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The role of high-latitude circulation and moisture transport in Arctic climate variability and change during winter
Stockholm University, Faculty of Science, Department of Meteorology . (Climate Dynamics)
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis examines the connections between atmospheric circulation in the high-latitudes, northward moisture transport, and Arctic climate variability and change during winter. An event based approach is taken by objectively defining phenomena termed “moisture intrusions” -- filamentary flows of anomalously moist air which originate at 70°N and cross the entire Arctic basin. They typically emanate from within the poleward advecting branches of mid-latitude cyclones held in place by blocking patterns to the east. Moisture intrusions contribute only a minority of the total northward moisture transport at 70°N, yet drive a significant proportion of the inter annual variability in surface temperature and downward longwave radiation over the entire polar cap. A positive trend in the frequency of these events, in response to a moistening of the atmosphere, is shown to have driven approximately 45% and 35% of the observed warming and sea ice decline in the Barents Sea during Dec-Jan over the past two decades. Moisture intrusions act to erode the temperature inversion and thus contribute to bottom amplified warming even in the absence of sea ice loss. Negative sea ice anomalies induced by intrusions persist for up to weeks at a time -- promoting upward turbulent heat fluxes and further bottom amplified warming. Systematic biases in the statistics of moisture intrusions are discovered in the CMIP5 models. The biases are predominantly a result of misrepresentation of the intense moisture fluxes and are almost entirely due to biases in the meridional velocity. Moisture intrusion biases explain only about 17% of the temperature bias in the Atlantic sector. The predicted biases, while small in amplitude, are very highly correlated with the true biases in the models however, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback. A analysis of the uncertainties in computed turbulent air-sea flux (TASF) climatologies arising due to the parameterisation of bulk formulae is also presented. TASF climatologies are computed over a series of sensitivity experiments using surface state variables from ERA-Interim. The largest source of uncertainty is related to the computation of the transfer coefficients and hence the choice of bulk algorithm itself. The majority of parameter approximations have small impacts when tested individually, but can lead to large disagreements when implemented in tandem.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2016. , 38 p.
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-134787ISBN: 978-91-7649-575-9ISBN: 978-91-7649-576-6OAI: oai:DiVA.org:su-134787DiVA: diva2:1038517
Public defence
2016-12-02, Högbomsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2016-11-09 Created: 2016-10-18 Last updated: 2016-11-16Bibliographically approved
List of papers
1. Large-scale circulation associated with moisture intrusions into the Arctic during winter
Open this publication in new window or tab >>Large-scale circulation associated with moisture intrusions into the Arctic during winter
2013 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 17, 4717-4721 p.Article in journal (Refereed) Published
Abstract [en]

We examine the poleward transport of water vapor across 70 degrees N during boreal winter in the ERA-Interim reanalysis product, focusing on intense moisture intrusion events. We analyze the large-scale circulation patterns associated with these intrusions and the impacts they have at the surface. A total of 298 events are identified between 1990 and 2010, an average of 14 per season, accounting for 28% of the total poleward transport of moisture across 70 degrees N. They are concentrated over the main ocean basins at that latitude in the Labrador Sea, North Atlantic, Barents/Kara Sea, and Pacific. Composites of sea level pressure and potential temperature on the 2 potential vorticity unit surface during intrusions show a large-scale blocking pattern to the east of each basin, deflecting midlatitude cyclones and their associated moisture poleward. The interannual variability of intrusions is strongly correlated with variability in winter-mean surface downward longwave radiation and skin temperature averaged over the Arctic.

Keyword
moisture, transport, Arctic, intrusions, circulation, temperature
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-96111 (URN)10.1002/grl.50912 (DOI)000325490300046 ()
Note

AuthorCount:3;

Available from: 2013-11-13 Created: 2013-11-11 Last updated: 2016-10-24Bibliographically approved
2. The Role of Moist Intrusions in Winter Arctic Warming and Sea Ice Decline
Open this publication in new window or tab >>The Role of Moist Intrusions in Winter Arctic Warming and Sea Ice Decline
2016 (English)In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, 4473-4485 p.Article in journal (Refereed) Published
Abstract [en]

This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a “cold clear” state with a strong inversion to a “warm opaque” state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 70°N during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades.

Keyword
Circulation/Dynamics, Energy transport, Physical Meteorology and Climatology, Climate change, Energy budget/balance, Ice loss/growth, Ice thickness, Moisture/moisture budget
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-134774 (URN)10.1175/JCLI-D-15-0773.1 (DOI)
Funder
Swedish e‐Science Research CenterSwedish Research Council
Available from: 2016-10-18 Created: 2016-10-18 Last updated: 2016-10-24Bibliographically approved
3. Representation of Arctic moist intrusions in CMIP5 models and implications for winter climate biases
Open this publication in new window or tab >>Representation of Arctic moist intrusions in CMIP5 models and implications for winter climate biases
(English)Manuscript (preprint) (Other academic)
Keyword
Arctic, general circulation models, CMIP5, biases, moisture transport
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-134780 (URN)
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Available from: 2016-10-18 Created: 2016-10-18 Last updated: 2016-10-24Bibliographically approved
4. Climatologically significant effects of some approximations in the bulk parameterizations of turbulent air-sea fluxes
Open this publication in new window or tab >>Climatologically significant effects of some approximations in the bulk parameterizations of turbulent air-sea fluxes
2016 (English)In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485Article in journal (Refereed) Epub ahead of print
Abstract [en]

This paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulae on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments are examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the 1982-2014 period are computed using SST and surface meteorological state variables from the ERA-Interim reanalysis. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulae. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically of the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%), but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the salt-related reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity, lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid- and subpolar latitudes.

Keyword
Turbulent fluxes, parameterisation, general circulation model, error, climatology
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-134784 (URN)10.1175/JPO-D-16-0169.1 (DOI)
Available from: 2016-10-18 Created: 2016-10-18 Last updated: 2016-10-24

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