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Dynamics and Variability of the Circulation in the North-Atlantic Subpolar Seas
Stockholm University, Faculty of Science, Department of Meteorology .
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with the dynamics and circulation in the northern North Atlantic and the Nordic Seas, processes of crucial importance for the mild climate of Scandinavia and Northern Europe. High-resolution ADCP scans of currents from Greenland to Scotland in the top 400 m demonstrate that the Reykjanes Ridge is a very effective separator of flow towards the Nordic and Labrador Seas, respectively. It was found that the meridional overturning circulation has weakened by ~1.7 Sv (1 Sv = 106 m3 s-1) during the 18-year period when altimetric data were available. This trend may be an effect of the Atlantic Multidecadal Oscillation, but is certainly not due to the North Atlantic Oscillation (NAO). By studying the circulation in the Faroe-Shetland Channel, which is an important choke point for the global thermohaline circulation, it was concluded that the contraction of the Norwegian-Sea gyre during low NAO periods plays an important role for disturbing the flow pattern. This specifically affects the regional ocean climate by leading to an accumulation of warm and saline Atlantic waters in the channel. During high NAO phases the circulation is strongly topographically controlled. The Norwegian Atlantic Slope Current (NwASC) is the main flow branch linking the North Atlantic to the Arctic and Barents Sea. It was found that the NwASC is largely coherent over seasonal to interannual time-scales. However, on shorter time-scales the coherency of the flow shows a sustained and pronounced weakening downstream of Lofoten. Intense eddy-shedding from the slope into the Lofoten Basin damps the coherent structure of the flow. The eddies take about two months to propagate to and to merge with the semi-permanent anticyclonic vortex above the deepest part of the Lofoten Basin. These results have implications for how flow/hydrographic anomalies are transferred through the Nordic Seas towards the Arctic. Anomalous transports of warm water into the Arctic and Barents Sea via the NwASC are found to be driven by a combination of the NAO and the other two leading modes of atmospheric variability in the North Atlantic. The results reported in the thesis may be of importance for achieving a correct representation of the heat conveyed polewards in climate models.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2014. , 38 p.
National Category
Meteorology and Atmospheric Sciences Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-101305ISBN: 978-91-7447-840-2 (print)OAI: oai:DiVA.org:su-101305DiVA: diva2:700617
Public defence
2014-04-11, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted. Paper 5: Manuscript.

Available from: 2014-03-20 Created: 2014-03-04 Last updated: 2014-03-11Bibliographically approved
List of papers
1. On the spatial structure and temporal variability of poleward transport between Scotland and Grennland:  
Open this publication in new window or tab >>On the spatial structure and temporal variability of poleward transport between Scotland and Grennland:  
2014 (English)In: Journal of Geophysical Research: Oceans, ISSN 2169-9291, Vol. 119, no 2, 824-841 p.Article in journal (Refereed) Published
Abstract [en]

[1] The flow north of warm subtropical water though the northeastern Atlantic is known to have many pathways that vary over time. Here we use a combination of upper ocean current measurements between Greenland and Scotland near 60°N and satellite altimetry to examine the space-time variability of poleward transport. The high-resolution scans of currents in the top 400 m show that the Reykjanes Ridge serves as a very effective separator of flow toward the Nordic and Labrador Seas, respectively. Whereas the Labrador Sea branch exhibits two mean flows to the north on the western slope of the Reykjanes Ridge, the eastern branch flows north in roughly equal amounts over the deep Maury channel and east of Hatton Bank including the Slope Current. There is also a well-defined southward flow along the eastern slope of the Reykjanes Ridge. The satellite altimetric sea surface height (SSH) data show good overall agreement with geostrophically determined -level difference from the repeat ADCP sections (1999–2002), but are unable to resolve the fine structure of the topographically defined mean circulation. The altimetric data show that variations in poleward flow west and east of the Reykjanes Ridge are strongly anticorrelated. They further reveal that the two eastern subbranches also exhibit anticorrelated variability, but offset in time with respect to the Labrador Sea branch. Remarkably, all these variations cancel out for the entire Greenland-Scotland section leaving a gradual decrease in sea-level difference of about 0.06 m over the 1993 to the end of 2010 observation period.

Keyword
Poleward transport, ADCP, Satellite altimetry, Topographic control, inter-annual variability, MOC
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-101309 (URN)10.1002/2013JC009287 (DOI)000336261200012 ()
Available from: 2014-03-04 Created: 2014-03-04 Last updated: 2014-07-07Bibliographically approved
2. The Response of the Circulation in the Faroe-Shetland Channel to the North Atlantic Oscillation
Open this publication in new window or tab >>The Response of the Circulation in the Faroe-Shetland Channel to the North Atlantic Oscillation
2012 (English)In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 64, 18423- p.Article in journal (Refereed) Published
Abstract [en]

This study, based on satellite-derived sea-surface heights and temperatures as well as hydrographic data, attempts to shed some light on the role of the extreme phases of the North Atlantic Oscillation (NAO) for the local dynamics of the Faroe-Shetland Channel (FSC). During the low-NAO event 2009-10 the Shetland-slope current showed a significant deflection from its usual path above the maximal gradient of the bathymetry, ultimately resulting in an anticyclone. This led to an accumulation of North Atlantic Water (NAW) over the deeper parts of the channel, manifested as a pronounced deepening of the halocline. Leading this deflection of the slope current by around 2 weeks, a cyclonic eddy associated with a doming of the halocline and originating from north of the Faroes (and hence constituted by Modified North Atlantic Waters) had moved southwards in the channel, coming to rest at its southern entrance. Assessing the influence of the NAO on these regional dynamics using 1992-2010 altimetric data, it was found that for positive phases of the NAO, the surface circulation tended to be strongly bathymetrically constrained and thus resembles the mean regional circulation. The negative phases of the NAO are associated with a regional weakening of the wind-stress curl, which leads to a contraction of the Norwegian-Sea gyre and a linked northward migration of the FSC recirculation involving a deflected path of the Shetland-slope current. This change in the circulation under negative NAO conditions may have an impact on the regional ocean climate through the accumulation of saline NAW in the channel.

Keyword
satellite altimetry, slope current, North Atlantic Oscillation, mesoscale dynamics, Norwegian-Sea gyre, topographic control
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-82082 (URN)10.3402/tellusa.v64i0.18423 (DOI)000310834000001 ()
Available from: 2012-11-07 Created: 2012-11-07 Last updated: 2017-12-07Bibliographically approved
3. Transport Coherency and Eddy Activity Along the Norwegian Atlantic Slope Current
Open this publication in new window or tab >>Transport Coherency and Eddy Activity Along the Norwegian Atlantic Slope Current
(English)In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485Article in journal (Refereed) Submitted
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-101306 (URN)
Available from: 2014-03-04 Created: 2014-03-04 Last updated: 2017-12-05
4. The Lofoten Vortex of the Nordic Seas
Open this publication in new window or tab >>The Lofoten Vortex of the Nordic Seas
(English)Article in journal (Refereed) Submitted
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-101307 (URN)
Available from: 2014-03-04 Created: 2014-03-04 Last updated: 2014-03-06
5. Atmospheric circulation patterns control the variability of the oceanic transport towards the Arctic region
Open this publication in new window or tab >>Atmospheric circulation patterns control the variability of the oceanic transport towards the Arctic region
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Meteorology and Atmospheric Sciences Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-101308 (URN)
Available from: 2014-03-04 Created: 2014-03-04 Last updated: 2014-03-06

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