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ON THE DRIVING PROCESSES OF THE ATLANTIC MERIDIONAL OVERTURNING CIRCULATION
, 2001
"... [1] Because of its relevance for the global climate the Atlantic meridional overturning circulation (AMOC) has been a major research focus for many years. Yet the question of which physical mechanisms ultimately drive the AMOC, in the sense of providing its energy supply, remains a matter of controv ..."
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Cited by 56 (3 self)
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[1] Because of its relevance for the global climate the Atlantic meridional overturning circulation (AMOC) has been a major research focus for many years. Yet the question of which physical mechanisms ultimately drive the AMOC, in the sense of providing its energy supply, remains a matter of controversy. Here we review both observational data and model results concerning the two main candidates: vertical mixing processes in the ocean’s interior and wind-induced Ekman upwelling in the Southern Ocean. In distinction to the energy source we also discuss the role of surface heat and freshwater fluxes, which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state. We conclude that both wind-driven upwelling and vertical mixing are likely contributing to driving the observed circulation. To quantify their respective contributions, future research needs to address some open questions, which we outline.
Bumke (2002), Comparison of surface layer and surface turbulent flux observations over the Labrador Sea with ECMWF analyses and NCEP reanalyses
- J. Phys. Oceanogr
"... Comparisons are made between a time series of meteorological surface layer observational data taken on board the R/V Knorr, and model analysis data from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the National Centers for Environmental Prediction (NCEP). The observational da ..."
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Cited by 42 (4 self)
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Comparisons are made between a time series of meteorological surface layer observational data taken on board the R/V Knorr, and model analysis data from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the National Centers for Environmental Prediction (NCEP). The observational data were gathered during a winter cruise of the R/V Knorr, from 6 February to 13 March 1997, as part of the Labrador Sea Deep Convection Experiment. The surface layer observations generally compare well with both model representations of the wintertime atmosphere. The biases that exist are mainly related to discrepancies in the sea surface temperature or the relative humidity of the analyses. The surface layer observations are used to generate bulk estimates of the surface momentum flux, and the surface sensible and latent heat fluxes. These are then compared with the model-generated turbulent surface fluxes. The ECMWF surface sensible and latent heat flux time series compare reasonably well, with overestimates of only 13 % and 10%, respectively. In contrast, the NCEP model overestimates the bulk fluxes by 51 % and 27%, respectively. The differences between the bulk estimates and those of the two models are due to different surface heat flux algorithms. It is shown that the roughness length formula used in the NCEP reanalysis project is inappropriate for moderate to high wind speeds. Its failings are acute for situations of large air–sea temperature
2006), The influence of sea ice on ocean heat uptake in response to increasing CO2
- J. Clim
"... Two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing CO2 are investigated with Community Climate System Model version 3 (CCSM3): a deep warming below 500 m and extending down several kilometers in the Southern Ocean and warming in a 200-m ..."
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Cited by 35 (15 self)
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Two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing CO2 are investigated with Community Climate System Model version 3 (CCSM3): a deep warming below 500 m and extending down several kilometers in the Southern Ocean and warming in a 200-m layer just below the surface in the Arctic Ocean. Ocean heat uptake caused by sea ice retreat is isolated by running the model with the sea ice albedo reduced artificially alone. This integration has a climate response with strong ocean heat uptake in the Southern Ocean and modest ocean heat uptake in the subsurface Arctic Ocean. The Arctic Ocean warming results from enhanced ocean heat transport from the northern North Atlantic. At the time of CO2 doubling, about 1/3 of the heat transport anomaly results from advection of anomalously warm water and 2/3 results from strengthened inflow. At the same time the overturning circulation is strengthened in the northern North Atlantic and Arctic Oceans. Wind stress changes cannot explain the circulation changes, which instead appear related to strengthened convection along the Siberian shelves. Deep ocean warming in the Southern Ocean is initiated by weakened convection, which is mainly a result of surface freshening through altered sea ice and ocean freshwater transport. Below about 500 m, changes in convection reduce the vertical and meridional temperature gradients in the Southern Ocean, which significantly reduce isopycnal diffusion of heat upward around Antarctica. The geometry of the sea ice cover and its influence on convection have a strong influence on ocean temperature gradients, making sea ice an important player in deep ocean heat uptake in the Southern Ocean. 1.
Past and Future Reorganizations in the Climate System
- Quat. Sci. Rev
, 2000
"... High-resolution records of past climatic changes during the last glacial have revealed a number of abrupt changes on time scales of decades or less. Climate models suggest that the deep ocean circulation has the potential to act as a pacemaker of such changes. Based on results from ice cores from ..."
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Cited by 29 (4 self)
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High-resolution records of past climatic changes during the last glacial have revealed a number of abrupt changes on time scales of decades or less. Climate models suggest that the deep ocean circulation has the potential to act as a pacemaker of such changes. Based on results from ice cores from both polar regions, and the reference to a common time scale based on the methane record, it is suggested that the ocean is involved in the 24 Dansgaard-Oeschger events. For the longer events, northern and southern hemispheres are strongly coupled and exhibit climate changes of opposite sign. For the shorter events, the hemispheres are not coupled. The specific global response depends upon the forcing, and probably, the state of the ocean prior to the onset of these events. While such abrupt climate changes appear to be caused by a unique mechanism (changes in the sea surface freshwater balance), models suggest that the response of the ocean circulation depends on the amplitude and ...
2005), Estimating eddy stresses by fitting dynamics to observations using a residual‐mean ocean circulation model and its adjoint
- J. Phys. Oceanogr
"... A global ocean circulation model is formulated in terms of the “residual mean ” and used to study eddy–mean flow interaction. Adjoint techniques are used to compute the three-dimensional eddy stress field that minimizes the departure of the coarse-resolution model from climatological observations of ..."
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Cited by 27 (9 self)
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A global ocean circulation model is formulated in terms of the “residual mean ” and used to study eddy–mean flow interaction. Adjoint techniques are used to compute the three-dimensional eddy stress field that minimizes the departure of the coarse-resolution model from climatological observations of temperature. The resulting 3D maps of eddy stress and residual-mean circulation yield a wealth of infor-mation about the role of eddies in large-scale ocean circulation. In eddy-rich regions such as the Southern Ocean, the Kuroshio, and the Gulf Stream, eddy stresses have an amplitude comparable to the wind stress, of order 0.2 N m2, and carry momentum from the surface down to the bottom, where they are balanced by mountain form drag. From the optimized eddy stress, 3D maps of horizontal eddy diffusivity are inferred. The diffusivities have a well-defined large-scale structure whose prominent features are 1) large values of (up to 4000 m2 s1) in the western boundary currents and on the equatorial flank of the Antarctic Circumpolar Current and 2) a surface intensification of , suggestive of a dependence on the stratification N 2. It is shown that implementation of an eddy parameterization scheme in which the eddy diffusivity has an N 2 dependence significantly improves the climatology of the ocean model state relative to that obtained using a spatially uniform diffusivity. 1.
Convective mixing and the thermohaline circulation
- J. Phys. Oceanogr
, 1999
"... ABSTRACT An idealized three-dimensional model of buoyancy-driven flow in a single hemisphere is used to investigate the relationship between the meridional overturning and the efficiency by which convective mixing eliminates static instability. In the ''fast'' limit (mixing time ..."
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Cited by 24 (1 self)
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ABSTRACT An idealized three-dimensional model of buoyancy-driven flow in a single hemisphere is used to investigate the relationship between the meridional overturning and the efficiency by which convective mixing eliminates static instability. In the ''fast'' limit (mixing timescale hours to weeks), the meridional overturning is not rate limited by the efficiency of convective mixing. If convective mixing is made less efficient, the model's meridional overturning strength increases. Moreover, the dominant downwelling occurs not at the highest surface density; hence the deep ocean is relatively buoyant. The numerical results are explained by the different influences of convective mixing and downward advection on the deep-ocean heat budget; they underscore the fundamentally three-dimensional nature of the meridional overturning. In addition, the narrowness of deep downwelling is related to the geostrophic dynamics of deep temperature anomalies near the eastern wall. The model results presented here are in contrast to the expectation that deep-water formation by convective mixing is a necessary, if not rate-limiting, ingredient to the existence of a thermohaline circulation.
Interannual variability of phytoplankton abundances
- in the North Atlantic. Deep-Sea Research II
, 2001
"... A framework is developed for examining spatial patterns of interannual variability in springtime chloro-phyll concentrations as a response to physical changes. A simpli"ed, two-layer bio-physical model reveals regional responses to interannual variability of convective mixing. Vertical mixing c ..."
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Cited by 22 (4 self)
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A framework is developed for examining spatial patterns of interannual variability in springtime chloro-phyll concentrations as a response to physical changes. A simpli"ed, two-layer bio-physical model reveals regional responses to interannual variability of convective mixing. Vertical mixing can promote productivity in the surface waters through enhanced nutrient supply, but also can retard productivity due to the transport of phytoplankton below Sverdrup's critical depth. The balance of these processes determines the regimes of response in the two-layer model. The regimes may be identi"ed by the ratio of the thickness of Sverdrup's critical layer during spring and the end of winter mixed layer, h /h. The responses predicted by the simpli"ed model are found in a more sophisticated four-compartment, nitrogen-based ecosystem model, driven by a general circulation model of the North Atlantic. Anomalously strong convective mixing leads to enhanced chlorophyll concentrations in regions of shallow mixed layers (h /h &1), such as the subtropics. In contrast, in the subpolar regions, where mixed layers are deeper (h /h;1), the sensitivity to convective mixing is weaker, and increased mixing can lead to lower phytoplankton abundances. The numerical model also reveals regions of more complex behavior, such as the inter-gyre boundary, where advective supply of nutrients plays a signi"cant role on interannual timescales. Preliminary analyses of in situ and remote observations from the Bermuda Atlantic Time-Series, Ocean Weather Station `Indiaa and the Coastal Zone Color Scanner also show qualitative agreement. The conceptual framework provides a tool for the analysis of ongoing remote ocean-color observations. 2001 Elsevier Science Ltd. All rights reserved. 1.
Variability of sea ice cover in the Chukchi Sea (western Arctic Ocean) during the Holocene
- Paleoceanography
, 2005
"... [1] Dinocysts from cores collected in the Chukchi Sea from the shelf edge to the lower slope were used to reconstruct changes in sea surface conditions and sea ice cover using modern analogue techniques. Holocene sequences have been recovered in a down-slope core (B15: 2135 m, 75440N, sedimentation ..."
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Cited by 11 (0 self)
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[1] Dinocysts from cores collected in the Chukchi Sea from the shelf edge to the lower slope were used to reconstruct changes in sea surface conditions and sea ice cover using modern analogue techniques. Holocene sequences have been recovered in a down-slope core (B15: 2135 m, 75440N, sedimentation rate of 1 cm kyr1) and in a shelf core (P1: 201 m, 73410N, sedimentation rate of 22 cm kyr1). The shelf record spanning about 8000 years suggests high-frequency centennial oscillations of sea surface conditions and a significant reduction of the sea ice at circa 6000 and 2500 calendar (cal) years B.P. The condensed offshore record (B15) reveals an early postglacial optimum with minimum sea ice cover prior to 12,000 cal years B.P., which corresponds to a terrestrial climate optimum in Bering Sea area. Dinocyst data indicate extensive sea ice cover (>10 months yr1) from 12,000 to 6000 cal years B.P. followed by a general trend of decreasing sea ice and increasing sea surface salinity conditions, superimposed on large-amplitude millennial-scale oscillations. In contrast, d18O data in mesopelagic foraminifers (Neogloboquadrina pachyderma) and benthic foraminifers (Cibicides wuellerstorfi) reveal maximum subsurface temperature and thus maximum inflow of the North Atlantic water around 8000 cal years B.P., followed by a trend toward cooling of the subsurface to bottom water masses. Sea-surface to subsurface conditions estimated from dinocysts and d18O data in foraminifers thus suggest a decoupling between the surface water layer and the intermediate North Atlantic water mass with the
Heat transfer and large-scale dynamics in turbulent Rayleigh-Benard convection
- REV MOD. PHYS
, 2009
"... The progress in our understanding of several aspects of turbulent Rayleigh-Bénard convection is reviewed. The focus is on the question of how the Nusselt number and the Reynolds number depend on the Rayleigh number Ra and the Prandtl number Pr, and on how the thicknesses of the thermal and the kinet ..."
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Cited by 11 (2 self)
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The progress in our understanding of several aspects of turbulent Rayleigh-Bénard convection is reviewed. The focus is on the question of how the Nusselt number and the Reynolds number depend on the Rayleigh number Ra and the Prandtl number Pr, and on how the thicknesses of the thermal and the kinetic boundary layers scale with Ra and Pr. Non-Oberbeck-Boussinesq effects and the dynamics of the large-scale convection-roll are addressed as well. The review ends with a list of challenges for future research on the turbulent Rayleigh-Bénard system.
Dynamics of convectively driven banded jets in the laboratory
- J. Atmos. Sci
"... The banded organization of clouds and zonal winds in the atmospheres of the outer planets has long fascinated observers. Several recent studies in the theory and idealized modeling of geostrophic turbulence have suggested possible explanations for the emergence of such organized patterns, typically ..."
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Cited by 9 (1 self)
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The banded organization of clouds and zonal winds in the atmospheres of the outer planets has long fascinated observers. Several recent studies in the theory and idealized modeling of geostrophic turbulence have suggested possible explanations for the emergence of such organized patterns, typically involving highly anisotropic exchanges of kinetic energy and vorticity within the dissipationless inertial ranges of turbulent flows dominated (at least at large scales) by ensembles of propagating Rossby waves. The results from an attempt to reproduce such conditions in the laboratory are presented here. Achievement of a distinct inertial range turns out to require an experiment on the largest feasible scale. Deep, rotating convection on small horizontal scales was induced by gently and continuously spraying dense, salty water onto the free surface of the 13-m-diameter cylindrical tank on the Coriolis platform in Grenoble, France. A “planetary vorticity gradient ” or “ effect ” was obtained by use of a conically sloping bottom and the whole tank rotated at angular speeds up to 0.15 rad s1. Over a period of several hours, a highly barotropic, zonally banded large-scale flow pattern was seen to emerge with up to 5–6 narrow, alternating, zonally aligned jets across the tank, indicating the development of an anisotropic field of geostrophic turbulence. Using particle image velocimetry (PIV) techniques, zonal jets are shown to have arisen from nonlinear
