ALBERT

Description: 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°.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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 ).

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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).

Description: : 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: [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.

Description: 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.

Description: [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.

Description: [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.

Description: 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.

Description: [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%.

Description: [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.

Description: [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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: Spur and groove (SAG) formations are found on the forereefs of many coral reefs worldwide. Modeling results have shown that SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter-rotating circulation cells, but these have never been observed in the field. We present results from two separate field studies of SAG formations on Palmyra Atoll which show their effect on waves to be small, but reveal a persistent order 1 cm/s depth-averaged Lagrangian offshore flow over the spur and onshore flow over the grooves. This circulation was stronger for larger, directly-incident waves and low alongshore flow conditions, consistent with predictions from modeling. Favorable forcing conditions must be maintained on the order of one hour to accelerate and develop the SAG circulation cells. The primary cross- and alongshore depth-averaged momentum balances were between the pressure gradient, radiation stress gradient and nonlinear convective terms, and the bottom drag was similar to values found on other reefs. The vertical structure of these circulation cells was previously unknown and the results show a complex horizontal offshore Lagrangian flow over the spurs near the surface driven by alongshore variability in radiation stress gradients. Vertical flow was downward over the spur and upward over the groove, likely driven by alongshore differences in bottom stress and not by vortex forcing. This article is protected by copyright. All rights reserved.

Description: Shipboard current measurements in the equatorial Indian Ocean in October and November of 2011 revealed oscillations in the meridional velocity with amplitude ~ 0.10m/s. These were clearest in a layer extending from ~300 to 600 m depth and had periods near 3 weeks. Phase propagation was upward. Measurements from two sequential time series at the equator, four meridional transects and one zonal transect are used to identify the oscillation as a Yanai wave packet and to establish its dominant frequency and vertical wavelength. The Doppler shift is accounted for, so that measured wave properties are translated into the reference frame of the mean zonal flow. We take advantage of the fact that, in the depth range where the wave signal was clearest, the time-averaged current and buoyancy frequency were nearly uniform with depth, allowing application of the classical theoretical representation of vertically propagating plane waves. Using the theory, we estimate wave properties that are not directly measured, such as the group velocity and the zonal wavelength and phase speed. The theory predicts a vertical energy flux that is comparable to that carried by midlatitude near-inertial waves. We also quantify the wave-driven meridional heat flux and the Stokes drift. This article is protected by copyright. All rights reserved.

Description: Understanding variability in the chlorophyll-specific absorption of marine phytoplankton, a ph * Chl (λ) , is essential for primary production modelling, calculation of underwater light field characteristics, and development of algorithms for remote sensing of chlorophyll concentrations. Previous field and laboratory studies have demonstrated significant apparent variability in a ph * Chl (λ) for natural samples and algal cultures. However, the potential impact of measurement uncertainties on derived values of a ph * Chl (λ) has received insufficient study. This study presents an analysis of measurement uncertainties for a data set collected in the Ligurian Sea in Spring and assesses the impact on estimates of a ph * Chl (λ) . It is found that a large proportion of apparent variability in this set of a ph * Chl (λ) can be attributed to measurement errors. Application of the same analysis to the global NOMAD data set suggests that a significant fraction of variability in a ph * Chl (λ) may also be due to measurement errors. This article is protected by copyright. All rights reserved.

Description: The South Atlantic is an important pathway for the inter-basin exchanges of heat and freshwater with strong influence on the global meridional overturning stability and variability. Along the 34°S parallel, a quarterly, high resolution XBT transect (AX18) samples the temperature structure in the upper ocean. The AX18 transect has been shown to be a useful component of a meridional overturning monitoring system of the region. However, a feasible, cost-effective design for an XBT-based system has not yet been developed. Here we use a high-resolution ocean assimilation product to simulate an XBT-based observational system across the South Atlantic. The sensitivity of the meridional heat transport, meridional overturning circulation, and geostrophic velocities to key observational and methodological assumptions is studied. Key assumptions taken into account are horizontal and temporal sampling of the transect, salinity and deep temperature inference, as well as the level of reference for geostrophic velocities. With the current sampling strategy, the largest errors in the meridional overturning and heat transport estimations are the reference (barotropic) velocity and the western boundary resolution. We show how altimetry can be used along with hydrography to resolve the barotropic component of the flow. We use the results obtained by the state estimation under observational assumptions to make recommendations for potential improvements in the AX18 transect implementation. This article is protected by copyright. All rights reserved.

Description: Observations of the nearbed velocity field over a rippled sediment bed under asymmetric wave forcing conditions were collected using a submersible particle image velocimetry (PIV) system. To examine the role of bedform-induced dynamics in the total momentum transfer, a double-averaging technique was implemented on the two-dimensional time-dependent velocity field by means of the full momentum equation. This approach allows for direct determination of the bedform-induced stresses, ie. stresses that arise due to the presence of bedforms, which are zero in flat bed conditions. This analysis suggests that bedform-induced stresses are closely related to the presence of coherent motions and may be partitioned from the turbulent stresses. Inferences of stress provided by a bedload transport model suggest that total momentum transfer obtained from the double-averaging technique is capable of reproducing bedform mobilization. Comparisons between the total momentum transfer and stress estimates obtained from local velocity profiles show significant variability across the ripple, and suggest that an array of sensors is necessary to reproduce bedform evolution. The imbalance of momentum obtained by resolving the different terms constituting the near-bed momentum balance (i.e. acceleration deficit, stress gradient, and bedform-induced skin friction) provides an estimate of the bedform-induced pressure that is consistent with flow separation. This analysis reveals three regions in the flow: the free-stream, where all terms are relatively balanced; the near-bed, where momentum imbalance is significant during flow weakening; and below ripple crests, where bedform-induced pressure is the leading order mechanism.

Description: An analysis is presented for the spatial and intensity distributions of North Atlantic extreme atmospheric events crossing the buoyant Amazon-Orinoco freshwater plume. The sea surface cooling amplitude in the wake of an ensemble of storm tracks travelling in that region is estimated from satellite products for the period 1998-2012. For the most intense storms, cooling is systematically reduced by ~50% over the plume area compared to surroundings open ocean waters. Historical salinity and temperature observations from in situ profiles indicate that salt-driven vertical stratification, enhanced oceanic heat content and barrier-layer presence within the plume waters are likely key oceanic factors to explain these results. Satellite SMOS surface salinity data combined with in situ observations are further used to detail the oceanic response to Category 4 hurricane Igor in 2010. Argo and satellite measurements confirm the haline stratification impact on the cooling inhibition as the hurricane crossed the river plume. Over this region, the SSS mapping capability is further tested and demonstrated to monitor the horizontal distribution of the vertical stratification parameter. SMOS SSS data can thus be used to consistently anticipate the cooling inhibition in the wake of TCs travelling over the Amazon-Orinoco plume region.

Description: The mixed layer salinity (MLS) budget of the tropical Pacific is investigated using results from a model of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). The results focusing on the western Pacific freshwater pool indicate that the long-term averaged surface freshwater flux is well balanced by ocean dynamics, in which the subsurface processes account for the major part. The MLS budget shows significant seasonal and interannual variability, as a consequence of interplay among surface freshwater flux, advection, mixing, and vertical entrainment. On seasonal time scale, both the MLS and mixed layer depth are largely controlled by surface freshwater flux. The opposite phase between the subsurface processes and the barrier layer thickness confirms the important influence of the barrier layer on vertical mixing and entrainment from below. On interannual time scale, all the MLS budget terms show significant ENSO signal, which in turn is highly correlated with the salinity front and barrier layer thickness in the equatorial Pacific.

Description: To study the relationship of solar heat input into the Arctic open water and the variations of sea ice extent, improved satellite based estimates of shortwave radiative (SWR) fluxes and most recent observations of ice extent are used. The SWR flux estimates are based on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and from the Advanced Very High Resolution Radiometer (AVHRR) for the period of 1984-2009. Ice extent information at 25-km resolution comes from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data as generated with the NASA Team algorithm developed by the Oceans and Ice Branch, Laboratory for Hydrospheric Processes, NASA Goddard Space Flight Center. The trends of the solar heat input into the ocean and the open water fraction for 1984-2009 are found to be positive: 0.3%/year and 0.8% / year respectively at a 99% confidence level. There is an obvious transition region separating the 26 years into two periods: one with moderate change: 1984-2002; one with an abrupt growth in both solar heat input and open water fraction: 2003-2009. The impact of the observed changes on the reduction of winter ice growth in 2007 is estimated to be about 44 cm, and a delay in fall freeze-up as about 10~36 days.

Description: Numerous studies have shown the primary importance of wind stress curl in coastal upwelling dynamics. The main goal of this new analysis is to describe the QuikSCAT surface wind stress curl at various scales in the Benguela and Canary upwelling systems. The dominant spatial pattern is characterized by cyclonic curl near continental boundaries and anticyclonic curl offshore, in association with equatorward alongshore (upwelling favorable) wind stress. At a smaller scale, we demonstrate the sensitivity of the QuikSCAT wind stress curl to coastal processes related to sea surface temperature (SST) mesoscale fluctuations by presenting a linear relationship between the curl and crosswind SST gradients. Despite the spatial and temporal sensitivity of the underlying thermal coupling coefficient, a local analysis of the fraction of the curl ascribed to SST variability shows that SST is a main driver of the wind stress curl variability and magnitude over the upwelling extension zone (~100 to 300 km from the coast) in both the Canary and Benguela systems. Closer to the shore, the curl patterns derived from QuikSCAT observations are only loosely related to SST-wind interactions. As a working hypothesis, they can also be associated with the coastline geometry and orographic effects that are likely to play an important role in local cooling processes.

Description: To provide a probable explanation on the field observed rapid sedimentation process near river mouths, we investigate the convective sedimentation in stably stratified saltwater using 3D numerical simulations. Guided by the linear stability analysis (Yu et al. 2013, J. Geophys. Res. Oceans, 118, 256-272), this study focuses on the nonlinear interactions of several mechanisms, which lead to various sediment finger patterns, and the effective settling velocity for sediment ranging from clay (single-particle settling velocity V 0 = 0.0036 and 0.0144 mm/s, or particle diameter d = 2 and 4 μ m) to silt ( V 0 =0.36 mm/s, or d =20 μ m). For very fine sediment with V 0 =0.0036 mm/s, the convective instability is dominated by double diffusion, characterized by millimeter-scale fingers. Gravitational settling slightly increases the growth rate; however, it has notable effect on the downward development of vertical mixing shortly after the sediment interface migrates below the salt interface. For sediment with V 0 = 0.0144 mm/s, Rayleigh-Taylor instabilities become dominant before double-diffusive modes grow sufficiently large. Centimeter-scale and highly asymmetric sediment fingers are obtained due to nonlinear interactions between different modes. For sediment with V 0 = 0.36 mm/s, Rayleigh-Taylor mechanism dominates and the resulting centimeter-scale sediment fingers show a plume-like structure. The flow pattern is similar to that without ambient salt stratification. Rapid sedimentation with effective settling velocity on the order of 1 cm/s is likely driven by convective sedimentation for sediment with V 0 greater than 0.1 mm/s at concentration greater than 10 to 20 g/L.

Description: The surface layer of the southeast Pacific Ocean (SEP) requires an input of cold, fresh water to balance heat gain and evaporation from air-sea fluxes. Models typically fail to reproduce the cool sea surface temperatures (SST) of the SEP, limiting our ability to understand the variability of this climatically important region. We estimate the annual heat budget of the SEP for the period 2004 - 2009, using data from the upper 250 m of the Stratus mooring, located at 85°W 20°S, and from Argo floats. The surface buoy measures meteorological conditions and air-sea fluxes; the mooring line is heavily instrumented, measuring temperature, salinity, and velocity at more than 15 depth levels. We use a new method for estimating the advective component of the heat budget that combines Argo profiles and mooring velocity data, allowing us to calculate monthly profiles of heat advection. Averaged over the 6 year study period, we estimate a cooling advective heat flux of -41 ± 29 W m-2, accomplished by a combination of the mean gyre circulation, Ekman transport, and eddies. This compensates for warming fluxes of 32 ± 4 W m-2 due to air-sea fluxes and 7 ± 9 W m-2 due to vertical mixing and Ekman pumping. A salinity budget exhibits a similar balance, with advection of freshwater (-60 psu m) replenishing the freshwater lost through evaporation (47 psu m) and Ekman pumping (14 psu m).

Description: Acoustic-gravity waves are compression-type waves propagating with amplitudes governed by the restoring force of gravity. They are generated, among others, by wind-wave interactions, surface waves interactions, and submarine earthquakes. We show that acoustic-gravity waves contribute to deep ocean currents and circulation; they cause chaotic flow trajectories of individual water parcels, which can be transported by a few centimetres per second.

Description: The Berau Continental Shelf is located close to the Equator in the Indonesian Archipelago, hosting a complex of coral reefs along its oceanic edge. The Berau coral reefs have a very high biodiversity, but the area is under serious risk due to river-derived nutrients and sediments. The region is characterized by weak winds, moderate tides and almost absent Coriolis forcing. Existing knowledge about river plume behaviour in tropical environments is limited. The aim of this paper is to investigate the influence of the subtle physical forcing on the dynamics of the Berau river plume. A three-dimensional model (ECOMSED) was calibrated with observational data. The model was forced by freshwater input from the Berau river distributaries, tides at the open boundaries and measured hourly wind. The model reproduces the freshwater dynamics on the shelf adequately and highlights that the river plume spreads symmetrically for river forcing only. Tides cause vertical mixing and suppress the cross-shelf spreading of the river plume. However, the spreading of the river plume over the shelf is mainly controlled by the weak monsoonal winds, resulting in a seasonal development. During the Southeast Monsoon, the southerly winds push the plume northeastward and cause a stratified water column in the northern part of the continental shelf. Northerly winds during the Northwest Monsoon disperse the plume to the south, promoting a vertically well-mixed water column. The results can be used to predict the possible impact of land-use changes in the steadily developing Berau region on coral reef health. This article is protected by copyright. All rights reserved.

Description: This study investigates the effect of adaptive (or targeted) observation on improving the mid-range (30 days) forecast skill of ocean state of the South China Sea (SCS). A region associated with the South China Sea Western Boundary Current (SCSWBC) is chosen as the “target” of the adaptive observation. The Conditional Nonlinear Optimal Perturbation (CNOP) approach is applied to a 3-dimensional ocean model and its adjoint model for determining the sensitive region. Results show that the initial errors in the sensitive region determined by the CNOP approach have significant impacts on the forecast of ocean state in the target region; thus, reducing these initial errors through adaptive observation can lead to a better 30-day prediction of ocean state in the target region. Our results suggest that implementing adaptive observation is an effective and cost-saving way to improve an ocean model's forecast skill over the SCS. This article is protected by copyright. All rights reserved.

Description: We present a year-round assessment of the hydrographic variability within the East Greenland Coastal Current on the Greenland shelf from five synoptic crossings and four years of moored hydrographic data. From the five synoptic sections the current is observed as a robust, surface intensified flow with a total volume transport of 0.66 ± 0.18 Sv and a freshwater transport of 42 ± 12 mSv. The moorings showed heretofore unobserved variability in the abundance of Polar and Atlantic water masses in the current on synoptic scales. This is exhibited as large vertical displacement of isotherms (often greater than 100 m). Seasonally, the current is hemmed into the coast during the fall by a full depth Atlantic Water layer that has penetrated onto the inner shelf. The Polar Water layer in the current then thickens through the winter and spring seasons increasing the freshwater content in the current; the timing implies that this is probably driven by the seasonally varying export of freshwater from the Arctic and not the local runoff from Greenland. The measured synoptic variability is enhanced during the winter and spring period due to a lower halocline and a concurrent enhancement in the along-coast wind speed. The local winds force much of the high-frequency variability in a manner consistent with downwelling, but variability distinct from downwelling is also visible. This article is protected by copyright. All rights reserved.

Description: The performance of several numerical ocean models is assessed with respect to their simulation of sea surface height (SSH) in the Arctic Ocean, and the main patterns of SSH variability and their causes over the past 40 years (1970-2009) are analyzed. In comparison to observations, all tested models broadly reproduce the mean SSH in the Arctic and reveal a good correlation with both tide gauge data and SSH anomalies derived from satellite observations. Although the models do not represent the positive Arctic SSH trend observed over the last two decades, their interannual-to-decadal SSH variability is in reasonable agreement with available measurements. Focusing on results from one of the models for a detailed analysis it is shown that the decadal-scale SSH variability over shelf areas and deep parts of the Arctic Ocean have pronounced differences that are determined mostly by salinity variations. A further analysis of the three time periods 1987-1992, 1993-2002 and 2003-2009, corresponding to the transition times between cyclonic and anticyclonic regimes of the atmospheric circulation over the Arctic, revealed an unusual increase of SSH in the Amerasian basin during 2003-2009. Results from this model support the recent finding that the increase is caused mainly by changes in freshwater content brought about by the freshwater export through the Canadian Arctic Archiplago and increased Ekman pumping in the Amerasian basin and partly by lateral freshwater transport changes, leading to a re-distribution of low-salinity shelf water. Overall we show that present day models can be used for investigating the reasons for low-frequency SSH variability in the region. This article is protected by copyright. All rights reserved.

Description: In this study, a neural network-based four-band model (NNFM) for the global oceanic and coastal waters has been developed in order to retrieve the total absorption coefficients a ( λ ). The applicability of the quasi-analytical algorithm (QAA) and NNFM models is evaluated by five independent datasets. Based on the comparison of a ( λ ) predicted by these two models with the field measurements taken from the global oceanic and coastal waters, it was found that both the QAA and NNFM models had good performances in deriving a ( λ ), but that the NNFM model works better than the QAA model. The results of the QAA model-derived a ( λ ), especially in highly turbid waters with strong backscattering properties of optical activity, was found to be lower than the field measurements. The QAA and NNFM models-derived a ( λ ) could be obtained from the MODIS data after atmospheric corrections. When compared with the field measurements, the NNFM model decreased by a 0.86 to 24.15% uncertainty (root mean square relative error) of the estimation from the QAA model in deriving a ( λ ) from the Bohai, Yellow, and East China seas. Finally, the NNFM model was applied to map the global climatological seasonal mean a (443) for the time range of July, 2002 to May, 2014. As expected, the a (443) value around the coastal regions was always larger than the open ocean around the equator. Viewed on a global scale, the oceans at a high latitude exhibited higher a (443) values than those at a low latitude. This article is protected by copyright. All rights reserved.

Description: In this study, we investigate the interannual variability of the sea surface temperature (SST) in the South China Sea (SCS) associated with two types of El Niño, namely, the eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño. First, double warm peaks can occur during both types of El Niño events in the SCS. However, the strong warm basin mode can only develop in the EP El Niño, while the warm semi-basin mode exists during the CP El Niño. Associated with an anomalous positive (negative) net surface heat flux in the EP (CP) El Niño, along with a shallower thermocline with weaker (stronger) northeasterly wind anomalies, the SST anomalies become warmer (cooler) in the developing autumn. Over the background of cooling SST in autumn of CP El Niño, therefore, only a weak warming can occur in the subsequent years, which is limited in the western boundary area under the forcing of warm ocean advection. Second, the SST oscillation periods are different in these two types of El Niño. The SST evolution in the EP El Niño is negative-positive with a quasi-biennial oscillation, but that in the CP El Niño is positive-negative-positive-negative with an annual oscillation. It seems that the double cooling in the CP El Niño is phase-locked to the late autumn season. This article is protected by copyright. All rights reserved.

Description: Following the idea that analysis of in-situ information in the salt budget could be used as a surrogate for global “ocean rain gauge”, the annual mean oceanic net freshwater flux (E-P) was estimated from the Argo profiles and the wind stress data on a global scale. The comparison between the independent E-P estimation from Argo and the E-P product sets, including the combination of precipitation from TRMM, GPCP, CMAP and evaporation from OAFlux, GSSTF3 and IFREMER and E-P set from NEWS formed from satellite, generally show similar spatial patterns, particularly on the larg scale. However, there are differences among the different satellite-based E-P estimates and between satellite estimates and independent in-situ estimates. Based on the pattern correlation and the RMSD, the evaporation and precipitation from OAFlux and TRMM agrees best with the E-P estimated from the independent Argo-based estimates.

Description: Observations of wave and sediment processes collected at two locations on the Atchafalaya inner shelf show that wave dissipation in shallow, muddy environments is strongly coupled to bed-sediment reworking by waves. During an energetic wave event (2 m significant wave height in 5 m water depth), acoustic backscatter records suggest that sediment in the surficial bed layer evolves from consolidated mud through liquefaction, fluid mud formation, and hindered settling to gelled, under-consolidated mud. Net swell dissipation increases steadily during the storm from negligible prestorm values, consistent with bed softening, but shows no correlation with detectable fluid mud layers. Remarkably, the maximum dissipation rate occurs poststorm, when no fluid mud layers are present. In the waning stage of the storm, the contribution of different wave-forcing processes to wave dissipation is analyzed using an inverse modeling approach based on a nonlinear three-wave interaction model. Although wave-mud interaction dominates dissipative processes, nonlinear three-wave interactions control the shape of the frequency distribution of the dissipation rate. In the wake of the storm, the viscosity values predicted by the inverse modeling converge toward measured values characteristic for gelled mud in a trend that is consistent with a fluid mud entering dewatering and consolidation stages.

Description: Causes of the coupled model bias in simulating the zonal sea surface temperature (SST) gradient in the equatorial Atlantic are examined in three versions of the same coupled general circulation model (CGCM) differing only in the cumulus convection scheme. One version of the CGCM successfully simulates the mean zonal SST gradient of the equatorial Atlantic, in contrast to the failure of the Coupled Model Intercomparison Project phase 3 models. The present analysis shows that key factors to be successful are high skills in simulating the meridional location of the Intertropical Convergence Zone, the precipitation over northern South America, and the southerly winds along the west coast of Africa associated with the West African monsoon in boreal spring. Model biases in the Pacific contribute to the weaker precipitation over northern South America. Uncoupled experiments with the atmospheric component further confirm the importance of remote influences on the development of the equatorial Atlantic bias.

Description: Oblique nonlinear interactions based on the Kadomtsev-Petviashvili equation (also known as the two-dimensional Korteweg–de Vries equation) are extended to internal solitary waves (ISWs) to explain why the amplitude does not decrease owing to the geometric spreading of the cylindrical wavefronts in the South China Sea (SCS). This resonance theory is used to explain a satellite image exhibiting special features, and it is proposed that wave arcs of different amplitude resonate, providing a mechanism for reinforcing a wave by boosting the amplitude. The present theory suggests the amplitude of the ISW that propagates across the SCS basin depends on the interaction of ISWs originating from different sources; hence, studying the generation of an ISW from a single source location cannot predict the ISW correctly.

Description: On western margins of ocean basins, such as the eastern continental shelf of the United States, rates of biological productivity are higher than in the open ocean, in spite of the mean downwelling circulation. We use a nonhydrostatic, three-dimensional, process study ocean model with idealized shelf-slope geometry, wind forcing, and tracers to explore the interplay between the circulation and the biogeochemistry of the shelf and slope; the pathways that can transport nutrients from the deep ocean and from the sediments to the surface ocean euphotic zone. Cross-shelf exchange between the open and coastal ocean is regulated by a shelf break front that separates light waters on the shelf from denser waters on the slope. The wind direction and strength influence both the position and slope of the isopycnals at the front, which become more vertical in response to northerly winds and flatten in response to southerly winds. When the wind direction oscillates between northerly and southerly, it pumps nutrient and gas-rich bottom boundary layer water up to the sea surface. Nutrients tend to accumulate in the benthic boundary layer during southerly winds and are pumped to the surface during periods of northerly winds. Stratification of the water column in summertime reduces the shelf break pump by dampening the effect of the winds on the movement of the front. When extrapolated over the northeast coast of the United States, the nutrients supplied by the shelf break pump from the open ocean to the coastal ocean are three times the estimated nitrogen delivered to the shelf from estuaries.

Description: Ocean acidification resulting from increases in present and future atmospheric CO2 levels could seriously affect diverse coastal and oceanic ecosystems. In this work, we determine that a significant trend in ocean acidification is superposed on the large seasonal and interannual variabilities of acidity in surface waters off the south coast of Honshu, Japan, based on our repeated observations of partial pressure of CO2 (pCO2), total inorganic carbon (TCO2), and pH. Multiple regression analysis of TCO2 as a function of temperature, salinity, and timing of observations shows that TCO2 increased at a rate of +1.23 ± 0.40 μmol kg−1 yr−1 for the period 1994–2008, while no long-term change has been determined for total alkalinity calculated from TCO2 and pCO2 in seawater. These results indicate that pH and the aragonite saturation state (Ωarag) are decreasing at a rate of −0.020 ± 0.007 decade−1 and −0.12 ± 0.05 decade−1, respectively. If future atmospheric CO2 levels keep increasing as predicted by the Intergovernmental Panel on Climate Change emission scenario A1FI, which postulates intensive fossil fuel use associated with very rapid economic growth, a further reduction of −0.8 to −1.0 in Ωarag is likely in the next 50 years. Such a rapid reduction of Ωarag could have negative impacts on a variety of calcareous organisms.

Description: A barotropic tidal model, with a parameterization term to account for the internal wave drag energy dissipation, is used to examine areas of possible M2 internal tide generation in the Kerguelen Plateau region. Barotropic energy flux and a distribution of wave drag dissipation are computed. The results suggest important conversion of barotropic energy into baroclinic tide generation over the northern Kerguelen Plateau shelf break, consistent with a theoretical criterion based on ocean stratification, tidal forcing frequency, and bathymetric gradients. The sea surface height signatures of time-coherent internal tides are studied using TOPEX/Poseidon and Jason-1 altimeter data, whose ascending tracks cross nearly perpendicular to the eastern and western Kerguelen Plateau shelf break. Oscillations of a few centimeters associated with phase-locked internal tides propagate away from the plateau over distances of several hundred kilometers with a ∼110 km wavelength. When reaching the frontal area of the Antarctic Circumpolar Current, the internal tide cannot be identified because of the aliasing of mesoscale variability into the same alias band as M2. Finally, using altimeter data, we estimate the M2 barotropic tidal power converted through the internal tide generation process. We find consistent values with the barotropic model parameterization estimation, which is also in good agreement with global internal tide model estimates. Combined with modeling, this study has shown that altimetry can be used to estimate internal tide dissipation.

Description: Variability in sea level at the longest periods observable in modern records has recently been found to be well correlated with local atmospheric pressure. At shorter periods, however, longshore winds are known to be one of the primary mechanisms for forcing sea level variability on the eastern margins of the ocean. There is a remarkable ∼80 mm drop in mean sea level on both the eastern North Pacific and North Atlantic coasts between the late 1800s and early 1900s; it is found here to be in agreement with longshore wind forcing from the equator up to the latitude of the observed tide stations. Better-resolved data beginning in 1960 show that the delay near the annual period between wind forcing and sea level is approximately 1 month. The relative high in sea level in the late 1800s on the west coast of Europe appears to have propagated westward across the Atlantic as a long Rossby wave and then to have been seen on the east coast of the United States. Because many features in long-term sea level variability are correlated with wind forcing on time scales from annual to decades, it will be prudent to base conclusions about long-term sea level rise on the longest records available. The results here are based on forcing by longshore winds; the related issue of the extent to which open ocean wind curl may also be responsible is not addressed here, nor the extent to which coastal sea level observations are representative of the open ocean.

Description: Analysis of long time series of current meter data from a mooring at 77°E and the equator during 2003–2007, along with mean sea level anomaly data, throws light on the occurrence of the lower-frequency (24 to 40 day) Yanai waves in the upper water column of the central equatorial Indian Ocean (EIO) during the positive Indian Ocean dipole (IOD) years of 2003, 2004, 2006, and 2007 and its absence during the negative IOD year 2005. This result is in contrast with the earlier studies that observed only the higher-frequency (biweekly period) Yanai wave in this region. We propose a new notion for the generation of the lower-frequency Yanai wave in the upper central EIO owing to the positive IOD phenomenon. The strong meridional current shear created by the northward shifting and strengthening of the westward flowing south equatorial current associated with positive IOD and the eastward flowing southwest monsoon current provides energy for the generation of lower-frequency Yanai waves. Vertical stratification of the water column appears to be responsible for the trapping of the different frequency of Yanai waves, with only the higher-frequency Yanai wave in the region of lower pycnocline. During positive IOD the strongly stratified upper water column responds to the lower-frequency Yanai wave, while the deeper ocean (4000 m) exhibited a longer-period (47 day) oscillation. The expected surface signature of Madden-Julian oscillation seems to be suppressed by strong easterlies during the positive IOD years.

Description: A numerical model of saline density currents across a triple-bend sinuous submerged channel enclosed by vertical sidewalls is developed. The unsteady, non-Boussinesq, turbulent form of the Reynolds Averaged Navier-Stokes equations is employed to study the flow structure in a quasi-steady state. Recursive tests are performed with axial slopes of 0.08°, 0.43°, 1.5°, and 2.5°. For each numerical experiment, the downstream and vertical components of the fluid velocity, density, and turbulent kinetic energy are presented at four distinct locations within the channel cross section. It is observed that a crucial change in the flow pattern at the channel bends is observed as the axial slope is increased. At low values of the axial slope a typical river-like pattern is found. At an inclination of 1.5°a transition starts to occur. When the numerical test is repeated with an axial slope of 2.5°, a clearly visible river-reversed secondary circulation is achieved. The change in the cross-sectional flow pattern appears to be associated with the spatial displacement of the core of the maximum downstream fluid velocity. Therefore, the axial slope in this series of experiments is linked to the velocity structure of the currents, with the height of the velocity maximum decreasing as a function of increasing slope. As such, the axial slope should be regarded also as a surrogate for flows with enhanced density or sediment stratification and higher Froude numbers. The work unifies the apparently paradoxical experimental and numerical results on secondary circulation in submarine channels.

Description: The recent detection of a central Pacific type of El Niño has added a new dimension to the El Niño-Southern Oscillation climatic puzzle. Sea surface salinity (SSS) observations collected during 1977–2008 in the tropical Pacific are used to contrast the three eastern Pacific (EP) (1982–1983, 1991–1992, 1997–1998) and seven central Pacific (CP) (1977–1978, 1986–1988, 1990–1991, 1992–1995, 2002–2003, 2004–2005, 2006–2007) types of El Niño events, as well as the six EP (1985–1986, 1988–1989, 1995–1996, 1999–2001, 2005–2006, 2007–2008) and two CP (1983–1984, 1998–1999) types of La Niña events. The EP El Niño events result in large (∼30° longitude) eastward displacements of the eastern edge of the low-salinity warm pool waters in the equatorial band, a resulting well-marked SSS freshening (∼−1) near the dateline, and a SSS increase (∼+1) below the mean position of the South Pacific Convergence Zone (SPCZ). The CP El Niño events are characterized by smaller (50%) eastward displacements of the eastern edge, a ∼15° longitude westward shift of the equatorial SSS freshening, and a comparatively reduced (∼50%) SSS increase in the SPCZ. A qualitative analysis indicates that changes in zonal currents and precipitation can account for the observed contrasted signature in SSS. Eastward current anomalies appear over most of the equatorial band during EP El Niño events. In contrast, there is a tendency for zonal current convergence slightly west of the dateline during CP El Niño events, consistent with the confinement of the warm/fresh pool in the western central equatorial basin, the related quasi-inexistent northeastward migration of the SPCZ, and associated heavy precipitation regime.

Description: We investigate the coupling between the dynamics in the Gulf of Aqaba (Gulf of Eilat, northern Red Sea) and the exchange flow through the Straits of Tiran in response to seasonally varying surface fluxes and northern Red Sea hydrographic conditions. Because the gulf is a relatively small basin, winter mixing between the surface and intermediate layers occurs over most places in the gulf including in the vicinity of the straits, and leads to a maximal exchange flow in the strait. During the spring, warming in the Red Sea forces an influx of warm Red Sea water into the gulf, the surface layer is refilled, and consequently the exchange flow in the straits changes from a maximal to a submaximal flow regime. As a result, the dense wintertime water formation in the gulf and the exchange flow through the strait are strongly coupled on seasonal time scales. In addition, the hydrographic conditions in the northern Red Sea undergo strong seasonality. These characteristics prevent the direct application of current theories for strait-marginal sea systems, which commonly assume steady conditions in the “open ocean” side of the strait and/or an annual mean surface flux over the marginal sea. We explain why the exchange of volume and heat between the Gulf of Aqaba (Gulf of Eilat) and the northern Red Sea is larger during spring-summer despite the net surface buoyancy input into the gulf and why it diminishes during fall-winter despite the large buoyancy loss to the atmosphere. The applicability of the results to other systems is discussed.

Description: Ocean tides under the large Weddell Sea ice shelves are among the least well observed on Earth. Here we present new, spatially extensive observations of the vertical tidal motion of the Filchner-Ronne and Larsen C ice shelves using Global Positioning System (GPS) data spanning a few weeks to years. We pay particular attention to the major tidal constituents (M2, S2, O1, K1) as well as important GRACE aliasing periods (K2 and S1). We compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves. The root-sum-square errors are 7–8 cm (CATS2008a and TPXO7.2) and 11–12 cm (GOT4.7 and FES2004) with the energetic M2 (RMSE = 4–8 cm) and S2 (4–5 cm) generally dominating these statistics. The FES2004 K1 is particularly inaccurate near the Larsen C Ice Shelf, with errors approaching 20 cm, meaning that GRACE Release 4 estimates of mass change in the northern Antarctic Peninsula will be biased. We find tidal energy at 3, 4, 5, 6 and, weakly, at 7 cycles per day at all of our sites. The largest amplitudes within these bands are at M4, MO3 and SP3 and approach 30 mm, although significant spatial variations exist. We show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones. Comparing model estimates with our M4, MS4 and MN4 values shows that models do not accurately represent these terms.

Description: Optical remote sensing is used to measure flow patterns in the swash zone. Timestack images are analyzed to measure the asymmetry and the relative duration of the inflow into the swash zone. This varies significantly between individual swashes, contrary to the classical analytical swash model for runup induced by bores, which predicts a similar flow pattern for all events. For swash forced by breaking bores, the gradient of the x-t locus of flow reversal varies over a wide range and flow reversal can occur simultaneously across the whole swash zone. This variation of the gradient of the locus of flow reversal in x-t space can be parameterized in terms of a single free variable in recent solutions to the nonlinear shallow water equations, which fully defines the swash boundary inflow condition. Consistent with the theory, the horizontal runup, the swash period, and the swash similarity parameter were observed to be independent of the swash inflow conditions but the flow asymmetry is not. Only a weak correlation was observed between the swash boundary condition and the Iribarren number and beach slope. Conversely, the analysis suggests that the degree of swash-swash interaction does influence the swash boundary condition and the resulting internal flow kinematics. The variation in inflow conditions is expected to influence the magnitudes of the velocity moments within the swash zone and therefore sediment transport rates.

Description: The global ocean biogeochemical models that are used in order to assess the ocean role in the global carbon cycle and estimate the impact of the climate change on marine ecosystems are getting more and more sophisticated. They now often account for several phytoplankton functional types that play particular roles in marine food webs and the ocean carbon cycle. These phytoplankton functional types have specific physiological characteristics, which are usually poorly known and therefore add uncertainties to model results. Indeed, this evolution in model complexity is not accompanied by a similar increase in the number and diversity of in situ data sets necessary for model calibration and evaluation. Thus, it is of primary importance to develop new methods to improve model performance using existing biogeochemical data sets, despite their current limitations. In this paper, we have optimized 45 physiological parameters of the PISCES global model, using a variational optimal control method. In order to bypass a global 3-D ocean variational assimilation, which would require enormous computation and memory storage, we have simplified the estimation procedure by assimilating monthly climatological in situ observations at five contrasted oceanographic stations of the JGOFS program in a 1-D version of the PISCES model. We began by estimating the weight matrix in the cost function by using heuristic considerations. Then we used this matrix to estimate the 45 parameters of the 1-D version of the PISCES model by assimilating the different monthly profiles (observed profiles at the five stations) in the same variational procedure on a time window of 1 year. This set of optimized parameters was then used in the standard 3-D global PISCES version to perform a 500 year global simulation. The results of both the standard and the optimized versions of the model were compared to satellite-derived chlorophyll-a images, which are an independent and global data set, showing that our approach leads to significant improvements in simulated surface chlorophyll-a in most of the regions of the world ocean. Besides demonstrating that we have improved the accuracy of the PISCES model, this study proposes a sound methodology that could be used to efficiently account for in situ data in biogeochemical ocean models.

Description: Velocity estimation from an image sequence is one of the most challenging inverse problems in computer vision, geosciences, and remote sensing applications. In this paper a nonlinear model has been created for estimating motion field under the constraint of conservation of intensity. A linear differential form of heat or optical flow equation is replaced by a nonlinear temporal integral form of the intensity conservation constraint equation. Iterative equations with Gauss-Newton and Levenberg-Marguardt algorithms are formulated based on the nonlinear equations, velocity field modeling, and a nonlinear least squares model. An algorithm with progressive relaxation of the overconstraint to improve the performance of the velocity estimation is also proposed. The new estimator is benchmarked using a numerical simulation model. Both angular and magnitude error measurements based on the synthetic surface heat flow from the numerical model demonstrate that the performance of the new approach with the nonlinear model is much better than the results of using a linear model of heat or optical flow equation. Four sequences of NOAA Advanced Very High Resolution Radiometer (AVHRR) images taken in the New York Bight fields is also used to demonstrate the performance of the nonlinear inverse model, and the estimated velocity fields are compared with those measured with the Coastal Ocean Dynamics Radar array. The experimental results indicate that the nonlinear inverse model provides significant improvement over the linear inverse model for real AVHRR data sets.

Description: Periods of high astronomically generated tides contribute to the occurrence of extreme sea levels. Over interannual time scales, two precessions associated with the orbit of the Moon cause systematic variation of high tides. A global assessment of when these tidal modulations occur allows for the prediction of periods when the enhanced risk of coastal flooding is likely in different parts of the world. This paper uses modeled tides to assess the influence of the 18.61 year lunar nodal cycle and the 8.85 year cycle of lunar perigee (which affects high tidal levels as a quasi 4.4 year cycle) on high tidal levels on a global scale. Tidal constituents from the TPXO7.2 global tidal model are used, with satellite modulation corrections based on equilibrium tide expectations, to predict multidecadal hourly time series of tides on a one-quarter degree global grid. These time series are used to determine the amplitude and phase of tidal modulations using harmonic analysis fitted to 18.61, 9.305, 8.85, and 4.425 year sinusoidal signals. The spatial variations in the range and phase of the tidal modulations are related to the global distribution of the main tidal constituents and tidal characteristics (diurnal or semidiurnal and tidal range). Results indicate that the 18.61 year nodal cycle has the greatest influence in diurnal regions with tidal ranges of 〉4 m and that the 4.4 year cycle is largest in semidiurnal regions where the tidal range is 〉6 m. The phase of the interannual tidal modulations is shown to relate to the form of the tide.

Description: This study examines the structure and dynamics of wind-forced intraseasonal zonal current variability in the equatorial Indian Ocean. We take advantage of a variety of satellite and in situ data sets, including unprecedented 4–8 year-long velocity time series records from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) program. Spectral analysis reveals prominent intraseasonal zonal currents variations along the equator with periods of 30–70 days. These oscillations are vertically in phase above the thermocline and propagate eastward with the local zonal winds. In the thermocline, intraseasonal zonal velocity variations also propagate eastward across a broad range of phase speeds expected for low baroclinic equatorial Kelvin waves; amplitudes decrease with depth, with deeper levels leading those near surface. Collectively, these results suggest that the near-surface layer responds directly to intraseasonal zonal wind stress forcing and that subsequently energy radiates downward and eastward in the thermocline in the form of wind-forced equatorial Kelvin waves. In addition, intraseasonal zonal current variability on the equator is coherent with off-equatorial sea surface height fluctuations in the eastern and central of the basin. This coherence is primarily due to the fact that equatorial zonal wind variations are associated with off-equatorial wind stress curls that can generate local Ekman pumping and westward propagating Rossby waves.

Description: Eight autonomous profiling floats equipped with miniaturized radiometers and fluorimeters have collected data in Pacific, Atlantic, and Mediterranean offshore zones. They measured in particular 0–400 m vertical profiles of the downward irradiance at three wavelengths (412, 490, and 555 nm) and of the chlorophyll a fluorescence. Such autonomous sensors collect radiometric data regardless of sky conditions and collect essentially uncalibrated fluorescence data. Usual processing and calibration techniques are no longer usable in such remote conditions and have to be adapted. The proposition here is an interwoven processing by which missing parts of irradiance profiles (due to intermittent cloud occurrence) are interpolated by accounting for possible changes in optical properties (detected by the fluorescence signal) and by which the attenuation coefficient for downward irradiance, used as proxy for [Chl a] (the chlorophyll a concentration), allows the fluorescence signal to be calibrated in absolute units (mg m−3). This method is successfully applied to about 600 irradiance and fluorescence profiles. Validation of the results in terms of [Chl a] is made by matchup with satellite (MODIS-A) chlorophyll (24.3% RMSE, N = 358). Validation of the method is obtained by applying it on similar field data acquired from ships, which, in addition to irradiance and fluorescence profiles, include the [Chl a] HPLC determination, used for final verification.

Description: The statistics of the temperature and its spatial derivatives at a wind-driven air-water interface were obtained from a comprehensive data set of high resolution infrared imagery for wind speeds ranging from 2 ms−1 to 10 ms−1. We focus our effort on considerations of the anisotropy, symmetry, and intermittency of the surface turbulence. The analysis reveals that the root-mean-square surface temperature, when made nondimensional by using the surface heat flux and friction velocity, is nearly independent of Richardson number (Ri, defined in section 2). In addition, the derivatives of the thermal field appear also to converge to a limiting value at low Ri. The skewness of the temperature field, though slightly positive for the lowest wind speed (2 ms−1), is otherwise negative. On the other hand, the skewness of the derivative of the temperature field in the along-wind direction is strictly positive, while the skewness in the cross-wind direction is essentially zero, owing to the spanwise symmetry of the flow. This has the consequence that wind direction can be estimated by computing the skewness of the directional derivative of the temperature field. The flatness of the temperature field is observed to be near the Gaussian value of 3 throughout the wind speed range, while the along-wind and cross-wind derivatives show non-Gaussian behavior, indicating the presence of intermittency in the thermal fields at small scales. All probability density functions of the temperature derivatives are seen to have Gaussian cores, with distinct exponential tails.

Description: Above sand waves on the seafloor, surface short waves, which are responsible for the radiance distribution in remote sensing imagery, are modulated gradually by the submarine topography. The relaxation rate μr characterizes the rate at which the short waves reach their saturation range after being disturbed. It is a key parameter in the weak hydrodynamic interaction theory and is also a most important parameter in the imaging mechanism used for mapping submarine bottom topography. In this study, a robust expression containing intensity and phase (advection effect) modulations of the perturbed action spectrum of short waves was deduced, by using the first-order weak hydrodynamic interaction theory. On the basis of the phase modulation, a method was developed to determine the relaxation rate in the Sun glitter imaging mechanism. The relaxation rates were estimated using in situ data measured on a cruise over the sand waves of the Taiwan Banks, a sea area between the East China Sea and the South China Sea, on 28–29 August 2006. Results showed that, under a wind speed of 5.0 m s−1, the relaxation rate of short waves was about 0.055 s−1 in response to current variations and about 0.025 s−1 equivalently in response to sea bottom topographic variations. The former value could be applied to interpret the amplitude of submarine topography by using satellite imagery, while the latter one (equivalent relaxation rate μ′r) could help to more accurately calibrate the spatial position of the retrieved sea bottom topography.

Description: An idealized mathematical model of tsunami evolution in deep sea and across the continental shelf is proposed. The initial value problem in deep sea is related to the well-known Cauchy- Poisson problem and the tsunami propagation across the continental shelf is derived using the linearized shallow water equations. When analyzing different cases of tsunamis in deep sea, it was found that tsunamis evolve into two basic wave types. One resembles a single wave and the other a wave packet. The analysis of different cases of tsunamis at the shoreline reveals that the continental shelf, due to its geometrical properties, serves as a tsunami amplifier, producing tsunami amplitudes up to 20 times larger than those at the edge of the continental shelf. A comparison with tsunami measurements suggests that the idealized model may be used for a reliable assessment of the principle hydrodynamic properties of the tsunami, such as the tsunami amplitude and its half period.

Description: The dynamic structure of an ocean eddy in the eddy-abundant South China Sea has rarely been captured by measurements and has seldom been discussed in the literature. In the present study, in situ current and hydrographic measurements from a weeklong cruise and concurrent satellite altimeter observations were utilized to examine the three-dimensional structure and physical properties of a cold eddy in the southwestern South China Sea. The underlying forcing mechanism for the formation of this cyclonic cold eddy was found to be tightly associated with the recirculation in a coastal baroclinic jet that had separated off the Vietnamese coast. The eddy was significantly influenced by a coexisting, anticyclonic warm eddy in the separated jet. With relatively steady intensity and radius, the cold eddy endured for two weeks after its swift formation in late August and prior to its quick dissipation in mid-September. This cold eddy was horizontally and vertically heterogeneous. Asymmetric currents with much stronger magnitude were found on its southeastern flank, next to the warm eddy, where a front in the pycnocline was responsible for the sharp decrease in the cold eddy's intensity in the water below. The distributions of temperature, vorticity, and vertical velocity in the cold eddy were spatially asymmetric and not overlapping. The intensity of the cold eddy gradually decreased with the depth and the eddy extended downward for more than 250 m with a vertically tilted central axis. The upward velocities around the center of the eddy and the downward velocities to the southwest and to the east of the center jointly formed the upward domes of isotherms and isohalines in the central part of the cold eddy.

Description: Using the Regional Ocean Model System, the ocean circulation on the East China Sea (ECS) shelf was examined by a fine-resolution model which was nested in a coarse-resolution Pacific Ocean model. The high-resolution simulation shows an accurate volume transport of 2.70 Sv (Sv ≡ 106 m3s−1) through the Tsushima Strait, which is more consistent with the previous 5.5 year observation value (2.64 Sv) than former model results. For the Taiwan Strait it also shows a close volume transport (1.03 Sv) to a recent estimate (1.20 Sv). At the same time the model results reproduced almost all of the known circulation structure on the ECS shelf. In addition, the hindcast of 2009 shows a Kuroshio Bottom Branch Current to the northeast of Taiwan (KBBCNT). The KBBCNT is confirmed by the observational bottom high-salinity water (from 15 August to 2 September 2009) whose distribution is also reproduced by the model results. Tracer and particle experiments were carried out to elucidate the formation of the high-salinity water and the pathway of the KBBCNT. In light of the field observation and numerical experiments, a new pathway of the KBBCNT is proposed: bifurcated from the subsurface water of Kuroshio northeast of Taiwan, it upwells northwestward gradually from 300 to 60 m, then turns to northeast in the region around 27.5°N, 122°E, and finally reaches 31°N off the mouth of the Changjiang River along ∼60 m isobaths, forming the bottom saline water off the coast of Zhejiang province, China.

Description: The rapidly changing East Siberian Arctic Shelf (ESAS) receives large amounts of terrestrial organic carbon (OC) from coastal erosion and Russian-Arctic rivers. Climate warming increases thawing of coastal Ice Complex Deposits (ICD) and can change both the amount of released OC, as well as its propensity to be converted to greenhouse gases (fueling further global warming) or to be buried in coastal sediments. This study aimed to unravel the susceptibility to degradation, and transport and dispersal patterns of OC delivered to the ESAS. Bulk and molecular radiocarbon analyses on surface particulate matter (PM), sinking PM and underlying surface sediments illustrate the active release of old OC from coastal permafrost. Molecular tracers for recalcitrant soil OC showed ages of 3.4-13 14 C-ky in surface PM and 5.5-18 14 C-ky in surface sediments. The age difference of these markers between surface PM and surface sediments is larger (i) in regions with low OC accumulation rates, suggesting a weaker exchange between water column and sediments, and (ii) with increasing distance from the Lena River, suggesting preferential settling of fluvially-derived old OC nearshore. A dual-carbon end-member mixing model showed that (i) contemporary terrestrial OC is dispersed mainly by horizontal transport while being subject to active degradation, (ii) marine OC is most affected by vertical transport and also actively degraded in the water column, and (iii) OC from ICD settles rapidly and dominates surface sediments. Preferential burial of ICD-OC released into ESAS coastal waters might therefore lower the suggested carbon cycle climate feedback from thawing ICD permafrost. This article is protected by copyright. All rights reserved.

Description: The Arctic sea ice cover undergoes large changes over an annual cycle. In winter and spring the ice cover consists of large, snow-covered plate-like ice floes, with very little open water. By the end of summer the snow cover is gone and the large floes have broken into a complex mosaic of smaller, rounded floes surrounded by a lace of open water. This evolution strongly affects the distribution and fate of the solar radiation deposited in the ice-ocean system and consequently the heat budget of the ice cover. In particular, increased floe perimeter can result in enhanced lateral melting. We attempt to quantify the floe evolution process through the concept of a floe size distribution that is modified by lateral melting and floe breaking. A time series of aerial photographic surveys made during the SHEBA field experiment is analyzed to determine evolution of the floe size distribution from spring through summer. Based on earlier studies, we assume the floe size cumulative distribution could be represented by a power law D -α where D is the floe diameter. The exponent α as well as the number density of floes N tot are estimated from measurements of total ice area and perimeter. As summer progressed, there was an increase in α as the size distribution shifted towards smaller floes and the number of floes increased. Lateral melting causes the distribution to deviate from a power law for small floe sizes. This article is protected by copyright. All rights reserved.

Description: The responses of the ocean planktonic ecosystem to finite-amplitude perturbations are investigated using an ocean planktonic ecosystem model. Through changing the higher predation rate on zooplankton, multiple equilibria of the model, namely “high-nutrient” and “low-nutrient” states, are obtained under certain parameter values. Based on these states, the perturbations with maximum nonlinear growth are determined using the conditional nonlinear optimal perturbation (CNOP) method. The linear and nonlinear evolutions of the CNOP perturbation are compared. The results show that the nonlinear evolution of the perturbation leads to predator–prey oscillations with larger amplitude than the linear evolution. Besides, after the perturbation amplitude exceeds a critical value, the nonlinear evolution of the perturbation will induce the linearly stable ecosystem state to lose the stability and become nonlinearly unstable. This implies that nonlinear processes have important impacts on the stability of the ecosystem. Specifically, we identify the nonlinear processes related to zooplankton grazing to impact the stability most for the high-nutrient state, while for the low-nutrient state the main nonlinear process affecting the stability is the uptake process. These results help to improve our understanding of the sensitivity of the oceanic ecosystem model to finite-amplitude perturbations and the underlying nonlinear stability properties. This article is protected by copyright. All rights reserved.

Description: We evaluated the impacts of summertime mesoscale convective systems (MCS) on the heat balance and diel surface mixed layer (SML) dynamics of the Brazilian Amazon's Tucuruí Hydroelectric Reservoir (THR). We used a synergistic approach that combines in situ data, remote sensing data and three-dimensional (3D) modeling to investigate the typical behavior of the components of the heat balance and the SML dynamics. During the study period (the austral summer of 2012-2013), 22 days with MCS activity were identified. These events occurred approximately every 4 days, and they were most frequent during January (50% of the observations). An analysis of local meteorological data showed that when MCS occur, the environmental conditions at THR change significantly ( p-value 〈 0.01). The net longwave flux, which was the heat balance component most strongly impacted by MCS, increased more than 32% on days with MCS activity. The daily integrated heat balance became negative (-54 W m -2 ) on MCS days, while the balance was positive (19 W m -2 ) on non MCS days. In response to the changes in the heat balance, the SML dynamics changed when a MCS was over the THR. The SML depth was typically 28% higher on the days with MCS (~1.6 m) compared with the days without MCS (~1.3 m). The results indicate that MCS are one of the main meteorological disturbances driving the heat balance and the mixing dynamics of Amazonian hydroelectric reservoirs during the summer. These events may have implications for the water quality and greenhouse gas emissions of Amazonian reservoirs. This article is protected by copyright. All rights reserved.

Description: A strong upwelling off the continental shelf of the northern South China Sea (SCS) in 1998 summer is reinvestigated using a suite of new satellite measurements and numerical modeling. Previous studies indicate that the upwelling in the western SCS, especially off the Vietnam coast, almost disappears during 1998 summer following the El Niño because of the weakened southwest monsoon. This study identifies that the coastal upwelling in the adjacent northern SCS (NSCS) is significantly strengthened during 1998 summer, and the alongshore wind stress is dramatically enhanced over the region. As a result, the offshore Ekman transport in 1998 summer is the strongest, almost twice the average of the other 17 years during 1997–2007. The Chl a concentrations in the representative upwelling regions are much higher than any in other years. Further analysis suggests that two adjacent basin-scale upwellings in the SCS have different responses and maintaining mechanisms because of the anticyclonic atmospheric circulation anomaly over the SCS and northwest Pacific. The northern flank of the atmospheric circulation anomaly intensifies the monsoonal winds off the NSCS coast, while the southern flank suppresses the southwesterly winds along the Vietnam coast.

Description: Ventilation of waters in and around the Sea of Okhotsk was investigated using simulations of chlorofluorocarbons (CFCs) in the northwestern North Pacific. We used an ocean general circulation model coupled with a sea ice model. The model reproduces the distributions of CFCs similar to observed values and indicates the importance of tidal mixing along the Kuril Islands and brine rejection to ventilation of waters in and around the Sea of Okhotsk. To clarify the role of each process, numerical experiments excluding one of the two processes were carried out. Results show that brine rejection transports CFCs into the intermediate layer as deep as 200–400 m along the path of dense shelf water in the western Sea of Okhotsk, but hardly to other areas and layers. On the other hand, tidal mixing transports CFCs into the intermediate and deeper layers throughout the Sea of Okhotsk. We conclude that the tidal mixing has a greater influence than brine rejection on the ventilation of layers below the winter mixed layer.