A document by Meinrat Andreae. Click on the document to view its contents.

We study the impacts of a continental slope on instability and mesoscale eddy fluxes in idealized 3-layer numerical model simulations. The simulations are inspired by and mimic the situation in the Arctic Ocean’s Beaufort Gyre where anti-cyclonic winds drive anti-cyclonic currents that are guided by the continental slope. The forcing and currents are retrograde with respect to topographic Rossby waves. The focus of the analysis is on eddy potential vorticity (PV) fluxes and eddy-mean flow interactions under the Transformed Eulerian Mean framework. Lateral momentum fluxes in the upper layer dominate over the actual continental slope where eddy form drag, i.e.\ vertical momentum flux, is suppressed due to the topographic PV gradient. The diagnosis also shows that while eddy momentum fluxes are up-gradient over parts of the slope, the total quasi-geostrophic PV flux is down-gradient everywhere. We then calculate the linearly unstable modes of the time-mean state and find that the most unstable mode contains several key features of the observed finite-amplitude fluxes over the slope, including down-gradient PV fluxes. When accounting for additional unstable modes, all qualitative features of the observed eddy fluxes in the numerical model are reproduced.

The northeastern part of the North Atlantic subpolar gyre is a key passage for the Atlantic Meridional Overturning Circulation upper cell. To this day, the precise pathway and intensity of bottom currents in this area have not reached a consensus. In this study, we make use of regional high resolution numerical modeling to suggest that the main bottom current flowing south of Iceland originates from both the Faroe-Bank Channel and the Iceland-Faroe Ridge (with about equal contributions) and then flows along the topographic slope centered. When flowing over the rough topography, this bottom current generates a bottom mixed layer reaching 200 m height. We further demonstrate that many submesoscale structures are generated at the southernmost tip of the Icelandic shelf, thus spreading water masses in the open Iceland Basin. These findings have major implication in the better understanding of the transport of dense water masses in the North Atlantic.

Vertical motions associated with mesoscale ocean eddies modulate the light and nutrient environment, stimulating anomalies in phytoplankton biomass and chlorophyll. Phytoplankton populations can be subsequently trapped by the horizontal circulation or laterally diluted. In a time-varying flow, Lagrangian methods can be used to quantify eddy trapping, also known as Lagrangian coherency. From two decades of remote sensing observations in the North Pacific Subtropical Gyre, we compared coincident Eulerian and Lagrangian eddy atlases to assess the impact of eddy trapping on chlorophyll concentration. We found higher chlorophyll within Lagrangian coherent boundaries than in Eulerianeddies and outside-eddy waters. Yet, there are differences regionally and seasonally. For example, chlorophyll is most enriched within coherent boundaries of the Hawaiian Lee eddies and to the south of 23N in fall and winter. Our results suggest that by not accounting for lateral dilution, Eulerian analyses may underestimate the role of mesoscale eddies in enhancing chlorophyll.

12 Iceberg ID Footprint (m²) Avg. P w from surface to keel depth (kg m-3) P atm at DEM construction (kg m-3)

Abstract. The mean dynamic topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents. CNES-CLS mean dynamic topography (MDT) solutions are calculated by merging information from altimeter data, GRACE, and GOCE gravity field and oceanographic in situ measurements (drifting buoy velocities, High Frequency radar velocities, hydrological profiles). The objective of this paper is to present the newly updated CNES-CLS22 MDT. The main improvement of this new CNES-CLS22 MDT over the previous CNES-CLS18 MDT is in the Arctic, with better coverage, no artifacts and a more realistic solution. This is due to the use of a new first guess estimated with the CNES-CLS22 MSS and the GOCO06s geoid to which optimal filtering has been applied, as well as Lagrangian filtering at the coast to reduce the intensity of normal currents at the coast. Improvements also include updating the drifting buoy and T/S profile databases, as well as processing to obtain synthetic mean geostrophic velocities and synthetic mean heights. In addition, a new data type, HF radar data, was processed to extract physical content consistent with MDT in the Mid Atlantic Bight region. The study of this region in particular has shown the improvements of the CNES-CLS22 MDT, but that there is still work to be done to obtain a more physical solution over the continental shelf. The CNES-CLS22 MDT has been evaluated against independent height and velocity data in comparison with the previous version, the CNES-CLS18. The new solution presents slightly better results, although not identical in all regions of the globe.

Current eddy-permitting and eddy-resolving ocean models require dissipation to prevent a spurious accumulation of enstrophy at the grid scale. We introduce a new numerical scheme for momentum advection in large-scale ocean models that involves upwinding through a weighted essentially non-oscillatory (WENO) reconstruction. The new scheme provides implicit dissipation and thereby avoids the need for an additional explicit dissipation that may require calibration of unknown parameters. This approach uses the rotational, "vector invariant" formulation of the momentum advection operator that is widely employed by global general circulation models. A novel formulation of the WENO "smoothness indicators" is key for avoiding excessive numerical dissipation of kinetic energy and enstrophy at grid-resolved scales. We test the new advection scheme against a standard approach that combines explicit dissipation with a dispersive discretization of the rotational advection operator in two scenarios: (i) two-dimensional turbulence and (ii) three-dimensional baroclinic equilibration. In both cases, the solutions are stable, free from dispersive artifacts, and achieve increased "effective" resolution compared to other approaches commonly used in ocean models.

A data-driven emulator for the baroclinic double gyre ocean simulation is presented in this study. Traditional numerical simulations using partial differential equations (PDEs) often require substantial computational resources, hindering real-time applications and inhibiting model scalability. This study presents a novel approach employing neural operators to address these challenges in an idealized double-gyre ocean simulation. We propose a deep learning approach capable of learning the underlying dynamics of the ocean system, complementing the classical methods, and effectively replacing the need for explicit PDE solvers at inference time. By leveraging neural operators, we efficiently integrate the governing equations, providing a data-driven and computationally efficient framework for simulating the double-gyre ocean circulation. Our approach demonstrates promising results in terms of accuracy and computational efficiency, showcasing the potential for advancing ocean modeling through the fusion of neural operators and traditional oceanographic methodologies. In comparison to a dynamical numerical model, we obtain 600x speedups allowing us to create 2000-day ensembles in tens of seconds instead of hours.

The South Atlantic subtropical mode water (STMW) is characterized by a volume of water confined between the seasonal and permanent thermoclines. It is formed during the months of winter to early spring, July to October, near the Brazil-Malvinas Confluence region and the Brazil Current recirculation gyre. The STMW presents a homogeneous temperature (T) and salinity distribution in the horizontal and vertical, on average reaching up to 170 m of layer thickness. We investigate the impact of STMW in the nutrient's distribution and the nutricline depth in the western South Atlantic. Three data sets were used to identify the mode water and determine nutrients concentration:i) My Ocean biogeochemical model reanalysis; ii) World Ocean Atlas 2013 (WOA13) monthly climatology; and iii) the In Situ Analysis System (ISAS) data. Based on WOA13, during the summer (January, February and March) the STMW is found at subsurface from 100 m to 225 m deep; it has a mean layer thickness of 112 ± 5.0 m, reaching a maximum of 145 m. During the winter, the maximum layer thickness is 250 m while the mean is 155 ± 20.3 m. In the summer, STMW is in subsurface from 100 m to 225 m deep. As expected, the formation of mode water occured mostly in the winter to mid-spring. Using the model and ISAS data, we found a correlation between a deepening of the surface layer of minimum nitrate concentration and the mode water formation period (from June to October) in vertical profiles. From 2002 to 2012, greater nutrients concentration were found during the STMW formation period, from surface to 200 m depth, than in other periods with STMW absence. We also found a peak chlorophyll-a concentration during STMW formation period, suggesting that STMW presence in the southwest South Atlantic is important for biological processes in the upper ocean.

A document by Peter Chu. Click on the document to view its contents.

The role that continental margin sediments play in the global carbon cycle and the mitigation of climate change is currently not well understood. Recent research has indicated that these sediments might store large amounts of organic carbon; however, Blue Carbon research continues to focus on vegetated coastal ecosystems as actionable Blue Carbon. Marine sediments are considered emerging Blue Carbon ecosystems, but to decide whether they are actionable requires better quantifications of organic carbon stocks, accumulation rates, and the mitigation potential from avoided emissions. To close some of these knowledge gaps, we spatially predicted organic carbon content, dry bulk density and sediment accumulation rates across the Norwegian margin. The resulting predictions were used to estimate organic carbon stocks in surface sediments and their accumulation rates. We found that organic carbon stocks are two orders of magnitude higher than those of vegetated coastal ecosystems and comparable to terrestrial ecosystems in Norway. Accumulation rates of organic carbon are spatially highly variable and linked to geomorphology and associated sedimentary processes. We identify shelf valleys with a glacial origin as hotspots of organic carbon accumulation with a potentially global role due to their widespread occurrence on formerly glaciated continental margins. The complex and heterogenous nature of continental margins regarding organic carbon accumulation means that to close existing knowledge gaps requires detailed spatial predictions that account for those complexities. Only in this way will it be possible to evaluate whether margin sediments might be actionable Blue Carbon ecosystems.

Optical and acoustic sensors have been widely used in laboratory experiments and field studies to investigate suspended particulate matter concentration and particle size over the last four decades. Both methods face a serious challenge as laboratory and in-situ calibrations are usually required. Furthermore, in coastal and estuarine environments, the coexistence of mud and sand often results in multimodal particle size distributions, amplifying erroneous measurements. This paper proposes a new approach of combining a pair of optical-acoustic signals to estimate the total concentration and sediment composition of a mud/sand mixture in an efficient way without an extensive calibration. More specifically, we first carried out a set of 54 bimodal size regime experiments to derive empirical functions of optical-acoustic signals, concentrations, and mud/sand fractions. The functionalities of these relationships were then tested and validated using more complex multimodal size regime experiments over 30 optical-acoustic pairs of 5 wavelengths (420, 532, 620, 700, 852 nm) and 6 frequencies (0.5, 1, 2, 4, 6, 8 MHz). In the range of our data, without prior knowledge of particle size distribution, combinations between optical wavelengths 620-700 nm and acoustic frequencies 4-6 MHz predict mud/sand fraction and total concentration with the variation < 10% for the former and < 15% for the later. This approach therefore enables the robust estimation of suspended sediment concentration and composition, which is particularly useful in cases where calibration data is insufficient.

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the redistribution of heat, freshwater and carbon. At 26.5oN, the meridional heat transport has traditionally been partitioned geometrically into vertical and horizontal circulation contributions; however, attributing these components to the AMOC and Subtropical Gyre (STG) flow structures remains widely debated. Using water parcel trajectories evaluated within an eddy-rich ocean hindcast, we present the first Lagrangian decomposition of the meridional heat transport at 26.5oN. We find that water parcels recirculating within the STG account for 37% (0.36 PW) of the total heat transport across 26.5oN, more than twice that of the classical horizontal “gyre” component (15%). Rather than being distinct from the overturning circulation, the heat transport associated with the STG is due to the formation of subtropical mode waters via a shallow downward spiral, which ultimately feeds the northward limb of the AMOC.

Fram Strait is the primary pathway for sea ice export from the Arctic Ocean, yet estimates of volume export are constrained by observations of ice thickness and drift. Using a new year-round CryoSat-2 ice thickness product we determine an average annual export of 1,712 ± 452 km^3 from 2011-2022. 15% of the Arctic Oceans sea ice volume is exported annually, while 3.2% of the volume lost during the melt season is exported. Comparing high- and low-resolution ice drift products reveals the latter underestimate export by 30%. Comparing volume export between 82°N and 79°N reveals a high melt rate of 1 cm d-1, reducing export by 53%. September sea ice volume declines by 286 km^3 for every 100 km^3 exported during summer, highlighting how export amplifies the ice-albedo feedback. Our estimates of volume export provide new insight into Fram Straits role as a sea ice sink and freshwater source.

Biweekly sea surface temperature (SST) variability significantly contributes to over 50% of the intraseasonal variability in the eastern equatorial Pacific (EEP) and Atlantic (EEA). Our study investigates this biweekly variability, employing a blend of in-situ and reanalysis datasets. The research identifies biweekly signals in SST, meridional wind, and ocean currents, notably in September-November in EEP and June-August in EEA. Biweekly southerly (northerly) drives simultaneous northward (southward) ocean currents in EEP, but with a 1-2-day phase delay in EEA. Consequently, these currents lead to SST anomalies with a 3-4-day lag in both EEP and EEA due to the presence of the cold tongue. The study reveals the origin of biweekly wind fluctuations in the western Pacific for EEP and the subpolar Pacific for EEA, connected by Rossby waves validated through a linearized non-divergent barotropic model. This research affirms the influence of subtropical and subpolar atmospheric forcing on equatorial SST.

Storms can have a direct impact on sea ice, but whether their effect is seen weeks to months later has received little attention. The immediate and longer term impacts of an idealized open water wind storm are investigated with a one-dimensional coupled ice-ocean model. Storms with different momentum, duration and date of occurrence are tested. During the storm, the mechanical forcing causes a deepening of the mixed layer, leading to an increase in mixed layer heat content, despite a decrease in mixed layer temperature. This results in a delay in sea ice formation that ranges between a few hours to weeks compared to the control run, depending on the storm characteristics. Throughout the freezing period, the storm-induced thick mixed layer experiences little variability, preventing warm water entrainment at the base of the mixed layer. This leads to faster sea ice growth compared to the control run, resulting in sea ice thickness differences of a few millimeters to around 10 cm before the melting onset. These results are stronger for runs with higher momentum storms which cause greater mixed layer deepening. Storms occurring in early August, when the ocean surface heat flux is positive, also amplify the results by forcing a greater increase in mixed layer heat content. The impacts of the storms are sensitive to the initial stratification, and amplified for a highly stratified ocean. We suggest that localized storms could significantly influence the seasonal dynamics of the mixed layer and consequently impact sea ice conditions.

A document by Nora Fried. Click on the document to view its contents.

A document by Yu Gao. Click on the document to view its contents.

The Northwest Tropical Atlantic (NWTA) is a region with complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8ºN that leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km2. These two processes affect the spatial variability of the region’s surface latent heat flux (LHF). First, the presence of surface freshwater modifies the vertical stratification of the ocean limiting the amount of heat that can be released to the atmosphere. Second, they create a highly heterogeneous mesoscale sea-surface temperature (SST) field that directly influences near-surface atmospheric circulation. These effects are illustrated byd from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC4A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m2, of which 100 W m2 are associated with wind speed changes and 40 W m2 with SST variations. Surface currents or stratification-change associated heat release remain as second-order contributions with LHF variations of less than 10 W m2 each. Although this study is limited by the paucity of collocated observations, it highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales.

Weekly to quarterly beach elevation surveys spanning 700-800 m alongshore and 8 years at two beaches were each supplemented with several months of ∼100 sub-weekly surveys. These beaches, which have different sediment types (sand vs. sand-cobble mix), both widen in summer in response to the seasonal wave climate, in agreement with a generic equilibrium model. Results suggest differences in backshore erodability contribute to differing beach responses in the stormiest (El Niño) year. At both sites, the time dependence of the equilibrium modeled shoreline resembles the first mode of an EOF decomposition of the observations. With sufficient training, an equilibrium-informed Extra Tree Regression model, that includes features motivated by equilibrium modelling, can significantly outperform a generic equilibrium model.

Shore-oblique bathymetric features occur around the world and have been statistically correlated with enhanced shoreline retreat on sandy beaches. However, the physical mechanisms that explain a causal relationship are not well understood. In this study, radar remote sensing observations and results from a phase-resolved numerical model explore how complex morphology alters nearshore hydrodynamics. Observations at selected times during high-energy storm events as well as a suite of idealized simulations indicate that shore-oblique features induce strong spatial variations in the water surface elevation and wave breaking patterns. Re-emergent offshore flows and longshore current accelerations occur near the apex of the oblique nearshore features. The results suggest that complex bathymetric morphology exerts a powerful control on nearshore hydrodynamics and increases the potential for enhanced cross-shore and alongshore sediment transport, thus contributing to localized erosional zones.

A solid understanding of the mechanisms behind the presently observed, rapid warming of the northwest North Atlantic Continental Shelf is lacking. We hypothesize that a weakening of the Labrador Current System (LCS), especially the shelfbreak jet along the Scotian Shelf, is contributing to these changes and that the future evolution of the LCS will be key to accurate projections. Here we analyze the response of a transient simulation of the high-resolution GFDL Climate Model 2.6 (CM2.6) which realistically simulates the regional circulation but includes only a highly simplified representation of ocean biogeochemistry. Then, we dynamically downscale CM2.6 using a medium-complexity regional biogeochemical ocean model to obtain projections of several ecosystem-relevant variables. In the simulation, the shelfbreak jet weakens throughout the century because of a reduction of the along-shelf pressure gradient caused by a buoyancy gain of the upper water column along the shelf edge. This buoyancy gain is the result of an increased presence of subtropical waters in the continental slope. Importantly, we find that the weakening of the shelfbreak jet is not in response to a northward shift of the Gulf Steam, as has been hypothesized by others, and that previous reports of a northward shift of the Gulf Stream North Wall (GSNW) are an artifact of the temperature-based GSNW criterion in common use. The projected weakening of the shelfbreak jet is likely to lead to a reduction in nutrient availability and a subsequent decline in productivity on the Scotian Shelf, Gulf of St. Lawrence, and Grand Banks.

Dense overflows from marginal seas are critical pathways of oxygen supply to the Arabian Sea Oxygen Minimum Zone (OMZ), yet these remain inadequately understood. Climate models struggle to accurately reproduce the observed extent and intensity of the Arabian Sea OMZ due to their limited ability to capture processes smaller than their grid scale, such as dense overflows. Multi-month repeated sections by underwater gliders off the coast of Oman resolve the contribution of dense Persian Gulf Water (PGW) outflow to oxygen supply within the Arabian Sea OMZ. We characterize PGW properties, seasonality, transport and mixing mechanisms to explain local processes influencing water mass transformation and oxygen fluxes into the OMZ. Atmospheric forcing at the source region and eddy mesoscale activity in the Gulf of Oman control spatiotemporal variability of PGW as it flows along the shelf of the northern Omani coast. Subseasonally, it is modulated by stirring and shear-driven mixing driven by eddy-topography interactions. The oxygen transport from PGW to the OMZ is estimated to be 1.3 Tmol yr-1 over the observational period, with dramatic inter- and intra-annual variability (±1.6 Tmol yr-1). We show that this oxygen is supplied to the interior of the OMZ through the combined action of double-diffusive and shear-driven mixing. Intermittent shear-driven mixing enhances double-diffusive processes, with mechanical shear conditions (Ri<0.25) prevailing 14% of the time at the oxycline. These findings enhance our understanding of fine-scale processes influencing oxygen dynamics within the OMZ that can provide insights for improved modeling and prediction efforts.

The Antarctic Slope Front and the associated Antarctic Slope Current are central in determining the dynamics along and the exchanges across the continental shelf break around Antarctica. Here, we present new, four-year-long (2017-2021) records from two moorings deployed on the upper part of the continental slope (530 m and 738 m depth) just upstream of the Filchner Trough in the southern Weddell Sea. We use the records to describe the mean state and the seasonal variability of the shelf break current and the regional hydrography. We find that (i) the current is bottom enhanced, (ii) the isotherms slope upwards towards the shelfbreak, and more so for warmer isotherms, and (iii) the monthly mean thermocline depth is shallowest in February-March and deepest in May-June while (iv) the current is strongest in April-June. On monthly timescales, we show that (v) positive (warm) temperature anomalies of the de-seasoned records are associated with weaker-than-usual currents. Our results contribute to the understanding of how warm ocean waters propagate southward and potentially affect basal melt rates at the Filchner-Ronne Ice Shelf.