ALBERT

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Description: Fire plays an important role in structuring vegetation in fire-prone regions worldwide. Progress has been made towards documenting the effects of individual fire events and fire regimes on vegetation structure; less is known of how different fire history attributes (e.g., time-since-fire, fire frequency) interact to affect vegetation. Using the temperate eucalypt ‘foothill’ forests of south-eastern Australia as a case-study system, we examine two hypotheses about such interactions: 1) that post-fire vegetation succession (e.g., time-since-fire effects) is influenced by other fire regime attributes, and 2) that the severity of the most recent fire overrides the effect of preceding fires on vegetation structure. Empirical data on vegetation structure were collected from 540 sites distributed across central and eastern Victoria, Australia. Linear mixed models were used to examine these hypotheses, and determine the relative influence of fire and environmental attributes on vegetation structure. Fire history measures, particularly time-since-fire, affected several vegetation attributes including ground and canopy strata; others such as low and sub-canopy vegetation were more strongly influenced by environmental characteristics like rainfall. There was little support for the hypothesis that post-fire succession is influenced by fire history attributes other than time-since-fire: only canopy regeneration was influenced by another variable (‘fire type’, representing severity). Our capacity to detect an overriding effect of the severity of the most recent fire was limited by a consistently weak effect of preceding fires on vegetation structure. Overall, results suggest the primary way that fire affects vegetation structure in foothill forests is via attributes of the most recent fire, both its severity and time since its occurrence: other attributes of fire regimes (e.g., fire interval, frequency) have less influence. The strong effect of environmental drivers such as rainfall and topography on many structural features show that foothill forest vegetation is also influenced by factors outside human control. While fire is amenable to human management, results suggest that at broad scales, structural attributes of these forests are relatively resilient to the effects of current fire regimes. Nonetheless, the potential for more frequent severe fires at short intervals, associated with a changing climate and/or fire management, warrant further consideration. This article is protected by copyright. All rights reserved.

Description: Integral projection models (IPMs) have a number of advantages over matrix-model approaches for analyzing size-structured population dynamics, because the latter require parameter estimates for each age or stage transition. However, IPMs still require appropriate data. Typically they are parameterized using individual-scale relationships between body size and demographic rates, but these are not always available. Here we present an alternative approach for estimating demographic parameters from time series of size-structured survey data using a Bayesian state-space IPM (SSIPM). By fitting an IPM in a state-space framework, we estimate unknown parameters and explicitly account for process and measurement error in a dataset to estimate the underlying process model dynamics. We tested our method by fitting SSIPMs to simulated data; the model fit the simulated size distributions well and estimated unknown demographic parameters accurately. We then illustrated our method using 9 years of annual surveys of the density and size distribution of two fish species (blue rockfish, Sebastes mystinus , and gopher rockfish, S. carnatus ) at seven kelp forest sites in California. The SSIPM produced reasonable fits to the data, and estimated fishing rates for both species that were higher than our Bayesian prior estimates based on coast-wide stock assessment estimates of harvest. That improvement reinforces the value of being able to estimate demographic parameters from local-scale monitoring data. We highlight a number of key decision points in SSIPM development (e.g., open vs. closed demography, number of particles in the state-space filter) so that users can apply the method to their own datasets. This article is protected by copyright. All rights reserved.

Description: Extensive outbreaks of bark beetles have killed trees across millions of hectares of forests and woodlands in western North America. These outbreaks have led to spirited scientific, public and policy debates about consequential increases in fire risk, especially in the wildland-urban interface (WUI), where homes and communities are at particular risk from wildfires. At the same time, large wildfires have become more frequent across this region. Widespread expectations that outbreaks increase extent, severity and/or frequency of wildfires are based partly on visible and dramatic changes in foliar moisture content and other fuel properties following outbreaks, as well as associated modeling projections. A competing explanation is that increasing wildfires are driven primarily by climatic extremes, which are becoming more common with climate change. However, the relative importance of bark beetle outbreaks versus climate on fire occurrence has not been empirically examined across very large areas and remains poorly understood. The most extensive outbreaks of tree-killing insects across the western United States have been of mountain pine beetle (MPB; Dendroctonus ponderosae ), which have killed trees over 〉 650,000 km 2 , mostly in forests dominated by lodgepole pine ( Pinus contorta ). Here we show that outbreaks of MPB in lodgepole pine forests of the western United States have been less important than climatic variability for the occurrence of large fires over the past 29 years. In lodgepole pine forests in general, as well as those in the WUI, occurrence of large fires was determined primarily by current and antecedent high temperatures and low precipitation but was unaffected by preceding outbreaks. Trends of increasing co-occurrence of wildfires and outbreaks are due to a common climatic driver rather than interactions between these disturbances. Reducing wildfire risk hinges on addressing the underlying climatic drivers, rather than treating beetle-affected forests. This article is protected by copyright. All rights reserved.

Description: Extensive mortality of whitebark pine, beginning in the early to mid-2000s, occurred in the Greater Yellowstone Ecosystem (GYE) of the western US, primarily from mountain pine beetle but also from other threats such as white pine blister rust. The climatic drivers of this recent mortality and the potential for future whitebark pine mortality from mountain pine beetle are not well understood, yet are important considerations in whether to list whitebark pine as a threatened or endangered species. We sought to increase the understanding of climate influences on mountain pine beetle outbreaks in whitebark pine forests, which are less well understood than in lodgepole pine, by quantifying climate-beetle relationships, analyzing climate influences during the recent outbreak, and estimating the suitability of future climate for beetle outbreaks. We developed a statistical model of the probability of whitebark pine mortality in the GYE that included temperature effects on beetle development and survival, precipitation effects on host tree condition, beetle population size, and stand characteristics. Estimated probability of whitebark pine mortality increased with higher winter minimum temperature, indicating greater beetle winter survival; higher fall temperature, indicating synchronous beetle emergence; lower two-year summer precipitation, indicating increased potential for host tree stress; increasing beetle populations; stand age; and increasing percent composition of whitebark pine within a stand. The recent outbreak occurred during a period of higher-than-normal regional winter temperatures, suitable fall temperatures, and low summer precipitation. In contrast to lodgepole pine systems, area with mortality was linked to precipitation variability even at high beetle populations. Projections from climate models indicate future climate conditions will likely provide favorable conditions for beetle outbreaks within nearly all current whitebark pine habitat in the GYE by the middle of this century. Therefore, when surviving and regenerating trees reach ages suitable for beetle attack, there is strong potential for continued whitebark pine mortality due to mountain pine beetle. This article is protected by copyright. All rights reserved.

Description: The prediction of mosquito abundance is of central interest in addressing mosquito population dynamics and in forecasting the associated emerging and re-emerging diseases. However, little work has focused on the systematic evaluation of how well adult mosquito abundance can be predicted as a function of observational resolutions, aggregation scales, and prediction lead time. Here we use a state space reconstruction (SSR) approach to compare the predictability of mosquito population dynamics at weekly, biweekly, and monthly scales. We focus on the analysis of Aedes vexans and Culiseta melanura populations monitored in Brunswick County (NC – USA) and find that prediction over a 7-day lead time is improved when daily observations are used, compared to the commonly used once-per-week sample. Our results demonstrate that daily observations of mosquito abundance contribute to improving mosquito predictability in two ways: (1) daily observations better capture fluctuations over short time scales, which are missed when sampling at coarser resolutions, (2) the aggregation of daily abundance observations reduces the impact of noise, thereby increasing the predictability of mosquito population dynamics as the aggregation scale is increased. We show that the evaluation of population dynamical models based on observed and predicted abundance can lead to a spuriously high apparent performance, due to the high auto-correlation in the observations used to update the model state at each successive time step. We show that the comparison of predicted and observed population change, expressed through per capita growth rates, leads to a more informative performance measure. This article is protected by copyright. All rights reserved.

Description: Climate conditions, such as temperature or precipitation averaged over several decades strongly affect species distributions, as evidenced by experimental results and a plethora of models demonstrating statistical relations between species occurrences and long-term climate averages. However, long-term averages can conceal climate changes that have occurred in recent decades and may not capture actual species occurrence well because the distributions of species, especially at the edges of their range, are typically dynamic and may respond strongly to short-term climate variability. Our goal here was to test whether bird occurrence models can be predicted by either covariates based on short-term climate variability or on long-term climate averages. We parameterized species distribution models (SDMs) based on either short-term variability or long-term average climate covariates for 320 bird species in the conterminous U.S., and tested whether any life-history trait-based guilds were particularly sensitive to short-term conditions. Models including short-term climate variability performed well based on their cross-validated AUC score (0.85), as did models based on long-term climate averages (0.84). Similarly, both models performed well compared to independent presence/absence data from the North American Breeding Bird Survey (independent AUC of 0.89 and 0.90, respectively). However, models based on short-term variability covariates more accurately classified true absences for most species (73% of true absences classified within the lowest quarter of environmental suitability versus 68%). In addition, they have the advantage that they can reveal the dynamic relationship between species and their environment because they capture the spatial fluctuations of species potential breeding distributions. With this information we can identify which species and guilds are sensitive to climate variability, identify sites of high conservation value where climate variability is low, and assess how species’ potential distributions may have already shifted due recent climate change. However, long-term climate averages require less data and processing time and may be more readily available for some areas of interest. Where data on short-term climate variability are not available, long-term climate information is a sufficient predictor of species distributions in many cases. However, short-term climate variability data may provide information not captured with long-term climate data for use in SDMs. This article is protected by copyright. All rights reserved.

Description: Increases in natural or non-crop habitat surrounding agricultural fields have been shown to be correlated with declines in insect crop pests. However, these patterns are highly variable across studies suggesting other important factors, such as abiotic drivers, which are rarely included in landscape models, may also contribute to variability in insect population abundance. The objective of this study was to explicitly account for the contribution of temperature and precipitation, in addition to landscape composition, on the abundance of a widespread insect crop pest, the soybean aphid ( Aphis glycines Matsumura), in Wisconsin soybean fields. We hypothesized that higher soybean aphid abundance would be associated with higher heat accumulation (e.g., growing degree days), and increasing non-crop habitat in the surrounding landscape, due to the presence of the overwintering primary hosts of soybean aphid. To evaluate these hypotheses, we used an ecoinformatics approach that relied on a large dataset collected across Wisconsin over a 9-year period (2003 – 2011), for an average of 235 sites per year (n=2,110 fields total). We determined surrounding landscape composition (1.5-km radius) using publicly available satellite-derived land cover imagery and interpolated daily temperature and precipitation information from the National Weather Service COOP weather station network. We constructed linear mixed models for soybean aphid abundance based on abiotic and landscape explanatory variables and applied model averaging for prediction using an information theoretic framework. Over this broad spatial and temporal extent in Wisconsin, we found that variation in growing season precipitation was positively related to soybean aphid abundance, while higher precipitation during the non-growing season had a negative effect on aphid populations. Additionally, we found that aphid populations were higher in areas with proportionally more forest, but were lower in areas where minor crops, such as small grains, were more prevalent. Thus, our findings support our hypothesis that including abiotic drivers increases our understanding of crop pest abundance and distribution. Moreover, by explicitly modeling abiotic factors, we may be able to explore how variable climate in tandem with land cover patterns may affect current and future insect populations, with potentially critical implications for crop yields and agricultural food webs. This article is protected by copyright. All rights reserved.

Description: Predator-prey interactions shape ecosystem structure and function, potentially limiting the productivity of valuable species. Simultaneously, stochastic environmental forcing affects species productivity, often through unknown mechanisms. The interacting effects of trophic and environmental conditions complicate management of exploited ecosystems and have motivated calls for more holistic management via ecosystem-based approaches, yet the limitations to these approaches are not widely appreciated. The Chignik salmon fishery in Alaska is managed to achieve maximum sustainable yield for sockeye salmon, though research suggests that predation by less economically valuable, and thus not targeted, coho salmon during juvenile rearing limits the productivity of sockeye salmon. We examined the relationship between historical sockeye salmon recruitment and coho salmon abundance observed in the Chignik system and could not detect a clear effect of coho salmon abundance on sockeye salmon productivity, given existing data. Using simulation models, we examined the probability of detecting a known predation effect on sockeye salmon recruitment in the presence of observation error in coho salmon abundance and stochasticity in sockeye salmon recruitment. Increased recruitment stochasticity reduced the ability to detect predator effects in recruitment, an effect further strengthened when low frequency environmental variation was added to the system. Further, increased observation error biased estimates of predator effects towards zero. Thus, in systems with high observation error on predator abundances, estimates of predation effects will be substantially weaker than true effects. We examined the effects of stochasticity on the ability of an adaptive management program to learn about ecosystem structure and detect an effect of management actions intended to release a prey species from its predators. Simulation models revealed that even under scenarios of large predation effects on sockeye salmon, stochastic recruitment masked detection of an effect of increased coho salmon harvest for nearly a decade. These results highlight the challenges inherent in ecosystem-based management of predator-prey systems due to mismatched time-scales of ecosystem dynamics and the willingness of stakeholders to risk losses in order to test uncertain hypotheses. It is critical for stakeholders considering EBFM and adaptive management strategies to be aware of the potential timelines of perceiving ecosystem change. This article is protected by copyright. All rights reserved.

Description: Timber harvest can adversely affect forest biota. Recent research and application suggest that retention of mature forest elements (‘retention forestry’), including unharvested patches (or ‘aggregates’) within larger harvested units, can benefit biodiversity compared to clearcutting. However, it is unclear whether these benefits can be generalized among the diverse taxa and biomes in which retention forestry is practiced. Lack of comparability in methods for sampling and analysing responses to timber harvest and edge creation presents a challenge to synthesis. We used a consistent methodology (similarly spaced plots or traps along transects) to investigate responses of vascular plants and ground-active beetles to aggregated retention at replicate sites in each of four temperate and boreal forest types on three continents: Douglas-fir forests in Washington, USA; aspen forests in Minnesota, USA; spruce forests in Sweden; and wet eucalypt forests in Tasmania, Australia. We assessed (i) differences in local (plot-scale) species richness and composition between mature (intact) and regenerating (previously harvested) forest; (ii) the lifeboating function of aggregates (capacity to retain species of unharvested forest); and whether intact forests and aggregates (iii) are susceptible to edge effects and (iv) influence the adjacent regenerating forest. Intact and harvested forests differed in composition but not richness of plants and beetles. The magnitude of this difference was generally similar among regions, but there was considerable heterogeneity of composition within and among replicate sites. Aggregates within harvest units were effective at lifeboating for both plant and beetle communities. Edge effects were uncommon even within the aggregates. In contrast, effects of forest influence on adjacent harvested areas were common and as strong for aggregates as for larger blocks of intact forest. Our results provide strong support for the widespread application of aggregated retention in boreal and temperate forests. The consistency of pattern in four very different regions of the world suggests that, for forest plants and beetles, responses to aggregated retention are likely to apply more widely. Our results suggest that through strategic placement of aggregates, it is possible to maintain the natural heterogeneity and biodiversity of mature forests managed for multiple objectives. This article is protected by copyright. All rights reserved.

Description: Distributions of foliar nutrients across forest canopies can give insight into their plant functional diversity and improve our understanding of biogeochemical cycling. We used airborne remote sensing and Partial Least Squares Regression (PLSR) to quantify canopy foliar nitrogen (N) across ~164 km 2 of wet lowland tropical forest in the Osa Peninsula, Costa Rica. We determined the relative influence of climate and topography on the observed patterns of canopy foliar N using a gradient boosting model (GBM) technique. At a local scale, where climate and substrate where constant, we explored the influence of slope position on canopy N by quantifying canopy N on remnant terraces, their adjacent slopes and knife edged ridges. In addition, we climbed and sampled 540 trees and analyzed foliar N in order to quantify the role of species identity (phylogeny) and environmental factors in predicting canopy N. Observed canopy N heterogeneity reflected environmental factors working at multiple spatial scales. Across the larger landscape, elevation and precipitation had the highest relative influence on predicting canopy foliar N (30 and 24%), followed by soils (15%), site exposure (9%), compound topographic index (8%), substrate (6%), and landscape dissection (6%). Phylogeny explained ~75% of the variation in the filed collected foliar N data, suggesting that phylogeny largely underpins the response to the environmental factors. Taken together, these data suggest that a large fraction of the variance in canopy N across the landscape is proximately driven by species composition, though ultimately this is likely a response to abiotic factors such as climate and topography. Future work should focus on the mechanisms and feedbacks involved, and how shifts in climate may translate to changes in forest function. This article is protected by copyright. All rights reserved.

Description: Escalating wildfire in subalpine forests with stand-replacing fire regimes is increasing the extent of early-seral forests throughout the western US. Post-fire succession generates the fuel for future fires, but little is known about fuel loads and their variability in young post-fire stands. We sampled fuel profiles in 24-year-old post-fire lodgepole pine ( Pinus contorta var. latifolia ) stands ( n =82) that regenerated from the 1988 Yellowstone Fires to answer three questions. (1) How do canopy and surface fuel loads vary within and among young lodgepole pine stands? (2) How do canopy and surface fuels vary with pre- and post-fire lodgepole pine stand structure and environmental conditions? (3) How have surface fuels changed between 8 and 24 years post-fire? Fuel complexes varied tremendously across the landscape despite having regenerated from the same fires. Available canopy fuel loads and canopy bulk density averaged 8.5 Mg ha -1 [range 0.0-46.6] and 0.24 kg m 3 [range: 0.0-2.3], respectively, meeting or exceeding levels in mature lodgepole pine forests. Total surface-fuel loads averaged 123 Mg ha -1 [range: 43 - 207], and 88% was in the 1000-hr fuel class. Litter, 1-hr, and 10-hr surface fuel loads were lower than reported for mature lodgepole pine forests, and 1000-hr fuel loads were similar or greater. Among-plot variation was greater in canopy fuels than surface fuels, and within-plot variation was greater than among-plot variation for nearly all fuels. Post-fire lodgepole pine density was the strongest positive predictor of canopy and fine surface fuel loads. Pre-fire successional stage was the best predictor of 100-hr and 1000-hr fuel loads in the post-fire stands and strongly influenced the size and proportion of sound logs (greater when late successional stands had burned) and rotten logs (greater when early successional stands had burned). Our data suggest that 76% of the young post-fire lodgepole pine forests have 1000-hr fuel loads that exceed levels associated with high-severity surface fire potential, and 63% exceed levels associated with active crown fire potential. Fire rotations in Yellowstone National Park are predicted to shorten to a few decades and this prediction cannot be ruled out by a lack of fuels to carry repeated fires. This article is protected by copyright. All rights reserved.

Description: Management of spatially structured species poses unique challenges. Despite a strong theoretical foundation, practitioners rarely have sufficient empirical data to evaluate how populations interact. Rather, assumptions about connectivity and source-sink dynamics are often based on incomplete, extrapolated or modeled data, if such interactions are even considered at all. Therefore, it has been difficult to evaluate whether spatially structured species are meeting conservation goals. We evaluated how estimated metapopulation structure responded to estimates of population sizes and dispersal probabilities, and to the set of populations included. We then compared outcomes of alternative management strategies that target conservation of metapopulation processes. We illustrated these concepts for Chinook salmon ( Oncorhynchus tshawytscha ) in the Snake River, USA. Our description of spatial structure for this metapopulation was consistent with previous characterizations. We found substantial differences in estimated metapopulation structure when we had incomplete information about all populations and when we used different sources of data (3 empirical, 2 modeled) to estimate dispersal, whereas responses to population size estimates were more consistent. Together, these findings suggest that monitoring efforts should target all populations occasionally and populations that play key roles frequently, and that multiple types of data should be collected when feasible. When empirical data are incomplete or of uneven quality, analyses using estimates produced from an ensemble of available datasets can help conservation planners and managers weigh near-term options. Doing so, we found tradeoffs in connectivity and source dominance in metapopulation-level responses to alternative management strategies that suggest which types of approaches may be inherently less risky. This article is protected by copyright. All rights reserved.

Description: Increasing tree density that followed fire exclusion after the 1880s in the southwestern United States (US) may have also altered nutrient cycles and led to a carbon (C) sink that constitutes a significant component of the US C budget. Yet, empirical data quantifying century-scale changes in C or nutrients due to fire exclusion are rare. We used tree-ring reconstructions of stand structure from five ponderosa pine–dominated sites from across northern Arizona to compare live tree C, nitrogen (N), and phosphorus (P) storage between the 1880s and 1990s. Live tree biomass in the 1990s contained up to 3 times more C, N, and P than in 1880s. However, the increase in C storage was smaller than values used in recent US C budgets. Furthermore, trees that had established prior to the 1880s accounted for a large fraction (28 to 66%) of the C, N, and P stored in contemporary stands. Overall, our century-scale analysis revealed that forests of the 1880s were on a trajectory to accumulate C and nutrients in trees even in the absence of fire exclusion, either because growing conditions became more favorable after the 1880s or because forests in the 1880s included age or size cohorts poised for accelerated growth. These results may lead to a reduction in the C sink attributed to fire exclusion, and they refine our understanding of reference conditions for restoration management of fire-prone forests. This article is protected by copyright. All rights reserved.

Description: The Conservation Effects Assessment Project (CEAP) was created in response to a request from the Office of Management and Budget that the U.S. Department of Agriculture - Natural Resource Conservation Service (USDA – NRCS) document the societal benefits anticipated to accrue from a major increase in conservation funding authorized by the 2002 Farm Bill. A comprehensive evaluation of the efficacy of rangeland conservation practices cost-shared with private landowners was unable to evaluate conservation benefits because outcomes were seldom documented. Four interrelated suppositions are presented to examine the causes underlying minimal documentation of conservations outcomes. These suppositions are: 1) benefits of conservation practices are considered a certainty so that documentation in not required, 2) minimal knowledge exchange between the USDA-NRCS and research organizations, 3) a paucity of conservation-relevant science, and 4) inadequate technical support for land owners following implementation of conservation practices. We then follow with recommendations to overcome potential barriers to documentation of conservation outcomes identified for each supposition. Collectively, this assessment indicates that the existing conservation practice standards are insufficient to effectively administer large conservation investments on rangelands and that modification of these standards alone will not achieve the goals explicitly stated by CEAP. We recommend that USDA-NRCS modify its conservation programs around a more comprehensive and integrative platform that is capable of implementing evidence-based conservation. Collaborative monitoring organized around landowner-agency-scientist partnerships would represent the focal point of a Conservation Program Assessment Network (CPAN). The primary network objective would be to establish missing information feedback loops between conservation practices and their agricultural and environmental outcomes to promote learning, adaptive management and innovation. Network information would be archived and made available to guide other, related conservation programs in relevant ecoregions. Restructuring conservation programs as recommend above would: 1) provide site specific information, learning and accountability that has been requested by CEAP and, 2) further advance balanced delivery of agricultural production and environmental quality goals. This article is protected by copyright. All rights reserved.

Description: To investigate the underlying mechanisms that control long-term recovery of tundra carbon (C) and nutrients after fire, we employed the Multiple Element Limitation (MEL) model to simulate 200-year post-fire changes in the biogeochemistry of three sites along a burn severity gradient in response to increases in air temperature, CO 2 concentration, nitrogen (N) deposition and phosphorus (P) weathering rates. The simulations were conducted for severely burned, moderately burned, and unburned arctic tundra. Our simulations indicated that recovery of C balance after fire was mainly determined by the internal redistribution of nutrients among ecosystem components (controlled by air temperature), rather than the supply of nutrients from external sources (e.g., nitrogen deposition and fixation, phosphorus weathering). Increases in air temperature and atmospheric CO 2 concentration resulted in 1) a net transfer of nutrient from soil organic matter to vegetation, and 2) higher C:nutrient ratios in vegetation and soil organic matter. These changes led to gains in vegetation biomass C but net losses in soil organic C stocks. Under a warming climate, nutrients lost in wildfire were difficult to recover because the warming-induced acceleration in nutrient cycles caused further net nutrient loss from the system through leaching. In both burned and unburned tundra, the warming-caused acceleration in nutrient cycles and increases in ecosystem C stocks were eventually constrained by increases in soil C:nutrient ratios, which increased microbial retention of plant-available nutrients in the soil. Accelerated nutrient turnover, loss of C, and increasing soil temperatures will likely result in vegetation changes, which further regulate the long-term biogeochemical succession. Our analysis should help in the assessment of tundra C budgets and of the recovery of biogeochemical function following fire, which is in turn necessary for the maintenance of wildlife habitat and tundra vegetation. This article is protected by copyright. All rights reserved.

Description: Restoring forest to hundreds of millions of hectares of degraded land has become a centerpiece of international plans to sequester carbon and conserve biodiversity. Forest landscape restoration will require scaling up ecological knowledge of secondary succession from small-scale field studies to predict forest recovery rates in heterogeneous landscapes. However, ecological field studies reveal widely divergent times to forest recovery, in part due to landscape features that are difficult to replicate in empirical studies. Seed rain can determine reforestation rate and depends on landscape features that are beyond the scale of most field studies. We develop mathematical models to quantify how landscape configuration affects seed rain and forest regrowth in degraded patches. The models show how landscape features can alter the successional trajectories of otherwise identical patches, thus providing insight into why some empirical studies reveal a strong effect of seed rain on secondary succession, while others do not. We show that seed rain will strongly limit reforestation rate when patches are near a threshold for arrested succession, when positive feedbacks between tree canopy cover and seed rain occur during early succession, and when directed dispersal leads to between-patch interactions. In contrast, seed rain has weak effects on reforestation rate over a wide range of conditions, including when landscape-scale seed availability is either very high or very low. Our modeling framework incorporates growth and survival parameters that are commonly estimated in field studies of reforestation. We demonstrate how mathematical models can inform forest landscape restoration by allowing land managers to predict where natural regeneration will be sufficient to restore tree cover. Translating quantitative forecasts into spatially-targeted interventions for forest landscape restoration could support target goals of restoring millions of hectares of degraded land and help mitigate global climate change. This article is protected by copyright. All rights reserved.

Description: Successful pest-mammal eradications from remote islands have resulted in important biodiversity benefits. Near-shore islands can also serve as refuges for native biota but require ongoing effort to maintain low-pest or pest-free status. Three management options are available in the presence of reinvasion risk: (1) control-to-zero density, in which immigration may occur but reinvaders are removed; (2) sustained population suppression (to relatively low numbers); or (3) no action. Biodiversity benefits can result from options 1 and 2. The management challenge is to make evidence-based decisions on the selection of an appropriate objective and to identify a financially feasible control strategy that has a high probability of success. This requires understanding the pest species population dynamics and how it will respond to a range of potential management strategies, each with an associated financial cost. We developed a 2-stage modelling approach that consisted of: (1) Bayesian inferential modelling to estimate parameters for a model of pest population dynamics and control; and (2) a forward projection model to simulate a range of plausible management scenarios and quantify the probability of obtaining zero density within four years. We applied the model to an ongoing, six-year trapping program to control stoats ( Mustela erminea ) on Resolution Island, New Zealand. Zero density has not yet been achieved. Results demonstrate that management objectives were impeded by a combination of a highly fecund population, insufficient trap attractiveness and a substantial proportion of the population that did not enter traps. Immigration is known to occur because the founding population arrived on the island by swimming from the mainland. However, immigration rate during this study was indistinguishable from zero. The forward projection modelling showed that control-to-zero density was feasible but required greater than a 2-fold budget increase to intensify the trapping rate relative to population growth. The 2-stage modelling provides the foundation for a management program in which broad-scale trials of additional trapping effort or improved trap lures would test model predictions and increase our understanding of system dynamics. This article is protected by copyright. All rights reserved.

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

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

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

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

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

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

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

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

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

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

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

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

Description: The application of physiological measures to conservation monitoring has been gaining momentum and, while a suite of physiological traits are available to ascertain disturbance and condition in wildlife populations, glucocorticoids (i.e., GCs: cortisol and corticosterone) are the most heavily employed. The interpretation of GC levels as sensitive indicators of population change necessitates that GCs and metrics of population persistence are linked. However, the relationship between GCs and fitness may be highly context-dependent, changing direction, or significance, depending on the GC measure, fitness metric, life history stage, or other intrinsic and extrinsic contexts considered. We examined the relationship between baseline plasma corticosterone (CORT) levels measured at two periods of the breeding season and three metrics of fitness (offspring quality, reproductive output, and adult survival) in female tree swallows ( Tachycineta bicolor ). Specifically, we investigated whether: i) a relationship between baseline CORT metrics and fitness exists in our population; ii) whether the inclusion of energetic contexts such as food availability, reproductive investment, or body mass could alter or improve the strength of the relationship between CORT and fitness; iii) whether energetic contexts could better predict fitness compared to CORT metrics. Importantly, we investigated these relationships in both natural conditions and under an experimental manipulation of foraging profitability (feather clipping) to determine the influence of an environmental constraint on GC-fitness relationships. We found a lack of relationship between baseline CORT and both short- and long-term metrics of fitness in control and clipped birds. In contrast, loss in body mass over reproduction positively predicted reproductive output (number of chicks leaving the nest) in control birds; however, the relationship was characterized by a low R 2 (5%), limiting the predictive capacity, and therefore the application potential, of such a measure in a conservation setting. Our results stress the importance of ground-truthing GC-fitness relationships and indicate that baseline GCs will likely not be easily employed as conservation biomarkers across many species and life history stages. Given the accumulating evidence of temporally-dynamic, inconsistent, and context-dependent GC-fitness relationships, placing effort towards directly measuring fitness traits, rather than plasma GC levels, will likely be more worthwhile for many conservation endeavours. This article is protected by copyright. All rights reserved.

Description: Ecological processes operate across temporal and spatial scales. Anthropogenic disturbances impact these processes, but examinations of scale dependence in impacts are infrequent. Such examinations can provide important insight to wildlife-human interactions and guide management efforts to reduce impacts. We assessed spatiotemporal scale dependence in habitat selection of mule deer ( Odocoileus hemionus ) in the Piceance Basin of Colorado, USA, an area of ongoing natural gas development. We employed a newly developed animal movement method to assess habitat selection across scales defined using animal-centric spatiotemporal definitions ranging from the local (defined from 5 hour movements) to the broad (defined from weekly movements). We extended our analysis to examine variation in scale dependence between night and day and assess functional responses in habitat selection patterns relative to the density of anthropogenic features. Mule deer displayed scale invariance in the direction of their response to energy development features, avoiding well pads and the areas closest to roads at all scales, though with increasing strength of avoidance at coarser scales. Deer displayed scale-dependent responses to most other habitat features, including land cover type and habitat edges. Selection differed between night and day at the finest scales, but homogenized as scale increased. Deer displayed functional responses to development, with deer inhabiting the least developed ranges more strongly avoiding development relative to those with more development in their ranges. Energy development was a primary driver of habitat selection patterns in mule deer, structuring their behaviors across all scales examined. Stronger avoidance at coarser scales suggests that deer behaviorally mediated their interaction with development, but only to a degree. At higher development densities than seen in this area, such mediation may not be possible and thus maintenance of sufficient habitat with lower development densities will be a critical best management practice as development expands globally. This article is protected by copyright. All rights reserved.

Description: In saltmarsh plant communities bottom-up pressure from nutrient enrichment is predicted to increase productivity, alter community structure, decrease biodiversity, and alter ecosystem functioning. Previous work supporting these predictions has been based largely on short-term, plot-level (e.g., 1-300 m 2 ) studies, which may miss landscape-level phenomena that drive ecosystem-level responses. We implemented an ecosystem-scale, 9-year nutrient experiment to examine how saltmarsh plants respond to simulated conditions of coastal eutrophication. Our study differed from previous saltmarsh enrichment studies in that we applied realistic concentrations of nitrate (70-100 μM NO 3 - ), the most common form of coastal nutrient enrichment, via tidal water at the ecosystem scale ( ca . 60,000 m 2 creeksheds). Our enrichments added a total of 1,700 kg N creek −1 y −1 , which increased N loading 10-fold versus reference creeks (low-marsh: 171 g N m −2 y −1 ; high-marsh: 19 g N m −2 y −1 ). Nutrients increased the shoot mass and height of low marsh, tall Spartina alterniflora ; however, declines in stem density resulted in no consistent increase in aboveground biomass. High-marsh plants S. patens and stunted S. alterniflora did not respond consistently to enrichment. Nutrient enrichment did not shift community structure, contrary to the prediction of nutrient-driven dominance of S. alterniflora and Distichlis spicata over S. patens . Our mild responses may differ the results of previous studies for a number of reasons. First, the limited response of the high marsh may be explained by loading rates orders of magnitude lower than previous work. Low loading rates in the high marsh reflect infrequent inundation, arguing that inundation patterns must be considered when predicting responses to estuarine eutrophication. Additionally, we applied nitrate instead of the typically-used ammonium, which is energetically favored over nitrate for plant uptake. Thus, the form of nitrogen enrichment used, not just N-load, may be important in predicting plant responses. Overall, our results suggest that when coastal eutrophication is dominated by nitrate and delivered via flooding tidal water, aboveground saltmarsh plant responses may be limited despite moderate-to-high water-column N concentrations. Furthermore, we argue that the methodological limitations of nutrient studies must be considered when using results to inform management decisions about wetlands. This article is protected by copyright. All rights reserved.

Description: Most species that are negatively impacted when their densities are low aggregate to minimize this effect. Aggregation has the potential to change how Allee effects are expressed at the population level. We studied the interplay between aggregation and Allee effects in the mountain pine beetle ( Dendroctonus ponderosae Hopkins), an irruptive bark beetle that aggregates to overcome tree defenses. By cooperating to surpass a critical number of attacks per tree, the mountain pine beetle is able to breach host defenses,oviposit and reproduce. Mountain pine beetles and Hymenopteran parasitoids share some biological features, the most notable of which is obligatory host death as a consequence of parasitoid attack and development. We developed spatiotemporal models of mountain pine beetle dynamics that were based on the Nicholson-Bailey framework but which featured beetle aggregation and a tree-level attack threshold. By fitting our models to data from a local mountain pine beetle outbreak, we demonstrate that due to aggregation, attack thresholds at the tree level can be overcome by a surprisingly low ratio of beetles per susceptible tree at the stand level. This results confirms the importance of considering aggregation in models of organisms that are subject to strong Allee effects. This article is protected by copyright. All rights reserved.

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

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

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

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

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

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

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

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

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

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

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