1,721,050 research outputs found
Wind-driven and buoyancy-driven circulation in the subtropical North Atlantic Ocean
No full textContinuous observations of ocean circulation at 26°N in the subtropical Atlantic Ocean have been made since April 2004 to quantify the strength and variability in the Atlantic Meridional overturning circulation (AMOC), in which warm, upper waters flow northward and colder deep waters below 1100 m depth return southward. The principal components of the AMOC are northward western boundary current transport in the Gulf Stream and Antilles Current, northward surface Ekman transport and southward thermocline recirculation, all of which are generally considered to be part of the wind-driven circulation. Southward flowing deep waters below 1100 m depth are usually considered to represent the buoyancy-driven circulation. We argue that the Gulf Stream is partially wind-driven but also partially buoyancy-driven as it returns upper waters upwelled in the global ocean back to water mass formation regions in the northern Atlantic. Seasonal to interannual variations in the circulation at 26°N are principally wind-driven. Variability in the buoyancy-driven circulation occurred in a sharp reduction in 2009 in the southward flow of Lower North Atlantic Deep Water when its transport decreased by 30% from pre-2009 values. Over the 14-year observational period from 2004 to 2018, the AMOC declined by 2.4 Sv from 18.3 to 15.9 Sv
Michael John Robert Fasham. 29 May 1942 — 7 June 2008
Full text linkProfessor Michael Fasham played a pioneering role in the development of marine ecosystem models for the study of nutrient and carbon cycling in the ocean. He is articularly celebrated for his famous Fasham–Ducklow–McKelvie model, which was the first of its kind to separate new and regenerated forms of nutrient, as well as including microbial recycling pathways. Fasham’s models provided key understanding of the links between primary production, carbon cycling and export (of organic matter from the surface to deep ocean) based on both deep and insightful parameterization inspired by his many collaborations with leading experimental and field biologists of the day, and by his expert use of data for model calibration and validation. He had the ability to see the big picture, linking observation and models to achieve a unified understanding of system dynamics. As well as the direct contributions of his own science, Fasham played a pivotal role in steering the international scientific agenda, notably his leadership of the Joint Global Ocean Flux Study which had the aim of understanding ocean carbon cycling and sinks via the coordination of extensive field programmes, synthesis and modelling. He will be remembered by those who knew him for his openness, enthusiasm and modesty, a man who was fun to know and to work with and who loved the thrill of scientific adventure and discovery
Changing currents: a strategy for understanding and predicting the changing ocean circulation
Full text linkWithin the context of UK marine science, we project a strategy for ocean circulation research over the next 20 years. We recommend a focus on three types of research: (i) sustained observations of the varying and evolving ocean circulation, (ii) careful analysis and interpretation of the observed climate changes for comparison with climate model projections, and (iii) the design and execution of focused field experiments to understand ocean processes that are not resolved in coupled climate models so as to be able to embed these processes realistically in the models. Within UK-sustained observations, we emphasize smart, cost-effective design of the observational network to extract maximum information from limited field resources. We encourage the incorporation of new sensors and new energy sources within the operational environment of UK-sustained observational programmes to bridge the gap that normally separates laboratory prototype from operational instrument. For interpreting the climate-change records obtained through a variety of national and international sustained observational programmes, creative and dedicated UK scientists should lead efforts to extract the meaningful signals and patterns of climate change and to interpret them so as to project future changes. For the process studies, individual scientists will need to work together in team environments to combine observational and process modelling results into effective improvements in the coupled climate models that will lead to more accurate climate predictions
A strategy for UK marine science for the next 20 years
No full textFifty years ago, there was ‘A discussion on progress and needs of marine science’ at the Royal Society [1]. George Deacon wrote in the Introduction (p. 286): ‘A hundred years ago the Society often listened to papers about the ocean, but the rapid growth of science … has led to some neglect of large-scale natural processes’. Today, marine science is at the core of many of the most substantial challenges and opportunities facing society. The large-scale natural processes referred to by Deacon, encompassing physics, chemistry and biology, are being perturbed by anthropogenic inputs to the atmosphere, most notably carbon dioxide, and directly and indirectly into the ocean, with substances ranging from radioactive elements to nutrients to endocrine disrupters. Ocean resources are increasingly being exploited. Offshore oil and gas are resources of growing importance to many developing nations, while marine renewable energy is a small but likely to be significant aspect for many coastal communities. Questions over the exploitation of the ocean's living resources can lead to clashes between science and politics. In terms of climate change, we are presently performing a global experiment by putting large amounts of carbon dioxide into the atmosphere. How the turbulent ocean and atmosphere will react to the resulting changes in radiative forcing and interact with land and ice forms are fascinating scientific problems of intrinsic interest but also with serious ramifications for mankind. Marine science is at the base of addressing these issues. Fundamental research is needed to deepen understanding of ocean processes, understanding that may enlarge or constrain the options for addressing the challenges facing society. What are the critical marine science issues that should be addressed in the next 20 years
How wind-forcing and air-sea heat exchange determine the meridional temperature gradient and stratification for the Antarctic Circumpolar Current
Full text linkMesoscale oceanic eddies have a profound effect on the meridional circulation in the Antarctic Circumpolar region. Previous studies have shown that eddies transport heat poleward to balance the heat lost by the ocean to the atmosphere in the waters around Antarctica and also transport eastward momentum downward at a rate comparable to the amount of momentum put into the water column by the wind stress. Using the poleward eddy heat fluxes to relate air-sea heat loss and wind stress, we find that the meridional temperature gradient in the circumpolar region is determined by the ratio of the air-sea heat loss to the zonal wind stress. Including a widely used parameterisation where poleward eddy heat fluxes are proportional to the mean meridional temperature gradient, we find that the slope of isotherms (or isopycnals) across the circumpolar zone is determined by the wind stress. Further assuming no cross-isopycnal mixing in the interior ocean, we find that the stratification in the deep ocean is determined by the ratio of air-sea heat loss to zonal wind stress squared. The poleward eddy heat fluxes also represent a southward eddy mass transport in the upper water column in density coordinates. The vertical gradient of this eddy mass transport is the meridional component of the eddy Stokes drift which is equal to the difference between the Lagrangian and Eulerian mean velocities in the circumpolar region. Because there can be no zonally averaged geostrophic Eulerian meridional velocity across the circumpolar region above the topography, this southward eddy Stokes drift provides a mechanism by which circumpolar deep water can flow southward across the circumpolar zone. Eddy heat fluxes, which are central to the mass, momentum, heat and energy balances in the circumpolar region, provide a catalyst for relating the roles of wind and buoyancy forcing in setting the overall circulation for the Southern Ocean
Low efficiency of nutrient translocation for enhancing oceanic uptake of carbon dioxide
No full textAnthropogenic emissions of carbon dioxide (CO2) are steadily increasing the concentration of this greenhouse gas in the Earth's atmosphere. The possible long-term consequences of this elevated concentration have led to proposals for a number of large-scale geoengineering schemes that aim to enhance or augment natural sinks for CO2. One such scheme proposes deploying a large number of floating “pipes” in the ocean that act to translocate nutrient-rich seawater from below the mixed layer to the ocean's surface: the nutrient supplied should enhance the growth of phytoplankton and consequently the export of organic carbon to the deep ocean via the biological pump. Here we examine the practical consequences of this scheme in a global ocean general circulation model that includes a nitrogen-based ecosystem and the biogeochemical cycle of carbon. While primary production is generally enhanced by the modeled pipes, as expected, the effect on the uptake of CO2 from the atmosphere is much smaller, may be negative, and shows considerable spatiotemporal variability. <br/
Mixing in the Deep Waters of the Western Mediterranean
No full textIn terms of the overall mixing environment, the Mediterranean Sea has relatively small tides and relatively weak winds compared with the greater ocean environment, so vertical diffusion due to mechanical mixing is likely to be generally smaller than in the open ocean. As the western Mediterranean deep water (WMDW) is naturally fresher and colder than the Levantine intermediate water (LIW), salinity and temperature both decrease downward below the LIW core toward the deep water. In the halocline-thermocline between the core of LIW and the deep water, warmer saltier waters overlie colder fresher waters and in such a region salt finger mixing processes can be effective mixing agents transporting salt, heat, and density downward. For the deep western Mediterranean, vertical diffusion due to mechanical mixing is expected to be small; and downward mixing of heat salt and density is expected to be substantial in the halocline-thermocline 400–1500-m depth
Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26°N
Full text linkThe Coupled Model Intercomparison Project (CMIP) allows the assessment of the representation of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. While CMIP Phase 6 models display a large spread in AMOC strength, the multi-model mean strength agrees reasonably well with observed estimates from RAPID1, but this does not hold for the AMOC s various components. In CMIP Phase 6 (CMIP6), the present-day AMOC is characterized by a lack of lower North Atlantic Deep Water (lNADW) due to the small scale of Greenland Iceland Scotland Ridge overflow and too much mixing. This is compensated for by increased recirculation in the subtropical gyre and more Antarctic Bottom Water (AABW). Deep-water circulation is dominated by a distinct deep western boundary current (DWBC) with minor interior recirculation compared with observations. The future decline in the AMOC of 7 Sv by 2100 under a Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) emission scenario is associated with decreased northward western boundary current transport in combination with reduced southward flow of upper North Atlantic Deep Water (uNADW). In CMIP6, wind stress curl decreases with time by 14 % so that wind-driven thermocline recirculation in the subtropical gyre is reduced by 4 Sv (17 %) by 2100. The reduction in western boundary current transport of 11 Sv is more than the decrease in wind-driven gyre transport, indicating a decrease over time in the component of the Gulf Stream originating from the South Atlantic.</p
South Atlantic overturning circulation at 24°S
Full text linkTo estimate the size of the meridional overturning circulation and the meridional heat and freshwater transports in the South Atlantic, we made a new transatlantic hydrographic section along 24°S in 2009 and we compare the resulting transports with those estimated for a historical section made in 1983. For the two sections, the overturning is estimated to be 21.5 Sv (2009) or 16.5 Sv (1983), the heat transport is northward at 0.7 PW (2009) or 0.4 PW (1983), and the freshwater transport is small but northward at 0.04 Sv (2009) or 0.17 Sv (1983). The differences in transports are primarily due to the different strengths of the southward Brazil Current transport during the occupation of the sections, 4.9 Sv (2009) or 12.3 Sv (1983). The freshwater transport associated with the meridional overturning circulation is estimated by two different methods for each of the two sections and is always southward ranging from -0.09 Sv to -0.34 Sv which means that the Atlantic meridional overturning circulation transports freshwater southward at 24°S. On the basis of theoretical studies, such southward freshwater transport at the southern boundary of the Atlantic Ocean means that the present Atlantic circulation has multiple equilibrium states, and that the one it occupies at present may be unstable to a sufficiently large freshwater event
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