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Symmetric and asymmetric modes of the thermohaline circulation
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
2006 (English)In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 58, no 5, 616-627 p.Article in journal (Refereed) Published
Abstract [en]

On the basis of a zonally averaged two-hemisphere ocean model, this study investigates how the asymmetric thermohaline circulation depends on the equator-to-pole as well as the pole-to-pole density difference. Numerical experiments are conducted with prescribed surface density distributions as well as with mixed boundary conditions. Further, two different representations of the small-scale vertical mixing are considered, viz. constant and stability-dependent vertical diffusivity. The two mixing representations yield the opposite overturning responses when the equator-to-pole density difference is changed, keeping the shape of the surface density field invariant. However, the overturning responses of the two representations are qualitatively similar when the degree of asymmetry of the surface density field is changed, keeping the density difference invariant. This applies essentially when the freshwater forcing is increased for fixed thermal boundary conditions. For a fixed freshwater forcing, on the other hand, an increase of the equator-to-pole temperature difference yields a weaker asymmetric circulation when the stability-dependent diffusivity is employed, whereas the reverse holds true for the constant diffusivity representation. Further, the numerical experiments show that the hysteresis characteristics of the asymmetric thermohaline circulation may be sensitive the nature of the small-scale vertical mixing.

Place, publisher, year, edition, pages
2006. Vol. 58, no 5, 616-627 p.
National Category
Oceanography, Hydrology, Water Resources
Research subject
Oceanography
Identifiers
URN: urn:nbn:se:su:diva-23747DOI: 10.1111/j.1600-0870.2006.00194.xOAI: oai:DiVA.org:su-23747DiVA: diva2:194333
Note
Part of urn:nbn:se:su:diva-473Available from: 2005-04-22 Created: 2005-04-22 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Some aspects of the Atlantic ocean circulation
Open this publication in new window or tab >>Some aspects of the Atlantic ocean circulation
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis deals with the ocean circulation from two viewpoints: Pro primo, the dependence of the global thermohaline ocean circulation (THC) on the parameterization of the small-scale vertical mixing processes in the interior of the ocean, and, pro secundo, the dynamics of the circulation in the Nordic Seas. The THC is found be crucially dependent on the parameterization of the small-scale vertical mixing, two types of which have been compared: The commonly used constant diffusivity and a, physically more plausible, stability-dependent parameterization. For constant diffusivity the circulation weakens when the equator-to-pole surface density difference is decreased, consonant with commonly held prejudices. However, for stability-dependent diffusivity the circulation is enhanced. This conclusion has been reached using two investigative techniques, viz. a scale analysis as well as a numerical zonally-averaged and equatorially symmetric THC model. However, if asymmetric flows are considered, the dynamics become more complex to interpret. It has, nevertheless, been concluded that when the degree of asymmetry of the surface-density distribution is taken to be fixed, the response of the circulation to changes of the surface-density distribution corresponds to that from the symmetric investigation.

The studies of the Nordic Seas are mainly based on satellite-altimetric data providing Sea-Level Anomalies (SLAs). These are utilized to estimate the seasonal cycle as well as the inter-annual variability of the depth-integrated flows. The seasonal cycle is examined using the winter-to-summer difference of the barotropic flow, with focus on the entire region as well as on two sections extending from a common point in the central Norwegian Sea to Svinøy on the Norwegian coast and to the Faroe Islands, respectively. The total barotropic transport is estimated to be around 10 Sv larger during winter than in summer, of which 8 Sv are associated with the barotropic re-circulation gyre in the interior of the Norwegian Sea, the remainder being linked to the Atlantic inflow across the Iceland-Scotland Ridge. The inter-annual variability of the circulation in the Nordic Seas is investigated on the basis of a theoretical analysis permitting independent calculation of the barotropic flow along closed isobaths using SLA data as well as wind data. The barotropic flow based on SLA data is found to co-vary with the flow estimated using wind data.

Place, publisher, year, edition, pages
Stockholm: Meteorologiska institutionen (MISU), 2005. 23 p.
Keyword
Oceanography, Thermohaline circulation, Vertical mixing, Altimetry, Nordic Seas
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-473 (URN)91-7155-047-X (ISBN)
Public defence
2005-05-20, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 10:00
Opponent
Supervisors
Available from: 2005-04-22 Created: 2005-04-22Bibliographically approved

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Tellus. Series A, Dynamic meteorology and oceanography
Oceanography, Hydrology, Water Resources

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