Abstract
The North Atlantic Ocean is the most intense marine sink for anthropogenic carbon dioxide (CO
2
) in the world’s oceans, showing high variability and substantial changes over recent decades. ...However, the contribution of biology to the variability and trend of this sink is poorly understood. Here we use
in situ
plankton measurements, alongside observation-based sea surface CO
2
data from 1982 to 2020, to investigate the biological influence on the CO
2
sink. Our results demonstrate that long term variability in the CO
2
sink in the North Atlantic is associated with changes in phytoplankton abundance and community structure. These data show that within the subpolar regions of the North Atlantic, phytoplankton biomass is increasing, while a decrease is observed in the subtropics, which supports model predictions of climate-driven changes in productivity. These biomass trends are synchronous with increasing temperature, changes in mixing and an increasing uptake of atmospheric CO
2
in the subpolar North Atlantic. Our results highlight that phytoplankton play a significant role in the variability as well as the trends of the CO
2
uptake from the atmosphere over recent decades.
The transport of dissolved oxygen (O₂) from the surface ocean into the interior is a critical process sustaining aerobic life in mesopelagic ecosystems, but its rates and sensitivity to climate ...variations are poorly understood. Using a circulation model constrained to historical variability by assimilation of observations, the study shows that the North Pacific thermocline effectively takes up O₂ primarily by expanding the area through which O₂-rich mixed layer water is detrained into the thermocline. The outcrop area during the critical winter season varies in concert with the Pacific decadal oscillation (PDO). When the central North Pacific Ocean is in a cold phase, the winter outcrop window for the central mode water class (CMW; a neutral density range of γ = 25.6–26.6) expands southward, allowing more O₂-rich surface water to enter the ocean’s interior. An increase in volume flux of water to the CMW density class is partly compensated by a reduced supply to the shallower densities of subtropical mode water (γ = 24.0–25.5). The thermocline has become better oxygenated since the 1980s partly because of strong O₂ uptake. Positive O₂ anomalies appear first near the outcrop and subsequently downstream in the subtropical gyre. In contrast to the O₂ variations within the ventilated thermocline, observed O₂ in intermediate water (density range of γ = 26.7–27.2) shows a declining trend over the past half century, a trend not explained by the open ocean water mass formation rate.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The oceanic mixed layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer, all hydrographic ocean properties are set for months to millennia. A vast area of the ...Southern Ocean is seasonally capped by sea‐ice, which alters the characteristics of the ocean mixed layer. The interaction between the ocean mixed layer and sea‐ice plays a key role for water mass transformation, the carbon cycle, sea‐ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under‐ice mixed layer are poorly understood due to the sparseness of in situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the polar regions. Working with elephant seal‐derived, ship‐based, and Argo float observations, we describe the seasonal cycle of the ocean mixed‐layer characteristics and stability of the ocean mixed layer over the Southern Ocean and specifically under sea‐ice. Mixed‐layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed‐layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea‐ice freshwater flux for the salt budget and by air‐sea flux for the heat budget. Ekman advection, vertical diffusivity, and vertical entrainment play only secondary roles. Our results suggest that changes in regional sea‐ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed layer, and impact large‐scale water mass formation and transformation with far reaching consequences for ocean ventilation.
Key Points
Climatological seasonal cycle of under‐ice mixed layer in the Southern Ocean is produced with unprecented number of observations
Under‐ice seasonal cycle of mixed‐layer buoyancy content and stability is primarily driven by their haline contributions
Buoyancy content of under‐ice mixed layer is predominantly explained by ice‐ocean and vertical entrainment fluxes
Ocean models predict a decline in the dissolved oxygen inventory of the global ocean of one to seven per cent by the year 2100, caused by a combination of a warming-induced decline in oxygen ...solubility and reduced ventilation of the deep ocean. It is thought that such a decline in the oceanic oxygen content could affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries and coastal economies. Regional observational data indicate a continuous decrease in oceanic dissolved oxygen concentrations in most regions of the global ocean, with an increase reported in a few limited areas, varying by study. Prior work attempting to resolve variations in dissolved oxygen concentrations at the global scale reported a global oxygen loss of 550 ± 130 teramoles (10
mol) per decade between 100 and 1,000 metres depth based on a comparison of data from the 1970s and 1990s. Here we provide a quantitative assessment of the entire ocean oxygen inventory by analysing dissolved oxygen and supporting data for the complete oceanic water column over the past 50 years. We find that the global oceanic oxygen content of 227.4 ± 1.1 petamoles (10
mol) has decreased by more than two per cent (4.8 ± 2.1 petamoles) since 1960, with large variations in oxygen loss in different ocean basins and at different depths. We suggest that changes in the upper water column are mostly due to a warming-induced decrease in solubility and biological consumption. Changes in the deeper ocean may have their origin in basin-scale multi-decadal variability, oceanic overturning slow-down and a potential increase in biological consumption.
A warming and freshening trend of the mixed layer in the upper southeastern tropical Atlantic Ocean (SETA) is observed by the Argo float array during the time period of 2006–2020. The associated ...ocean surface density reduction impacts upper-ocean stratification that intensified by more than 30% in the SETA region since 2006. The initial typical subtropical stratification with a surface salinity maximum is shifting to more tropical conditions characterized by warmer and fresher surface waters and a subsurface salinity maximum. During the same period isopycnal surfaces in the upper 200 m are shoaling continuously. Observed wind stress changes reveal that open ocean wind curl-driven upwelling increased, however, partly counteracted by reduced coastal upwelling due to weakened alongshore southerly winds. Weakening southerly winds might be a reason why tropical surface waters spread more southward reaching further into the SETA region. The mixed layer warming and freshening and associated stratification changes might impact the marine ecosystem and pelagic fisheries in the Angolan and northern Namibian upwelling region.
Ocean processes at the Antarctic continental slope Heywood, Karen J.; Schmidtko, Sunke; Heuzé, Céline ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
07/2014, Letnik:
372, Številka:
2019
Journal Article
Recenzirano
Odprti dostop
The Antarctic continental shelves and slopes occupy relatively small areas, but, nevertheless, are important for global climate, biogeochemical cycling and ecosystem functioning. Processes of water ...mass transformation through sea ice formation/melting and ocean-atmosphere interaction are key to the formation of deep and bottom waters as well as determining the heat flux beneath ice shelves. Climate models, however, struggle to capture these physical processes and are unable to reproduce water mass properties of the region. Dynamics at the continental slope are key for correctly modelling climate, yet their small spatial scale presents challenges both for ocean modelling and for observational studies. Cross-slope exchange processes are also vital for the flux of nutrients such as iron from the continental shelf into the mixed layer of the Southern Ocean. An iron-cycling model embedded in an eddy-permitting ocean model reveals the importance of sedimentary iron in fertilizing parts of the Southern Ocean. Ocean gliders play a key role in improving our ability to observe and understand these small-scale processes at the continental shelf break. The Gliders: Excellent New Tools for Observing the Ocean (GENTOO) project deployed three Seagliders for up to two months in early 2012 to sample the water to the east of the Antarctic Peninsula in unprecedented temporal and spatial detail. The glider data resolve small-scale exchange processes across the shelf-break front (the Antarctic Slope Front) and the front's biogeochemical signature. GENTOO demonstrated the capability of ocean gliders to play a key role in a future multi-disciplinary Southern Ocean observing system.
The Atlantic Subtropical Cells Inferred from Observations Tuchen, Franz Philip; Lübbecke, Joke F.; Schmidtko, Sunke ...
Journal of geophysical research. Oceans,
November 2019, 2019-11-00, 20191101, Letnik:
124, Številka:
11
Journal Article
Recenzirano
Odprti dostop
The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres, they are ...characterized by equatorward transport at thermocline level, upwelling at the equator, and poleward Ekman transport in the surface layer. This study uses recent data from Argo floats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward flow and the thermocline equatorward flow. The latter is bounded by the 26.0 kg m−3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ± 1.1 Sv from the southern hemisphere compared to 2.9 ± 1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ± 3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ± 0.8 Sv at 10°S and 6.4 ± 1.4 Sv at 10°N). A diapycnal transport of about 3 Sv across the 26.0 kg m−3 isopycnal is required in order to maintain the mass balance of the STC circulation.
Plain Language Summary
The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropics to the subtropical regions within the upper 300 m. In both hemispheres, they are characterized by equatorward transport at subsurface level and poleward transport in the surface layers. They are closed by upwelling at the equator and subduction in the subtropics. STCs are suggested to impact sea‐surface temperature variability in tropical upwelling regions thereby influencing, for example, precipitation patterns. The boundary between the two branches is approximated by the depth at which the meridional velocity reverses. The lower boundary of the deep equatorward branch is defined by an isoline of potential density. We find that at subsurface level, the equatorward branches converge in the tropics with more transport coming from the southern hemisphere. At the surface, a more symmetric divergence of water mass is observed in the tropics. The surface layers are also influenced by geostrophic transport generally counteracting the wind‐driven divergence. In total, the net surface divergence and the subsurface convergence yield a residual. It is suggested that this water mass volume deficit originates from below the STCs and enters the subsurface layers in the tropics where it is lifted to the surface.
Key Points
Equatorward and poleward transports associated with the Atlantic Subtropical Cells are estimated from observations and reanalysis data
Estimates show asymmetry in thermocline transports (three times more transport from the south) and symmetric flow divergence at the surface
Transport budget reveals a residual of 3 Sv likely linked to the upper‐layer flow of the Atlantic meridional overturning circulation
A recent analysis of observed oxygen changes shows a 2% decline in marine oxygen during the 50 years since 1960. However, these oxygen changes vary on time scales related to climate modes and by ...regions, including areas of increasing oxygen. Hence, any local oxygen change is related to various subsets of these drivers for the different regions and time scales. Here we provide an overview of drivers presently known for the different regions in the upper and deep ocean and the regional influence of climate modes, focussing on decadal and longer time scales for open ocean regions. We identify and compile regions where changes in solubility, stratification, decadal to multidecadal variability, source waters (either increases or decreases), overturning circulation or circulation-driven changes, and biological or nutrient stimulation have been shown to play a role in oxygen changes. The superposition and interaction of drivers and processes makes the decomposition of the impact on oxygen distribution difficult. Nevertheless, the description of the different drivers identified will help in better understanding the oxygen changes observed and lead to better verification of numerical models of future ocean oxygen levels.
The surface mixed layer of the world ocean regulates global climate by controlling heat and carbon exchange between the atmosphere and the oceanic interior
. The mixed layer also shapes marine ...ecosystems by hosting most of the ocean's primary production
and providing the conduit for oxygenation of deep oceanic layers. Despite these important climatic and life-supporting roles, possible changes in the mixed layer during an era of global climate change remain uncertain. Here we use oceanographic observations to show that from 1970 to 2018 the density contrast across the base of the mixed layer increased and that the mixed layer itself became deeper. Using a physically based definition of upper-ocean stability that follows different dynamical regimes across the global ocean, we find that the summertime density contrast increased by 8.9 ± 2.7 per cent per decade (10
-10
per second squared per decade, depending on region), more than six times greater than previous estimates. Whereas prior work has suggested that a thinner mixed layer should accompany a more stratified upper ocean
, we find instead that the summertime mixed layer deepened by 2.9 ± 0.5 per cent per decade, or several metres per decade (typically 5-10 metres per decade, depending on region). A detailed mechanistic interpretation is challenging, but the concurrent stratification and deepening of the mixed layer are related to an increase in stability associated with surface warming and high-latitude surface freshening
, accompanied by a wind-driven intensification of upper-ocean turbulence
. Our findings are based on a complex dataset with incomplete coverage of a vast area. Although our results are robust within a wide range of sensitivity analyses, important uncertainties remain, such as those related to sparse coverage in the early years of the 1970-2018 period. Nonetheless, our work calls for reconsideration of the drivers of ongoing shifts in marine primary production, and reveals stark changes in the world's upper ocean over the past five decades.
Climate models with biogeochemical components predict declines in oceanic dissolved oxygen with global warming. In coastal regimes oxygen deficits represent acute ecosystem perturbations. Here, we ...estimate dissolved oxygen differences across the global tropical and subtropical oceans within the oxygen minimum zone (200–700-dbar depth) between 1960–1974 (an early period with reliable data) and 1990–2008 (a recent period capturing ocean response to planetary warming). In most regions of the tropical Pacific, Atlantic, and Indian Oceans the oxygen content in the 200–700-dbar layer has declined. Furthermore, at 200
dbar, the area with O
2 <70
μmol
kg
−1, where some large mobile macro-organisms are unable to abide, has increased by 4.5 million
km
2. The tropical low oxygen zones have expanded horizontally and vertically. Subsurface oxygen has decreased adjacent to most continental shelves. However, oxygen has increased in some regions in the subtropical gyres at the depths analyzed. According to literature discussed below, fishing pressure is strong in the open ocean, which may make it difficult to isolate the impact of declining oxygen on fisheries. At shallower depths we predict habitat compression will occur for hypoxia-intolerant taxa, with eventual loss of biodiversity. Should past trends in observed oxygen differences continue into the future, shifts in animal distributions and changes in ecosystem structure could accelerate.
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