27 research outputs found
Cold vs. warm water route â sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model
The northward flow of the upper limb of the
Atlantic Meridional Overturning Circulation (AMOC) is fed by waters entering
the South Atlantic from the Indian Ocean mainly via the Agulhas Current (AC)
system and by waters entering from the Pacific through Drake Passage (DP),
commonly referred to as the âwarmâ and âcoldâ water routes, respectively.
However, there is no final consensus on the relative importance of these two
routes for the upper limb's volume transport and thermohaline properties. In
this study we revisited the AC and DP contributions by performing Lagrangian
analyses between the two source regions and the North Brazil Current (NBC) at
6â S in a realistically forced high-resolution (1â20â)
ocean model.
Our results
agree with the prevailing conception that the AC contribution is the major
source for the upper limb transport of the AMOC in the tropical South
Atlantic. However, they also suggest a non-negligible DP contribution of
around 40 %, which is substantially higher than estimates from
previous Lagrangian studies with coarser-resolution models but now better
matches estimates from Lagrangian observations. Moreover, idealized analyses
of decadal changes in the DP and AC contributions indicate that the ongoing
increase in Agulhas leakage indeed may have induced an increase in the AC
contribution to the upper limb of the AMOC in the tropics, while the DP
contribution decreased. In terms of thermohaline properties, our study
highlights the fact that the AC and DP contributions cannot be unambiguously
distinguished by their temperature, as the commonly adopted terminology may
imply, but rather by their salinity when entering the South Atlantic. During
their transit towards the NBC the bulk of DP waters experiences a net density
loss through a net warming, whereas the bulk of AC waters experiences a
slight net density gain through a net increase in salinity. Notably, these
density changes are nearly completely captured by Lagrangian particle
trajectories that reach the surface mixed layer at least once during their
transit, which amount to 66 % and 49 % for DP and AC
waters, respectively. This implies that more than half of the water masses
supplying the upper limb of the AMOC are actually formed within the South
Atlantic and do not get their characteristic properties in the Pacific and
Indian Oceans.</p
Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods
The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich oceanâsea-ice model INALT20 (1/20â horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasizing the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2âSv, but the variability and trend of the transport are similar across these estimates. During the transit from the Agulhas Current at 32ââS to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong airâsea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic
Propagation and transformation of upper North Atlantic deep water from the subpolar gyre to 26.5°N
Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to reâexamine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddyârich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the MidâAtlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the MidâAtlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer
Propagation and Transformation of Upper North Atlantic Deep Water From the Subpolar Gyre to 26.5°N
Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to reâexamine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddyârich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the MidâAtlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the MidâAtlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer
Regional Imprints of Changes in the Atlantic Meridional Overturning Circulation in the Eddy-rich Ocean Model VIKING20X
A hierarchy of global 1/4° (ORCA025) and Atlantic Ocean 1/20° nested (VIKING20X) ocean/sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50â60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation, and in particular the long-term temporal evolution, of the Atlantic Meridional Overturning Circulation (AMOC) strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5° N pointing at a dominant role of the forcing. Regional observations in western boundary current systems at 53° N, 26.5° N and 11° S are explored in respect to their ability to represent the AMOC and to monitor the temporal evolution of the AMOC. Apart from the basin-scale measurements at 26.5° N, it is shown that in particular the outflow of North Atlantic Deepwater at 53° N is a good indicator of the subpolar AMOC trend during the recent decades, if the latter is provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable in representing realistic forcing-related and ocean-intrinsic trends
Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation
The Lagrangian method-where current location and intensity are determined by tracking the movement of flow along its path-is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid-20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice-free oceans and begin to construct basin-scale, and eventually global-scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC
Restricted dispersal in a sea of gene flow
Howfar domarine larvae disperse in the ocean? Decades of population genetic
studies have revealed generally low levels of genetic structure at large spatial
scales (hundreds of kilometres). Yet this result, typically based on discrete
sampling designs, does not necessarily imply extensive dispersal. Here, we
adopt a continuous sampling strategy along 950 km of coast in the northwestern
Mediterranean Sea to address this question in four species. In line
with expectations, we observe weak genetic structure at a large spatial scale.
Nevertheless, our continuous sampling strategy uncovers a pattern of isolation
by distance at small spatial scales (few tens of kilometres) in two species. Individual-
based simulations indicate that this signal is an expected signature of
restricted dispersal. At the other extreme of the connectivity spectrum, two
pairs of individuals that are closely related genetically were found more
than 290 km apart, indicating long-distance dispersal. Such a combination of
restricted dispersal with rare long-distance dispersal events is supported by
a high-resolution biophysical model of larval dispersal in the study area,
and we posit that it may be common in marine species. Our results bridge
population genetic studies with direct dispersal studies and have implications
for the design of marine reserve networksVersiĂłn del edito
Influence of Microplastics on Microbial Structure, Function, and Mechanical Properties of Stream Periphyton
Este artĂculo contiene 17 pĂĄginas, 5 figuras, 4 tablas.Periphyton is a freshwater biofilm composed of prokaryotic and eukaryotic communities that
occupy rocks and sediments, forming the base of the food web and playing a key role in
nutrient cycling. Given the large surface that periphyton comprises, it may also act as a sink for
a diverse range of man-made pollutants, including microplastics (MP). Here we investigated
the effect of 1â4 ÎŒm and 63â75 ”m sized, spherical polyethylene MP with native and ultraviolet
(UV)-weathered surface on developing natural stream periphyton communities over 28 days.
In order to ensure proper particle exposure, we first tested MP suspension in water or in water
containing either Tween 80, extracellular polymeric substances â EPS, fulvic acids, or protein.
We found the extract of EPS from natural periphyton to be most suitable to create MP
suspensions in preparation of exposure. Upon exposure, all tested types of MP were found to
be associated with the periphyton, independent of their size and other properties. While
biomass accrual and phenotypic community structure of the photoautotrophs remained
unchanged, the prokaryotic and eukaryotic communities experienced a significant change in
composition and relative abundances. Moreover, alpha diversity was affected in eukaryotes,
but not in prokaryotes. The observed changes were more prominent in periphyton exposed to
UV-treated as compared with native surface MP. Mechanical properties, as assessed by
compression rheology, showed that MP-exposed periphyton had longer filamentous
streamers, higher stiffness, lower force recovery and a higher viscoelasticity than control
periphyton. Despite the observed structural and mechanical changes of periphyton, functional
parameters (i.e., photosynthetic yield, respiration and nutrient uptake efficiencies) were not
altered by MP, indicating the absence of MP toxicity, and suggesting functional redundancy in
the communities. Together, our results provide further proof that periphyton is a sink for MP
and demonstrate that MP can impact local microbial community composition and mechanical
properties of the biofilms. Consequences of these findings might be a change in dislodgement behavior of periphyton, a propagation through the food chains and impacts on nutrient cycling
and energy transfer. Hence, taking the omnipresence, high persistence and material and size
diversity of MP in the aquatic environment into account, their ecological consequences need
further investigation.The study was financially supported by the Velux foundation,
project number 1039, Switzerland. Additional lab work was
funded by Tailwind grant of Eawag Switzerland. Open access
funding was provided by EawagâSwiss Federal Institute of
Aquatic Science And Technology.Peer reviewe
Mapping the local viscosity of non-Newtonian fluids flowing through disordered porous structures
Flow of non-Newtonian fluids through topologically complex structures is ubiquitous in most biological, industrial and environmental settings. The interplay between local hydrodynamics and the fluidâs constitutive law determines the distribution of flow paths. Consequently the spatial heterogeneity of the viscous resistance controls mass and solute transport from the micron to the meter scale. Examples range from oil recovery and groundwater engineering to drug delivery, filters and catalysts. Here we present a new methodology to map the spatial variation of the local viscosity of a non-Newtonian fluid flowing through a complex pore geometry. We use high resolution image velocimetry to determine local shear rates. Knowing the local shear rate in combination with a separate measurement of the fluidâs constitutive law allows to quantitatively map the local viscosity at the pore scale. Our experimental resultsâwhich closely match with three-dimensional numerical simulationsâdemonstrate that the exponential decay of the longitudinal velocity distributions, previously observed for Newtonian fluids, is a function of the spatial heterogeneity of the local viscosity. This work sheds light on the relationship between hydraulic properties and the viscosity at the pore scale, which is of fundamental importance for predicting transport properties, mixing, and chemical reactions in many porous systems.ISSN:2045-232
The Agulhas Current System as an important driver for oceanic and terrestrial climate
The Agulhas Current system around South Africa combines the dynamics of strong ocean currents in the Indian Ocean with eddyâmean flow interactions. The system includes an associated interoceanic transport towards the Atlantic, Agulhas leakage, which varies on both interannual and decadal timescales. Agulhas leakage is subject to a general increase under increasing greenhouse gases, with higher leakage causing a warming and salinification of the upper ocean in the South Atlantic. The far-field consequences include the impact of the Agulhas Current on the Benguela Upwelling system, a major eastern boundary upwelling system that supports a lucrative fishing industry. Through sea surface temperatures and associated airâsea fluxes, the Agulhas Current system also influences regional climate in southern Africa, leading to a heterogeneous pattern of rainfall over southern Africa and to a reduction of precipitation in most areas under global warming conditions. Changes in the Agulhas Current system and the regional climate also cause changes in regional sea-level and wind-induced waves that deviate from global trends. Combining these oceanic changes with extreme precipitation events, global warming can considerably amplify flood impacts along the coast of South Africa if no adaptation measures are implemented