scholarly journals The Response of an Idealized Ocean Basin to Variable Buoyancy Forcing

2005 ◽  
Vol 35 (5) ◽  
pp. 601-615 ◽  
Author(s):  
M. A. Lucas ◽  
J. J. Hirschi ◽  
J. D. Stark ◽  
J. Marotzke
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean Basin ◽  
Meridional Overturning Circulation ◽  
Quasi Equilibrium ◽  
Vertical Diffusion ◽  
Strong Response ◽  
Buoyancy Forcing

Abstract The response of an idealized ocean basin to variable buoyancy forcing is examined. A general circulation model that employs a Gent–McWilliams mixing parameterization is forced by a zonally constant restoring surface temperature profile, which varies with latitude and time over a period P. In each experiment, 17 different values of P are studied, ranging from 6 months to 32 000 yr. The model's meridional overturning circulation (MOC) exhibits a very strong response on all time scales greater than 15 yr, up to and including the longest forcing time scales examined. The peak-to-peak values of the MOC oscillations reach up to 125% of the steady-state maximum MOC and exhibit resonance-like behavior, with a maximum at centennial to millennial forcing periods (depending on the vertical diffusivity). This resonance-like behavior stems from the existence of two adjustment time scales, one of which is set by the vertical diffusion and the other of which is set by the basin width. Furthermore, the linearity of the response as well as its lag with the forcing varies with the forcing period. The considerable deviation from the quasi-equilibrium response at all time scales above 15 yr is surprising and suggests a potentially important role of the ocean circulation for climate, even at Milankovich time scales.

scholarly journals The Role of Oceans and Sea Ice in Abrupt Transitions between Multiple Climate States

2013 ◽  
Vol 26 (9) ◽  
pp. 2862-2879 ◽  
Author(s):  
Brian E. J. Rose ◽  
David Ferreira ◽  
John Marshall
Keyword(s):  
Sea Ice ◽  
Time Scales ◽  
Ocean Circulation ◽  
Radiative Forcing ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Heating And Cooling ◽  
Equable Climate ◽  
Polar Ocean

Abstract The coupled climate dynamics underlying large, rapid, and potentially irreversible changes in ice cover are studied. A global atmosphere–ocean–sea ice general circulation model with idealized aquaplanet geometry is forced by gradual multi-millennial variations in solar luminosity. The model traverses a hysteresis loop between warm ice-free conditions and cold glacial conditions in response to ±5 W m−2 variations in global, annual-mean insolation. Comparison of several model configurations confirms the importance of polar ocean processes in setting the sensitivity and time scales of the transitions. A “sawtooth” character is found with faster warming and slower cooling, reflecting the opposing effects of surface heating and cooling on upper-ocean buoyancy and, thus, effective heat capacity. The transition from a glacial to warm, equable climate occurs in about 200 years. In contrast to the “freshwater hosing” scenario, transitions are driven by radiative forcing and sea ice feedbacks. The ocean circulation, and notably the meridional overturning circulation (MOC), does not drive the climate change. The MOC (and associated heat transport) collapses poleward of the advancing ice edge, but this is a purely passive response to cooling and ice expansion. The MOC does, however, play a key role in setting the time scales of the transition and contributes to the asymmetry between warming and cooling.

scholarly journals The Global Zonally Integrated Ocean Circulation, 1992–2006: Seasonal and Decadal Variability

2009 ◽  
Vol 39 (2) ◽  
pp. 351-368 ◽  
Author(s):  
Carl Wunsch ◽  
Patrick Heimbach
Keyword(s):  
Southern Ocean ◽  
Time Scales ◽  
Ocean Circulation ◽  
General Circulation ◽  
Decadal Variability ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Oceanic Circulation ◽  
Meridional Transport

Abstract The zonally integrated meridional and vertical velocities as well as the enthalpy transports and fluxes in a least squares adjusted general circulation model are used to estimate the top-to-bottom oceanic meridional overturning circulation (MOC) and its variability from 1992 to 2006. A variety of simple theories all produce time scales suggesting that the mid- and high-latitude oceans should respond to atmospheric driving only over several decades. In practice, little change is seen in the MOC and associated heat transport except very close to the sea surface, at depth near the equator, and in parts of the Southern Ocean. Variability in meridional transports in both volume and enthalpy is dominated by the annual cycle and secondarily by the semiannual cycle, particularly in the Southern Ocean. On time scales longer than a year, the solution exhibits small trends with complicated global spatial patterns. Apart from a net uptake of heat from the atmosphere (forced by the NCEP–NCAR reanalysis, which produces net ocean heating), the origins of the meridional transport trends are not distinguishable and are likely a combination of model disequilibrium, shifts in the observing system, other trends (real or artificial) in the meteorological fields, and/or true oceanic secularities. None of the results, however, supports an inference of oceanic circulation shifts taking the system out of the range in which changes are more than small perturbations. That the oceanic observations do not conflict with an apparent excess heat uptake from the atmosphere implies a continued undersampling of the global ocean, even in the upper layers.

Impact of Accurate Geoid Fields on Estimates of the Ocean Circulation

2007 ◽  
Vol 24 (8) ◽  
pp. 1464-1478 ◽  
Author(s):  
Detlef Stammer ◽  
Armin Köhl ◽  
Carl Wunsch
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Formal Proof ◽  
Meridional Overturning Circulation ◽  
Dynamic Topography ◽  
Model Errors ◽  
Geoid Model ◽  
Meridional Heat Transport ◽  
The Impact

Abstract The impact of new geoid height models on estimates of the ocean circulation, now available from the Gravity Recovery and Climate Experiment (GRACE) spacecraft, is assessed, and the implications of far more accurate geoids, anticipated from the European Space Agency’s (ESA) Gravity and Ocean Circulation Explorer (GOCE) mission, are explored. The study is based on several circulation estimates obtained over the period 1992–2002 by combining most of the available ocean datasets with a global general circulation model on a 1° horizontal grid and by exchanging only the EGM96 geoid model with two different geoid models available from GRACE. As compared to the EGM96-based solution, the GRACE geoid leads to an estimate of the ocean circulation that is more consistent with the Levitus temperature and salinity climatology. While not a formal proof, this finding supports the inference of a substantially improved GRACE geoid skill. However, oceanographic implications of the GRACE model are only modest compared to what can be obtained from ocean observations alone. To understand the extent to which this is merely a consequence of a not-optimally converged solution or if a much more accurate geoid field could in principle play a profound role in the ocean estimation procedure, an additional experiment was performed in which the geoid error was artificially reduced relative to all other datasets. Adjustments occur then in all elements of the ocean circulation, including 10% changes in the meridional overturning circulation and the corresponding meridional heat transport in the Atlantic. For an optimal use of new geoid fields, improved error information is required. The error budget of existing time-mean dynamic topography estimates may now be dominated by residual errors in time-mean altimetric corrections. Both these and the model errors need to be better understood before improved geoid estimates can be fully exploited. As is commonly found, the Southern Ocean is of particular concern.

scholarly journals A salty deep ocean as a prerequisite for glacial termination

2021 ◽  
Vol 14 (12) ◽  
pp. 930-936
Author(s):  
Gregor Knorr ◽  
Stephen Barker ◽  
Xu Zhang ◽  
Gerrit Lohmann ◽  
Xun Gong ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Late Quaternary ◽  
Temperature Response ◽  
Circulation Model ◽  
Deep Ocean ◽  
Ocean General Circulation Model ◽  
Glacial Maximum ◽  
Meridional Overturning Circulation ◽  
Salinity Stratification

AbstractDeglacial transitions of the middle to late Pleistocene (terminations) are linked to gradual changes in insolation accompanied by abrupt shifts in ocean circulation. However, the reason these deglacial abrupt events are so special compared with their sub-glacial-maximum analogues, in particular with respect to the exaggerated warming observed across Antarctica, remains unclear. Here we show that an increase in the relative importance of salt versus temperature stratification in the glacial deep South Atlantic decreases the potential cooling effect of waters that may be upwelled in response to abrupt perturbations in ocean circulation, as compared with sub-glacial-maximum conditions. Using a comprehensive coupled atmosphere–ocean general circulation model, we then demonstrate that an increase in deep-ocean salinity stratification stabilizes relatively warm waters in the glacial deep ocean, which amplifies the high southern latitude surface ocean temperature response to an abrupt weakening of the Atlantic meridional overturning circulation during deglaciation. The mechanism can produce a doubling in the net rate of warming across Antarctica on a multicentennial timescale when starting from full glacial conditions (as compared with interglacial or subglacial conditions) and therefore helps to explain the large magnitude and rapidity of glacial terminations during the late Quaternary.

scholarly journals Meridional Overturning Circulation in a multi-basin model. Part II: Sensitivity to diffusivity and wind in warm and cool climates

2021 ◽  
Author(s):  
Jonathan A. Baker ◽  
Andrew J. Watson ◽  
Geoffrey K. Vallis
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
The Pacific ◽  
Primary Determinant ◽  
Meridional Overturning

AbstractThe response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind forcing and Pacific basin vertical diffusivity is investigated under varying buoyancy forcings, corresponding to ‘warm’, ‘present-day’ and ‘cold’ states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all buoyancy forcings. The sensitivity of the AMOC to wind stress increases as the buoyancy forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to buoyancy forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively-driven upwelling sustain the AMOC in the warmer (present-day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the buoyancy forcing are also needed to decouple the two basins.

Bjerknes Compensation at High Northern Latitudes: The Ocean Forcing the Atmosphere

2007 ◽  
Vol 20 (24) ◽  
pp. 6023-6032 ◽  
Author(s):  
E. van der Swaluw ◽  
S. S. Drijfhout ◽  
W. Hazeleger
Keyword(s):  
Heat Transport ◽  
Time Scales ◽  
General Circulation ◽  
Circulation Model ◽  
Open Water ◽  
Meridional Overturning Circulation ◽  
Atmospheric Response ◽  
Using Data ◽  
Hadley Centre ◽  
Sst Anomalies

Abstract The mechanisms for Bjerknes compensation of heat transport variations through the atmosphere and ocean on decadal time scales are investigated, using data output from a preindustrial control run of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It has recently been shown that Bjerknes compensation occurs on decadal time scales in a long preindustrial control run of HadCM3. This result is elaborated on by performing lead/lag correlations of the atmospheric and oceanic heat transports. By using statistical analysis, Bjerknes compensation is observed on decadal time scales at latitudes between 50° and 80°N. A maximum compensation rate of ∼55% occurs at 70°N. At this latitude, the correlation rate peaks when the ocean leads the atmosphere by one year. The mechanisms by which Bjerknes compensation occurs at this latitude are investigated. Anomalies in oceanic heat transport appear to be associated with variations in the strength of the Atlantic meridional overturning circulation (MOC). The associated sea surface temperature (SST) anomalies are in general too weak to assert a significant impact on the atmosphere. At 70°N, however, such SST anomalies are a prelude to the transition from sea ice coverage to open water after which the associated changes in heat exchange with the atmosphere are strong enough to force an atmospheric response. Because of the presence of a strong MOC component in the Atlantic Ocean, this interaction is confined to the region where the northeast Atlantic and Arctic Oceans connect. The atmospheric response to increased (decreased) heating from below is a decreased (increased) poleward temperature gradient, leading to a decreased (increased) heat transport by baroclinic eddies. The anomalous thermal low that is set up by heating from the ocean is associated with anomalous advection of cold air from the Greenland landmass.

scholarly journals Southern Ocean controls of the vertical marine <i>δ</i><sup>13</sup>C gradient – a modelling study

2018 ◽  
Vol 15 (23) ◽  
pp. 7205-7223 ◽  
Author(s):  
Anne L. Morée ◽  
Jörg Schwinger ◽  
Christoph Heinze
Keyword(s):  
Gas Exchange ◽  
Southern Ocean ◽  
Ocean Circulation ◽  
General Circulation ◽  
Dissolved Inorganic Carbon ◽  
Circulation Model ◽  
Ocean Basin ◽  
Global Ocean ◽  
Basin Scale ◽  
Poc Export

Abstract. δ13C, the standardised 13C ∕ 12C ratio expressed in per mille, is a widely used ocean tracer to study changes in ocean circulation, water mass ventilation, atmospheric pCO2, and the biological carbon pump on timescales ranging from decades to tens of millions of years. δ13C data derived from ocean sediment core analysis provide information on δ13C of dissolved inorganic carbon and the vertical δ13C gradient (i.e. Δδ13C) in past oceans. In order to correctly interpret δ13C and Δδ13C variations, a good understanding is needed of the influence from ocean circulation, air–sea gas exchange and biological productivity on these variations. The Southern Ocean is a key region for these processes, and we show here that Δδ13C in all ocean basins is sensitive to changes in the biogeochemical state of the Southern Ocean. We conduct a set of idealised sensitivity experiments with the ocean biogeochemistry general circulation model HAMOCC2s to explore the effect of biogeochemical state changes of the Southern and Global Ocean on atmospheric δ13C, pCO2, and marine δ13C and Δδ13C. The experiments cover changes in air–sea gas exchange rates, particulate organic carbon sinking rates, sea ice cover, and nutrient uptake efficiency in an unchanged ocean circulation field. Our experiments show that global mean Δδ13C varies by up to about ±0.35 ‰ around the pre-industrial model reference (1.2 ‰) in response to biogeochemical change. The amplitude of this sensitivity can be larger at smaller scales, as seen from a maximum sensitivity of about −0.6 ‰ on ocean basin scale. The ocean's oldest water (North Pacific) responds most to biological changes, the young deep water (North Atlantic) responds strongly to air–sea gas exchange changes, and the vertically well-mixed water (SO) has a low or even reversed Δδ13C sensitivity compared to the other basins. This local Δδ13C sensitivity depends on the local thermodynamic disequilibrium and the Δδ13C sensitivity to local POC export production changes. The direction of both glacial (intensification of Δδ13C) and interglacial (weakening of Δδ13C) Δδ13C change matches the direction of the sensitivity of biogeochemical processes associated with these periods. This supports the idea that biogeochemistry likely explains part of the reconstructed variations in Δδ13C, in addition to changes in ocean circulation.

Anomalies of Meridional Overturning: Mechanisms in the North Atlantic

2005 ◽  
Vol 35 (8) ◽  
pp. 1455-1472 ◽  
Author(s):  
Armin Köhl
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
General Circulation ◽  
Numerical Models ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract Optimal observations are used to investigate the overturning streamfunction in the North Atlantic at 30°N and 900-m depth. Those observations are designed to impact the meridional overturning circulation (MOC) in numerical models maximally when assimilated and therefore establish the most efficient observation network for studying changes in the MOC. They are also ideally suited for studying the related physical mechanisms in a general circulation model. Optimal observations are evaluated here in the framework of a global 1° model over a 10-yr period. Hydrographic observations useful to monitor the MOC are primarily located along the western boundary north of 30°N and along the eastern boundary south of 30°N. Additional locations are in the Labrador, Irminger, and Iberian Seas. On time scales of less than a year, variations in MOC are mainly wind driven and are made up through changes in Ekman transport and coastal up- and downwelling. Only a small fraction is buoyancy driven and constitutes a slow response, acting on time scales of a few years, to primarily wintertime anomalies in the Labrador and Irminger Seas. Those anomalies are communicated southward along the west coast by internal Kelvin waves at the depth level of Labrador Sea Water. They primarily set the conditions at the northern edge of the MOC anomaly. The southern edge is mainly altered through Rossby waves of the advective type, which originate from temperature and salinity anomalies in the Canary Basin. Those anomalies are amplified on their way westward in the baroclinic unstable region of the subtropical gyre. The exact meridional location of the maximum MOC response is therefore set by the ratio of the strength of these two signals.

scholarly journals Effects of orbital forcing on atmosphere and ocean heat transports in Holocene and Eemian climate simulations with a comprehensive Earth system model

2009 ◽  
Vol 5 (5) ◽  
pp. 2311-2341 ◽  
Author(s):  
N. Fischer ◽  
J. Jungclaus
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Quasi Equilibrium ◽  
Orbital Forcing ◽  
Atmospheric Circulation Patterns ◽  
The North ◽  
Heat And Moisture

Abstract. Orbital forcing does not only exert direct insolation effects but also alters climate indirectly through feedback mechanisms that modify atmosphere and ocean dynamics and meridional heat and moisture transfers. We investigate the regional effects of these changes by detailed analysis of atmosphere and ocean circulation and heat transports in a coupled atmosphere-ocean-sea ice-biosphere general circulation model (ECHAM5/JSBACH/MPI-OM). We perform long term quasi equilibrium simulations under pre-industrial, mid-Holocene (6000 years before present – yBP), and Eemian (125 000 yBP) orbital boundary conditions. Compared to pre-industrial climate, Eemian and Holocene temperatures show generally warmer conditions at higher and cooler conditions at lower latitudes. Changes in sea-ice cover, ocean heat transports, and atmospheric circulation patterns lead to pronounced regional heterogeneity. Over Europe, the warming is most pronounced over the north-eastern part in accordance with recent reconstructions for the Holocene. We attribute this warming to enhanced ocean circulation in the Nordic Seas and enhanced ocean-atmosphere heat flux over the Barents Shelf in conduction with retreat of sea ice and intensified winter storm tracks over northern Europe.

scholarly journals The Influence of Diapycnal Mixing on Quasi-Steady Overturning States in the Indian Ocean

2007 ◽  
Vol 37 (9) ◽  
pp. 2290-2304 ◽  
Author(s):  
Matthew D. Palmer ◽  
Alberto C. Naveira Garabato ◽  
John D. Stark ◽  
Joël J-M. Hirschi ◽  
Jochem Marotzke
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Quasi Equilibrium ◽  
Time Step ◽  
Overturning Circulation ◽  
Southern Boundary ◽  
Current Observation ◽  
The Indian Ocean

Abstract A regional general circulation model (GCM) of the Indian Ocean is used to investigate the influence of prescribed diapycnal diffusivity (Kd) on quasi-steady states of the meridional overturning circulation (MOC). The model has open boundaries at 35°S and 123°E where velocity, temperature, and salinity are prescribed at each time step. The results suggest that quasi-steady overturning states in the Indian Ocean are reached on centennial time scales. The size and structure of the MOC are controlled by the distribution of Kd and the southern boundary conditions. The distribution of Kd required to support an overturning circulation in the model interior of a magnitude equal to that prescribed at the southern boundary is estimated using a 1D advection–diffusion balance in isopycnal layers. Implementing this approach, 70%–90% of the prescribed deep inflow can be supported in quasi-steady state. Thus one is able to address the systematic discrepancy between past estimates of the deep MOC based on hydrographic sections and those based on GCM results. However, the Kd values required to support a substantial MOC in the model are much larger than current observation-based estimates, particularly for the upper 3000 m. The two estimates of the flow field near 32°S used to force the southern boundary imply a highly nonuniform distribution of Kd, as do recent estimates of Kd based on hydrographic observations. This work highlights the need to improve and implement realistic estimates of (nonuniform) Kd in ocean and coupled ocean–atmosphere GCMs when investigating quasi-equilibrium model states.

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