Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
On the Arctic Seasonal Cycle
Stockholm University, Faculty of Science, Department of Meteorology .
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The seasonal cycle of snow and sea ice is a fundamental feature of the Arctic climate system. In the Northern Hemisphere, about 55 million km2 of sea ice and snow undergo complete melt and freeze processes every year. Because snow and sea ice are much brighter (higher albedo) than the underlying surface, their presence reduces absorption of incoming solar energy at high latitudes. Therefore, changes of the sea-ice and snow cover have a large impact on the Arctic climate and possibly at lower latitudes. One of the most important determining factors of the seasonal snow and sea-ice cover is the timing of the seasonal melt-freeze transitions. Hence, in order to better understand Arctic climate variability, it is key to continuously monitor these transitions.

This thesis presents an algorithm for obtaining melt-freeze transitions using scatterometers over both the land and sea-ice domains. These satellite-borne instruments emit radiation at microwave wavelengths and measure the returned signal. Several scatterometers are employed: QuikSCAT (1999–2009), ASCAT (2009–present), and OSCAT (2009–present). QuikSCAT and OSCAT operate at Ku-band (λ=2.2 cm) and ASCAT at C-band (λ=5.7 cm), resulting in slightly different surface interactions. This thesis discusses these dissimilarities over the Arctic sea-ice domain, and juxtaposes the time series of seasonal melt-freeze transitions from the three scatterometers and compares them with other, independent datasets.

The interactions of snow and sea ice with other components of the Arctic climate system are complex. Models are commonly employed to disentangle these interactions. But this hinges upon robust and well-formulated models, reached by perpetual testing against observations. This thesis also presents an evaluation of how well eleven state-of-the-art global climate models reproduce the Arctic sea-ice cover and the summer length—given by the melt-freeze transitions—using surface observations of air temperature.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2014. , 38 p.
Keyword [en]
Arctic climate, Seasonal melt-freeze transitions, Arctic sea ice and snow, Active microwave measurements, Climate model evaluation
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-100008ISBN: 978-91-7447-846-4 (print)OAI: oai:DiVA.org:su-100008DiVA: diva2:690478
Public defence
2014-02-28, 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 1: In press. Paper 4: Submitted.

Available from: 2014-02-06 Created: 2014-01-23 Last updated: 2014-01-28Bibliographically approved
List of papers
1. Evaluation of pan-Arctic melt-freeze onset in CMIP5 climate models and reanalyses using surface observations
Open this publication in new window or tab >>Evaluation of pan-Arctic melt-freeze onset in CMIP5 climate models and reanalyses using surface observations
2014 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 42, no 7-8, 2239-2257 p.Article in journal (Refereed) Published
Abstract [en]

The seasonal melt-freeze transitions are fun- damental features of the Arctic climate system. The representation of the pan-Arctic melt and freeze onset (north of 60°N) is assessed in two reanalyses and eleven CMIP5 global circulation models (GCMs). The seasonal melt-freeze transitions are retrieved from surface air temperature (SAT) across the land and sea-ice domains and evaluated against surface observations. While monthly averages of SAT are reasonably well represented in models, large model-observation and model–model disparities of timing of melt and freeze onset are evident. The evaluation against surface observations reveals that the ERA-Interim reanalysis performs the best, closely followed by some of the climate models. GCMs and reanalyses capture the seasonal melt-freeze transitions better in the central Arctic than in the marginal seas and across the land areas. The GCMs project that during the 21st century, the summer length—the period between melt and freeze onset—will increase over land by about 1 month at all latitudes, and over sea ice by 1 and 3 months at low and high latitudes, respectively. This larger summer-length increase over sea ice at pro- gressively higher latitudes is related to a retreat of summer sea ice during the 21st century, since open water freezes roughly 40 days later than ice-covered ocean. As a consequence, by the year 2100, the freeze onset is projected to be initiated within roughly 10 days across the whole Arctic Ocean, whereas this transition varies by about 80 days today.

Keyword
Arctic melt-freeze transitions, Climate model evaluation, Arctic climate change, CMIP5 future projections, Reanalysis, Arctic ocean
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-99927 (URN)10.1007/s00382-013-1811-z (DOI)000334068100033 ()
Available from: 2014-01-21 Created: 2014-01-21 Last updated: 2017-12-06Bibliographically approved
2. Mapping of seasonal freeze-thaw transitions across the pan-Arctic land and sea ice domains with satellite radar
Open this publication in new window or tab >>Mapping of seasonal freeze-thaw transitions across the pan-Arctic land and sea ice domains with satellite radar
Show others...
2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, C08004- p.Article in journal (Refereed) Published
Abstract [en]

To monitor the pan-Arctic seasonal freeze-thaw transitions of the land surface and sea ice, we analyze daily backscatter data from satellite scatterometry to examine the time series on an annual basis by applying an optimal edge detection scheme, and iterate against an internal median climatology to mitigate unreasonable outliers. By applying this novel algorithm to resolution-enhanced QuikSCAT data from 1999 to 2009, we have mapped a decade of seasonal freeze-thaw transitions across the landmass and sea ice north of 60 degrees N at a spatial resolution better than 5 km. The data set has been validated against surface air temperature measurements and snow depth obtained from a distributed network of weather stations and drift buoys. Most retrieved timings from surface and QuikSCAT measurements agree to less than a week at thaw transition for both land and sea ice and at freeze transition for sea ice, indicating successful retrieval over a range of surface covers. While the spatial pattern of freeze-thaw transition changes substantially from year to year, the interannual variability of the mean transition timing over a particular surface is small.

Keyword
active microwave, cryosphere, freeze-thaw, pan-Arctic, sea ice, terrestrial
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-81554 (URN)10.1029/2012JC008001 (DOI)000307468100009 ()
Note

AuthorCount:5;

Available from: 2012-10-30 Created: 2012-10-25 Last updated: 2017-12-07Bibliographically approved
3. Extending the QuikSCAT record of seasonal melt–freeze transitions over Arctic sea ice using ASCAT
Open this publication in new window or tab >>Extending the QuikSCAT record of seasonal melt–freeze transitions over Arctic sea ice using ASCAT
Show others...
2014 (English)In: Remote Sensing of Environment, ISSN 0034-4257, E-ISSN 1879-0704, Vol. 141, no 5, 214-230 p.Article in journal (Refereed) Published
Abstract [en]

The seasonal melt–freeze transitions are important to continuously monitor over Arctic sea ice in order to better understand Arctic climate variability. The Ku-band scatterometer QuikSCAT (13.4 GHz), widely used to retrieve pan-Arctic seasonal transitions, discontinued its decadal long record in 2009. In this study, we show that the C-band scatterometer ASCAT (5.3 GHz), in orbit since 2006 and with an anticipated lifetime through 2021, can be used to extend the QuikSCAT record of seasonal melt–freeze transitions. This is done by (1) comparing back- scatter measurements over multiyear and first-year ice, and by (2) retrieving seasonal transitions from resolution-enhanced ASCAT and QuikSCAT measurements and comparing the results with independent datasets. Despite operating in different frequencies, ASCAT and QuikSCAT respond similarly to surface transitions. However, QuikSCAT measurements respond slightly stronger to the early melt of first-year ice, making it less sensitive to sea-ice dynamics. To retrieve the transitions, we employed an improved edge-detector algorithm, which was iterated and constrained using sea-ice concentration data, efficiently alleviating unreasonable outliers. This gives melt–freeze transitions over all Arctic sea ice north of 60°N at a 4.45 km resolution during 1999–2009 and 2009–2012 for QuikSCAT and ASCAT, respectively. Using the sensor overlap period, we show that the retrieved transitions retrieved from the different instruments are largely consistent across all regions in the Arctic sea-ice domain, indicating a robust consistency.

Keyword
Active microwave measurements, Satelliteborne scatterometry, Arctic sea ice and snow, Surface processes, Melt–freeze retrieval, Arctic climate
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-99928 (URN)10.1016/j.rse.2013.11.004 (DOI)000331662600018 ()
Available from: 2014-01-21 Created: 2014-01-21 Last updated: 2017-12-06Bibliographically approved
4. OSCAT as a successor to QuikSCAT: a comparison over Arctic sea ice with emphasis on the seasonal melt-freeze transitions
Open this publication in new window or tab >>OSCAT as a successor to QuikSCAT: a comparison over Arctic sea ice with emphasis on the seasonal melt-freeze transitions
Show others...
(English)In: Annals of Glaciology, ISSN 0260-3055, E-ISSN 1727-5644Article in journal (Refereed) Submitted
Abstract [en]

It is important to continuously monitor the seasonal melt-freeze transitions because of their influence on the Arctic climate system. The Ku-band scatterometer QuikSCAT was widely used to retrieve the seasonal transitions before its antenna failed in November 2009. In this study, we show that OSCAT, a Ku-band scatterometer launched in September 2009, can serve as a successor to QuikSCAT over Arctic sea ice. This is done by (1) comparing backscatter measurements acquired in overlapping time periods, and by (2) retrieving the seasonal melt-freeze transitions over Arctic sea ice from resolution-enhanced QuikSCAT and OSCAT data and comparing the results, also with other datasets. The main difference between the instruments, in terms of backscatter measurements, is the incidence angle in which backscatter is acquired, which yields backscatter discrepancies of 1-2 dB. This discrepancy is significantly smaller than the response of both sensors to surface melting and freezing processes. An edge-detection algorithm is employed that retrieves seasonal transitions from QuikSCAT (1999-2009) and OSCAT (2011-2013) at a 4.45-km spatial resolution. A comparison with transitions retrieved from the C-band scatterometer ASCAT (2009-2013) and from passive microwave radiometers (1999-2013) indicates that the transitions from scatterometers are largely consistent across all regions in the Arctic sea-ice domain.

Keyword
surface melt, remote sensing, sea-ice dynamics, sea-ice growth and decay, snow/ice surface processes
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-99929 (URN)
Available from: 2014-01-21 Created: 2014-01-21 Last updated: 2017-12-06Bibliographically approved

Open Access in DiVA

fulltext(11956 kB)5604 downloads
File information
File name FULLTEXT01.pdfFile size 11956 kBChecksum SHA-512
bf90e8876ef081615fc01cd768e4ef6dca63d51283f4e5fcfda49af8a1478c2ed293ea5f76d291ea4ebd17d35b3f8d03fc0f964e91c87794bd3cfc5e4c3ae801
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Mortin, Jonas
By organisation
Department of Meteorology
Climate Research

Search outside of DiVA

GoogleGoogle Scholar
Total: 5604 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 313 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf