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

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

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

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

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

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

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

Predicting sudden stratospheric warming 2018 and its climate impacts with a multi‐model ensemble

Sudden stratospheric warmings (SSWs) are significant source of enhanced subseasonal predictability, but whether this source is untapped in operational models remains an open question. Here we report on the prediction of the SSW on 12 February 2018, its dynamical precursors, and surface climate impacts by an ensemble of dynamical forecast models. The ensemble forecast from 1 February predicted 3 times increased odds of an SSW compared to climatology, although the lead time for SSW prediction varied among individual models. Errors in the forecast location of a Ural high and underestimated magnitude of upward wave activity flux reduced SSW forecast skill. Although the SSW's downward influence was not well forecasted, the observed northern Eurasia cold anomaly following SSW was predicted, albeit with a weaker magnitude, due to persistent tropospheric anomalies. The ensemble forecast from 8 February predicted the SSW, its subsequent downward influence, and a long‐lasting cold anomaly at the surface

Seasonal forecasts of the exceptional Northern Hemisphere Winter of 2020

The winter of 2019-20 was dominated by an extremely strong stratospheric polar vor20 tex and positive tropospheric Arctic Oscillation (AO). Here, we analyze forecasts from 6 different prediction systems contributing to the C3S seasonal forecast database. Most performed very strongly, with consistently high skill for January–March 2020 from fore23 casts launched through October–December 2019. Although the magnitude of the anoma24 lies was underestimated, the performance of most prediction systems was extremely high for a positive AO winter relative to the common hindcast climate. Ensemble members which better predicted the extremely strong stratospheric vortex better predicted the extreme tropospheric state. We find a significant relationship between forecasts of the anomalous mid-latitude tropospheric wave pattern in early winter, which destructively interfered with the climatological stationary waves, and the strength of the stratospheric vortex later in the winter. Our results demonstrate a strong interdependence between the accuracy of stratospheric vortex and AO forecasts

The tropical influence on sub‐seasonal predictability of wintertime stratosphere and stratosphere–troposphere coupling

A unique set of relaxation experiments with a forecast model initialized during December–January 1999–2019 is used to explore tropical influence on the Northern Hemisphere polar stratosphere and stratosphere–troposphere coupling, to quantify predictability benefits due to the perfect knowledge of the tropical variability. On average, predictability of the polar stratosphere, as represented by the 50 hPa geopotential height anomalies north of 50°N (Z50), increases from 17 days in freely running (control) forecasts to 21 days in the tropical relaxation experiments. At sub‐seasonal time‐scales, a statistically significant improvement in weekly mean skill scores can be demonstrated in 14%–20% of individual forecast ensembles, mostly in cases when the skill of the corresponding control forecast is worse than average. In these forecasts, root‐mean‐square errors and forecast spread of Z50 during forecast weeks 3–5 are decreased by 10%–15%. Stratospheric improvements are detected during periods of both vortex strengthening and vortex weakening, including most major Sudden Stratospheric Warmings that occurred during the study period, via modulation of the upward wave activity fluxes. An active Madden–Julian Oscillation (MJO) is found in most of these events with MJO phase 5–7 preceding vortex weakening and MJO phase 3–4 preceding vortex strengthening. Forecasts with improved stratospheric circulation also have improved tropospheric circulation during and after the periods when improvements are detected in the stratosphere. We attribute these improvements to both stratosphere–troposphere coupling and tropospheric tropical teleconnections