Convergent coastal-plain estuaries have been shortened by dam-like structures worldwide. We used 31 long-term water level stations and a semi-analytical tide model to investigate the influence of a dam and landward-funneling on tides and storm surge propagation in the greater Charleston Harbor region, South Carolina, where three rivers meet: the Ashley, Cooper, and Wando. Our analysis shows that the principle tidal harmonic (M2), storm surge, and long-period setup-setdown (~4–10 days) propagate as long waves with the greatest amplification and celerity observed in the M2 wave. All waves attenuate in landward regions, but, as they approach the dam on the Cooper River, a frequency dependent response in amplitude and phase progression occurs. Dam-induced amplification scales with wave frequency, causing the greatest amplification in M2 overtides. Model results show that funneling and the presence of a dam amplify tidal waves through partial and full reflection, respectively. The different phase progression of these reflected waves, however, can ultimately reduce the total wave amplification. We use a friction-convergence parameter space to demonstrate how amplification is largest for partial reflection, when funneling and wave periods are not extreme (often the case of dominant tides), and for full reflection, when funneling and/or wave periods are small. The analysis also shows that in the case of long period events (>day), such as storm surges, dams may attenuate the wave in funneling estuaries. However, dams may amplify the most intense storm surges (short, high) more than funneling with unexpected consequence that can greatly increase flood exposure.

Glacial fjord circulation determines the import of oceanic heat to the Greenland Ice Sheet and the export of ice sheet meltwater to the ocean. However, limited observations and the presence of both glacial and coastal forcing - such as coastal-trapped waves - make uncovering the physical mechanisms controlling fjord-shelf exchange difficult. Here we use multi-year, high-resolution, realistically forced numerical simulations of Sermilik Fjord in southeast Greenland to evaluate the exchange flow. We compare models, with and without a plume, to differentiate between the exchange flow driven by shelf variability and that driven by subglacial discharge. We use the Total Exchange Flow framework to quantify the exchange volume fluxes. We find that a decline in offshore wind stress from January through July drives a seasonal reversal in the exchange flow increasing the presence of warm Atlantic Water at depth, that the exchange flux in the summer doubles with the inclusion of glacial plumes, and that the plume-driven circulation is more effective at renewal with a flushing time 1/3 that of the shelf-driven circulation near the fjord head.

A document by Xiang Yang. Click on the document to view its contents.

Currents and pressure records from the DeepLev mooring station (Eastern Levantine Basin) are analyzed to identify the dominant tidal constituents and their seasonal and depth variability. Harmonic and spectral analysis on seasonal segments of currents and pressure reveal attributes of the tidal regime in the Eastern Levantine Basin: (1) Dominant semidiurnal sea-level variability; (2) seasonal variation of semidiurnal and diurnal tides found in both currents and pressure datasets; and (3) significant diurnal currents with weak semidiurnal currents in all seasons. The most dominant tidal constituent found from the pressure dataset is the M2 (12.4 h). Results from pressure datasets generally agree with previous models and observations of semidiurnal tides, while the diurnal tides are larger than previously reported by 8-9 cm in the winter and 1-2 cm in the summer. The surface current variability differs from the one reported before in the Eastern Levantine Basin, with M2 magnitudes weaker by 1 cm, while the diurnal tides (K1, O1) are 1-2 cm larger. Seasonal segments showed seasonal differences in the local tidal regime’s amplitudes, with the K1 (7 cm difference between winter and fall) and S2 (4 cm difference between summer and fall) the most pronounced. We analyzed the M2 and S2 tides using surface drifters near DeepLev at different dataset lengths while considering the time constraints needed to resolve the tides adequately. The longer the dataset, the higher the resolution of the tidal analysis and the lower the amplitude leakages from nearby frequencies resulting in weaker tidal currents.

Estuaries in the northern California current system (NCCS) experience seasonally reversing wind stress, which is expected to impact the origin and properties of shelf water which enters NCCS estuaries (’shelf inflow’). Wind stress has been shown to affect the source of shelf inflow by driving alongshelf currents. However, the effects of wind-driven Ekman dynamics and shelf currents from larger-scale forcing on shelf inflow have yet to be explored. Variations in shelf inflow to the Salish Sea and the Columbia River estuary, two large NCCS estuarine systems, were studied using a realistic hydrodynamic model. The paths and source of shelf water were identified using particles released on the shelf. Particles were released every two weeks of 2017 and tracked for sixty days. Shelf inflow was identified as particles that crossed the estuary mouths. Mean wind stress during each release was compared with initial horizontal and vertical positions and physical properties of shelf inflow particles. For both the Salish Sea and the Columbia River estuary, upwelling-favorable wind stress was correlated with a shelf inflow source north of the estuary mouth. Depth was not correlated with wind stress for either estuary, but relative depth (depth scaled by isobath) increased during upwelling-favorable winds for both. Properties of inflow changed from cold and fresh during upwelling to warm and salty during downwelling, reflecting seasonal changes in NCCS shelf waters. These results may be extended to predict the source and properties of shelf inflow to estuaries in other regions with known wind or shelf current patterns.

This study examines the impact of ocean advection and surface freshwater flux on the non-seasonal, upper-ocean salinity variability in two climate model simulations with eddy-resolving and eddy-parameterized ocean components (HR and LR, respectively). We assess the realism of each simulation by comparing their sea surface salinity (SSS) variance with satellite and Argo float estimates. Our results show that, in the extratropics, the HR variance is about five times larger than that in LR and agrees with the Argo estimates. In turn, the extratropical satellite SSS variance is smaller than that from HR and Argo by about a factor of two, potentially reflecting the low sensitivity of radiometers to SSS in cold waters. Using a simplified salinity conservation equation for the upper-50-m ocean layer, we find that the advection-driven variance in HR is, on average, one order of magnitude larger than the surface flux-driven variance, reflecting the action of mesoscale processes.

The Gulf of Mexico (GoM) is home to some of the most energetic eddies in the ocean. They detach from the Loop-Current and drift through the basin, transporting large amounts of heat and salt. These eddies, known as Loop Current rings (LCRs) have a crucial role in the GoM’s dynamics and in the weather of the eastern US, and this role is largely conditioned by their longevity and decay properties. Here, we use an empirical method to estimate the energy evolution of all LCRs detached since 1993. We found that, contrary to the commonly accepted idea that LCRs conserve their energy as they drift through the GoM and decay suddenly against the western platform, LCRs’ energy decays faster in the eastern basin, and they typically lose three-quarter of their energy before encountering the continental shelf. We also show that wind-current feedback largely contributes to the energy decay and conversion.

North Atlantic sea surface temperatures (NASST), particularly in the subpolar region, are among the most predictable locations in the world’s oceans. However, the relative importance of atmospheric and oceanic controls on their variability at multidecadal timescales remain uncertain. Neural networks (NNs) are trained to examine the relative importance of oceanic and atmospheric predictors in predicting the NASST state in the Community Earth System Model 1 (CESM1). In the presence of external forcings, oceanic predictors outperform atmospheric predictors, persistence, and random chance baselines out to 25-year leadtimes. Layer-wise relevance propagation is used to unveil the sources of predictability, and reveal that NNs consistently rely upon the Gulf Stream-North Atlantic Current region for accurate predictions. Additionally, CESM1-trained NNs do not need additional transfer learning to successfully predict the phasing of multidecadal variability in an observational dataset, suggesting consistency in physical processes driving NASST variability between CESM1 and observations.

Decaying gelatinous zooplankton (GZ) originating from surface waters has been proposed as a possible major contributor to the biological carbon pump. However, studies arrived at largely diverging conclusions concerning the role of decaying GZ as organic matter supply for the deep-sea heterotrophic biota. We complement previous approaches to GZ sinking by proposing the first dynamically consistent physical model coupling GZ sinking speed and its mass. We evaluate GZ contribution to deep-ocean carbon sequestration and to the soft-tissue carbon pump by solving the model equations on the global ocean grid employing monthly climatological temperature fields and published exponential and linear temperature dependencies of mass decay rates. We present the global ocean distribution of the fraction of GZ-mass sinking out of the euphotic zone (200 m depth), twilight zone (1000 m depth) and the fraction of mass reaching the global ocean floor. Solutions in the upper water column are strongly dependent on the mass decay rate. Since most of the decay happens in the initial phase of the sinking process, the sinking-decay coupling exerts a substantial impact on sinking rates but has limited effect on the fraction of mass reaching the bathypelagic and abyssal ocean. Our model approach indicates that there are substantial latitudinal differences in the potential supply of GZ detrital matter to the deep sea. While at low latitudes only negligible amounts of GZ biomass are deposited at the ocean floor, high latitudes allow for substantial GZ detrital mass transport to depths below 1000 m.

Today, the ocean absorbs ~25% of the human-induced carbon emissions. Earth System Models (ESMs) indicate that the absorption increases by 0.79±0.07PgC per ppm of atmospheric CO2 increase (carbon-concentration feedback), but diminishes by -17.3±5.5PgC per degree of warming (carbon-climate feedback). Due to limited computational capacity, ESMs parameterize flows at scales smaller than their horizontal grid resolution, typically ~1°. We conduct simulations of global warming using increasingly finer horizontal resolutions (from 1° to 1/27°), with an ocean-biogeochemical model, in an idealized mid-latitude double-gyre circulation. Our findings demonstrate that these ocean carbon cycle feedbacks are highly influenced by resolution. This sensitivity primarily stems from how the overturning circulation’s mean state depends on resolution, as well as how it responds to global warming. Although being a fraction of the intricate response to climate change, it emphasizes the significance of an accurate representation of small-scale ocean processes to better constrain the future ocean carbon uptake.

A document by Roy Barkan. Click on the document to view its contents.

Sea ice-waves interactions have been widely studied in the marginal ice zone, at relatively low wind speeds and wave frequencies. Here, we focus on very different conditions typical of coastal polynyas: extremely high wind speeds and locally-generated, short, steep waves. We overview available parameterizations of relevant physical processes (nonlinear wave-wave interactions, energy input by wind, whitecapping and ice-related dissipation) and discuss modifications necessary to adjust them to polynya conditions. We use satellite-derived data and spectral modelling to analyze waves in ten polynya events in the Terra Nova Bay, Antarctica. We estimate the wind-input reduction factor over ice in the wave-energy balance equation at 0.56. By calibrating the model to satellite observations we show that exact treatment of quadruplet wave-wave interactions (as opposed to the default Discrete Interaction Approximation) is necessary to fit the model to data, and that the power n>4 in the sea-ice source term S_ice~f^n (where f denotes wave frequency) is required to reproduce the observed very strong attenuation in spectral tail in frazil streaks. We use a very-high resolution satellite image of a fragment of one of the polynyas to determine whitecap fraction. We show that there are more than twofold differences in whitecap fraction over ice-free and ice-covered regions, and that the model produces realistic whitecap fractions without any tuning of the whitecapping source term. Finally, we estimate the polynya-area-integrated wind input, energy dissipation due to whitecapping, and whitecap fraction to be on average below 25%, 10% and 30%, respectively, of the corresponding open-water values.

Mesoscale eddies play an important role in transporting water properties, enhancing air-sea interactions, and promoting large-scale mixing of the ocean. They are generally referred to as “coherent” structures because they are organized, rotating fluid elements that propagate within the ocean and have long lifetimes (months or even years). Eddies have been sampled by sparse in-situ vertical profiles, but because in-situ ocean observations are limited, they have been characterized primarily from satellite observations, numerical simulations, or relatively idealized geophysical fluid dynamics methods. However, each of these approaches has its limitations. Many questions about the general structure and “coherence” of ocean eddies remain unanswered. In this study, we investigate the properties of 7 mesoscale eddies sampled with relative accuracy during 4 different field experiments in the Atlantic. Our results suggest that the Ertel Potential Vorticity (EPV) is a suitable parameter to isolate and characterize the eddy cores and their boundaries. The latter appear as regions of finite horizontal extent, characterized by a local extremum of the vertical and horizontal components of the EPV. These are found to be closely related to the presence of a different water mass in the core (relative to the background) and the steepening of the isopycnals due to eddy occurrence and dynamics. Based on these results, we propose a new criterion for defining eddies. We test our approach using a theoretical framework and explore the possible magnitude of this new criterion, including its upper bound.

A significant shift in Earth’s climate characterizes the Neogene, transitioning from a single-ice-sheet planet to the current bipolar configuration. This climate evolution is closely linked to changing ocean currents, but globally-distributed continuous high-resolution sedimentary records are needed to fully capture this interaction. The Ocean Drilling Program (ODP) Site 752, located on Broken Ridge in the Indian Ocean, provides such a Miocene-to-recent archive. We use X-ray fluorescence (XRF) core scanning to build an eccentricity-tuned age-depth model and reconstruct paleoceanographic changes since 23 Ma. We find two intervals of enhanced productivity, during the early and middle Miocene (18.5 – 13.7 Ma) and late Pliocene/early Pleistocene (3 – 1 Ma). We also report a mixed eccentricity-obliquity imprint in the XRF-derived paleoproductivity proxy. In terms of grain size, three coarsening steps occur between 19.2 – 16 Ma, 10.8 – 8 Ma, and since 2.6 Ma. The steps respectively indicate stronger current winnowing in response to vigorous Antarctic Intermediate Water flow over Broken Ridge in the early Miocene, the first transient onset of Tasman Leakage in the Late Miocene, and the intensification of global oceanic circulation at the Plio-Pleistocene transition. High-resolution iron and manganese series provide a detailed Neogene dust record. This study utilized a single hole from an ODP legacy-site. Nevertheless, we managed to provide novel perspectives on past Indian Ocean responses to astronomical forcing. We conclude that Neogene sediments from Broken Ridge harbor the potential for even more comprehensive reconstructions. Realizing this potential necessitates re-drilling of these sedimentary archives utilizing modern drilling strategies.

An ensemble of experiments based on a ¼° global NEMO configuration is presented, including tidally forced and non-tidal simulations, and using both the default z* geopotential vertical coordinate and the z~ filtered Arbitrary Lagrangian-Eulerian coordinate, the latter being known to reduce numerical mixing. This is used to investigate the sensitivity of numerical mixing, and the resulting model drifts and biases, to both tidal forcing and the choice of vertical coordinate. The model is found to simulate an acceptably realistic external tide, and the first-mode internal tide has a spatial distribution consistent with estimates from observations and high-resolution tidal models, with vertical velocities in the internal tide of over 50 meters per day. Tidal forcing with the z* coordinate increases numerical mixing in the upper ocean between 30°S and 30°N where strong internal tides occur, while the z~ coordinate substantially reduces numerical mixing and biases in tidal simulations to levels below those in the z* non-tidal control. The implications for the next generation of climate models are discussed.

We explore the sensitivity of Southern Ocean surface and deep ocean temperature and salinity biases in the FOCI coupled climate model to atmosphere-ocean coupling time step and to lateral diffusion in the ocean with the goal to reduce biases common to climate models. The reference simulation suffers from a warm bias at the sea surface which also extends down to the seafloor in the Southern Ocean and is accompanied by a too fresh surface, in particular along the Antarctic coast. Reducing the atmosphere-ocean coupling time step from 3 hours to 1 hour results in increased sea-ice production on the shelf and enhanced melting to the north which reduces the fresh bias of the shelf water while also strengthening the meridional density gradient favouring a stronger Antarctic Circumpolar Current (ACC). With the shorter coupling step we also find a stronger meridional overturning circulation with more upwelling and downwelling south and north of the ACC respectively, as well as a reduced warm bias at almost all depths. Tuning the lateral ocean mixing has only a small effect on the model biases, which contradicts previous studies using a similar model configuration. We note that the latitude of the surface westerly wind maximum has a northward bias in the reference simulation and that this bias is unchanged as the surface temperature and sea-ice biases are reduced in the coupled simulations. Hence, the surface wind biases over the Southern Hemisphere midlatitudes appear to be unrelated to biases in sea-surface conditions.

Recent marine heatwaves in the Gulf of Alaska have had devastating and lasting impacts on species from various trophic levels. As a result of climate change, total heat exposure in the upper ocean has become longer, more intense, more frequent, and more likely to happen at the same time as other environmental extremes. The combination of multiple environmental extremes can exacerbate the response of sensitive marine organisms. Our hindcast simulation provides the first indication that more than 20 % of the bottom water of the Gulf of Alaska continental shelf was exposed to quadruple heat, positive [H+], negative Ωarag, and negative [O2] compound extreme events during the 2018-2020 marine heat wave. Natural intrusion of deep and acidified water combined with the marine heat wave triggered the first occurrence of these events in 2019. During the 2013-2016 marine heat wave, surface waters were already exposed to widespread marine heat and positive [H+] compound extreme events due to the temperature effect on the [H+]. We introduce a new Gulf of Alaska Downwelling Index (GOADI) with short-term predictive skill, which can serve as indicator of past and near-future positive [H+], negative Ωarag, and negative [O2] compound extreme events on the shelf. Our results suggest that the marine heat waves may have not been the sole environmental stressor that led to the observed ecosystem impacts and warrant a closer look at existing in situ inorganic carbon and other environmental data in combination with biological observations and model output.

In the northern Gulf of Mexico shelf, the Mississippi-Atchafalaya River System (MARS) impacts the carbonate system by delivering freshwater with a distinct seasonal pattern in both total alkalinity (Alk) and dissolved inorganic carbon (DIC), and promoting biologically-driven changes in DIC through nutrient inputs. However, how and to what degree these processes modulate the interannual variability in calcium carbonate solubility have been poorly documented. Here, we use an ocean-biogeochemical model to investigate the impact of MARS’s discharge and chemistry on interannual anomalies of aragonite saturation state (ΩAr). Based on model results, we show that the enhanced mixing of riverine waters with a low buffer capacity (low Alk-to-DIC ratio) during high-discharge winters promotes a significant ΩAr decline over the inner-shelf. We also show that increased nutrient runoff and vertical stratification during high-discharge summers promotes strong negative anomalies in bottom ΩAr, and less intense but significant positive anomalies in surface ΩAr. Therefore, increased MARS discharge promotes an increased frequency of suboptimal ΩAr levels for nearshore coastal calcifying species. Additional sensitivity experiments further show that reductions in the Alk-to-DIC ratio and nitrate concentration from the MARS significantly modify the simulated ΩAr spatial patterns, weakening the positive surface ΩAr anomalies during high-discharge summers or even producing negative surface ΩAr anomalies. Our findings suggest that riverine water carbonate chemistry is a main driver of interannual variability in ΩAr over river dominated ocean margins.

A document by Wenda Zhang. Click on the document to view its contents.

The effects of contemporary increases in riverine freshwater into the Arctic Ocean are estimated from ocean model simulations, using two runoff data sets. One runoff data set which is based on older climatological data, which has no inter-annual variability after 2007 and as such does not represent the observed increases in river runoff into the Arctic. The other data set comes from a hydrological model developed for the Arctic drainage basin, which includes contemporary changes in the climate. In the pan-Arctic this new data set represents an approximately 11% increase in runoff, compared with the older climatological data. Comparing two ocean model runs forced with the different runoff data sets, overall changes in different freshwater markers across the basin were found to be between 5-10%, depending on the area investigated. The strongest increases were seen from the Siberian rivers, which in turn caused the strongest freshening in the Eastern Arctic.

The Beaufort Sea High is a high-pressure system located in the Beaufort Sea and influences ocean circulation in the western Arctic known as the Beaufort Gyre. Wrangel Island, located in the western Chukchi Sea, typically experiences easterly sea ice motion due to the Beaufort Gyre. We find that under these climatological conditions, moving ice is blocked by the island and piles up on its eastern side, while ice on its western side continues to drift. This results in an ice thickness dipole across the island. A reversal in the sense of the oceanic and atmospheric circulation across the western Arctic results in a dipole with the opposing sign. We find the dipole is present throughout the year and is strongest in January when the ice thickness difference is approximately 1m. During the spring, it is associated with the transient opening of a polynya to the west of the island. The dipole is the result of opposing ice divergence and convergence across the western Arctic and may impact ocean circulation and ecosystems within the Chukchi Sea.

The spring phytoplankton bloom plays a major role in pelagic ecosystems; however, its dynamics is overlooked due to insufficient, highly-resolved observational data. Here we investigate the start, peak and decline of a two-week phytoplankton spring bloom in Frohavet, located at the coast of mid-Norway. We used observations from an uncrewed surface vehicle (USV) combined with buoy measurements, satellite images, discrete water sampling and modelling approaches. The spring bloom (March-June 2022) consisted of multiple peaks (up to 5 mg m-3), with a long peak in April, coincident with the period when the USV captured the temporal and spatial dynamics of the bloom. Short-term (5 days) episode of calm weather in the spring, such as clear skies and consistent low wind speed (< 7 m s-1) shoaled the mixed layer depth (< 15 m), after strong wind speed (average wind speed up to 20 m s-1 in March) and mixing events in winter. These rapid changes in the environment promoted the rapid development of the spring bloom - from 1 to 5 mg m-3 in 5 days. Likewise, the collapse of the bloom was rather quick, 1-2 days and coincides with low nitrate values and rapid increase in wind speed (> 10 m s-1), suggesting strong influence of the environment on phytoplankton dynamics during early stages of the spring bloom. Understanding the dynamics of the spring bloom is crucial for the management of marine resources. Integration of distinct observational platforms has the potential to unveil the environmental factors underlying phytoplankton bloom dynamics.

The Beaufort Sea has experienced a significant decline in sea ice, with thinner first-year ice replacing thicker multi-year ice. This transition makes the ice cover weaker and more mobile, making it more vulnerable to breakup during winter. Using a coupled ocean-sea-ice model, we investigated the impact of these changes on sea-ice breakup events and lead formation from 2000 to 2018. The simulation shows an increasing trend in the Beaufort Sea lead area fraction during winter, with a pronounced transition around 2007. A high lead area fraction in winter promotes a significant growth of new, thin ice within the Beaufort region while also leading to enhanced sea ice transport out of the area. The export offsets ice growth, resulting in negative volume anomalies and preconditioning a thinner and weaker ice pack at the end of the cool season. Our results indicate that large breakup events may become more frequent as the sea-ice cover thins and that such events only became common after 2007. This result highlights the need to represent these processes in global-scale climate models to improve projections of the Arctic.

Observations indicate that symmetric instability is active in the East Greenland Current during strong northerly wind events. Theoretical considerations suggest that baroclinic instability may also be enhanced during these events. An ensemble of idealised numerical ocean models, forced with northerly winds show that the short time-scale response (from two to four weeks) to the increased baroclinicity of the flow is the excitation of symmetric instability, which sets the potential vorticity of the flow to zero. The high latitude of the current means that the zero potential vorticity state has low stratification, and symmetric instability destratifies the water column. On longer time scales (greater than four weeks), baroclinic instability is excited and the associated slumping of isopycnals restratifies the water column. Eddy-resolving models that fail to resolve the submesoscale should consider using submesoscale parameterisations to prevent the formation of overly stratified frontal systems following down-front wind events. The mixed layer in the current deepens at a rate proportional to the square root of the time-integrated wind stress. Peak water mass transformation rates vary linearly with the time-integrated wind stress. The duration of a wind event leads to a saturation of mixing rates which means increasing the peak wind stress in an event leads to no extra mixing. Using ERA5 reanalysis data we estimate that between 1.5Sv and 1.8Sv of East Greenland Coastal Current Waters are produced by mixing with lighter surface waters during wintertime by down-front wind events. Similar amounts of East Greenland-Irminger Current water are produced at a slower rate.