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The thermohaline circulation during the Last Glacial Maximum and in the Present-Day climate
Stockholm University, Faculty of Science, Department of Meteorology .
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thermohaline circulation (THC) corresponds to the large time- and spatial-scales ocean circulation associated with the transport of heat and salt, and is known to be an important factor controlling the climate variability. The large scales involved in the THC make it difficult to observe, and therefore the synergy of numerical models and climate proxy reconstructions is particularly relevant to study the characteristics of this circulation in the present and past climates.

In this doctoral thesis, the THC during the Last Glacial Maximum (LGM) and the Present-Day (PD) is explored using a state-of-the-art Ocean General Circulation Model in its high- and low-resolution regimes. By comparing the LGM model outputs with the paleo-proxy reconstructions, it is shown that the high-resolution simulation improves the representation of the sea surface tem- peratures in the regions where the current structures appear to be complex, i.e., the western boundary currents (Agulhas, Kuroshio, Gulf Stream) and the Antarctic Circumpolar Current, although statistical comparisons with paleo- proxy reconstructions are not significantly improved on a global scale.

The THC involves a superposition of processes acting at widely different spatial and temporal scales, from the geostrophic large-scale and slowly-varying flow to the mesoscale turbulent eddies and at even smaller-scale, the mixing generated by the internal wave field. Not all these processes can be properly resolved in numerical models, and thus need to be parameterized. Analyzing the THC in an eddy-permitting numerical model, it was found that the temporal scales required for diagnosing the Southern Ocean circulation should not exceed 1 month and the spatial scales needed to be taken into account must be smaller than 1°. Important changes in the nature and intensity of the THC were observed between the LGM and PD simulations. An estimation of the turnover times (i.e., the time it takes for the water parcel to make and entire loop on the Conveyor Belt) revealed that the LGM THC could be more vigorous than under the PD conditions. As a result, the ocean transports of heat and freshwater, the oceanic uptake of CO2, the ventilation of the deep ocean and the reorganization of the passive and active tracers (e.g., temperature, salinity, greenhouse gases, nutrients) can be altered in these different regimes.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2013. , 52 p.
Keyword [en]
ocean model, circulation, LGM
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-89011ISBN: 978-91-7447-697-2 (print)OAI: oai:DiVA.org:su-89011DiVA: diva2:615109
Public defence
2013-05-24, Högbomsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 5: Manuscript. 

Available from: 2013-05-02 Created: 2013-04-08 Last updated: 2014-01-31Bibliographically approved
List of papers
1. The world ocean thermohaline circulation
Open this publication in new window or tab >>The world ocean thermohaline circulation
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2012 (English)In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 42, no 9, 1445-1460 p.Article in journal (Refereed) Published
Abstract [en]

A new global streamfunction is presented and denoted the thermohaline streamfunction. This is defined as the volume transport in terms of temperature and salinity (hence no spatial variables). The streamfunction is used to analyze and quantify the entire World Ocean conversion rate between cold/warm and fresh/saline waters. It captures two main cells of the global thermohaline circulation, one corresponding to the conveyor belt and one corresponding to the shallow tropical circulation. The definition of a thermohaline streamfunction also enables a new method of estimating the turnover time as well as the heat and freshwater transports of the conveyor belt. The overturning time of the conveyor belt is estimated to be between 1000 and 2000 yr, depending on the choice of stream layer. The heat and freshwater transports of these two large thermohaline cells have been calculated by integrating the thermohaline streamfunction over the salinity or temperature, yielding a maximum heat transport of the conveyor belt of 1.2 PW over the 34.2-PSU salinity surface and a freshwater transport of 0.8 Sv (1 Sv = 10(6) m(3) s(-1)) over the 9 degrees C isotherm. This is a measure of the net interocean exchange of heat between the Atlantic and Indo-Pacific due to the thermohaline circulation.

National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-81829 (URN)10.1175/JPO-D-11-0163.1 (DOI)000309018500004 ()
Note

AuthorCount:5;

Available from: 2012-11-07 Created: 2012-11-01 Last updated: 2017-12-07Bibliographically approved
2. The residual circulation of the Southern Ocean: Which spatio-temporal scales are needed?
Open this publication in new window or tab >>The residual circulation of the Southern Ocean: Which spatio-temporal scales are needed?
2013 (English)In: Ocean Modelling, ISSN 1463-5003, E-ISSN 1463-5011, Vol. 64, 46-55 p.Article in journal (Refereed) Published
Abstract [en]

The Southern Ocean circulation consists of a complicated mixture of processes and phenomena that arise at different time and spatial scales which need to be parametrized in the state-of-the-art climate models. The temporal and spatial scales that give rise to the present-day residual mean circulation are here inves- tigated by calculating the Meridional Overturning Circulation (MOC) in density coordinates from an eddy-permitting global model. The region sensitive to the temporal decomposition is located between 38°S and 63°S, associated with the eddy-induced transport. The ‘‘Bolus’’ component of the residual circu- lation corresponds to the eddy-induced transport. It is dominated by timescales between 1 month and 1 year. The temporal behavior of the transient eddies is examined in splitting the ‘‘Bolus’’ component into a ‘‘Seasonal’’, an ‘‘Eddy’’ and an ‘‘Inter-monthly’’ component, respectively representing the correlation between density and velocity fluctuations due to the average seasonal cycle, due to mesoscale eddies and due to large-scale motion on timescales longer than one month that is not due to the seasonal cycle. The ‘‘Seasonal’’ bolus cell is important at all latitudes near the surface. The ‘‘Eddy’’ bolus cell is dominant in the thermocline between 50°S and 35°S and over the whole ocean depth at the latitude of the Drake Passage. The ‘‘Inter-monthly’’ bolus cell is important in all density classes and is maximal in the Brazil– Malvinas Confluence and the Agulhas Return Current. The spatial decomposition indicates that a large part of the Eulerian mean circulation is recovered for spatial scales larger than 11.25°, implying that small-scale meanders in the Antarctic Circumpolar Current (ACC), near the Subantarctic and Polar Fronts, and near the Subtropical Front are important in the compensation of the Eulerian mean flow. 

Keyword
Overturning, Stream function, Southern Ocean, Bolus, Deacon Cell
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-88439 (URN)10.1016/j.ocemod.2013.01.005 (DOI)000316193600004 ()
Funder
Swedish Research Council
Available from: 2013-03-15 Created: 2013-03-15 Last updated: 2017-12-06Bibliographically approved
3. A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation
Open this publication in new window or tab >>A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation
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2013 (English)In: Climate of the Past Discussions, ISSN 1814-9340, E-ISSN 1814-9359, Vol. 9, 297-328 p.Article in journal (Refereed) Published
Abstract [en]

Most state-of-the-art climate models include a coarsely resolved oceanic compo- nent, which has difficulties in capturing detailed dynamics, and therefore eddy- permitting/eddy-resolving simulations have been developed to reproduce the observed World Ocean. In this study, an eddy-permitting numerical experiment is conducted to simulate the global ocean state for a period of the Last Glacial Maximum (LGM, ∼26500 to 19000yr ago) and to investigate the improvements due to taking into account these higher spatial scales. The ocean general circulation model is forced by a 49-yr sample of LGM atmospheric fields constructed from a quasi-equilibrated climate-model simulation. The initial state and the bottom boundary condition conform to the Paleoclimate Modelling Intercomparison Project (PMIP) recommendations. Be- fore evaluating the model efficiency in representing the paleo-proxy reconstruction of the surface state, the LGM experiment is in this first part of the investigation, compared with a present-day eddy-permitting hindcast simulation as well as with the available PMIP results. It is shown that the LGM eddy-permitting simulation is consistent with the quasi-equilibrated climate-model simulation, but large discrepancies are found with the PMIP model analyses, probably due to the different equilibration states. The strongest meridional gradients of the sea-surface temperature are located near 40° N and S, this due to particularly large North-Atlantic and Southern-Ocean sea-ice covers. These also modify the locations of the convection sites (where deep-water forms) and most of the LGM Conveyor Belt circulation consequently takes place in a thinner layer than today. Despite some discrepancies with other LGM simulations, a glacial state is captured and the eddy-permitting simulation undertaken here yielded a useful set of data for comparisons with paleo-proxy reconstructions. 

National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-88440 (URN)10.5194/cpd-9-297-2013 (DOI)
Available from: 2013-03-15 Created: 2013-03-15 Last updated: 2017-12-06Bibliographically approved
4. A Last Glacial Maximum World-Ocean simulation at eddy-permitting resolution – Part 2: Confronting the paleo-proxy data
Open this publication in new window or tab >>A Last Glacial Maximum World-Ocean simulation at eddy-permitting resolution – Part 2: Confronting the paleo-proxy data
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2013 (English)In: Climate of the Past Discussions, ISSN 1814-9340, E-ISSN 1814-9359, Vol. 9, 329-350 p.Article in journal (Refereed) Published
Abstract [en]

Previous investigations concerning the design of an eddy-permitting LGM oceanic sim- ulation are here extended with focus on whether this type of simulation is capable of improving the numerical results with regard to the available paleo-proxy reconstructions. Consequently, an eddy-permitting and two coarse-grid simulations of the same LGM period are confronted with a dataset from the Multiproxy Approach for the Recon- struction of the Glacial Ocean Sea Surface Temperatures (MARGO SSTs) and a num- ber of sea-ice reconstructions. From a statistical analysis it was found that the eddy- permitting simulation does not significantly improve the SST representation with regard to the paleo-reconstructions. The western boundary currents are better resolved in the high-resolution experiment than in the coarse simulations, but, although these more detailed SST structures yield a locally improved consistency between modelled pre- dictions and proxies, they do not contribute significantly to the global statistical score. As in the majority of the PMIP2 simulations, the modelled sea-ice conditions are still inconsistent with the paleo-reconstructions, probably due to the choice of the model equilibrium. 

National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-88441 (URN)10.5194/cpd-9-329-2013 (DOI)
Available from: 2013-03-15 Created: 2013-03-15 Last updated: 2017-12-06Bibliographically approved
5. The vigorous large-scale ocean circulations during the Last Glacial Maximum
Open this publication in new window or tab >>The vigorous large-scale ocean circulations during the Last Glacial Maximum
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The representation of the ocean thermohaline circulation (THC) under glacial and interglacial climate conditions is investigated using a new global thermohaline stream function. Consequently, the interglacial and glacial THCs are compared from two experiments based on an ocean general circulation model forced at the surface by conditions representing the present-day and the period of the Last Glacial Maximum (LGM, ≈ 21kyr ago). It is shown  that the LGM THC is amplified by the salinity/density contrast between the Atlantic and the Pacific basins, as well as in the abyss due to larger salinity gradients. Even though the circuit along the Conveyor Belt loop is not drastically changed, the water mass transformations can regionally differ between the two periods. Additionally, the LGM Conveyor Belt Cell is more  isolated from the abyss and its turnover time is between two and three times shorter than in the present-day simulation, suggesting vigorous large-scale circulation. 

National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-89010 (URN)
Available from: 2013-04-08 Created: 2013-04-08 Last updated: 2014-11-10Bibliographically approved

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