53 research outputs found

    Dynamical processes in the stratosphere and upper troposphere and their influence on the distribution of trace gases in the polar atmosphere

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    Transport plays an important role in the distribution of long-lived gases such as ozone and water vapour in the atmosphere. Understanding of observed variability in these gases as well as prediction of the future changes depends therefore on our knowledge of the relevant atmospheric dynamics. This dissertation studies certain dynamical processes in the stratosphere and upper troposphere which influence the distribution of ozone and water vapour in the atmosphere. The planetary waves that originate in the troposphere drive the stratospheric circulation. They influence both the meridional transport of substances as well as parameters of the polar vortices. In turn, temperatures inside the polar vortices influence abundance of the Polar Stratospheric Clouds (PSC) and therefore the chemical ozone destruction. Wave forcing of the stratospheric circulation is not uniform during winter. The November-December averaged stratospheric eddy heat flux shows a significant anticorrelation with the January-February averaged eddy heat flux in the midlatitude stratosphere and troposphere. These intraseasonal variations are attributable to the internal stratospheric vacillations. In the period 1979-2002, the wave forcing exhibited a negative trend which was confined to the second half of winter only. In the period 1958-2002, area, strength and longevity of the Arctic polar vortices do not exhibit significant long-term changes while the area with temperatures lower than the threshold temperature for PSC formation shows statistically significant increase. However, the Arctic vortex parameters show significant decadal changes which are mirrored in the ozone variability. Monthly ozone tendencies in the Northern Hemisphere show significant correlations (|r|=0.7) with proxies of the stratospheric circulation. In the Antarctic, the springtime vortex in the lower stratosphere shows statistically significant trends in temperature, longevity and strength (but not in area) in the period 1979-2001. Analysis of the ozone and water vapour vertical distributions in the Arctic UTLS shows that layering below and above the tropopause is often associated with poleward Rossby wave-breaking. These observations together with calculations of cross-tropopause fluxes emphasize the importance of poleward Rossby wave breaking for the stratosphere-troposphere exchange in the Arctic.Kuljetuksella on tärkeä rooli otsonin ja vesihöyryn leviämisessä ilmakehässä. Siksi näiden kaasujen havaitun vaihtelun ymmärtäminen ja tulevaisuuden muutoksen ennustaminen vaatii tietoa asiaan vaikuttavasta ilmakehän dynamiikasta. Tämä väitöskirja tutkii eräitä otsonin ja vesihöyryn kuljetukseen liittyviä dynaamisia prosesseja stratosfäärissä ja ylemmässä troposfäärissä. Planetaariset aallot, jotka syntyvät troposfäärissä, säätelevät stratosfäärin meridionaalista kiertoliikettä, joka puolestaan kuljettaa sekä materiaa että lämpöä päiväntasaajalta korkeammille leveysasteille ja siten mm. vaikuttaa polaarivorteksin ominaisuuksiin. Vorteksin lämpötilat vaikuttavat vuorostaan polaaristratosfääripilvien runsauteen ja siten otsonikatoon. Aaltovaikutus stratosfäärin kiertoon vaihtelee talven aikana. Marras-joulukuun keskimääräinen stratosfäärin lämpövuo näyttää selkeitä merkkejä antikorrelaatiosta tammi-helmikuun keskimääräisen stratosfäärin ja tropospfäärin lämpövuon kanssa. Nämä vuodenaikavaihtelut ovat johdettavissa sisäisesta stratosfäärin värähtelystä. Aaltoaktiivisuus tarkastelukauden 1979-2002 aikana näytti tammi-helmikuussa negatiivista trendiä. Arktisen vorteksin pinta-ala, voimakkuus, ja kesto jakson 1958-2002 aikana eivät näyttäneet selkeitä merkkejä pitkäaikaisesta muutoksesta. Sen sijaan pinta-ala, jolla lämpötila on riittävän kylmä polaaristratosfääripilvien muodostumiselle, kasvoi tarkastelujaksolla hieman. Arktisen polaarivorteksin pitkäaikaiset muutokset muistuttavat otsonikerroksen muutoksia. Kuukausittain tarkasteltuna otsonitendenssit pohjoisella pallonpuoliskolla korreloivat selkeästi (|r|=0.7) stratosfäärin kiertoa edustavien muuttujien kanssa. Antarktisen alueella kevätaikainen polaarivorteksi jakson 1979-2001 aikana näyttää selkeää laskevaa lämpötilatrendiä. Vorteksin voimakkuus ja kesto olivat kasvussa, mutta vorteksin pinta-alassa ei havaita merkittävää muutosta. Otsonin ja vesihöyryn pystykuljetuksen analyysi Arktisessa ylätroposfäärissä ja alastratosfäärissä osoittaa että anomaaliset kerrokset tropopausin ala- ja yläpuolella usein liittyvät napasuuntaisen Rossby-aallon murtumiseen. Nämä havainnot sekä laskelmat kaksisuuntaisesta kuljetuksesta tropopaussin läpi osoittavat kuinka tärkeä merkitys napasuuntaisen Rossby-aallon murtumisella on stratosfääri-troposfäärivaihdossa arktisessa yläilmakehässä

    Mixed layer temperature response to the southern annular mode: Mechanisms and model representation

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    Previous studies have shown that simulated sea surface temperature (SST) responses to the southern annular mode (SAM) in phase 3 of the Coupled Model Intercomparison Project (CMIP3) climate models compare poorly to the observed response. The reasons behind these model inaccuracies are explored. The ocean mixed layer heat budget is examined in four of the CMIP3 models and by using observations- reanalyses. The SST response to the SAM is predominantly driven by sensible and latent heat flux and Ekman heat transport anomalies. The radiative heat fluxes play a lesser but nonnegligible role. Errors in the simulated SST responses are traced back to deficiencies in the atmospheric response to the SAM. The models exaggerate the surface wind response to the SAM leading to large unrealistic Ekman transport anomalies. During the positive phase of the SAM, this results in excessive simulated cooling in the 40°-65°S latitudes. Problems with the simulated wind stress responses, which relate partly to errors in the simulated winds themselves and partly to the transfer coefficients used in the models, are a key cause of the errors in the SST response. In the central Pacific sector (90°-150°W), errors arise because the simulated SAM is too zonally symmetric. Substantial errors in the net shortwave radiation are also found, resulting from a poor repre- sentation of the changes in cloud cover associated with the SAM. The problems in the simulated SST re- sponses shown by this study are comparable to deficiencies previously identified in the CMIP3 multimodel mean. Therefore, it is likely that the deficiencies identified here are common to other climate models

    Stratosphere-troposphere coupling enhances subseasonal predictability of Northern Eurasian cold spells

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    Here we explore the stratospheric influence on the predictability of Eurasian cold-spell events using the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble hindcasts obtained from the Subseasonal-to-Seasonal (S2S) archive. To isolate the stratospheric influence, we subsampled two groups of hindcasts according to the strength of the stratospheric polar vortex preceding the cold spells at the surface. The predicted probability of cold spells, defined as the lowest 10th percentile of weekly mean temperature anomalies over northern Eurasia (10 degrees W-130 degrees E and 50 degrees N-65 degrees N), is systematically higher, by 0.05-0.2, at lags 7-24 days in the forecasts initialized during the weak stratospheric vortex compared to the strong stratospheric vortex group, extending the predictability of cold spells by 3-5 days. Our results suggest that, in the case of the weak polar vortex, stratosphere-troposphere coupling favors the negative Northern Annular Mode (NAM) regime and the cold-air outbreaks in Eurasia. As a consequence, the long stratospheric predictability extends the predictability of the cold spells. On the other hand, when the polar vortex is strong, the stratospheric anomalies do not favor the observed negative NAM regime, which thus results from the internal tropospheric processes only. In this case the predictability of cold-air spells is limited. Furthermore, we show that the extended predictability of cold spells arising from the stratosphere-troposphere coupling is captured by a simple statistical model, suggesting that governing large-scale dynamics behave effectively linearly over some limited periods. Quantified contribution of the stratosphere-troposphere coupling to the enhanced skill of the extended-range cold-spell forecasts documented in our paper may prove useful in the development of forecasting tools.Peer reviewe

    Predictibilidad del clima en la estratosfera

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    Atmospheric Circulation Response to Anomalous Siberian Forcing in October 2016 and its Long‐Range Predictability

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    Abstract: The warm Arctic-cold continent pattern was of record strength in October 2016, providing the opportunity to test its proposed influence on large-scale atmospheric circulation. We find a record weak polar stratospheric vortex and negative North Atlantic Oscillation in November-December 2016 and link them to increased planetary wave generation associated with cold Siberian anomalies followed by troposphere-stratosphere dynamical coupling. At the same time the warm Arctic anomalies, in particular those over the Barents-Kara Seas, do not appear to play an important role in forcing the atmospheric circulation. Long-range forecasts initialized on 1 October 2016 reproduced both the weak polar vortex and negative North Atlantic Oscillation, as well as their link with the Siberian temperatures. Our results support the stratospheric pathway for atmospheric circulation forcing associated with Siberian surface anomalies and uncover a source of skill for subseasonal forecasts from October to December. Plain Language Summary: The warm Arctic-cold continent pattern is an observed, large-scale pattern of near-surface temperatures where the Arctic is warmer than average and Siberia is colder than average. This pattern was of record strength in October 2016, providing the opportunity to test its influence on the Northern Hemisphere atmospheric circulation and the possibility of skillful long-range forecasts. It has been proposed that the warm Arctic-cold continent pattern can drive large atmospheric waves, which are able to travel from the troposphere into the stratosphere, where they weaken the strong wintertime winds that make up the stratospheric polar vortex. A weakened polar vortex can then lead to changes in the surface pressure that can affect weather patterns. We find a record weak polar stratospheric vortex in late autumn 2016 and link that to cold Siberian anomalies. At the same time the warm Arctic anomalies do not appear to play an important role in forcing the atmospheric circulation. Long-range forecasts initialized in October 2016 reproduced both the weak polar vortex and resulting surface pressure patterns. Our results support the stratospheric pathway for atmospheric circulation forcing by Siberian surface anomalies and uncover a source of skill for subseasonal forecasts in the Northern Hemisphere autumn.Peer reviewe

    Links between Arctic sea ice and extreme summer precipitation in China: an alternative view

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    Potential links between the Arctic sea-ice concentration anomalies and extreme precipitation in China are explored. Associations behind these links can be explained by physical interpretations aided by visualisations of temporarily lagged composites of variables such as atmospheric mean sea level pressure and sea surface temperature. This relatively simple approach is verified by collectively examining already known links between the Arctic sea ice and rainfall in China. For example, similarities in the extreme summer rainfall response to Arctic sea-ice concentration anomalies either in winter (DJF) or in spring (MAM) are highlighted. Furthermore, new links between the Arctic sea ice and the extreme weather in India and Eurasia are proposed. The methodology developed in this study can be further applied to identify other remote impacts of the Arctic sea ice variability

    Advancing Our Understanding of Eddy-driven Jet Stream Responses to Climate Change – A Roadmap

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    Purpose of Review: Extratropical jets and associated storm tracks significantly influence weather and regional climate across various timescales. Understanding jet responses to climate change is essential for reliable regional climate projections. This review serves two main purposes: (1) to provide an accessible overview of extratropical jet dynamics and a comprehensive examination of current challenges and uncertainties in predicting jet responses to greenhouse gas increases and (2) to suggest innovative experiments to advance our understanding of these responses. Recent Findings: While successive generations of climate model ensembles consistently project a mean poleward shift of the midlatitude zonal-mean maximum winds, there remains considerable intermodel spread and large uncertainty across seasonal and regional jet responses. Of particular note is our limited understanding of how these jets respond to the intricate interplay of multiple concurrent drivers, such as the strong warming in polar and tropical regions, and the relative importance of each factor. Furthermore, the difficulty of simulating processes requiring high resolution, such as those linked to sharp sea surface temperature gradients or diabatic effects related to tropical convection and extratropical cyclones, has historically hindered progress. Summary: We advocate for a collaborative effort to enhance our understanding of the jet stream response to climate change. We propose a series of new experiments that take advantage of recent advances in computing power and modelling capabilities to better resolve small-scale processes such as convective circulations, which we consider essential for a good representation of jet dynamics
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