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Atmospheric & Oceanic Applications of Eulerian and Lagrangian Transport Modelling
Stockholm University, Faculty of Science, Department of Meteorology .
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents several ways to understand transports of air and water masses in the atmosphere and ocean, and the transports of energy that they imply. It presents work using various kinds of observations as well as computer simulations of the atmosphere and oceans. One of the main focuses is to identify similarities and differences between models and observations, as well as between different models.

The first half of the thesis applies Lagrangian methods to study flows in the atmosphere and oceans. Part of the work focuses on understanding how particles follow the currents in the Baltic Sea and how they disperse. It is suggested that the commonly used regional ocean model for the Baltic Sea, RCO, underestimates the transport and the dispersion of the particles, which can have consequences for studies of e.g. biogeochemistry as well as for operational use. A similar methodology is used to study how particles are transported between the tropics and mid-latitudes by the large-scale atmospheric circulation. It is found that the mass transport associated with northbound and southbound particles can cancel in the zonally averaged circulation, and we propose that the degree of cancellation depends on the method of averaging.

The latter half of the thesis focuses on Eulerian stream functions and specifically a thermodynamic stream function that combines the zonal and meridional circulations of the atmosphere into a single circulation. The results are used to study the inter-annual variability of the intensity and thermodynamic properties of the global atmospheric circulation. A significant correlation to ENSO variability is found both in reanalysis and the EC-Earth coupled climate model. It is also shown that a set of models from the CMIP5 project show a slowdown of the atmospheric circulation as a result of global warming and associated changes in near-surface moisture content and upper-level radiative cooling.

Abstract [sv]

Denna avhandling presenterar olika metoder för att studera datormodeller av atmosfä- ren, haven, och klimatsystemet. Metoderna använder såväl Lagrangeska synsätt dvs att betrakta atmosfären eller haven som individuella partiklar i rörelse, som Eulerska synsätt där atmosfären och haven ses som gas eller vätska i rörelse. I artikel 1 sjö- sätts ett antal “surface drifters” i Östersjön som driver fritt med havsströmmarna och vars hastighet mäts av satelliter. Genom att modellera Lagrangeska partiklars rörelser i Östersjön och jämföra med dessa “surface drifters” kan det visas att datormodeller kan underskatta både medelhastigheten av partiklarna samt deras utbredning. I ar- tikel 2 simuleras luftmassornas rörelser mellan tropikerna och mellanbreddgraderna (∼ 45◦N/S). Ett medelvärde över all longituder tenderar att ignorera betydande mass- och energitransporter mellan tropikerna och mellanbredderna, och dessa kvantifieras i detalj i artikel 2.

Artiklarna 3 och 4 presenterar en metod för att studera atmosfärens storskaliga rörelser utifrån ett termodynamiskt perspektiv där luftmassornas värme och fukt studeras. Det visas att variationer ytvattentemperatur vid ekvatorn i Stilla havet kan få atmosfären att, i ett globalt medelvärde, bli fuktigare och varmare samtidigt som masstransporter- na saktar ner. På samma sätt visas att en global uppvärmning till följd av ökade utsläpp av växthusgaser kan få atmosfären att bli varmare, fuktigare och att masstransporterna kan sakta ner. 

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2014. , 46 p.
Keyword [en]
Eulerian, Lagrangian, trajectory, climate, atmosphere, ocean, modelling, drifter, thermodynamic, climate change
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-97348ISBN: 978-91-7447-823-5 (print)OAI: oai:DiVA.org:su-97348DiVA: diva2:683779
Public defence
2014-02-07, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
BalticWay
Funder
EU, FP7, Seventh Framework Programme, 217246
Note

At the time of the doctoral defence the following papers were unpublished and had a status as follows: Paper 3: In press; Paper 4: Manuscript.

Available from: 2014-01-16 Created: 2013-12-09 Last updated: 2014-01-08Bibliographically approved
List of papers
1. Surface drifters and model trajectories in the Baltic sea
Open this publication in new window or tab >>Surface drifters and model trajectories in the Baltic sea
2012 (English)In: Boreal environment research, ISSN 1239-6095, E-ISSN 1797-2469, Vol. 17, no 6, 447-459 p.Article in journal (Refereed) Published
Abstract [en]

Results from recent deployments of surface drifters in the Baltic Sea are presented. For the first time ever, the realism of model-generated trajectories was assessed by a statistical com- parison with trajectories of SVP drifters. The absolute dispersion (i.e. the distance from the initial point as a function of time) was found to be somewhat underestimated by the model trajectories. A severe underestimation of the relative dispersion (pair separation) was also noted, which may, to some extent, be due to the limited resolution of the model. However, the relative dispersion was also found to be very dependent on the initial separation of the model trajectory pairs. After filtering the inertial oscillations, a good agreement of the veloc- ity auto-correlations between the drifters and model trajectories was found. A discussion on the impact of these results on future trajectory modelling in the Baltic Sea is also provided. 

Keyword
Surface drifter, trajectory, RCO, Baltic Sea
National Category
Oceanography, Hydrology, Water Resources
Research subject
Oceanography
Identifiers
urn:nbn:se:su:diva-81644 (URN)000311772500004 ()
Projects
BalticWay
Available from: 2012-10-30 Created: 2012-10-29 Last updated: 2017-12-07Bibliographically approved
2. Lagrangian decomposition of the Hadley and Ferrel cells
Open this publication in new window or tab >>Lagrangian decomposition of the Hadley and Ferrel cells
2012 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, L15807- p.Article in journal (Refereed) Published
Abstract [en]

The meridional overturning circulation of the atmosphere between 45°S and 45°N is decomposed using simulated 3D Lagrangian trajectories for calculating the Lagrangian overturning streamfunctions. This permits an analysis of meridional mass transports which otherwise cancel in time-averaged zonal-mean Eulerian streamfunctions. Overturning circulations inferred from trajectories of no net meridional transport are qualitatively similar to the Eulerian mean, but yield half the Hadley cell amplitude, and twice that of the Ferrel cell. Cross-equatorial transports of some 130 Sv result in two cells that account for the remainder of the Hadley cells. The overturning in midlatiudes is partly cancelled by large (≈150 Sv) meridional transports approximately following isentropes. Changes and implications for various coordinate systems, e.g., isentropic, are also discussed.

Keyword
Ferrel cell, Hadley cell, Lagrangian trajectories, meridional overturning circulation, 0545 Computational Geophysics: Modeling (1952, 4255, 4316), 1620 Global Change: Climate dynamics (0429, 3309), 3319 Atmospheric Processes: General circulation (1223), 3346 Atmospheric Processes: Planetary meteorology (5445, 5739), 3394 Atmospheric Processes: Instruments and techniques
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-79043 (URN)10.1029/2012GL052420 (DOI)000307464100004 ()
Available from: 2012-09-04 Created: 2012-08-24 Last updated: 2017-12-07Bibliographically approved
3. The Atmospheric General Circulation in Thermodynamical Coordinates
Open this publication in new window or tab >>The Atmospheric General Circulation in Thermodynamical Coordinates
2014 (English)In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 71, no 3, 916-928 p.Article in journal (Refereed) Published
Abstract [en]

The zonal and meridional components of the atmospheric general circulation are used to define a global thermodynamic stream function in dry static energy versus latent heat coordinates. Diabatic motions in the tropical circulations and fluxes driven by midlatitude eddies are found to form a single, global thermodynamic cycle. Calculations based on the ERA-Interim reanalysis dataset indicate that the cycle has a peak transport of 428 Sv (Sv = 109 kg s−1). The thermodynamic cycle encapsulates a globally interconnected heat and water cycle comprising ascent of moist air where latent heat is converted into dry static energy, radiative cooling where dry air loses dry static energy, and a moistening branch where air is warmed and moistened. It approximately follows a tropical moist adiabat and is bounded by the Clausius-Clapeyron relationship for near-surface air. The variability of the atmospheric general circulation is related to ENSO events using reanalysis data from recent years (1979-2009) and historical simulations from the EC-Earth coupled climate model (1850-2005). The thermodynamic cycle in both EC-Earth and ERA-Interim widens and weakens with positive ENSO phases and narrows and strengthens during negative ENSO phases with a high correlation coefficient. Weakening in amplitude suggests a reduction in moist convection in the tropics, while widening suggests an increase in mean tropical near-surface moist static energy.

Keyword
Climate, Atmosphere, Mass transport, Stream function, Thermodynamic, Hydrothermal
National Category
Climate Research
Research subject
Meteorology; Oceanography
Identifiers
urn:nbn:se:su:diva-96549 (URN)10.1175/JAS-D-13-0173.1 (DOI)000331927800004 ()
Available from: 2013-12-09 Created: 2013-11-22 Last updated: 2017-12-06Bibliographically approved
4. Slowdown of the Global Atmospheric Circulation with Global Warming
Open this publication in new window or tab >>Slowdown of the Global Atmospheric Circulation with Global Warming
(English)Manuscript (preprint) (Other academic)
Keyword
slowdown, atmosphere, general circulation, stream function, energy, global warming, climate change
National Category
Climate Research
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-97347 (URN)
Available from: 2013-12-09 Created: 2013-12-09 Last updated: 2014-01-07

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