Intense atmosphere-ocean-ice interactions in the Ross Sea play a vital role in global overturning circulation by supplying saline and dense shelf waters. Since the 1960s, freshening of the Ross Sea shelf water has led to a decline in Antarctic Bottom Water formation. Since the early 2010s, however, the salinity of the western Ross Sea has rebounded. This study adopts an ocean-sea ice model to investigate the causes of this salinity rebound. Model-based salinity budget analysis indicates that the salinity rebound was driven by increased brine rejection from sea ice formation, triggered by the nearly equal effects of local anomalous winds and surface heat flux. The local divergent wind anomalies promoted local sea ice formation by creating a thin ice area, while a cooling heat flux anomaly decreased the surface temperature, increasing sea ice production. This highlights the importance of understanding local climate variability in projecting future dense shelf water change.

The concentration of chlorophyll-a (CHL) is an important proxy for autotrophic biomass and primary production in the ocean. Quantifying trends and variability in CHL are essential to understanding how marine ecosystems are affected by climate change. Previous analyses have focused on assessing trends in CHL mean, but little is known about observed changes in CHL extremes and variance. Here we apply a quantile regression model to detect trends in CHL distribution over the period of 1997-2022 for several quantiles. We find that the magnitude of trends in upper quantiles of global CHL (>90th) are larger than those in lower quantiles (≤50th) and in the mean, suggesting a growing asymmetry in CHL distribution. On a regional scale, trends in different quantiles are statistically significant at high latitude, equatorial, and oligotrophic regions. Assessing changes in CHL distribution has potential to yield a more comprehensive understanding of climate change impacts on CHL.

Phytoplankton stoichiometry modulates the interaction between carbon, nitrogen and phosphorus cycles, yet most biogeochemical models represent phytoplankton C:N:P as constants. This simplification has been linked to Earth System Model (ESM) biases and potential misrepresentation of biogeochemical responses to climate change. Here we integrate key elements of the Adaptive Trait Optimization Model (ATOM) for phytoplankton stoichiometry with the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) ocean biogeochemical model. Within a series of global ocean-ice-ecosystem retrospective simulations, ATOM-COBALT reproduced observations of particulate organic matter N:P, and compared to static N:P, exhibited reduced phytoplankton P-limitation, enhanced N-fixation, and increased low-latitude export, leading to improved consistency with observations. Two mechanisms together drove these patterns: the growth hypothesis and frugal P-utilization during scarcity. The addition of translation compensation- differential temperature dependencies of photosynthetic relative to biosynthetic processes- led to relatively modest strengthening of N:P variations and biogeochemical responses relative to growth-plus-frugality. Comparison of the multi-mechanism model herein against frugality-only models suggest that both can capture observed N:P patterns and produce qualitatively similar biogeochemical effects. There are, however, quantitative response differences and different responses across N:P mechanisms are expected under climate change- with the growth rate mechanism adding a distinct biogeochemical footprint in highly-productive low-latitude regions. These results suggest that variable phytoplankton N:P makes some biogeochemical processes resilient to environmental changes, and support using dynamic N:P formulations with the ocean biogeochemical component of next generation of ESMs.

Coastal wetlands play a critical role in maintaining the health of our planet by providing essential ecosystem services such as flood control, water purification, and critical habitat for a vast variety of species. However, their vulnerability to climate change and sea-level rise poses a significant threat to these services. Therefore, to provide long-term protection against erosion and sea-level rise, a shoreline restoration project was designed in coastal North Carolina (US) to use dredged sediments and rebuild the historic footprint of an eroded shoreline marsh adjacent to a regional airport's runway. To evaluate the potential benefits of this restoration project, the Sea Level Affecting Marsh Model (SLAMM) was employed. The developed model was run at a high spatial resolution (1m cell size) to investigate the effects of sea-level rise on the wetland communities and estimate the potential benefits of using dredged sediment to increase surface elevation. The results of the SLAMM model indicated that the restoration project offers substantial benefits in terms of shoreline marsh persistence through 2050, under all sea-level rise scenarios. This finding is significant because it shows that the restoration project can provide immediate benefits and help sustain the coastal wetlands in the face of sea-level rise. However, the benefits of the restoration project start to diminish after 2050, and differences among marsh areas in the restored and unrestored scenario decrease with increasing rates of sea-level rise. Therefore, it is essential to develop adaptive management strategies to ensure the long-term persistence of coastal wetlands and their ecosystem services. Overall, this study shows that the beneficial use of dredged sediments as a nature-based solution can effectively sustain coastal habitats threatened by sea-level rise and erosion.

The Congolese upwelling system (CUS), located along the West African coast north of the Congo River, is one of the most productive and least studied systems in the Gulf of Guinea. The Sea Surface Temperature minimum in the CUS occurs in austral winter, when the winds are weak and not particularly favorable to coastal upwelling. Here, for the first time, we use a high-resolution regional ocean model to identify the key atmospheric and oceanic processes that control the seasonal evolution of the mixed-layer temperature in a 1°-wide coastal band from 6°S to 4°S. The model is in good agreement with observations on seasonal timescales, and in particular reproduces the signature of the surface upwelling during the austral winter, the shallow mixed-layer due to salinity stratification, and the signature of coastal wave propagation. The analysis of the mixed-layer heat budget reveals a competition between warming by air-sea fluxes, dominated by the solar flux throughout the year, and cooling by vertical mixing at the base of the mixed-layer, as other tendency terms remain weak. The seasonal cooling is induced by vertical mixing, but is not controlled by the local wind. A subsurface analysis shows that remotely-forced coastal trapped waves raise the thermocline from April to August, which strengthens the vertical temperature gradient at the base of the mixed-layer and leads to the mixing-induced seasonal cooling in the Congolese upwelling system.

The Surface Ocean CO2 Atlas (SOCAT) of CO2 fugacity (fCO2) observations is a key resource supporting annual assessments of CO2 uptake by the ocean and its side effects on the marine ecosystem. SOCAT data are usually released with a lag of up to 1.5 years which hampers timely quantification of recent variations of carbon fluxes between the Earth System components, not only with the ocean. This study uses a statistical ensemble approach to analyse fCO2 with a latency of one month only based on the previous SOCAT release and a series of predictors. A retrospective prediction for the years 2021-2022 is made to test the model skill, followed by the generation of fCO2 and fluxes from January to August in 2023. Results indicate a modest degradation of the model skill in prediction mode and open the possibility to provide robust information about marine carbonate system variables with low latency.

A document by Shelley Stall. Click on the document to view its contents.

Marine extreme events such as marine heatwaves, ocean acidity extremes and low oxygen extremes can pose a substantial threat to marine organisms and ecosystems. Such extremes might be particularly detrimental (i) when they occur compounded in more than one stressor, and (ii) when the extremes extend substantially across the water column, restricting the habitable space for marine organisms. Here, we use daily output from a hindcast simulation (1961-2020) from the ocean component of the Community Earth System Model (CESM) to characterise such column-compound extreme events (CCX), employing a relative threshold approach to identify the extremes and requiring them to extend vertically over at least 50m. The diagnosed CCXs are prevalent, occupying worldwide in the 1960s about 1% of the volume contained within the top 300m. Over the duration of our simulation, CCXs become more intense, last longer, and occupy more volume, driven by the trends in ocean warming and ocean acidification. For example, the triple CCX have expanded 24-fold, now last 3-times longer, and have become 6-times more intense since the early 1960s. Removing this effect with a moving baseline permits us to better understand the key characteristics of the CCXs. They last typically about 10 to 30 days and predominantly occur in the tropics and the high latitudes, regions of high potential biological vulnerability. Overall, the CCXs fall into 16 clusters, reflecting different patterns and drivers. Triple CCX are largely confined to the tropics and the North Pacific, and tend to be associated with the El Nino-Southern Oscillation.

Near-horizontal shore platforms display highly irregular plan shapes, but little is known about the way in which these irregularities influence the significant wave height (Hs) on the platforms and the frequency components of the nearshore wave field. We use a nonlinear Boussinesq wave model to conduct harmonic and bispectral mode decomposition analyses, studying the control of concave and convex platform edges over wind (WW: 0.125 - 0.33 Hz), swell (SW: 0.05 - 0.125 Hz) and infragravity (IG: 0.008 - 0.05 Hz) frequencies. For breaking and non-breaking waves, increasing the platform edge concavity intensified wave divergence and subsequent attenuation of SW and IG across the outer platforms, reducing by up to 25%. Increasing the platform edge convexity intensified focusing and amplification of SW and WW over the outer platforms, increasing by up to 18% and 55% for breaking and non-breaking waves. In the presence of breaking, IG amplification depended on the generation of wave divergence across the inner platform, a condition determined by a critical convex curvature threshold (Κ=1.8) balancing wave focusing from refraction and defocusing from breaking. We find that convex curvature can determine the relative dominance of WW, SW and IG across platforms. Alongshore, coherent wave interactions governed IG stationary patterns defined by a node near the platform centreline and two antinodes on either side of concave edges. A node was generated at the platform centreline, and two antinodes were observed on either side of the convex edges for Κ>1.8, with the opposite pattern observed for Κ<1.8.

The global distribution of high Remote-sensing reflectance (Rrs) waters visible from satellite, likely associated with coccolithophore blooms, has changed markedly over the past 40 years. Over that period there has globally been an overall decrease in bloom area of 1.15 million km2 but with notable Rrs increases in the Barents Sea and the Antarctic Ocean. The primary drivers of these fundamental changes to ocean biogeochemistry have been investigated using Machine Learning techniques together with contemporaneous global multi-decadal time-series of sea-surface temperature (SST); wind speed and stress; sea level anomaly (SLA); photosynthetically available radiation (PAR) and; mixed layer depth (MLD). When split into ocean provinces different drivers of positive and negative trends in Rrs were found to dominate in different regions, but generally increases were found to coincide with changes to SST, PAR and reductions to wind-speed.

Satellite-based remote sensing missions have revolutionized our understanding of the Ocean state and dynamics. Among them, spaceborne altimetry provides valuable measurements of Sea Surface Height (SSH), which is used to estimate surface geostrophic currents. However, due to the sensor technology employed, important gaps occur in SSH observations. Complete SSH maps are produced by the altimetry community using linear Optimal Interpolations (OI) such as the widely-used Data Unification and Altimeter Combination System (DUACS). However, OI is known for producing overly smooth fields and thus misses some mesostructures and eddies. On the other hand, Sea Surface Temperature (SST) products have much higher data coverage and SST is physically linked to geostrophic currents through advection. We design a realistic twin experiment to emulate the satellite observations of SSH and SST to evaluate interpolation methods. We introduce a deep learning network able to use SST information, and a trainable in two settings: one where we have no access to ground truth during training and one where it is accessible. Our investigation involves a comparative analysis of the aforementioned network when trained using either supervised or unsupervised loss functions. We assess the quality of SSH reconstructions and further evaluate the network's performance in terms of eddy detection and physical properties. We find that it is possible, even in an unsupervised setting to use SST to improve reconstruction performance compared to SST-agnostic interpolations. We compare our reconstructions to DUACS's and report a decrease of 41\% in terms of root mean squared error.

South Georgia is a heavily glaciated sub-Antarctic island in the Southern Ocean. Cumberland Bay is the largest fjord on the island, split into two arms, each with a large marine-terminating glacier at the head. Although these glaciers have shown markedly different retreat rates over the past century, the underlying drivers of such differential retreat are not yet understood. This study uses observations and a new high-resolution oceanographic model to characterize oceanographic variability in Cumberland Bay and to explore its influence on glacier retreat. While observations indicate a strong seasonal cycle in temperature and salinity, they reveal no clear hydrographic differences that could explain the differential glacier retreat. Model simulations suggest the subglacial outflow plume dynamics and fjord circulation are sensitive to the bathymetry adjacent to the glacier, though this does not provide persuasive reasoning for the asymmetric glacier retreat. The addition of a postulated shallow inner sill in one fjord arm, however, significantly changes the water properties in the resultant inner basin by blocking the intrusion of colder, higher salinity waters at depth. This increase in temperature could significantly increase submarine melting, which is proposed as a possible contribution to the different rates of glacier retreat observed in the two fjord arms. This study represents the first detailed description of the oceanographic variability of a sub-Antarctic island fjord, highlighting the sensitivity of fjord oceanography to bathymetry. Notably, in fjords systems where temperature decreases with depth, the presence of a shallow sill has the potential to accelerate glacier retreat.

The access of heat to the Antarctic ice shelf cavities is regulated by the Antarctic Slope Front, separating relatively warm offshore water masses from cold water masses on the continental slope and inside the cavity. Previous observational studies along the East Antarctic continental slope have identified the drivers and variability of the front and the associated current, but a complete description of their seasonal cycle is currently lacking. In this study, we utilize two years (2019-2020) of observations from two oceanographic moorings east of the prime meridian to further detail the slope front and current seasonality. In combination with climatological hydrography and satellite-derived surface velocity, we identify processes that explain the hydrographic variability observed at the moorings. These processes include (i) an offshore spreading of seasonally formed Antarctic Surface Water, resulting in a lag in salinity and thermocline depth seasonality toward deeper isobaths, and (ii) the crucial role of buoyancy fluxes from sea ice melt and formation for the baroclinic seasonal cycle. Finally, data from two sub-ice-shelf moorings below Fimbulisen show that flow at the main sill into the cavity seasonally coincides with a weaker slope current in spring/summer. The flow is directed out of the cavity in autumn/winter when the slope current is strongest. The refined description of the variability of the slope current and front contributes to a more complete understanding of processes important for ice-shelf-ocean interactions in East Antarctica.

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

We show that steric sea-level varies with a period of 18.6 years along the western European coast. We hypothesize that this variation originates from the modulation of semidiurnal tides by the lunar nodal cycle and associated changes in ocean mixing. Accounting for the steric sea level changes in the upper 400 m of the ocean solves the discrepancy between the nodal cycle in mean sea level observed by tide gauges and the theoretical equilibrium nodal tide. Namely, by combining the equilibrium tide with the nodal modulation of steric sea level, we close the gap with the observations. This result supports earlier findings that the observed phase and amplitude of the 18.6-year cycle do not always correspond to the equilibrium nodal tide.

Ocean eddies play an important role in the distribution of heat, salt, and other tracers in the global ocean. But while surface eddies have been studied extensively, deeper eddies are less well understood. Here we study deep coherent vortices (DCVs) in the Northeast Atlantic Ocean using a high resolution numerical simulation. We perform a census of the DCVs on the $27.60$ kg/m$^3$ isopycnal, at the depth of $700-1500$ m, where DCVs of Mediterranean water (meddies) propagate. We detect a large number of DCVs, with maxima around continental shelves, and islands, dominated by small and short-lived cyclones. However, the large and long-lived DCVs are mostly anticyclonic. Among the long-lived DCVs, anticyclonic meddies, stand out. They grow in size by merging with other anticyclonic meddies. Cyclonic meddies are also regularly formed, but most of them are destroyed near their formation sites due to the presence of the energetic anticyclonic meddies, which destroy cyclones by straining and wrapping the positive vorticity around their core. During their life cycle, as they propagate to the southwest, anticyclonic meddies can interact with other DCVs, including anticyclones containing Antarctic Intermediate Water generated near the Moroccan coast, Canary anticyclonic DCVs and cyclonic DCVs generated south of $30^\circ$N along the African continental shelf. With these latter, they can form dipoles, and with the former, they co-rotate pro tempore. Thus, a more detailed view of the life cycle of anticyclonic meddies is proposed: they grow by merging, undergo multiple interactions along their path, and they decay at low latitudes.

We use seismic ambient noise recorded by dense ocean bottom nodes (OBNs) in the Gorgon gas field, Western Australia, to compute time-lapse seafloor models of shear-wave velocity. The extracted hourly cross-correlation (CC) functions in the frequency band 0.1 – 1 Hz contain mainly Scholte waves with very high signal to noise ratio. We observe temporal velocity variations (dv/v) at the order of 0.1% with a peak velocity change of 0.8% averaged from all station pairs, from the conventional time-lapse analysis with the assumption of a spatially homogeneous dv/v. With a high-resolution reference (baseline) model from full waveform inversion of Scholte waves, we present an elastic wave equation based double-difference inversion (EW-DD) method, using arrival time differences between the reference and time-lapsed Scholte waves, for mapping temporally varying dv/v in the heterogeneous subsurface. The time-lapse velocity models reveal increasing/decreasing patterns of shear-wave velocity in agreement with those from the conventional analysis. The velocity variation exhibits a ~24-hour cycling pattern, which appears to be inversely correlated with sea level height, possibly associated with dilatant effects for porous, low-velocity shallow seafloor and rising pore pressure with higher sea level. This study demonstrates the feasibility of using dense passive seismic surveys for quantitative monitoring of subsurface property changes in the horizontal and depth domain.

The Paleocene-Eocene Thermal Maximum (PETM) was a transient global warming event recognised in the geologic record by a prolonged negative carbon isotope excursion (CIE). The onset of the CIE was the result of a rapid influx of 13C-depleted carbon into the ocean-atmosphere system. However, the mechanisms required to sustain the negative CIE remains unclear. Previous studies have identified enhanced mobilisation of petrogenic organic carbon (OCpetro) and argued that this was likely oxidised, increasing atmospheric carbon dioxide (CO2) concentrations after the onset of the CIE. With existing evidence limited to the mid-latitudes and subtropics, we determine whether: (i) enhanced mobilisation and subsequent burial of OCpetro in marine sediments was a global phenomenon; and (ii) whether it occurred throughout the PETM. To achieve this, we utilised a lipid biomarker approach to trace and quantify OCpetro burial in a global compilation of PETM-aged shallow marine sites (n = 7, including five new sites). Our results confirm that OCpetro mass accumulation rates (MARs) increased within the subtropics and mid-latitudes during the PETM, consistent with evidence of higher physical erosion rates and intense episodic rainfall events. The high-latitude sites do not exhibit distinct changes in the organic carbon source during the PETM. This may be due to the more stable hydrological regime and/or additional controls. Crucially, we also demonstrate that OCpetro MARs remained elevated during the recovery phase of the PETM. Although OCpetro oxidation was likely an important positive feedback mechanism throughout the PETM, we show that this feedback was both spatially and temporally variable.

This paper introduces the new eddy-resolving global Simple Ocean Data Assimilation version 4 (SODA4) ocean/sea ice reanalysis. As with its predecessor SODA3, sequential data assimilation is used to constrain the evolving temperature and salinity fields using World Ocean Database profiles, in-situ and satellite sea surface temperature observations, and sea ice thickness estimates as constraints. The numerical model is based on NOAA/Geophysical Fluid Dynamics Laboratory MOM5.1/SIS1 numerics with nominal l/10° horizontal resolution, and 75 levels in the vertical. Surface forcing is provided by a bias-corrected version of the European Center for Medium Range Weather Forecasts ERA5 forcing, while continental discharge is provided by a separate monthly observation-based discharge dataset. A 13-year (2010-2022) reanalysis experiment (SODA4.15.2) is compared to the coarser resolution, but otherwise similar SODA3.15.2 and to assimilated and independent observations. These comparisons show that the greatest reduction in time mean bias occurs near strong fronts, which become narrower and stronger. Eddy variability is increased, increasing eddy heat transport. Improved vertical resolution produces shallow summer temperature and salinity stratification which is most noticeable in midlatitudes and the eastern tropical Pacific and Atlantic in Northern Hemisphere winter.

In the tropical Pacific, weak ventilation and intense microbial respiration at depth give rise to a low dissolved oxygen (O2) environment that is thought to be ventilated primarily by the equatorial current system (ECS). The role of mesoscale eddies and diapycnal mixing as potential pathways of O2 supply in this region, however, remains poorly known due to sparse observations and coarse model resolution. Using an eddy resolving simulation of ocean circulation and biogeochemistry, we assess the contribution of these processes to the O2 budget balance and find that turbulent mixing of O2 and its modulation by mesoscale eddies contribute substantially to the replenishment of O2 in the upper equatorial Pacific thermocline, complementing the advective supply of O2 by the ECS and meridional circulation at depth. These transport processes are strongly sensitive to seasonal forcing by the wind, with elevated mixing of O2 into the upper thermocline during summer and fall when the vertical shear of the lateral flow and eddy kinetic energy are intensified. The tight link between eddy activity and the downward mixing of O2 arises from the modulation of equatorial turbulence by Tropical Instability Waves via their eddy impacts on the vertical shear. This interaction of ocean processes across scales sustains a local pathway of O2 delivery into the equatorial Pacific interior and highlights the need for adequate observations and model representation of turbulent mixing and mesoscale processes for understanding and predicting the fate of the tropical Pacific O2 content in a warmer and more stratified ocean.

A document by Evi Wubben. Click on the document to view its contents.

A document by Bianca Robin Spiering. Click on the document to view its contents.

The baroclinic component of the sea surface height, referred to as steric height, is governed by geostrophically balanced motions and unbalanced internal waves, and thus is an essential indicator of ocean interior dynamics. Using yearlong measurements from a mooring array, we assess the distribution of upper-ocean steric height across frequencies and spatial scales of O(1-20 km) in the northeast Atlantic. Temporal decomposition indicates that the two largest contributors to steric height variance are large-scale atmospheric forcing (32.8%) and mesoscale eddies (34.1%), followed by submesoscale motions (15.2%), semidiurnal internal tides (8%), super-tidal variability (6.1%) and near-inertial motions (3.8%). Structure function diagnostics further reveal the seasonality and scale dependence of steric height variance. In winter, steric height is dominated by balanced motions across all resolved scales, whereas in summer, unbalanced internal waves become the leading-order contributor to steric height at scales of a few kilometers.

Excessive nutrient loading is a well-established driver of hypoxia in aquatic ecosystems. However, recent limnological research has illuminated the role of Chromophoric Dissolved Organic Matter (CDOM) in exacerbating hypoxic conditions, particularly in freshwater lakes. In coastal ocean environments, the influence of CDOM on hypoxia remains an underexplored area of investigation. This study seeks to elucidate the intricate relationship between CDOM and hypoxia by employing a nitrogen-based model within the context of Chesapeake Bay, a large estuary with unique characteristics including salinity stratification and the localization of hypoxia/anoxia in a 30-meter-deep channel aligned with the estuary’s primary stem. Our findings indicate that the impact of CDOM on nutrient dynamics and productivity varies significantly across different regions of Chesapeake Bay. In the upper Bay, the removal of CDOM reduces light limitation, thus promoting increased productivity, resulting in the generation of more detritus and burial, which, in turn, contributes to elevated levels of hypoxia. As we transition to the middle and lower Bay, the removal of CDOM can cause a decline in integrated primary productivity due to nutrient uptake in the upper Bay. This decrease in productivity is associated with reduced burial and denitrification, ultimately leading to a decrease in hypoxia levels. Streamflow modulates this impact. The time integral of the hypoxic volume during low-flow years is particularly sensitive to CDOM removal, while in high-flow years, it is relatively unchanged. This research underscores the necessity for a comprehensive understanding of the intricate interactions between CDOM and hypoxia in coastal ecosystems.