Impact of Wave-Vortical Interactions on Oceanic Submesoscale Lateral Dispersion

Submesoscale lateral transport of Lagrangian particles in pycnocline conditions is investigated by means of idealized numerical simulations with reduced-interaction models. Using a projection technique, the models are formulated in terms of wave-mode and vortical-mode nonlinear interactions, and they range in complexity from full Boussinesq to waves-only and vortical-modes-only (QG) models. We find that, on these scales, most of the dispersion is done by vortical motions, but waves cannot be discounted because they play an important, albeit indirect, role. In particular, we show that waves are instrumental in filling out the spectra of vortical-mode energy at smaller scales through non-resonant vortex-wave-wave triad interactions. We demonstrate that a richer spectrum of vortical modes in the presence of waves enhances the effective lateral diffusivity, compared to QG. Waves also transfer energy upscale to vertically sheared horizontal flows which are a key ingredient for internal-wave shear dispersion. In the waves-only model, the dispersion rate is an order of magnitude smaller and is attributed entirely to internal-wave shear dispersion.

The importance of freeze/thaw cycles on lateral tracer transport in ice-wedge polygons

A significant portion of the Arctic coastal plain is classified as polygonal tundra and plays a vital role in soil carbon cycling. Recent research suggests that lateral transport of dissolved carbon could exceed vertical carbon releases to the atmosphere. However, the details of lateral subsurface flow in polygonal tundra have not been well studied. We incorporated a subsurface transport process into an existing state-of-art hydrothermal model. The model captures the physical effects of freeze/thaw cycles on lateral flow in polygonal tundra. The new modeling capability enables non-reactive tracer movement within subsurface. We utilized this new capability to investigate the impact of freeze/thaw cycle on lateral flow in the polygon polygonal tundra. Our study indicates the important role of freeze/thaw cycle and freeze-up effect on lateral tracer transport, suggesting that dissolved species could be transported from the middle of the polygon to the sides within a couple of thaw seasons. Introducing carbon lateral transport in the climate models could substantially reduce the uncertainty associated with the impact of thawing permafrost.

Changes in global terrestrial live biomass over the 21st century

Live woody vegetation is the largest reservoir of biomass carbon, with its restoration considered one of the most effective natural climate solutions. However, terrestrial carbon fluxes remain the largest uncertainty in the global carbon cycle. Here, we develop spatially explicit estimates of carbon stock changes of live woody biomass from 2000 to 2019 using measurements from ground, air, and space. We show that live biomass has removed 4.9 to 5.5 PgC year−1 from the atmosphere, offsetting 4.6 ± 0.1 PgC year−1 of gross emissions from disturbances and adding substantially (0.23 to 0.88 PgC year−1) to the global carbon stocks. Gross emissions and removals in the tropics were four times larger than temperate and boreal ecosystems combined. Although live biomass is responsible for more than 80% of gross terrestrial fluxes, soil, dead organic matter, and lateral transport may play important roles in terrestrial carbon sink.

Seasonality of density currents induced by differential cooling

When lakes experience surface cooling, the shallow littoral region cools faster than the deep pelagic waters. The lateral density gradient resulting from this differential cooling can trigger a cold downslope density current that intrudes at the base of the mixed layer during stratified conditions. This process is known as thermal siphon (TS). TS flushes the littoral region and increases water exchange between nearshore and pelagic zones, with possible implications on the lake ecosystem. Past observations of TS in lakes are limited to specific cooling events. Here, we focus on the seasonality of the TS-induced lateral transport and investigate how the seasonally varying forcing conditions control the occurrence and intensity of TS. We base our analysis on one year of observations of TS in Rotsee (Switzerland), a small wind-sheltered temperate lake composed of an elongated shallow region. We demonstrate that TS occurs for more than 50 % of the days from late summer to winter and efficiently flushes the littoral region in ~10 hours. We further quantify the seasonal evolution of the occurrence, intensity and timing of TS. The conditions for the formation of TS are optimal in autumn, when the duration of the cooling phase is longer than the initiation timescale of TS. The decrease in surface cooling by one order of magnitude from summer to winter reduces the lateral transport by a factor of two. We interpret this transport seasonality with scaling relationships relating the daily averaged cross-shore velocity, unit-width discharge and flushing timescale to the surface buoyancy flux, mixed layer depth and lake bathymetry. The timing and duration of the diurnal flushing by TS are associated with the duration of the daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing, compared to the summer period. Our findings based on scaling arguments can be extended to other aquatic systems to assess, at a global scale, the relevance of TS in lakes and reservoirs.

Particle tracking model results suggest little lateral transport bias in inorganic and organic SST proxies in the Mediterranean Sea

<p>Ocean currents can transport sinking particles hundreds of kilometers from their origin at the ocean surface to their burial location, resulting in an offset between sea surface temperatures (SSTs) above the burial site and the particle’s origin. Quantifying this offset in particles carrying molecules used in SST proxies can reduce uncertainty in paleoclimate reconstructions. In the Mediterranean Sea, where δ<sup>18</sup>O<sub>foraminifera</sub>, U<sup>K’</sup><sub>37</sub>- and TEX<sub>86</sub>-based SSTs can exhibit large offsets from surface conditions, understanding the possible contribution of lateral transport to proxy bias can provide additional insight when interpreting paleoclimate records.</p><p>In this study, Lagrangian particle tracking experiments are performed using the NEMO flow field to simulate transport and allow for a quantitative estimate of transport bias. The model determines the ocean surface origin locations of foraminifera and sedimentary particles that carry alkenones or GDGTs to compare with surface sediment datasets for δ<sup>18</sup>O<sub>foraminifera</sub>, U<sup>K’</sup><sub>37</sub> and TEX<sub>86</sub>, respectively. A range of sinking speeds appropriate for the export of organic matter (6, 12, 25, 50, 100, 250, and 500 m/d) is used in the model to represent different export modes (i.e., individual coccoliths, coccospheres, aggregates), where the three fastest sinking speeds can also represent sinking foraminifera. Results show that lateral transport bias is generally small within the Mediterranean Sea and cannot explain the large offsets in proxy-based SST reconstructions in this basin.</p>

Warm core mesoscale eddies along the boundary current and in the Sofia Deep north of Svalbard

<p>The Atlantic water boundary current north of Svalbard is a major heat and salt source to the Arctic Ocean. Yet, the mechanisms controlling the lateral transport of Atlantic water properties are not well understood. Model simulations suggest mesoscale eddies may be important for transporting heat away from the boundary current, but supporting observations are sparse.</p><p>Between September and November in 2018, a Seaglider was deployed north of Svalbard as part of the Nansen Legacy project to investigate intraseasonal variations in the boundary current and the transformation of Atlantic water. It made several transects across the boundary current and a transect across the Sofia deep. Warm core eddies originating from the boundary current were detected in the Sofia deep. Combining the Seaglider data with two year-long mooring arrays north of Svalbard, deployed in 2018 within the Nansen Legacy framework, we investigate mesoscale eddies using eddy recognition algorithms applied to glider transects and timeseries from moorings. Initial results indicate that mesoscale eddies frequently occur in the boundary current, with radius less than 10 km and velocity maxima as high as 0.35 m/s.</p>

A 3-D Model of Antarctic Ice Shelf Surface Hydrology

<p>The formation of surface meltwater has been linked with the disintegration of many ice shelves in the Antarctic Peninsula over the last several decades. Despite the importance of surface meltwater production and transport to ice shelf stability, knowledge of these processes is still lacking. Understanding the surface hydrology of ice shelves is an essential first step to reliably project future sea level rise from ice sheet melt.<br><br>In order to better understand the processes driving meltwater distribution on ice shelves, we present results from case studies using a new 3-D model of surface hydrology for Antarctic ice shelves. It is the first comprehensive model of surface hydrology to be developed for Antarctic ice shelves, enabling us to incorporate key processes such as the lateral transport of surface meltwater. Recent observations suggest that surface hydrology processes on ice shelves are more complex than previously thought, and that processes such as lateral routing of meltwater across ice shelves, ice shelf flexure and surface debris all play a role in the location and influence of meltwater. Our model allows us to account for these and is calibrated and validated through both remote sensing and field observations. Here we present results from in depth studies from selected ice shelves with significant surface melt features.<br><br>This community-driven, open-access model has been developed with input from observations, and allows us to provide new insights into surface meltwater distribution on Antarctica’s ice shelves. This enables us to answer key questions about their past and future evolution under changing atmospheric conditions and vulnerability to meltwater driven hydrofracture and collapse.</p>

The SPLASH Action Group – Towards standardized sampling strategies along the soil-to-hydrosystems continuum in permafrost landscapes

<p><span>The Action Group called ‘Standardized methods across Permafrost Landscapes: from Arctic Soils to Hydrosystems’ (SPLASH), funded by the International Permafrost Association, is a community-driven effort aiming to provide a suite of standardized field strategies for sampling mineral and organic components in soils, sediments, surface water bodies and coastal environments across permafrost landscapes. This unified approach will allow data to be shared and compared, thus improving our understanding of the processes occurring during lateral transport in circumpolar Arctic watersheds. This is an international and transdisciplinary effort aiming to provide a fieldwork “tool box” of the most relevant sampling schemes and sample conservation procedures for mineral and organic permafrost pools.</span></p><p><span>With climate change, permafrost soils are undergoing drastic transformations. B</span><span>oth localized abrupt thaw (thermokarst) and gradual ecosystem shifts (e.g., active layer thickening, vegetation changes) drive changes in hydrology and biogeochemical cycles (carbon, nutrients, and contaminants). Mineral and organic components interact along the “lateral continuum” (i.e., from soils to aquatic systems) changing their composition and reactivity across the different interfaces. The circumpolar Arctic region is characterized by high spatial heterogeneity (e.g., geology, topography, vegetation, and ground-ice content) and large inter-annual and seasonal variations in local climate and biophysical processes. Common sampling strategies, applied in different seasons and locations, could help to tackle the spatial and temporal complexity inextricably linked to biogeochemical processes. </span><span>This unified approach developed in permafrost landscapes will allow us to overcome the following challenges: (1) identifying interfaces where detectable changes in mineral and organic components occur; (2) allowing spatial comparison of these detectable changes; and (3) capturing temporal (inter-/intra-annual) variations at these interfaces. </span><span>In order to build on the great effort to better assess the permafrost feedback to climate change, there is an urgent need for a set of community-based protocols to capture changes the dynamics of organics and minerals during their lateral transport. </span></p><p><span>Here, we present the first results from an online survey recently conducted among researchers from different disciplines. The survey inputs provide valuable information about the common approaches currently applied along the “soil-to-hydrosystems” continuum and the specific challenges associated with permafrost studies. These results about the ‘WHAT, WHERE, WHEN, and HOW’ of field sampling (e.g., sample collection, filtration, conservation...) allow for identifying the most relevant sampling strategies and also the current knowledge gaps. Finally, we present examples of the protocols available to investigate organic and mineral components from soils to marine environments,</span> on which a synoptic sampling strategy can be built. <span>A</span><span>ll forthcoming contributions from our community are still welcome, helping the SPLASH team </span><span>to</span> <span>fill</span><span> up the most adapted tool box to Arctic permafrost landscapes</span><span>.</span></p>