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: Accurate parameterization of spatially-variable diffusivity in complex shelf regions such as the Great Barrier Reef (GBR) lagoon is an unresolved issue for hydrodynamic models. This leads to large uncertainties to the flushing time derived from them and to the evaluation of ecosystem resilience to terrestrially-derived pollution. In fact, numerical hydrodynamic models and analytical cross-shore diffusion models have predicted very different flushing times for the GBR lagoon. Nevertheless, scarcity of in-situ measurements used previously in the latter method prevents derivation of detailed diffusivity profiles. Here, detailed cross-shore profiles of diffusivity were calculated explicitly in a closed form for the first time from the steady state transects of sea surface temperature for different sections of the GBR lagoon. We find that diffusivity remains relatively constant within the inner lagoon (〈 ∼20km) where tidal current is weak, and increases linearly with sufficiently large tidal amplitude in reef-devoid regions, but increases dramatically where the reef matrixes start and fluctuates with reef size and density. The cross-shelf profile of steady-state salinity calculated using the derived diffusivity values agrees well with field measurements. The calculated diffusivity values are also consistent with values derived from satellite-tracked drifters. Flushing time by offshore diffusion is of the order of 1 month, suggesting the important role of turbulent diffusion in flushing the lagoon, especially in reef-distributed regions. The results imply that previous very large residence times predicted by numerical hydrodynamic models may result from underestimation of diffusivity. Our findings can guide parameterization of diffusivity in hydrodynamic modelling. This article is protected by copyright. All rights reserved.

Description: On the basis of five oceanographic cruises carried out in the Eastern Tropical Pacific off Mexico, relationships between the larval fish habitats (areas inhabited by larval fish assemblages) and the environmental circulation scales (mesoscale, seasonal and interannual) were examined. Analysis of in situ data over a grid of hydrographic stations and oblique zooplankton hauls with bongo net (505 µm) was combined with orthogonal robust functions decomposition applied to altimetry anomalies obtained from satellite. During both cool (March and June) and warm (August and November) periods, Bray-Curtis dissimilarity Index defined three recurrent larval fish habitats which varied in species composition and extent as a function of the environmental scales. The variability of the Tropical larval fish habitat (characterized by high species richness, and dominated by Vinciguerria lucetia, Diogenichthys laternatus and Diaphus pacificus ) was associated with the seasonal changes. The Transitional-California Current larval fish habitat (dominated by V. lucetia and D. laternatus , with lower mean abundance and lower species richness than in the Tropical habitat) and Coastal-and-Upwelling larval fish habitat (dominated by Bregmaceros bathymaster ) was associated mainly with mesoscale activity induced by eddies and with coastal upwelling. During February 2010, the Tropical larval fish habitat predominated offshore and the Transitional-California Current larval fish habitat was not present, which we attribute to the effect of El Niño conditions. Thus the mesoscale, seasonal and interannual environmental scales affect the composition and extension of larval fish habitats. This article is protected by copyright. All rights reserved.

Description: In the western North Atlantic, warm and saline water is brought by the North Atlantic Current (NAC) from the subtropics into the subpolar gyre. Four Inverted Echo Sounders with high precision pressure sensors (PIES) were moored between 47°40'N and 52°30'N to study the main pathways of the NAC from the western into the eastern basin. The array configuration that forms three segments (northern, central and southern) allows partitioning of the NAC and some assessment of NAC flow paths through the different Mid-Atlantic Ridge fracture zones. We exploit the correlation between the NAC transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of NAC transports to the period from 1992 to 2013. The mean NAC transport over the entire 21-years is 27±5 Sv, consisting of 60% warm water of subtropical origin, and 40% subpolar water. We did not find a significant trend in the total transport time series, but individual segments had opposing trends, leading to a more focused NAC in the central subsection and decreasing transports in the southern and northern segments. The spectral analysis exhibits several significant peaks. The two most prominent are around 120 days, identified as the time scale of meanders and eddies, and at 4-9 years, most likely related to the NAO. Transport composites for the years of highest and lowest NAO indices showed a significantly higher transport (+2.9 Sv) during strong NAO years, mainly in the southern segment. This article is protected by copyright. All rights reserved.

Description: This paper is the first of a two part series that investigates passive buoyant tracers in the ocean surface boundary layer. The first part examines the influence of equilibrium wind-waves on vertical tracer distributions, based on large eddy simulations (LES) of the wave-averaged Navier-Stokes equation. The second part applies the model to investigate observations of buoyant microplastic marine debris, which has emerged as a major ocean pollutant. The LES model captures both Langmuir turbulence (LT) and enhanced turbulent kinetic energy input due to breaking waves (BW) by imposing equilibrium wind-wave statistics for a range of wind and wave conditions. Concentration profiles of LES agree well with analytic solutions obtained for an eddy diffusivity profile that is constant near the surface and transitions into the K-Profile Parameterization (KPP) profile shape at greater depth. For a range of wind and wave conditions, the eddy diffusivity normalized by the product of water-side friction velocity and mixed layer depth, $h$, mainly depends on a single non-dimensional parameter, the peak wavelength (which is related to Stokes drift decay depth) normalized by $h$. For smaller wave ages, BW critically enhances near surface mixing, while LT effects are relatively small. For greater wave ages, both BW and LT contribute to elevated near surface mixing and LT significantly increases turbulent transport at greater depth. We identify a range of realistic wind and wave conditions for which only Langmuir (and not BW or shear-driven) turbulence is capable of deeply submerging buoyant tracers. This article is protected by copyright. All rights reserved.

Description: We investigate the characteristics of shear–generated turbulence in the natural environment by considering data from a number of cruises in the western equatorial Pacific. In this region the vertical shear of the flow is dominated by flow structures that have a relatively small vertical scale of O(10m). Combining data from all cruises we find a strong relationship between the turbulent dissipation rate, ϵ , vertical shear, S , and buoyancy frequency, N . Examination of ϵ at a fixed value of Richardson number, Ri = N 2 ∕ S 2 , shows that for a wide range of values of N , where u t is an appropriate velocity scale which we assume to be the horizontal velocity scale of the turbulence. The implied vertical length scale, ℓ v = u t ∕ N , is consistent with theoretical and numerical studies of stratified turbulence. Such behavior is found for Ri  〈 0.4. The vertical diffusion coefficient then scales as at a fixed value of Richardson number. The amplitude of ϵ is found to increase with decreasing Ri , but only modestly, and certainly less dramatically than suggested by some parameterization schemes. Provided the shear generating the turbulence is resolved our results point to a way to parameterize the unresolved turbulence. This article is protected by copyright. All rights reserved.

Description: Global simulations are presented focusing on the atmosphere-ice-ocean (AIO) surface layer (SL) in the Arctic. Results are produced using an ocean model (NEMO) coupled to two different sea ice models: the Louvain-La-Neuve single-category model (LIM2) and the Los Alamos multi-category model (CICE4). A more objective way to adjust the sea ice-ocean drag is proposed compared to a coefficient tuning approach. The air-ice drag is also adjusted to be more consistent with the atmospheric forcing dataset. Improving the AIO SL treatment leads to more realistic results, having a significant impact on the sea ice volume trend, sea ice thickness and the Arctic freshwater (FW) budget. The physical mechanisms explaining this sensitivity are studied. Improved sea ice drift speeds result in less sea ice accumulation in the Beaufort Sea, correcting a typical ice thickness bias. Sea ice thickness and drag parameters affect how atmospheric stress is transferred to the ocean, thereby influencing Ekman transport and FW retention in the Beaufort Gyre (BG). Increasing sea ice-ocean roughness reduces sea ice growth in winter by reducing ice deformation and lead fractions in the BG. It also increases the total Arctic FW content by reducing sea ice export through Fram Strait. Similarly, increasing air-ice roughness increases the total Arctic FW content by increasing FW retention in the BG. This article is protected by copyright. All rights reserved.

Description: Based on observed temperature and velocity in 2005 in northwestern South China Sea, the shallow ocean responses to three tropical cyclones were examined. The oceanic response to Washi was similar to common observations with 2°C cooling of the ocean surface and slight warming of the thermocline resulted from vertical entrainment. Moreover, the wave field was dominated by first mode near-inertial oscillations which were red-shifted and trapped by negative background vorticity leading to an e -folding time scale of 12 days. The repeated reflections by the surface and bottom boundaries were thought to yield the successive emergence of higher modes. The oceanic responses to Vicente appeared to be insignificant with cooling of the ocean surface by only 0.5°C and near-inertial currents no larger than 0.10 m/s as a result of a deepened surface mixed layer. However, the oceanic responses to Typhoon Damrey were drastic with cooling of 4.5°C near the surface and successive barotropic-like near-inertial oscillations. During the forced stage, the upper ocean heat content decreased conspicuously by 11.65% and the stratification was thoroughly destroyed by vertical mixing. In the relaxation stage, the water particle had vertical displacement of 20 to 30 m generated by inertial pumping. The current response to Damrey was weaker than Washi due to the deepened mixed layer and the destroyed stratification. Our results suggested that the shallow water oceanic responses to tropical cyclones varied significantly with the intensity of tropical cyclones, and was affected by local stratification and background vorticity. This article is protected by copyright. All rights reserved.

Description: The Arctic halocline forms a cold stratified barrier between the seasonally-modified near-surface layers and deeper Atlantic-derived waters. Its low temperature is maintained by intrusions of cold water formed over Arctic shelves in winter. Surprisingly, cold salty (33) water capable of halocline ventilation (Beaufort Sea Winter Water: BSWW) has been observed in the Beaufort Sea during some winters despite the low salinity (20-25) of shelf waters there in summer. This study uses year-round data from moored instruments on the Beaufort shelf and slope during 2009-2011 to investigate the mechanisms involved. Our analysis reveals that four air-sea interaction processes contribute to the formation of BSWW – flushing of the low salinity surface water from the shelf via Ekman transport in late summer and early fall, compensatory upwelling of more saline halocline water onto the shelf, net seaward ice drift that promotes ice production by maintaining a flaw lead, and entrainment of dense upwelled water into the freezing surface layer on the inner shelf. This work moves beyond earlier studies in revealing that while weather conditions were more favourable to BSWW formation in the winter of 2010-11 than in 2009-10, the difference was more strongly influenced by Ekman transport (offshore at the surface, onshore at the seabed) than by differences in cumulative brine injection from ice growth. The strength of the Ekman circulation over the Canadian Beaufort shelf in winter, and its inter-annual variation, have significance for surface nutrient renewal and for the cross-shelf transport of pollutants at the surface and the seabed. This article is protected by copyright. All rights reserved.

Description: This study presents an optimal analysis method for estimating from observations the large and small scale components of a field. This technique relies on an iterative generalized least squares procedure to determine the statistics of the small scale fluctuations directly from the data and is thus especially valuable when such information is not known a priori. The use of spherical radial basis functions in fitting the large scale signal is suggested, particularly when the domain is sufficiently large. Two test cases illustrate several of the properties of this procedure, demonstrate its utility, and provide practical guidelines for its use. This method is then applied to observations collected by the Argo array of profiling floats to produce global gridded absolute geostrophic velocity estimates. This article is protected by copyright. All rights reserved.

Description: Field studies suggested that the biogeochemical settings and community structures are substantial different between the central Northern South China Sea (NSCS) and the upstream Kuroshio Current (KC). In particular, the water column of KC is characterized by substantially lower nutrients and productivity but higher Trichodesmium abundance and nitrogen fixation compared to the NSCS. The mechanism driving the difference of the two marine ecosystems, however, remains inadequately understood. Here, a one-dimensional biogeochemical model was developed to simulate the long-term variability of lower-trophic planktonic ecosystem for two pelagic stations in the NSCS and the KC near the Luzon Strait. The physical model included the vertical mixing driven by air-sea interaction and the Ekman pumping induced by wind stress curl. The biological model was constructed by modifying a nitrogen-based NPZD model with the incorporation of phosphorus cycle and diazotroph nitrogen fixation. After validation by several field datasets, the model was used to study the impact of long-term physical forcing on ecosystem variability in the two distinct stations. Our results suggested that nutrient transport above nitracline during summer was largely controlled by vertical turbulent mixing, while Ekman pumping was important for nutrient transport below the nitracline. Our results also indicated that diazotroph community structure and N 2 fixation in the NSCS and the KC could be strongly influenced by physical processes through the impacts on vertical nutrient fluxes. The disadvantage of diazotroph in the NSCS in compared to the KC during the summer could be attributed to its high nitrate fluxes from subsurface leading to outcompete of diazotrophs by faster growing non-diazotroph phytoplankton. This article is protected by copyright. All rights reserved.

Description: We present a new methodology that allows quantifying the impact of individual terms of the temperature and salinity conservation within the mixed layer on mixed layer depth (MLD). The method is applied to output from an ocean general circulation model in the Indian Ocean to investigate variability and changes in MLD. On seasonal timescales and for most areas of the Indian Ocean variability of MLD is tightly linked to all thermohaline budget terms. In the Indian Ocean at approximately 20 o S the MLD co-varies with surface heat and freshwater fluxes on intraseasonal and interannual timescales. The geography of the region includes the Leeuwin Current, plus the tropical eastern Indian Ocean for interannual surface freshwater fluxes. The range of seasonal amplitudes of MLD variability varies with individual budget terms but is typically within 1 m/month to 100 m/month. The ocean footprints of an intraseasonal tropical cyclone, tropical and mid-latitude seasonal temperature and salinity budgets and interannual variability associated with the Indian Ocean Dipole Mode are analyzed. The results reveal close relationships of the thermohaline budgets within the mixed layer with the variability of the MLD. The associated tendencies of changes in MLD are consistent with Argo and satellite-based observations of tendencies within the mixed layer and sea-surface temperature and salinity. This article is protected by copyright. All rights reserved.

Description: High-temporal resolution measurements in the Labrador Sea surface layer are presented using an upwardly-profiling autonomous microstructure instrument, which captures an internal wave in the act of breaking at the base of the surface mixed layer, driving turbulence levels two to three orders of magnitude above the background. While lower-frequency (near-inertial) internal waves are known to be important sources of turbulence, we report here a higher frequency internal wave breaking near the ocean surface. Due to observational limitations, the exact nature of the instability cannot be conclusively identified, but the interaction of wave-induced velocity with unresolved background shear appears to be the most likely candidate. These observations add a new process to the list of those currently being considered as potentially important for near-surface mixing. The geographical distribution and global significance of such features is unknown, and underscores the need for more extensive small-scale, rapid observations of the ocean surface layer. This article is protected by copyright. All rights reserved.

Description: Thermal infrared (IR) imagery is used to quantify the high spatial and temporal variability of dissipation due to wave breaking in the surf zone. The foam produced in an actively breaking crest, or wave roller, has a distinct signature in IR imagery. A retrieval algorithm is developed to detect breaking waves and extract wave roller length using measurements taken during the Surf Zone Optics 2010 experiment at Duck, NC. The remotely-derived roller length and an in situ estimate of wave slope are used to estimate dissipation due to wave breaking by means of the wave-resolving model by Duncan [1981]. The wave energy dissipation rate estimates show a pattern of increased breaking during low tide over a sand bar, consistent with in situ turbulent kinetic energy dissipation rate estimates from fixed and drifting instruments over the bar. When integrated over the surf zone width, these dissipation rate estimates account for 40-69% of the incoming wave energy flux. The Duncan [1981] estimates agree with those from a dissipation parameterization by Janssen and Battjes [2007], a wave energy dissipation model commonly applied within nearshore circulation models. This article is protected by copyright. All rights reserved.

Description: Interactions of currents and waves with a rippled seabed in the inner part of the coastal ocean bottom boundary layer are studied using particle image velocimetry, ADV and bottom roughness measurements. Mean velocity profiles collapse with appropriate scaling in the log layer, but vary substantially in the roughness sublayer. When wave induced motions are similar or greater than the mean current, the hydrodynamic roughness ( z 0 ) determined from velocity profiles is substantially larger than directly measured values. The roughness signature in turbulent energy spectra persists with elevation when its scale falls in the dissipation range, but decays in the log layer for larger roughness elements. Reynolds shear stress profiles peak in the lower parts of the log layer, diminishing below it, and gradually decaying at higher elevations. In contrast, wave shear stresses are negligible within the log layer, but become significant within the roughness sublayer. This phenomenon is caused by an increase in the magnitude and phase lag of the vertical component of wave-induced motion. No single boundary layer length scale collapses the Reynolds stresses, but both the Prandtl mixing length and eddy viscosity profiles agree well with the classical model of linear increase with elevation, especially near the seabed. Within the log region, profiles of shear production and dissipation rates of turbulence converge. Below it, dissipation rapidly increases, peaking near the seabed. Conversely, the shear production decays near the seabed, in agreement with the eddy viscosity model, but in contrast to both laboratory and computational rough wall studies. This article is protected by copyright. All rights reserved.

Description: Although much of the focus on future sea level rise concerns the long-term trend associated with anthropogenic warming, on shorter timescales, internal climate variability can contribute significantly to regional sea level. Such sea level variability should be taken into consideration when planning efforts to mitigate the effects of future sea level change. In this study, we quantify the contribution to regional sea level of the El Niño-Southern Oscillation (ENSO). Through cyclostationary empirical orthogonal function analysis (CSEOF) of the long reconstructed sea level dataset and of a set of United States tide gauges, two global modes dominated by Pacific Ocean variability are identified and related to ENSO and, by extension, the Pacific Decadal Oscillation. By estimating the combined contribution of these two modes to regional sea level, we find that ENSO can contribute significantly on short time scales, with contributions of up to 20 cm along the west coast of the U.S. The CSEOF decomposition of the long tide gauge records around the U.S. highlights the influence of ENSO on the U.S. east coast. Tandem analyses of both the reconstructed and tide gauge records also examine the utility of the sea level reconstructions for near-coast studies. This article is protected by copyright. All rights reserved.

Description: This paper presents a comprehensive numerical study into the infragravity-wave dynamics at a field site, characterised by a gently-sloping barred beach. The non-hydrostatic wave-flow model SWASH was used to simulate the local wave field for a range of wave conditions (including mild and storm conditions). The extensive spatial coverage of the model allowed us to analyse the infragravity-wave dynamics at spatial scales not often covered before. Overall, the model predicted a wave field that was representative of the natural conditions, supporting the model application to analyse the wave dynamics. The infragravity-wave field was typically dominated by leaky waves, except near the outer bar where bar-trapped edge waves were observed. Relative contributions of bar-trapped waves peaked during mild conditions, when they explained up to 50% of the infragravity variance. Near the outer bar, the infragravity wave growth was partly explained by nonlinear energy transfers from short-waves. This growth was strongest for mild conditions, and decreased for more energetic conditions when short-waves were breaking at the outer bar. Further shoreward, infragravity waves lost most of their energy, due to a combination of nonlinear transfers, bottom friction, and infragravity-wave breaking. Nonlinear transfers were only effective near the inner bar, whereas near the shoreline (where losses were strongest) the dissipation was caused by the combined effect of bottom friction and breaking. This study demonstrated the model's potential to study wave dynamics at field scales not easily covered by in-situ observations. This article is protected by copyright. All rights reserved.

Description: Microstructure and current velocity measurements were collected at a cross-channel transect in the James River under spring and neap tidal conditions in May 2010 to study cross-estuary variations in vertical mixing. Results showed that near-surface mixing was related to lateral circulation during the ebb phase of a tidal cycle, and that the linkage was somewhat similar from neap to spring tides. During neap tides, near-surface mixing was generated by the straining of lateral density gradients influenced by the advection of fresh, riverine water on the right side (looking seaward) of the transect. Spring tide results revealed similar findings on the right side of the cross-section. However on the left side, the straining by velocity shears acted in concert with density straining. Weak along-estuary velocities over the left shoal were connected to faster velocities in the channel via a clockwise lateral circulation (looking seaward). These results provided evidence that in the absence of direct wind forcing, near-surface vertical mixing can occur from mechanisms uncoupled from bottom friction. This article is protected by copyright. All rights reserved.

Description: This study uses the analysis of variance approaches to quantify the climate change signal and uncertainty in multi-model ensembles of statistical simulations of significant wave height ( H s ), which are based on the CMIP5 historical, RCP4.5 and RCP8.5 forcing scenario simulations of sea level pressure. Here, the signal of climate change refers to the temporal variations caused by the prescribed forcing. “Significant” means “significantly different from zero at 5% level”. In a 4-model ensemble of H s simulations, the common signal - the signal that is simulated in all the 4 models - is found to strengthen over time. For the historical followed by RCP8.5 scenario, the common signal in annual mean H s is found to be significant in 16.6% and 82.2% of the area by year 2005 and 2099, respectively. The global average of the variance proportion of the common signal increases from 0.75% in year 2005 to 12.0% by year 2099. The signal is strongest in the eastern tropical Pacific (ETP), featuring significant increases in both the annual mean and maximum of H s in this region. The climate model uncertainty (i.e., inter-model variability) is significant nearly globally; its magnitude is comparable to or greater than that of the common signal in most areas, except in the ETP where the signal is much larger. In a 20-model ensemble of H s simulations for the period 2006-2099, the model uncertainty is found to be significant globally; it is about 10 times as large as the variability between the RCP4.5 and 8.5 scenarios. This article is protected by copyright. All rights reserved.

Description: The subannual variability of total alkalinity (TA) distributions in the northeastern Gulf of Mexico was examined through the use of TA data from ship-based water sampling, historical records of riverine TA, and contemporaneous model output of surface currents and salinity. TA variability was restricted to the upper 150 m of the water column, where relationships between salinity and TA were controlled primarily by subannual variations in the extent of mixing between seawater and river water. A transition in TA distribution patterns between the river-dominated northern margin (near the Mississippi–Atchafalaya River System) and the ocean current-dominated eastern margin (West Florida Shelf) was observed. An index for riverine alkalinity input was formulated to provide insights about riverine alkalinity contributions in the upper water column. Spatial and temporal variations of total alkalinity in the northeastern Gulf of Mexico are primarily controlled by riverine TA inputs and ocean currents. This article is protected by copyright. All rights reserved.

Description: The interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi-3D nearshore community model, NearCoM-TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semi-diurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave-current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward-directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel-shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primarily by waves. This article is protected by copyright. All rights reserved.

Description: Arctic primary production is sensitive to reductions in sea ice cover, and will likely increase into the future. Whether this increased primary production (PP) will translate into increased export of particulate organic carbon (POC) is currently unclear. Here we report on the POC export efficiency during summer 2012 in the Atlantic sector of the Arctic Ocean. We coupled 234-Thorium based estimates of the export flux of POC to onboard incubation based estimates of PP. Export efficiency (defined as the fraction of PP that is exported below 100 m depth: ThE -ratio) showed large variability (0.09 ± 0.19 to 1.3 ± 0.3). The highest ThE -ratio (1.3 ± 0.3) was recorded in a mono-specific bloom of Phaeocystis pouchetii located in the ice edge. Blooming diatom dominated areas also had high ThE -ratios (0.1 ± 0.1 to 0.5 ± 0.2), while mixed and/or pre-bloom communities showed lower ThE -ratios (0.10 ± 0.03 to 0.19 ± 0.05). Furthermore, using oxygen saturation, bacterial abundance, bacterial production, and zooplankton oxygen demand, we also investigated spatial variability in the degree to which this sinking material may be remineralised in the upper mesopelagic (〈 300 m). Our results suggest that blooming diatoms and P. pouchetii can export a significant fraction of their biomass below the surface layer (100 m) in the open Arctic Ocean. Also, we show evidence that the material sinking from a P. pouchetii bloom may be remineralised (〉100m) at a similar rate as the material sinking from diatom blooms in the upper mesopelagic, contrary to previous findings. This article is protected by copyright. All rights reserved.

Description: Correlations of Trichodesmium colony abundance with the eddy field emerged in two segments of Video Plankton Recorder observations made in the southwestern North Atlantic during fall 2010 and spring 2011. In fall 2010, local maxima in abundance were observed in cyclones. We hypothesized surface Ekman transport convergence as a mechanism for trapping buoyant colonies in cyclones. Idealized models supported the potential of this process to influence the distribution of buoyant colonies over timescales of several months. In spring 2011, the highest vertically integrated colony abundances were observed in anticyclones. These peaks in abundance correlated with anomalously fresh water, suggesting riverine input as a driver of the relationship. These contrasting results in cyclones and anticyclones highlight distinct mechanisms by which mesoscale eddies can influence the abundance and distribution of Trichodesmium populations of the southwestern North Atlantic. This article is protected by copyright. All rights reserved.

Description: Previous studies have suggested that ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment (GRACE) can be used to measure the depth-averaged, or barotropic, transport variability of the Antarctic Circumpolar Current (ACC). Here, we use GRACE OBP observations to calculate transport variability in a region of the Southern Indian Ocean encompassing the major fronts of the ACC. We use a statistical analysis of a simulated GRACE-like data set to determine the uncertainty of the estimated transport for the 2003.0-2013.0 time period. We find that when the transport is averaged over 60° of longitude, the uncertainty (one standard error) is close to 1 Sv (1 Sv = 10 6 m 3 s −1 ) for low-pass filtered transport, which is significantly smaller than the signal and lower than previous studies have found. The interannual variability is correlated with the Southern Annual Mode (SAM) (0.61), but more highly correlated with circumpolar zonally averaged winds between 45°S – 65°S (0.88). GRACE transport reflects significant changes in transport between 2007 and 2009 that is observed in the zonal wind variations but not in the SAM index. We also find a statistically significant trend in transport (-1.0 ± 0.4 Sv yr −1 , 90% confidence) that is correlated with a local deceleration in zonal winds related to an asymmetry in the SAM on multi-decadal periods. This article is protected by copyright. All rights reserved.

Description: We present full-depth hydrographic and velocity observations across a solitary meander within the Agulhas Current, and assess the degree to which the meander causes upwelling, cross-frontal mixing, and diapycnal transport. These events can input nutrients onto the continental shelf as well as advect larvae offshore. We find that the meander drives inshore upwelling with vertical velocities of at least 13 m day – 1 . The meander also causes diapycnal transport resulting in 1°C cooler and 0.25 fresher central waters and 1°C warmer, 0.25 saltier intermediate waters below the thermocline. We introduce a new coordinate system that separates these changes into kinematic changes due to meandering and property changes along transport streamlines. This reveals that most of the observed diapycnal transport below the thermocline is due to property changes. We find a small amount of enhanced cross-frontal mixing associated with solitary meanders of the Agulhas Current, but it is statistically insignificant. We believe this is due to the strongly barotropic nature of the meandering Agulhas Current, which maintains a deep steering level that inhibits mixing. This article is protected by copyright. All rights reserved.

Description: Global sea levels have been rising through the past century and are projected to rise at an accelerated rate throughout the 21 st century. This has motivated a number of authors to search for already existing accelerations in observations, which would be, if present, vital for coastal protection planning purposes. No scientific consensus has been reached yet as to how a possible acceleration could be separated from intrinsic climate variability in sea level records. This has led to an intensive debate on its existence and, if absent, also on the general validity of current future projections. Here we shed light on the controversial discussion from a methodological point of view. To do so we provide a comprehensive review of trend methods used in the community so far. This resulted in an overview of 30 methods, each having its individual mathematical formulation, flexibilities and characteristics. We illustrate that varying trend approaches may lead to contradictory acceleration–deceleration inferences. As for statistics-oriented trend methods we argue that checks on model assumptions and model selection techniques yield a way out. However, since these selection methods all have implicit assumptions, we show that good modeling practices are of importance too. We conclude at this point that (i) several differently characterized methods should be applied and discussed simultaneously, (ii) uncertainties should be taken into account to prevent biased or wrong conclusions, and (iii) removing internally generated climate variability by incorporating atmospheric or oceanographic information helps to uncover externally forced climate change signals. This article is protected by copyright. All rights reserved.

Description: The West Greenland Current system (WGCS) transports heat and freshwater into the Labrador Sea, influencing the formation of Labrador Sea Water, a key component of the Atlantic Meridional Overturning Circulation. Notwithstanding its importance, relatively little is known about the structure and transport of this current system and its seasonal and interannual variability. Here we use historical hydrographic data from 1992 to 2008, combined with AVISO satellite altimetry, to diagnose the mean properties as well as seasonal and interannual variability of the boundary current system. We find that while the surface, fresh, cold West Greenland Current is amplified in summer, the subsurface warm, salty Irminger Current has maximum transport in winter, when its waters are also warmer and saltier. Seasonal changes in the total transport are thus mostly due to changes in the baroclinic structure of the current. By contrast, we find a trend towards warmer/saltier waters and a slow down of the WGCS, within the period studied. The latter is attributed to changes in the barotropic component of the current. Superimposed on this trend, warm and salty anomalies transit through the system in 1997 and 2003 and are associated with a rapid increase in the transport of the boundary current due to changes in the baroclinic component. The boundary current changes precede similar changes in the interior with a one-two year lag, indicating that anomalies advected into the region by the boundary current can play an important role in the modulation of convection in the Labrador Sea. This article is protected by copyright. All rights reserved.

Description: The structure of the Antarctic Slope Front (ASF) and the associated Antarctic Slope Current (ASC) on the Scotia Sea side of the Weddell-Scotia Confluence (WSC) is described using data from a hydrographic survey and three 1-year long moorings across the continental slope. The ASC in this region flows westwards along isobaths with an annual mean speed of ∼0.2 m s – 1 , with time variability dominated by the K 1 and O 1 tidal diurnal constituents, a narrowband oscillation with ∼2-week period attributable to the spring/neap tidal cycle, and seasonal variability. Realistic and idealized high-resolution numerical simulations are used to determine the contribution of tides to the structure of the ASF and the speed of the ASC. Two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured ASF/ASC cross-slope structure, the time variability at our moorings, and the reduced stratification within the WSC. Two idealized simulations run with tide-only forcing, one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous, show that tides can generate the ASC and ASF through volume flux convergence along the slope initiated by effects including the Lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior. Climate models that exclude the effects of tides will not correctly represent the ASF and ASC or their influence on the injection of intermediate and dense waters from the WSC to the deep ocean. This article is protected by copyright. All rights reserved.

Description: To better describe the state of sea ice in the marginal ice zone (MIZ) with floes of varying thicknesses and sizes, both an ice thickness distribution (ITD) and a floe size distribution (FSD) are needed. In this work, we have developed a FSD theory that is coupled to the ITD theory of Thorndike et al . [1975] in order to explicitly simulate the evolution of FSD and ITD jointly. The FSD theory includes a FSD function and a FSD conservation equation in parallel with the ITD equation. The FSD equation takes into account changes in FSD due to ice advection, thermodynamic growth, and lateral melting. It also includes changes in FSD because of mechanical redistribution of floe size due to ice ridging and, particularly, ice fragmentation induced by stochastic ocean surface waves. The floe size redistribution due to ice fragmentation is based on the assumption that wave-induced breakup is a random process such that when an ice floe is broken, floes of any smaller sizes have an equal opportunity to form, without being either favored or excluded. To focus only on the properties of mechanical floe size redistribution, the FSD theory is implemented in a simplified ITD and FSD sea ice model for idealized numerical experiments. Model results show that the simulated cumulative floe number distribution (CFND) follows a power law as observed by satellites and airborne surveys. The simulated values of the exponent of the power law, with varying levels of ice breakups, are also in the range of the observations. It is found that floe size redistribution and the resulting FSD and mean floe size do not depend on how floe size categories are partitioned over a given floe size range. The ability to explicitly simulate multicategory FSD and ITD together may help to incorporate additional model physics, such as FSD-dependent ice mechanics, surface exchange of heat, mass, and momentum, and wave-ice interactions. This article is protected by copyright. All rights reserved.

Description: A simple scheme is developed to represent Sea Surface Diurnal Cycling (SSDC) in Coupled General Circulation Models (CGCM). It follows Zeng and Beljaars [2005], but in addition to a night-time deep well mixed ocean boundary layer and a deep day-time stable layer, a shallow sub-grid-scale stable diurnal boundary layer is allowed to develop during the day, followed by a deepening convective layer. These four regimes have empirical property profiles and their governing parameters are determined by comparison of idealized experiments with published in situ and satellite observations. Mixing across the base of the shallow stable layer is governed by a gradient Richardson number, so prognostic equations are solved for salinity and current, as well as temperature. A conclusion is that the timing of peak warming depends on diurnal shear. The SSDC is implemented in the Community Earth System Model (CESM) for multiple purposes: the maximum diurnal amplitude of warming is found to exceed 5 ∘ C and to be more than 2 ∘ C over most of the ocean; the global distribution of average daytime minus night-time SST is used to validate the SSDC against a satellite SST product; and the mean seasonal surface heat flux and precipitation from an uncoupled CESM atmosphere are used to show the climate impacts that might be expected in a CGCM. Two major conclusions are that these impacts are not negligible and that much of the observed signals of diurnal cycling are captured by SSDC without the computational expense of resolving the relevant ocean processes. This article is protected by copyright. All rights reserved.

Description: The oligotrophic regions of the subtropical gyres cover a significant portion of the global ocean, and exhibit considerable but poorly understood intraseasonal, interannual, and longer-term variations in spatial extent. Here, using historical observations of surface ocean nitrate, wind, and currents, we have investigated how horizontal and vertical supplies of nitrate control seasonal changes in the size and shape of oligotrophic regions of the subtropical North Atlantic. In general, the oligotrophic region of the subtropical North Atlantic is associated with the region of weak vertical supply of nitrate. Though the total vertical supply of nitrate here is generally greater than the total horizontal supply, we find that seasonal expansion and contraction of the oligotrophic region is consistent with changes in horizontal supply of nitrate. In this dynamic periphery of the subtropical gyre, the seasonal variations in chlorophyll are linked to variations in horizontal nitrate supply that facilitate changes in intracellular pigment concentrations, and to a lesser extent, phytoplankton biomass. Our results suggest that horizontal transports of nutrient are crucial in setting seasonal cycles of chlorophyll in large expanses of the subtropical North Atlantic, and may play a key and underappreciated role in regulating interannual variations in these globally important marine ecosystems. This article is protected by copyright. All rights reserved.

Description: Climate variations influence North Atlantic winter storm intensity and resultant variations in wave energy levels. A 60-year hindcast allows investigation of the influence of decadal climate variability on long-term trends of North Atlantic wave power, P W , spanning the 1948-2008 epoch. P W variations over much of the eastern North Atlantic are strongly influenced by the fluctuating North Atlantic Oscillation (NAO) atmospheric circulation pattern, consistent with previous studies of significant wave height, Hs . Wave activity in the western Atlantic also responds to fluctuations in Pacific climate modes, including the Pacific North American (PNA) pattern and the El Niño/Southern Oscillation. The magnitude of upward long-term trends during winter over the northeast Atlantic is strongly influcenced by heightened storm activity under the extreme positive phase of winter NAO in the early 1990's. In contrast, P W along the United States East Coast shows no increasing trend, with wave activity there most closely associated with the PNA. Strong wave power “events” exhibit significant upward trends along the Atlantic coasts of Iceland and Europe during winter months. Importantly, in opposition to the long-term increase of P W , a recent general decrease in P W across the North Atlantic from 2000 to 2008 occurred. The 2000-2008 decrease was associated with a general shift of winter NAO to its negative phase, underscoring the control exerted by fluctuating North Atlantic atmospheric circulation on P W trends. This article is protected by copyright. All rights reserved.

Description: The short-term (less than 20 days) response of surface circulation to wind has been determined in waters of the southeastern Bay of Biscay, using wind impulse response (time domain) and transfer (frequency domain) functions relating high-frequency radar currents and reanalysis winds. The response of surface currents is amplified at the near-inertial frequency and the low-frequency and it varies spatially. The analysis indicates that the response of the ocean to the wind is slightly anisotropic, likely due to pressure gradients and friction induced by the bottom and coastline boundaries in this region. Thus, the transfer function at the near-inertial frequency decreases onshore due to the coastline inhibition of circularly-polarized near-inertial motion. In contrast, the low-frequency transfer function is enhanced towards the coast as a result of the geostrophic balance between the cross-shore pressure gradient and the Coriolis forces. The transfer functions also vary with season. In summer, the current response to wind is expected to be stronger but shallower due to stratification; in winter, the larger mixed layer depth results in a weaker but deeper response. The results obtained are consistent with the theoretical description of wind-driven circulation and can be used to develop a statistical model with a broad range of applications including accurate oceanic forecasting and understanding of the coupled atmosphere-ocean influence on marine ecosystems. This article is protected by copyright. All rights reserved.

Description: Alongshore force balances, including the role of nonlinear advection, in the shoaling and surf zones onshore of a submarine canyon are investigated using a numerical modeling system (Delft3D/SWAN). The model is calibrated with waves and alongshore flows recorded over a period of 1.5 months at 26 sites along the 1.0-, 2.5-, and 5.0-m depth contours spanning about 2km of coast. Field-observation-based estimates of the alongshore pressure and radiation-stress gradients are reproduced well by the model. Model simulations suggest that the alongshore momentum balance is between the sum of the pressure and radiation-stress gradients and the sum of the nonlinear advective terms and bottom stress, with the remaining terms (e.g., wind stress and turbulent mixing) being negligible. The simulations also indicate that unexplained residuals in previous field-based estimates of the momentum balance may be owing to the neglect of the nonlinear advective terms, which are similar in magnitude to the sum of the forcing (pressure and radiations stress gradients) and to the bottom stress. This article is protected by copyright. All rights reserved.

Description: Measurements of currents and turbulence beneath a geostationary ship in the equatorial Indian Ocean during a period of weak surface forcing revealed unexpectely strong turbulence beneath the surface mixed layer. Coincident with the turbulence was a marked reduction of the current speeds registered by shipboard Doppler current profilers, and an increase in their variability. At a mooring 1 km away, measurements of turbulence and currents showed no such anomalies. Correlation with the shipboard echosounder measurements indicate that these nighttime anomalies were associated with fish aggregations beneath the ship. The fish created turbulence by swimming against the strong zonal current in order to remain beneath the ship, and their presence affected the Doppler speed measurements. The principal characteristics of the resultant ichthyogenic turbulence are i) low wavenumber rolloff of shear spectra in the inertial subrange relative to geophysical turbulence, ii) Thorpe overturning scales that are small compared with the Ozmidov scale, and iii) low mixing efficiency. These factors extend previous findings by Gregg and Horne [2009] to a very different biophysical regime, and support the general conclusion that the biological contribution to mixing the ocean via turbulence is negligible. This article is protected by copyright. All rights reserved.

Description: We analyze a concurrent simulation result of the ocean circulation and tidal currents using a data assimilative ocean general circulation model covering the Western North Pacific with horizontal resolution of 1/36to investigate possible interactions between them. Four sites of active M 2 internal tide variability in open ocean (hot spots): Tokara Strait, Izu Ridge, Luzon Strait, and Ogasawara Ridge, are detected from both the satellite observation and the simulation. Energy cycle analysis of the simulated M 2 baroclinic tide indicates two types of the hot spots: dissipation (Tokara Strait and Izu Ridge) and radiation (Luzon Strait and Ogasawara Ridge) dominant sites. Energy conversion from barotropic to baroclinic M 2 tides at the hot spots is modulated considerably by the lower-frequency changes in the density field. Modulation at the two spots (Tokara Strait and Izu Ridge) is affected by the Kuroshio path variation together with the seasonal variation of the shallow thermocline. At the other two sites, influence from changes in the relatively deep stratification through the Kuroshio intrusion into South China Sea (Luzon Strat) and mesoscale eddy activity (Ogasawara Ridge) is dominant in the modulation. This article is protected by copyright. All rights reserved.

Description: The net ecosystem metabolism of the seagrass Thalassia testudinum was studied across a nutrient and productivity gradient in Florida Bay, Florida, using the Eulerian control volume, eddy covariance, and biomass addition techniques. In situ oxygen fluxes were determined by a triangular Eulerian control volume with sides 250m long and by eddy covariance instrumentation at its center. The biomass addition technique evaluated the aboveground seagrass productivity through the net biomass added. The spatial and temporal resolutions, accuracies, and applicability of each method were compared. The eddy covariance technique better resolved the short-term flux rates and the productivity gradient across the bay, which was consistent with the long-term measurements from the biomass addition technique. The net primary production rates from the biomass addition technique, which were expected to show greater autotrophy due to the exclusion of sediment metabolism and belowground production, were 71, 53, and 30 mmol carbon m −2 d −1 at 3 sites across the bay. The net ecosystem metabolism was 35, 25, and 11 mmol oxygen m −2 d −1 from the eddy covariance technique and 10, -103, and 14 mmol oxygen m −2 d −1 from the Eulerian control volume across the same sites, respectively. The low-flow conditions in the shallow bays allowed for periodic stratification and long residence times within the Eulerian control volume that likely limited its precision. Overall, the eddy covariance technique had the highest temporal resolution while producing accurate long-term flux rates that surpassed the capabilities of the biomass addition and Eulerian control volume techniques in these shallow coastal bays. This article is protected by copyright. All rights reserved.

Description: During the summer of 2012, we used laser diffractometry to investigate the temporal and vertical variability of the particle size spectrum (1.25-100 µm in equivalent diameter) in the euphotic zone of the North Pacific Subtropical Gyre. Particles measured with this optical method (1.25-100 µm) accounted for ∼40% of the particulate carbon stocks in the upper euphotic zone (25-75 m), as estimated using an empirical formula to transform particle volume to carbon concentrations. Over the entire vertical layer considered (20-180 m), the largest contribution to particle volume corresponded to particles between 3 and 10 µm in diameter. Although the exponent of a power-law parameterization suggested that larger particles had a lower relative abundance than in other regions of the global ocean, this parameter, and hence conclusions about relative particle abundance, are sensitive to the shape of the size distribution and to the curve fitting method. Results on the vertical distribution of particles indicate that different size fractions varied independently with depth. Particles between 1.25 and 2 µm reached maximal abundances coincident with the depth of the chlorophyll a maximum (averaging 121 ± 10 m), where eukaryotic phytoplankton abundances increased. In contrast, particles between 2 and 20 µm tended to accumulate just below the base of the mixed layer (41 ± 14 m). Variability in particle size tracked changes in the abundance of specific photoautotrophic organisms (measured with flow cytometry and pigment concentration), suggesting that phytoplankton population dynamics are an important control of the spatiotemporal variability in particle concentration in this ecosystem. This article is protected by copyright. All rights reserved.

Description: Antarctic ice sheet mass loss has been linked to an increase in oceanic heat supply, which enhances basal melt and thinning of ice shelves. Here, we detail the interaction of modified Circumpolar Deep Water (mCDW) with the Amery Ice Shelf, the largest ice shelf in East Antarctica, and provide the first estimates of basal melting due to mCDW. We use sub–ice shelf ocean observations from a borehole site (AM02) situated ∼70 km inshore of the ice shelf front, together with open ocean observations in Prydz Bay. We find that mCDW transport into the cavity is about 0.22 ± 0.06 Sv (1Sv = 10 6 m 3 s −1 ). The inflow of mCDW drives a net basal melt rate of up to 2 ± 0.5 m yr −1 during 2001 (23.9 ± 6.52 Gt yr −1 from under about 12,800 km 2 of the north–eastern flank of the ice shelf). The heat content flux by mCDW at AM02 shows high intra–annual variability (up to 40%). Our results suggest two main modes of sub–ice shelf circulation and basal melt regimes: (1) the “ice pump”/high salinity shelf water circulation, on the western flank and (2) the mCDW meltwater–driven circulation in conjunction with the “ice pump”, on the eastern flank. These results highlight the sensitivity of the Amery's basal melting to changes in mCDW inflow. Improved understanding of such ice shelf–ocean interaction is crucial to refining projections of mass loss and associated sea-level rise. This article is protected by copyright. All rights reserved.

Description: We consider the effect of rain on wind wave generation and dissipation. Rain falling on a wavy surface may have a marked tendency to dampen the shorter waves in the tail of the spectrum, the related range increasing with the rain rate. Historical and sailors' reports suggest this leads to calmer wave conditions, certainly so for the action of breakers. We have explored this situation using a fully coupled meteorological-wave model system, adding an artificial rain rate dependent damping of the tail. Contrarily to direct marine experience, the experimental results show higher wind speeds and wave heights. A solid indication of the truth is achieved with the direct comparison between operational model (where rain effect is ignored) and measured data. These strongly support the sailors' claims of less severe wave conditions under heavy rain. This leads to a keen analysis of the overall process, in particular on the role of the tail of the spectrum in modulating the wind input and the white-capping, and how this is presently modeled in operational activity. We suggest that some revision is due and that the relationship between white-capping and generation by wind is deeper and more implicative than presently generally assumed. This article is protected by copyright. All rights reserved.

Description: River-tide dynamics remain poorly understood, in part because of conventional harmonic analysis (HA) does not cope effectively with non-stationary signals. To explore non-stationary behavior of river tides and the modulation effects of river discharge, this work analyzes tidal signals in the Yangtze River estuary using both HA in a non-stationary mode and continuous wavelet transforms (CWT). The Yangtze is an excellent natural laboratory in which to analyze river tides, because of its high and variable flow, its length, and the fact that there are do dams or reflecting barriers within the tidal part of the system. Analysis of tidal frequencies by CWT and analysis of subtidal water level and tidal ranges reveal a broad range of subtidal variations over fortnightly, monthly, semi-annual, and annual frequencies, driven by subtidal variations in friction and by variable river discharges. We employ HA in a non-stationary mode (NSHA) by segregating data within defined flow ranges into separate analyses. NSHA quantifies the decay of the principal tides and the modulation of M 4 tide with increasing river discharges. M 4 amplitudes decrease far upriver (landward portion of the estuary) and conversely increase close to the ocean as river discharge increases. The fortnightly frequencies reach an amplitude maximum upriver of that for over tide frequencies, due to the longer wavelength of the fortnightly constituents. These methods and findings should be applicable to large tidal rivers globally, and have broad implications regarding management of navigation channels and ecosystems in tidal rivers. This article is protected by copyright. All rights reserved.

Description: Ocean tracers such as carbon dioxide, nutrients, plankton, and oil advect, diffuse, and react primarily in the oceanic mixed layer where air-sea gas exchange occurs and light is plentiful for photosynthesis. There can be substantial heterogeneity in the spatial distributions of these tracers due to turbulent stirring, particularly in the submesoscale range where partly geostrophic fronts and eddies and small-scale three-dimensional turbulence are simultaneously active. In this study, large eddy simulations spanning horizontal scales from 20km down to 5m are used to examine the effects of multiscale turbulent mixing on nonreactive passive ocean tracers from interior and sea-surface sources. The simulations include the effects of both wave-driven Langmuir turbulence and submesoscale eddies, and tracers with different initial and boundary conditions are examined in order to understand the respective impacts of small-scale and submesoscale motions on tracer transport. Tracer properties are characterized using spatial fields and statistics, multiscale fluxes, and spectra, and the results detail how tracer mixing depends on air-sea tracer flux rate, tracer release depth, and flow regime. Although vertical fluxes of buoyancy by submesoscale eddies compete with mixing by Langmuir turbulence, vertical fluxes of tracers are often dominated by Langmuir turbulence, particularly for tracers that are released near the mixed layer base or that dissolve rapidly through the surface, even in regions with pronounced submesoscale activity. Early in the evolution of some tracers, negative eddy diffusivities occur co-located with regions of negative potential vorticity, suggesting that symmetric instabilities or other submesoscale phenomenon may act to oppose turbulent mixing. This article is protected by copyright. All rights reserved.

Description: Previous observational studies have found increasing primary production (PP) in response to declining sea ice cover in the Arctic Ocean. In this study, under-ice PP was assessed based on three coupled ice-ocean-ecosystem models participating in the Forum for Arctic Modeling and Observational Synthesis (FAMOS) project. All models showed good agreement with under-ice measurements of surface chlorophyll-a concentration and vertically integrated PP rates during the main under-ice production period, from mid-May to September. Further, modeled thirty-year (1980-2009) mean values and spatial patterns of sea ice concentration compared well with remote sensing data. Under-ice PP was higher in the Arctic shelf seas than in the Arctic Basin, but ratios of under-ice PP over total PP were spatially correlated with annual mean sea ice concentration, with higher ratios in higher ice concentration regions. Decreases in sea ice from 1980 to 2009 were correlated significantly with increases in total PP and decreases in the under-ice PP/total PP ratio for most of the Arctic, but nonsignificantly related to under-ice PP, especially in marginal ice zones. Total PP within the Arctic Circle increased at an annual rate of between 3.2 and 8.0 Tg C/year from 1980 to 2009. This increase in total PP was due mainly to a PP increase in open water, including increases in both open water area and PP rate per unit area, and therefore much stronger than the changes in under-ice PP. All models suggested that, on a pan-Arctic scale, the fraction of under-ice PP declined with declining sea ice cover over the last three decades. This article is protected by copyright. All rights reserved.

Description: Four moorings were deployed near “Dragon Flag”, an active hydrothermal vent in the valley of the Southwest Indian Ridge. The goal was to examine the variability of currents and temperature, which will guide the trajectory of spreading plumes. The mean current was cross-isobath, and the circulation was characterized by a sub-mesoscale circulation. Observed currents also showed fluctuations with periods of 1-15 days. The inferred phase speed and wavelength for the wave with a period of 4.4 days are 10.4 km⋅d −1 and 45.8km, respectively, which are consistent with the topographic Rossby wave theory. The persistent warming tendency with corresponding variation of salinity based on background θ - S properties may be caused by background circulation and divergence of the water column. The warming or cooling episodes were most likely as signatures of isopycnal surface depression or uplifting induced by the moving of mesoscale eddies. Well resolved rotary spectra exhibited important nonlinear interactions between inertial and semi-diurnal tide in the velocity and temperature records. Amplification of near-inertial currents in the near bottom are also exposed. These discoveries provided new evidence for the non-linear interaction and trapped near-inertial waves by the ridge, which occurred in the deep ocean of the Southern Hemisphere. Such nonlinear interaction may represent a significant energy loss pathway for the internal waves, and part of the decay of such motion would likely result in increased mixing to maintain the abyssal stratification. Enhanced near-inertial motions can play a major role for the local advection of hydrothermal plumes. This article is protected by copyright. All rights reserved.

Description: We examine four years (2011-2014) of sea surface heights (SSH) from CryoSat-2 (CS-2) over the ice-covered Arctic and Southern Oceans. Results are from a procedure that identifies and determines the heights of sea surface returns. Along 25-km segments of satellite ground tracks, variability in the retrieved SSHs is between ∼2-3 cm (standard deviation) in the Arctic and slightly higher (∼3 cm) in the summer and the Southern Ocean. Average sea surface tilts (along these 25-km segments) are 0.01±3.8 cm/10 km in the Arctic, and slightly lower (0.01±2.0 cm/10 km) in the Southern Ocean. Intra-seasonal variability of CS-2 dynamic ocean topography (DOT) in the ice-covered Arctic is nearly twice as high as that of the Southern Ocean. In the Arctic, we find a correlation of 0.92 between three years of DOT and dynamic heights (DH) from hydrographic stations. Further, correlation of four years of area-averaged CS-2 DOT near the North Pole and time-variable ocean-bottom pressure from a pressure gauge and from GRACE, yields coefficients of 0.83 and 0.77, with corresponding differences of 〈3 cm (RMS). These comparisons contrast the length scale of baroclinic and barotropic features and reveal the smaller amplitude barotropic signals in the Arctic Ocean. Broadly, the mean DOT from CS-2 for both poles compares well with those from the ICESat campaigns and the DOT2008A and DTU13MDT fields. Short length scale topographic variations, due to oceanographic signals and geoid residuals, are especially prominent in the Arctic Basin but less so in the Southern Ocean. This article is protected by copyright. All rights reserved.

Description: Here we present the first record of Sr/Ca variability in a massive Porites lutea coral from the Lakshadweep Archipelago, Arabian Sea. The annual mean sea surface temperature (SST) in this region and the surrounding areas has increased steadily in the recent past. During some major El Niño events, SSTs are even higher, imposing additional thermal-stress on corals, episodically leading to coral bleaching. We infer from the coral-Sr/Ca record (1981 to 2008) that during some of these events high and persistent SSTs lead to a dampening of the temperature signal in coral-Sr/Ca, impairing the coral's ability to record full scale warming. Thus, coral-Sr/Ca may provide a history of past El Niño Southern-Oscillation (ENSO) induced thermal-stress episodes, which are a recurrent feature also seen in cross-spectral analysis between coral-Sr/Ca and the Nino3.4 index. Despite the impact of episodical thermal-stress during major El Niño events, our coral proxy faithfully records the seasonal monsoon-induced summer cooling on the order of ∼2.3 ˚C. Calibration of coral-Sr/Ca with instrumental grid-SST data shows significant correlation to regional SST and monsoon variability. Hence, massive Porites corals of this region are highly valuable archives for reconstructing long-term changes in SST, strongly influenced by monsoon variability on seasonal scales. More importantly, our data show that this site with increasing SST is an ideal location for testing the future effects of the projected anthropogenic SST increase on coral reefs that are already under thermal-stress worldwide. This article is protected by copyright. All rights reserved.

Description: The Intergovernmental Panel on Climate Change (IPCC) fifth assessment of projected global and regional ocean temperature change is based on global climate models that have coarse (∼100-km) ocean and atmosphere resolutions. In the Northwest Atlantic, the ensemble of global climate models has a warm bias in sea surface temperature due to a misrepresentation of the Gulf Stream position; thus, existing climate change projections are based on unrealistic regional ocean circulation. Here we compare simulations and an atmospheric CO 2 doubling response from four global climate models of varying ocean and atmosphere resolution. We find that the highest resolution climate model (∼10-km ocean, ∼50-km atmosphere) resolves Northwest Atlantic circulation and water mass distribution most accurately. The CO 2 doubling response from this model shows that upper-ocean (0-300 m) temperature in the Northwest Atlantic Shelf warms at a rate nearly twice as fast as the coarser models and nearly three times faster than the global average. This enhanced warming is accompanied by an increase in salinity due to a change in water mass distribution that is related to a retreat of the Labrador Current and a northerly shift of the Gulf Stream. Both observations and the climate model demonstrate a robust relationship between a weakening Atlantic Meridional Overturning Circulation (AMOC) and an increase in the proportion of Warm-Temperate Slope Water entering the Northwest Atlantic Shelf. Therefore, prior climate change projections for the Northwest Atlantic may be far too conservative. These results point to the need to improve simulations of basin and regional-scale ocean circulation. This article is protected by copyright. All rights reserved.

Description: Wave directions and mean currents observed for two 1-month-long periods in 7- and 2-m water depths along 11 km of the southern shoreline of Martha's Vineyard, MA have strong tidal modulations. Wave directions are modulated by as much as 70 ° over a tidal cycle. The magnitude of the tidal modulations to the wave field decreases alongshore to the west, consistent with the observed decrease in tidal currents from 2.1 to 0.2 m/s along the shoreline. A numerical model (SWAN and Deflt3D-FLOW) simulating waves and currents reproduces the observations accurately. Model simulations with and without wave-current interaction and tidal depth changes demonstrate that the observed tidal modulations of the wave field primarily are caused by wave-current interaction and not by tidal changes to water depths over the nearby complex shoals. This article is protected by copyright. All rights reserved.

Description: A composite radar scattering model composed of the atmosphere radiative transfer model and the ocean surface Bragg wave theory is developed to analyze the impact of hurricane rain on the normalized radar-backscatter cross-section (NRCS) measured in the VV- and cross-polarized C-band Synthetic Aperture Radar (SAR) channels. The model results are validated against SAR and SFMR measured wind speeds and rain rates for two hurricane cases. The contribution of rain to the NRCS is backscatter from two parts: the atmosphere column and the ocean surface. In the atmosphere, microwave attenuation and the rain-induced volume backscattering are simulated by the model. We find that the impact of raindrops in the atmosphere is almost negligible for the VV polarization, but important for the cross-polarization. On the ocean surface, comparisons between our model and other existing models without rain lead to the conclusion that the VV polarization NRCS can be simulated reasonably well without considering the non-Bragg scattering mechanisms. Similar to the wave breaking mechanism, the microwave diffraction on the craters, crowns and stalks, produced by rain drops, are also negligible for VV polarization. However, the non-Bragg scattering is important for the cross-polarized NRCS simulations. Finally, we performed simulations to understand the VV-polarized NRCS behavior under different wind speeds at various rain rates. This article is protected by copyright. All rights reserved.

Description: Surface wave transformation and the resulting nearshore circulation along a section of coast with strong alongshore bathymetric gradients outside the surf zone are modeled for a consecutive four-week time period. The modeled hydrodynamics are compared to in situ measurements of waves and currents collected during the Nearshore Canyon Experiment and indicate that for the entire range of observed conditions, the model performance is similar to other studies along this stretch of coast. Strong alongshore wave height gradients generate rip currents that are observed by remote sensing data and predicted qualitatively well by the numerical model. Previous studies at this site have used idealized scenarios to link the rip current locations to undulations in the offshore bathymetry but do not explain the dichotomy between permanent offshore bathymetric features and intermittent rip current development. Model results from the month-long simulation are used to track the formation and location of rip currents using hourly statistics, and results show that the direction of the incoming wave energy strongly controls whether rip currents form. In particular, most of the offshore wave spectra were bi-modal and we find that the ratio of energy contained in each mode dictates rip current development, and the alongshore rip current position is controlled by the incident wave period. Additionally, model simulations performed with and without updating the nearshore morphology yield no significant change in the accuracy of the predicted surf zone hydrodyanmics indicating that the large scale offshore features (e.g. submarine canyon) predominately control the nearshore wave-circulation system. This article is protected by copyright. All rights reserved.

Description: In this paper a new methodology is proposed for the computation of Background Errors in wave data assimilation systems. Background errors define the spatial influence of an observation in the model domain. Since at present the directional wave spectrum is the fundamental variable of both state-of-the-art numerical models and most modern instrumentation, this is at the core of the proposed methodology. The advantage of the spectral approach is that the wave spectrum contains detailed information of the different wave systems and physical processes at work (e.g., wind sea or swells). These systems have different origins and may be driven by different mechanisms, having therefore different spatial structures, length scales and sensitivity to local wind conditions. The presented method enables making consistent and specific corrections to each component of the spectrum, in time and space. The innovations presented here require an integral look at the data assimilation algorithm for which a suitable scheme is also proposed. Examples of computed background errors are presented for shelf and oceanic basins showing the spatial structures of the different wave systems active in these areas. This article is protected by copyright. All rights reserved.

Description: The heat influx of the Atlantic water and its interannual variability through the Fram Strait toward the Arctic Ocean are examined by using a realistically configured ice-ocean general circulation model. The modeled routes of the Atlantic water and high eddy activity around the Fram Strait are consistent with many observations. Two-thirds of the heat transported by the Atlantic water passing through the Fram Strait (78°N) is lost by the westward transport and the sea surface cooling, and the other one-third is injected to the Arctic Ocean. The contribution of oceanic eddy to the westward heat transport is 5% of that of mean current. The variability of sea level pressure anomaly centered at the Nordic Seas explains the interannual variability of the heat passing through the Fram Strait, transported westward, and cooled at the sea surface in the north of the Fram Strait. The interannual variabilities of these heat fluxes have significant correlations with the NAO. The interannual variability of heat transported by the Atlantic water and entering the Arctic Ocean is caused by the variability of the Siberian high. This article is protected by copyright. All rights reserved.

Description: Near-bed sediment pickup is critical for predictions of intrawave suspension and in turn net sediment transport in coastal models. In the present study, numerical results from a two-dimensional Reynolds-averaged Navier-Stokes model are used to assess the functional relationship of intrawave ripple-averaged sediment pickup above steep ripples. The numerical model provides intrawave time histories of ripple-averaged near-bed velocities and turbulence, which are qualitatively interrogated to determine pickup functional relationships. Several specific sediment pickup formulations are implemented within the numerical model: expressions relating pickup to near-bed velocity or near-bed turbulent kinetic energy via the bed shear stress; and expressions relating pickup to near-bed shear production of turbulent kinetic energy. These are then tested via model-data comparisons of near-bed suspended sediment concentration. The results show that the traditional functions relating sediment pickup to near-bed velocity cannot lead to reasonable intrawave suspension predictions above vortex ripples under a ripple-averaged framework. Instead, relating sediment pickup to near-bed turbulence quantities, such as turbulent kinetic energy or shear production of turbulent kinetic energy, significantly improves the numerical predictions for these conditions. This article is protected by copyright. All rights reserved.

Description: Basal melting of floating ice shelves and iceberg calving constitute, the two almost equal paths of freshwater flux between the Antarctic, ice cap and the Southern Ocean. The largest icebergs (〉100km 2 ), transport most of the ice volume but their basal melting is small, compared to their breaking into smaller icebergs that constitute thus, the major vector of freshwater. The archives of nine altimeters have, been processed to create a small icebergs (〈8km 2 ) database, of positions, sizes and volumes spanning the 1992-2014 period. The, inter-calibrated monthly ice volumes from the different altimeters, have been merged in an homogeneous 23 year climatology. The iceberg, size distribution, covering the 0.1-10000 km 2 range, estimated, by combining small and large icebergs size measurements follows well, a power law of slope -1.52±0.32 close to the -3/2 laws observed, and modeled for brittle fragmentation. The global volume of ice and, its distribution between the ocean basins present a very strong inter-annual, variability only partially explained by the number of large icebergs., Indeed, vast zones of the Southern Ocean free of large icebergs are, largely populated by small iceberg drifting over thousands of km., The correlation between the global small and large icebergs volumes, shows that small icebergs are mainly generated by large ones breaking., Drifting and trapping by sea ice can transport small icebergs for, long period and distances. Small icebergs act as an ice diffuse process, along large icebergs trajectories while sea ice trapping acts as a, buffer delaying melting. This article is protected by copyright. All rights reserved.