The formation of sea ice in the Arctic Ocean, as well as other physical processes such as injection of air and rapid cooling, plays a crucial role in determining the physical and chemical properties of its waters, which in turn drive the circulation in the Arctic [1]. Such processes can be constrained by conservative tracers which are biologically and chemically nonreactive, such as the noble gases. The full suite of stable noble gases (He, Ne, Ar, Kr and Xe) have been measured for the first time in the Arctic Ocean—along with CFC-12, SF6, and other transient tracers—during the Ventilation and Anthropogenic Carbon in the Arctic Ocean (VACAO) project of the wider Synoptic Arctic Survey 2021 (SAS21) [2]. The noble gas profiles indicate a water column strongly influenced by rapid cooling and excess air injection, with a surface signature characteristic of solute rejection by sea ice formation.We have compared multiple Arctic Ocean gas exchange models (based on similar models used in the Antarctic by Loose et al. [3] and in the Labrador Sea by Hamme et al. [4]) to constrain the fractions of Arctic water composed of Pacific, Atlantic and sea ice melt-derived origin waters, as well as the amount of sea ice being formed and air being injected into the water via bubbles. These parameters are estimated using a χ­2-minimisation procedure, where the misfit between fitted parameters and data is minimised. Preliminary results indicate a non-negligible sea ice term in the equations describing gas saturation anomalies in the Arctic.Another key goal of VACAO is to use transient tracers to study the ventilation timescales of the Arctic Ocean, with application towards the study of the efficacy of its CO2 solubility pump/storage. CFC-12/SF6 is one tracer pair with which this is attempted; water dating with this pair requires knowledge of the concentration history of the tracers in the surface water, for which there are no direct measurements. Thus, the physical gas exchange parameters modeled from the noble gas data can be used in conjunction with observed atmospheric histories to more accurately describe the surface water histories of CFC-12 and SF6. This in turn can be used to better constrain the Transit Time Distribution parameters used when dating Arctic waters [5]. Comparisons with other VACAO age tracer data (39Ar and 129I/236U) may act as validation tools to this “correction” to CFC-12/SF6 dating.References for Abstract[1] Rudels, B., and Carmack, E. 2022. Arctic ocean water mass structure and circulation. Oceanography, 35(3–4), 52–65 pp.[2] Snoeijs-Leijonmalm, P. and the SAS-Oden 2021 Scientific Party (2022). Expedition Report SWEDARCTIC Synoptic Arctic Survey 2021 with icebreaker Oden. Swedish Polar Research Secretariat. 300 pp.[3] Loose, B., Stammerjohn, S., Sedwick, P., & Ackley, S. (2023). Sea ice formation, glacial melt and the solubility pump boundary conditions in the Ross Sea. Journal of Geophysical Research: Oceans, 128, e2022JC019322.[4] Hamme, R. C., Emerson, S. R., Severinghaus, J. P., Long, M. C., & Yashayaev, I. (2017). Using noble gas measurements to derive air-sea process information and predict physical gas saturations. Geophysical Research Letters, 44, 9901–9909 pp.[5] Jeansson, E., Tanhua, T., Olsen, A., Smethie, W. M., Rajasakaren, B., Ólafsdóttir, S. R., & Ólafsson, J. (2023). Decadal changes in ventilation and anthropogenic carbon in the Nordic Seas. Journal of Geophysical Research: Oceans, 128, e2022JC019318.

Karen J Wang1,2,*, Yongsong Huang1,2,*, Tyler Kartzinel2,3, Markus Majaneva4, Nora Richter1,2,5, Sian Liao2,6Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, 02912, USAInstitute at Brown for Environment and Society, Brown University, Providence, RI, 02912, USADepartment of Ecology and Evolutionary Biology, Brown University, Providence, RI, 02912, USANorwegian Institute for Nature Research (NINA), NO-7485, Trondheim, NorwayDepartment of Marine Microbiology & Biogeochemistry, NIOZ Royal Netherlands Institute for Sea Research, 1790 AB Den Burg, the NetherlandsDepartment of Chemistry, Brown University, Providence, RI, 02912, USA

A document by Freya Muir. Click on the document to view its contents.

The Atlantic Meridional Overturning Circulation (AMOC) is a critical component of the climate system, strongly influencing the climate via ocean heat transport. The AMOC had different characteristics during glacial periods and is expected to change under anthropogenic climate forcing. To reconstruct past AMOC strength, the Pa/Th (protactinium-231 to thorium-230) ratio measured in marine sediments serves as a unique proxy. However, this ratio reflects not only circulation changes, but also effects from biological particle export and benthic nepheloid layers. Therefore, it remains an open question which regions exhibit a reliable AMOC signal in their sedimentary Pa/Th. This study, utilising the Bern3D model and a compilation of sediment cores with 11 newly published cores, suggests that equatorial West Atlantic Pa/Th is as sensitive to AMOC changes as the Bermuda Rise region. Additionally, the Pa/Th response to AMOC changes observed in part of the northern North Atlantic, which is opposite to regions further south, is caused by AMOC-induced changes in particle production. Cores in this region are promising to reconstruct AMOC strength, despite exhibiting an AMOC-to-Pa/Th relationship opposite from usual and high levels of opal. Additional cores in the North Atlantic at 40-60°N between 1 and 2 km depth are desirable for the application of Pa/Th. Our results suggest a new focus of Pa/Th reconstructions on the equatorial West Atlantic and the northern North Atlantic, which appear to be better suited to quantify past AMOC strength.

Macroalgae re- and afforestation are frequently proposed carbon dioxide removal (CDR) strategies, but the carbon storage is often uncertain. In nature, kelp forests grow on hard or sandy substrate leading to as little as ∼0.4% of their primary production being buried in the kelp’s habitat (Krause-Jensen and Duarte, 2016) with the remainder exported. Export of dissolved and particulate carbon from kelp forests is strongly affected by the exchange of water between the kelp forest and the surrounding water. The permanence of storage of the exported carbon is determined by its destination, which is affected by the flow in and around the forest. Additionally, the potential for forests to grow can be limited by nutrients dissolved in the water, the availability of which is determined by the same water exchange rates.Here, we use OceanBioME to build a numerical model of coupled flow/kelp interactions to study how tidal currents interact with a giant kelp (Macrocystis pyrifera) forest. We first validate our model using observations of currents within and surrounding a kelp forest in Southern California. By varying the kelp density within our model and tracking dissolved tracer released from the kelp forest, we analyse the timescales of water exchange to better understand how the flow influences carbon export and nutrient uptake. We find a density that maximizes the export of tracers which coincides with the density typical of natural kelp forests. Additionally, the drag results in a mean circulation through the forest and a mean displacement of the individuals suggesting that the physical dynamics of the kelp should be an important consideration in future studies and when planning re/aforestation projects.

Solar heating of the upper ocean is a primary energy input to the ocean-atmosphere system, and the vertical heating profile is modified by the concentration of phytoplankton in the water, with consequences for sea surface temperature and upper ocean dynamics. Despite the development of increasingly complex modeling approaches for radiative transfer in the atmosphere and upper ocean, the simple parameterizations of radiant heating used in most ocean models are plagued by errors and inconsistencies. There remains a need for a parameterization that is reliable in the upper meters and contains an explicitly spectral dependence on the concentration of biogenic material, while maintaining the computational simplicity of the parameterizations currently in use. In this work, we assemble simple, observationally-validated physical modeling tools for the key controls on ocean radiant heating, and simplify them into a parameterization that fulfills this need. We then use observations from 64 spectroradiometer depth casts across 6 cruises, 13 surface hyperspectral radiometer deployments, and 2 UAV flights to probe the accuracy and uncertainty associated with the new parameterization. We conclude with a case study using the new parameterization to demonstrate the impact of chlorophyll concentration on the structure of diurnal warm layers, an investigation that was not possible to conduct accurately using previous parameterizations. The parameterization presented in this work equips researchers to better model global patterns of sea surface temperature, diurnal warming, and mixed-layer depths, without a prohibitive increase in complexity.

In this study, we explore the impact of oceanic moisture fluxes on atmospheric blocks using the ECMWF Integrated Forecast System. Artificially suppressing surface latent heat flux over the Gulf Stream region leads to a significant reduction (up to 30%) in atmospheric blocking frequency across the northern hemisphere. Affected blocks show a shorter lifespan (-6%), smaller spatial extent (-12%), and reduced intensity (-0.4%), with an increased detection rate (+17%). These findings are robust across various blocking detection thresholds. Analysis indicates a resolution-dependent response, with resolutions lower than Tco639 (~18km) showing no significant change in some blocking characteristics, even with reduced blocking frequency. Exploring the broader Rossby wave pattern, we observe that diminished moisture flux favours eastward propagation and higher zonal wavenumbers, while air-sea interactions promotes stationary and westward-propagating waves with zonal wavenumber 3. This study underscores the critical role of western boundary current’s moisture fluxes in modulating atmospheric blocking.

The subpolar North Atlantic plays an outsized role in the atmosphere-to-ocean carbon sink. The central Irminger Sea is home to well-documented deep winter convection and high phytoplankton production, which drive strong seasonal and interannual variability in regional carbon cycling. We use observational data from moored carbonate system chemistry sensors and annual turn-around cruise samples at the Ocean Observatories Initiative’s Global Irminger Sea Array to construct a near-continuous time series of mixed layer dissolved inorganic carbon (DIC), pCO2, and total alkalinity from summer 2015 to summer 2022. We use these carbonate system chemistry time series to deconvolve the physical and biological drivers of surface ocean carbon cycling in this region on seasonal, annual, and interannual time scales. We find high annual net community production within the seasonally-varying mixed layer, averaging 9.7±1.7 mol m-2 yr-1 with high interannual variability (range of 6.0 to 13.7 mol m-2 yr-1). The highest daily net community production rates occur during the late winter and early spring, prior to the observed high chlorophyll concentrations associated with the spring phytoplankton bloom. As a result, the winter and early spring play a much larger role in biological carbon export from the mixed layer than traditionally thought.

Using nine years of full-depth profiles from 55 Deep Argo floats in the Southwest Pacific Basin collected between 2014 and 2023, we find consistent warm anomalies compared to a long-term climatology below 2000 m ranging between 11\(\pm\)2 to 34\(\pm\)2m\(^o\)C, most pronounced between 3500 and 5000 m. Over this period, a cooling trend is found between 2000-4000 m and a significant warming trend below 4000 m with a maximum rate of 4.1\(\pm\)0.31 m\(^o\)C yr\(^{-1}\) near 5000 m, with a possible acceleration over the second half of the period. The integrated Steric Sea Level expansion below 2000 m was 7.9\(\pm\) 1 mm compared to the climatology with a trend of 1.3\(\pm\) 1.6 mm dec\(^{-1}\) over the Deep Argo era, contributing significantly to the local sea level budget. We assess the ability to close a full Sea Level Budget, further demonstrating the value of a full-depth Argo array.

This work investigates observations of gradual upward phase shifting of temperature oscillations in a steep tropical reservoir, which differ from the π radians sharp shifts that are usually accepted for the description of baroclinic motions in terms of normal modes. Supported on numerical modeling and theoretical inviscid wave ray tracing, we show that the gradual upward phase shifting is the signature of vertically propagating seiches, which refer to basin-scale oscillations that are stationary in the horizontal but propagate downwards in the vertical. We show that the vertically propagating seiche occurs due to the predominant supercritical reflection of the internal wave rays at the lake boundaries, which focuses the internal wave energy downwards with a minor fraction of the energy reflected upwards, resulting in a net downward energy propagation. The net downward energy flux precludes the formation of standing waves, with potential implications for the common framework of the energy flux path at the interior of stratified lakes. The analysis supports that vertically propagating seiches and standing mode preclusion are expected to occur in any given lake, but their signatures are more evident in steep sided lakes, with a wide metalimnion and/or for lower forcing frequencies, characteristic of higher order vertical modes.

Large scale climate indices such as the North Pacific Gyre Oscillation (NPGO) have been linked to variability in both phytoplankton and zooplankton, yet the mechanisms by which they are linked remain unknown. We used a three-dimensional coupled biophysical model, SalishSeaCast, to determine the mechanistic links between the NPGO and plankton dynamics in the Strait of Georgia, Canada. First, we compared bottom-up processes during NPGO positive (cold-phase) and negative (warm-phase) years. Then, we conducted a series of model experiments to determine the effects of the NPGO on local physical drivers by switching individual parameters between a typical warm and cold year. The model showed that higher SST and weaker winds contributed to an earlier increase in spring diatom biomass during warm-phase years. Due to the conditions set up during the spring, warm-phase years exhibited lower overall summer diatom biomass and an earlier shift to nanoflagellate-dominance compared to cold-phase years. Our systematic model experiments revealed that variability in wind-driven resupply of nutrients to the surface waters during the summer had the most significant impact on diatom biomass, and ultimately on the food available to zooplankton grazers. The Z1 and Z2 model classes grazed on a higher proportion of nanoflagellates during the summer of warm-phase years, suggestive of a poorer quality diet consumed during warm years. Results from this study are relevant in the context of other climate signals (e.g., El Niño) favouring weaker winds or increased stratification, which would limit the amount of nutrients being replenished to the surface waters.

At marine-terminating glaciers, the interplay between meltwater buoyancy and local currents control turbulent exchanges. Because of challenges in making centimeter-scale measurements at glaciers, turbulent dynamics at near-vertical ice-ocean boundaries are poorly constrained. Here we present the first observations from instruments robotically-bolted to an underwater ice face, and use these to elucidate the tug-of-war between meltwater-derived buoyancy and externally-forced currents in controlling boundary-layer dynamics. Our observations captured two limiting cases of the flow. When external currents are weak, meltwater buoyancy energizes the turbulence and dominates the near-boundary stress. When external currents strengthened, the plume diffused far from the boundary and the associated turbulence decreases. As a result, even relatively weak buoyant melt plumes are as effective as moderate shear flows in delivering heat to the ice. These are the first in-situ observations to demonstrate how buoyant melt plumes energize near-boundary turbulence, and why their dynamics are critical in predicting ice melt.

Wind, wave, and acoustic observations are used to test a scaling for ambient sound levels in the ocean that is based on the relative penetration depth of active bubbles during surface wave breaking. The focus is on acoustic frequencies in the range 1-10 kHz, which are typically scaled by wind speed alone. Wind and wave information are combined in a parametric form to describe the depth of the active bubble layer (which produces sound) relative to the depth of the passive bubble layer (which attenuates sound). The relative depth scaling has a primary dependence on wind speed and a secondary dependence on any departure of significant wave height from fully-developed, open-ocean conditions. The scaling is tested with long time-series observations of winds and waves at Ocean Station Papa (North Pacific Ocean), as well as with a case study with fetch limitation near the island of Jan Mayen (Norwegian Sea). When waves are less developed (e.g., limited by fetch) at a given wind speed, the attenuating layer is relatively thin and the sound levels are higher. The scaling is a plausible explanation for the observed reduction in sound levels during high wind events (winds greater than 15 m/s).

Future sea level rise under global warming poses serious risks of extreme sea level events in coastal regions worldwide. Numerous state-of-the-art climate models, with their relatively coarse horizontal resolution, may not adequately resolve coastal wave dynamics, leading to uncertainties in coastal sea level variability representation. This study compared eddy-resolving and non-eddying ocean models in reproducing sea level variability, focusing on the probability distribution along the western coast of India. The eddy-resolving model can simulate intraseasonal sea level variations associated with coastal waves driven by equatorial wind anomalies. The non-eddying model fails to capture over 81% of observed extreme sea level events, as shown in the probability distribution for intraseasonal time series. Although capable of simulating Indian Ocean Dipole-related low-frequency sea level anomalies, the non-eddying model does not replicate their connection to intraseasonal extreme events. The results suggest that climate model projections may underestimate future changes in extreme sea level events.

Improved ocean biogeochemistry (BGC) parameters in Earth System Models can enhance the representation of the global carbon cycle. We aim to demonstrate the potential of parameter estimation (PE) using an ensemble data assimilation method to optimise five key BGC parameters within the Norwegian Earth System Model (NorESM). The optimal BGC parameter values are estimated with an iterative ensemble smoother technique, applied a-posteriori to the error of monthly climatological estimates of nitrate, phosphate and oxygen produced by a coupled reanalysis that assimilates monthly ocean physical observed climatology. Reducing the ocean physics biases while keeping the default parameters (DP) initially reduces BGC state bias in the intermediate depth but deteriorates near the surface, suggesting that the DP are tuned to compensate for physical biases. Globally uniform and spatially varying estimated parameters from the first iteration effectively mitigate the deterioration and reduce BGC errors compared to DP, also for variables not used in the PE (such as C0$_2$ fluxes and primary production). While spatial PE performs superior in specific regions, global PE performs best overall. A second iteration can further improve the performance of global PE for near-surface BGC variables. Finally, we assess the performance of the global estimated parameters in a 30-year coupled reanalysis, assimilating time-varying temperature and salinity observations. It reduces error by 20\%, 18\%, 7\%, and 27\% for phosphate, nitrate, oxygen, and dissolved inorganic carbon, respectively, compared to the default version of NorESM. The proposed PE approach is a promising innovative tool to calibrate ESM in the future.

Antarctic Bottom Water (AABW) formation and transport constitute a key component of the global ocean circulation. Direct observations suggest that AABW volumes and transport rates may be decreasing, but these observations are too temporally or spatially sparse to determine the cause. To address this problem, we develop a new method to reconstruct AABW transport variability using data from the GRACE (Gravity Recovery and Climate Experiment) satellite mission. We use an ocean general circulation model to investigate the relationship between ocean bottom pressure and AABW: we calculate both of these quantities in the model, and link them using a regularised linear regression. Our reconstruction from modelled ocean bottom pressure can capture 65-90% of modelled AABW transport variability, depending on the ocean basin. When realistic observational uncertainty values are added to the modelled ocean bottom pressure, the reconstruction can still capture 30-80% of AABW transport variability. Using the same regression values, the reconstruction skill is within the same range in a second, independent, general circulation model. We conclude that our reconstruction method is not unique to the model in which it was developed and can be applied to GRACE satellite observations of ocean bottom pressure. These advances allow us to create the first global reconstruction of AABW transport variability over the satellite era. Our reconstruction provides information on the interannual variability of AABW transport, but more accurate observations are needed to discern AABW transport trends.

The representation of turbulent fluxes during oceanic convective events is important to capture the evolution of the oceanic mixed layer. To improve the accuracy of turbulent fluxes, we examine the possibility of adding a non-gradient component in their expression in addition to the usual downgradient part. To do so, we extend the $k-\varepsilon$ algebraic second-moment closure by relaxing the assumption on the equilibrium of the temperature variance $\overline{\theta’^2}$. With this additional transport equation for the temperature variance, we obtain a $k - \varepsilon - \overline{\theta’^2}$ model (the “$k \varepsilon t$” model) which includes a non-gradient term for the temperature flux. We validate this new model against Large Eddy Simulations (LES) in both wind-forced and buoyancy-driven regimes. In both cases, we find that the vertical profile of temperature is well captured by the $k \varepsilon t$ model. Particularly, for the buoyancy-driven regime, the non-gradient term increases the portion of the mixed layer that is stably stratified. This is an improvement since this portion is too small with the $k - \varepsilon$ parameterization. Finally, a comparison of the non-gradient term with the KPP non-local term gives insights for refining the KPP’s ad hoc shape polynomial.

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We developed a coupled overland and river model for modeling compound flooding using the kinematic wave approximation on inland sections of an unstructured mesh, and the diffusive wave approximation on riverine sections. A finite element method is used for spatial discretization and a Crank-Nicolson scheme is used for time discretization. A wetting and drying algorithm is implemented for improved efficiency in the model. Pluvial conditions and tidal conditions are implemented as source terms in the river model. The results show that effects from compound inundation could be captured using this framework.

Vertical profiles of temperature microstructure at 95 stations were obtained over the Beaufort shelf and shelfbreak in the southern Canada Basin during a November 2018 research cruise. Two methods for estimating the dissipation rates of temperature variance and turbulent kinetic energy were compared using this dataset. Both methods require fitting a theoretical spectrum to observed temperature gradient spectra, but differ in their assumptions. The two methods agree for calculations of the dissipation rate of temperature variance, but not for that of turbulent kinetic energy. After applying a rigorous data rejection framework, estimates of turbulent diffusivity and heat flux are made across different depth ranges. The turbulent diffusivity of temperature is typically enhanced by about one order of magnitude in profiles on the shelf compared to near the shelfbreak, and similarly near the shelfbreak compared to profiles with bottom depth >1000 m. Depth bin means are shown to vary depending on the averaging method (geometric means tend to be smaller than arithmetic means and maximum likelihood estimates). The statistical distributions of heat flux within the surface, cold halocline, and Atlantic water layer change with depth. Heat fluxes are typically <1 Wm−2, but are greater than 50 Wm−2 in ∼8% of the overall data. These largest fluxes are located almost exclusively within the surface layer, where temperature gradients can be large.

Most studies focus on the impact of climate change on the mean state of phytoplankton and primary productivity, but little is known about whether and how climate change is impacting variance and extremes. In this study, we assess changes in chlorophyll-a concentration (CHL), which is an important proxy for primary production of marine ecosystems. Previous study suggests a decreasing variability in phytoplankton chlorophyll in both observational period and the model ensemble [1]. Our objective is to understand whether and how climate change impacts the whole distribution of variability in primary productivity. We utilize a series of statistical methods and reanalysis of CHL datasets to assess the variance and extremes of all distributions of CHL.

Eleven Earth System Models (ESMs) contributing to the Coupled Model Intercomparison Project (CMIP6) were evaluated with respect to climate-related stressors impacting Arctic marine ecosystems (temperature, sea ice, oxygen, ocean acidification). Stressors show regional differences and varying differences over time and space among models. Trend magnitudes increase over time and are highest by end-of-century for temperature and O2. Differences between scenarios SSP2-4.5 and SSP5-8.5 for these variables vary among models and regions, mainly driven by sea-ice retreat. Differences in biogeochemical parameterizations contribute to acidification differences. Projections indicate consistent ocean acidification until 2040 and faster progression for the higher emission scenario thereafter. For SSP5-8.5 all Arctic regions show aragonite undersaturation by 2080, and calcite undersaturation for all but two regions by 2100 for all models. Most regions can avoid calcite undersaturation with lower emissions (SSP2-4.5). All variables show increases in seasonal amplitude, most prominently for temperature and oxygen. Calcium carbonate saturation state (Ω) shows little change to the seasonal range and a suggestion of temporal shifts in extrema. Seasonal changes in Ω may be underestimated due to lacking carbon cycle processes within sea ice in CMIP6 models. The analysis emphasizes regionally varying threats from multiple stressors on Arctic marine ecosystems and highlights the propagation of uncertainties from sea ice to temperature and biogeochemical variables. Large model differences in seasonal cycles emphasize the need for improved model constraints, predominantly the representation of sea-ice decline, to enhance the applicability of CMIP models in multi-stressor impacts assessments.