Upwelling systems play a key role in the global carbon and
nitrogen cycles and are also of local relevance due to their high productivity and fish
resources. To capture and understand the high ...spatial and temporal variability in
physical and biogeochemical parameters found in these regions, novel measurement
techniques have to be combined in an interdisciplinary manner. Here we use
high-resolution glider-based physical–biogeochemical observations in combination with
ship-based underwater vision profiler, sensor and bottle data to investigate the drivers
of oxygen and nitrate variability across the shelf break off Mauritania in June 2014.
Distinct oxygen and nitrate variability shows up in our glider data. High-oxygen and
low-nitrate anomalies were clearly related to water mass variability and probably linked
to ocean transport. Low-oxygen and high-nitrate patches
co-occurred with enhanced turbidity signals close to the seabed, which suggests locally
high microbial respiration rates of resuspended organic matter near the sea floor. This
interpretation is supported by high particle abundance observed by the underwater vision
profiler and enhanced particle-based respiration rate estimates close to the seabed.
Discrete in situ measurements of dissolved organic carbon and amino acids suggest the
formation of dissolved organic carbon due to particle dissolution near the seabed fueling
additional microbial respiration. During June an increase in the oxygen concentration on
the shelf break of about 15 µmol kg−1 was observed. These changes go
along with meridional circulation changes but cannot be explained by typical water mass
property changes. Thus our high-resolution interdisciplinary observations highlight the
complex interplay of remote and local physical–biogeochemical drivers of oxygen and
nitrate variability off Mauritania, which cannot be captured by classical shipboard
observations alone.
The formation of a subsurface anticyclonic eddy in the Peru‐Chile Undercurrent (PCUC) in January and February 2013 is investigated using a multiplatform four‐dimensional observational approach. ...Research vessel, multiple glider, and mooring‐based measurements were conducted in the Peruvian upwelling regime near 12°30'S. The data set consists of >10,000 glider profiles and repeated vessel‐based hydrography and velocity transects. It allows a detailed description of the eddy formation and its impact on the near‐coastal salinity, oxygen, and nutrient distributions. In early January, a strong PCUC with maximum poleward velocities of ∼0.25 m/s at 100–200 m depth was observed. Starting on 20 January, a subsurface anticyclonic eddy developed in the PCUC downstream of a topographic bend, suggesting flow separation as the eddy formation mechanism. The eddy core waters exhibited oxygen concentration of <1 μmol/kg, an elevated nitrogen deficit of ∼17 μmol/L, and potential vorticity close to zero, which seemed to originate from the bottom boundary layer of the continental slope. The eddy‐induced across‐shelf velocities resulted in an elevated exchange of water masses between the upper continental slope and the open ocean. Small‐scale salinity and oxygen structures were formed by along‐isopycnal stirring, and indications of eddy‐driven oxygen ventilation of the upper oxygen minimum zone were observed. It is concluded that mesoscale stirring of solutes and the offshore transport of eddy core properties could provide an important coastal open ocean exchange mechanism with potentially large implications for nutrient budgets and biogeochemical cycling in the oxygen minimum zone off Peru.
Key Points:
Multiplatform observations of a subsurface anticyclone formation off Peru
Flow separation is suggested as the formation mechanism
The eddy provides an important coastal open ocean exchange mechanism for solutes
Abstract
Coastal upwelling rates are classically determined by the intensity of the upper-ocean offshore Ekman transport. But (sub-)mesoscale turbulence modulates offshore transport, hence the net ...upwelling rate. Eddy effects generally oppose the Ekman circulation, resulting in so-called “eddy cancellation”, a process well studied in the Southern Ocean. Here we investigate how air-sea heat/buoyancy fluxes modulate eddy cancellation in an idealized upwelling model. We run CROCO simulations with constant winds but varying heat fluxes with and without submesoscale-rich turbulence. Eddy cancellation is consistently evaluated with three different methods that all account for the quasi-isopycnal nature of ocean circulation away from the surface. For zero heat fluxes the release of available potential energy by baroclinic instabilities is strongest and leads, near the coast, to nearly full cancellation of the Ekman cross-shore circulation by eddy effects,
i.e.
, zero net mean upwelling flow. With increasing heat fluxes eddy cancellation is reduced and the transverse flow progressively approaches the classical Ekman circulation. Sensitivity of the eddy circulation to synoptic changes in air-sea heat fluxes is felt down to 125 m depth despite short experiments of tens of days. Mesoscale dynamics dominate the cancellation effect in our simulations which might also hold for the real ocean as the relevant processes act below the surface boundary layer. Although the idealized setting overemphasis the role of eddies and thus studies with more realistic settings should follow, our findings have important implications for the overall understanding of upwelling system dynamics.
The influence of chlorophyll shading on ocean dynamics has been usually disregarded in eastern boundary upwelling systems modeling studies in spite of their very high primary productivity. Here, we ...study how this effect impacts on the Peru upwelling system using a regional mesoscale‐resolving physical biogeochemical coupled model. We show that the shading effect leads to a surface cooling of up to 1°C on the shelf due to subsurface cooling of the source waters during their transit toward the shelf. The shading effect leads to a more realistic subsurface stratification, a slowdown of the alongshore currents, and a shoaling of the oxycline. Impacts on the regional model biases show that the shading effect needs to be taken into account in both physical and coupled physical‐biogeochemical regional models of upwelling systems.
Plain Language Summary
The chlorophyll pigments of phytoplankton capture the downward penetrating solar energy to produce photosynthesis and warm the surface of the ocean. However, this effect is seldom taken into account in ocean models, in particular in upwelling systems where chlorophyll concentration is very high. In this study, we show that taking into account this effect in a model of the Peruvian upwelling system, one of the most productive systems in the world, modifies not only the temperature, circulation, and turbulence, but also stimulates nearshore phytoplankton production and deoxygenation over the shelf. This study shows that this effect needs to be parameterized in future modeling studies.
Key Points
The influence of surface chlorophyll shading on the Peru upwelling system is investigated using a physical biogeochemical coupled model
The shading effect leads to surface cooling, nutricline and oxycline shoaling, and phytoplankton increase on the shelf
The shading effect should be taken into account in high‐resolution eastern boundary upwelling System models
Members of the gammaproteobacterial clade SUP05 couple water column sulfide oxidation to nitrate reduction in sulfidic oxygen minimum zones (OMZs). Their abundance in offshore OMZ waters devoid of ...detectable sulfide has led to the suggestion that local sulfate reduction fuels SUP05-mediated sulfide oxidation in a so-called "cryptic sulfur cycle". We examined the distribution and metabolic capacity of SUP05 in Peru Upwelling waters, using a combination of oceanographic, molecular, biogeochemical and single-cell techniques. A single SUP05 species,
Thioglobus perditus, was found to be abundant and active in both sulfidic shelf and sulfide-free offshore OMZ waters. Our combined data indicated that mesoscale eddy-driven transport led to the dispersal of
T. perditus and elemental sulfur from the sulfidic shelf waters into the offshore OMZ region. This offshore transport of shelf waters provides an alternative explanation for the abundance and activity of sulfide-oxidizing denitrifying bacteria in sulfide-poor offshore OMZ waters.
The Peruvian upwelling system encompasses the most intense and shallowest oxygen minimum zone (OMZ) in the ocean. This system shows pronounced submesoscale activity like filaments and fronts. We ...carried out glider‐based observations off Peru during austral summer 2013 to investigate whether submesoscale frontal processes ventilate the Peruvian OMZ. We present observational evidence for the subduction of highly oxygenated surface water in a submesoscale cold filament. The subduction event ventilates the oxycline but does not reach OMZ core waters. In a regional submesoscale‐permitting model we study the pathways of newly upwelled water. About 50% of upwelled virtual floats are subducted below the mixed layer within 5 days emphasizing a hitherto unrecognized importance of subduction for the ventilation of the Peruvian oxycline.
Key Points
Observed subduction of oxygenated surface water in a submesoscale filament off Peru
Submesoscale‐permitting model simulations confirm that subduction occurs in filaments off Peru
Lagrangian float diagnostics suggest that 50% of the newly upwelled water is subducted
Understanding how climate change will affect oceanic fluid transport is crucial for environmental applications and human activities. However, a synoptic characterization of the influence of climate ...change on mesoscale stirring and transport in the surface ocean is missing. To bridge this gap, we exploit a high‐resolution, fully coupled climate model of the Mediterranean basin using a Network Theory approach. We project significant increases of horizontal stirring and kinetic energies in the next century, likely due to increments of available potential energy. The future evolution of basin‐scale transport patterns hints at a rearrangement of the main hydrodynamic provinces, defined as regions of the surface ocean that are well mixed internally but with minimal cross‐flow across their boundaries. This results in increased heterogeneity of province sizes and stronger mixing in their interiors. Our approach can be readily applied to other oceanic regions, providing information for the present and future marine spatial planning.
Plain Language Summary
Transport and mixing of water masses driven by ocean currents influence a variety of fundamental processes, including heat redistribution, ecosystem functioning, and pollutants spreading. Therefore, understanding how fluid transport will be affected by climate change is crucial, in particular in the ocean surface, where marine life and human activities are concentrated. Here, we exploit a state‐of‐the‐art climate model over the Mediterranean basin using a novel methodology which integrates Network Theory concepts with Lagrangian modeling. We assess past conditions and future changes at climatic scales of ocean stirring and transport over the entire basin. Our results reveal a significant increment of surface stirring linked to an increase of currents kinetic energy, which in turn could be ascribed to increments of available potential energy. We then provide a regionalization of the ocean surface based on hydrodynamic provinces that are well mixed internally but with little leaking across their boundaries. Our model project an increased heterogeneity of province sizes and a stronger mixing in their interiors, while their mean area and coherence remain unaffected. Our approach could be applied to other oceanic domains and help designing adaptive strategies for marine spatial planning.
Key Points
We exploit a coupled climate model over the Mediterranean combining Network Theory with a Lagrangian approach
Entropy and kinetic energy analyses project a significant increase of stirring for the next century
Future transport patterns result in larger areas variability and stronger internal mixing of hydrodynamic provinces
•Generalization of the energy budget backscatter for a sub-grid eddy parameterization.•Rossby number-dependent dissipation scaling controls the forward energy-cascade.•Parameterization is tested ...successfully in a double gyre basin shallow water model.•The energy cycle of the model is considerably improved at low computational cost.
The parameterization of sub-grid scale processes is one of the key challenges towards improved numerical simulations of the atmospheric and oceanic circulation. Numerical weather prediction models as well as climate models would benefit from more sophisticated turbulence closures that allow for less spurious dissipation at the grid-scale and consequently higher and more realistic levels of eddy kinetic energy (EKE). Recent studies propose to use a hyperviscous closure in combination with an additional deterministic forcing term as a negative viscosity to represent backscatter of energy from unresolved scales. The sub-grid EKE is introduced as an additional prognostic variable that is fed by dissipation at the grid scale, and enables recycling of EKE via the backscatter term at larger scales. This parameterization was previously shown to work well in zonally re-entrant channel configurations. Here, a generalization in the form of a Rossby number-dependent scaling for the strength of the backscatter is introduced to represent the emergence of a forward energy-cascade in unbalanced flows near the boundaries. We apply the parameterization to a shallow water model of a double gyre basin and provide evidence for its general applicability. In terms of mean state and variability, a low resolution model is considerably improved towards a high resolution control run at low additional computational cost.
The eastern tropical South Pacific (ETSP) hosts the Peruvian upwelling
system, which represents one of the most productive areas in the world ocean.
High primary production followed by rapid ...heterotrophic utilization of
organic matter supports the formation of one of the most intense oxygen
minimum zones (OMZs) in the world ocean, where dissolved oxygen
(O2) concentrations reach less than 1 µmol kg−1.
The high productivity leads to an accumulation of dissolved organic matter
(DOM) in the surface layers that may serve as a substrate for heterotrophic
respiration. However, the importance of DOM utilization for O2
respiration in the Peruvian upwelling system in general and for shaping the
upper oxycline in particular remains unclear so far. This study reports the
first estimates of diapycnal fluxes and supply of O2, dissolved
organic carbon (DOC), dissolved organic nitrogen, dissolved hydrolysable
amino acids (DHAA) and dissolved combined carbohydrates (DCCHO) for the ETSP
off Peru. Diapycnal flux and supply estimates were obtained by combining
measured vertical diffusivities and solute concentration gradients. They were
analysed together with the molecular composition of DCCHO and DHAA to infer
the transport of labile DOM into the upper OMZ and the potential role of DOM
utilization for the attenuation of the diapycnal O2 flux that
ventilates the OMZ. The observed diapycnal O2 flux
(50 mmol O2 m−2 d−1 at maximum) was limited to the
upper 80 m of the water column; the O2 supply of ∼1 µmol kg−1 d−1 was comparable to previously published
O2 consumption rates for the North and South Pacific OMZs. The
diapycnal DOM flux (31 mmol C m−2 d−1 at maximum) was limited
to ∼30 m water depth, suggesting that the labile DOM is extensively
consumed within the upper part of the shallow oxycline off Peru. The analyses
of DCCHO and DHAA composition support this finding, suggesting that DOM
undergoes comprehensive remineralization within the upper part of the
oxycline, as the DOM within the core of the OMZ was found to be largely
altered. Estimated by a simple equation for carbon combustion, aerobic
respiration of DCCHO and DHAA, supplied by diapycnal mixing
(0.46 µmol kg−1 d−1 at maximum), could account for up
to 38 % of the diapycnal O2 supply in the upper oxycline, which
suggests that DOM utilization plays a significant role for shaping the upper
oxycline in the ETSP.
Stability Analysis of the Labrador Current THOMSEN, Sören; EDEN, Carsten; CZESCHEL, Lars
Journal of physical oceanography,
02/2014, Letnik:
44, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
Mooring observations and model simulations point to an instability of the Labrador Current (LC) during winter, with enhanced eddy kinetic energy (EKE) at periods between 2 and 5 days and ...much less EKE during other seasons. Linear stability analysis using vertical shear and stratification from the model reveals three dominant modes of instability in the LC: 1) a balanced interior mode with along-flow wavelengths of about 30–45 km, phase velocities of 0.3 m s−1, maximal growth rates of 1 day−1, and surface-intensified but deep-reaching amplitudes; 2) a balanced shallow mode with along-flow wavelengths of about 0.3–1.5 km, phase velocities of 0.55 m s−1, about 3 times larger growth rates, but amplitudes confined to the mixed layer (ML); and 3) an unbalanced symmetric mode with the largest growth rates, vanishing phase speeds, and along-flow structure, and very small cross-flow wavelengths, also confined to the ML. Both balanced modes are akin to baroclinic instability but operate at moderate-to-small Richardson numbers Ri with much larger growth rates as for the quasigeostrophic limit of Ri ≫ 1. The interior mode is found to be responsible for the instability of the LC during winter. Weak stratification and enhanced vertical shear due to local buoyancy loss and the advection of convective water masses from the interior result in small Ri within the LC and up to 3 times larger growth rates of the interior mode in March compared to summer and fall conditions. Both the shallow and the symmetric modes are not resolved by the model, but it is suggested that they might also play an important role for the instability in the LC and for lateral mixing.
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