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Freshwater processes and water mass transformation in the Arctic Ocean
Stockholm University, Faculty of Science, Department of Meteorology .
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis explores freshwater-related processes and water mass transformation in the Arctic Ocean. Knowledge of these processes is important from both a local and a global perspective. Globally, because the export of cold and low saline water and sea ice might influence the North Atlantic and global meridional overturning circulation. Locally, because freshwater processes affect the vertical stratification and permit favorable conditions for the ice cover.

Models of different complexity are the main tools of the present work. A part of the material considers how these models can be used to examine the key processes governing freshwater balance. Additionally, the freshwater budgets amongst 10 different ocean general circulation models (OGCMs) are compared and robust features and weaknesses identified.

A large part considers the freshwater processes governing the stratification with an emphasis on the low saline upper parts. The interactions between freshwater sources and sinks are studied in an OGCM using passive tracers. It is found that the composition, pathways and shelf-basin exchange of low saline water primarily involve processes linked to Siberian runoff, Pacific water and sea-ice melting and formation. Motivated by observed changes and paleorecords the sensitivity of the stratification is further explored in freshwater perturbation experiments with an OGCM. The response yields a deeper halocline for decreasing freshwater input, in line with a theoretical model.

The final part focuses on a new framework for analyzing water mass transformations. In the framework volume, heat and salt budgets are computed in salinity-temperature space. Using different OGCMs it is shown how surface and interior processes transform inflowing waters towards colder and fresher waters and how the halocline renewal rate can be estimated. Limiting cases for the water mass transformation balance are identified by separating contributions from surface, internal and boundary fluxes.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2014. , 44 p.
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-107594ISBN: 978-91-7447-998-0 (print)OAI: oai:DiVA.org:su-107594DiVA: diva2:751037
Public defence
2014-11-05, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, 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: 2014-10-14 Created: 2014-09-22 Last updated: 2017-03-03Bibliographically approved
List of papers
1. Arctic Ocean freshwater: How robust are model simulations?
Open this publication in new window or tab >>Arctic Ocean freshwater: How robust are model simulations?
Show others...
2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, C00D16- p.Article in journal (Refereed) Published
Abstract [en]

The Arctic freshwater (FW) has been the focus of many modeling studies, due to the potential impact of Arctic FW on the deep water formation in the North Atlantic. A comparison of the hindcasts from ten ocean-sea ice models shows that the simulation of the Arctic FW budget is quite different in the investigated models. While they agree on the general sink and source terms of the Arctic FW budget, the long-term means as well as the variability of the FW export vary among models. The best model-to-model agreement is found for the interannual and seasonal variability of the solid FW export and the solid FW storage, which also agree well with observations. For the interannual and seasonal variability of the liquid FW export, the agreement among models is better for the Canadian Arctic Archipelago (CAA) than for Fram Strait. The reason for this is that models are more consistent in simulating volume flux anomalies than salinity anomalies and volume-flux anomalies dominate the liquid FW export variability in the CAA but not in Fram Strait. The seasonal cycle of the liquid FW export generally shows a better agreement among models than the interannual variability, and compared to observations the models capture the seasonality of the liquid FW export rather well. In order to improve future simulations of the Arctic FW budget, the simulation of the salinity field needs to be improved, so that model results on the variability of the liquid FW export and storage become more robust. Citation: Jahn, A., et al. (2012), Arctic Ocean freshwater: How robust are model simulations?, J. Geophys. Res., 117, C00D16, doi: 10.1029/2012JC007907.

National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-88749 (URN)10.1029/2012JC007907 (DOI)000315347800001 ()
Funder
FormasEU, FP7, Seventh Framework Programme, GA212643
Note

AuthorCount:15;

Available from: 2013-03-26 Created: 2013-03-26 Last updated: 2017-12-06Bibliographically approved
2. Arctic Ocean freshwater composition, pathways and transformations from a passive tracer simulation
Open this publication in new window or tab >>Arctic Ocean freshwater composition, pathways and transformations from a passive tracer simulation
2014 (English)In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, 23988Article in journal (Refereed) Published
Abstract [en]

Freshwater (FW) induced transformations in the upper Arctic Ocean were studied using a coupled regional sea ice-ocean model driven by winds and thermodynamic forcing from a reanalysis of data during the period 1948-2011, focusing on the mean state during 1968-2011. Using passive tracers to mark a number of FW sources and sinks, their mean composition, pathways and export were examined. The distribution of the simulated FW height reproduced the known features of the Arctic Ocean and volume-integrated FW content matched climatological estimates reasonably well. Input from Eurasian rivers and extraction by sea-ice formation dominate the composition of the Arctic FW content whilst Pacific water increases in importance in the Canadian Basin. Though pathways generally agreed with previous studies the locus of the Eurasian runoff shelf-basin transport centred at the Alpha-Mendeleyev ridge, shifting the Pacific-Atlantic front eastwards. A strong coupling between tracers representing Eurasian runoff and sea-ice formation showed how water modified on the shelf spreads across the Arctic and mainly exits through the Fram Strait. Transformation to salinity dependent coordinates showed how Atlantic water is modified by both low-salinity shelf and Pacific waters in an estuary-like overturning producing water masses of intermediate salinity that are exported to the Nordic Seas. A total halocline renewal rate of 1.0 Sv, including both shelf-basin exchange and cross-isohaline flux, was estimated from the transports: both components were of equal magnitude. The model’s halocline shelf-basin exchange is dominated by runoff and sea-ice processes at the western shelves (the Barents and Kara seas) and Pacific water at the eastern shelves (the Laptev, East Siberian and Chukchi seas).

Keyword
Arctic Ocean, freshwater, pathways, water mass transformation, passive tracers
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-107528 (URN)10.3402/tellusa.v66.23988 (DOI)
Funder
Swedish Research Council Formas, 214-2009-389
Available from: 2014-09-19 Created: 2014-09-18 Last updated: 2017-12-05Bibliographically approved
3. Arctic Ocean Water Mass Transformation in S-T Coordinates
Open this publication in new window or tab >>Arctic Ocean Water Mass Transformation in S-T Coordinates
2015 (English)In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 45, no 4, 1025-1050 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, watermass transformations in the Arctic Ocean are studied using a recently developed salinity-temperature (S-T) framework. The framework allows the water mass transformations to be succinctly quantified by computing the surface and internal diffusive fluxes in S-T coordinates. This study shows how the method can be applied to a specific oceanic region, in this case the Arctic Ocean, by including the advective exchange of water masses across the boundaries of the region. Based on a simulation with a global ocean circulation model, the authors examine the importance of various parameterized mixing processes and surface fluxes for the transformation of water across isohaline and isothermal surfaces in the ArcticOcean. The model-based results reveal a broadly realistic Arctic Ocean where the inflowing Atlantic and Pacific waters are primarily cooled and freshened before exiting back to the North Atlantic. In the model, the water mass transformation in the T direction is primarily accomplished by the surface heat flux. However, the surface freshwater flux plays a minor role in the transformation of water toward lower salinities, which is mainly driven by a downgradient mixing of salt in the interior ocean. Near the freezing line, the seasonal melt and growth of sea ice influences the transformation pattern.

National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-117335 (URN)10.1175/JPO-D-14-0197.1 (DOI)000352542800006 ()
Note

AuthorCount:4;

Available from: 2015-05-18 Created: 2015-05-18 Last updated: 2017-12-04Bibliographically approved
4. The response of the central Arctic Ocean stratification to freshwater perturbations
Open this publication in new window or tab >>The response of the central Arctic Ocean stratification to freshwater perturbations
2016 (English)In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 121, no 1, 792-817 p.Article in journal (Refereed) Published
Abstract [en]

Using a state-of-the-art coupled ice-ocean-circulation model, we perform a number of sensitivity experiments to examine how the central Arctic Ocean stratification responds to changes in river runoff and precipitation. The simulations yield marked changes in the cold halocline and the Arctic Atlantic layer. Increased precipitation yields a warming of the Atlantic layer, which primarily is an advective signal, propagated through the St. Anna Trough, reflecting air-sea heat flux changes over the Barents Sea. As the freshwater supply is increased, the anticyclonic Beaufort Gyre is weakened and a greater proportion of the Arctic Ocean freshwater is exported via the Fram Strait, with nearly compensating export decreases through the Canadian Arctic Archipelago. The corresponding reorganization of the freshwater pool appears to be controlled by advective processes, rather than by the local changes in the surface freshwater flux. A simple conceptual model of the Arctic Ocean, based on a geostrophically controlled discharge of the low-salinity water, is introduced and compared with the simulations. Key predictions of the conceptual model are that the halocline depth should decrease with increasing freshwater input and that the Arctic Ocean freshwater storage should increase proportionally to the square root of the freshwater input, which are in broad qualitative agreement with the sensitivity experiments. However, the model-simulated rate of increase of the freshwater storage is weaker, indicating that effects related to wind forcing and rerouting of the freshwater-transport pathways play an important role for the dynamics of the Arctic Ocean freshwater storage.

Keyword
Arctic Ocean, halocline, freshwater, conceptual model, ocean model, Atlantic water
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-128572 (URN)10.1002/2015JC011003 (DOI)000371432200046 ()
Available from: 2016-06-10 Created: 2016-03-30 Last updated: 2017-11-28Bibliographically approved

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