagic, rather than benthic, environment. To date, however, neither sea ice extent nor the timing of its retreat have undergone long-term changes in the Eastern Bering Sea.

Beschreibung: [1]  Ocean colour data from the 13-year Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission are used to examine the distribution of surface chlorophyll a in the south west Pacific Ocean. The mean surface chlorophyll field is similar to that found by previous workers, with elevated levels in the Subtropical Front and around the Subantarctic Islands that have an associated shelf. The annual cycle in surface chlorophyll shows a ubiquitous summer bloom in Subantarctic Water, with autumn, winter and spring blooms variously in Subtropical Water and across the Subtropical Front. The autumn blooms progresses equatorwards with time at the same rate as wind stress. This supports the idea that the autumn bloom develops in response to increased wind stress, with a likely mechanisms for the bloom being mixing to the surface of the deep chlorophyll maximum, and/or increased production due to entrainment of nutrients. The spring bloom progresses polewards with time. It starts after the mixed layer reaches its deepest, and its timing appears to be linked to the reduction in wind stress in spring. Under the assumption that all tracers are well mixed to the seasonal thermocline in autumn and winter, we conclude that vertically-integrated chlorophyll increases at all latitudes in Subtropical Water during autumn and winter. Unfortunately carbon to chlorophyll ratios are not known well enough to determine whether the same is true for vertically-integrated carbon biomass. Individual spring blooms show significant spatial structure and are different from year to year. This leads to low spatial coherence for the temporal variability in surface chlorophyll.

Beschreibung: [1]  Interaction of warm, Atlantic-origin water (AW) and colder, polar origin water (PW) advecting southward in the East Greenland Current (EGC) influences the heat content of water entering Greenland's outlet glacial fjords. Here we use depth and temperature data derived from deep-diving seals to map out water mass variability across the continental shelf and to augment existing bathymetric products. We compare depths derived from the seal dives with the IBCAO Version 3 bathymetric database over the shelf and find differences up to 300 m near several large submarine canyons. In the vertical temperature structure, we find two dominant modes: a cold mode, with the typical AW/PW layering observed in the EGC, and a warm mode, where AW is present throughout the water column. The prevalence of these modes varies seasonally and spatially across the continental shelf, implying distinct AW pathways. In addition, we find that satellite sea surface temperatures (SST) correlate significantly with temperatures in the upper 50 m ( R  = 0.54), but this correlation decreases with depth ( R  = 0.22 at 200 m), and becomes insignificant below 250 m. Thus, care must be taken in using SST as a proxy for heat content, as AW mainly resides in these deeper layers. Sample Unit Level Copyright

Beschreibung: [1]  The Seychelles Dome refers to the shallow climatological thermocline in the southwestern Indian Ocean, where ocean wave dynamics efficiently affect sea surface temperature, allowing sea surface temperature anomalies to be predicted up to 1–2 years in advance. Accurate reproduction of the dome by ocean-atmosphere coupled general circulation models (CGCMs) is essential for successful seasonal predictions in the Indian Ocean. This study examines the Seychelles Dome as simulated by 35 CGCMs, including models used in phase five of the Coupled Model Intercomparison Project (CMIP5). Among the 35 CGCMs, 14 models erroneously produce an upwelling dome in the eastern half of the basin whereas the observed Seychelles Dome is located in the southwestern tropical Indian Ocean. The annual mean Ekman pumping velocity in these models is found to be almost zero in the southern off-equatorial region. This result is inconsistent with observations, in which Ekman upwelling acts as the main cause of the Seychelles Dome. In the models reproducing an eastward-displaced dome, easterly biases are prominent along the equator in boreal summer and fall, which result in shallow thermocline biases along the Java and Sumatra coasts via Kelvin wave dynamics and a spurious upwelling dome in the region. Compared to the CMIP3 models, the CMIP5 models are even worse in simulating the dome longitudes.

Beschreibung: [1]  Tidal junctions play a crucial role in the transport of water, salt and sediment through a delta distributary network. Water, salt and sediment are exchanged at tidal junctions, thereby influencing the transports in the connecting branches and the overall dynamics of the system. This paper presents observations of water, salt and sediment transports in three channels that connect at a stratified tidal junction. Flow variation in one channel was found to lag behind flow variation in a connected channel by more than one hour, which is largely attributed to channel length differences from the junction to the sea. The water columns in the three channels were periodically stratified during spring tide, whereas the salinity structure represented a salt wedge during neap tide. Salinity differences between the three channels were substantial. The channels contain water bodies of different salinity and act largely independently. Flow velocities in the upper and lower layers differed substantially. Flow in the lower layer generally was in the direction of acceleration produced by the baroclinic pressure gradient. Interestingly, baroclinic pressure gradients were sometimes directed landward, indicating the presence of saltier water at the land side of the estuary. In sharp channel bends close to the junction, secondary flow was strongest at the highest axial flow velocity during spring tide. In one channel bend these circulations steered the suspended sediment towards the innerbend, which affected the suspended sediment division.

Beschreibung: [1]  Using a two-way nested-grid ocean general circulation model driven by a repeat annual cycle forcing, the Kuroshio path variation is examined by varying the strength of wind forcing. For climatological or 10 % weaker wind forcing, both large-meander (LM) and non-large-meander (NLM) paths alternately appear, with each type of path continuing for a few years to a decade. This time scale and path transition processes are generally consistent with observations. For the LM path, the main balance in the depth-integrated vorticity equation for the upper ocean is shown to be between the beta, advection, and stretching terms. The stretching term is comparable to and has the same sign as the beta term at the western side of the meandering part, indicating that the stretching term has the effects of shortening the wavelength and stabilizing the LM path. The stretching term is mainly determined by downwelling caused by crossing of the upper and deep flows. Contrary to the above two cases, only the NLM path appears for 10 % stronger wind forcing. It is suggested that the strength of climatological wind forcing is near the upper limit that allows the LM path to occur. In this study, the control experiment is carried out by choosing parameterizations and parameter values to reproduce the Kuroshio path variation as realistic as possible. Therefore, the results from the two sensitivity experiments are expected to represent a realistic response of the Kuroshio path variation.

Beschreibung: [2]  The Greenland Ice Sheet releases large amounts of freshwater into the fjords around Greenland and many fjords are in direct contact with the ice sheet through tidewater outlet glaciers. Here we present the first seasonal hydrographic observations from the inner part of a sub-Arctic fjord, relatively close to and within 4 to 50 km of a fast-flowing tidewater outlet glacier. This region is characterized by a dense glacial and sea ice cover. Freshwater from runoff, subglacial freshwater (SgFW) discharge, glacial and sea ice melt are observed above 50-90 m depth. During summer SgFW and subsurface glacial melt mixed with ambient water are observed as a layered structure in the temperature profiles below the low-saline summer surface layer (〈7 m). During winter the upper water column is characterized by step-wise halo- and thermoclines formed by mixing between deeper layers and the surface layer influenced by ice melt. The warm ( T  〉 1 °C) intermediate water mass is a significant subsurface heat source for ice melt. We analyze the temperature and salinity profiles observed in late summer by a thermodynamic mixing model and determine the total freshwater content in the layer below the summer surface layer to be between 5 and 11%. The total freshwater contribution in this layer from melted glacial ice was estimated to be 1-2 %, while the corresponding SgFW was estimated to be 3-10%. The winter measurements in the subsurface halocline layer showed a total freshwater content of about 1% and with no significant contribution from SgFW.

Beschreibung: [1]  Through its influence on the structure of pelagic ecosystems, phytoplankton size distribution (pico-, nano-, and micro-plankton) is believed to play a key role in “the biological pump”. In this paper, an algorithm is proposed to estimate phytoplankton size fractions (PSF) for micro-, nano-, and pico-plankton ( f m , f n , and f p , respectively) from the spectral features of remote sensing data. From remote sensing reflectance spectrum ( Rrs ( λ )), the algorithm constructs four types of spectral features: a normalized Rrs ( λ ), band ratios, continuum removed spectra and spectral curvatures. Using support vector machine recursive feature elimination, the algorithm ranks the constructed spectral features and Rrs ( λ ) according to their sensitivities to PSF which is then regressed against the sensitive spectral features through support vector machine regression.The algorithm is validated with: (1) simulated Rrs ( λ ) and PSF; and (2) Rrs ( λ ) obtained by Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and PSF determined from High-Performance Liquid Chromatography (HPLC) pigments. The validation results show the overall effectiveness of the algorithm in estimating PSF, with R 2 of: (1) 0.938 ( f m ) for the simulated SeaWiFS dataset; and (2) 0.617 ( f m ), 0.475 ( f n ) and 0.587 ( f p ) for the SeaWiFS satellite dataset. The validation results also indicate that continuum removed spectra and spectral curvatures are the dominant spectral features sensitive to PSF with their wavelengths mainly centered on the pigment-absorption domain. Global spatial distributions of f m , f n and f p were mapped with monthly SeaWiFS images. Overall, their biogeographical distributions are consistent with our current understanding that pico-plankton account for a large proportion of total phytoplankton biomass in oligotrophic regions, nano-plankton in transitional areas and micro-plankton in high-productivity regions.

Beschreibung: [1]  A three-dimensional mixture theory model for flow and sediment transport in the seafloor boundary layer, SedMix3D, was used to simulate the flow over and the resulting sediment entrainment and evolution of rippled beds. SedMix3D treats the fluid-sediment mixture as a continuum of varying density and viscosity with the concentration of sediment and velocity of the mixture simulated by the Navier Stokes equations coupled with a sediment flux equation for the mixture. Model validation was performed by comparing simulated time-dependent quantities and bulk statistics with measurements obtained in the laboratory under scaled forcing conditions. Two-dimensional planes extracted from a three-dimensional simulation were compared to observations made using planar Particle Image Velocimetry (PIV) in a laboratory flume. The simulated results of time-averaged velocities and time-dependent quantities of vorticity and swirling strength were in good agreement with the observations. The model was used to analyze the three-dimensionality of vortex formation and ejection produced by oscillatory flow over vortex ripples, a process that cannot be observed in the laboratory with planar PIV measurements. The three-dimensional simulated results showed that the swirling strength varied significantly in the cross-flow direction, indicating that the vortices formed and dissipated non-uniformly due torandom fluctuations. Subsequently, an order of magnitude difference in offshore sediment flux was observed using two different methods to calculate sediment fluxes (spatially-averaging and at a point). The results suggest that while a two-dimensionalplane may be sufficient to examine the general hydrodynamics over ripples, three-dimensional analysis is necessary for a complete understanding of sediment transport.

Beschreibung: [1]  The Santa Marta Grande Cape (CSM) area, a relatively steep and narrow continental shelf off southeast Brazil, is under the influence of the Brazil Current and intense seasonal variations of the wind field that affect the composition of water masses and their circulation. Hydrographic data, satellite derived wind stress and numerical simulations are used to characterize the seasonal and shorter period variability of the oceanographic conditions off CSM, in the area between 27 o S and 30 o S. The study is focused on the upwelling episodes and the dynamical mechanisms associated with these events. Field data and numerical results indicate significant seasonal changes in the oceanographic structure, the water mass composition and the dynamical balance. During fall and winter, when southwesterly winds prevail and the frequency of meteorological frontal systems increases the region is characterized by waters of continental origin, the Plata Plume Water (PPW). During spring and summer deeper waters reach the surface layer due to the action of northeasterly winds. Coastal upwelling events are revealed by surface temperature minima observed south of CSM. Synoptic data show that these events occur in pulses associated with changes in the wind direction. The analysis of the numerical results show that this particular location of the upwelling band results from the synergy between shelf-break upwelling and the regional shelf circulation induced by the northeasterly winds both of which are modulated by changes in the coastline orientation and shelf width.

Beschreibung: [1]  We evaluate a three-dimensional, nested-grid near shore model of Lake Michigan for its ability to describe key aspects of hydrodynamics and solute transport using data from a field study conducted in summer 2008. Velocity comparisons with observations from five bottom-mounted ADCPs at different depths in the coastal boundary layer (CBL) show that the numerical model was able to simulate currents and flow reversals accurately within the inertial boundary layer, however model accuracy reduced close to the shoreline. Power spectra of observed and simulated velocity time series at different locations showed that the hydrodynamic model was able to describe the energy contained in the inertial scales, but over-predicted turbulent dissipation rates. As a result, model-predicted values of energy in the smaller, dissipation scales were lower compared to values based on observations in the CBL. Differences between energy contained in the observed and simulated velocity spectra increased as the shoreline is approached. Observations showed that vertical variations in the alongshore and cross-shore velocities were dominated by inertial waves, therefore inaccuracies in representing energy dissipation rates and processes not explicitly described in the hydrodynamic model (e.g., anisotropy, waves, and wave-current interactions) could potentially contribute to errors in describing transport in the CBL. Measurements from a continuous dye release experiment from a riverine outfall in southern Lake Michigan were used to estimate a mean dispersion coefficient of 5.6 m 2 /s. Improved representations of physical processes (such as turbulence, internal waves and wave-current interactions) can be expected to provide better descriptions of solute transport in the CBL.

Beschreibung: [1]  In this contribution a detailed investigation of surface thermal patterns on the water surface is presented, with wind speeds ranging from 1ms − 1 to 7ms − 1 and various surface conditions. Distinct structures can be observed on the surface - small scale short lived structures termed fish scales and larger scale cold streaks that are consistent with the footprints of Langmuir Circulations. The structure of the surface heat pattern depends strongly on wind induced stress. Consistent behavior regarding the spacing of cold streaks can be observed in a range of laboratory facilities when expressed as a function of water sided friction velocity, u ∗ . This behavior systematically decreased until a point of saturation at u ∗  = 0.7 cm/s. We present a new image processing based approach to the analysis of the spacing of cold streaks based on a machine learning approach to classify the thermal foot prints of near surface turbulence. Comparison is made with studies of Langmuir circulation and the following key points are found. Results suggest a saturation in the tangential stress, anticipating that similar behavior will be observed in the open ocean. A relation to Langmuir numbers shows that thermal footprints in infrared images are consistent with Langmuir circulations and depend strongly on wind wave conditions.

Beschreibung: [1]  The hydrodynamic characteristics of mesoscale eddies of the southern Gulf of California are described from observations made in August, 2004. Vertical profiles to ~1000 m were made with a CTD with dissolved oxygen and fluorescence sensors, and a Lowered Acoustic Doppler Profiler. The four sampled eddies were aligned along the Gulf axis and had alternating sense of rotation. The Okubo-Weiss parameter was used to calculate the eddy core radii, and it also revealed that the current field of almost all the sampled area was eddy-dominated. The mean radii of the eddy cores varied between 32 and 36 km. Maximum surface tangential velocities were 0.4-0.5 m/s. The anticyclonic eddies were 500-700 m deep, while the cyclones were 450-500 m deep. The depression of the isolines associated with the anticyclonic eddies was ~100 m at depths between 200 m and 400 m; however, the thermocline/pycnocline, which spanned from ~20 to 75 m, was domed by ~10 m. The cyclonic eddies lifted the isolines by ~70 m at depths above 500 m, but the isolines were flat above ~60 m. This suggests that different dynamics operate in the layer above the strong and shallow pycnocline. In particular, the wind stress curl can affect circulation and pycnocline topography, and therefore the patterns that appear in chlorophyll satellite images. In our case, the eddies were detectable in those images because of the chlorophyll-enhanced streamers along their edges, and, for anticyclones, probably because the chlorophyll maximum found at 40-50 m depth was domed following the pycnocline.

Beschreibung: [1]  Every summer a large area (15,000 km 2 on average) over the Texas-Louisiana shelf in the northern Gulf of Mexico turns hypoxic due to decay of organic matter that is primarily derived from nutrient inputs from the Mississippi/Atchafalaya River System. Interannual variability in the size of the hypoxic zone is large. The 2008 Action Plan put forth by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, an alliance of multiple state and federal agencies and tribes, calls for a reduction of the size of the hypoxic zone through nutrient management in the watershed. Comprehensive models help build mechanistic understanding of the processes underlying hypoxia formation and variability and are thus indispensable tools for devising efficient nutrient reduction strategies and for building reasonable expectations as to what responses can be expected for a given nutrient reduction. Here we present such a model, evaluate its hypoxia simulations against monitoring observations and assess the sensitivity of the hypoxia simulations to model resolution, variations in sediment oxygen consumption and choice of physical horizontal boundary conditions. We find that hypoxia simulations on the shelf are very sensitive to the parameterization of sediment oxygen consumption, a result of the fact that hypoxic conditions are restricted to a relatively thin layer above the bottom over most of the shelf. We show that the strength of vertical stratification is an important predictor of dissolved oxygen concentration in bottom waters and that modification of physical horizontal boundary conditions can have a large effect on hypoxia simulations because it can affect stratification strength.

Beschreibung: [1]  Satellite remote sensing offers one of the best spatial and temporal observational approaches. However, well-validated satellite imagery has remained elusive for Lake Superior. Lake Superior's optical properties are highly influenced by colored dissolved organic matter (CDOM), which has hindered the retrieval of chlorophyll concentration through band-ratio algorithms. This study evaluatedseven existing inversion algorithms. The top performing inversion algorithm was tuned to a Lake Superior optical dataset and applied to satellite imagery. The retrieval of chlorophyll concentration via inversion algorithms was not possible due to errors in derived CDOM absorption being greater than phytoplanktonabsorption values andthe very small contribution of phytoplankton absorption to the overall absorption budget. However, the retrieval of absorption due to CDOM from satellite imagery was encouraging. To ensure the best satellite remotely sensed reflectance estimates were used in the retrieval of absorption due to CDOM, several atmospheric correction schemes were evaluated. The absorption due to CDOM was greatest in the western arm of Lake Superior and near river mouths and decreased with distance offshore. The absorption due to CDOM had a bimodal distribution over the annual cycle with the greatest peak in fall and a smaller peak in spring.

Beschreibung: [1]  This study presents aspects of the spatial and temporal variability of abyssal water masses in the Ionian Sea, as derived from recent temperature, salinity, dissolved oxygen and velocity observations and from comparisons between these and former observations. Previous studies showed how in the Southern Adriatic Sea the Adriatic Deep Water (AdDW) became fresher (ΔS ≈ −0.08) and colder (ΔT ≈ −0.1°C) after experiencing warming and salinification between 2003 and 2007. Our data, collected from October 2009 to July 2010 from two bottom moorings, one within the Strait of Otranto and the other in the northern Ionian, confirm this tendency: a bottom vein of southward-flowing AdDW, whose temperature and salinity continuously decreased during the observation time, was detected there. Typically, the vein travel time between the two stations ranged between 45 and 50 days. This gave us a temporal estimate for AdDW anomaly propagation towards the Ionian abyss from their Adriatic generation region. The density excess of the observed vein was always enough to enable its existence as a bottom-arrested current. This evidence confirms that, at that time (2009 and 2010), the Adriatic Sea was greatly contributing to the formation of Eastern Mediterranean Deep Water (EMDW), the bottom water of the Eastern Mediterranean. Hence, based on these results and on the evidence that, from 2003 to 2009, abyssal Ionian waters became saltier and warmer under the time-lagged influence of AdDW, possible future changes in the EMDW characteristics, as a response to Adriatic variability, are discussed.

Beschreibung: ABSTRACT [1]  A three-dimensional circulation model with a relatively simple dissolved oxygen model is used to examine the role that physical forcing has on controlling hypoxia and anoxia in Chesapeake Bay. The model assumes that the biological utilization of dissolved oxygen is constant in both time and space, isolating the role that physical forces play in modulating oxygen dynamics. Despite the simplicity of the model it demonstrates skill in reproducing the observed variability of dissolved oxygen in the Bay, highlighting the important role that variations in physical forcing have on the seasonal cycle of hypoxia. Model runs demonstrate significant changes in the annual integrated hypoxic volume as a function of river discharge, water temperature and wind speed and direction. Variations in wind speed and direction had the greatest impact on the observed seasonal cycle of hypoxia and large impacts on the annually integrated hypoxic volume. The seasonal cycle of hypoxia was relatively insensitive to synoptic variability in river discharge, but integrated hypoxic volumes were sensitive to the overall magnitude of river discharge at annual time scales. Increases in river discharge were shown to increase hypoxic volumes, independent from the associated biological response to higher nutrient delivery. However, increases in hypoxic volume were limited at very high river discharge because increased advective fluxes limited the overall length of the hypoxic region. Changes in water temperature, and its control on dissolved oxygen saturation, were important to both the seasonal cycle of hypoxia, as well as the overall magnitude of hypoxia in a given year.

Beschreibung: [1]  This paper examines the net transport through a multi-inlet bay under a combined force of strong wind and tide, with observations and a model experiment. The observations were made in central Georgia in the Sapelo and Altamaha Sounds between Sept. 13 and 17, 2000. Wind was weak in the beginning of the survey. An air pressure trough (as a weak cold front) passed the area on Sept 16, when the wind changed to from the northeast and increased in magnitude. This front was associated with a mid-latitude cyclone in the New England area. This weather event with an episode of strong northeasterly winds prompted a numerical model experiment on an idealized three-inlet bay, with a set of nonlinear 2-D shallow water equations on an f-plane, which provides some insight to the wind-driven circulation under the presence of tidal forcing. It is found that tidally-induced currents are small compared to wind induced flows. When the wind direction is not perpendicular to the alignment of the three inlets, the net outward flow tends to occur at the inlet further away in the downwind direction. This is associated with a net inward transport in the inlet opposite of the downwind direction. As a result, the middle inlet has the minimum of the net flow. When the wind is perpendicular to the barrier islands, and if the three inlets have different maximum depth values, the deeper inlet tends to have a net flow against the wind, while the shallower inlet tends to have a net flow in the direction of wind. Offshore (onshore) currents may develop outside of the inlet with outward (inward) flow, as an effect of fluxes through the inlets on the coastal ocean.

Beschreibung: [1]  Velocities of surface drifters are analyzed to study tidal currents throughout the Adriatic Sea. Spectral and harmonic analyses indicate that the M2, S2 and K1 constituents dominate. Maps of tidal characteristics show that M2 and S2 are rectilinear currents (reversing tides) aligned with the main axis of the Adriatic basin with maximum amplitude (~7 cm/s for M2 and ~4 cm/s for S2) in the northern area off the Istrian Peninsula. Near the northern coast, semi-diurnal tidal currents decrease in amplitude and rotate in the counterclockwise sense. Near the Po River delta, M2 (S2) motions rotate in the counterclockwise (clockwise) sense. S2 rotation is also counterclockwise near the northeastern coast. M2 phases increase from about 130º on the eastern Croatian coast to 190º on the western Italian side. S2 phases range in 150º-200º. In the middle and southern Adriatic, the semi-diurnal tides are small (~1 cm/s). The diurnal tidal currents (K1) are strong across the basin at the levels of Monte Conero and the Gargano Peninsula with speed larger than 5 cm/s and mainly clockwise rotation, and also in coastal areas (e.g., on the Albanian shelf and close to the Otranto Channel). Phases increase from the east to the west coasts (by as much as 150º). These new results compare satisfactorily with previous observations and numerical simulations, although tidal amplitudes are under-estimated with respect to mooring measurements. They extend for the first time the description of the Adriatic tidal currents to the entire basin based on direct velocity observations.

Beschreibung: [1]  This study considered cross-frontal exchange as a possible mechanism for the observed along-front freshening and cooling between the 27.0 and 27.3 kg m -3 isopycnals north of the Subantarctic Front (SAF) in the southeast Pacific Ocean. This isopycnal range, which includes the densest Subantarctic Mode Water (SAMW) formed in this region, is mostly below the mixed layer, and so experiences little direct air-sea forcing. Data from two cruises in the southeast Pacific were examined for evidence of cross-frontal exchange; numerous eddies and intrusions containing Polar Frontal Zone (PFZ) water were observed north of the SAF, as well as a fresh surface layer during the summer cruise that was likely due to Ekman transport. These features penetrated north of the SAF, even though the potential vorticity structure of the SAF should have acted as a barrier to exchange. An optimum multiparameter (OMP) analysis incorporating a range of observed properties was used to estimate the cumulative cross-frontal exchange. The OMP analysis revealed an along-front increase in PFZ water fractional content in the region north of the SAF between the 27.1 and 27.3 kg m -3 isopycnals; the increase was approximately 0.13 for every 15° oflongitude. Between the 27.0 and 27.1 kg m -3 isopycnals, the increase was approximately 0.15 for every 15° of longitude. A simple bulk calculation revealed that this magnitude of cross-frontal exchange could have caused the downstream evolution of SAMW temperature and salinity properties observed by Argo profiling floats.

Beschreibung: [1]  Estimating the mixed-layer heat budget is a key issue for understanding the cold tongue development in the eastern equatorial Atlantic. A high-resolution ocean regional model is used to diagnose the mixed-layer heat budget online during the EGEE-3 experiment from May to August 2006. The heat budget shows the major role of the horizontal advection and turbulent mixing in the mixed-layer temperature balance in the cold tongue. The surface net heat flux and entrainment processes play a minor role. The equatorial cooling is mainly induced by low-frequency advection, which is balanced by high-frequency zonal and meridional advections. The high-frequency advections are organized in patterns along the northern edge of the cold tongue, where they are associated with strong sea surface temperature gradients and well-developed tropical instability waves in the western Atlantic. Special attention is paid to the wind energy flux, which controls horizontal advection and turbulent mixing. We suggest that the wind energy flux drives the vertical velocity, which in turn adjusts the mixed-layer depth, its stratification, and the vertical shear of the horizontal current. Although vertical advection is not essential in providing cold water in the Atlantic cold tongue, it is shown that the vertical velocity plays a central role in preconditioning the mixed layer and maximizes the turbulent mixing.

Beschreibung: . [1]  A hydrographic survey offshore Ningaloo reef, North-West Australia, in austral autumn 2010 revealed relatively stable subsurface water masses in the region, despite the influence of inter-annual variability of the Leeuwin Current (LC). The surface water mass seems slightly more variable at seasonal time-scale probably due to various contributions of the geographically distinct source-waters of the LC. A subsurface nitrate maximum (~ 110-230 m), a prominent feature of the Ningaloo area during autumn, was consistently observed within different water masses. Tightly coupled variations of subsurface nitrate and oxygen at small vertical scale suggest it is due to local in-situ remineralisation of organic matter likely to accumulate along sharp physical interfaces and possibly favoured by injections of oxygenated subsurface waters. Offshore, enhanced levels of surface chlorophyll- a within the downwelling favourable LC are associated with deeper mixed layer depth, eroding the shallow source of nutrients. Close to the continental shelf, these nutrients are observed to be efficiently uplifted within the core of quasi-persistent topographically-trapped submesoscale cyclonic eddies dominated by non-linear effects. A wind-driven coastal upwelling event occurred in autumn and was characterized by a relatively deep source (~100-150 m), coinciding with the subsurface maximum of nutrients, thus promoting coastal productivity locally and further north by alongshore advection within the Ningaloo current.

Beschreibung: [1]  Partitioning of the total non-water absorption coefficient of seawater, a nw ( λ ) (i.e., the light absorption coefficient after subtraction of pure water contribution), into phytoplankton, a ph ( λ ), and non-phytoplankton, a dg ( λ ), components is important in the areas of ocean optics, biology, and biogeochemistry. We propose a partitioning model based on stacked-constraints approach, which requires input of a nw ( λ ) at a minimum of six specific light wavelengths. Compared with existing models, our approach requires much less restrictive assumptions about the spectral slope of a dg ( λ ) and the spectral shape of a ph ( λ ). Our model is based on several inequality constraints determined from an extensive, quality-verified set of field data covering oceanic and coastal waters from low to high-latitudes. With these constraints, the model first derives a wide range of speculative solutions for a dg ( λ ) and a ph ( λ ) and then identifies feasible solutions. Final model outputs include the optimal solution and a range of feasible solutions for a dg ( λ ) and a ph ( λ ). The optimal solutions agree well with measurements. For example, the median ratio of the model-derived optimal solutions to measured a dg ( λ ) and a ph ( λ ) at 443 nm is very close to 1, i.e., 1.004 and 0.988, respectively. The mean absolute percent difference between the optimal solutions and measured values of a dg (443) and a ph (443) is 6.5% and 12%, respectively. The range of feasible solutions encompasses the measured a dg ( λ ) and a ph ( λ ) with a probability 〉 90% at most wavelengths. Our results support the prospect for the applications of the partitioning model using the input data of a nw ( λ ) collected from various oceanographic and remote-sensing platforms.

Beschreibung: [1]  Tidal flow through the Luzon Strait produces large internal waves that propagate westward into the South China Sea and eastward into the Pacific. Underwater gliders gathered sustained observations of internal waves during 7 overlapping missions from April 2007 through July 2008. A particular focus is high frequency internal waves, where the operational definition of high involves periods shorter than a glider profile taking 3–6 hours. Internal wave vertical velocity is estimated from measurements of pressure and glider orientation through two methods: (1) use of a model of glider flight balancing buoyancy and drag along the glider path, and (2) high-pass filtering of the observed glider vertical velocity. By combining high frequency vertical velocities from glider flight with low frequency estimates from isopycnal depth variations between dives, a spectrum covering 5 decades of frequency is constructed. A map of the standard deviation of vertical velocity over the survey area shows decay from the Luzon Strait into the Pacific. The growth of high frequency vertical velocity with propagation into the South China Sea is observed through two two-week time series stations. The largest observed vertical velocities are greater than 0.2 m s -1 , and are associated with displacements approaching 200 m. The high frequency waves are observed at regular intervals of one day as they ride on diurnal tidal internal waves generated in the Strait.

Beschreibung: [1]  We investigate the responses of the Southern Hemisphere subpolar gyres to projected climate changes over the 21st century by CMIP3 and CMIP5 models. Under increased greenhouse gas forcing, the Southern Hemisphere westerly winds consistently become intensified, resulting in increased cyclonic wind forcing in the subpolar region in these models. Under such wind forcing changes, it is a robust feature that there are consistent increases in the westward flow close to the coast of Antarctica, with strong implications to the mass balance of the Antarctic ice shelves and ice sheets. However, there are large discrepancies in the responses of the gyre axes and overall gyre strengths. Some models show equatorward expansions of the southern gyre limbs, resulting in consistent and large gyre strength increases, while some other models show poleward contractions of the gyres, and generally small and less consistent gyre strength changes. These uncertainties are primarily a result of the uncertain simulations of eddy-driven circulations in the Antarctic Circumpolar Current. The associated buoyancy forcing changes play a secondary role in driving these oceanic responses. [2]  This study reveals that there are large uncertainties in the projections of the subpolar circulation in the current generation of coupled climate models, though CMIP5 models have considerably smaller inter-model spreads in the present-day and projected gyre strengths. To predict the subpolar circulation changes, future improved modelling studies need to particularly reduce the uncertainties in the projections of the westerly jet and to reduce the uncertainties in the eddy-driven circulation responses to wind forcing changes.