ALBERT

Description: Opportunistic observations captured the coupled estuarine-shelf interactions as the Alabama coastal region transitioned from a period of low to flood river discharge conditions. The period of focus was February 18 to April 10, 2011 during which time a combination of in-situ (water level, salinity and velocity) and remote sensing (ocean color) data provided information on the estuarine and shelf environment prior to, during, and post a major river discharge event that captured a relatively rare spatially synoptic view of the structural evolution of a discharge plume in response to changing forcing conditions. The discharge event generated major changes in the hydrographic conditions and forcing responses within the estuary and on the shelf. The resulting surface advected plume was observed for approximately two weeks, during which time the observed differences in shelf circulation were directly linked to the discharge plume and a plume bulge with anticyclonic circulation was identified at times throughout the event. The plume was exposed to a range of wind conditions which modulated the surface structure: downwelling winds elongated the plume structure and upwelling winds reversed and widened the plume. The influence of wind forcing, even during very low wind (〈3.75 m s −1 ) and large outflow (∼7,000 m 3 s −1 ) conditions, was apparent, as a result of the shallow and wide characteristics of the plume. Anticyclonic bulge regions have only been identified in a few systems and the occurrence of this feature on the Alabama shelf has significant implications on transport and fate of river discharge in this region. This article is protected by copyright. All rights reserved.

Description: Five quantitative methodologies (metrics) that may be used to assess the skill of sea ice models against a control field are analyzed. The methodologies are Absolute Deviation, Root Mean Square Deviation, Mean Displacement, Hausdorff Distance, and Modified Hausdorff Distance. The methodologies are employed to quantify similarity between spatial distribution of the simulated and control scalar fields providing measures of model performance. To analyze their response to dissimilarities in 2-dimensional fields (contours), the metrics undergo sensitivity tests (scale, rotation, translation, and noise). Furthermore, in order to assess their ability to quantify resemblance of 3-dimensional fields the metrics are subjected to sensitivity tests where tested fields have continuous random spatial patterns inside the contours. The Modified Hausdorff Distance approach demonstrates the best response to tested differences, with the other methods limited by weak responses to scale and translation. Both Hausdorff Distance and Modifed Hausdorff Distance metrics are robust to noise, as opposed to the other methods. The metrics are then employed in realistic cases that validate sea ice concentration fields from numerical models and sea ice mean outlook against control data and observations. The Modified Hausdorff Distance method again exhibits high skill in quantifying similarity between both 2-dimensional (ice contour) and 3-dimensional (ice concentration) sea ice fields. The study demonstrates that the Modified Hausdorff Distance is a mathematically tractable and efficient method for model skill assessment and comparison providing effective and objective evaluation of both 2-dimensional and 3-dimensional sea ice characteristics across data sets. This article is protected by copyright. All rights reserved.

Description: The Extended Ellett Line (EEL) hydrographic section extends from Scotland to Iceland crossing the Rockall Trough, Hatton-Rockall Basin and Iceland Basin. With 61 full-depth stations at a horizontal resolution of 10 to 50 km, the EEL samples the upper limb of the Atlantic Meridional Overturning Circulation flowing across the Iceland-Scotland Ridge into the Nordic Seas. The Rockall Trough has been sampled nearly four times per year from 1975 to 1996, and the full section annually since 1996. The EEL is an exceptionally long timeseries of deep-ocean temperatures and salinities. This study extends prior work in the Rockall Trough, and examines for the first time 18 year records in the Iceland and Hatton-Rockall Basins. We quantify errors in the timeseries from two sources: observational errors and aliasing. The data quality and annual sampling are suitable for observing interannual to decadal variability because the variability exceeds our error estimates. The upper waters of all 3 basins are cooler/fresher from 1997 to 2001, warmer/more saline 2001 to 2006, and cooler/fresher from 2006 to 2014. A reference level for geostrophic shear is developed heuristically and by comparison with sea-surface altimetry. The mean northward transport in the upper waters is 6.7±3.7 Sv and there is a 6.1±2.5 Sv southward flow below the thermocline. Although the magnitude of the Iceland Basin overturning circulation (4.3±1.9 Sv) is greater than in the Rockall Trough (3.0±3.7 Sv), the variability is greater in the Rockall Trough. We discuss the results in the context of our understanding of drivers of variability. This article is protected by copyright. All rights reserved.

Description: In the oligotrophic waters to the east of Madagascar a large phytoplankton bloom is found to occur in late austral summer. This bloom is composed of nitrogen fixers and can cover up to ∼1% of the world's ocean surface area. Satellite observations show that its spatial structure is closely tied to the underlying mesoscale eddy field in the region. The causes of the bloom and its temporal behavior (timing of its initiation and termination) and spatial variability are poorly understood, in part due to a lack of in situ observations. Here an eddy resolving 1/12˚ resolution ocean general circulation model and Lagrangian particle tracking are used to examine the hypothesis that iron from sediments around Madagascar could be advected east by the mesoscale eddy field to fertilize the bloom, and that variability in advection could explain the significant interannual variability in the spatial extent of the bloom. The model results suggest that this is indeed possible and furthermore imply that the bloom could be triggered by warming of the mixed layer, leading to optimal conditions for nitrogen fixers to grow, while its termination could be due to iron exhaustion. It is found that advection of Madagascan iron could re-supply the bloom region with this micronutrient in the period between the termination of one bloom and the initiation of the next in the following year. This article is protected by copyright. All rights reserved.

Description: This study evaluates capability of the Argo observation network for monitoring ocean variation, especially for eddy-scale variation, by using an optimum interpolation (OI) procedure. Sea surface dynamic height anomalies (DHAs) are derived from Argo temperature and salinity profile data, and DHA fields are obtained by the OI based on the space-time correlation scales estimated from along-track sea level anomaly (SLA) data by satellite altimetry. The DHA fields are compared with the SLA fields derived from the same OI applied to the along-track SLA data. The results show that the equatorial Kelvin waves and tropical instability waves are well captured by Argo floats. Eddies are also monitored effectively in the subtropical western North Pacific. The OI results of DHA do not agree well with those of SLA in the high latitudes. A simple test of the space-time OI analysis shows that more than six data in the e-folding domain, where the correlation coefficient of ocean variation is above e −1 , are required for the reliable analysis with 99% confidence level. Argo floats provide sufficient number of observations for the reliable analysis in the low latitudes and some areas in the North Pacific. Two to three times more Argo data would be required in most of mid-latitudes and much more in high latitudes for capturing eddy-scale variation. This article is protected by copyright. All rights reserved.

Description: Once oil plumes such as those originating from underwater blowouts reach the ocean mixed layer (OML), their near-surface dispersion is influenced heavily by wind and wave-generated Langmuir turbulence. In this study, the complex oil spill dispersion process is modeled using large-eddy simulation (LES). The mean plume dispersion is characterized by performing statistical analysis of the resulting fields from the LES data. Although the instantaneous oil concentration exhibits high intermittency with complex spatial patterns such as Langmuir-induced striations, it is found that the time-averaged oil distribution can still be described quite well by smooth Gaussian-type plumes. LES results show that the competition between droplet rise velocity and vertical turbulent diffusion due to Langmuir turbulence is crucial in determining both the dilution rate and overall direction of transport of oil plumes in the OML. The smoothness of the mean plume makes it feasible to aim at modeling the oil dispersion using Reynolds-averaged type formulations, such as the K-profile parameterization (KPP) with sufficient vertical resolution to capture vertical profiles in the OML. Using LES data, we evaluate the eddy viscosity and eddy diffusivity following the KPP framework. We assess the performance of previous KPP models for pure shear turbulence and Langmuir turbulence by comparing them with the LES data. Based on the assessment a modified KPP model is proposed, which shows improved overall agreement with the LES results for both the eddy viscosity and the eddy diffusivity of the oil dispersion under a variety of flow conditions and droplet sizes. This article is protected by copyright. All rights reserved.

Description: The region of the Southern Ocean that encompasses the Subantarctic Front (SAF) to the north and the Polar Front (PF) to the south contains most of the transport of the Antarctic Circumpolar Current. Here, skewness of sea level anomaly (SLA) from 1992-2013 is coupled with a meandering Gaussian jet model to estimate the mean position, meridional width, and the percent variance that each front contributes to total SLA variability. The SAF and PF have comparable widths (85km) in the circumpolar average, but their widths differ significantly in the East Pacific Basin (85km and 60km respectively). Interannual variability in the positions of the SAF and PF are also estimated using annual subsets of the SLA data from 1993 to 2012. The PF position has enhanced variability near strong topographic features such as the Kerguelen Plateau, the Campbell Plateau east of New Zealand, and downstream of Drake Passage. Neither the SAF nor the PF showed a robust meridional trend over the 20-year period. The Southern Annular Mode was significantly correlated with basin-averaged SAF and PF positions in the East Pacific and with the PF south of Australia. A correlation between the PF and the basin-scale wind stress curl anomaly was also found in the Western extratropical Pacific but not in other basins. This article is protected by copyright. All rights reserved.

Description: The sensitivity and variability of spatial tsunami inundation footprints in coastal cities and towns due to a mega-thrust subduction earthquake in the Tohoku region of Japan are investigated by considering different fault geometry and slip distributions. Stochastic tsunami scenarios are generated based on the spectral analysis and synthesis method with regards to an inverted source model. To assess spatial inundation processes accurately, tsunami modeling is conducted using bathymetry and elevation data with 50-m grid resolutions. Using the developed methodology for assessing variability of tsunami hazard estimates, stochastic inundation depth maps can be generated for local coastal communities. These maps are important for improving disaster preparedness by understanding the consequences of different situations/conditions, and by communicating uncertainty associated with hazard predictions. The analysis indicates that the sensitivity of inundation areas to the geometrical parameters (i.e. top-edge depth, strike, and dip) depends on the tsunami source characteristics and the site location, and is therefore complex and highly nonlinear. The variability assessment of inundation footprints indicates significant influence of slip distributions. In particular, topographical features of the region, such as ria coast and near-shore plain, have major influence on the tsunami inundation footprints. This article is protected by copyright. All rights reserved.

Description: Siberian river water is a first-order contribution to the Arctic freshwater budget, with the Ob, Yenisey, and Lena supplying nearly half of the total surface freshwater flux. However, few details are known regarding where, when and how the freshwater transverses the vast Siberian shelf seas. This paper investigates the mechanism, variability and pathways of the fresh Kara Sea outflow through Vilkitsky Strait towards the Laptev Sea. We utilize a high-resolution ocean model and recent shipboard observations to characterize the freshwater-laden Vilkitsky Strait Current (VSC), and shed new light on the little-studied region between the Kara and Laptev Seas, characterized by harsh ice conditions, contrasting water masses, straits and a large submarine canyon. The VSC is 10-20 km wide, surface-intensified, and varies seasonally (maximum from August-March) and interannually. Average freshwater (volume) transport is 500 ± 120 km 3 a −1 (0.53 ± 0.08 Sv), with a baroclinic flow contribution of 50-90%. Interannual transport variability is explained by a storage-release mechanism, where blocking-favorable summer winds hamper the outflow and cause accumulation of freshwater in the Kara Sea. The year following a blocking event is characterized by enhanced transports driven by a baroclinic flow along the coast that is set up by increased freshwater volumes. Eventually, the VSC merges with a slope current and provides a major pathway for Eurasian river water towards the Western Arctic along the Eurasian continental slope. Kara (and Laptev) Sea freshwater transport is not correlated with the Arctic Oscillation, but rather driven by regional summer pressure patterns. This article is protected by copyright. All rights reserved.

Description: Ice export from the vast Arctic Siberian shelf is calculated using δ 18 O values and salinity data for water samples collected during the International Siberian Shelf Study between August and September 2008 (ISSS-08). The samples represent a wide range of salinities and δ 18 O values due to river water inputs and sea ice removal. We estimate the fraction of water that has been removed as ice by interpreting observed δ 18 O values and salinities as a result of mixing between river water and sea water end-members as well as to fractional ice removal. This method does not assume an ice end-member of fixed composition, which is especially important when applied on samples with large differences in salinity. The results show that there is net transport of ice from both the Laptev and the Eastern Siberian Seas, and in total 3000 km³ of sea ice is exported from the shelf. The annual total export of ice from the entire region, calculated from the residence time of water on the shelf, is estimated to be 860 km 3 yr −1 . Thus, changes in ice production on the shelf may have great impact on sea ice export from the Arctic Ocean. This article is protected by copyright. All rights reserved.

Description: The seasonal evolution of melt ponds has been well-documented on multiyear and landfast first-year sea ice, but is critically lacking on drifting, first-year sea ice, which is becoming increasingly prevalent in the Arctic. Using 1-meter resolution panchromatic satellite imagery paired with airborne and in situ data, we evaluated melt pond evolution for an entire melt season on drifting first-year and multiyear sea ice near the 2011 Applied Physics Laboratory Ice Station (APLIS) site in the Beaufort and Chukchi seas. A new algorithm was developed to classify the imagery into sea ice, thin ice, melt pond, and open water classes on two contrasting ice types: first-year and multiyear sea ice. Surprisingly, melt ponds formed ∼3 weeks earlier on multiyear ice. Both ice types had comparable mean snow depths, but multiyear ice had 0 - 5-cm deep snow covering ∼37% of its surveyed area, which may have facilitated earlier melt due to its low surface albedo compared to thicker snow. Maximum pond fractions were 53 ± 3% and 38 ± 3% on first-year and multiyear ice, respectively. APLIS pond fractions were compared with those from the Surface Heat Budget of the Arctic Ocean (SHEBA) field campaign. APLIS exhibited earlier melt and double the maximum pond fraction, which was in part due to the greater presence of thin snow and first-year ice at APLIS. These results reveal considerable differences in pond formation between ice types, and underscore the importance of snow depth distributions in the timing and progression of melt pond formation. This article is protected by copyright. All rights reserved.

Description: Results from the laboratory experiments on the evolution of baroclinically unstable flows generated in a rotating tank with topographic β-effect are presented. We study zonal jets of alternating direction which occur in these flows. The primary system we model includes lighter fluid in the South and heavier fluid in the North with resulting slow meridional circulation and fast mean zonal motion. In a two-layer system the velocity shear between the layers results in baroclinic instability which equilibrates with time and, due to interaction with β-effect generates zonal jets. This system is archetypal for various geophysical systems including the general circulation and jet streams in the Earth's atmosphere, the Antarctic Circumpolar Current or the areas in the vicinity of western boundary currents where baroclinic instability and multiple zonal jets are observed. The gradient of the surface elevation and the thickness of the upper layer are measured in the experiments using the Altimetric Imaging Velocimetry and the Optical Thickness Velocimetry techniques respectively. Barotropic and baroclinic velocity fields are then derived from the measured quantities. The results demonstrate that the zonal jets are driven by “eddy forcing” due to continuously created baroclinic perturbations. The flow is baroclinic to a significant degree and the jets are “surface intensified”. The meridional wavelength of the jets varies linearly with the baroclinic radius of deformation and is also in a good agreement with a modified Rhines scale. This suggests a linear dependence of the perturbation velocity in the equilibrated baroclinically unstable flow on the β-parameter. This article is protected by copyright. All rights reserved.

Description: In the 4 major Eastern Boundary Upwelling Systems (EBUS), mesoscale eddies are known to modulate the biological productivity and transport near-coastal seawater properties toward the offshore ocean, however little is known about their main characteristics and vertical structure. This study combines 10 years of satellite-altimetry data and Argo float profiles of temperature and salinity, and our main goals are i ) to describe the main surface characteristics of long-lived eddies formed in each EBUS and their evolution, and ii ) to depict the main vertical structure of the eddy-types that co-exist in these regions. A clustering analysis of the Argo profiles surfacing within the long-lived eddies of each EBUS allows us to determine the proportion of surface and subsurface-intensified eddies in each region, and to describe their vertical structure in terms of temperature, salinity and dynamic height anomalies. In the Peru–Chile Upwelling System, 55% of the sampled anticyclonic eddies (AEs) have subsurface-intensified maximum temperature and salinity anomalies below the seasonal pycnocline, whereas 88% of the cyclonic eddies (CEs) are surface-intensified. In the California Upwelling System, only 30% of the AEs are subsurface-intensified and all of the CEs show maximum anomalies above the pycnocline. In the Canary Upwelling System, ∼40% of the AEs and ∼60% of the CEs are subsurface-intensified with maximum anomalies extending down to 800 m depth. Finally, the Benguela Upwelling System tends to generate ∼40-50% of weak surface-intensified eddies and ∼50-60% of much stronger subsurface-intensified eddies with a clear geographical distribution. The mechanisms involved in the observed eddy vertical shapes are discussed. This article is protected by copyright. All rights reserved.

Description: Hydrographic data, chlorofluorocarbon-12 (CFC-12) and sulfur hexafluoride (SF 6 ) measurements collected in March 2010 and September-October 2011 in the Red Sea, as well as an idealized numerical experiment are used to study the formation and spreading of Red Sea Outflow Water (RSOW) in the Red Sea. Analysis of inert tracers, potential vorticity distributions and model results confirm that RSOW is formed through mixed layer deepening caused by sea surface buoyancy loss in winter in the northern Red Sea and reveal more details on RSOW spreading rates, pathways, and vertical structure. The southward spreading of RSOW after its formation is identified as a layer with minimum potential vorticity, and maximum CFC-12 and SF 6 . Ventilation ages of seawater within the RSOW layer, calculated from the partial pressure of SF 6 (pSF 6 ), range from 2 years in the northern Red Sea to 15 years at 17°N. The distribution of the tracer ages is in agreement with the model circulation field which shows a rapid transport of RSOW from its formation region to the southern Red Sea where there are longer circulation pathways and hence longer residence time due to basin wide eddies. The mean residence time of RSOW within the Red Sea estimated from the pSF 6 age is 4.7 years. This time scale is very close to the mean transit time (4.8 years) for particles from the RSOW formation region to reach the exit at the Strait of Bab el Mandeb in the numerical experiment. This article is protected by copyright. All rights reserved.

Description: Infragravity waves are oceanic surface gravity waves but with wavelengths (10's km) and periods (〉30s) much longer than wind waves and swell. Mostly studied in shallow water, knowledge of infragravity waves in deep water has remained limited. Recent interest in deep-water infragravity waves has been motivated by the error they may contribute to future high-resolution satellite radar altimetry measurements of sea level. Here, deep-water infragravity waves offshore of the Pacific Northwest of the USA were studied using Differential Pressure Gauges which were deployed as part of the Cascadia Initiative array from September 2012-May 2013. Cross-correlation of the records revealed direction of infragravity wave propagation across the array, from which source regions were inferred. The dominant source was found to be the coastline to the east, associated with large wind waves and swell incident on the eastern side of the basin. The source shifted southward during northern-hemisphere summer, and on several days in the record infragravity waves arrived from the western side of the Pacific. Asymmetry of cross-correlation functions for five of these westerly arrivals was used to calculate the ratio of seaward to shoreward propagating energy, and hence estimate the strength of infragravity wave reflection at periods of 100-200s. Reflection of these remote arrivals from the west appeared to be strong, with a lower bound estimate of r=0.49±0.29 (reflection coefficient ± standard error) and an upper bound estimate of r=0.74±0.06. These results suggest that reflection at ocean boundaries may be an important consideration for infragravity waves in the deep ocean. This article is protected by copyright. All rights reserved.

Description: We present the horizontal kinetic energy (KE) balance of near-inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15.26°N in the Arabian Sea during the southwest monsoon. The highly sheared and stratified transition layer at the mixed-layer base varies between 5∼m and 35∼m and correlates negatively with the wind stress. Results from the mixed layer near-inertial KE (NIKE) balance suggest that wind energy at times can energize the transition layer and at other times is fully utilized within the mixed layer. A simple two layer model is utilized to study the shear evolution in the transition layer and shown to match well with observations. The shear production in this model arises from alignment of wind stress and shear. Although the winds are unidirectional during the monsoon, the shear in the transition layer is predominantly near-inertial. The near-inertial shear bursts in the observations show the same phasing and magnitude at near-inertial frequencies as the wind-shear alignment term. This article is protected by copyright. All rights reserved.

Description: Cross- and along-shelf winds drive cross-shelf transport that promotes the exchange of tracers and nutrients to the open sea. The shelf response to cross-shelf winds is studied in the north shelf of the Ebro Delta (Mediterranean Sea), where those winds are prevalent and intense. Offshore winds in the region exhibit strong intensities (wind stress larger than 0.8 Pa) during winter and fall. The monthly average flow observed in a one-year current meter record at 43.5 m was polarized following the isobaths with the along-shelf variability being larger than the cross-shelf. Prevalent southwestward along-shelf flow was induced by the 3-dimensional regional response to cross-shelf winds and the coastal constraint. Seaward near-surface velocities occurred predominantly during offshore wind events. During intense wind periods, the surface cross-shelf water transport exceeded the net along-shelf transport. During typically stratified seasons, the intense cross-shelf winds resulted in a well-defined two-layer flow and were more effective at driving offshore transport than during unstratified conditions. While transfer coefficients between wind and currents were generally around 1%, higher cross-shelf transfer coefficients were observed in the near-inertial band. The regional extent of the resulting upwelling during energetic cross-shelf winds events, estimated using surface temperature, was concentrated around the region of the wind jet. Cross-shelf transport due to along-shelf winds was only effective during northeast wind events. During along-shelf wind conditions, the transport was estimated to be between 10% and 50% of the theoretical Ekman transport. This article is protected by copyright. All rights reserved.

Description: We have investigated the phytoplankton dynamics of the Senegalo-Mauritanian upwelling region, which is a very productive region, by processing a 13-year set of SeaWiFS satellite ocean-color measurements using a PHYSAT-like method. We clustered the spectra of the ocean-color normalized reflectance (reflectance normalized by a reflectance dependent on chlorophyll- a concentration only) into 10 significant spectral classes using a Self Organized Map (SOM) associated with a hierarchical ascendant classification (HAC). By analyzing a 13-year climatology of these classes, we have been able to outline a coherent scenario describing the Senegalo-Mauritanian upwelling region in terms of spatio-temporal variability of phytoplankton groups: during the onset of the upwelling (December to February), we mainly observed nanoeukaryote-type phytoplankton in the coastal area; in April–May, the period corresponding to the maximum chlorophyll- a concentration, the nanoeukaryote types were replaced by diatom types. This scenario is in agreement with microscope phytoplankton cell observations done during several past cruises. This article is protected by copyright. All rights reserved.

Description: The Baltic Sea is a marginal sea, located in a highly industrialized region in Central Northern Europe. Salt water inflows from the North Sea and associated ventilation of the deep exert crucial control on the entire Baltic Sea ecosystem. This study explores the impact of anticipated sea level changes on the dynamics of those inflows. We use a numerical oceanic general circulation model covering both the Baltic and the North Sea. The model sucessfully retraces the essential ventilation dynamics throughout the period 1961 to 2007. A suite of idealized experiments suggests that rising sea level is associated with intensified ventilation as salt water inflows become stronger, longer and more frequent. Expressed quantitatively as a salinity increase in the deep central Baltic Sea we find that a sea level rise of 1 m triggers a saltening of more than 1 PSU. This substantial increase in ventilation is the consequence of the increasing cross section in the Danish Straits amplified by a reduction of vertical mixing. This article is protected by copyright. All rights reserved.

Description: The Persian Gulf feeds a warm and salty outflow in the Gulf of Oman (northern Arabian Sea). The salt climatological distribution is relatively smooth in the Gulf of Oman, and the signature of a slope current carrying salty waters is difficult to distinguish hundreds of kilometers past the Strait of Hormuz, in contrast to other outflows of the world ocean. This study focuses on the mechanisms involved in the spreading of Persian Gulf Water (PGW) in the Gulf of Oman, using a regional primitive equation numerical simulation. The authors show that the dispersion of PGW occurs through a regime that is distinct from, for example, the one responsible for the Mediterranean outflow dispersion. The background mesoscale eddy field is energetic and participates actively to the spreading of PGW. Remotely formed eddies propagate into the Gulf of Oman and interact with the topography, leading to submesoscales formation and PGW shedding. Eddy-topography interactions are isolated in idealized simulations and reveal the formation of intense frictional boundary layers, generating submesoscale coherent vortices (SCVs). Interactions take place at depths encompassing the PGW depth, thus SCVs trap PGW and contribute to its redistribution from the boundaries to the interior of the Gulf of Oman. The overall efficiency of these processes is confirmed by a strong contribution of eddy salt fluxes in the interior of the basin, and is quantified using particle statistics. It is found to be a highly dispersive regime, with an approximated eddy diffusivity of ∼1700 m 2 s −1 . This article is protected by copyright. All rights reserved.

Description: Velocities of surface drifters are analyzed to study tidal currents throughout the World Ocean. The global drifter dataset spanning the period 1979-2013 is used to describe the geographical structure of the surface tidal currents at global scale with a resolution of 2 degrees. Harmonic analysis is performed with two semi-diurnal, two diurnal and four inferred tidal constituents. Tidal current characteristics (amplitude of semi-major axis, rotary coefficient, tidal ellipse inclination and Greenwich phase) are mapped over the World Ocean from direct observations. The M2 currents dominate on all the shallow continental shelves with magnitude exceeding 60 cm/s. They are also substantial (4-5 cm/s) over the main deep topographic features such as the Mid-Atlantic Ridge, the Southwest Indian Ridge and the Mariana Ridge. The S2 currents have amplitudes typically half the size of the M2 currents, with a maximum of about 30 cm/s. The K1 and O1 currents are important in many shallow seas. They are large in the vicinity of the turning latitudes near 30°N/S where they merge with inertial motions of the same frequency. They are also substantial in the South China Sea and Philippine Sea. Maps of rotary coefficients indicate that all tidal motions are essentially clockwise (anticlockwise) in the Northern (Southern) Hemisphere. The rotary coefficient of the tidal currents are compared with the theory of freely and meridionally propagating baroclinic inertia-gravity waves. The Greenwich phase of the M2 constituent has large scale coherent propagation patterns which could be interpreted as the propagation of the barotropic tide. This article is protected by copyright. All rights reserved.

Description: Satellite altimetry sea surface height measurements reveal high mesoscale eddy activity in the southeastern tropical Indian Ocean (SETIO). In this study, the characteristics of mesoscale eddies in the SETIO are investigated by analyzing 564 cyclonic eddy (CE) tracks and 695 anticyclonic eddy (AE) tracks identified from a new version of satellite altimetry data with a daily temporal resolution. The mean radius, lifespan, propagation speed and distance of CEs (AEs) are 149 (153) km, 50 (46) days, 15.3 (16.6) cm s −1 , and 651 (648) km, respectively. Some significant differences exist in the eddy statistical characteristics between the new-version daily altimeter data and the former weekly data. Mean vertical structures of anomalous potential temperature, salinity, geostrophic current, as well as heat and salt transports of the composite eddies, are estimated by analyzing Argo profile data matched to altimeter-detected eddies. The composite analysis shows that eddy-induced ocean anomalies are mainly confined in the upper 300 dbar. In the eddy core, CE (AE) could induce a cooling (warming) of 2ºC between 60 and 180 dbar and maximum positive (negative) salinity anomalies of 0.1 (-0.3) psu in the upper 50 (110) dbar. The meridional heat transport induced by the composite CE (AE) is southward (northward), whereas the salt transport of CE (AE) is northward (southward). Most of the meridional heat and salt transports are carried in the upper 300 dbar. This article is protected by copyright. All rights reserved.

Description: The cross-shore evolution of individual wave celerity is investigated using two high-resolution laboratory experiments on bichromatic waves. Individual waves are tracked during their onshore propagation and their characteristics, including celerity, are estimated. The intra-wave variability in celerity is low in the shoaling zone, but increases strongly after breaking. It is maximum when the infragravity wave height to water depth ratio is the largest, that is to say close to the shoreline. There, the observed range of individual wave celerity can be as large as the mean celerity value. This variability can be largely explained by the variations in water depth and velocity induced by the infragravity waves. The differences in celerity are such that they lead to the merging of the waves in the inner surf zone for most of the wave conditions considered. Again, the location at which the first waves start merging strongly correlates with the infragravity wave height to water depth ratio. The consequences of these findings for celerity-based depth-inversion techniques are finally discussed. Surprisingly, accounting for the infragravity-wave modulation of the velocity field in the celerity estimate does not significantly improve depth estimation in the surf zone. However, it is shown that the occurrence of bore merging decreases significantly the coherence of the wave field in the surf zone. This loss of coherence could hamper celerity estimation from pixel intensity time-series, and explain, at least partly, the relatively poor performance of depth-inversion techniques in the inner surf zone. This article is protected by copyright. All rights reserved.

Description: A decade of moored measurements from the Arctic Ocean's northwestern Beaufort Gyre (collected as a component of the Beaufort Gyre Exploration Project) are analyzed to examine the range of mesoscale eddies over the water column, and the dynamical processes that set eddy vertical scales. A total of 58 eddies were identified in the moored record, all anticyclones with azimuthal velocities ranging from 10 cm/s to 43 cm/s. These are divided into three classes based on core depths. Shallow eddies (core depths around 120 m) are shown to be vertically confined by the strong stratification of the halocline; typical thicknesses are around 100 m. Deep eddies (core depths around 1200 m) are much taller (thicknesses around 1300 m) owing to the weaker stratification at depth, consistent with a previous study. Eddies centered around mid-depths all have two cores (vertically aligned and separated in depth) characterized by velocity maxima and anomalous temperature and salinity properties. One core is located at the base of the halocline (around 200 m depth) and the other at the depth of the Atlantic Water layer (around 400 m depth). These double-core eddies have vertical scales between those of the shallow and deep eddies. The strongly decreasing stratification in their depth range motivates a derivation for the quasi-geostrophic adjustment of a nonuniformly stratified water column to a potential vorticity anomaly. The result aids in interpreting the dynamics and origins of the double-core eddies, providing insight into transport across a major water mass front separating Canadian and Eurasian Water. This article is protected by copyright. All rights reserved.

Description: In this article we show that the class of low frequency (sub-inertial) waves known as coastal-trapped waves (CTWs) are a significant agent of water volume exchange in a west Svalbard fjord, and by extension more widely along the west Svalbard and east Greenland margins where similar conditions prevail. We show that CTWs generated by weather systems passing across the sloping topography of the shelf break propagate into the fjord, steered by the topography of an across-shelf trough. The CTWs have characteristic periods of ∼two days, set by the passage time of weather systems. Phase speeds and wavelengths vary seasonally by a factor of two, according to stratification: winter (summer) values are C p = 0.25 ms − 1 (0.5 ms − 1 ) and λ = 40 km (84 km). CTW-induced flow velocities in excess of 0.2 ms − 1 at 100 m water depth are recorded. Observationally-scaled CTW model results allow their explicit role in volume exchange to be quantified. Of the estimated exchange terms, estuarine exchange is weakest ( m 3 s −1 ), followed by barotropic tidal pumping ( m 3 s −1 ), with intermediary exchange dominating ( m 3 s −1 ). Oscillatory flows display greatest activity in the one to five day period band, and CTW activity is identified as the likely source of variability in the 40 to 60 hour period band. Within that band intermediary exchange driven by CTWs is estimated as m 3 s −1 ; an exchange rate exceeding both barotropic and estuarine exchange estimates. This article is protected by copyright. All rights reserved.

Description: The California Undercurrent transports warm, salty, nutrient-rich, oxygen-depleted water along the continental slope from the equatorial Pacific to the Aleutian Islands. We use multi-year acoustic Doppler current profiler records collected simultaneously at two mooring sites off Vancouver Island to detail the regional structure of the undercurrent and to show that much of its variability is attributable to the passage of remotely forced, coastal-trapped waves. We also document two subsurface currents missed by earlier current measurements. The undercurrent becomes evident in spring, intensifies through summer and fall, and merges with the wind-driven poleward surface flow in winter. During intensification at the southern mooring site (A1), the undercurrent shoals from 250±50 m in early summer to 150±50 m depth in late fall. At the northern site (BP2), 225 km to the northwest of A1, the current is weaker and maintains a year-round depth of 150±50 m. Temporal variability in the undercurrent velocity attains highest coherence with winds along the southern Oregon-northern California coast, with peak coherence occurring for “synoptic” (10-40 day period) alongshore winds off Cape Blanco in southern Oregon. The undercurrent lag of 3±2 days relative to the Cape Blanco winds at synoptic periods is consistent with low mode, poleward propagating, coastally trapped waves. For periods 〉 40 days, the wind-current coherence remains high for winds off the Oregon-California coast but lags are often negative, indicating possible forcing by alongshore baroclinic pressure gradients. At interannual time scales, the undercurrent variations have links to climate-scale processes in the equatorial Pacific. This article is protected by copyright. All rights reserved.

Description: In this study the forecast skill of the U.S. Navy operational Arctic sea ice forecast system, the Arctic Cap Nowcast/Forecast System (ACNFS), is presented for the period Feb 2014 – June 2015. ACNFS is designed to provide short term, 1-7 day forecasts of Arctic sea ice and ocean conditions. Many quantities are forecast by ACNFS; the most commonly used include ice concentration, ice thickness, ice velocity, sea surface temperature, sea surface salinity, and sea surface velocities. Ice concentration forecast skill is compared to a persistent ice state and historical sea ice climatology. Skill scores are focused on areas where ice concentration changes by ±5% or more, and are therefore limited to primarily the marginal ice zone. We demonstrate that ACNFS forecasts are skillful compared to assuming a persistent ice state, especially beyond 24 hours. ACNFS is also shown to be particularly skillful compared to a climatologic state for forecasts up to 102 hours. Modeled ice drift velocity is compared to observed buoy data from the International Arctic Buoy Programme. A seasonal bias is shown where ACNFS is slower than IABP velocity in the summer months and faster in the winter months. In February 2015 ACNFS began to assimilate a blended ice concentration derived from Advanced Microwave Scanning Radiometer 2 (AMSR2) and the Interactive Multisensor Snow and Ice Mapping System (IMS). Preliminary results show that assimilating AMSR2 blended with IMS improves the short-term forecast skill and ice edge location compared to the independently derived National Ice Center Ice Edge product. This article is protected by copyright. All rights reserved.

Description: An improved extended optimum multi-parameter (eOMP) analysis was applied to hydrographic (temperature and salinity), and water chemistry data, including dissolved oxygen (O 2 ), nutrients (nitrate plus nitrite, phosphate, and silicate), dissolved inorganic carbon (DIC), and total alkalinity (TAlk) data collected during late spring and summer from 2006 to 2012 in bottom waters off the Louisiana coast, to explore the dynamics and stoichiometry of DIC production during the development and maintenance of summer hypoxia. Our analysis demonstrated that DIC in bottom water was relatively low from April to June, but increased significantly in July, peaked in August, and dropped slightly in September. Furthermore, DIC production resulted from both aerobic organic carbon (OC) respiration and denitrification, as well as substantial loss due to vertical mixing with surface water. The average summer gross OC respiration rate was estimated to be 0.19 g C m −2 d −1 , with the highest values occurring in late summer when hypoxic conditions dominated. We also found that C org /N/P/-O 2 remineralization ratios for aerobic respiration were generally consistent with the classic Redfield ratio (106/16/1/138) except individual C/N and C/P ratios were slightly lower, indicating that marine OC was the major source of the DIC production in the bottom water. This study quantified the role of temporal bottom-water microbial respiration to seasonal DIC dynamics and provided a means for studying the stoichiometry of biogeochemical processes in coastal waters. This article is protected by copyright. All rights reserved.

Description: With the advent of Argo floats, it now seems feasible to study the interannual variations of upper ocean hydrographic properties of the historically undersampled Southern Ocean. To do so, scattered hydrographic profiles often first need to be mapped. To investigate biases and errors associated both with the limited space-time distribution of the profiles and with the mapping methods, we colocate the mixed layer depth (MLD) output from a state-of-the-art 1/12° DRAKKAR simulation onto the latitude, longitude and date of actual in-situ profiles from 2005 to 2014. We compare the results obtained after remapping using a nearest-neighbor (NN) interpolation and an objective analysis (OA) with different spatio-temporal grid resolutions and decorrelation scales. NN is improved with a coarser resolution. OA performs best with low decorrelation scales, avoiding too strong a smoothing, but returns values over larger areas with large decorrelation scales and low temporal resolution, as more points are available. For all resolutions OA represents better the annual extreme values than NN. Both methods underestimate the seasonal cycle in MLD. MLD biases are lower than 10 m on average but can exceed 250 m locally in winter. We argue that current Argo data should not be mapped to infer decadal trends in MLD, as all methods are unable to reproduce existing trends without creating unrealistic extra ones. We also show that regions of the subtropical Atlantic, Indian and Pacific Oceans, and the whole ice-covered Southern Ocean, still cannot be mapped even by the best method because of the lack of observational data. This article is protected by copyright. All rights reserved.

Description: Arctic sea ice concentration from satellite passive microwave measurements is analysed to assess the form and timing of the onset of decline of recent ice loss, and the regional dependence of the response. The timing of the onset is estimated using an objective method, and suggests differences of up to 20 years between the various subregions. A clear distinction can be drawn between the recent onset times of the Atlantic sector (beginning in 2003) and the much earlier onset times associated with the Pacific sector, where the earliest transition to rapid loss is found in 1992. Rates of decline prior to and following the transition points are calculated, and suggest that the post-onset rate of loss is greatest in the Barents Sea, and weakest in the Pacific sector. Covariability between the seasons is noted in the SIC response, both at interannual and longer time scales. For two case regions, potential mechanisms for the onset time transitions are briefly analysed. In the Barents Sea, the onset time coincides with a redistribution of the pathways of ice circulation in the region, whilst along the Alaskan coast, the propagation of the regional signal can be traced in the age of the sea ice. The results presented here indicate a series of spatially self-consistent regional responses, and may be useful in understanding the primary drivers of recent sea ice loss. This article is protected by copyright. All rights reserved.

Description: Super typhoon Haiyan struck the Philippines on November 8, 2013, marking one of the strongest typhoons at landfall in recorded history. Extreme storm waves attacked the Pacific coast of Eastern Samar where the violent typhoon first made landfall. Our field survey confirmed storm overwash heights of 6–14 m above mean sea level were distributed along the southeastern coast and extensive inundation occurred in some coastal villages in spite of natural protection by wide fringing reefs. A wave model based on Boussinesq-type equations was constructed to simulate wave transformation over shallow fringing reefs and validated against existing laboratory data. Wave propagation and runup on the Eastern Samar coast are then reproduced using offshore boundary conditions based on a wave hindcast. The model results suggests that extreme waves on the shore are characterized as a superposition of the infragravity wave and sea-swell components. The balance of the two components is strongly affected by the reef width and beach slope through wave breaking, frictional dissipation, reef-flat resonances and resonant runup amplification. Therefore, flood characteristics significantly differ from site to site due to a large variation of the two topographic parameters on the hilly coast. Strong coupling of infragravity waves and sea swells produces extreme runup on steep beaches fronted by narrow reefs, whereas the infragravity waves become dominant over wide reefs and they evolve into bores on steep beaches. This article is protected by copyright. All rights reserved.

Description: The effect of three-dimensional wave-induced streaming on the seabed boundary layer is investigated for following and opposing waves and current where the wave propagation forms a non-zero angle with the current. It is shown that the sea bed boundary layer flow results from an interaction between the classical wave-current interaction (reducing the mean velocity relative to current alone), Longuet-Higgins streaming (forcing the flow in the wave propagation direction) and streaming caused by turbulence asymmetry in successive wave half-cycles (forcing the flow against the wave propagation direction). For waves and current which are not colinear, the mean velocity profile exhibits a veering behaviour which is strongly affected by streaming, particularly for the most wave-dominated situations. The effect of streaming on the boundary layer flow has been investigated for different wave-current conditions and bottom roughnesses. Visualizations are given by mean Eulerian and Lagrangian velocity profiles, as well as three-dimensional seabed boundary layer particle trajectories. The effect of streaming decreases as the flow becomes more current-dominated. The mean velocity in the current direction decreases as the roughness increases. However, the mean velocity orthogonal to the current direction increases as the roughness increases due to the lack of wave-current interaction in this direction. An excellent agreement between the predicted and recently measured velocity profiles ( Yuan and Madsen [2015]) beneath horizontally uniform asymmetric forcing is obtained. This article is protected by copyright. All rights reserved.

Description: In the coastal ocean off the Northeast U.S., the sea surface temperature (SST) in the first half of 2012 was the highest on the record for the past roughly 150 years of recorded observations. The underlying dynamical processes responsible for this extreme event are examined using a numerical model, and the relative contributions of air-sea heat flux versus lateral ocean advective heat flux are quantified. The model accurately reproduces the observed vertical structure and the spatiotemporal characteristics of the thermohaline condition of the Gulf of Maine and the Middle Atlantic Bight waters during the anomalous warming period. Analysis of the model results show that the warming event was primarily driven by the anomalous air-sea heat flux, while the smaller contribution by the ocean advection worked against this flux by acting to cool the shelf. The anomalous air-sea heat flux exhibited a shelf-wide coherence, consistent with the shelf-wide warming pattern, while the ocean advective heat flux was dominated by localized, relatively smaller scale processes. The anomalous cooling due to advection primarily resulted from the along-shelf heat flux divergence in the Gulf of Maine, while in the Middle Atlantic Bight the advective contribution from the along- and cross-shelf heat flux divergences were comparable. The modeling results confirm the conclusion of the recent analysis of in situ data by Chen et al. (2014a) that the changes in the large-scale atmospheric circulation in the winter of 2011-2012 primarily caused the extreme warm anomaly in the spring of 2012. The effect of along-shelf or cross-shelf ocean advection on the warm anomalies from either the Scotian Shelf or adjacent continental slope was secondary. This article is protected by copyright. All rights reserved.

Description: Isoprene (C 5 H 8 ) and three volatile organic iodine compounds (VOIs: CH 3 I, C 2 H 5 I, and CH 2 ClI) in surface seawater were measured in the western Arctic, Northwest Pacific, Indian, and Southern oceans during the period 2008–2012. These compounds are believed to play an important role in the marine atmospheric chemistry after their emission. The measurements were performed with high time-resolution (1–6 h intervals) using an online equilibrator gas-chromatography mass-spectrometer. C 5 H 8 was most abundant in high-productivity transitional waters and eutrophic tropical waters. The chlorophyll-a normalized production rates of C 5 H 8 were high in the warm sub-tropical and tropical waters, suggesting the existence of a high emitter of C 5 H 8 in the biological community of the warm waters. High concentrations of the three VOIs in highly productive transitional water were attributed to biological productions. For CH 3 I, the highest concentrations were widely distributed in the basin area of the oligotrophic subtropical NW Pacific, probably due to photochemical production and/or high emission rates from phytoplankton. In contrast, the lowest concentrations of C 2 H 5 I in subtropical waters were attributed to photochemical removal. Enhancement of CH 2 ClI concentrations in the shelf–slope areas of the Chukchi Sea and the transitional waters of the NW Pacific in winter suggested that vertical mixing with subsurface waters by regional upwelling or winter cooling act to increase the CH 2 ClI concentrations in surface layer. Sea–air flux calculations revealed that the fluxes of CH 2 ClI were the highest among the three VOIs in shelf–slope areas; the CH 3 I flux was highest in basin areas. This article is protected by copyright. All rights reserved.

Description: The Intertropical Convergence Zone (ITCZ) is a major source of the surface freshwater input to the tropical open ocean. Under the ITCZ, sea-surface salinity (SSS) fronts that extend zonally across the basins are observed by the Aquarius/SAC-D mission and Argo floats. This study examined the evolution and forcing mechanisms of the SSS fronts. It is found that, although the SSS fronts are sourced from the ITCZ-freshened surface waters, the formation, structure, and propagation of these fronts are governed by the trade-wind driven Ekman processes. Three features characterize the governing role of Ekman forcing. First, the SSS fronts are associated with near-surface salinity minimum zones (SMZs) of 50-80 m deep. The SMZs are formed during December-March when the near-equatorial Ekman convergence zone concurs with an equatorward displaced ITCZ. Second, after the formation, the SMZs are carried poleward away at a speed of ∼3.5 km day −1 by Ekman transport. The monotonic poleward propagation is a sharp contrast to the seasonal north/south oscillation of the ITCZ. Lastly, each SMZ lasts about 12 – 15 months until dissipated at latitudes beyond 10°N/S. The persistence of more than one calendar year allows two SMZs to coexist during the formation season (December – March), with the newly formed SMZ located near the equator while the SMZ that is formed in the previous year located near the latitudes of 10-15° poleward after one year's propagation. The contrast between the ITCZ and SMZ highlights the dominance of Ekman dynamics on the relationship between the SSS and the ocean water cycle. This article is protected by copyright. All rights reserved.

Description: The European Unions' Marine Strategy Framework Directive aims to limit anthropogenic influences in the marine environment. But marine ecosystems are characterised by high variability, and it is not trivial to define its natural state. Here, we use the physical environment as a basis for marine classification, as it determines the conditions in which organisms must operate to survive and thrive locally. We present a delineation of the North Sea into five distinct regimes, based on multi-decadal stratification characteristics. Results are based on a 51-year simulation of the region using the coupled hydrobiogeochemical model GETM-ERSEM-BFM. The five identified regimes are: permanently stratified, seasonally stratified, intermittently stratified, permanently mixed and Region Of Freshwater Influence (ROFI). The areas characterised by these regimes show some interannual variation in geographical coverage, but are overall remarkable stable features within the North Sea. Results also show that 29% of North Sea waters fail to classify as one of the defined stratification regimes, due to high interannual variability. Biological characteristics of these regimes differ from diatom-based food webs in areas with prolonged stratification to Phaeocystis -dominated food webs in areas experiencing short-lived or no stratification. The spatial stability of the identified regimes indicates that carefully selected monitoring locations can be used to represent a substantive area of the North Sea. This article is protected by copyright. All rights reserved.

Description: Material transport and dispersion near the mouth of a tidal inlet (New River Inlet, NC) are investigated using GPS-tracked drifters and numerical models. For ebb tide releases, velocities are largest (〉1 ms −1 ) in 2 approximately 30-m wide channels that bisect the 1-3 m deep ebb shoal. In the channels, drifter and subsurface current meter velocities are similar, consistent with strong vertical mixing and 2D hydrodynamics. Drifters were preferentially entrained in the channelized jets where drifter cluster lateral spreading rates μ in were small ( μ in ≈ 0.5m 2 s −1 ). At the seaward edge of the ebb shoal, jet velocities decrease linearly with distance (to ≤ 0.2 ms −1 , about 1 km from shore), and cluster spreading rates are larger with μ out ≈ 3m 2 s −1 . Although the models COAWST and NearCom generally reproduce the observed trajectory directions, certain observed drifter properties are poorly modeled. For example, modeled mean drifter velocities are smaller than observed, and upon exiting the inlet, observed drifters turn north more than modeled drifters.} The model simulations do reproduce qualitatively the spreading rates observed in the inner inlet, the flow deceleration, and the increase in μ out observed in the outer inlet. However, model spreading rates increase only to μ out  〈 1 m 2 s −1 . Smaller modeled than observed μ out may result from using unstratified models. Non-coincident (in space) observations show evidence of a buoyant plume (Δρ = 1kgm −3 ) in the outer inlet, likely affecting drifter lateral spreading. Generally, drifter based model performance is good within the inlet channels where tidal currents are strongest, whereas model-data differences are significant farther offshore. This article is protected by copyright. All rights reserved.

Description: Time-space varying uncertainty maps of monthly mean Arctic summer ice drift are presented. To assess the error statistics of two low-resolution Eulerian ice drift products, we use high-resolution Lagrangian ice motion derived from synthetic aperture radar (SAR) imagery. The Lagrangian trajectories from the SAR data are converted to an Eulerian format to serve as reference for the error assessment of the Eulerian products. The statistical error associated with the conversion is suppressed to an acceptable level by applying a threshold for averaging. By using the SAR ice drift as a reference, we formulate the uncertainty of monthly mean ice drift as an empirical function of drift speed and ice concentration. The empirical functions are applied to derive uncertainty maps of Arctic ice drift fields. The estimated uncertainty maps reasonably capture an increase of uncertainty with the progress of summer melting season. The uncertainties range from 1.0 cm s −1 to 2.0 cm s −1 , which indicates that the low-resolution Eulerian products for summer seasons are of practical use for climate studies, model validation and data assimilation, if their uncertainties are appropriately taken into account. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Twelve-year satellite observations between 2002 and 2013 from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the satellite Aqua are used to quantitatively assess the water property changes in the Aral Sea. The shortwave infrared (SWIR) atmospheric correction algorithm is required and used to derive normalized water-leaving radiance spectra nL w ( λ ) in the Aral Sea. We used radiance ratio nL w (555)/ nL w (443) as a surrogate to characterize the spatial and temporal variations of chlorophyll-a (Chl-a) in the Aral Sea. Both seasonal variability and significant interannual changes were observed when the Aral Sea desiccated between 2002 and 2013. All three separated regions of the Aral Sea show increased nL w (555)/ nL w (443) ratio (a surrogate for Chl-a) and the diffuse attenuation coefficient at the wavelength of 490 nm ( K d (490)) during the fall season. Of the three regions, the North Aral Sea has had the least interannual variability, while South-East (SE) Aral Sea experienced drastic changes. Waters in the SE Aral Sea are the most turbid with significantly higher K d (490) than those in the other two sub-regions. K d (490) gradually increased from ∼2 m −1 in 2002 to ∼3.5 m −1 after 2008 in the SE Aral Sea. In comparison, both radiance ratio nL w (555)/ nL w (443) and K d (490) were relatively stable for the North Aral Sea. In the South-West (SW) Aral Sea, however, nL w (555)/ nL w (443) values reached peaks in the fall of 2007 and 2010. A possible link between the Aral Sea water property change and the regional climate variation is also discussed. This article is protected by copyright. All rights reserved.

Description: Global and regional probability density functions and higher statistical moments are analyzed for anomalies of the surface geostrophic velocity components inferred from the 3-year Jason-1 TOPEX/POSEIDON Tandem mission and for sea level anomalies (SLA) observed through the TOPEX/POSEIDON, Jason-1 and 2 altimetric missions, together covering a 19-year period. Results are compared with those obtained from the AVISO 19 year, 1/3° gridded SLA space-time objective analysis and associated geostrophic velocity anomalies. The study reveals that eddy variability appears to be Gaussian over most parts of the ocean, outside the influence of energetic current systems, and that specific flow regimes in the ocean can be identified through higher statistical moments of the flow field and SLA observations. However, the moment-ratio diagrams of skewness and kurtosis reveal that in energetic boundary currents the ocean does not follow Gaussian statistics, but rather behaves like an exponential distribution. Higher statistical moments of SLA and velocity anomalies do vary seasonally and thereby provide valuable information about the seasonal changes of the oceans' flow field. This article is protected by copyright. All rights reserved.

Description: Offshore-penetrating tongues of coastal water have been frequently observed during the downwelling-favorable monsoon season at specific locations in waters off the Min-Zhe Coast, a region influenced by a buoyant coastal current originating from the Changjiang River. This process plays an important role in cross-shelf material exchange in the East China Sea (ECS), but the underlying mechanisms are not fully understood. This study suggests that the penetrating fronts are the response of buoyant coastal water to along-isobath undulation of the ambient pycnocline that is controlled by the temperature stratification in seawater. When the ambient pycnocline descends sharply in the downshelf direction, coastal water is transported offshore due to the joint effect of baroclinicity and relief (JEBAR), and thus generates a penetrating front. Along-isobath pycnocline undulation in the ECS can arise from non-uniform tidal mixing due to tidal wave divergence off the Min-Zhe Coast. On-shelf intrusion of cold and dense Kuroshio subsurface water prevents thorough mixing of the pycnocline. Different from the common cross-shelf transport phenomena induced by winds or frontal instabilities, such a tidal mechanism should produce penetrating fronts at specific locations, in agreement with observations. This article is protected by copyright. All rights reserved.

Description: A two-week field experiment investigated the hydrodynamics of a strongly tidally-forced tropical intertidal reef platform in the Kimberley region of northwestern Australia, where the spring tidal range exceeds 8 m. At this site, the flat and wide (∼1.4 km) reef platform is located slightly above mean sea level, such that during low tide the offshore water level can fall 4 m below the platform. While the reef always remained submerged over each tidal cycle, there were dramatic asymmetries in both the water levels and velocities on the reef, i.e., the flood duration lasted only ∼2 hr versus ∼10 hr for the ebb. These dynamics were investigated using a one-dimensional numerical model (SWASH) to solve the nonlinear shallow water equations with rapid (sub- to super-critical) flow transitions. The numerical model revealed that as water drains off the reef, a critical flow point was established near the reef edge prior to the water discharging down the steep forereef. Despite this hydraulic control, bottom friction on the reef was still found to make a far greater contribution to elevating water levels on the reef platform and keeping it submerged over each tidal cycle. Finally, a simple analytical model more broadly shows how water levels on intertidal reef platforms functionally depend on properties of reef morphology, bottom roughness, and tidal conditions, revealing a set of parameters (a reef draining time-scale and friction parameter) that can be used to quantify how the water depth will fall on a reef during ebb tide. This article is protected by copyright. All rights reserved.

Description: The Nordic Seas have been assumed to be a net sink of the ozone-depleting greenhouse gas N 2 O; however, few studies have been conducted in this region. The N 2 O profile data obtained during the 5th Chinese National Arctic Research Expedition demonstrate that the N 2 O distribution pattern in the Nordic Seas differs from that of most other oceans. The N 2 O sink characteristics of this region are confirmed by the undersaturation of N 2 O in the water column. The distributions of N 2 O in three subbasins of the Nordic Seas vary in the upper 1000 m but are homogenous below 1000 m due to a shared origin in the Greenland Basin (GB). Air-sea exchange and vertical convection are thought to be the dominant factors in the N 2 O distribution in the GB, resulting in a distribution pattern that correlates significantly with the atmospheric mixing ratio variation over the past 40 years. Although recent studies have shown that weakened convection and/or enhanced Arctic outflow below the mid-depth has occurred, our results show that these variations have yet to significantly affect the above relationship. The distribution could be considered a “historical record” that can be used to evaluate the air-to-sea flux over the past 40 years in the GB. The annual amount of N 2 O absorbed by the GB is ∼0.016-0.029 Tg N, which is equal to 0.4-0.8% of the world ocean emissions. This amount should not be simply neglected because it is absorbed by a region whose area accounts for only 0.03% of the world ocean area. This article is protected by copyright. All rights reserved.

Description: An improved understanding of the temporal variability and the spatial distribution of snowmelt on Antarctic sea ice is crucial to better quantify atmosphere-ice-ocean interactions, in particular sea-ice mass and energy budgets. It is therefore important to understand the mechanisms that drive snowmelt, both at different times of the year and in different regions around Antarctica. In this study, we combine diurnal brightness temperature differences (dT B (37GHz)) and ratios (T B (19GHz)/T B (37GHz)) to detect and classify snowmelt processes. We distinguish temporary snowmelt from continuous snowmelt to characterize dominant melt patterns for different Antarctic sea ice regions from 1988/89 to 2014/15. Our results indicate four characteristic melt types. On average, 38.9±6.0% of all detected melt events are diurnal freeze-thaw cycles in the surface snow layer, characteristic of temporary melt (Type A). Less than 2% reveal immediate continuous snowmelt throughout the snowpack, i.e. strong melt over a period of several days (Type B). In 11.7±4.0%, Type A and B take place consecutively (Type C), and for 47.8±6.8% no surface melt is observed at all (Type D). Continuous snowmelt is primarily observed in the outflow of the Weddell Gyre and in the northern Ross Sea, usually 17 days after the onset of temporary melt. Comparisons with Snow Buoy data suggest that also the onset of continuous snowmelt does not translate into changes in snow depth for a longer period but might rather affect the internal stratigraphy and density structure of the snowpack. Considering the entire data set, the timing of snowmelt processes does not show significant temporal trends. This article is protected by copyright. All rights reserved.

Description: A large hypoxic zone forms every summer on the Texas-Louisiana Shelf in the northern Gulf of Mexico due to nutrient and freshwater inputs from the Mississippi/Atchafalaya River System. Efforts are underway to reduce the extent of hypoxic conditions through reductions in river nutrient inputs, but the response of hypoxia to such nutrient load reductions is difficult to predict because biological responses are confounded by variability in physical processes. The objective of this study is to identify the major physical model aspects that matter for hypoxia simulation and prediction. In order to do so we compare three different circulation models (ROMS, FVCOM and NCOM) implemented for the northern Gulf of Mexico, all coupled to the same simple oxygen model, with observations and against each other. By using a highly simplified oxygen model we eliminate the potentially confounding effects of a full biogeochemical model and can isolate the effects of physical features. In a systematic assessment we found that 1) model-to-model differences in bottom water temperatures result in differences in simulated hypoxia because temperature influences the uptake rate of oxygen by the sediments (an important oxygen sink in this system), 2) vertical stratification does not explain model-to-model differences in hypoxic conditions in a straightforward way, and 3) the thickness of the bottom boundary layer, which sets the thickness of the hypoxic layer in all three models, is key to determining the likelihood of a model to generate hypoxic conditions. These results imply that hypoxic area, the commonly used metric in the northern Gulf which ignores hypoxic layer thickness, is insufficient for assessing a model's ability to accurately simulate hypoxia, and that hypoxic volume needs to be considered as well. This article is protected by copyright. All rights reserved.

Description: Over the California Current upwelling system in summer, the prevailing upwelling-favorable winds episodically weaken (relax) or reverse direction for a few days. Near Pt. Conception, California, the wind usually does not reverse, but wind relaxation allows poleward oceanic coastal flow with ecological consequences. To determine the offshore extent and synoptic forcing of these wind relaxations, we formed composite averages of wind stress from the QuikSCAT satellite and atmospheric pressure from the North American Regional Reanalysis (NARR) using 67 wind relaxations during summer 2000–2009. Wind relaxations at Pt. Conception are the third stage of an event sequence that repeatedly affects the west coast of North America in summer. First, 5–7 dy before the wind weakens near Pt. Conception, the wind weakens or reverses off Oregon and northern California. Second, the upwelling-favorable wind intensifies along central California. Third, the wind relaxes at Pt. Conception, and the area of weakened winds extends poleward to northern California over 3–5 dy. The NARR underestimates the wind stress within ∼200 km of coastal capes by a factor of 2. Wind relaxations at Pt. Conception are caused by offshore extension of the desert heat low. This synoptic forcing is related to event cycles that cause wind reversal as in Halliwell and Allen [1987] and Mass and Bond [1996], but includes weaker events. The wind relaxations extend ∼600 km offshore, similarly to the California-scale hydraulic expansion fan shaping the prevailing winds, and ∼1000 km alongshore, limited by an opposing pressure gradient force at Cape Mendocino. This article is protected by copyright. All rights reserved.

Description: A three dimensional velocity field constructed from Argo observations and sea surface heights (called Argo & SSH, hereinafter) is used to estimate meridional overturning volume transport and meridional heat transport (MHT) across 20°S, 25°S, 30°S, and 35°S for the years 2000 to 2014 in the South Atlantic. Volume transport in the upper branch of Meridional Overturning Circulation (MOC) and MHT from the observations are consistent with the previous observations, but are higher than the estimates derived from three data assimilative ocean models, at some of the latitudes. Both the observations and models show strong correlations between the strength of MOC and MHT at all the latitudes. The corresponding change in MHT for 1 Sv change of MOC strength, in the observations, increases from 0.046 PW in 25°S, 30°S and 35°S to 0.056 PW across 20°S. A comparison of model-based transports at 35°S at the boundaries and in the interior with those from Argo & SSH shows significant differences between them with respect to the contributions in the three segments of the section. In addition, the contributions also vary greatly between the different models. An analysis of the seasonality of MOC in the models and in the observations reveals that MOC anomalies in the models mostly show strong annual cycles at all the latitudes, whereas those derived from Argo & SSH exhibit annual cycles at three latitudes (35°S, 30°S and to a lesser extent at 25°S) and a semiannual cycle at 20°S This article is protected by copyright. All rights reserved.

Description: Reconstruction of historical Arctic sea level is generally difficult due to the limited coverage and quality of both tide gauge and altimetry data in the area. Here a strategy to achieve a stable and plausible reconstruction of Arctic sea level from 1950 to today is presented. This work is based on the combination of tide gauge records and a new 20-year reprocessed satellite altimetry derived sea level pattern. Hence the study is limited to the area covered by satellite altimetry (68ºN and 82ºN). It is found that timestep cumulative reconstruction as suggested by Church and White (2000) may yield widely variable results and is difficult to stabilize due to the many gaps in both tide gauge and satellite data. A more robust sea level reconstruction approach is to use datum adjustment of the tide gauges in combination with satellite altimetry, as described by (Ray and Douglas, 2011). In this approach, a datum-fit of each tide gauges is used and the method takes into account the entirety of each tide gauge record. This makes the Arctic sea level reconstruction much less prone to drifting. From our reconstruction, we found that the Arctic mean sea level trend is around 1.5 mm +/- 0.3 mm/y for the period 1950 to 2010, between 68ºN and 82ºN. This value is in good agreement with the global mean trend of 1.8 +/- 0.3 mm/y over the same period as found by Church and White (2004). This article is protected by copyright. All rights reserved.

Description: Often described as oligotrophic, the west Florida continental shelf supports abundant fisheries, experiences blooms of the harmful algae, Karenia brevis and exhibits subsurface chlorophyll maxima evident in shipboard and glider surveys. Renewal of inorganic nutrients by the upwelling of deeper ocean water onto the shelf may account for this, but what are the origins and pathways by which such new water may broach the shelf break and advance toward the shoreline? We address these questions via numerical model simulations of pseudo-Lagrangian, isopycnic water parcel trajectories. Focus is on 2010, when the west Florida shelf was subjected to an anomalously protracted period of upwelling caused by Gulf of Mexico Loop Current interactions with the shelf slope. Origins and pathways are determined by integrating trajectories over successive 45 day intervals, beginning from different locations along the shelf break and at various locations and depths along the shelf slope. Waters upwelling across the shelf break are found to originate from relatively shallow depths along the shelf slope. Even for the anomalous 2010 year, much of this upwelling occurs from about 150 m and above, although waters may broach the shelf break from 300 m depth, particularly in the Florida Panhandle. Such inter-annual renewal of west Florida shelf waters appears to have profound effects on west Florida shelf ecology. This article is protected by copyright. All rights reserved.

Description: Anomalous sea surface temperature (SST) cooling south of Java, initiated during May-July, is an important precursor to positive Indian Ocean Dipole (pIOD) events. As shown previously, the Java SST anomalies are spatially and temporally coincident with seasonal upwelling induced locally by southeasterly trade winds. However, we confirm earlier findings that interannual variability of the Java cooling is primarily driven by remote wind forcing from coastal Sumatra and the equatorial Indian Ocean (EqIO); we also find an inuence from winds along the Indonesian Throughow. The wind forcing in the EqIO and along coastal Sumatra does not initiate SST cooling locally due to a deep thermocline and thick barrier layer, but can force upwelling Kelvin waves that induce substantial surface cooling once they reach the seasonally shallower thermocline near the coast of Java. Satellite altimetry is used to obtain a Kelvin wave coefficient that approximates Kelvin wave amplitude variations along the equator. All pIOD years in the satellite record have anomalous levels of upwelling Kelvin wave activity along the equator during April-June, suggesting that upwelling waves during this season are necessary for pIOD event development. However, a change to wind-forced downwelling Kelvin waves during July-August can abruptly terminate cool Java SST anomalies and weaken the pIOD event. Upwelling Kelvin wave activity along the equator and wind stress anomalies west of Sumatra are both robust predictors of the IOD index later in the calendar year, while values of the Kelvin wave coefficient are the most reliable predictor of pIOD events specifically. This article is protected by copyright. All rights reserved.

Description: We reconstructed changes in biogenic opal export productivity (BOEP) in the southern Bering Sea (BS) over the last 4.3Ma, based on mass accumulation rate (MAR) of biogenic opal from Integrated Ocean Drilling Program (IODP) Site U1340. The results show that the BOEP in the BS was high and variable between ∼4.3Ma and ∼1.9Ma, extremely low and relatively stable from ∼1.9Ma to ∼1.1Ma, and then fluctuated frequently (generally high during interglacials and low during glacials) during the last ∼1.1Ma. One interval of enhanced BOEP from 4.3Ma∼3.2Ma is a response to the Late Miocene–Early Pliocene “Biogenic Bloom Event”. Another interval from 2.8Ma∼1.9Ma correlates with global opal burial shifting from high-latitude oceans to upwelling-influenced regions following the intensification of the Northern Hemisphere Glaciation (NHG). Whereas, the increase in BS opal export productivity during the last 1.1Ma tends to be a “local” phenomenon. Overall, the BOEP shows a similar trend and good correspondence to the input of the Alaskan Stream (AS), which can be traced using the Na 2 O/K 2 O ratio. We thus conclude that the AS may be the direct, and primary factor on BOEP variability in the BS during the last ∼4.3Ma. In addition, although the poor correlation between opal MAR and volcanic glass suggests that BOEP variability was not controlled by long-term variations in the volcanism or ash abundance, increased ash abundance indicated by high contents of volcanic glasses was also a possible reason for enhanced BOEP during the period from ∼4.3Ma to ∼3.2Ma and the last ∼0.5Ma. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Field observations collected in Chesapeake Bay demonstrate how wind-driven circulation interacts with estuarine bathymetry to control when and where the vertical mixing of dissolved oxygen occurs. In the across-Bay direction, the lateral Ekman response to along-Bay wind forcing contributes to the vertical mixing of dissolved oxygen in two ways. First, the lateral tilting of the pycnocline/oxycline, consistent with the thermal wind relationship, advects the region of high vertical gradient into the surface and bottom boundary layers where mixing can occur. Secondly, upwelling of low oxygen water to the surface enhances the atmospheric influx. In the along-Bay direction, the abrupt change in bottom depth associated with Rappahannock Shoal results in surface convergence and downwelling, leading to localized vertical mixing. Water that is mixed on the shoal is entrained into the up-Bay residual bottom flow resulting in increases in bottom dissolved oxygen that propagate up the system. The increases in dissolved oxygen are often associated with increases in temperature and decreases in salinity, consistent with vertical mixing. However, the lagged arrival moving northward suggests that the propagation of this signal up the Bay is due to advection. This article is protected by copyright. All rights reserved.

Description: Gulf of Mexico Loop Current (LC) interactions with the West Florida Shelf (WFS) slope play an important role in shelf ecology through the upwelling of new inorganic nutrients across the shelf break. This is particularly the case when the LC impinges upon the shelf slope in the southwest portion of the WFS near the Dry Tortugas. By contacting shallow water isobaths at this “pressure point” the LC forcing sets the entire shelf into motion. Characteristic patterns of LC interactions with the WFS and their occurrences are identified using unsupervised neural network, Self-Organizing Map, from 23 years (1993 – 2015) of altimetry data. The duration of the occurrences of such LC patterns is used as an indicator of offshore forcing of anomalous upwelling. Consistency is found between the altimetry-derived offshore forcing and the occurrence and severity of WFS coastal blooms of the toxic dinoflagellate, Karenia brevis : years without major blooms tend to have prolonged LC contact at the “pressure point,” whereas years with major blooms tend not to have prolonged offshore forcing. Resetting the nutrient state of the shelf by the coastal ocean circulation in response to deep-ocean forcing demonstrates the importance of physical oceanography in shelf ecology. A satellite altimetry-derived seasonal predictor for major K. brevis blooms is also proposed. This article is protected by copyright. All rights reserved.

Description: A comprehensive set of autonomous, ice-ocean measurements were collected across the Canada Basin to study the summer evolution of the ice-ocean boundary layer (IOBL) and ocean mixed layer (OML). Evaluation of local heat and freshwater balances and associated turbulent forcing reveals that melt ponds (MP's) strongly influence the summer IOBL-OML evolution. Areal expansion of MP's in mid-June start the upper ocean evolution resulting in significant increases to ocean absorbed radiative flux (19 Wm −2 in this study). Buoyancy provided by MP drainage shoals and freshens the IOBL resulting in a 39 MJm −2 increase in heat storage in just 19 days (52% of the summer total). Following MP drainage, a near-surface fresh layer deepens through shear-forced mixing to form the summer mixed layer (sML). In late summer, basal melt increases due to stronger turbulent mixing in the thin sML and the expansion of open water areas due in part to wind forced divergence of the sea ice. Thermal heterogeneities in the marginal ice zone (MIZ) upper ocean led to large ocean-to-ice heat fluxes (100-200 Wm −2 ) and enhanced basal ice melt (3-6 cm-day −1 ), well away from the ice edge. Calculation of the upper ocean heat budget show that local radiative heat input accounted for at least 89% of the observed latent heat losses and heat storage (partitioned 0.77/0.23). These results suggest that the extensive area of deteriorating sea ice observed away from the ice edge during the 2014 season, termed the “thermodynamically forced MIZ,” was driven primarily by local radiative forcing. This article is protected by copyright. All rights reserved.

Description: The Antarctic continental shelf supports a high level of marine primary productivity and is a globally important carbon dioxide (CO 2 ) sink through the photosynthetic fixation of CO 2 via the biological pump. Sustaining such high productivity requires a large supply of the essential micronutrient iron (Fe); however, the pathways for Fe delivery to these zones vary spatially and temporally. Our study is the first to report a previously unquantified source of concentrated bio-available Fe to Antarctic surface waters. We hypothesize that Fe derived from subglacial processes is delivered to euphotic waters through the accretion (Fe storage) and subsequent melting (Fe release) of a marine-accreted layer of ice at the base of the Amery Ice Shelf (AIS). Using satellite-derived Chlorophyll-a data, we show that the soluble Fe supplied by the melting of the marine ice layer is an order of magnitude larger than the required Fe necessary to sustain the large annual phytoplankton bloom in Prydz Bay. Our finding of high concentrations of Fe in AIS marine ice and recent data on increasing rates of ice shelf basal melt in many of Antarctica's ice shelves [ Paolo et al ., 2015] should encourage further research into glacial and marine sediment transport beneath ice shelves and their sensitivity to current changes in basal melt. Currently, the distribution, volume and Fe concentration of Antarctic marine ice is poorly constrained. This uncertainty, combined with variable forecasts of increased rates of ice shelf basal melt, limits our ability to predict future Fe supply to Antarctic coastal waters. This article is protected by copyright. All rights reserved.

Description: The California Undercurrent transports Pacific Equatorial Water (PEW) into the Southern California Bight from the eastern tropical Pacific Ocean. PEW is characterized by higher temperatures and salinities, with lower pH, representing a source of potentially corrosive (aragonite, Ω〈1) water to the region. We use ichthyoplankton assemblages near the cores of the California Current and the California Undercurrent to determine whether PEW influenced fish diversity. We use hydrographic data to characterize the inter-annual and seasonal variability of estimated pH and aragonite saturation with depth. Although there is substantial variability in PEW presence as measured by spice on the 26.25-26.75 isopycnal layer, as well as in pH and aragonite staturation, we found fish diversity to be stable over the decades 1985-1996 and 1999-2011. We detected significant difference in species structure during the 1998 La Niña period, due to reduced species evenness. Species richness due to rare species was higher during the 1997/98 El Niño compared to the La Niña but the effect on species structure was undetectable. Lack of difference in the species abundance structure in the decade before and after the 1997/99 ENSO event showed that the assemblage reverted to its former structure following the ENSO perturbation, indicating resilience While the inter-decadal species structure remained stable, the long tail of the distributions shows that species richness increased between the decades consistent with intrusion of warm water with more diverse assemblages into the southern California region. This article is protected by copyright. All rights reserved.

Description: Early ice retreat and ocean warming are changing various facets of the Arctic marine ecosystem, including the biogeographic distribution of marine organisms. Here, an endemic copepod species, Calanus glacialis, was used as a model organism, to understand how and why Arctic marine environmental changes may induce biogeographic boundary shifts. A copepod individual-based model was coupled to an ice-ocean-ecosystem model to simulate temperature- and food-dependent copepod life history development. Numerical experiments were conducted for two contrasting years: a relatively cold and normal sea ice year (2001) and a well-known warm year with early ice retreat (2007). Model results agreed with commonly known biogeographic distributions of C. glacialis , which is a shelf/slope species and cannot colonize the vast majority of the central Arctic basins. Individuals along the northern boundaries of this species' distribution were most susceptible to reproduction timing and early food availability (released sea ice algae). In the Beaufort, Chukchi, East Siberian, and Laptev Seas where severe ocean warming and loss of sea ice occurred in summer 2007, relatively early ice retreat, elevated ocean temperature (about 1-2°C higher than 2001), increased phytoplankton food, and prolonged growth season created favorable conditions for C. glacialis development and caused a remarkable poleward expansion of its distribution. From a pan-Arctic perspective, despite the great heterogeneity in the temperature and food regimes, common biogeographic zones were identified from model simulations, thus allowing a better characterization of habitats and prediction of potential future biogeographic boundary shifts. This article is protected by copyright. All rights reserved.

Description: Inertial Oscillations are a ubiquitous feature of the surface ocean. Here we combine new observations with a numerical model to investigate the role of inertial oscillations in driving deepening of the surface mixed layer in a seasonally stratified sea. Observations of temperature and current structure, from a mooring in the Western Irish Sea, reveal episodes of strong currents (〉0.3ms −1 ) lasting several days, resulting in enhanced shear across the thermocline. Whilst the episodes of strong currents are coincident with windy periods, the variance in the shear is not directly related to the wind stress. The shear varies on a sub-inertial timescale with the formation of shear maxima lasting several hours occurring at the local inertial period of 14.85h. These shear maxima coincide with the orientation of the surface current being at an angle of approximately 90° to the right of the wind direction. Observations of the water column structure during windy periods reveal deepening of the surface mixed layer in a series of steps which coincide with a period of enhanced shear. During the periods of enhanced shear gradient Richardson number estimates indicate Ri −1 ≥4 at the base of the surface mixed layer, implying the deepening as a result of shear instability. A one-dimensional vertical exchange model successfully reproduces the magnitude and phase of the shear spikes as well as the step like deepening. The observations and model results therefore identify the role of wind-shear alignment as a key entrainment mechanism driving surface mixed layer deepening in a shallow, seasonally stratified sea. This article is protected by copyright. All rights reserved.

Description: To better understand the statistical and theoretical characteristics of nonlinear internal waves (NLIWs) in the broad continental shelf of the northeastern East China Sea (ECS), historical hydrographic data collected over 50 years between 1962 and 2011 are analyzed to calculate monthly climatology. Based on KdV and extended KdV models under the two-layer approximation (i.e., mode-1 NLIWs), the monthly climatology for propagating speed and characteristic width is constructed, ranging from 0.8 to 1.2 m s −1 and from O(10 2 ) to O(10 3 ) m, respectively. The result is consistent with a few previous in situ observations in the region. When NLIWs originating in the southeastern slope area approach the shallower regime (northwestward propagation), they propagate more slowly with neither break nor extinction, but with a shorter width, since both the Iribarren and Ostrovsky numbers are small ( I r ≪ 0.45 and O s ≪ 1, respectively). Limitations of the two-layered KdV-type models are discussed (e.g., an importance of mode-2 waves) in the context of occasional extension of the low-salinity Changjiang Discharged Water onto the area, which implies distinct effects on the kinematic parameters of NLIWs in the ECS. This article is protected by copyright. All rights reserved.

Description: Using the new high-resolution operational model of ECMWF, we revisit the storm during which the Draupner freak wave of January 1, 1995 was recorded. The modeling system gives a realistic evolution of the storm highlighting the crucial role played by the southward propagating polar low in creating the extreme wave conditions present at the time the freak wave was recorded. We also discuss the predictability of the meteorological event. The hindcast wave spectra allow a new analysis of the probability of occurrence of the Draupner wave that we analyze not only in time at a specific position, but also in space. This leads us to discuss how exceptional the so-called freak waves really are. For a given sea state, as characterized by the significant wave height, they are namely part of the reality of the ocean, the key point being the probability of encountering them. In this respect, the often considered record at a specific location can be misleading because the probability of detecting a freak wave must be considered both in space and time. This article is protected by copyright. All rights reserved.

Description: In limited-area ocean models, open boundary conditions (OBCs) often create dynamic inconsistencies and perform poorly in resolving tidal or subtidal flow when both forces exist. Orlanski-type radiation OBCs are reasonably efficient at treating the subtidally forced flow, and Flather-type OBCs are commonly adapted for the tidally forced flow. However, neither of them performs well when tidal and subtidal forces simultaneously drive the flows. We have developed a novel OBC that integrates the active OBC in Gan and Allen [2005] and a Flather-type OBC. This new OBC accommodates the concurrent Tidal and Subtidal (TST) forcing, and the respective tidal or subtidal forcing, at the open boundary of a limited-area model. This new TST-OBC treats the tidal component with a Flather-type OBC, and it separates subtidal barotropic and baroclinic components into local (forced) and global (unforced) components. Then an unforced Orlanski-type OBC can be applied to the global part. We applied the TST-OBC to all model variables to reduce dynamic inconsistence. Using the Regional Ocean Modeling System, we applied the TST-OBC to the shallow East China Sea shelf where strong tidal and subtidal forces over complex topography govern the circulation. Our numerical experiments and analyses suggest that the TST-OBC was robust for both concurrent tidal-subtidal forcing and solely tidal or subtidal forcing at the open boundary. It reduced spurious energy reflection, and, overall, it performed better than an Orlanski-type or Flather-type OBC in reproducing realistic tidal and subtidal shelf circulation. This article is protected by copyright. All rights reserved.

Description: Global M 2 tidal surface currents are predicted using a global baroclinic ocean model with horizontal grid spacing of 1/12° and 19 z-levels in the vertical. After first showing the predicted tidal elevations are in reasonable agreement with observations made by bottom pressure recorders and altimeters, the predicted tidal surface currents are evaluated by comparing them with independent estimates based on observed drifter trajectories. Both predicted and observed tidal surface currents can exceed 0.1 m s – 1 in the deep ocean. Internal tides are shown to make a significant contribution to the predicted tidal surface currents. Phase locking of the surface and internal tides causes spatial changes in the predicted tidal surface currents that vary with approximately the same wavenumber as that of the lowest mode internal tide. Qualitatively similar, small-scale variations are also detected in the observed estimates but the variations do not line up exactly with the predictions. Possible explanations for the mismatch are given. The seasonal variation of M 2 tidal surface currents, and the energy conversion rate from surface to internal tides, is also predicted by initializing, and restoring, the model to an observed seasonal climatology of temperature and salinity. Compared to tidal elevation, the seasonal change of tidal surface current can be large (order 10% for each hemisphere). It is caused by seasonal variations in the vertical structure of the baroclinic modes and the energy conversion rate. In the vicinity of major bathymetric features, the seasonal variation of second and higher order modes can be much larger (up to 50%). This article is protected by copyright. All rights reserved.

Description: Seismic Oceanography is a developing research topic where new acoustic methods allow high-resolution teledetection of the thermohaline structure of the ocean. First implementations to study the Ocean Surface Boundary Layer have recently been achieved but remain very challenging due to the weakness and shallowness of such seismic reflectors. In this article, we develop a multifrequency seismic analysis of hydrographic datasets collected in a seasonally stratified midlatitude shelf by ARGO network floats to assess the detectability issue of shallow thermoclines. This analysis, for which sensitivity to the data reduction scheme used by ARGO floats for the transmission of the profiles is discussed, allows characterizing both the depth location and the frequency dependency of the dominant reflective feature of such complex structures. This approach provides the first statistical distribution of the range of variability of the frequency-dependent seismic reflection amplitude of the midlatitude seasonal thermoclines. We introduce a new parameter to quantify the overall capability of a multichannel seismic setup, including the source strength, the fold and the ambient noise level, to detect shallow thermoclines. Seismic source signals are approximated by Ricker wavelets, providing quantitative guidelines to help in the design of seismic experiments targeting such oceanic reflectors. For shallow midlatitude seasonal thermoclines, we show that the detectability is optimal for seismic peak frequencies between 200 and 400 Hz: this means that airgun and Sparker sources are not well suited and that significant improvements of source devices will be necessary before seismic imaging of OSBL structures can be reliably attempted. This article is protected by copyright. All rights reserved.

Description: Turbulent mixing and energy dissipation have important roles in the global circulation but are not resolved by ocean models. We use direct numerical simulations of a geostrophic circulation, resolving turbulence and convection, to examine the rates of dissipation and mixing. As a starting point, we focus on circulation in a rotating rectangular basin forced by a surface temperature difference but no wind stress. Emphasis is on the geostrophic regime for the horizontal circulation, but also on the case of strong buoyancy forcing (large Rayleigh number), which implies a turbulent convective boundary layer. The computed results are consistent with existing scaling theory that predicts dynamics and heat transport dependent on the relative thicknesses of thermal and Ekman boundary layers, hence on the relative roles of buoyancy and rotation. Scaling theory is extended to describe the volume-integrated rate of mixing, which is proportional to heat transport and decreases with increasing rotation rate or decreasing temperature difference. In contrast, viscous dissipation depends crucially on whether the thermal boundary layer is laminar or turbulent, with no direct Coriolis effect on the turbulence unless rotation is extremely strong. For strong forcing, in the geostrophic regime, the mechanical energy input from buoyancy goes primarily into mixing rather than dissipation. For a buoyancy-driven circulation in a basin comparable to the North Atlantic we estimate that the total rate of mixing accounts for over $95$\% of the mechanical energy supply, implying that buoyancy is an efficient driver of mixing in the oceans. This article is protected by copyright. All rights reserved.

Description: Detailed measurements are presented of velocities and turbulence under a large-scale regular plunging breaking wave in a wave flume. Measurements were obtained at 12 cross-shore locations around a mobile medium-sand breaker bar. They focused particularly on the dynamics of the wave bottom boundary layer (WBL) and near-bed turbulent kinetic energy (TKE), measured with an Acoustic Concentration and Velocity Profiler (ACVP). The breaking process and outer-flow hydrodynamics are in agreement with previous laboratory and field observations of plunging waves, including a strong undertow in the bar trough region. The WBL thickness matches with previous studies at locations offshore from the bar crest, but it increases near the breaking-wave plunge point. This relates possibly to breaking-induced TKE or to the diverging flow at the shoreward slope of the bar. Outer-flow TKE is dominated by wave breaking and exhibits strong spatial variation with largest TKE above the breaker bar crest. Below the plunge point, breaking-induced turbulence invades the WBL during both crest and trough half-cycle. This results in an increase in the time-averaged TKE in the WBL (with a factor 3) and an increase in peak onshore and offshore near-bed Reynolds stresses (with a factor 2) from shoaling to breaking region. A fraction of locally-produced TKE is advected offshore over a distance of a few meters to shoaling locations during the wave trough phase, and travels back onshore during the crest half-cycle. The results imply that breaking-induced turbulence, for large-scale conditions, may significantly affect near-bed sediment transport processes. This article is protected by copyright. All rights reserved.

Description: Internal tides in the Middle Atlantic Bight region are found to be noticeably influenced by the presence of the shelfbreak front and the Gulf Stream, using a combination of observations, equations, and data-driven model simulations. To identify the dominant interactions of these waves with subtidal flows, vertical-mode momentum and energy partial differential equations are derived for small-amplitude waves in a horizontally- and vertically-sheared mean flow and in a horizontally- and vertically-variable density field. First, the energy balances are examined in idealized simulations with mode-1 internal tides propagating across and along the Gulf Stream. Next, the fully-nonlinear dynamics of regional tide-mean-flow interactions are simulated with a primitive-equation model, which incorporates realistic summer-mesoscale features and atmospheric forcing. The shelfbreak front, which has horizontally-variable stratification, decreases topographic internal-tide generation by about 10% and alters the wavelengths and arrival times of locally-generated mode-1 internal tides on the shelf and in the abyss. The (sub)-mesoscale variability at the front and on the shelf, as well as the summer stratification itself, also alter internal-tide propagation. The Gulf Stream produces anomalous regions of (20 mW m −2 ) mode-1 internal-tide energy-flux divergence, which are explained by tide-mean-flow terms in the mode-1 energy balance. Advection explains most tide-mean-flow interaction, suggesting that geometric wave theory explains mode-1 reflection and refraction at the Gulf Stream. Geometric theory predicts that offshore-propagating mode-1 internal tides that strike the Gulf Stream at oblique angles (more than thirty degrees from normal) are reflected back to the coastal ocean, preventing their radiation into the central North Atlantic. This article is protected by copyright. All rights reserved.

Description: The hypothesis that the impingement of mesoscale eddy flows on small-scale topography regulates diapycnal mixing and meridional overturning across the deep Southern Ocean is assessed in an idealised model. The model simulates an eddying circumpolar current coupled to a double-celled meridional overturning with properties broadly resembling those of the Southern Ocean circulation, and represents lee wave-induced diapycnal mixing using an online formulation grounded on wave radiation theory. The diapycnal mixing generated by the simulated eddy field is found to play a major role in sustaining the lower overturning cell in the model, and to underpin a significant sensitivity of this cell to wind forcing. The vertical structure of lower overturning is set by mesoscale eddies, which propagate the effects of near-bottom diapycnal mixing by displacing isopycnals vertically. This article is protected by copyright. All rights reserved.

Description: This study compiles co-located oceanic observations of high-resolution vertical profiles of nitrate concentration and turbulent microstructure around the Svalbard shelf slope, covering both the permanently ice-free Fram Strait and the pack ice north of Svalbard. The authors present an overview over the seasonal evolution of the distribution of nitrate and its relation to upper ocean stratification. The average upward turbulent diffusive nitrate flux across the seasonal nitracline during the Arctic summer season is derived, with average values of 0.3 and 0.7 mmolm −2 ,d −1 for stations with and without ice cover, respectively. The increase under ice-free conditions is attributed to different patterns of stratification under sea ice versus open water. The nitrate flux obtained from microstructure measurements lacked a seasonal signal. However, bottle incubations indicate that August nitrate uptake was reduced by more than an order of magnitude relative to the May values. It remains inconclusive whether the new production was limited by an unidentified factor other than supply in late summer, or the uptake was underestimated by the incubation method. This article is protected by copyright. All rights reserved.

Description: A coupled wave-circulation model is used to examine interactions between surface gravity waves and ocean currents over the eastern Canadian shelf and adjacent deep waters during three severe weather events. The simulated significant wave heights (SWHs) and peak wave periods reveal the importance of wave-current interactions (WCI) during and after the storm. In two fast-moving hurricane cases, the maximum SWHs are reduced by more than 11% on the right-hand side of the storm track and increased by about 5% on the left-hand side due to different WCI mechanisms on waves on two sides of the track. The dominate mechanisms of the WCI on waves include the current-induced modification of wind energy input to the wave generation, and current-induced wave advection and refraction. In the slow-moving winter storm case, the effect of WCI decreases the maximum SWHs on both sides of the storm track due to different results of the current-induced wave advection, which is affected greatly by the storm translation speed. The simulated sea surface temperature (SST) cooling induced by hurricanes and SST warming induced by the winter storm are also enhanced (up to 1.2 o C) by the WCI mechanisms on circulation and hydrography. The 3D wave forces can affect water columns up to 200 m in all three storm cases. By comparison, the effect of breaking wave-induced mixing in the ocean upper layer is more important under strong stratification conditions in two hurricane cases than under weak stratification conditions in the winter storm case. This article is protected by copyright. All rights reserved.

Description: ABSTRACT This study investigates the relevance of infragravity (IG) waves at Albufeira Lagoon Inlet, a shallow wave-dominated inlet located on the Western Coast of Portugal. A field experiment carried out in September 2010 revealed the occurrence of low-frequency oscillations (i.e. 25 to 300 s) in water levels and current velocities. While these fluctuations were present over the ebb- tidal delta along the whole tidal cycle, they only appeared between the beginning of the flood and up to two hours after high tide inside the lagoon. The XBeach modeling system was applied to Albufeira Lagoon Inlet and reproduced the generation and propagation of IG waves and their blocking during the ebb. This behavior was explained by blocking due to opposing tidal currents reaching 2.5 m.s −1 in shallow water depths. Numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of IG waves in the inlet. IG waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100% of their magnitude. The fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon. This hypothesis will have to be verified, namely under storm wave conditions. This article is protected by copyright. All rights reserved.

Description: Near-global and continuous measurements from satellite altimetry have provided accurate estimates of global mean sea level in the past two decades. Extending these estimates further into the past is a challenge using the historical tide gauge records. Not only is sampling non-uniform in both space and time, tide gauges are also affected by vertical land motion (VLM) that creates a relative sea level change not representative of ocean variability. To allow for comparisons to the satellite altimetry estimated global mean sea level (GMSL), typically the tide gauges are corrected using glacial isostatic adjustment (GIA) models. This approach, however, does not correct other sources of VLM that remain in the tide gauge record. Here, we compare Global Positioning System (GPS) VLM estimates at the tide gauge locations to VLM estimates from GIA models, and assess the influence of non-GIA related VLM on GMSL estimates. We find that the tide gauges, on average, are experiencing positive VLM (i.e. uplift) after removing the known effect of GIA, resulting in an increase of 0.24 +/- 0.08 mm year −1 in GMSL trend estimates from 1900 to present when using GPS-based corrections. While this result is likely dependent on the subset of tide gauges used and the actual corrections used, it does suggest that non-GIA VLM plays a significant role in 20 th century estimates of GMSL. Given the relatively short GPS records used to obtain these VLM estimates, we also estimate the uncertainty in the GMSL trend that results from limited knowledge of non-GIA related VLM. This article is protected by copyright. All rights reserved.

Description: During springtime in the Arctic, bromine explosion events occur when high concentrations of reactive bromine species are observed in the boundary layer with the concurrence of ozone depletion events and mercury depletion events. While a variety of substrates such as snow, sea ice, frost flowers and aerosols have been proposed to be the substrate and/or source of bromine activation in the Arctic, recent studies have highlighted the role of snow. Here we report concentration profiles of halides (Br − and Cl − ), Na + , and oxidized mercury across the snow-sea ice-seawater interface at a coastal marine site in the Canadian Arctic Archipelago in March and June 2014, as well as in an experimental sea ice mesocosm in Winnipeg in January 2014. The occurrence of bromine activation at the Arctic site in March was indicated by the high mercury concentrations in snowpack. At both the Arctic and mesocosm sites, the molar ratios of Br − /Na + were nearly constant throughout the sea ice depth, but highly variable in the upper layer of the overlying snowpack, revealing that bromine activation takes place in the sunlit snow instead of sea ice. This is supported by calculations showing that the loss of Br – from the upper layer of the snowpack is large enough to produce the observed concentrations of reactive bromine in the atmospheric boundary layer. However, the upper layer of the Arctic snowpack tends to be generally enriched in Br – due to the net addition of Br – -enriched gases and non-sea-salt aerosols. This article is protected by copyright. All rights reserved.

Description: This work demonstrates the feasibility of 2D time-domain, adjoint-state acoustic full-waveform inversion (FWI) to retrieve high-resolution models of ocean physical parameters such as sound speed, temperature and salinity. The proposed method is first described and then applied to pre-stack multi-channel seismic (MCS) data acquired in the Gulf of Cadiz (SW Iberia) in 2007 in the framework of the Geophysical Oceanography project. The inversion strategy flow includes specifically-designed data pre-conditioning for acoustic noise reduction, followed by the inversion of sound speed in the shotgather domain. We show that the final sound speed model has a horizontal resolution of ∼ 70 m , which is two orders of magnitude better than that of the initial model constructed with coincident eXpendable Bathy Thermograph (XBT) data, and close to the theoretical resolution of O ( λ ). Temperature (T) and salinity (S) are retrieved with the same lateral resolution as sound speed by combining the inverted sound speed model with the thermodynamic equation of seawater and a local, depth-dependent T-S relation derived from regional conductivity-temperature-depth (CTD) measurements of the National Oceanic and Atmospheric Administration (NOAA) database. The comparison of the inverted T and S models with XBT and CTD casts deployed simultaneously to the MCS acquisition shows that the thermohaline contrasts are resolved with an accuracy of 0.18 o C for temperature and 0.08 PSU for salinity. The combination of oceanographic and MCS data into a common, pseudo-automatic inversion scheme allows to quantitatively resolve submeso-scale features that ought to be incorporated into larger-scale ocean models of oceans structure and circulation. This article is protected by copyright. All rights reserved.

Description: Photosynthesis parameters are routinely estimated from in vitro measurements of primary production under constant light reaching each incubation bottle, by fitting a photosynthesis-irradiance function to the measurements. Here we take one such function and integrate it in time for variable light input, similar to natural conditions, to obtain the analytical solution for the vertical profile of daily phytoplankton production in the field. This solution is then fitted to in situ measurements of primary production profiles in the same manner as a photosynthesis-irradiance function is fitted to in vitro measurements under controlled and constant light conditions to retrieve the photosynthesis-irradiance parameters. The method is tested on the Hawaii Ocean Time-series data set. The solution explained 97.88% of the variance in measured normalized production at individual depths. The recovered parameters were then used to model the normalized daily water-column production. The model explained 99.21% of variance in normalized watercolumn production of the entire data set. The seasonal cycle of the photosynthesis parameters recovered with the analytical solution was further studied for the Hawaii Ocean Time-series. With respect to the photosynthesis parameter determination, the solution bridges the gap between classical photosynthesis-irradiance measurements under controlled light conditions and in situ measurements which are made under natural, variable light conditions. It presents a new tool for the estimation of photosynthesis parameters from in situ measurements of primary production. This article is protected by copyright. All rights reserved.

Description: Marine seismic reflection technique is used to observe the strong ocean dynamic process of nonlinear internal solitary waves (ISWs or solitons) in the near-surface water. Analysis of ISWs is problematical because of their transient nature and limitations of classical physical oceanography methods. This work explores a Markov Chain Monte Carlo (MCMC) approach to recover the temperature and salinity of ISW field using the seismic reflectivity data and in-situ hydrographic data. The MCMC approach is designed to directly sample the posterior probability distributions of temperature and salinity which are the solutions of the system under investigation. The principle improvement is the capability of incorporating uncertainties in observations and prior models which then provide quantified uncertainties in the output model parameters. We tested the MCMC approach on two acoustic reflectivity datasets one synthesized from a CTD cast and the other derived from multichannel seismic reflections. This method finds the solutions faithfully within the significantly narrowed confidence intervals from the provided priors. Combined with a low frequency initial model interpreted from seismic horizons of ISWs, the MCMC method is used to compute the finescale temperature, salinity, acoustic velocity, and density of ISW field. The statistically derived results are equivalent to the conventional linearized inversion method. However, the former provides us the quantified uncertainties of the temperature and salinity along the whole section whilst the latter does not. These results are the first time ISWs have been mapped with sufficient detail for further analysis of their dynamic properties. This article is protected by copyright. All rights reserved.

Description: Time-depth variations of the equatorial currents over the upper 1000 m depth in the western Pacific Ocean were directly measured by acoustic Doppler current profiler moorings at 2°N, 140°E and 4.7°N, 140°E during January-August 2014. Intra-seasonal variations of the equatorial currents, with periods of 37-73 days, were observed encompassing the North Equatorial Countercurrent (NECC), northern branch of the South Equatorial Current (SEC), Equatorial Undercurrent (EUC), Equatorial Intermediate Current (EIC), North Intermediate Countercurrent (NICC), and North Equatorial Subsurface Current (NESC). Compared with previous studies based mainly on shipboard synoptic surveys, the 8-month time series of velocity profiles provided direct evidence for the existence of NESC; captured reversals of the EIC in May and the NESC in June from westward to eastward direction; and revealed larger vertical extensions of the SEC and NESC, and greater depths of the EIC and NICC than previously thought. According to a global analysis product of ocean surface current, during January-April 2014 the NECC was located around its southernmost position and with its the weakest intensity over the past twenty years. Some of the anomalous characteristics of these flows may be related to the fickle El Niño of 2014. This article is protected by copyright. All rights reserved.

Description: Wave focusing of energetic swell fields can result in small-scale variations associated with coherent interference that can be important for nearshore circulation and beach dynamics. However, coherent interference is difficult to measure with conventional in-situ instruments and is not accounted for in operational wave models. As a result, such effects are generally ignored. In this work we analyze X-band radar observations collected at Ocean Beach, San Francisco using a Wigner-Ville or coupled-mode spectrum, to show how long-dwell remote sensing technology allows us to identify coherent wave interference. Our analysis demonstrates that during energetic swell events, the nearshore wave field consists of two non-collinear, but coherent, swell patterns that originate from the same offshore source but are directionally separated due to refraction over the San Francisco Bar. The length scale of the associated alongshore wave height variability (200m) is consistent with the wavenumber separation obtained from the coupled mode analysis. This confirms that the small-scale variability is primarily due to coherent interference. In addition, our analysis shows that the shoreline exhibits a strong localized response near the radar site on the 200 m scale, which suggests that coherent interference effects can affect wave-driven nearshore transport processes and localized erosion. This article is protected by copyright. All rights reserved.

Description: This paper provides a detailed analysis of momentum, angular momentum, vorticity, and energy budgets of a submesoscale front undergoing frontogenesis driven by an upper-ocean, submesoscale eddy field in a Large Eddy Simulation (LES). The LES solves the wave-averaged, or Craik-Leibovich, equations in order to account for the Stokes forces that result from interactions between nonbreaking surface waves and currents, and resolves both submesoscale eddies and boundary layer turbulence down to 4.9m × 4.9m × 1.25m grid scales. It is found that submesoscale frontogenesis differs from traditional frontogenesis theory due to four effects: Stokes forces, momentum and kinetic energy transfer from submesoscale eddies to frontal secondary circulations, resolved turbulent stresses, and unbalanced torque. In the energy, momentum, angular momentum, and vorticity budgets for the frontal overturning circulation, the Stokes shear force is a leading-order contributor, typically either the second or third largest source of frontal overturning. These effects violate hydrostatic and thermal wind balances during submesoscale frontogenesis. The effect of the Stokes shear force becomes stronger with increasing alignment of the front and Stokes shear and with a nondimensional scaling. The Stokes shear force and momentum transfer from submesoscale eddies significantly energize the frontal secondary circulation along with the buoyancy. This article is protected by copyright. All rights reserved.

Description: The polar front in the North Atlantic is bound to the ridge between Iceland and the Faroe Islands, where about one-half of the northward transport of warm Atlantic Water into the Nordic Seas occurs, as well as about one sixth of the equatorward dense overflow. We find a low salinity water mass at the surface of the Iceland-Faroe Front (IFF), which in wintertime subducts along outcropping isopycnals and is found in much modified form on the Atlantic side of the Iceland-Faroe Ridge (IFR) crest. The features found on the Atlantic side of the crest at depth have temperature and salinity characteristics which are clearly traceable to the surface outcrop of the IFF. The presence of coherent low salinity layers on the Atlantic side of the IFR crest has not been previously reported. Warm waters above the IFR primarily feed the Faroe Current, and injection of a low salinity water mass may play an early role in the water mass transformation taking place in the Nordic Seas. The seasonality of the intrusive features suggests a link between winter convection, mixed layer instability and deep frontal subduction. These low salinity anomalies (as well as a low oxygen water mass from the Iceland Basin) can be used as tracers of the intermediate circulation over the IFR. This article is protected by copyright. All rights reserved.

Description: We investigate the individual and joint decadal variability of Southern Ocean state quantities, such as the strength of the Ross and Weddell Gyres, Drake Passage transport, and sea ice area, using the National Institute of Water and Atmospheric Research UK Chemistry and Aerosols (NIWA-UKCA) model and CMIP5 models. Variability in these quantities is stimulated by strong deep reaching convective events in the Southern Ocean, which produce an Antarctic Bottom Water-like water mass and affect the large-scale meridional density structure in the Southern Ocean. An increase in the (near) surface stratification, due to freshwater forcing, can be a pre-condition for subsequent strong convection activity. The combination of enhanced-gyre driven sea ice and freshwater export, as well as ongoing subsurface heat accumulation, lead to a time lag between changes in oceanic freshwater and heat content. This causes an ongoing weakening of the stratification until sudden strong mixing events emerge and the heat is released to the atmosphere. We find that strong convection reduces sea ice cover, weakens the subpolar gyres, increases the meridional density gradient and subsequently results in a positive Drake Passage transport anomaly. Results of available CMIP5 models confirm that variability in sea ice, Drake Passage transport and the Weddell Gyre strength is enhanced if models show strong open ocean convective events. Consistent relationships between convection, sea ice, Drake Passage transport and Ross Gyre strength variability are evident in most models, whether or not they host open-ocean convection. This article is protected by copyright. All rights reserved.

Description: Global mass balance calculations indicate the majority of particulate organic carbon (POC) exported from shelf seas is transferred via downslope exchange processes. Here we demonstrate the downslope flux of POC from the Hebrides Shelf is approximately 3-to-5-fold larger per unit length/area than the global mean. To reach this conclusion we quantified the offshore transport of particulate and dissolved carbon fractions via the “Ekman Drain”, a strong downwelling feature of the NW European Shelf circulation, and subsequently compared these fluxes to simultaneous regional air-sea CO 2 fluxes and on-shore wind-driven Ekman fluxes to constrain the carbon dynamics of this shelf. Along the shelf break we estimate a mean offshelf total carbon (dissolved + particulate) flux of 4.2 tonnes C m −1 d −1 compared to an onshelf flux of 4.5 tonnes C m −1 d −1 . Organic carbon represented 3.3% of the onshelf carbon flux but 6.4% of the offshelf flux indicating net organic carbon export. Dissolved organic carbon represented 95% and POC 5% of the exported organic carbon pool. When scaled along the shelf break the total offshelf POC flux (0.007 Tg C d −1 ) was found to be three times larger than the regional air-sea CO 2 ingassing flux (0.0021 Tg C d −1 ), an order of magnitude larger than the particulate inorganic carbon flux (0.0003 Tg C d −1 ) but far smaller than the DIC (2.03 Tg C d −1 ) or DOC (0.13 Tg C d −1 ) fluxes. Significant spatial heterogeneity in the Ekman drain transport confirms that offshelf carbon fluxes via this mechanism are also spatially heterogeneous. This article is protected by copyright. All rights reserved.

Description: The vorticity balance of the Antarctic Circumpolar Current in Drake Passage is examined using four years of observations from current- and pressure-recording Inverted Echo Sounders. The time-varying vorticity, planetary and relative vorticity advection, and bottom pressure torque are calculated in a two-dimensional array in the eddy-rich Polar Frontal Zone (PFZ). Bottom pressure torque is also estimated at sites across Drake Passage. Mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local (50-km-scale) vorticity budget in the PFZ, and are balanced to first order by the divergence of horizontal velocity. Most of this divergence comes from the ageostrophic gradient flow, which also provides a second-order adjustment to the geostrophic relative vorticity advection. Bottom pressure torque is approximately one-third the size of the local depth-integrated divergence. Although the cDrake velocity fields exhibit significant turning with depth throughout Drake Passage even in the mean, surface vorticity advection provides a reasonable representation of the depth-integrated vorticity balance. Observed near-bottom currents are strongly topographically steered, and bottom pressure torques grow large where strong near-bottom flows cross steep topography at small angles. Upslope flow over the northern continental slope dominates the bottom pressure torque in cDrake, and the mean across this Drake Passage transect, 3 to 4 × 10 −9 m s −2 , exceeds the mean wind stress curl by a factor of 15 to 20. This article is protected by copyright. All rights reserved.

Description: A full-scale experimental study of turbulent boundary layer flows under irregular waves and currents is conducted with the primary objective to investigate the equivalent-wave concept by Madsen [1994]. Irregular oscillatory flows following the bottom-velocity spectrum under realistic surface irregular waves are produced over two fixed rough bottoms in an oscillatory water tunnel, and flow velocities are measured using a Particle Image Velocimetry. The root-mean-square (RMS) value and representative phase lead of wave velocities have vertical variations very similar to those of the first-harmonic velocity of periodic wave boundary layers, e.g. the RMS wave velocity follows a logarithmic distribution controlled by the physical bottom roughness in the very near-bottom region. The RMS wave bottom shear stress and the associated representative phase lead can be accurately predicted using the equivalent-wave approach. The spectra of wave bottom shear stress and boundary layer velocity are found to be proportional to the spectrum of free-stream velocity. Currents in the presence of irregular waves exhibit the classic two-log-profile structure with the lower log-profile controlled by the physical bottom roughness and the upper log-profile controlled by a much larger apparent roughness. Replacing the irregular waves by their equivalent sinusoidal waves virtually makes no difference for the co-existing currents. These observations, together with the excellent agreement between measurements and model predictions, suggest that the equivalent-wave representation adequately characterizes the basic wave-current interaction under irregular waves. This article is protected by copyright. All rights reserved.

Description: A simple box model is used to examine oxygen depletion in an idealized ocean-margin upwelling system. Near-bottom oxygen depletion is controlled by a competition between flushing with oxygenated offshore source waters and respiration of particulate organic matter produced near the surface and retained near the bottom. Upwelling-supplied nutrients are consumed in the surface box, and some surface particles sink to the bottom where they respire, consuming oxygen. Steady states characterize the potential for hypoxic near-bottom oxygen depletion; this potential is greatest for faster sinking rates, and largely independent of production timescales except in that faster production allows faster sinking. Timescales for oxygen depletion depend on upwelling and productivity differently, however, as oxygen depletion can only be reached in meaningfully short times when productivity is rapid. Hypoxia thus requires fast production, to capture upwelled nutrients, and fast sinking, to deliver the respiration potential to model bottom waters. Combining timescales allows generalizations about tendencies toward hypoxia. If timescales of sinking are comparable to or smaller than the sum of those for respiration and flushing, the steady state will generally be hypoxic, and results indicate optimal timescales and conditions exist to generate hypoxia. For example, the timescale for approach to hypoxia lengthens with stronger upwelling, since surface particle and nutrient are shunted off-shelf, in turn reducing subsurface respiration and oxygen depletion. This suggests that if upwelling winds intensify with climate change the increased forcing could offer mitigation of coastal hypoxia, even as the oxygen levels in upwelled source waters decline. This article is protected by copyright. All rights reserved.

Description: Icebergs calved from the Antarctic continent act as moving sources of freshwater while drifting in the Southern Ocean. The lifespan of these icebergs strongly depends on their original size during calving. In order to investigate the effects (if any) of the calving size of icebergs on the Southern Ocean, we use a coupled general circulation model with an iceberg component. Iceberg calving length is varied from 62 m up to 2.3 km, which is the typical range used in climate models. Results show that increasing the size of calving icebergs leads to an increase in the westward iceberg freshwater transport around Antarctica. In simulations using larger icebergs, the reduced availability of meltwater in the Amundsen and Bellingshausen Seas suppresses the sea-ice growth in the region. In contrast, the increased iceberg freshwater transport leads to increased sea-ice growth around much of the East Antarctic coastline. These results suggest that the absence of large tabular icebergs with horizontal extent of tens of kilometers in climate models may introduces systematic biases in sea-ice formation, ocean temperatures and salinities around Antarctica. This article is protected by copyright. All rights reserved.

Description: Near-bottom water flowing over the Kerama Gap's sills is thought to ventilate the deep water below ∼1100 m depth in the Okinawa Trough and then upwell with 5‒10 years residence time. The present study follows up on this phenomenon, using comprehensive profile data of temperature, salinity, dissolved oxygen, currents and turbulence obtained by intensive shipboard observations performed in June 2013 and June 2014 in the region. Strong near-bottom sub-tidal flow with speeds exceeding 0.5 m s −1 was observed within a layer of about 100 m thickness over the western side of the peak of the main sill. Temperature and salinity sections along the Kerama Gap indicated some depressions and overturns of the deep water downstream of the strong overflow, suggesting the existence of breaking internal gravity waves and hydraulic jumps. Associated vertical diffusivities, estimated using the Thorpe scale and the buoyancy frequency, were three to four orders of magnitude larger than typical values observed in the thermocline of the open ocean (∼10 −5 m 2 s −1 ). The dissolved oxygen section also indicated strong vertical mixing and associated upwelling with the entrainment of the near-bottom overflow water into the lower thermocline beneath the Kuroshio in the Okinawa Trough. The present study not only supports the previous conceptual model but also provides new evidence that the Okinawa Trough is an upwelling location where nutrient rich Philippine Sea intermediate water is sucked up into the lower thermocline below the Kuroshio. This article is protected by copyright. All rights reserved.

Description: Water properties from moored measurements (2010 – 2015) near Icy Cape on the eastern Chukchi shelf have been examined in relation to satellite observations of ice cover. Atlantic Water (AW), with temperature 〉 -1°C and salinity 〉 33.6, has been observed to upwell from deeper than 200 m in the Arctic Basin onto the Chukchi Shelf via Barrow Canyon. Most previous observations of AW on the Chukchi shelf have been in or near Barrow Canyon; observations of AW farther onto the shelf are rare. Despite mooring location on the shelf ∼225 km from the head of Barrow Canyon, five AW events have been observed at mooring C1 (70.8°N, 163.2°W) in four years of data. All but one of the events occurred under openings in the sea ice cover (either a polynya or the ice edge). No events were observed during the winter of 2011/2012, a year with little polynya activity in the region. In addition to changes in temperature and salinity, the AW events are typically associated with southwestward winds and currents, changes in sea-ice cover, and increased nutrient concentrations in the bottom water. Estimates of heat content associated with the AW events suggest that the Chukchi Polynya can often be classified as a hybrid sensible heat/wind-driven polynya. This article is protected by copyright. All rights reserved.

Description: We are in an era of unprecedented data volumes generated from observations and model simulations. This is particularly true from satellite Earth Observations (EO) and global scale oceanographic models. This presents us with an opportunity to evaluate large scale oceanographic model outputs using EO data. Previous work on model skill evaluation has led to a plethora of metrics. The paper defines two new model skill evaluation metrics. The metrics are based on the theory of universal multifractals and their purpose is to measure the structural similarity between the model predictions and the EO data. The two metrics have the following advantages over the standard techniques: a) they are scale-free, b) they carry important part of information about how model represents different oceanographic drivers. Those two metrics are then used in the paper to evaluate the performance of the FVCOM model in the shelf seas around the south-west coast of the UK. This article is protected by copyright. All rights reserved.

Description: ABSTRACT To clarify the formation process of the salinity minimum in the Kuroshio–Oyashio mixed water region and understand the mechanism of meridional heat transport between the subtropical and subpolar gyres, 16 profiling floats were deployed within a warm-core anticyclonic eddy off Hokkaido from June 2012 to December 2013. Then, the evolution of an anticyclonic eddy was examined using time series of the water properties. The largest fluctuations in water properties were observed in April and May 2013, when the anticyclonic eddy first moved south to interact with a warm front, then back north. Salinity in the salinity minimum layer increased during the interaction. After the eddy detached from the frontal structure, low-salinity water was again observed with small intrusive structures, which eventually converged to a smooth zigzag structure in the potential temperature-salinity diagram, suggesting that both vertical mixing and vertical heaving played a role in the temporal changes observed after the eddy detached from the front. Since the salinity variation during the interaction event was about half the total salinity change during the whole experimental period, the interaction of an eddy with a front might be important for modifying the water properties of the eddy, and, therefore, for the meridional transport of heat and fresh water. This article is protected by copyright. All rights reserved.

Description: Conditions prevailing in regions of deep water formation imprint their signature in the concentrations of dissolved noble gases, which are conserved in the deep ocean. Such “recharge conditions” including temperature, salinity, and interactions with sea ice are important in view of ocean-atmosphere CO 2 partitioning. Noble gases, especially the temperature sensitive Kr and Xe, are well-established tracers to reconstruct groundwater recharge conditions. In contrast, tracer oceanography has traditionally focused on He isotopes and the light noble gases Ne and Ar, which could be analyzed at the required high precision. Recent developments of analytical and data interpretation methods now provide fresh perspectives for noble gases in oceanography. This article is protected by copyright. All rights reserved.

Description: Coral reefs are threatened worldwide, and there is a need to develop new approaches to monitor reef health under natural conditions. Because simultaneous measurements of net community production (NCP) and net community calcification (NCC) are used as important indicators of reef health, tools are needed to assess them in situ . Here, we present the Benthic Ecosystem and Acidification Measurement System (BEAMS), to provide the first fully autonomous approach capable of sustained, simultaneous measurements of reef NCP and NCC under undisturbed, natural conditions on timescales ranging from tens of minutes to weeks. BEAMS combines the chemical and velocity gradient in the benthic boundary layer to quantify flux from the benthos for a variety of parameters to measure NCP and NCC. Here, BEAMS was used to measure these rates from two different sites with different benthic communities on the western reef terrace at Palmyra Atoll for two weeks in September, 2014. Measurements were made every ∼15 minutes. The trends in metabolic rates were consistent with the benthic communities between the two sites with one dominated by fleshy organisms and the other dominated by calcifiers (degraded and healthy reefs, respectively). This demonstrates the potential utility of BEAMS as a reef health monitoring tool. NCP and NCC were tightly coupled on timescales of minutes to days, and light was the primary driver for the variability of daily integrated metabolic rates. No correlation between CO 2 levels and daily integrated NCC was observed, indicating that NCC at these sites were not significantly affected by CO 2 . This article is protected by copyright. All rights reserved.

Description: Measurements conducted in the southern Bay of Bengal (BoB) as a part of the ASIRI-EBoB Program portray the characteristics of high-frequency internal waves in the upper pycnocline as well as the velocity structure with episodic events of shear instability. A 20-hour time series of CTD, ADCP and acoustic backscatter profiles down to 150 m as well as temporal CTD measurements in the pycnocline at z = 54 m were taken to the east of Sri Lanka. Internal waves of periods ∼ 10 to 40 min were recorded at all depths below a shallow (∼ 20 – 30 m) surface mixed layer in the background of an 8-m amplitude internal tide. The absolute values of vertical displacements associated with high-frequency waves followed the Nakagami distribution with a median value of 2.1 m and a 95% quintile 6.5 m. The internal wave amplitudes are normally distributed. The tails of the distribution deviate from normality due to episodic high-amplitude displacements. The sporadic appearance of internal waves with amplitudes exceeding ∼ 5 m usually coincided with patches of low Richardson numbers, pointing to local shear instability as a possible mechanism of internal-wave induced turbulence. The probability of shear instability in the summer BoB pycnocline based on an exponential distribution of the inverse Richardson number, however, appears to be relatively low, not exceeding 4% for Ri  〈 0.25 and about 10% for Ri  〈 0.36 (K-H billows). The probability of the generation of asymmetric breaking internal waves and Holmboe instabilities is above ∼ 25%. This article is protected by copyright. All rights reserved.

Description: The impact of Antarctic bottom water (AABW) formation on the Weddell Gyre and its northward propagation characteristics are studied using a 4000-yr long control run of the GFDL CM2.1 model as well as sensitivity experiments. In the control run, the AABW cell and Weddell Gyre are highly correlated when the AABW cell leads the Weddell Gyre by several years, with an enhanced AABW cell corresponding to a strengthened Weddell Gyre and vice versa. An additional sensitivity experiment shows that the response of the Weddell Gyre to AABW cell changes is primarily attributed to interactions between the AABW outflow and ocean topography, instead of the surface wind stress curl and freshwater anomalies. As the AABW flows northward, it encounters topography with steep slopes that induce strong downwelling and negative bottom vortex stretching. The anomalous negative bottom vortex stretching induces a cyclonic barotropic streamfunction over the Weddell Sea, thus leading to an enhanced Weddell Gyre. The AABW cell variations in the control run have significant meridional coherence in density space. Using passive dye tracers, it is found that the slow propagation of AABW cell anomalies south of 35 o S corresponds to the slow tracer advection time scale. The dye tracers escape the Weddell Sea through the western limb of the Weddell Gyre and then go northwestward to the Argentine Basin through South Sandwich Trench and Georgia Basin. This slow advection by deep ocean currents determines the AABW cell propagation speed south of 35 o S. North of 35 o S the propagation speed is determined both by advection in the deep western boundary current and through Kelvin waves. This article is protected by copyright. All rights reserved.

Description: The eye of Hurricane Isaac passed through the center of an array of six deepwater water-column current meter moorings deployed in the northern Gulf of Mexico. The trajectory of the hurricane provided for a unique opportunity to quantify differences in the full water-column oceanic response to a hurricane to the left and right of the hurricane trajectory. Prior to the storm passage, relative vorticity on the right side of the hurricane was strongly negative; while on the left, relative vorticity was positive. This resulted in an asymmetry in the near inertial frequencies oceanic response at depth and horizontally. A shift in the response to a slightly larger inertial frequencies ∼1.11 f was observed and verified by theory. Additionally, the storm passage coincided with an asymmetric change in relative vorticity in the upper 1000 m, which persisted for ∼15 inertial periods. Vertical propagation of inertial energy was estimated at 29 m/day, while horizontal propagation at this frequency was approximately 5.7 km/day. Wavelet analysis showed two distinct sub-inertial responses, one with a period of 2-5 days and another with a period of 5-12 days. Analysis of the sub-inertial bands reveals that the spatial and temporal scales are shorter and less persistent than the near-inertial variance. As the array is geographically located near the site of the Deepwater Horizon oil spill, the spatial and temporal scales of response have significant implications for the fate, transport, and distribution of hydrocarbons following a deepwater spill event. This article is protected by copyright. All rights reserved.

Description: ESA's satellite magnetometer mission Swarm is supposed to lower the limit of observability for oceanic processes. While periodic magnetic signals from ocean tides are already detectable in satellite magnetometer observations, changes in the general ocean circulation are yet too small or irregular for a successful separation. An approach is presented that utilizes the good detectability of tidal magnetic signals to detect changes in the oceanic electric conductivity distribution. Ocean circulation, tides and the resultant magnetic fields are calculated with a global general ocean circulation model coupled to a 3D electromagnetic induction model. For the decay of the meridional overturning circulation, as an example, the impact of climate variability on tidal oceanic magnetic signals is demonstrated. Total overturning decay results in anomalies of up to 0.7 nT in the radial magnetic M2 signal at sea level. The anomalies are spatially heterogeneous and reach in extended areas 30% or more of the unperturbed tidal magnetic signal. The anomalies should be detectable in long time series from magnetometers on land or at the ocean bottom. The anomalies at satellite height (430 km) reach 0.1 nT and pose a challenge for the precision of the Swarm mission. Climate variability induced deviations in the tide system (e.g., tidal velocities and phases) are negligible. Changes in tidal magnetic fields are dominated by changes in sea water salinity and temperature. Therefore, it is concluded that observations of tidal magnetic signals could be used as a tool to detect respective state changes in the ocean. This article is protected by copyright. All rights reserved.