ALBERT

Description: Abstract A series of five realistic, nested, hydrostatic numerical ocean model simulations are used to study semidiurnal internal tide generation and propagation from the continental slope, through the shelf break and to the midshelf adjacent to Point Sal, CA. The statistics of modeled temperature and horizontal velocity fluctuations are compared to midshelf observations (30‐ to 50‐m water depth). Time‐ and frequency‐domain methods are used to decompose internal tides into components that are coherent and incoherent with the barotropic tide, and the incoherence fraction is 0.5–0.7 at the midshelf locations in both the realistic model and observations. In contrast, the incoherence fraction is at the most 0.45 for a simulation with idealized stratification, and neither atmospheric forcing nor mesoscale currents. Negligible conversion from barotropic to baroclinic energy occurs at the local shelf break. Instead, the dominant internal tide energy sources are regions of small‐scale near‐critical to supercritical bathymetry on the Santa Lucia escarpment (1,000–3,000 m), 70–80 km from the continental shelf. Near the generation region, semidiurnal baroclinic energy is primarily coherent and rapidly decays adjacent to the shelf break. In the realistically forced model, incoherent energy is less than 10% in the generation region, with a steady increase in incoherence fraction from the continental slope to the midshelf. Backward ray tracing from the midshelf to the Santa Lucia escarpment identifies multiple energy pathways potentially leading to spatial interference. As internal tides shoal on the predominantly subcritical slope/shelf system, temporally variable stratification and Doppler shifting from mesoscale and submesoscale features appear equally important in leading to the loss of coherence.

Description: Abstract Variability of the flow across the Solomon Sea's southern entrance was examined using end point subsurface moorings and seafloor pressure sensors, reconstructed velocity profiles based on satellite‐derived surface velocity and bottom pressure‐derived subsurface velocity, and 1993–2017 proxy volume transport based on satellite altimetry. The reconstructed velocity correctly represents the fluctuating surface flow and subsurface core providing a high‐frequency continuous observing system for this sea. The mean equatorward volume transport over 0‐ to 500‐m depth layer is 15.2 Sv (1 Sv ≡ 106 m3/s) during July 2012 to May 2017. The measurements resolve the full spectrum of the volume transport including energetic subseasonal variability that fluctuates by as much as 25 Sv over one week. At low‐frequency timescales, the study finds that linear Rossby waves forced by Ekman pumping in the interior of the Pacific influence not only seasonal fluctuations as found by previous studies but also interannual variability. As found previously, the El Niño–Southern Oscillation highly influences interannual volume transport. During the 2015/2016 El Niño, observations show the seasonal cycle to be suppressed from the second half of 2014, prior to the mature phase of the El Niño, to September 2016 along with an increase in across‐transect transport. At subseasonal timescales, local Ekman pumping and remote wind stress curl are responsible for a third of the subseasonal variance. The study highlights the importance of high‐frequency observations at the southern entrance of the Solomon Sea and the ability of a linear Rossby model to represent the low‐frequency variability of the transport.

Description: Abstract A direct method is presented to obtain the meridional overturning and heat transport in oceanic basins from observations under the sole assumptions of geostrophy and hydrostatics,. The method is made possible because of the rising Argo float displacements data base which can provide a reference level at 1000 dbar for the time mean circulation at 1° × 1° resolution. To achieve the overturning and heat transport objectives, the absolute geostrophic time mean circulation must have non divergent barotropic transports and this requires the solutions of two Poisson equations with suitable boundary conditions, one for the geopotential at 1000 dbar and one for the barotropic streamfunction. Applied to the subpolar Atlantic for the period 2000‐2009, an overturning of 16‐18 Sv is found around 40o‐50oN, a meridional heat transport of 0.59 PW is found at 40oN (0.23 PW at 60oN) so that on average ~50 Wm‐2 are exported from ocean to atmosphere to feed the atmospheric storm track. The zonally averaged flow (the overturning) falls short of explaining the observed heat transport and the barotropic component of the circulation accounts for up to 50% of the heat transport poleward of 55oN. With the rising Argo float data base, the method offers high potential to reconstruct the World Ocean time mean circulation and its heat transport away from the equator at higher resolution. The drawback is that it requires in some critical places additional current observations on the shallow shelves which are not sampled by the Argo floats.

Description: Abstract Here we present an observation‐based study of the coupled land‐ocean regions of influence for the transformation of precipitation over land into coastal river plume structure in the Gulf of Mexico (GoM). First, we locate the regions on land for which precipitation and runoff generation have the strongest relationship with local river discharge. Then we map, on average, the apparent unique contribution of individual river discharge forcing to specific features of river plume structure across the GoM. To this end, we employ a spatial‐temporal lagged correlation analysis that relates satellite‐based precipitation, soil moisture, and sea surface salinity observations to in situ river discharge for the three primary freshwater input sources for the GoM. On land, we find a likely source region for the northeastern GoM in the southeastern Mississippi basin at 16‐day lead time, a likely source region for the northeastern GoM in the Mobile Bay basin at 3‐day lead time and a likely source region for the Central GoM from the Texas basin region at 4‐day lead time. In the ocean, we find statistically significant regions of distinct contribution for each of the three sources of freshwater on plume structure at lag times from weeks to several months. Though a statistical approach is limited in its interpretability, this result advances progress toward a predictive framework for mapping of the impacts of hydrological flood events from land into the ocean using observations alone.

Description: Abstract This paper evaluates the intraseasonal variability of sea surface temperature (SST) along the Sumatra‐Java southern coast using available satellite‐derived oceanic and atmospheric data combined with output from a numerical model. The result reveals that the intraseasonal variability of SST is greater during boreal summer–fall (June–October) than during boreal winter–spring (November–May). Composite analysis shows a correlation between positive/negative intraseasonal SST variabilities and coastal downwelling/upwelling, as well as onshore/offshore Ekman transport during summer–fall. During this period, with the significantly increasing role of oceanic advection, oceanic processes are evidently enhanced and dominate the intraseasonal variability of SST. Meanwhile, the contribution of atmospheric processes drops by 67%. During winter–spring, the intraseasonal SST is primarily contributed by atmospheric processes but has a nonsignificant relationship with sea level anomalies. Intraseasonal SST anomalies vary out of phase with surface wind anomalies. The result also shows a relatively small contribution by vertical processes throughout the year, with the maximum in April and the minimum during August–September. Further analysis reveals that the alternating dominance of atmospheric and oceanic processes on intraseasonal variability of SST is responsible for the seasonality along the Sumatra‐Java southern coast. Moreover, the result indicates that the seasonality in intraseasonal SST is different in the eastern Indonesian Seas, which tends to be relatively strong in boreal winter. Distinct dominance of atmospheric and oceanic processes in intraseasonal SST is the main reason for these differences in seasonal variation characteristics.

Description: Abstract The Surface Water Ocean Topography (SWOT) satellite mission is planned for launch in 2021. It will use the technique of radar interferometry to measure sea surface height over a 120‐km‐wide swath with a 20‐km gap around the satellite's nadir track. The oceanographic objectives of the mission are to study ocean circulation at scales down to 15 km. To prepare for the evaluation of the mission's performance, we are undertaking a series of studies to explore the efficacy of an assimilative high‐resolution modeling system for estimating the state of the ocean based on independent observations from both spaceborne and in situ measurements. The system is based on the heritage of a multiscale approach to data assimilation by the Regional Ocean Modeling System. Observing System Simulation Experiments were first conducted in the setup of an identical twin experiment to assess the system's performance near the calibration/validation site of SWOT off the coast of California. The system was applied to a nested model domain with 1‐km resolution. Simulated satellite observations of SSH, sea surface temperature, salinity, in situ observations of upper ocean temperature, and salinity by profiling floats and a dedicated notional array of station‐keeping gliders were assimilated by the system. The results indicate that such an observing system can accurately estimate the state of the ocean, and in particular SSH for the evaluation of SWOT performance.

Description: Abstract Sea ice data assimilation can greatly improve forecasts of Arctic sea ice evolution. Many previous sea ice data assimilation studies were conducted without assimilating ocean state variables, even though the sea ice evolution is closely linked to the oceanic conditions, both dynamically and thermodynamically. Based on the method of a localized ensemble error subspace transform Kalman filter, satellite‐retrieved sea ice concentration and sea ice thickness are assimilated into an Arctic sea ice‐ocean model. As a new addition, sea surface temperature (SST) data are also assimilated. The additional assimilation of SST improves not only the simulated ocean temperature in the mixed layer of the ocean substantially but also the accuracy of sea ice edge position, sea ice extent, and sea ice thickness in the marginal sea ice zone. The improvement in the simulated potential temperature in the upper 1,000 m can be attributed to the enhanced vertical convection processes in the regions where the assimilated observational SST is colder than the simulated SST without assimilation. The improvements in the sea ice edge position and sea ice thickness simulations are primarily caused by the SST data assimilation reducing biases in the simulated SST and the associated coupled ocean‐sea ice processes. Our investigation suggests that, due to the complex interaction between the sea ice and ocean, assimilating ocean data should be an indispensable component of numerical polar sea ice forecasting systems.

Description: Abstract Despite the importance of the large‐scale Atlantic circulation for the climate system and sea level, most of the interior flow field is only known qualitatively, and neither the mean nor the variability and trends are quantified. We investigate the meridional flow field in the western Atlantic at 47°N between 44°W and 31°W, combining moored pressure inverted echo sounders and current meter moorings with lowered acoustic Doppler current profiler and Argo data. Correlations with altimetry are used to extend each of the transport time series back to 1993. At the Canadian continental margin the boundary current exports −23.1 ± 1.5 Sv to the south. Nearby, the northward flowing North Atlantic Current (NAC) imports 105.9 ± 3.4 Sv into the subpolar gyre. Constrained mainly by topography, about half of that flow recirculates in close proximity to the NAC (−58.8 ± 3.9 Sv). NAC and recirculation are significantly anticorrelated. The flow east of 37°W (−27.8 ± 2.1 Sv) has no permanent regional features and is not correlated to the NAC. The sum of the interior components (19.3 ± 3.3 Sv) shows a significant trend in the time period 1993–2018 of −0.60 Sv/year. This decline is dominated by the significant increase of the southward flow east of 37°W (−0.44 Sv/year). The trends of the other individual components are not significant, but the sum of the interior and boundary current transport is (−0.71Sv/year). The trends are most likely caused by regionally different warming.

Description: Abstract Mixing in the ocean and shelf seas is critical for the vertical distribution of dynamically active properties, such as density and biogeochemical tracers. Eight different decadal simulations are used to assess the skill of vertical Turbulent Mixing Schemes (TMS) in a 3D regional model of tidally active shelf seas. The TMS differ in the type of stability functions used and in the Ozmidov/Deardorff/Galperin limiter of the turbulence length scales. We review the dependence of the critical Richardson and Prandtl numbers to define the “diffusiveness” of the TMS. The skill in representing bias and variability of stratification profiles is assessed with 5 different metrics: surface and bottom temperatures; pycnocline depth, thickness and strength. The assessment is made against hydrography from three datasets (28,000 profiles in total). Bottom and surface temperatures are found to be as sensitive to TMS choice as to horizontal resolution or heat flux formulation, as reported in other studies. All TMS under‐represent the pycnocline depth and benthic temperatures. This suggests physical processes are missing from the model, and these are discussed. Different TMSs show the best results for different metrics, and there is no outright winner. Simulations coupled with an ecosystem model show the choice of TMS strongly affects the ecosystem behaviour: shifting the timing of peak chlorophyll by one month, showing regional chlorophyll differences of order 100%, and redistributing the production of chorophyll between the pycnocline and mixed layer.

Description: Abstract During the Lagrangian submesoscale experiment (LASER), 1000 drifters were launched to sample the surface ocean flow in the northern Gulf of Mexico. Due to half a dozen strong winter storms, about 40% of the drifters lost their drogue. This unintended situation facilitated documentation of both near surface (5 cm) and deeper (60 cm) flows. These depths are relevant to transport of oil spills, as well as marine debris, such as micro plastics, a rapidly‐growing environmental problem. Here, we improve the surface Lagrangian current prediction by combining a state‐of‐the‐art ocean forecast model with wind and wave data. The ocean surface velocities are obtained from the Navy Coordinate Ocean Model (NCOM) at 1 km horizontal resolution, while the wind and wave fields are from the UWIN‐CM coupled atmosphere‐wave‐ocean model. Two Lagrangian parameterizations are tested: one is based on Ekman dynamics, and the other directly on the surface winds. LASER dataset is then used to assess the performance of these formulations, as a function of wind/wave conditions, as well as geographic region. It is found that incorporation of wind and wave data into the ocean circulation model can lead to major prediction improvement, by reducing the average two‐day separation from the modeled and real LASER trajectories by a factor ranging from 1.4 to 4.9. This is a significant improvement for applications, where a rapid deployment of assets is needed, such as oil spill response, or other tracking problems.

Description: Abstract The observed seasonal and intraseasonal evolution of near‐surface meteorological and oceanographic variables in the Andaman Sea (AS) during March 2014 to December 2017 is examined using moored buoy observations at 10.5°N, 94°E. The amplitude of temperature inversions is very weak (0.2°C to 0.4°C) and they appeared primarily during winter and latter part of summer. The net surface heat flux plays a primary role and vertical processes term contributes secondarily to determine the seasonal ML heat storage variability. Consistent with the seasonal variations of formation and strength of temperature inversion, vertical processes term shows a positive tendency during winter. The sea surface salinity (SSS) shows large amplitude intraseasonal variability during fall and winter and it is attributed to the variability of horizontal circulation in the presence of large lateral SSS gradients at the mooring location. The sea surface temperature (SST) shows the presence of strong intraseasonal variability between 20–80 days, though its amplitude of oscillation is distinctly higher during May–October than November–April. Bandpass filtered (20–80 days) time series of different components of the ML heat budget shows that the net surface heat flux primarily determines the intraseasonal ML heat storage variability. Our analysis further shows that during May–October, both net shortwave radiation and latent heat flux together determine the modulation of the intraseasonal net surface heat flux. In contrast, latent heat flux acts as the sole factor to determine the modulation of the intraseasonal net surface heat flux during November–April.

Description: Abstract The upper oceanic thermal response induced by Tropical Cyclone Phailin (9th‐14th October 2013) under the influence of East India Coastal Current (EICC) and a cyclonic eddy are investigated and contrasted with the response from open ocean region using a high‐resolution HYbrid Coordinate Ocean Model (HYCOM) simulation. There is significant cooling (7°C) inside the cold core eddy and negligible cooling (0.5°C) within the EICC region characterized by the shallow and deeper thermocline, respectively. Our analysis of mixed layer heat budget terms showed that the horizontal advection plays a significant role in determining the temperature tendency for the location within the EICC, in contrary to the general dominance of vertical processes as reported in previous studies during the cyclone period. The analysis for the locations Inside Eddy (IE) and Open Ocean (OO) concur with the previous studies showing the dominance of vertical processes towards the temperature tendency. Further, near the coast, the surface cooling is minimal compared to the subsurface cooling, dominantly seen between 50m to 100m depth. This disparity indicates that the factors responsible for the surface temperature anomalies are different from those of subsurface. Our analysis of thermal signatures after the passage of cyclone showed that the EICC and cyclonic eddy contributes to the faster advection of cold wake and recovery of SST to the pre‐storm state.

Description: No abstract is available for this article.

Description: Abstract The south Indian Ocean (SIO) is a region of strong air‐sea heat loss due to the unique ocean circulation pattern influenced by the Indonesian Throughflow. In this study, the seasonal variation of the surface layer heat budget in the eastern SIO is investigated using 2 years of measurements from a mooring at 25°S, 100°E, the only colocated upper ocean and surface meteorology time series in the subtropical Indian Ocean. The mooring data are combined with other in situ and satellite data to examine the role of air‐sea fluxes and ocean heat transport on the evolution of mixed layer temperature using heat budget diagnostic models. Results show that on seasonal timescales, mixed layer heat storage in the eastern SIO is mostly balanced by a combination of surface fluxes and turbulent entrainment with a contribution from horizontal advection at times. Solar radiation dominates the seasonal cycle of net surface heat flux, which warms the mixed layer during austral summer (67 Wm‐2) and cools it during austral winter (‐44 Wm‐2). Entrainment is in good agreement with the heat budget residual for most of the year. Horizontal advection is spatially variable and appears to be dominated by the presence of mesoscale eddies and possibly annual and semi‐annual Rossby waves propagating from the eastern boundary. Results from the 2‐year mooring‐based data analysis are in reasonably good agreement with a 12‐year regional heat budget analysis around the mooring location using ocean reanalysis products.

Description: Abstract The spatial distributions of biogenic dimethylated sulfur compounds (BDSCs), including dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethylsulfoxide, were determined in the Yellow Sea and Bohai Sea during a survey in April–May 2014 and the occurrence and fate of BDSCs in the surface seawater were investigated. The concentrations of DMS and DMSP were significantly correlated with the stocks of chlorophyll a and a decreasing trend was observed from the inshore to the offshore areas. In situ incubation experiments indicated that more than half of the degraded dissolved DMSP (DMSPd) was transformed into DMS. Irradiation experiments showed that the photooxidation of DMS under ultraviolet B, ultraviolet A, and visible light accounted for 23.9%, 71.8%, and 4.3% of the total photooxidation of DMS, respectively. The sea‐to‐air fluxes of DMS ranged from 0.24 to 34.11 μmol m−2 day−1 with a mean of 8.84 μmol m−2 day−1. A comparison of the DMS production rate and main removal rates indicated that bioproduction cannot completely maintain the removal of DMS and might not be the only but the primary source of DMS in the surface seawater. Additionally, the average turnover times of microbial consumption, photooxidation, and sea‐to‐air exchange of DMS were 1.53, 1.16, and 4.28 day and the contributions of the three removal pathways were 40.0%, 41.2%, and 18.8% respectively; this indicated that microbial consumption and photooxidation played dominant roles in controlling the removal of DMS from the surface seawater.

Description: Abstract A large production of anomalous dense water in the North Western Mediterranean Sea during winter 2005 led to a widespread abrupt shift in Western Mediterranean deep waters characteristics. This new configuration, the so‐called Western Mediterranean Transition (WMT), involved a complex thermohaline structure that was tracked over time through a deep hydrographic station located NE of Minorca Island, sampled 37 times between 2004 and 2017. In this study, the thermohaline evolution of the WMT signal is analysed in detail. Using a 1‐D diffusion model sensitive to double‐diffusive mixing phenomena, the contribution to the heat and salt budgets of the deep Western Mediterranean in terms of ventilation and diffusive transference from the intermediate layers above is disentangled. Results show distinct stages in the evolution of the deep waters, driven by background diffusion and intermittent injections of new waters. The progression of a multi‐layered structure in the deep ocean is well represented through existing parameterizations of salt fingering and diffusive layering processes, and makes it possible to infer an independent estimate of regional background diffusivity consistent with current knowledge. Overall, the deep layers of the Western Mediterranean underwent substantial warming (0.059°C) and salt increase (0.021) between 2004 and 2017, mostly dominated by injections of dense waters in the 2005–2006 and 2011–2013 periods. Thus, within the WMT period, heat uptake rate in the deep Western Mediterranean was substantially higher than that of the intermediate levels in the global ocean.

Description: Abstract We use a numerical model, already validated for this purpose, to simulate the effect of wave frequency spread on wave transformation and swash amplitudes. Simulations are performed for planar beach slope cases and for offshore wave spectra whose frequency spread changes over realistic values. Results indicate that frequency spread, under normally approaching waves, affects swash amplitudes. For moderately dissipative conditions, the significant infragravity swash increases for increasing values of the offshore frequency spread. The opposite occurs under extremely dissipative conditions. The numerical analysis suggests that this inverted pattern is driven by the effect that different distributions of incoming long‐wave energy have on low‐frequency wave propagation and dissipation. In fact, with large frequency spreads, wave groups force relatively short subharmonic waves that are strongly enhanced in the shoaling zone. This process leads to an infragravity swash increase for increasing frequency spread under moderately dissipative conditions, in which low‐frequency energy dissipation in shallow water is negligible or small. However, under extremely dissipative conditions, the significant low‐frequency energy dissipation associated with large frequency spreads overturns the strong energy growth in the shoaling zone eventually yielding an infragravity swash decrease for increasing frequency spread.

Description: Abstract Buoyant microplastic in the ocean can be submerged to deeper layers through biofouling and the consequent loss of buoyancy or by wind‐induced turbulent mixing at the ocean surface. Yet the fact that particles in deeper layers are transported by currents that are different from those at the surface has not been explored so far. We compute 10‐year trajectories of 1 million virtual particles with the Parcels framework for different particle advection scenarios to investigate the effect of near‐surface currents on global particle dispersal. We simulate the global‐scale transport of passive microplastic for (i) particles constrained to different depths from the surface to 120‐m depth, (ii) particles that are randomly displaced in the vertical with uniform distribution, (iii) particles subject to surface mixing, and (iv) for a 3‐D passive advection model. Our results show that the so called “garbage patches” become more “leaky” in deeper layers and completely disappear at about 60‐m depth. At the same time, subsurface currents can transport significant amounts of microplastic from subtropical and subpolar regions to polar regions, providing a possible mechanism to explain why plastic is found in these remote areas. Finally, we show that the final distribution in the surface turbulent mixing scenario with particle rise speed wr = 0.003 m/s is very similar to the distribution of plastic at the surface. This demonstrates that it is not necessary to incorporate surface mixing for global long‐term simulations, although this might change on more local scales and for particles with lower rise speeds.

Description: Abstract The dynamics of shoal‐channel estuaries require consideration of lateral gradients and transport, which can create significant intratidal variability in stratification and circulation. When the shoal‐channel system is strongly coupled by tidal exchange with mudflats, marshes, or other habitats, the gradients driving intratidal stratification variations are expected to intensify. To examine this dynamic, hydrodynamic data were collected from 27 January 2017 to 10 February 2017 in Lower South San Francisco Bay, a small subembayment fringed by extensive shallow vegetated habitats. During this deployment, salinity variations were captured through instrumentation of six stations (arrayed longitudinally and laterally) allowing for mechanisms of stratification creation and destruction to be calculated directly and compared with observed time variability of stratification at the central station. We present observation‐based calculations of longitudinal straining, longitudinal advection, lateral straining, and lateral advection. The time dependence of stratification was observed directly and calculated by summing measured longitudinal and lateral mechanisms. We found that the stratification dynamics switch between being longitudinally dominated during the middle of ebb and flood tides to being laterally dominated during the tidal transitions. This variability is driven by the interplay between tidally variable lateral density gradients and turbulent mixing. Relatively constant along‐estuary density gradients are differentially advected during flood and ebb tides, resulting in maximal lateral density gradients around tidal transitions. Simultaneous decrease in turbulent mixing at slack tides allows lateral density‐driven exchange to stratify the estuary channel at the slack after flood. At the end of ebb, barotropic forcing drives negatively buoyant shoal waters toward the channel.

Description: Abstract In most places extreme high tides undergo a clear seasonal variation. It is well known that semidiurnal tides tend to peak during equinox seasons, and diurnals during solstice seasons. This is a consequence of the solar and lunar declinations, which when large maximize diurnal tides at the expense of semidiurnals. The semiannual range modulation of tidal extremes for a pure semidiurnal tide is determined mainly by the amplitude of the K2 constituent; a pure diurnal is determined mainly by P1. Mixed tidal regimes tend to experience maxima very roughly around the times of solstice, but not always, with the semiannual modulation generally a complicated function of constituent amplitudes and phases. These modulations are here mapped worldwide by analyzing tidal extremes predicted with a global tide model. The known 4.4‐year modulation in extreme tides is a consequence of declinational and perigean effects coming in and out of phase. The phase of the 4.4‐year modulation is controlled by the phase of the semiannual modulation, irrespective of whether the tide is diurnal, semidiurnal, or mixed.

Description: Abstract The three‐dimensional structure of the offshore export of Mississippi River (MR) waters is documented for the first time with in situ data. Numerical simulations and satellite data in the Gulf of Mexico (GoM) are also employed to study two pathways that were detected in summer of 2015, along the eastern and western sides of the Loop Current (LC). The initial formation of offshore branches was primarily due to the interaction of the anticyclonic LC and LC Eddy (which were close to the MR Delta and the Louisiana‐Texas shelf‐slope, respectively) with riverine waters that had been advected eastward by westerly winds (which reduced the westward buoyancy‐driven currents). The interaction of anticyclonic circulation patterns with cyclones (LC Frontal Eddies) was found to influence the dynamics and structure of the branches. Thickness variability and other vertical characteristics of the brackish plumes were investigated from their origin in the northern GoM through their extension in the Straits of Florida. In particular, offshore branch thickness increased near the LC and LC Frontal Eddy fronts. The two types of pathways revealed different factors contributing to the low‐salinity waters. Besides the MR input, precipitation contributed to the eastern pathway, while waters from additional northern GoM rivers contributed to the western pathway. The study offers new insights on the processes that control the formation and the offshore (southward) advection of low‐salinity waters. These processes have implications on the properties of waters hundreds of kilometers from the northern river sources, extending to the southern Gulf and the Straits of Florida.

Description: Abstract The Scotia Sea is the site of one of the largest spring phytoplankton blooms in the Southern Ocean. Past studies suggest that shelf‐iron inputs are responsible for the high productivity in this region, but the physical mechanisms that initiate and sustain the bloom are not well understood. Analysis of profiling float data from 2002 to 2017 shows that the Scotia Sea has an unusually shallow mixed‐layer depth during the transition from winter to spring, allowing the region to support a bloom earlier in the season than elsewhere in the Antarctic Circumpolar Current. We compare these results to the mixed‐layer depth in the 1/6° data‐assimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixed‐layer variability in the region. Results indicate the importance of lateral advection of Weddell Sea surface waters in setting the stratification. A Lagrangian particle release experiment run backward in time suggests that Weddell outflow constitutes 10% of the waters in the upper 200 m of the water column in the bloom region. This dense Weddell water subducts below the surface waters in the Scotia Sea, establishing a sharp subsurface density contrast that cannot be overcome by wintertime convection. Profiling float trajectories are consistent with the formation of Taylor columns over the region's complex bathymetry, which may also contribute to the unique stratification. Furthermore, biogeochemical measurements from 2016 and 2017 bloom events suggest that vertical exchange associated with this Taylor column enhances productivity by delivering nutrients to the euphotic zone.

Description: Abstract Upper‐ocean dynamics in the Northern Indian Ocean (NIO) depend on changes in the magnitude and location of the high salinity waters of the Arabian Sea and low salinity waters of the Bay of Bengal. The large sea surface salinity (SSS) differences between these two basins are related to the surface freshwater flux (evaporation minus precipitation), which is positive (negative) in the Arabian Sea (Bay of Bengal). To quantify large‐scale salinity changes on decadal time scale over the whole water column and to study trends in salinity and volume transport, we have analyzed Simple Ocean Data Assimilation (SODA) reanalysis product, HYbrid Coordinate Ocean Model (HYCOM) simulations, European Centre for Medium‐Range Weather Forecasting's ERA‐Interim reanalysis product, and riverine streamflow data from the National Centers for Atmospheric Research's Global River Flow and Continental Discharge Dataset for the NIO. We find increased freshening conditions in the Bay of Bengal and salinification conditions in the Arabian Sea that would support a stronger zonal SSS difference in the NIO but that it is partially compensated by positive (negative) salt transports into the Bay of Bengal (BoB) (Arabian Sea). Empirical orthogonal function analysis of SODA SSS indicates that the main factors of SSS variability are Indian Ocean Dipole and El Niño‐Southern Oscillation and seasonal currents. The trends in the volume transport reveal decadal changes in zonal equatorial currents in HYCOM and Somali Current in SODA.

Description: Abstract The interannual variability and trends of the Alaska Gyre and Gulf of Alaska (GOA) circulation are examined using meridional geostrophic transport from Argo temperature and salinity (2004–2017) and altimetric sea surface height (1993–2017). More than half of the top 1,500 m meridional transport variability in the Alaska Gyre is accounted for by a statistical mode strongly correlated with the Pacific Decadal Oscillation (PDO) index, consistent with the PDO exerting a major influence on North Pacific sea surface temperature variability. During a positive phase of the PDO, the zero‐transport streamline separating the subtropical from the Alaska Gyre is shifted to the south from its mean position, while more transport is diverted northward, associated with a stronger and larger Alaska Gyre. Additionally, over the 25‐year altimetric record there is a linear, increasing trend in strength of the Alaska Gyre (but not in areal extent), accompanied by an increasing trend for the incoming North Pacific Current. The effect of the PDO transport mode on GOA circulation is weak. Temperature and salinity volume averaged for the GOA covary with the PDO index, with warmer and fresher waters during a positive phase. Despite correlated anomalies for temperature, salinity, and northward transport into the GOA, however, geostrophic advection from the south contributes only minimally to the interannual variations of water properties in the GOA. An exception was the marine heat wave of 2013/2014 and its aftermath when temperature advection from the south played a more appreciable role for warming and subsequent cooling of the GOA.

Description: Abstract A new method for the detection of sea ice using GNSS‐R (Global Navigation Satellite Systems Reflectometry) is presented and applied to 33 months of data from the U.K. TechDemoSat‐1 mission. This method of sea ice detection shows the potential for a future GNSS‐R polar mission, attaining an agreement of over 98% and 96% in the Antarctic and Arctic, respectively, when compared to the European Space Agency's Climate Change Initiative sea ice concentration product. The algorithm uses a combination of two parameters derived from the delay‐Doppler Maps to quantify the spread of power in delay and Doppler. Application of thresholds then allows sea ice to be distinguished from open water. Differences between the TechDemoSat‐1 sea ice detection and comparison data sets are explored. The results provide information on the seasonal and multiyear changes in sea ice distribution of the Arctic and Antarctic. Future potential and applications of this technique are discussed.

Description: Abstract A strong decrease of volume and density of North Pacific Subtropical Mode Water (NPSTMW) in 1999, was analyzed in a regional high‐resolution (0.1°) numerical ocean circulation model simulation. Both shoaling of the bottom, and deepening of the top of the NPSTMW layer contributed equally to volume decrease. They were locally governed by different physical processes, but both seem to be associated with basin‐wide changes in wind. A westward propagating negative thermocline depth anomaly, that developed in the Central Pacific when the Pacific Decadal Oscillation index changed from a positive to a negative phase in 1998, caused shoaling of the bottom of the NPSTMW layer. Deepening of the top of the NPSTMW layer was due to an increase in the near surface stratification, caused by an increase in wind‐driven lateral heat transport convergence by the Kuroshio Extension jet starting in 1997. Both processes increased the potential vorticity (PV) in the NPSTMW region, decreasing the volume of water in the NPSTMW density range that satisfied the low PV constraint that is part of the definition of "mode water”. The strong near‐surface density decrease provided preconditioning for preferential surface formation of a lighter variety of NPSTMW, further decreasing its density. It also resulted in decrease of the outcrop window in the NPSTMW density range, strongly reducing its formation rate in 1998 and 1999 despite strong surface heat loss.

Description: Abstract Off the coast of central Chile, subsurface anticyclonic eddies are a salient feature of the oceanic circulation, transporting a significant fraction of coastal water that is rich in nutrients and poor in dissolved oxygen offshore. In this study, the formation mechanism of these eddies is analyzed through a high‐resolution (~0.3 km) and low‐resolution (~3 km) oceanic model that realistically simulate the regional mean circulation, including the Peru‐Chile Undercurrent (PCUC). An analysis of the vorticity and eddy kinetic energy in both simulations indicated that the subsurface eddies can be triggered through a combination of processes that are associated with instabilities of the PCUC. In the high‐resolution simulation, we observed that the interaction between the PCUC and topographic slope generates anticyclonic vorticity and potential vorticity close to zero in the bottom boundary layer. The separation of the undercurrent from the slope favors the intensification of anticyclonic vorticity. It reaches magnitudes that are larger than the planetary vorticity while kinetic energy is converted from the PCUC to the eddy flow. These processes set the necessary conditions for the development of centrifugal instabilities, which can form submesoscale structures. The coalescence of submesoscale structures generates a subsurface anticyclonic mesoscale eddy. In the low‐resolution simulations (〉3 km) centrifugal instabilities are not simulated, and the barotropic conversion of the mean kinetic energy into eddy kinetic energy appears as the main process of eddy formation. We showed that the vertical structure of these eddies is sensitive to the spatial resolution of the model.

Description: Abstract Oceans are an important natural source of the greenhouse gas nitrous oxide (N2O). The isotopomer signature of N2O provides a useful tool to differentiate the production processes of N2O in the oceans. Here we present the distribution of the concentration and stable isotopic composition of dissolved N2O in the water column of the shelf and slope region of the northern South China Sea (SCS) in June 2015. Dissolved N2O concentrations in surface waters ranged from 6.9 to 9.1 nM with an average of 7.7 ± 0.6 nM (136 ± 10% saturation). Higher N2O was found at the region influenced by coastal water entrained by eddies. Vertical profiles of dissolved N2O showed a general increase with depth below the mixed layer and reached a broad peak (23–29 nM) at around 700 m coinciding with the nitrate maximum and oxygen minimum. The SP values measured for N2O ranged between 10.2‰ and 18.8‰, suggesting that dissolved N2O in the water column had been produced from both nitrification (ammonium oxidation) and nitrifier denitrification (nitrite reduction). Nitrification dominated in the intermediate water (120–1,000 m) while nitrifier denitrification dominated in the euphotic zone. The sea‐to‐air fluxes of N2O were estimated to be 7.0 ± 6.1 and 6.9 ± 6.5 μmol m−2 day−1 using two different gas transfer relationships. N2O emissions from the shelf and slope regions of the northern SCS were estimated to be 0.25 Tg N2O year−1, suggesting that coastal areas like the SCS are net sources of N2O to the atmosphere.

Description: Abstract We present in situ observations of mean and turbulent bottom stresses in a shallow, wave‐ and current‐driven flow over a cohesive sediment bed on the eastern shoals of South San Francisco Bay. Data from a Nortek Vectrino Profiler deployed with its measurement volume overlapping the bed allowed us to calculate mean velocity profiles and turbulent Reynolds stresses over a 1.5 cm profile with 1 mm vertical resolution. Additional acoustic instrumentation and pressure sensors provided mean current and wave data. From these observations we found that biological roughness elements protruding from the sediment bed result in a mean velocity profile qualitatively similar to that found in canopy shear mixing layers. Despite fundamental differences between this measured velocity structure and that assumed by wave‐current boundary layer models, we also found that the addition of waves to mean currents increases the net drag felt by the flow. The near‐bed momentum flux was often dominated by a wave‐induced component, which was generated by interactions between the wavy flow and the rough bed. Finally, we estimated the friction velocity using several different calculation methods and compared results to the measured bottom stress. This analysis revealed that traditional methods (e.g. log law fitting and point turbulence measurements) are consistent with one another when measuring the stress outside the wave boundary layer, but were all poor approximations of the total stress at the bed.

Description: Abstract For ice concentrations less than 85%, internal ice stresses in the sea ice pack are small and sea ice is said to be in free drift. The sea ice drift is then the result of a balance between Coriolis acceleration and stresses from the ocean and atmosphere. We investigate sea ice drift using data from individual drifting buoys as well as Arctic‐wide gridded fields of wind, sea ice and ocean velocity. We perform probabilistic inverse modeling of the momentum balance of free‐drifting sea ice, implemented to retrieve the Nansen number, scaled Rossby number and stress turning angles. Since this problem involves a non‐linear, under‐constrained system, we used a Monte Carlo guided search scheme ‐ the Neighbourhood Algorithm ‐ to seek optimal parameter values for multiple observation points. We retrieve optimal drag coefficients of CA = 1.2×10‐3 and CO = 2.4×10‐3 from ten‐day averaged Arctic‐wide data from July 2014 that agree with the AIDJEX standard, with clear temporal and spatial variations. Inverting daily‐averaged buoy data give parameters that, whilst more accurately resolved, suggest that the forward model over‐simplifies the physical system at these spatial and temporal scales. Our results show the importance of the correct representation of geostrophic currents. Both atmospheric and oceanic drag coefficients are found to decrease with shorter temporal averaging period, informing the selection of drag coefficient for short time‐scale climate models.

Description: Abstract Scientific and societal interest in the relationship between the Atlantic Meridional Overturning Circulation (AMOC) and United States (US) east coast sea level has intensified over the past decade, largely due to: 1) projected, and potentially ongoing, enhancement of sea‐level rise associated with AMOC weakening; and 2) the potential for observations of US east coast sea level to inform reconstructions of North Atlantic circulation and climate. These implications have inspired a wealth of model‐ and observation‐based analyses. Here, we review this research, finding consistent support in numerical models for an anti‐phase relationship between AMOC strength and dynamic sea level (DSL). However, simulations exhibit substantial along‐coast and inter‐model differences in the amplitude of AMOC‐associated DSL variability. Observational analyses focusing on shorter (generally less than decadal) timescales show robust relationships between some components of the North Atlantic large‐scale circulation and coastal sea‐level variability, but the causal relationships between different observational metrics, AMOC, and sea level are often unclear. We highlight the importance of existing and future research seeking to understand relationships between AMOC and its component currents, the role of ageostrophic processes near the coast, and the interplay of local and remote forcing. Such research will help reconcile the results of different numerical simulations with each other and with observations, inform the physical origins of covariability, and reveal the sensitivity of scaling relationships to forcing, timescale, and model representation. This information will, in turn, provide a more complete characterization of uncertainty in relevant relationships, leading to more robust reconstructions and projections.

Description: Abstract High‐accuracy spectrophotometric pH measurements were taken during a summer cruise to study the pH dynamics and its controlling mechanisms in the northern Gulf of Mexico in hypoxia season. Using the recently available dissociation constants of the purified m‐cresol purple (Douglas & Byrne, 2017, https://doi.org/10.1016/j.marchem.2017.10.001; Müller & Rehder, 2018, https://doi.org/10.3389/fmars.2018.00177), spectrophotometrically measured pH showed excellent agreement with pH calculated from dissolved inorganic carbon (DIC) and total alkalinity over a wide salinity range of 0 to 36.9 (0.005 ± 0.016, n = 550). The coupled changes in DIC, oxygen, and nutrients suggest that biological production of organic matter in surface water and the subsequent aerobic respiration in subsurface was the dominant factor regulating pH variability in the nGOM in summer. The highest pH values were observed, together with the maximal biological uptake of DIC and nutrients, at intermediate salinities in the Mississippi and Atchafalaya plumes where light and nutrient conditions were favorable for phytoplankton growth. The lowest pH values (down to 7.59) were observed along with the highest concentrations of DIC and apparent oxygen utilization in hypoxic bottom waters. The nonconservative pH changes in both surface and bottom waters correlated well with the biologically induced changes in DIC, that is, per 100‐μmol/kg biological removal/addition of DIC resulted in 0.21 unit increase/decrease in pH. Coastal bottom water with lower pH buffering capacity is more susceptible to acidification from anthropogenic CO2 invasion but reduction in eutrophication may offset some of the increased susceptibility to acidification.

Description: Abstract Interannual variability of Ocean Heat Content (OHC) is intimately linked to ocean water mass changes. Water mass characteristics are imprinted at the ocean surface and are modulated by climate variability on interannual to decadal time scales. In this study, we investigate the water mass change and their variability using an isopycnal decomposition of the OHC. For that purpose, we address the thickness and temperature changes of these water masses using both individual temperature‐salinity profiles and optimal interpolated products from Argo data. Isopycnal decomposition allows us to characterize the water mass interannual variability and decadal trends of volume and OHC. During the last decade (2006–2015), much of interannual and decadal warming is associated with Southern Hemisphere Subtropical Mode Water and Subantarctic Mode Water, particularly in the South Pacific Eastern Subtropical Mode Water, the Southeastern Indian Subantarctic Mode Water, and the Southern Pacific Subantarctic Mode Water. In contrast, Antarctic Intermediate Water in the Southern Hemisphere and North Atlantic Subtropical Mode Water in the Northern Hemisphere have cooled. This OHC interannual variability is mainly explained by volume (or mass) changes of water masses related to the isopycnal heaving. The forcing mechanisms and interior dynamics of water masses are discussed in the context of the wind stress change and ocean adjustment occurring at interannual time scale.

Description: Abstract Revolutionary observational arrays, together with a new generation of ocean and climate models, have provided new and intriguing insights into the Atlantic Meridional Overturning Circulation (AMOC) over the last two decades. Theoretical models have also changed our view of the AMOC, providing a dynamical framework for understanding the new observations and the results of complex models. In this paper we review recent advances in conceptual understanding of the processes maintaining the AMOC. We discuss recent theoretical models that address issues such as the interplay between surface buoyancy and wind forcing, the extent to which the AMOC is adiabatic, the importance of mesoscale eddies, the interaction between the middepth North Atlantic Deep Water cell and the abyssal Antarctic Bottom Water cell, the role of basin geometry and bathymetry, and the importance of a three‐dimensional multiple‐basin perspective. We review new paradigms for deep water formation in the high‐latitude North Atlantic and the impact of diapycnal mixing on vertical motion in the ocean interior. And we discuss advances in our understanding of the AMOC's stability and its scaling with large‐scale meridional density gradients. Along with reviewing theories for the mean AMOC, we consider models of AMOC variability and discuss what we have learned from theory about the detection and meridional propagation of AMOC anomalies. Simple theoretical models remain a vital and powerful tool for articulating our understanding of the AMOC and identifying the processes that are most critical to represent accurately in the next generation of numerical ocean and climate models.

Description: Abstract Transverse‐vertical structure and temporal variability of the Kuroshio current across the Tokara Strait during 2003–2012 measured by a ferryboat acoustic Doppler current profiler with a 2‐km horizontal resolution and a two‐day interval are presented. The Kuroshio passing through the Tokara Strait exhibits a multicore velocity structure. Its seasonal volume transport variation is biannual for baroclinic components relative to 700 m, peaking in July and December–January. However, the barotropic transport component exhibits an annual cycle with a maximum in December. Empirical orthogonal function analysis of the cross‐sectional velocity is performed. The first two empirical orthogonal function modes reveal the north‐south shift of the Kuroshio current axis and the change in Kuroshio volume transport, respectively. Temporal variabilities of the leading two modes correspond to those of the Kuroshio Position Index and the sea level difference across the strait, respectively. The third empirical orthogonal function mode, with a relatively smaller horizontal scale, was examined in terms of turbulent mixing. The banded structure captured by this mode is likely induced by flow‐topography interaction because islands in the Kuroshio route could cause horizontal and vertical flow separation. Additional analysis based on high‐resolution reanalysis data suggested that (1) inertial instability, which is expected in the areas with negative Ertel's potential vorticity, arises to enhance vertical mixing around the islands in the Tokara Strait, and (2) when the Kuroshio directly impinges the islands, flow divergence in the lee of the islands drives upwelling and leads to uplift of isotherms.

Description: Abstract Near‐inertial waves (NIWs), a fundamental oceanic response to wind forcing, play an important role in dynamical processes related to ocean mixing and hence have attracted sustained interest. Herein, we investigate temporally and spatially varying NIW distributions in the mixed and deep layers of the East/Japan Sea (EJS) using high‐resolution hourly‐wind‐forced data‐assimilated ocean model outputs. Temporally, the kinetic energy of NIWs in the mixed and deep layers is higher in fall and winter than in spring and summer, showing maxima in December, corresponding to wind forcings of both wind stress and wind‐current resonance. Spatially, the NIW energy in the mixed layer is higher on the northern side of the subpolar front (SPF), although there are no significant spatial differences in the wind forcings. Because of intensive background currents and their vorticities in the upper layer on the southern side of the SPF, vertical transfer of NIW energy is facilitated, shown by a shorter e‐folding decay time scale of NIWs in the mixed layer on the southern side of the SPF. The NIW kinetic energy in the deep layer of 400–1,000 m is higher on the southern side than on the northern side, an opposite spatial pattern to that in the mixed layer, but consistent with a previous observational study. Our results confirm that energetic anticyclonic circulations with negative relative vorticity in the upper layer on the southern side enable vertical penetration of NIW energy from the mixed layer to the deep layer more effectively.

Description: Abstract A set of numerical simulations (with horizontal resolutions of 1/4 ° and 1/12 °) are conducted to study the Pacific Water pathway in the Arctic Ocean and the freshwater content in Beaufort Gyre. Passive tracer tags the Pacific Water entering through Bering Strait into the Arctic Ocean, and further reveal its circulation routes and spatial distribution. Both the 1/4 ° and 1/12 ° simulations show Pacific Water mainly follows the Transpolar Drift over the integration period of 2002‐2016, with a limited amount being able to flow eastward along the Alaskan coast to enter the Canadian Arctic Archipelago. However, the circulation pattern of Pacific Water within the Beaufort Gyre is quite different with a stronger and tighter anticyclonic circulation in the 1/12 ° simulation corresponding to the difference in freshwater content. The 1/12 ° simulation successfully reproduces the overall recent increasing trend in the freshwater content in the Beaufort Gyre while the 1/4 ° simulation fails to maintain the high freshwater content state after 2007. Budget analysis suggests that this difference in Beaufort Gyre freshwater storage is mainly caused by lateral advection. The lateral freshwater flux is decomposed into two components due to the slow‐varying circulation and meso‐scale eddies. The difference in the capability to resolve eddies in the two simulations causes the difference in the temporal evolution of both components of the lateral flux.

Description: Abstract During September 2016 an ice‐free Beaufort Sea was observed for only the second time. Like previous regional sea ice minima (1998, 2008 and 2012), seasonal preconditioning of the ice pack towards younger, thinner ice types contributed to premature breakup that accelerated the ice‐albedo feedback and enhanced summer melt. In 2016, anomalously high sea ice export and ice pack divergence during February and April promoted the unusual widespread formation of new ice within the Beaufort. Thin ice types reached a peak regional concentration of 30% in March, when the ice cover is typically dominated by thick first‐year and multiyear sea ice. Combined CryoSat‐2 and SMOS data indicate that the regional ice volume plateaued from December to March as export offset ice growth and ultimately culminated in a ‐30% volume anomaly in April 2016. This atypically thin ice cover broke up 7 weeks earlier than average, with open water not only forming within coastal flaw leads but also within the offshore pack ice. By July 2016, vast areas of open water within the highly fractured ice cover accelerated the ice‐albedo feedback and led to rapid melt. Though maintaining a partial ice cover during summer throughout the observational record, significant negative trends in September sea ice area within the Beaufort are now punctuated by two recent ice‐free Septembers (2012, 2016). As the Beaufort transitions towards a seasonally ice‐free sea, we examine the role of winter preconditioning through sea ice transport and its growing importance within an increasingly seasonal and mobile Arctic ice cover.

Description: Abstract The Solomon Sea is a marginal sea in the western Pacific warm pool that contains the South Pacific low latitude western boundary currents (LLWBCs). These LLWBCs chiefly exit the Solomon Sea through three channels (Vitiaz Strait, St. George [Reviewer 1, comment 1 and Reviewer 2, minor 1]'s Channel and Solomon Strait) and serve as the primary source water for the Equatorial Undercurrent (EUC). Simulations have shown that transport partitioning between the straits determines the water mass structure of the EUC, but the relative contributions of transport through each strait have not been observed before. As part of the Southwest Pacific Ocean Circulation and Climate Experiment (SPICE), an array of moorings was deployed simultaneously in the three outflow channels of the Solomon Sea from July 2012 until March 2014 to resolve transport and water properties in each strait. Above deep isopycnals (σ0 ≤ 27.5), Vitiaz and Solomon Straits account for [Reviewer 2, major 1] 54% 54.2% and [Reviewer 2, major 1]36% 36.2% of the mean transport, respectively with the remaining [Reviewer 2, major 1] 10% 9.6% exiting through St. George's Channel. The strongest subinertial transport variability is observed in Solomon Strait and dominates total [Reviewer 1, comment 2] Solomon Sea transport variability and a significant fraction of this variability is at intraseasonal timescales. Finally, a previously unobserved deep current at 1500 m depth is found to enter the Solomon Sea through Solomon Strait, with a deployment mean transport of 4.6 Sv (Sv ≡ 106 m3 s‐1).

Description: Abstract Numerical experiments show that in a zonally symmetric model of a tropical ocean forced only by transient winds both inertia‐gravity wave activity and the energy dissipation rate have a pronounced maximum in the pycnocline close to the equator regardless of the latitudinal distribution of the energy input into the ocean's mixed layer. We consider a number of factors that determine the spatial distribution of mixing and find that equatorial enhancement is due to a combination of three factors: a stronger superinertial component of the wind forcing close to the equator, wave action convergence at turning latitudes for equatorially trapped waves, and nonlinear wave‐wave interactions between equatorially trapped waves. The most important factor is wave action convergence at turning latitudes.

Description: Abstract We investigate suspended particles collected from the upper mixed layer in the inner shelf of the southern East China Sea during autumn 2013 for carbon and nitrogen (POC and PN) contents and their isotope compositions (δ13C and δ15N) along with hydrographic parameters to understand the sources and dynamics of particulate organic matter (POM) in the study area. Results indicated that the extensive hydrological processes affect the biogeochemical composition of suspended POM, as revealed by the horizontally mixing POM and the spatial variation of δ15N and molar C/N ratio. Low C/N (2.4–6.5) and a weak correlation between POC and in situ chlorophyll fluorescence suggested that POM is dominated by the recently formed and well‐preserved planktonic OM. By fitting the linear correlation between δ13C and POC data with a photosynthetic fractionation model, we further disentangled dynamic controls of phytoplankton production and species diversity on δ13C variability (−24.3 to −21.3‰), emphasizing the constant effect of productivity‐derived POC on δ13C (0.02‰ per μg/L). The δ15N variability (2.3–7.4‰) is largely controlled by the mixing of isotopically different nitrogen sources, in which the importance of biological nitrogen fixation is unfolded based on the small δ15N and the negative correlation between δ15N of POM and seawater temperature. This implies that Kuroshio‐induced biological N fixation plays an important role in supporting the marine production in the East China Sea. These hydrologically driven δ13C and δ15N changes of marine productivity‐derived POM suggest that internal biophysical dynamics rather than terrestrial versus marine OM mixing largely control the C and N compositional variability in the shelf seas.

Description: Abstract Extreme ocean waves are part of the climate system but responsible for significant impacts on coastal and offshore environments, structures, and populations. In the Indian Ocean (IO), the wind and wave climate can be significantly influenced by natural climate variability, such as the El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM), yet our understanding on their regional influence is limited, particularly for seasonal extremes. Here, seasonal extreme significant wave heights (SWHs) and winds in the IO are examined over the period 1957–2010 utilizing ERA‐20C reanalysis data and the nonstationary generalized extreme value distribution to understand climatic extremes, by considering climate indices as covariates. ENSO influence on extreme SWHs include increases in the Bay of Bengal, southwest tropical IO (TIO), southern IO (SIO; a broad extension south of Australia), and South China and Philippine (SCP) Seas, and decreases in the Arabian Sea in boreal summer during El Niño. Extreme SWH responses to the IOD include increases in the eastern TIO, southwest TIO, and SIO in boreal autumn during its positive phase. Lastly, Southern Annular Mode not only significantly affects the SIO year round but has a weak influence in the northern and tropical IO. Composite analysis of ENSO and IOD events further highlight in phase combinations display less significant influence than out of phase combinations during summer, but not autumn. Mean and extreme wind responses are consistent with SWH responses to natural climate variability, and climate mode teleconnection patterns help explain the seasonal variations.

Description: Abstract Hypoxia, defined as dissolved oxygen (DO) 〈 2 mg/L, in the central basin of Lake Erie has been studied since the mid‐1900s. Even so, spatial patterns of hypoxia, and episodic hypoxia in nearshore areas where drinking water plant intakes are located, are not well characterized owing to limited observations and short‐term dynamics. We evaluated a physically based, DO model with respect to patterns of hypoxia observed in Lake Erie. The DO model used assigned rates of sediment and water column oxygen demand that were temperature dependent but otherwise spatially and temporally uniform. The DO model was linked to National Oceanic and Atmospheric Administration's (NOAA) Lake Erie Operational Forecasting System hydrodynamic model, an application of the Finite Volume Community Ocean Model (FVCOM). Model temperature and DO were compared with observations from ship‐based studies, real‐time sensor networks and an array of moored sensors that we deployed in 2017. In years with dominant southwesterly winds, persistent downwelling occurred along the south shore, which resulted in a thinner thermocline and earlier initiation of hypoxia along the south shore than the north. Occasional northeast winds temporarily reversed this pattern, causing upwelling along the south shore that brought hypoxic water to nearshore locations and water intakes. The DO model reproduced observed spatial and temporal patterns of hypoxia and revealed locations subject to episodes of hypoxia, including nearshore Ohio, north of Pelee Island, and near the Bass Islands. Model skill was limited in some respects, highlighting the importance of accurate simulation of the thermal structure and spatial patterns of oxygen demand rates.

Description: Abstract Multi‐model Arctic Ocean ``Climate Response Function” (CRF) experiments are analyzed in order to explore the effects of anomalous wind forcing over the Greenland Sea (GS) on poleward ocean heat transport, Atlantic Water (AW) pathways, and the extent of Arctic sea ice. Particular emphasis is placed on the sensitivity of the AW circulation to anomalously strong or weak GS winds in relation to natural variability, the latter manifested as part of the North Atlantic Oscillation (NAO). We find that anomalously strong (weak) GS wind forcing, comparable in strength to a strong positive (negative) NAO index, results in an intensification (weakening) of the poleward AW flow, extending from south of the North Atlantic Subpolar Gyre, through the Nordic Seas, and all the way into the Canadian Basin. Reconstructions made utilizing the calculated CRFs explain ~50 % of the simulated AW flow variance; this is the proportion of variability that can be explained by GS wind forcing. In the Barents and Kara Seas there is a clear relationship between the wind‐driven anomalous AW inflow and the sea ice extent. Most of the anomalous AW heat is lost to the atmosphere, and loss of sea ice in the Barents Sea results in even more heat loss to the atmosphere, and thus effective ocean cooling. Release of passive tracers in a subset of the suite of models reveals differences in circulation patterns and shows that the flow of AW in the Arctic Ocean is highly dependent on the wind stress in the Nordic Seas.

Description: Abstract The rear face of the wave spectrum is described by an equilibrium and a saturation subrange. Although accurate information about these ranges are highly relevant for wave modeling and many practical applications, there have been inconsistencies between results originating from temporal and spatial measurements. These discrepancies have been explained by the Doppler shift and the harmonics of nonlinear waves. We present high‐frequency wave measurements from the Baltic Sea gathered with R/V Aranda using a wave staff array, which provided directional frequency‐wavenumber data. In addition to the traditional wavenumber and frequency spectra, F(k) and S(ω), we also define a new spectrum that is a function of the inverse phase speed. We denote this spectrum Q(ν), where ν=kω−1. The properties of this Q‐spectrum were studied using data from four different sites. A strongly forced fetch‐limited case showed an equilibrium‐to‐saturation transition in the Q‐spectrum, with less variations in the equilibrium constants compared to the frequency spectra. The transition to a saturation regime happened around Uν=3 in all spectra where an equilibrium range was identified. Most duration‐limited spectra had no equilibrium range in the inverse phase speed domain. The absence of an equilibrium range was consistent with the wavenumber domain, but the frequency spectra still showed an apparent equilibrium subrange extending to ωU/g=5. The consistency of the saturation ranges between the Q‐spectrum and the wavenumber spectrum indicate a weak Doppler shift effect. We deduced that the main factor distorting the frequency spectra was wave nonlinearities.

Description: Abstract The summer South Vietnam Upwelling (SVU) is a major component of the South China Sea circulation where it also influences ecosystems. Here we revisit the existing knowledge of the SVU interannual variability. Based on a set of 15‐year eddy‐resolving sensitivity simulations, we quantify the respective contributions from different factors (atmospheric, river and oceanic forcings, ocean intrinsic variability OIV, El‐Niño Southern Oscillation ENSO) to this interannual variability and explore the underlying mechanisms. Our sea surface temperature upwelling indices allow us to quantify the strong SVU interannual variability in terms of strength and spatial distribution. Strong SVU years are offshore‐dominant with SVU centers located within 11‐12oN and 110‐112oE whereas weak SVU years are coastal‐dominant with SVU centers located near the coast and spanning a larger 10‐14oN latitude range. Our study confirms the leading influence of the summer wind, and further reveals that coastal SVU variability is related to the variability of the eastward jet that develops from the coast, whereas offshore SVU variability is strongly driven by the spatio‐temporal collocation of wind stress curl with cyclonic eddies. OIV, and to a lesser extent perturbations induced by river discharge and lateral oceanic conditions, strongly triggers interannual variability of background eddy circulation, thus of the SVU. ENSO influences the SVU mainly through its direct influence on summer winds.

Description: Abstract During the period 2011–2017 the South Pacific subtropical sea surface salinity maximum (SSS‐max) exhibited significant variability. It nearly doubled in area, increased in salinity by ~0.1 pss, and moved northward by 1–2° of latitude and eastward by 10° of longitude. The change was not associated with change in air‐sea freshwater flux, but rather due to changes in the advection of the subtropical gyre and South Equatorial Current, and variation in Ekman transport and Ekman pumping. There was a substantial decrease in the dynamic height difference associated with the South Equatorial Current, the Ekman transport, and Ekman pumping northeast of the SSS‐max. The observed changes in the SSS‐max appear to be linked to those of subducted subtropical underwater observed in the interior. They are also strongly associated with the Pacific Decadal Oscillation (PDO). The lagged correlation between the annual average PDO index and the area of the SSS‐max, with PDO leading by 2 years, is 0.86.

Description: Abstract A systematic study of Benguela Niño and Benguela Niña events during 1958 to 2015 including those that developed before the satellite era (1982) is carried out using an ocean general circulation model in combination with a linear equatorial model. Altogether, 21 strong warm and cold anomalous coastal events are identified among which 6 undocumented extreme coastal events are reported. Results suggest that most of these extreme coastal events including the newly identified ones are linked to remote equatorial forcing via mode 2 equatorial Kelvin waves. The latter propagates after approaching the African coast poleward as coastally trapped waves leading surface temperature anomalies along the Angola‐Benguela current system by one month. One to two months before the peak of Benguela Niños or Niñas usually occurring in March–April, a large‐scale wind stress forcing is observed with both local (variations of alongshore coastal wind stress) and remote forcing developing simultaneously. Results further suggest that surface temperature anomalies off Southern Angola and in the Angola‐Benguela Front are associated with equatorial dynamics and meridional wind stress fluctuations off the southwestern African coast north of 15°S. Similar mechanisms are observed for Northern Namibia in combination with forcing by local meridional wind stress variations.

Description: Abstract We present a new method to identify phytoplankton functional types (PFTs) in the Mediterranean Sea from ocean color data (GlobColour data in the present study) and AVHRR sea surface temperature. The principle of the method is constituted by two very fine clustering algorithms, one mapping the relationship between the satellite data and the pigments and the other between the pigments and the PFTs. The clustering algorithms are constituted of two efficient self‐organizing maps, which are neural network classifiers. We were able to identify and estimate the percentage of six PFTs: haptophytes, chlorophytes, cryptophytes, Synechococcus, Prochlorococcus, and diatoms. We found that these PFTs present a peculiar variability due to the complex physical and biogeochemical characteristics of the Mediterranean Sea: Haptophytes and chlorophytes dominate during winter and mainly in the western Mediterranean basin, while Synechococcus and Prochlorococcus dominate during summer. The dominance of diatoms was mainly observed in spring in the Balearic Sea in response to deep water convection phenomena and near the coastline and estuaries due to important continental inputs. Cryptophytes present a weak concentration in the Aegean Sea in autumn. The validation tests performed on in situ matchups showed satisfying results and proved the ability of the method to reconstruct efficiently the spatiotemporal patterns of phytoplankton groups in the Mediterranean Sea. The method can easily be applied to other oceanic regions.

Description: No abstract is available for this article.

Description: Abstract Twenty‐five years of high‐resolution (1/12°) ocean reanalysis are used to examine the Confluence of the Malvinas Current (MC) with the Brazil Current (BC) from synoptic to interannual time scales. The model transports of the MC (38.0 Sv ± 7.4 Sv 57 at 41°S) and the BC (23.0 Sv ± 11 Sv at 36°S) agree with observations. The model shows the branching of the MC near the Confluence with an offshore branch returning south and an inner branch sinking below the BC and managing to continue northward along the continental slope. Northward velocities associated with the subsurface inner branch peak at 40 cm/s at 36°S at 700 m depth. The model documents the migrations of the Subantarctic (SAF) and Subtropical front (STF) at the Confluence. The SAF and STF positions vary over a large range at synoptic (800 km) and interannual scale (300 and 200 km respectively) compared to the rather small seasonal migrations of the STF (150 km) and SAF (50 km). While trends in the MC are small over the 25‐years of the reanalysis, the BC becomes more intense (12.5 cm/s), saltier (0.37 psu) and warmer (2.5°C) in the upper 1000 m. These trends are accompanied with a southward displacement of the STF and the SAF of 150 and 50 km.

Description: Abstract This study addresses the dynamics of the Agulhas inshore front in the submesoscale range upstream of 26°E. Submesoscale frontal eddies are observed in the vicinity of Port Elizabeth (26°E) from satellite images and in observations collected from under‐water gliders. Using a submesoscale‐resolving numerical model (dx ~ 0.75 km) we are able to simulate similar submesoscale eddies. Barotropic instability is confirmed as the generation mechanism by a 1D linear stability analysis and an eddy kinetic energy budget. Kinetic energy is transferred from the mean flow to the eddies through the mean horizontal shear, which is a signature of barotropic instability. When the Agulhas Current is in a non‐meandering state, submesoscale eddy generation is a recurrent process which locally drives the front's variability. Along the front, the spatial variability of barotropic instability is shaped by the background strain. A large strain aligned with the frontal axis intensifies the frontal shear upstream of 28°E while a weakening of the strain allows for barotropic instability to be triggered downstream. Although an intermittent process, the barotropic instability shows a dominant period of variability comparable with the variability of the Agulhas Current and Undercurrent.

Description: Abstract The salinity in the 50‐300 m water column of the South Pacific subtropical gyre (10°S‐30°S) increased from 2004 to 2016. The observed changes are primarily associated with changes in the South Pacific Tropical Water (SPTW) volume, which increased at a rate of (3.17 ± 0.25) ·1014 m3 per decade in the region of 10‐30°S and 150°E‐90°W. The increases in the SPTW volume are caused by increased SPTW production, which increased at a rate of 3.45 ± 2.65 Sv per decade. The temporal variability in the mixed layer accounts for 75 ± 32% of the observed increase in SPTW production. The increases in temporal changes in the mixed layer are due to a deeper mixed layer depth (MLD) in austral winter and a shallower MLD in austral spring. The possible drivers of MLD changes at the air‐sea surface are examined. Among the wind stress curl, buoyancy fluxes and the turbulence induced by wind stress, changes in the surface buoyancy fluxes play the most important role in the deepening of the MLD during austral winter, and changes in the wind stress play a leading role in the shoaling of the MLD during austral spring. Changes in the buoyancy fluxes during winter and changes in the wind stress in spring can be attributed to increases in the Southern Annular Mode (SAM) from 2004 to 2016. This study highlights the strong modulation of SPTW formation by decadal climate variability over the subtropical South Pacific.

Description: Abstract Over the past 150 years, the Lower Columbia River Estuary (LCRE) controlling depth has approximately doubled, the majority of historical wetlands and floodplain have been reclaimed, numerous infrastructure projects have altered and confined flow pathways, and significant natural and anthropogenic changes to the discharge hydrograph have occurred. To investigate the effect of these changes on tides, river slope, and flood water levels, we construct and validate numerical models that simulate flow over late 19th century and present‐day bathymetry. The models are validated using archival (1853‐1877) and modern tide measurements throughout the LCRE, and river stage measurements from the tidal river (1876‐present). Historical flood plain roughness and levee heights are validated iteratively, by requiring simulations to match the observed roll‐off in the river stage rating curve during floods. Measurements and model results show that environmental change has amplified tidal constituents, with peak change about 60km from the coast. By contrast, increased depth has reduced river slope for low and moderate river discharge. For rarely observed extreme floods of 30×103 m3s‐1, simulated modern water levels exceed historical in Portland (OR). These observations highlight competing hydrodynamic effects, which are investigated by scaling the St Venant equations for a simulated 25×103 m3s‐1 flood: while larger modern depth reduces frictional effects and decreases surface slope, reduced floodplain access confines modern flow into channels, increasing velocity, bed stress and water levels. However, the highly frictional historical floodplain conveyed little flow, limiting the effect of floodplain to storage effects; hence, most simulated historical floods exceed modern levels.

Description: Abstract The Southwestern Atlantic Ocean has one of the largest and most productive continental shelves of the southern hemisphere. Despite its relevance, its circulation patterns have been largely inferred from hydrographic observations and numerical models. Here we describe the variability of the shelf circulation based on the analysis of eleven months of multi‐level currents measured by two bottom‐mounted acoustic Doppler current profilers deployed over the continental shelf at 39°S. The record‐length mean is 12 cm s‐1 and 13 cm s‐1 in the upper layer and decreases to 6 cm s‐1 and 8 cm s‐1 near the bottom, at the deployment nearer and further from the coast respectively. The mean flow direction is towards the NE, following the orientation of the isobaths. Measurements at both sites show that the along‐shore barotropic component accounts for 83% of the variability observed and are well correlated (0.86), suggesting a relatively uniform flow, which is presumably driven by large‐scale forcing. Indeed, large scale wind stress patterns dominate the temporal variability of the in‐situ currents and the passage of atmospheric fronts induces significant changes in the observed currents at all depths. We found that for 12 % of the measurements the currents reverse the direction to the SW in response to these atmospheric patterns. Furthermore, the analysis of sea surface height reconstructed from bottom pressure measurements at both sites and from a coastal tide gauge reveals that the variability of the along‐shore currents is driven by the cross‐shore pressure gradients generated by the along‐shore wind stress.

Description: Abstract Pine Island Ice Shelf, in the Amundsen Sea, is losing mass due to increased heat transport by warm ocean water penetrating beneath the ice shelf and causing basal melt. Tracing this warm deep water and the resulting glacial meltwater can identify changes in melt rate and the regions most affected by the increased input of this freshwater. Here, optimum multi‐parameter analysis is used to deduce glacial meltwater fractions from independent water mass characteristics (standard hydrographic observations, noble gases and oxygen isotopes), collected during a ship‐based campaign in the eastern Amundsen Sea in February‐March 2014. Noble gases (neon, argon, krypton and xenon) and oxygen isotopes are used to trace the glacial melt and meteoric water found in seawater and we demonstrate how their signatures can be used to rectify the hydrographic trace of glacial meltwater, which provides a much higher resolution picture. The presence of glacial meltwater is shown to mask the Winter Water properties, resulting in differences between the water mass analyses of up to 4 g kg−1 glacial meltwater content. This discrepancy can be accounted for by redefining the ”pure” Winter Water endpoint in the hydrographic glacial meltwater calculation. The corrected glacial meltwater content values show a persistent signature between 150 ‐ 400 m of the water column across all of the sample locations (up to 535 km from Pine Island Ice Shelf), with increased concentration towards the west along the coastline. It also shows, for the first time, the signature of glacial meltwater flowing off‐shelf in the eastern channel.

Description: Abstract The month‐to‐month variation of the winter South China Sea (SCS) western boundary current (WBC) along the western slope is examined using drifter observations, satellite altimetry data and an ocean reanalysis. The most surprising phenomenon is that the WBC velocity at the sea surface reaches the maxima in November‐December, which cannot be explained by wind forcing and Kuroshio intrusion alone. Analysis results demonstrate that buoyancy effect should be considered to explain the month‐to‐month variation besides wind‐Kuroshio effects. In winter, cold‐and‐salty advection by the WBC from the north decreases/reverses the zonal density gradient in the seasonal pycnocline induced by wind forcing and Kuroshio intrusion, and therefore weakens wind‐Kuroshio‐induced WBC. Buoyancy effect on the winter SCS WBC is opposite to wind‐Kuroshio effects. In addition, buoyancy effect reaches the maximum in January, which is concurrent with wind‐Kuroshio effects. As a result of their competition, the zonal density gradient in the seasonal pycnocline is maximum in November‐December, resulting in the maximum surface velocity along the western slope occurring in November‐December. This study demonstrates the importance of buoyancy forcing to the winter SCS WBC.

Description: Abstract New fine‐scale observations from the central Ross Gyre reveal the presence of double‐diffusive staircase structures underlying the surface mixed layer. These structures are persistent over seasons, with more developed mixed layers within the double‐diffusive staircase in winter months. The sensitivity of the ice formation rate with respect to mixing processes within the main pycnocline (double‐diffusive versus purely turbulent mixing) is investigated with the 1D model. A scenario with purely turbulent mixing results in significant underestimates of sea ice thickness. However, a scenario when double‐diffusive mixing operates in the presence of weak shear yields plausible ranges for sea ice thickness that agrees well with the observations. The model results and observations suggest a peculiar feedback mechanism that promotes the self‐maintenance of double‐diffusive staircases. Suppression of the vertical heat fluxes due to the presence of a double‐diffusive staircase, compared to purely turbulent case, allows Upper Circumpolar Deep Water to be more exposed to surface buoyancy fluxes. Our results shed light on the process ‐ double diffusion ‐ that might account for estimated rates of winter water mass transformation in the central Ross Gyre.

Description: Abstract Since the early twentieth century the amplitudes of tidal constituents in the Gulf of Maine and Bay of Fundy display clear secular trends that are among the largest anywhere observed for a regional body of water. The M2 amplitude at Eastport, Maine, increased at a rate of 13.7±1.2 cm/century, although it temporarily dropped during 1980–1990, apparently in response to changes in the wider North Atlantic. Annual tidal analyses indicate M2 reached an all‐time high amplitude last year (2018). Here we report new estimates of tides derived from nineteenth century water‐level measurements found in the U.S. National Archives. Results from Eastport, Portland, and Pulpit Harbor (tied to Bar Harbor) do not follow the twentieth century trends and indicate that the Gulf of Maine tide changes commenced sometime in the late nineteenth or early twentieth centuries, coincident with a transition to modern rates of sea‐level rise as observed at Boston and Portland. General agreement is that sea level rise alone is insufficient to cause the twentieth‐century tide changes. A role for ocean stratification is suggested by the long‐term warming of Gulf of Maine waters; archival water temperatures at Boston, Portland, and Eastport show increases of ∼2°C since the 1880s. In addition, a changing seasonal dependence in M2 amplitudes is reflected in a changing seasonal dependence in water temperatures. The observations suggest that models seeking to reproduce Gulf of Maine tides must consider both sea level rise and long‐term changes in stratification.

Description: Abstract The shallow Chukchi Sea is a gateway to the Arctic Ocean for Pacific‐origin waters. While a substantial portion of the Pacific‐origin waters flows through Barrow Canyon in the northeast corner of the Chukchi Sea, little is known on the hydrography of the surrounding regions in winter. We present profiles of wintertime hydrography on the Chukchi slope from an autonomous profiling instrument, and mooring records in Barrow Canyon. The central and western sectors of Barrow Canyon in December 2016 ‐ February 2017 (DJF 2016‐2017) were anomalously warm (∼0.5 ° C warmer than the climatology) with the flow orienting toward the Arctic Ocean. Unlike the summertime warming near the surface, the warm outflow has a temperature maximum at 80 dbar, and this outflow modifies the water mass properties on the Chukchi slope 70 km north of the canyon. Based on our Barrow Canyon mooring records starting in 2002, this is the first time that such warming is recorded on the outflow in winter. We discuss that this is due to the combination of the Barrow Canyon outflow favourable wind pattern and warming in the southern Chukchi Sea (the Gulf of Alaska and the Bering Sea) before the winter.

Description: Abstract Uncertainty measures for global‐mean sea level (GMSL) estimates are quantified, resulting from limited (in space and time) along‐track altimetric sampling of the global sea level field by altimetric satellite missions. To estimate such sampling‐related uncertainty, sea surface height (SSH) fields simulated by the high‐resolution STORM/NCEP ocean circulation model were sub‐sampled along altimeter tracks for the period 1993‐2010 and subsequently processed into global SSH averages using similar techniques to those used by six groups worldwide. Results show that the underlying satellite space‐time sampling has a substantial impact on the accuracy of GMSL estimates. This uncertainty originates primarily from data missing over sea ice‐covered regions; omitted data from shallow seas also contributes. Uncertainties in GMSL estimates result both from interpolation techniques required to fill data gaps such as missing tracks, and the choice of the mean sea surface required to estimate SSH anomalies. Cumulative effects lead to errors in GMSL estimates from ~0.8 mm to ~3.2 mm (RMS, root mean squared), depending on the underlying details of the estimation method. Results suggest that sampling limitations in meridional direction are a fundamental constraint on the accuracy level reachable for any altimetric GMSL estimate, resulting in a systematic Gaussian uncertainty of about 1.2 mm (RMS), 50% of which occurs on monthly time scales while some fraction occurs on time scales of several years. In all cases, a significant fraction of the error results from a mass exchange between the global ocean and sea ice‐covered polar regions. Contributions from data processing details are measurable but less significant.

Description: Abstract Ocean surface waves propagating through sea ice are scattered and dissipated. The net attenuation occurs preferentially at the higher frequencies, and thus the spectral bandwidth of a given wave field is reduced, relative to open water. The reduction in bandwidth is associated with an increase in the groupiness of the wave field. Using SWIFT buoy data from the 2015 Arctic Sea State experiment, bandwidth is compared between pancake ice and open water conditions, and the linkage to group envelopes is explored. The enhancement of wave groups in ice is consistent with the simple linear mechanism of superposition of waves with narrowing spectral bandwidth. This is confirmed using synthetic data. Nonlinear mechanisms, which have been shown as significant in other ice types, are not found to be important in this data set.

Description: Abstract The oceanographic response and atmospheric forcing associated with downwelling along the Alaskan Beaufort Sea shelf/slope is described using mooring data collected from August 2002 to September 2004, along with meteorological time series, satellite data, and reanalysis fields. In total, 55 downwelling events are identified with peak occurrence in July and August. Downwelling is initiated by cyclonic low‐pressure systems displacing the Beaufort High and driving westerly winds over the region. The shelfbreak jet responds by accelerating to the east, followed by a depression of isopycnals along the outer shelf and slope. The storms last 3.25 ± 1.80 days, at which point conditions relax toward their mean state. To determine the effect of sea ice on the oceanographic response, the storms are classified into four ice seasons: open water, partial ice, full ice, and fast ice (immobile). For a given wind strength, the largest response occurs during partial ice cover, while the most subdued response occurs in the fast ice season. Over the 2‐year study period, the winds were strongest during the open water season; thus, the shelfbreak jet intensified the most during this period and the cross‐stream Ekman flow was largest. During downwelling, the cold water fluxed off the shelf ventilates the upper halocline of the Canada Basin. The storms approach the Beaufort Sea along three distinct pathways: a northerly route from the high Arctic, a westerly route from northern Siberia, and a southerly route from south of Bering Strait. Differences in the vertical structure of the storms are presented as well.

Description: Abstract A new inversion method for determining near‐surface shear currents from a measured wave spectrum is introduced. The method is straightforward to implement and starts from the existing state‐of‐the‐art technique of assigning effective depths to measured wavenumber‐dependent Doppler shift velocities. A polynomial fit is performed, with the coefficients scaled based on a simple derived relation to produce a current profile that is an improved estimate of the true profile. The method involves no user‐input parameters, with the optimal parameters involved in the polynomial fit being chosen based on a simple criterion involving the measured Doppler shift data only. The method is tested on experimental data obtained from a laboratory where current profiles of variable depth dependence could be created and measured by particle image velocimetry, which served as “truth” measurements. Applying the new inversion method to experimentally measured Doppler shifts resulted in a 〉3× improvement in accuracy relative to the state‐of‐the‐art for current profiles with significant near‐surface curvature. The experiments are dynamically similar to typical oceanographic flows such as wind‐drift profiles and our laboratory thus makes a suitable and eminently useful scale model of the real‐life setting. Our results show that the new method can achieve improved accuracy in reconstructing near‐surface shear profiles from wave measurements by a simple extension of methods which are currently in use, incurring little extra complexity and effort. A novel adaptation of the normalized scalar product method has been implemented, able to extract Doppler shift velocities as a function of wavenumber from the measured wave spectrum.

Description: Abstract This paper presents a novel quantification of the fraction of broken waves (Qb) in natural surf zones using data from seven micro‐tidal, wave‐dominated, sandy Australian beaches. Qb is a critical, but rarely quantified, parameter for parametric surf zone energy dissipation models which are commonly used as coastal management tools. Here, Qb is quantified using a combination of remote sensing and in‐situ data. These data and machine learning techniques enable quantification of Qb for a substantial dataset (〉330,000 waves). The results show that Qb is a highly variable parameter with a high degree of inter‐ and intra‐beach variability. Such variability could be correlated to environmental parameters: tidal variations correlated with changes in Qb of up to 70% for a given local water depth (h) on a low‐tide terrace beach, and increased infragravity relative to sea‐swell energy correlated to lower values of Qb at the surf‐swash boundary. Qb also correlates well with the Australian beach morphodynamic model: for more dissipative beaches Qb increases rapidly in the outer surf zone, whereas for more reflective beaches Qb increases slowly throughout the surf zone. Finally, when comparing data to existing models, three commonly used theoretical formulations for Qb are observed to be poor predictors with errors of the order of 40%. Existing theoretical Qb models are shown to improve (revised errors of the order of 10%) if the Rayleigh probability distribution that describes the wave height is in these models is replaced by the Weibull distribution.

Description: Abstract The evolution of air bubbles after breaking waves plays an important role in gas and particle exchange between water bodies and the atmosphere. To improve our understanding of the impacts of salinity on this process, we systematically investigate the effect of salt concentrations ranging from 0 to 40 g/kg on the volume and size distributions of subsurface bubble plumes and surface foams in a laboratory breaking wave analog. Our experimental setup utilizes an intermittent plunging sheet to simulate breaking waves, while two synchronized digital cameras were used to monitor the temporal evolution of bubble plumes and surface foams. We first highlight the importance of plunging sheet intermittency on surface foam evolution. We then show that increasing salinity enhances the entrainment of submillimeter bubbles but has a less significant effect on larger supramillimeter bubbles. We observed that the foam area in saltier waters is consistently higher than that in freshwater throughout the foam decay phase. Furthermore, our investigation of surface bubble sizes shows that salinity has a more distinct effect on smaller (sub 2 mm) than on larger bubbles. This suggests that salinity may have a more pronounced impact on jet than on film drops ejection mechanisms. Finally, we conclude that the change in salinity within the typical oceanographic range is likely not a major factor for bubble‐mediated interactions at the water surface during breaking waves. However, even low‐salt concentrations greatly alter air entrainment characteristics in freshwater systems.

Description: Abstract Cohesive sediments exist as flocs of different sizes, which are built and destroyed through flocculation processes including both aggregation and breakup. This study investigates sediment flocculation processes in wave‐driven Langmuir turbulence that is commonly observed in coastal ocean through embedding a size‐resolving flocculation model into a turbulence‐resolving hydrodynamic model. The specific research questions are how Langmuir turbulence affects flocculation processes and how flocculation processes impact the spatial and size distributions of suspended cohesive sediment. The results show that Langmuir turbulence suspends flocs in the water column and organizes flocs of different sizes. By modulating the encounter of flocs and redistributing flocs in the turbulence field, Langmuir turbulence enhances the aggregation and breakup rates of flocs that are located in similar regions with high turbulent dissipation rates and suppresses those of others. As an outcome of modulated flocculation processes, floc size distribution changes with depth and floc mass concentration profiles change with floc size. The addition of wave breaking increases the shear rate near the surface and reduces the median floc size and averaged settling velocity, leading to increase in total floc mass concentration in the whole water column. Wave breaking also increases cross‐shelf sediment transport by more than 15% under the simulated conditions, which is comparable to that due to Langmuir turbulence compared to shear turbulence. Both floc size distribution and floc concentration vary with wind and wave conditions.

Description: Abstract The work presented here aims at interpreting the pattern observed at low frequencies in wavenumber‐frequency (k,ω) representations (dispersion diagrams) of radar returns from sea surface. This pattern is sometimes referred to as the “group line” in the literature and is supposed to result from the non‐linear behavior of the signal with respect to surface profile. To confirm this assumption, we have calculated the patterns generated by various non‐linear terms of second order in sea surface height. It is shown that the resolution in frequency of most data is not sufficient to allow for an accurate representation of the group line through a FFT. A methodology is proposed to define an average slope of the group line, which can be interpreted in terms of velocity.

Description: Abstract Tsunami resonance and coupled oscillation of shelf and bays modes has been reported to be important in tsunami wave amplification. The main objective of this work is to study the spatial pattern of natural oscillation modes and to analyze the influence of several resonators on the coast of the central Chile, which has a complex morphology with several bays, submarine canyons, and a wide continental shelf. First, natural oscillation modes were computed by means of modal analysis of local and regional domains. Second, a dense network of tide gauges and pressure sensors was analyzed to obtain background spectra inside bays. Third, tsunami spectra were computed from both tsunami records and numerical simulations. The results show that the use of modal analysis and background and tsunami spectra is effective for identifying natural oscillation modes. In addition, a dense network of tide gauges is useful to validate the spatial pattern of these natural modes. It was observed that larger resonators and the shelf are important in coupling oscillation with local bays, such that large amplification can be observed. Finally, this analysis allowed the diverse effects of 2010 and 2011 tsunamis in the bays of central Chile to be explained, making it possible to better address tsunami mitigation measures and the preparedness of coastal communities.

Description: Abstract Temperature variability in the nearshore (from ≈6‐m depth to the shoreline) is influenced by many processes including wave breaking and internal waves. A nearshore heat budget resolving these processes has not been considered. A 7‐month experiment at the Scripps Institution of Oceanography Pier (shoreline to 6‐m depth) measured temperature and surface and cross‐shore heat fluxes to examine a nearshore heat budget with fine cross‐shore spatial (≈20 m) and temporal (5 day to 4 hr) resolution. Winds, waves, air and water temperature, and in particular, pier end stratification varied considerably from late Fall to late Spring. The largest heat flux terms were shortwave solar radiation and baroclinic advective heat flux both varying on tidal time scales. The net heat flux is coherent and in phase with the nearshore heat content change at diurnal and semidiurnal frequencies. The binned mean heat budget has squared correlation R2=0.97 and best‐fit slope of 0.76. Including an elevated breaking wave albedo parameterization reduced the residual heat flux and improved the best‐fit slope. Baroclinic and barotropic advective heat fluxes have significant noise, and removing them from the heat budget improves the best‐fit slope when stratification is weak. However, when daily mean stratification is large, baroclinic advective heat flux dominates variability and is required to capture large (≈3 °C h−1) internal wave events. At times, large heat budget residuals highlight neglected heat budget terms, pointing to surfzone alongshore advection of temperature anomalies.

Description: Abstract We use observations from the Quinault River, a small river that flows into an energetic surf zone on the West Coast of Washington state, to investigate the interaction between river and wave forcing. By synthesizing data from moorings, drifters, and Unmanned Aerial System video, we develop a conceptual model of this interaction based on three length scales: the surf zone width, LSZ; the near‐field plume length, LNF; and the cross‐shore extent of the channel, LC. The relationships between these length scales show how tidal variability and bathymetric effects change the balance of wave and river momentum. The most frequently observed state is LSZ〉LNF. Under these conditions the surf zone traps the outflowing river plume and the river water's initial propagation into the surf zone is set by LNF. When the river velocity is highest during low water, and when wave forcing is low, LNF〉LSZ and river water escapes the surf zone. At high water during low wave forcing, LC〉LSZ, such that minimal wave breaking occurs in the channel and river water escapes onto the shelf. Based on the discharge, wave, and tidal conditions, the conceptual model is used to predict the fate of river water from the Quinault over a year, showing that approximately 70% of the river discharge is trapped in the surf zone upon exiting the river mouth.

Description: Abstract Satellite altimetry reveals substantial decadal variability in sea level ζ across the tropical Pacific during 1993–2015. An ocean state estimate that faithfully reproduces the observations is used to elucidate the origin of these low‐frequency tropical Pacific ζ variations. Analysis of the hydrostatic equation reveals that recent decadal ζ changes in the tropical Pacific are mainly thermosteric in nature, related to changes in upper‐ocean heat content. A forcing experiment performed with the numerical model suggests that anomalous wind stress was an important driver of the relevant heat storage and thermosteric variation. Closed budget diagnostics further clarify that the wind‐stress‐related thermosteric ζ variation resulted from the joint actions of large‐scale ocean advection and local surface heat flux, such that advection controlled the budget over shorter, intraseasonal to interannual time scales, and local surface heat flux became increasingly influential at longer decadal periods. In particular, local surface heat flux was important in contributing to a recent reversal of decadal ζ trends in the tropical Pacific. Contributions from local surface heat flux partly reflect damping latent heat flux tied to wind‐stress‐driven sea‐surface‐temperature variations.

Description: Abstract Wavenumber‐frequency spectra are obtained by performing a two‐dimensional Fourier transform of range‐time NRCS or Doppler velocity maps. In such diagrams, some energy is present at low space‐time frequencies, resulting from the non‐linear behavior of the measured quantity related to the sea surface geometry. This feature is called the group line, since for a narrow‐band wave packet the energy is concentrated along a straight line with group velocity as slope. Which physical non‐linear process generates the group line remains an open question. Breaking waves have been proposed as the most probable contributor. However, numerical simulations from weakly nonlinear surfaces without breaking events and experiments performed at low winds also provide such feature. In a companion paper, a theoretical and numerical analysis has permitted to predict the energy distribution of the group line depending on the kind of non‐linearity. It provides some means to characterize a group line in a rigorous way. In this paper, it is used to analyze the group lines derived from the experimental MARLENE data. The group lines computed from the backscattering amplitude behave as the one of the square of sea surface slopes. The analysis of the Doppler velocities provides similar results, which significantly differ from what is expected if breaking waves are the main contributors and do break at velocities reported in the literature. Our results suggest that the group line mainly reflects the asymptotic behavior of the scattering amplitude at grazing incidence, of which leading non‐linear term is proportional to the square of surface slope.

Description: Abstract A 38‐day long time series obtained using a combination of moored Wirewalkers equipped with conductivity‐temperature‐depth profilers and bottom‐mounted and subsurface acoustic Doppler current profilers provided detailed high‐resolution observations that resolved near‐surface velocity and vertical and cross‐shelf density gradients of the Chesapeake Bay plume far field. This unprecedented data set allowed for a detailed investigation of the impact of wind forcing on the thermal wind shear of a river plume. Our results showed that thermal wind balance was a valid approximation for the cross‐shelf momentum balance over the entire water column during weak winds ( 0.075 Pa), and it was also valid within the interior during moderate downwelling (−0.125 0.075 Pa). Stronger wind conditions, however, resulted in the breakdown of the thermal wind balance in the Chesapeake Bay plume, with thermal wind shear overestimating the observed shear during downwelling and underestimating during upwelling conditions. A momentum budget analysis suggests that viscous stresses from wind‐generated turbulence are mainly responsible for the generation of ageostrophic shear.

Description: Abstract Global sea surface salinity (SSS) has been obtained from space since 2009 by the Soil Moisture and Ocean Salinity (SMOS) mission and has been further enhanced by Aquarius in 2011 and Soil Moisture Active‐Passive (SMAP) missions in 2015. Due to the differences between SMOS, Aquarius and SMAP in the instruments used, retrieval algorithms and error correction strategies, the quality of their gridded products are different. In this paper, we have assessed the accuracy of three satellite products using in situ gridded data and buoy data. Compared with gridded in situ salinity measurements, the monthly Aquarius data are of the best quality, reaching the mission target accuracy (0.2 PSU) in the open ocean. SMOS and SMAP agree well with in situ data in the open ocean between 40°S and 40°N (root‐mean‐square deviation (RMSD): SMOS 0.211 PSU, SMAP 0.233 PSU). The RMSD of SMAP is lower than that of SMOS at high latitudes, which may due to the fact that the roughness correction of SMAP is based on the Aquarius geophysical model function (GMF). Meanwhile, time series comparison of salinity measured at 1 m by moored buoys indicates that satellite SSS capture variability of SSS at weekly time scales with reasonably good accuracy (RMSD: SMOS 0.25 PSU, SMAP 0.26 PSU), when excluding suspicious buoy data. Synergetic analysis of satellite SSS and Argo data indicates that satellite SSS can be applied as real‐time quality control (QC) of buoy 1 m salinity data.

Description: Abstract We report direct measurement of drag forces due to tidal flow over a submerged seagrass bed in Ngeseksau Reef, Koror State, Republic of Palau. In our study, drag is computed using an array of high‐resolution pressure measurements, from which values of the drag coefficients, CD, referenced to measured depth‐average velocities, V, were inferred. Reflecting the fact that seagrass blades deflect in the presence of flow, we find that CD is O(1) when flows are weak and tends towards a value of 0.03 at the highest velocities, behavior that is consistent with existing theory for canopy flows with flexible canopy elements. A limited subset of velocity profiles obey the law of the wall, producing values of shear velocity that, while noisy, broadly agree with values inferred from the pressure measurements.

Description: Abstract The Beaufort Gyre (BG) is a large‐scale bathymetrically‐constrained circulation driven by a surface Ekman convergence that creates a bowl‐shaped halocline which stores a significant portion of the Arctic Ocean's freshwater. Theoretical studies suggest that in the gyre interior, the halocline is equilibrated by a balance between Ekman pumping and counteracting mesoscale eddy transport energized by baroclinic instability. However, the strongest anticyclonic flows occur over steep continental slopes and, despite bathymetric slopes being known to influence baroclinic instability, their large‐scale impacts on BG halocline remain unexplored. Here, we use an idealized eddy‐resolving BG model to demonstrate that the existence of continental slopes dramatically affects key gyre characteristics leading to deeper halocline, stronger anticyclonic circulation, and prolonged equilibration. Over continental slopes, the magnitude of the Eulerian‐mean circulation is dramatically reduced due to the Ekman overturning being compensated by the eddy‐momentum‐driven overturning. The eddy thickness flux overturning associated with lateral salt transport is also weakened over the slopes, indicating a reduction of eddy thickness diffusivity even if the isopycnal slopes are largest there. Using a theoretical halocline model, we demonstrate that it is the localized reduction in eddy diffusivity over continental slopes that is critical in explaining the halocline deepening and prolonged equilibration time. Our results emphasize the need for observational studies of eddy overturning dynamics over continental slopes and the development of slope‐aware mesoscale eddy parameterizations for low‐resolution climate models.

Description: Abstract The Coal Oil Point seep field is among the most active and studied hydrocarbon seep fields in the world. The water column of the Coal Oil Point seep field was acoustically surveyed from 31 August 2016 to 14 September 2016 with a 200 kHz split‐beam echosounder to map the distribution of natural hydrocarbons in the region. An in situ direct capture device was used to measure the volumetric gas flux of natural hydrocarbons for three localized seep sites while simultaneously collecting acoustic volume backscatter measurements of the hydrocarbons within the water column. The acoustic volume backscatter was calibrated with the measured volumetric gas flux and the resulting relationship was used to determine flux over the entire seep field. The estimate of integrated volumetric gas flow rate over a survey area of approximately 4.1 km2 was 23,800 m3/day. The estimates of integrated volumetric gas flow rate and volumetric gas flux were compared to measurements reported in previous studies and were two to seven times smaller than results obtained by Hornafius et al., (1999), which had a total survey area of 18 km2. However, differences between methodologies limit the ability to assess natural variability in the Coal Oil Point seep field.

Description: Abstract Submarine groundwater discharge (SGD) plays a critical role in coastal and ocean biogeochemistry. Elucidating spatially and temporally heterogeneous SGD fluxes is difficult. Here we use radium isotopes to explore the external sources and mixing regime along the eastern coast of South Africa. We demonstrate that the long‐lived radium isotope compositions are controlled by low inputs of low and high salinity terrestrial groundwater. While activities of 228Ra and 226Ra in beach porewaters are similar to coastal waters, 224Ra is enriched by inputs of 228Th from coastal seawater. Porewater ages, based on the production of 224Ra from 228Th, range from 0.3 to 2.3 d indicating rapid flushing of the beach system. Unlike radium, however, nutrients follow a more complex pathway. We hypothesize that high total dissolved nitrogen (TDN) and phosphorus concentrations in beach porewaters (TDN ranges from 1 to 〉700 μM) and the coastal ocean (TDN ranges from 1 to 〉40 μM) are derived from a source not enriched in radium. We speculate that this source is terrestrial water flowing below the dune barrier at depths exceeding our beach sampling depths. This water likely flows upward through breaches in the confining layer into the beach or enters the ocean directly through paleochannels. The presence of high nutrient concentrations in the coastal ocean unaccompanied by high 228Ra activities leads to the hypothesis of this additional nutrient source. These combined inputs may be of considerable importance to the coastal ecology of southeastern Africa, an oligotrophic ecosystem dominated by the nutrient poor Agulhas Current.

Description: Abstract The bumpy continental slope/shelf topography is a quite common feature in the northern South China Sea (SCS), yet its effect on the shoaling internal solitary waves (ISWs) remains poorly understood. Therefore, numerical simulations by a fully nonlinear, nonhydrostatic model are carried out to explore the bumpy continental slope/shelf topographic effects on the transbasin evolution of large‐amplitude ISW in the northern SCS. It is found that the prominent bumps over both continental slope and shelf regions play significant roles in modulating the evolution of transbasin ISW in the northern SCS. The bump over the continental slope is capable of triggering a solitary‐like mode‐2 internal wave packet, while the bump over the continental shelf can result in three wave groups, including a leading group of rank‐ordered mode‐1 ISW packet, two following groups of non‐rank‐ordered mode‐1 ISW packet and mode‐2 internal waves. The bumps can cause a peak‐to‐peak difference of the energy decay rate of ISW up to 10–20 KW/m over continental slope region and 3–5 KW/m over continental shelf region. The wave kinetic energy (KE) is found to exceed the available potential energy (APE) by as much as 50% over the continental shelf break region. Over the shelf region, however, the bumps can first make the KE drop to as low as only 80% of the APE, but later the KE might bounce back to approximately 1.1–1.2 times of the APE. Both onshore‐ and offshore‐propagating beam‐like disturbances are found to be excited by the bumps. Except for the onshore‐propagating mode‐2 ISW packet, the reflected offshore‐propagating waves in different internal modes are also formed. These onshore‐ and offshore‐propagating multimodal internal waves can be clarified by the beam scattering and local generation mechanism.

Description: Abstract A new dataset of temperature and salinity fields reconstructed from satellite altimetry data between January 1993 and April 2014 is combined with satellite observations of mesoscale eddies in the subpolar North Atlantic between 40°N ‐ 55°N and 43°W ‐ 20.5°W. The dataset is used to calculate the meridional heat and freshwater transports related to the propagation of eddies crossing 47°N, a latitude close to the boundary between the subpolar and the subtropical gyres. The largest heat and freshwater transports by eddies are observed in the western part of the Newfoundland Basin. Around 35%‐45% of the heat and freshwater transports by eddies across 47°N stem from individual isolated eddies with large thermohaline signatures. Northward moving cold and fresh cyclonic eddies carrying subpolar water from the Western Boundary Current make a considerable contribution to the overall heat and freshwater transport by eddies crossing 47°N. While the transport by individual eddies is negligible compared to the transport by the mean flow in this region, it can have a notable influence on the temporal variability. The analysis is repeated for a model simulation with 1/12° horizontal resolution for the period 2002–2013. The observed results are well reproduced in the model simulation, in particular, the modeled number of eddies crossing 47°N, the spatial distribution, and the associated heat and freshwater transports across this latitude are consistent with the observations.

Description: Abstract The momentum balance for the density‐driven, non‐tidal circulation of a partially mixed estuary is generally considered to be between the pressure gradient and vertical friction forces, the result being a two‐layered, mean estuarine circulation often referred to as gravitational convection. All estuaries, however, tend to have geometric complexities that may alter this simplistic view. Here we apply a very high resolution, numerical circulation model to diagnose the momentum balance distributions throughout Tampa Bay, a partially mixed estuary on Florida's west coast. With resolution as fine as 20 m, the model resolves the channels, inlets, bridge causeways and other geometric complexities that impact the momentum balance distributions. A point‐by‐point, three‐dimensional momentum balance closure analysis demonstrates that while the general expectation for the non‐tidal estuarine circulation is met, the distribution of terms within the balance is more complex than the simplistic view when real geometries are considered. With Tampa Bay geometries being typical of many partially mixed estuaries, the results herein may also be of a more general nature.

Description: Abstract The external inflow/outflow through straits on the periphery of the South China Sea (SCS), and the associated internal response of vertical transport over the broad continental slope form and sustain a cyclonic‐anticyclonic‐cyclonic (CAC) circulation in the upper‐middle‐lower layer in the SCS. We conduct a process‐oriented numerical study to investigate the underlying coupled external‐internal dynamics that remains unknown, despite that the dynamics plays a critical role in forming and sustaining the CAC circulation. External sandwich‐like inflow‐outflow‐inflow in the water column through the Luzon Strait forms a three‐dimensional CAC slope current over the curving slope topography in the SCS basin, in which the downward/upward transport associated with the cross‐slope motion is established. The along‐slope current provides vorticity through the beta effect and accounts for the most external planetary vorticity input through the strait in the upper layer of the SCS, while the vorticity induced by the vertical transport is the major response to the external vorticity flux in the middle and lower layers. We illustrate the critical role of the vertical transport in linking the vorticity among the different layers for the development and sustenance of the CAC circulation. The vertical transport is associated with the cross‐slope motion due to slope current‐topography interaction, which involves mainly in bottom frictional transport and geostrophic cross‐isobath transport by the pressure gradient force (PGF) in the along‐slope direction. PGF is generated by nonlinear vorticity advection and the beta effect of the background slope current.

Description: Abstract This study investigates the capacity of a Spartina alterniflora meadow to attenuate waves during storm events based on field observations in the Chesapeake Bay. These observations reveal that environmental conditions including the ratio between water depth and plant height, (hr); the ratio between wave height (HS) and water depth; and current directions impact the wave height decay. Further, we present empirical representations of the bulk drag coefficient (Cd) as a function of the Keulegan‐Carpenter (KC) and Reynolds (Re) numbers, and the hr ratio. When applying the distinction between current directions, this representation exhibits better agreement when using the Re (ρ2 = 54%) and hr (ρ2 = 77%) than with the KC (ρ2 = 39%). Furthermore, we show that the representation of Cd can be improved by using a hr‐based modified Re and KC formulation, yielding correlations of 76% (modified Re) and 78% (modified KC). The proposed expressions are validated during another storm and predicted HS computed within the marsh results in a root‐mean‐square error of 0.014m, overestimating the largest HS (0.22m) by 18%. Finally, these expressions are applied to several hypothetical sea conditions. Under similar vegetation characteristics, HS of 1.55m and 0.8m (close to a 10,000 and 100‐year recurrence interval storm) are attenuated by 50% and 70% respectively, at 250 m from the marsh edge. This study provides evidence that validates the saltmarsh wave attenuation capacity during storms, quantifies this attenuation and supports the transferability of the existing formulas in the literature across similar coastal marshes.

Description: Abstract The Beaufort Gyre is a key feature of the Arctic Ocean, acting as a reservoir for fresh water in the region. Depending on whether the prevailing atmospheric circulation in the Arctic is anticyclonic or cyclonic, either a net accumulation or release of fresh water occurs. The sources of fresh water to the Arctic Ocean are well established and include contributions from the North American and Eurasian rivers, the Bering Strait Pacific water inflow, sea ice meltwater and precipitation, but their contribution to the Beaufort Gyre fresh water accumulation varies with changes with the atmospheric circulation. Here, we use a Lagrangian backward tracking technique in conjunction with the 1/12° resolution NEMO model to investigate how sources of fresh water to the Beaufort Gyre have changed in recent decades, focusing on increase in the Pacific water content in the gyre between the late 1980s and early 2000s. Using empirical orthogonal functions (EOF) we analyse the change in the Arctic oceanic circulation that occurred between the 1980s and 2000s. We highlight a “waiting room” advective pathway that was present in the 1980s and provide evidence that this pathway was caused by a shift in the center of Ekman transport convergence in the Arctic. We discuss the role of these changes as a contributing factor to changes in the stratification, and hence potentially the biology, of the Beaufort Gyre region.

Description: Abstract The continental shelf seas of China (CSSC) broadly encompass the Bohai Sea, the Yellow Sea, and the East China Sea and exhibit highly variable optical properties. Accurate satellite estimates of particulate organic carbon (POC) remain challenging because optimal proxies for remotely sensed POC are largely obscure in these optically complex coastal waters. In this study, optical and biogeochemical data, including the particulate beam attenuation coefficient (cp), particulate backscattering coefficient (bbp), remote sensing reflectance (Rrs), POC, total suspended matter (TSM) and chlorophyll‐a (Chla), were collected over multiple seasons and years in the CSSC. We first classified the study area into three different water types with three different POC retrieval proxies: the TSM for high‐TSM (HT) waters, Chla for low‐TSM (LT) waters and Rrs ratio (Rrs(490)/Rrs(555)) for moderate‐TSM (MT) waters. A composite POC algorithm using these three optimal proxies was then developed for Geostationary Ocean Color Imager (GOCI) satellite data (hereafter called the POC_CSSC algorithm). The validation results indicated that the accuracy of GOCI‐derived POC was greatly improved with a mean relative error of 32.08%. Application of the POC_CSSC algorithm to GOCI data over a tidally impacted estuary demonstrated the robustness of the algorithm and that tides play different roles in the broad CSSC. More specifically, tides have the strongest influence on nearshore estuarine waters, regulating the progression of high‐POC water masses from estuary to offshore environments, while offshore waters were the least influenced by tides with less variable, low POC concentrations.

Description: Abstract To reach upwelling and downwelling zones deep within the Southern Ocean seasonal sea‐ice cover, water masses must move across the Antarctic Circumpolar Current and through current systems including the Ross Gyre, Weddell Gyre, and Antarctic Slope Current (ASC). In this study we focus our attention on the lagrangian exchange between the Ross Gyre and surrounding current systems. We conducted numerical experiments using 5‐day 3D velocity fields from the Southern Ocean State Estimate with a particle tracking package to identify pathways by which waters move from near the Antarctic (AA) coastal margins or Antarctic Circumpolar Current (ACC) into the interior of the Ross Gyre, and to identify the timescales of variability associated with these pathways. Waters from near the AA margins enter the Ross Gyre along the western and northern boundary of gyre until the gyre separates from the Pacific‐Antarctic Ridge (PAR) near fracture zones. At this juncture, ACC‐derived inflow dominates the across‐gyre transport up to the Antarctic margin. Transport and exchange associated with different time‐average components of flow are calculated to determine the relative contributions of high‐ and low‐frequency and time‐mean components.

Description: Abstract The dynamics of the poleward shift of the Pacific North Equatorial Current bifurcation latitude (NBL) is studied using a 5.5‐layer reduced‐gravity model. It is found that the poleward shift of the NBL is associated with the asymmetric intensity of the wind stress curl input to the Pacific tropical and subtropical gyres. Stronger wind stress curl in the subtropical gyre leads to equatorward transport in the interior upper ocean across the boundary between the two gyres, causing a poleward transport compensation at the western boundary. In the lower layer ocean, in turn, there is poleward (equatorward) transport at the interior (western boundary) due to Sverdrup balance which requires zero transport at the gyre boundary where zonally integrated wind stress curl is zero. Therefore, the NBL exhibits a titling feature, with its position being more equatorward in the upper layer and more poleward in the lower layer. The equatorial currents bifurcations in other basins are also characterized by the poleward‐titling vertical structure. The wind stress curl over the subtropical gyre is generally stronger than that over the tropical gyre, resulting in the bifurcations shifting poleward with increasing depth.

Description: Abstract We report the significant impact of near‐inertial waves (NIWs) on vertical mixing and air‐sea carbon dioxide (CO2) fluxes in the Southern Ocean (SO) using a biogeochemical model coupled to an eddy‐rich ocean circulation model. The effects of high‐frequency processes are quantified by comparing the fully coupled solution (ONLINE) to two offline simulations based on five‐day averaged output of the ONLINE simulation: one that uses vertical mixing archived from the ONLINE model (CTRL) and another in which vertical mixing is recomputed from the five‐day average hydrodynamic fields (5dAVG). In this latter simulation, processes with temporal variabilities of a few days including NIWs are excluded in the biogeochemical simulation. Suppression of these processes reduces vertical shear and vertical mixing in the upper ocean, leading to decreased supply of carbon‐rich water from below, less CO2 outgassing in austral winter, and more uptake in summer. The net change amounts up to 1/3 of the seasonal variability in SO CO2 flux. Our results clearly demonstrate the importance of resolving high frequency processes such as NIWs to better estimate the carbon cycle in numerical model simulations.

Description: Abstract The Arctic Ocean mixed layer interacts with the ice cover above and warmer, nutrient rich waters below. Ice‐Tethered Profiler observations in the Canada Basin of the Arctic Ocean over 2006‐2017 are used to investigate changes in mixed layer properties. In contrast to decades of shoaling since at least the 1980's, the mixed layer deepened by 9 m from 2006‐2012 to 2013‐2017. Deepening resulted from an increase in mixed layer salinity that also weakened stratification at the base of the mixed layer. Vertical mixing alone can explain less than half of the observed change in mixed layer salinity, and so the observed increase in salinity is inferred to result from changes in freshwater accumulation via changes to ice‐ocean circulation or ice melt/growth and river runoff. Even though salinity increased, the shallowest density surfaces deepened by 5 m on average suggesting that Ekman pumping over this time period remained downwards. A deeper mixed layer with weaker stratification has implications for the accessibility of heat and nutrients stored in the upper halocline. The extent to which the mixed layer will continue to deepen appears to depend primarily on the complex set of processes influencing freshwater accumulation.

Description: Abstract Wake eddies are frequently created by flow separation where ocean currents encounter abrupt topography in the form of islands or headlands. Most previous work has concentrated on wake eddy generation by either purely oscillatory (usually tidal) currents, or quasi‐steady mean flows. Here we report measurements near the point of flow separation at the northern end of the Palau island chain, where energetic tides and vertically sheared low‐frequency flows are both present. Energetic turbulence measured near the very steeply sloping ocean floor varied cubically with the total flow speed (primarily tidal). The estimated turbulent viscosity suggests a regime of flow separation and eddying wake generation for flows that directly feel this drag. Small‐scale (∼ 1 km), vertically sheared wake eddies of different vorticity signs were observed with a ship‐board survey on both sides of the separation point, and significantly evolved over several tidal periods. The net production and export of vorticity into the wake, expected to sensitively depend on the interplay of tidal and low frequency currents, is explored here with a simple conceptual model. Application of the model to a 10‐month mooring record suggests that inclusion of high frequency oscillatory currents may boost the net flux of vorticity into the ocean interior by a depth dependent factor of 2 to 25. Models that do not represent the effect of these high frequency currents may not accurately infer the net momentum or energy losses felt where strong flows encounter steep island or headland topography.

Description: Abstract The Western Boundary Current in the Bay of Bengal (BOB), also known as East India Coastal Current (EICC), is northward (southward) and continuous during pre– (post–) Indian Summer Monsoon (ISM), but discontinuous during ISM (June–September). This study investigates the features of this discontinuity and role of eddies, local winds and southern open boundary lateral forcing using AVISO TOPEX Poseidon, ERS and Jason1 combined sea surface height anomaly, Ocean Surface Current Analysis Real–time (OSCAR) surface currents and high resolution Regional Ocean Modelling System (ROMS) simulations. The study shows that the discontinuity is governed by westward and southwestward propagating eddies between two opposite facing flows along the boundary; northward of 10°N and southward of 21°N. This northward flow of 10°N is wind driven with lateral influence during September. The southward flow of 21°N is influenced by the Summer Monsoon Current but the local wind influence starts from July onwards. During ISM, about 79% cyclonic and 77% anticyclonic eddies move westward while 68% cyclonic and 69% anticyclonic eddies move southward. Three highly active eddy genesis areas, having more than 50% activities during ISM, are identified; northern and southwestern bay for cyclonic and off Visakhapatnam (17.68°N, 83.21°E) coast for anticyclonic eddies. The pre– (post–) ISM is dominated by anticyclonic (cyclonic) eddies, which are found across the western BOB and away from northward (southward) EICC. The ISM is dominated by westward and southwestward moving both cyclonic and anticyclonic eddies, which are confined to the boundary, where discontinuity is generally observed.

Description: Abstract In Eastern Boundary Upwelling Systems (EBUS), the upwelling‐favorable wind speeds decrease toward the coast in the so‐called wind drop‐off coastal strip, which has been shown to be influential on the coastal upwelling dynamics, particularly in terms of the relative contributions of Ekman drift and Ekman suction to coastal upwelling. Currently, the wind drop‐off length scale is not properly resolved by the atmospheric forcing of regional ocean models in EBUS, featuring a smoother cross shore wind profile that results in stronger near shore speeds that could partly explain the coastal cold bias often found in those model simulations. Here, as a case study for the upwelling system off Central Chile, the sensitivity of upwelling dynamics to the coastal wind reduction is investigated using a regional oceanic model (ROMS). Coastal wind profiles at different resolutions are first generated using a regional atmospheric model, validated from altimeter data, and then used to correct the coarse atmospheric wind forcing used for sensitivity experiments with ROMS. It is shown that the wind drop‐off correction induces a reduction in the oceanic coastal jet intensity, a stronger poleward undercurrent and a coherent offshore Ekman drift. It also yields a significant reduction of the cold bias along the coast compared to the simulation with “uncorrected” winds. Such reduction cannot be solely explained by the reduced Ekman transport only partially compensated by increase in Ekman suction. The analysis of the surface heat budget reveals in fact that an important contributor to the cooling reduction along the coast in the presence of coastal wind drop‐off is the heat flux term mediated by the reduction in the mixed‐layer depth. Overall, our results illustrate the non‐linear response of the upwelling dynamics to the coastal wind profiles in this region.

Description: Abstract This paper describes a framework in which artificial extreme sea levels (ESLs) can be generated for use in flood risk analyses. Such analyses require large numbers of events to accurately assess the risk associated with certain return water levels and quantify uncertainties surrounding the temporal variability of ESL events. Stochastic models satisfy this requirement as they are computationally inexpensive, and thus many thousands of events may be generated over a very short period of time. As a case study, we have developed a stochastic model for the German Baltic Sea coast capable of simulating the temporal behavior of ESLs. To do this, high‐resolution water level data from 45 tide‐gauges have been used as model input. At each location, observed ESLs are identified and parameterized. Artificial ESLs are generated using Monte‐Carlo‐Simulations based on the parametric distribution functions fitted to the parameterized observed ESLs. We show that the method outlined here provides an accurate representation of ESLs at all tide‐gauges tested. However, the model is limited by the availability, length and quality of high‐resolution water level data. Due to the rarity of ESLs in the German Baltic Sea, including historical measurements into the stochastic procedure allows for the generation of artificial ESLs more in‐line with past extremes.

Description: Abstract The shallow subtropical cells (STCs) in the Pacific Ocean are thought to modulate the background state that the El Niño Southern Oscillation (ENSO) operates in. This modulation is proposed to impact the frequency and intensity of ENSO events and their teleconnections. We use a high‐resolution ocean model to investigate the volume transports associated with the STC branches along 5°N and 5°S. We find three prominent differences between the Southern hemisphere (SH) STC and the Northern hemisphere (NH) STC: i) the NH STC varies 26% stronger than the SH STC; ii) the NH STC appears to lead the SH STC by 3 months which causes the NH and SH STCs to play different roles during the course of El Nino and La Nina events; iii) in spite of the relative symmetry of the wind stress trends the STCs have differing decadal trends, with the SH STC clearly dominating the changes in the post‐1993 period. To investigate the mechanisms driving the STC variability we identify winds that are linearly and non‐linearly related to ENSO to force the ocean model. The hemispheric difference in interannual variance as well as the phase difference between the STCs can be explained with ENSO forcing. Our results suggest ENSO to be an important factor in modulating its own background state, with a prominent role for the winds that are non‐linearly related to ENSO. The decadal trends and their interhemispheric disparity, however, cannot be reproduced by our targeted ENSO experiments.

Description: Abstract Particle scattering is an important process that determines both the light penetration through the water column and water‐leaving light. Backscattering, in combination with absorption, determines the remote‐sensing reflectance that is used in ocean colour algorithms. Additionally, the wavelength dependence of the backscattering ratio can be related to the particle composition in seawater. Here, we examine the magnitude and the spectral behaviour of the backscattering ratio against other bio‐optical properties based on a comprehensive set of continuous measurements collected in coastal waters of the Great Barrier Reef (GBR) over three years. The site is located offshore of a major river, and close to the cross‐shelf transition of bottom sediments from terrigenous muds to marine carbonates. The backscattering ratio measured at 650 nm clearly clustered the data into two types. Type 1, which we identified as terrigenous mud with a low organic fraction, is characterised by backscattering ratio below 0.011 with a mean value of 0.005. Type 2, which we identified as marine carbonate with a higher organic fraction, has backscattering ratio above 0.011 with a mean value of 0.02. Within 3 years, study site was exposed to type 1 dominated particles 13% of the time, and type 2 87%. The observed changes in the backscattering ratio at this one coastal site is as large as the variability seen throughout the global ocean. This work provides a better understanding of processes determining the optical characteristics and insights into optical parameterisations that can be used in process‐based optical modelling of the GBR.

Description: Abstract Empirical equations for wave breaking and wave setup are compared with archived shoreline wave setup measurements to investigate the contribution of wind‐waves to extreme Mean Total Water Levels (MTWL, the mean height of the shoreline), for natural beaches exposed to open ocean wind‐waves. A broad range of formulations are compared through linear regression and quantile regression analysis of the highest measured values. Shoreline wave setup equations are selected based on the availability of local beach slope data and the ability of the quantile regression to show a good representation of the highest measured levels. Wave parameters from an existing spectral wave hindcast are used as input to the selected equations and are combined with a storm‐tide time series to quantify the relative contribution of shoreline wave setup to the extreme MTWL climate along Australian beaches. A multi‐pass analysis is provided to understand the ability to capture the shoreline wave setup estimates with and without considering beach slope. The national scale analysis which does not include beach slope indicates there are multiple contributing factors to MTWL. Examples are provided at two locations of differing local beach slope to show the importance of including local beach slope in determining the contribution of waves to MTWL. A tool is in development for further investigation of wave setup for Australian beaches.

Description: Abstract Realistic simulation of nearshore (from the shoreline to approximately 10‐km offshore) Lagrangian material transport is required for physical, biological, and ecological investigations of the coastal ocean. Recently, high‐resolution simulations of the coastal ocean have revealed a shelf populated with small‐scale, rapidly evolving currents that arise at resolutions 100 m. However, many historical and recent investigations of coastal connectivity utilize circulation models with ≈1‐km resolution. Here we show a resolution sensitivity to simulated Lagrangian transport and coastal connectivity with a hierarchy of Regional Oceanic Modeling System simulations of the Santa Barbara Channel at Δx= 1, 0.3, 0.1, and 0.036 km. At higher resolution (  100 m), rapid alongshore and vertical transport occurs in regions less than 1 km from the shoreline due to submesoscale shelf currents that open up new transport pathways on the shelf: submesoscale fronts and filaments, topographic wakes, and narrow alongshore jets. Shallow‐water fronts and filaments induce early time downwelling and subsequent dispersal at depth of surface material; this is not captured at coarser resolution (Δx= 1 km). Differences in three‐dimensional and two‐dimensional transport are explored in a higher‐resolution simulation: In general, three‐dimensional trajectories are more dispersive than two‐dimensional, due to a separation in their respective trajectories.

Description: Abstract One of the foci of the Forum for Artic Modeling and Observational Synthesis (FAMOS) project is improving Arctic regional ice‐ocean models and understanding of physical processes regulating variability of Arctic environmental conditions based on synthesis of observations and model results. The Beaufort Gyre, centered in the Canada Basin of the Arctic Ocean, is an ideal phenomenon and natural laboratory for application of FAMOS modeling capabilities to resolve numerous scientific questions related to the origin and variability of this climatologic freshwater reservoir and flywheel of the Arctic Ocean. The unprecedented volume of data collected in this region is nearly optimal to describe the state and changes in the Beaufort Gyre environmental system at synoptic, seasonal and interannual time scales. The in situ and remote sensing data characterizing ocean hydrography, sea surface heights, ice drift, concentration and thickness, ocean circulation, and biogeochemistry have been used for model calibration and validation or assimilated for historic reconstructions and establishing initial conditions for numerical predictions. This special collection of studies contributes: time series of the Beaufort Gyre data, new methodologies in observing, modeling and analysis; interpretation of measurements and model output; and discussions and findings that shed light on the mechanisms regulating Beaufort Gyre dynamics as it transitions to a new state under different climate forcing.

Description: Abstract The carbon cycle in estuarine environments is difficult to quantify because of substantial spatiotemporal heterogeneity in the sources, exchanges, and fates of carbon. We overcame these challenges with a multidecade numerical modeling analysis of seasonal, interannual, and decadal variability in net ecosystem metabolism (NEM) and associated carbon fluxes in Chesapeake Bay. Interannual variability in NEM along the estuarine axis indicated a clear spatial dependency of NEM on riverine discharge, with elevated flows causing increasing upper bay heterotrophy and increasing lower bay autotrophy during wet years. Our 30‐year simulation suggested the Chesapeake Bay is somewhat unique among estuaries in its tendency toward net autotrophy as a consequence of its extremely high nutrient to organic matter input ratio and large size. Budgets of three different carbon pools revealed that the entire Chesapeake Bay is a CO2 source to the atmosphere and organic carbon source to the open shelf, providing quantitative export estimates for interpretation of anthropogenic perturbations to the regional carbon flux.