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Atlantic Water in the Nordic Seas: A satellite altimetry perspective on ocean circulation
Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University.
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Atlantic Water in the Nordic Seas contributes to the mild climate of Northern Europe and is the main oceanic source of heat for the Arctic. The northward bound transport of the warm and saline Atlantic Water is mediated by a topographically constrained cyclonic boundary current along the Norwegian continental slope. The analysis within this thesis is based on satellite observations of dynamic Sea Surface Heights (SSH) from 1993 to the recent present, combined with both hydrographic observations and modelling. It provides some new perspectives and results, as well as corroborates the essential role of bottom topography for the circulation in the Nordic Seas.

In the first part of the thesis, the topographic constraint is used in the analysis by examining the satellite-derived SSH along topographic contours. We find stationary along-contour anomalies that indicate deviations from strict topographic steering. However, we show that these deviations are dynamically consistent with, and can be explained by, potential vorticity conservation in an adiabatic steady-state model for flow over a topographic slope. The analysis along topographic contours is further developed to study northward-propagating, low-frequency ocean temperature signals. These signals have an expression in the SSH and their propagation speed is remarkably slow compared to the current speed. We propose a conceptual model of shear dispersion effects, in which the effective advection speed of a tracer is determined not only by the rapid current core, but by a mean velocity taken over the cross-flow extent of Atlantic Water. The model predicts a reduced effective tracer advection velocity, comparable to the one observed.

The close connection between anomalies in SSH and heat content is further used to study decadal variability in the Nordic Seas. There is a shift in decadal trends in the mid-2000s, from a period of strong increase in SSH and heat content to a more stagnant period. We find this variability to be forced remotely, rather than by local air-sea heat fluxes. By developing a conceptual model of ocean heat convergence, we are able to explain the broad features of the decadal changes with the temperature variability of the inflowing Atlantic Water from the subpolar North Atlantic.

In the final part of the thesis, satellite-derived surface geostrophic velocity fields are used as input to a Lagrangian trajectory model. Based on this, we study the fractionation of the Atlantic Water in the Nordic Seas between the two straits towards the Arctic Ocean: the Barents Sea Opening and the Fram Strait. This Lagrangian approach also provides insights on the origin of the water that reach the straits. We find that it is the frontal current branch, rather than the slope current, that contributes to the variability of the Barents Sea Opening inflow of warm Atlantic Water, and thus potentially to the climate of the Barents Sea and its sea ice cover.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2020. , p. 34
Keywords [en]
Arctic Ocean climate, ocean heat transport, sea surface height, topographic control, Lagrangian trajectories
National Category
Climate Research Oceanography, Hydrology and Water Resources Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-176273ISBN: 978-91-7797-939-5 (print)ISBN: 978-91-7797-940-1 (electronic)OAI: oai:DiVA.org:su-176273DiVA, id: diva2:1374032
Public defence
2020-01-31, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, 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: Manuscript.

Available from: 2020-01-08 Created: 2019-11-28 Last updated: 2019-12-18Bibliographically approved
List of papers
1. Stationary Sea Surface Height Anomalies in Cyclonic Boundary Currents: Conservation of Potential Vorticity and Deviations from Strict Topographic Steering
Open this publication in new window or tab >>Stationary Sea Surface Height Anomalies in Cyclonic Boundary Currents: Conservation of Potential Vorticity and Deviations from Strict Topographic Steering
2016 (English)In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 46, no 8, p. 2437-2456Article in journal (Refereed) Published
Abstract [en]

In high-latitude subpolar seas, such as the Nordic seas and the Labrador Sea, time-mean geostrophic currents mediate the bulk of the meridional oceanic heat transport. These currents are primarily encountered along the continental slopes as intense cyclonic boundary currents, which, because of the relatively weak stratification, should be strongly steered by the bottom topography. However, analyses of hydrographic and satellite altimetric data along depth contours in Nordic seas boundary currents reveal some remarkable, stationary, along-stream variations in the depth-integrated buoyancy and bottom pressure. A closer examination shows that these variations are linked to changes in steepness and curvature of the continental slope. To examine the underlying dynamics, a steady-state model of a cyclonic stratified boundary current over a topographic slope is developed in the limit of small Rossby numbers. Based on potential vorticity conservation, equations for the zeroth-and first-order pressure and buoyancy fields are derived. To the lowest order, the flow is completely aligned with the bottom topography. However, the first-order results show that where the lowest-order flow increases (decreases) its relative vorticity along a depth contour, the first-order pressure and depthintegrated buoyancy increase (decrease). This response is associated with cross-isobath flows, which induce stretching/compression of fluid elements that compensates for the changes in relative vorticity. The model-predicted along-isobath variations in pressure and depth-integrated buoyancy are comparable in magnitude to the ones found in the observational data from the Nordics Seas.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-134445 (URN)10.1175/JPO-D-15-0219.1 (DOI)000380799200011 ()
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2019-12-12Bibliographically approved
2. Shear dispersion and delayed propagation of temperature anomalies along the Norwegian Atlantic Slope Current
Open this publication in new window or tab >>Shear dispersion and delayed propagation of temperature anomalies along the Norwegian Atlantic Slope Current
2018 (English)In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 70, article id 1453215Article in journal (Refereed) Published
Abstract [en]

Using satellite altimetric sea surface height (ADT) data, we search for propagation of hydrographic anomalies along the Norwegian Atlantic Slope Current (NwASC) from the SvinOy section in the south to the Fram Strait in the north. Our analyses indicate that ADT anomalies, related to low-frequency temperature variations, propagate downstream with speeds of about 2cm s. Notably, this speed is nearly an order of magnitude slower than the speed of the NwASC, which in agreement with previously estimated propagation speeds of hydrographic anomalies along the flow. A conceptual tracer advection model, consisting of a thin current core interacting with an adjacent slow moving reservoir, is introduced to examine temperature anomaly propagation along the NwASC. It is shown that shear dispersion effects, resulting from cross-stream eddy mixing and velocity shear, can qualitatively explain the observed delayed propagation of hydrographic anomalies: low-frequency temperature anomalies move downstream with an effective velocity that corresponds to a mean velocity across the entire Atlantic Water layer, rather than the speed of Norwegian Atlantic Slope Current.

Keywords
Nordic Seas, propagation ocean heat anomalies towards the Arctic Ocean, satellite altimetry, shear dispersion and delayed propagation
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-156072 (URN)10.1080/16000870.2018.1453215 (DOI)000428839900001 ()
Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2019-12-12Bibliographically approved
3. Mechanisms of the time-varying sea surface height and heat content trends in the eastern Nordic Seas
Open this publication in new window or tab >>Mechanisms of the time-varying sea surface height and heat content trends in the eastern Nordic Seas
2019 (English)In: Ocean Science Discussions (OSD), ISSN 1812-0806, E-ISSN 1812-0822Article in journal (Other academic) Submitted
Abstract [en]

The Nordic Seas is the main ocean conveyor of heat between the North Atlantic Ocean and the Arctic Ocean. Although the decadal variability of the Subpolar North Atlantic has been given significant attention lately, especially regarding the cooling trend since mid-2000s, less is known about the potential connection downstream in the northern basins. Using sea surface heights from satellite altimetry over the past 25 years (1993–2017), we find significant variability on multiyear-to-decadal time scales in the Nordic Seas. In particular, the regional trends in sea surface height show signs of a slowdown since mid-2000s as compared to the rapid increase in the preceding decade since early 1990s. This change is most prominent in the Atlantic origin waters in the eastern Nordic Seas and is closely linked, as estimated from hydrography, to heat content. Furthermore, we formulate a simple heat budget for the eastern Nordic Seas to discuss the relative importance of local and remote sources of variability; advection of temperature anomalies in the Atlantic inflow is found to be the main mechanism. A conceptual model of ocean heat convergence, with only upstream temperature measurements at the inflow to the Nordic Seas as input, is able to reproduce key aspects of the decadal variability of the Nordic Seas' heat content. Based on these results, we argue that there is a strong connection with the upstream Subpolar North Atlantic. However, although the shift in trends in the mid-2000s is coincident in the Nordic Seas and the Subpolar North Atlantic, the eastern Nordic Seas has not seen a reversal of trends but instead maintain elevated sea surface heights and heat content in the recent decade considered here.

Keywords
Sea Surface Height, Variability, Ocean heat content
National Category
Oceanography, Hydrology and Water Resources Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-176079 (URN)10.5194/os-2019-109 (DOI)
Available from: 2019-11-20 Created: 2019-11-20 Last updated: 2020-01-03
4. A Satellite-based Lagrangian Perspective on Atlantic Water Fractionation between Arctic Gateways
Open this publication in new window or tab >>A Satellite-based Lagrangian Perspective on Atlantic Water Fractionation between Arctic Gateways
(English)Manuscript (preprint) (Other academic)
National Category
Oceanography, Hydrology and Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-176040 (URN)
Available from: 2019-11-18 Created: 2019-11-18 Last updated: 2019-12-16Bibliographically approved

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