Impact of air-sea coupling on northern hemisphere summer climate and the monsoon-desert teleconnection

Previous studies have established the existence of a large-scale teleconnection between ascent in the South Asian Summer Monsoon and subsidence over the Mediterranean (known as “the monsoon–desert mechanism”). Improving the representation of this mechanism could potentially improve the skill of seasonal forecasts for European summer weather patterns. In this study, the impact of air–sea coupling on the NH summer climate and the representation of the monsoon–desert mechanism is analysed in two 45-year experiments with the Met-Office Unified Model. In the first coupled experiment, the atmosphere is allowed to freely interact with a high-vertical-resolution mixed-layer ocean model. The diagnosed daily SSTs from this experiment are then used to force an atmosphere-only uncoupled experiment. The two experiments have a similar mean state, but the coupled experiment has a substantially more realistic representation of interannual precipitation variability over the Indian Summer Monsoon region. The coupled experiment can capture the observed westward propagating Rossby-wave trains excited by the Indian Summer Monsoon, while in the uncoupled experiment the Rossby-wave response is more local. It is shown that in the coupled experiment more moisture is transported inland and monsoon precipitation reaches further north, which favours westward Rossby wave propagation. Finally, evidence is shown that the coupled experiment can capture the observed interannual relationship between the Indian Summer Monsoon precipitation and precipitation over the Balkans/Black Sea region

Evidence for the influence of the North Atlantic Oscillation on the total ozone column at northern low latitudes and midlatitudes during winter and summer seasons.

The strong influence of the winter North Atlantic Oscillation (NAO) on the total ozone column (TOC) in the Northern Hemisphere has been reported in a number of previous studies. In this study we show that this influence is not restricted to the winter season but is also significant in summer. Especially interesting effects of the summer NAO (SNAO) on the TOC are observed over the eastern Mediterranean region, where a strongly positive SNAO index is related to the creation of a geopotential height-negative anomaly over Greece with maximum amplitude at 200 hPa. Another anomaly was observed west of the Iberian Peninsula with similar effects on the TOC. Analyzing 26 years of Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) data from the equator to midlatitudes (60°) in the Northern Hemisphere, we demonstrate that the SNAO accounts for up to 30% of the TOC variability with a strong latitudinal and longitudinal dependence. Additionally, we obtain significant correlations between the NAO index and the thermal tropopause pressure and also with the geopotential heights at 200 and 500 hPa. Finally, some indirect connections between NAO and the TOC through teleconnections are also discussed

How robust are trends in the Brewer-Dobson circulation derived from observed stratospheric temperatures?

Most global circulation models and climate-chemistry models forced with increasing greenhouse gases predict a strengthening of the Brewer-Dobson circulation (BDC) in the twenty-first century, and some of them claim that such strengthening has already begun at the end of the twentieth century. However, observational evidence for such a trend remains inconclusive. The goal of this paper is to examine the evidence for observed trends in the stratospheric overturning circulation using a suite of currently available observational stratospheric temperature data. Trends are examined as ''departures'' from the global mean temperature, since such trends reflect the effects of dynamics and spatially inhomogeneous radiative forcing and are to first order independent of the direct radiative effects of increasing well-mixed greenhouse gas concentrations. The primary conclusion of the study is that temperature observations do not reveal statistically significant trends in the Brewer-Dobson circulation over the period from 1979 to the present, as covered by Microwave Sounding Unit and Stratospheric Sounding Unit temperatures. The estimated trends in the BDC are weak in all datasets and not statistically significant at the 95% confidence level. In many cases, different data products yield very different results, particularly when the trends are stratified by season. Implications for the interpretation of recent stratospheric climate change are discussed. The results illustrate the essential need to better constrain the accuracy of future stratospheric temperature datasets

Observational evidence of European summer weather patterns predictable from spring

Forecasts of summer weather patterns months in advance would be of great value for a wide range of applications. However, seasonal dynamical model forecasts for European summers have very little skill, particularly for rainfall. It has not been clear whether this low skill reflects inherent unpredictability of summer weather or, alternatively, is a consequence of weaknesses in current forecast systems. Here we analyze atmosphere and ocean observations and identify evidence that a specific pattern of summertime atmospheric circulation--the summer East Atlantic (SEA) pattern--is predictable from the previous spring. An index of North Atlantic sea-surface temperatures in March-April can predict the SEA pattern in July-August with a cross-validated correlation skill above 0.6. Our analyses show that the sea-surface temperatures influence atmospheric circulation and the position of the jet stream over the North Atlantic. The SEA pattern has a particularly strong influence on rainfall in the British Isles, which we find can also be predicted months ahead with a significant skill of 0.56. Our results have immediate application to empirical forecasts of summer rainfall for the United Kingdom, Ireland, and northern France and also suggest that current dynamical model forecast systems have large potential for improvement

Current and emerging developments in subseasonal to decadal prediction

Weather and climate variations of subseasonal to decadal timescales can have enormous social, economic and environmental impacts, making skillful predictions on these timescales a valuable tool for decision makers. As such, there is a growing interest in the scientific, operational and applications communities in developing forecasts to improve our foreknowledge of extreme events. On subseasonal to seasonal (S2S) timescales, these include high-impact meteorological events such as tropical cyclones, extratropical storms, floods, droughts, and heat and cold waves. On seasonal to decadal (S2D) timescales, while the focus remains broadly similar (e.g., on precipitation, surface and upper ocean temperatures and their effects on the probabilities of high-impact meteorological events), understanding the roles of internal and externally-forced variability such as anthropogenic warming in forecasts also becomes important. The S2S and S2D communities share common scientific and technical challenges. These include forecast initialization and ensemble generation; initialization shock and drift; understanding the onset of model systematic errors; bias correct, calibration and forecast quality assessment; model resolution; atmosphere-ocean coupling; sources and expectations for predictability; and linking research, operational forecasting, and end user needs. In September 2018 a coordinated pair of international conferences, framed by the above challenges, was organized jointly by the World Climate Research Programme (WCRP) and the World Weather Research Prograame (WWRP). These conferences surveyed the state of S2S and S2D prediction, ongoing research, and future needs, providing an ideal basis for synthesizing current and emerging developments in these areas that promise to enhance future operational services. This article provides such a synthesis

The evolution of the Brewer-Dobson circulation and the ozone layer during the last three decades

[cat] El 1985 Joseph Farman i els seus col•legues de la British Antarctic Survey van publicar un article [Farman et al. 1985] que mostrava que la columna total d’ozó sobre l’Antàrtida durant la primavera havia disminuït de manera alarmant entre el 1975 i el 1984. Estudis posteriors van confirmar aquesta disminució i el terme “forat de la capa d’ozó” va esdevenir popular. L’ocurrència d’aquest fenomen va revitalitzar l’interès de la comunitat científica per l’estudi de l’estratosfera. Des d’aleshores s’ha avançat molt en la comprensió dels processos que hi tenen lloc. Aquests avenços han estat possibles gràcies al desen­volupament de les eines de teledetecció i de sofisticats models numèrics. Aquests models ens permeten contrastar els nostres coneixements teòrics amb observacions i així determinar-ne la validesa. La visió que es tenia en el passat de l’estratosfera com una capa isolada dels fenòmens de la troposfera ha canviat totalment. Avui és àmpliament acceptat que els processos que es desenvolupen a la troposfera afecten l’estat de l’estratosfera i a l’inrevés, els fenòmens que tenen lloc a l’estratosfera poden afectar la troposfera. Malgrat tots els avenços fets en els últims anys, el nostre coneixement sobre els pro­cessos de l’estratosfera no és complet. Hi ha importants incerteses sobre la manera com el canvi climàtic afectarà l’estratosfera i sobre la recuperació dels nivells d’ozó estratosfèric. En aquest context, la motivació d’aquesta tesi és contribuir a l’estudi de l’evolució de la capa d’ozó, de les temperatures de l’estratosfera i de la circulació de Brewer -Dobson (BDC) al llarg de les tres últimes dècades. Conèixer i quantificar els factors que determinen l’evolució d’aquests fenòmens i comprovar si els models numèrics ac­tuals són capaços de reproduir l’evolució observada és de vital importància si volem pronosticar la seva evolució futura en un escenari de canvi global. S’hi aborden aquestes preguntes: 1. En quina mesura l’oscil•lació de l’Atlàntic Nord (NAO) influeix en l’evolució de la columna total d’ozó a l’hemisferi nord durant l’hivern i l’estiu? 2. Com són de consistents les tendències observades de la BDC? 3. En quina mesura l’actual generació de models generals de circulació atmos­fèrica que consideren les interaccions amb l’oceà [“Atmosphere-Ocean Global Circulation Models” (AOGCMs)] i els actuals models químics-climàtics acoblats [coupled chemistry-climate models (CCMs)] reprodueixen les tendències en les temperatures observades a l’estratosfera? En el capítol 1 presentem a mode d’introducció alguns dels conceptes que seran usats en els capítols posteriors. Primerament, presentem una breu descripció del clima de l’estratosfera. A continuació introduïm les equacions que regeixen la circu­lació a l’estratosfera i les usem per descriure els mecanismes que forcen l’intercanvi de massa entre la troposfera i l’estratosfera. Finalment fem una breu descripció de les reaccions fotoquímiques de producció i destrucció de l’ozó estratosfèric. En el capítol 2 per tal d’intentar contestar la primera qüestió analitzem com l’Oscil•lació de l’Atlàntic Nord (NAO) afecta la distribució de la columna total d’ozó a l’hemisferi nord. Per dur a terme aquest estudi examinem el patró espa­cial de correlació entre la NAO i la columna total d’ozó, l’alçada de la tropopausa tèrmica i diferents alçades geopotencials tant a l’hivern com durant l’estiu. Aquest estudi mostra que la NAO influencia d’una manera molt important la distribució d’ozó tant a l’hivern com a l’estiu. L’estudi també mostra que aquesta influència no està limitada a l’Atlàntic Nord sinó que abasta tot l’hemisferi nord. La majoria del models prediuen una acceleració de la BDC en els pròxims cinquanta anys com a resposta a l’increment de la concentració dels gasos d’efecte hivernacle a l’atmosfera. Si aquesta acceleració es produís, alteraria de manera molt determi­nant la distribució de diferents compostos químics a l’estratosfera, com per exemple l’ozó. Aquesta predicció però, es fonamenta bàsicament en simulacions numèriques i l’evidència observacional és molt pobre. La pròpia de.nició de la BDC fa que aquesta sigui molt difícil de determinar per mitjans observacionals. La pretensió del capítol 3 és contestar la segona pregunta. Per això utilitzem les tendències de temperatures a l’estratosfera com un mitjà indirecte per inferir possi­bles tendències en la BDC al llarg de les últimes tres dècades. Primerament utilitzem el model químic-climàtic acoblat WACCM per analitzar la idoneïtat d’inferir tendèn­cies en la BDC a partir de l’anàlisi de les tendències de temperatura. Seguidament analitzem l’estructura meridional de les tendències de les diferències entre les mit­janes zonals i les mitjanes globals de temperatura. La diferència entre la mitjana zonal i la global és en primer ordre independent de l’augment de la concentració de CO2 i vapor d’aigua i respon primàriament a canvis en la circulació (com la BDC). L’anàlisi es realitzarà per quatre rangs d’altitud representatius de la baixa, mitja i alta estratosfera. S’analitzaran les tendències per anomalies provinents de radiosondatges, satèl•lits i reanàlisis. L’estudi demostra que tot i que l’estructura meridional de les tendències és coherent amb una acceleració de la BDC, aquestes no són estadísticament signi.catives en cap dels nivells estudiats. A més a més hi ha importants diferències entre diferents bases de dades. L’estudi remarca la necessitat de disposar de més i millors bases de dades de temperatura a l’estratosfera si volem validar les prediccions fetes pels models. En el capítol 4 tractem la tercera qüestió. Per això analitzarem la capacitat del conjunt de models AOGCMs del projecte CMIP5 i dels CCMs del projecte de val­idació CCMVal2 per reproduir l’evolució de les temperatures de l’estratosfera en les últimes tres dècades. Aquest capítol mostra que hi ha importants discrepàncies entre models i observacions i entre els mateixos models. En acabar, es proposen possibles causes que expliquin aquestes discrepàncies. Finalment, en el capítol 5 es presenta un sumari de la tesi i una prospectiva que suggereix possibles futures línies de recerca. La present tesi es basa en aquestes publicacions: • Ossó, A., Y. Sola, J. Bech, and J. Lorente, 2011. Evidence for the in.uence of the NAO on total ozone column at northern low-and mid-latitudes during winter and summer seasons. J. Geophys. Res., 116. • Ossó, A., Y. Sola, K. Rosenlof, B. Hassler, J. Bech, J. Lorente, 2014. How robust are trends in the Brewer-Dobson Circulation derived from observed stratospheric temperatures? (Submitted to Journal of Climate 04-17-2014). • Thompson, D.W.J., D.J. Seidel, W.J. Randel, C. Zou, A.H. Butler, C. Mears, A.Ossó, C. Long, R. Lin, 2012. The mystery of recent stratospheric tempera­ture trends. Nature 491, 692–697. • Bech J., Y. Sola, A. Ossó, J. Lorente, 2014. Analysis of 14 years of broadband ground based solar UV index observations in Barcelona. Int. J. Climatol. • Sola, Y., J. Lorente, A. Ossó, 2012. Analyzing UV-B narrowband solar ir­radiance: Comparison with erythemal and vitamin D production irradiances. Journal of Photochem. Photobiol. B 117, 90-96[eng] In 1985 Joseph Farman and his colleagues of the British Antarctic Survey published a paper [Farman et al. 1985] showing that the total ozone content of the atmosphere during the Antarctic spring had decreased dramatically between 1975 and 1984. Subsequent studies confirmed this decrease and the term Antarctic ozone hole became popular. The occurrence of this phenomenon revitalized the interest of the climate community in the stratosphere. Since then many remarkable advances have been made in our understanding of stratospheric processes. These advances have been possible by the advent of remote sensing measurements of the atmosphere together with the sophistication of the atmospheric simulations. Nowadays it is known that stratospheric climate is determined by a complex coupling of dynamic, radia­tive and chemical processes.The view of the stratosphere as an isolated layer have changed completely and today it is widely accepted that both the tropospheric pro­cesses a.ect the stratospheric climate, and the other way around, the stratospheric processes a.ect the climate in the troposphere. Despite all the progresses made, our knowledge is far to be complete. There are still large uncertainties about how climate change is a.ecting the stratosphere and to what extent it will in.uence the stratospheric ozone recovery. Moreover recent studies [e.g., Thompson et al. 2012] evidenced that there are important discrepancies among observational data sets and between observations and numerical simulations of recent stratospheric temperature trends. OVERVIEW The aim of this thesis is to contribute on the understanding of the past evolution of the ozone layer and the Brewer-Dobson circulation. With this objective the following question will be addressed: 1. To what extent are the long-term trends of total ozone a.ected by the North Atlantic Oscillation (NAO) during winter and summer seasons? 2. How robust are the observed trends in the Brewer-Dobson Circulation (BDC)? 3. Is the current generation of coupled atmosphere-ocean global circulation mod­els (AOGCMs) and coupled climate-chemistry models (CCMs) reproducing the observed stratospheric temperature trends over the last three decades? Chapter 1 presents an introduction to the basic concepts that are transverse to chapter 2 and chapter 3. First, a description of the mean state of the stratosphere is provided. Then, an introducing of the basic equations used to describe the strato­sphere followed by a description of the mechanisms involve in the troposphere­stratosphere exchange processes is presented. Finally a description of the basic processes involved in the production, transport and destruction of ozone are re­viewed. Chapter 2 deals with question 1 and is built on the bases of the article Ossó et al. [2012]. In this chapter the in.uence of the North Atlantic Oscillation on the total ozone content of the atmosphere is examined during winter and summer seasons. Chapter 3 deals with questions 2 and is built on the bases of the article Ossó et al. [2014]. First, a brief review of our current understanding of the stratospheric temperature trends is provided. Then, residual vertical velocity trends and temper­ature trends from the CCM WACCM output is examined. Finally, stratospheric temperatures are used to infer trends in the BDC during the last 3 decades. Chapter 4 presents a brief discussion on the performance of the current AOGCMs and CCMs on reproducing the stratospheric temperature trends derived from ob­servation. Finally a summary and outlook of the thesis is provided in chapter 5. This PhD study is primary built on the following peer-reviewed publications: • Ossó, A., Y. Sola, J. Bech, and J. Lorente, 2011: Evidence for the in.uence of the NAO on total ozone column at northern low-and mid-latitudes during winter and summer seasons. J. Geophys. Res., 116, D24. • Ossó, A., Y. Sola, K. Rosenlof, B. Hassler, J. Bech, J. Lorente, 2014: How robust are trends in the Brewer-Dobson Circulation derived from observed stratospheric temperatures? (Submitted to Journal of Climate 04-17-2014). • Thompson, D.W.J., D.J. Seidel, W.J. Randel, C. Zou, A.H. Butler, C. Mears, A.Ossó, C. Long, R. Lin, 2012: The mystery of recent stratospheric tempera­ture trends. Nature 491, 692–697. Other contributions made during this thesis: • Bech J., Y. Sola, A. Ossó, J. Lorente, 2014: Analysis of 14 years of broadband ground based solar UV index observations in Barcelona. Int. J. Climatol. • Sola, Y., J. Lorente, A. Ossó, 2012: Analyzing UV-B narrowband solar irra­diance: Comparison with erythemal and vitamin D production irradiances. Journal of Photochem.Photobiol. B 117, 90-96