The role of high-latitude circulation and moisture transport in Arcticclimate variability and change during winter

Cian Woods

The role of high-latitude circula-tion and moisture transport in Arc-tic climate variability and changeduring winter

Cian Woods

Cover image: The Ward Hunt Ice Shelf, September 2003.Credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC.File freely available from: http://visibleearth.nasa.gov/view.php?id=69068.

ISBN print: 978-91-7649-575-9ISBN digital: 978-91-7649-576-6c© Cian Woods, Stockholm 2016

Printed in Sweden by Holmbergs, Malmö 2016.

Distributor: Department of Meteorology, Stockholm University.

Abstract

This thesis examines the connections between atmospheric circulation in thehigh-latitudes, northward moisture transport, and Arctic climate variability andchange during winter. An event based approach is taken by objectively definingphenomena termed “moisture intrusions” – filamentary flows of anomalouslymoist air which originate at 70◦N and cross the entire Arctic basin. They typ-ically emanate from within the poleward advecting branches of mid-latitudecyclones held in place by blocking patterns to the east. Moisture intrusionscontribute only a minority of the total northward moisture transport at 70◦N,yet drive a significant proportion of the inter annual variability in surface tem-perature and downward longwave radiation over the entire polar cap. A posi-tive trend in the frequency of these events, in response to a moistening of theatmosphere, is shown to have driven approximately 45% and 35% of the ob-served warming and sea ice decline in the Barents Sea during Dec-Jan over thepast two decades. Moisture intrusions act to erode the temperature inversionand thus contribute to bottom amplified warming even in the absence of seaice loss. Negative sea ice anomalies induced by intrusions persist for up toweeks at a time – promoting upward turbulent heat fluxes and further bottomamplified warming. Systematic biases in the statistics of moisture intrusionsare discovered in the CMIP5 models. The biases are predominantly a resultof misrepresentation of the intense moisture fluxes and are almost entirely dueto biases in the meridional velocity. Moisture intrusion biases explain onlyabout 17% of the temperature bias in the Atlantic sector. The predicted bi-ases, while small in amplitude, are very highly correlated with the true biasesin the models however, suggesting that the temperature bias directly inducedby misrepresented intrusion statistics may be strongly amplified by sea icefeedback. A analysis of the uncertainties in computed turbulent air-sea flux(TASF) climatologies arising due to the parameterisation of bulk formulae isalso presented. TASF climatologies are computed over a series of sensitiv-ity experiments using surface state variables from ERA-Interim. The largestsource of uncertainty is related to the computation of the transfer coefficientsand hence the choice of bulk algorithm itself. The majority of parameter ap-proximations have small impacts when tested individually, but can lead to largedisagreements when implemented in tandem.

Sammanfattning

Denna avhandling undersöker sambanden mellan atmosfärens cirkulation påhöga latituder, nordlig fuktighetstransport och klimatvariationer och klimatför-ändringar i arktisk under vintern. En händelsebaserad metod utvecklas genomatt objektivt definiera ett fenomen som härmed benämns “fuktighets intrång”.Detta är onormalt fuktig luft som har sitt ursprung rund 70◦N och som trans-porteras över hela arktiska oceanen. Denna luft härstammar från lågtryck somstyrs på nordliga banor på grund av blockerande högtryck. Fuktighets intrång-en bidrar endast till en liten del av den totala nordliga fukttransporten vid 70◦N,men bidrar till en betydande andel av den årliga variationer i yttemperatur ochnedåtriktad långvågig strålning över hela polartäcket. En ökning av dessa hän-delser, på grund av att luftfuktigheten i atmosfären ökar, har visat sig stå för45% av den observerade uppvärmningen och 35% av havsisens nedgång i Ba-rents hav under Dec-Jan, under de senaste två decennierna. Fuktighets intrång-en försvagar temperatur inversionen och bidrar därmed till en markförstärktuppvärmning, även när det inte blir förlust av havsis. Negativa havsis avvikel-ser, inducerade av intrången, kan kvarstå flera veckor i taget. Dessa kan främ-ja uppåtriktade turbulenta värmeflöden och ytterligare bidra till markförstärktuppvärmningen. Systematiska avvikelser i statistiken av fuktighets intrångenhar upptäcks i CMIP5 modellerna. Dessa avvikelser beror i huvudsak på attde intensiva fuktighets flödena representeras felaktigt, vilket nästan uteslutan-de beror på fel antagande av meridian hastigheten. Avvikelser i beräkningenav den inträngande luftfuktigheten förklarar endast ca 17% av temperatur av-vikelserna i den Atlantiska sektorn. De förutspådda avvikelserna, även om dehar liten amplitud, är dock mycket starkt korrelerade med de verkliga avvikel-serna i modellerna. Detta tyder på att avvikelserna i temperaturen, som berorpå felaktigheter i intrångs statistik, kan förstärkas av feedback från havsisen.En osäkerhetsanalys i hur “computed turbulent air-sea flux” (TASF) klimatolo-gier uppstår på grund av parameteriseringen av bulk formler presenteras också.TASF klimatologier beräknas med en serie känslighetsexperiment

This thesis is dedicated to my parents

List of Papers

The following papers, referred to in the text by their Roman numerals, areincluded in this thesis.

PAPER I: Woods, C., R. Caballero, and G. Svensson (2013), Large-scalecirculation associated with moisture intrusions into the Arcticduring winter, Geophys. Res. Lett., 40: 4717–4721,doi: 10.1002/grl.50912

PAPER II: Woods, C. and R. Caballero (2016), The role of moist intrusionsin winter Arctic warming and sea ice decline, J. Climate, 29(12): 4473–4485, doi: 10.1175/jcli-d-15-0773.1

PAPER III: Woods, C., R. Caballero, and G. Svensson (2016), Representa-tion of Arctic moist intrusions in CMIP5 models and implica-tions for winter climate biases. Under revision in J. Climate.

PAPER IV: Brodeau, L., B. Bernier, S. Gulev, C. Woods (2016), Climato-logically significant effects of some approximations in the bulkparameterizations of turbulent air-sea fluxes, J. Phys. Oceanogr.,doi: 10.1175/jpo-d-16-0169.1. Accepted 19 September 2016.

Reprints were made with permission from the publishers.

A historical perspective of Arcticmeteorological research

The first meteorological observations of the Arctic were made in the early partof the nineteenth century by William Scoresby. Prior to this period, the harsh-ness and remoteness of the Arctic climate had hampered expeditions. Duringhis 1807–1818 expeditions, Scorseby provided the first known measurementsof the Arctic atmosphere, snow and ice (Scoresby 1820). Expeditions by theBritish Navy followed later during the 1880’s, and more and more expeditionsinto the Arctic were motivated by international interest in the Polar regionsas well as the race to reach the pole itself. The first measurements of tem-peratures profiles were made as early as 1914 using kite ascents over Siberia(Brooks 1931). These ascents showed that the vertical structure of the Arctic istypically characterised by a surface based inversion – whereby temperature ac-tually increases with height close to the surface. More detailed investigationsusing captive balloon ascents were carried out by Sverdrup during the Maudexpedition, 1918–1925 (Sverdrup 1933). Having embedded the Maud firmlywithin the sea ice, their plan was to drift with the ocean surface currents andpotentially make a pass of the north pole. Although they did not reach the pole,Sverdrups measurements provided the first information on the structure of theArctic inversion over a large area. The first attempt to describe the physicalmechanisms whereby temperature inversions emerged in the Arctic was madeby Wexler (1936). Wexler correctly concluded that the inversion was princi-pally a response to a radiative deficit at the surface. Later developments wouldshow the importance of horizontal heat advection in maintaining a persistentinversion, eventually leading to our current understanding ; a complex verti-cal structure controlled by radiative cooling at the surface, warm air advectionfrom lower latitudes, cloud properties and topography.

Prior to the 1950’s, the role of synoptic variability in the Arctic was con-siderably uncertain. This was due manly to the geographical paucity of thedata. Dzerdzeevskii (1945) was the first to correctly conclude that cyclonesare a common feature of the deep Arctic, particularly in summer. After the1950’s, Russian and American drifting stations began reporting regularly onthe synoptic conditions of the Arctic. This data was subsequently incorporatedinto sea level pressure (SLP) analyses, and helped to form our current under-

standing of the general circulation in the Arctic. Initially the Arctic circula-tion was viewed as being dominated by a basin-scale anti-cyclone at all times.This concept was first proposed by Helmholtz (1889) and expanded on subse-quently by Hobbs (1910, 1926, 1945). Hobbs formed his “Greenland glacialanticyclone” theory, which involved a persistent high pressure cell over theGreenland ice sheet that strongly affected the mid-latitude circulation. Despitecontradicting observational evidence collected during the 1930’s and 1940’s,this high pressure cell concept remained. Dzerdzeevskii (1945) analysed theRussian drifting icebreaker “Sedov” and “Northe Pole 1” drifting station dataand showed that cyclonic activity was a common feature in the Arctic, par-ticularly during summer. With the advent of the computer age and improvingSLP analyses, cyclone detection and climatologies began improving rapidly. Itbecame apparent that the Arctic circulation was broadly characterised, in thetime mean, by a high pressure anti-cyclone over the Beaufort Gyre and lowpressure cyclone over the Barents and Kara Seas. These new data were the ba-sis for a series of studies over the following decade (Group and Wilson 1958;Keegan 1958; Reed and Kunkel 1960). Our current understanding of Arcticdynamics can be traced back to this period of research, and has continued todevelop from the concepts formed by these early pioneers.

Contents

Abstract v

Sammanfattning vii

List of Papers ix

A historical perspective of Arctic meteorological research xi

1 Introduction 15

2 The Arctic and its role in Earths climate system 192.1 Earths climate system . . . . . . . . . . . . . . . . . . . . . . 192.2 The Arctic winter climate . . . . . . . . . . . . . . . . . . . . 232.3 Arctic amplification . . . . . . . . . . . . . . . . . . . . . . . 25

3 Data 273.1 Reanalysis and in-situ data . . . . . . . . . . . . . . . . . . . 273.2 Climate model data . . . . . . . . . . . . . . . . . . . . . . . 283.3 Modelling errors and uncertainties . . . . . . . . . . . . . . . 29

4 Conclusions 334.1 Summary of papers . . . . . . . . . . . . . . . . . . . . . . . 334.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Acknowledgements xxxvii

References xxxix

1. Introduction

The rapidly diminishing snow and sea ice content of the Arctic ocean is one ofthe most provocative features of recent climate change on our planet (Figure1.1). These changes have in turn generated a huge amount of media coverage,and for many people represents the poster-child of mans potentially climateaugmenting activities. Perhaps no other feature of global climate change isperceived as acutely amongst the public than that of the diminishing Arctic seaice.

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Figure 1.1: Arctic sea ice area, shown every 10 years since 1980. Data for thecurrent year (2016) are also shown. Adapted from the National Snow & Ice DataCenter (NSIDC) website, with partial support from the National Aeronauticsand Space Administration (NASA) – http://nsidc.org/arcticseaicenews/charctic-interactive-sea-ice-graph/.

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The loss of sea ice itself is tightly connected to another prominent featureof global warming – whereby the Arctic is warming at a rate about double thatof the global mean (Cohen et al. 2014). This tendency for the Arctic to warmmore rapidly than elsewhere is commonly referred to as Arctic amplificationin the literature (Serreze and Barry 2011). Of course the question of how theseArctic changes may influence the global climate system is a major uncertainty(Barnes and Screen 2015; Shaw et al. 2016). With the high albedo ice andsnow surface receding, the Earth becomes a more efficient absorber of the Sunsradiation. As such, changes in high latitudes influence the energy exchangesbetween the Earth and Sun. As these exchanges are the primary drivers of theglobal circulation, Arctic change may have global implications (Vihma 2014).

Outside of the atmospheric science community, more and more eyes are in-tently focused on the changes occurring in these northern high-latitudes, witha mind for expanding and emerging industries once a milder climate with per-manent stretches of open ocean becomes a reality. Recent estimates of undis-covered oil and gas reserves within the Arctic have certainly driven a lot ofthis interest (Figure 1.2). It is estimated by the United States Geological Sur-vey (USGS) that approximately 30% of the world’s undiscovered gas and 13%of the worlds undiscovered oil may be located in the Arctic ocean, and in manycases under no more that 500 m of water (Gautier et al. 2009).

Figure 1.2: Estimates of natural gas and oil reserves within the Arctic. Adaptedfrom Gautier et al. (2009).

The decline in sea ice extent, particularly during the summer, also rep-resents opportunities for the commercial shipping industry as longer ice freeconditions along the Siberian coast and potential new routes becoming naviga-ble during summer (Ho 2010; Smith and Stephenson 2013). These changes arein turn expected to bring increased tourism to the region (Stewart et al. 2007).

All evidence indicates that Arctic will under go a large increase in human

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occupancy before the end of the century. As as result, improvements in weatherforecasting in the region, particularly the forecasting of sea ice, constitute amajor area in need of improvement (Eicken 2013). In summary, understandingthe mechanisms that drive variability and climate change within the Arctic area major priority for both climate science as a whole and for the developmentof future industries in the region.

This kappa is outlined as follows. In chapter 2 I focus on the global circu-lation and Arctic climate as it is understood from our observations. The resultsof Paper I and Paper II, being drawn from observational evidence, are framedin this context. In chapter 3 I go on to describe the various types of data usedduring the course of this thesis, and then discuss some of the errors and uncer-tainties associated with computer modelling of the climate system. The resultsof Paper III and Paper IV, having focused on these issues, are framed in thiscontext. The kappa concludes with a summary of the papers included in thisthesis as well as a discussion of some interesting avenues for further research.

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2. The Arctic and its role inEarths climate system

This thesis has primarily focused on atmospheric circulation in the high north-ern latitudes and it’s relationship with Arctic climate variability and climatechange during winter. Given this, it is useful to outline the role the Arcticplays the larger global scale. This chapter aims to impart some basic conceptsand understandings of how the global climate system functions. The chapterbegins with a brief introduction to the climate system, and the role the Arcticplays in this global context. The second section describes the general climateof the Arctic during the winter season and discusses the results of Paper I inthis context. The final section discusses the observed phenomenon of Arcticamplification, the mechanisms by which it emerges and the contribution ofPaper II in this context.

2.1 Earths climate system

The global circulation of the atmosphere and oceans is primarily driven by apermanent temperature gradient maintained between the warm, moist equatorand the cold, dry poles. This temperature gradient emerges principally as aresult of Earths sphericity; which acts to diffuse solar radiation over largerareas towards higher latitudes, leading to a disproportionate warming of theequator with respect to the poles. Warmer air also has a propensity to containmore water vapour, a major greenhouse gas that traps heat radiated by theEarth towards space. Consequently, the global distribution of humidity closelyresembles the distribution of temperature.

The mean energy budget, averaged over the whole globe, is shown in Fig-ure 2.1a. Virtually all energy in the climate system is provided by the Sunsradiation, with a daily average of about 342 W m−2 received at the top of atmo-sphere (TOA). Not all of this radiation enters the the climate system however;while about 68% is absorbed directly by Earths surface (49%) and atmosphere(20%), the remaining 32% is reflected back to space by land surfaces (9%)and clouds (23%) (Kiehl and Trenberth 1997; Wild et al. 2013). This abilityof the Earth to reflect the Suns energy back to space is known as the albedo

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effect. Changes in the mean reflectivity of the Earth, through melting sea-icefor example, may therefore have important implications for the global energybalance, and hence the global climate.

Over time, the Earth remains largely in a state of radiative equilibriumat the TOA – with the net incoming solar shortwave radiation balanced byterrestrial longwave radiation emitted from the surface and atmosphere backto space. The zonal mean distribution of these radiative exchanges is shownin Figure 2.1b. One salient feature is that the energy fluxes are not balancedlocally – there is a net surplus of energy received at the equator, compensatedby a net deficit of energy emitted to space in the polar regions. The amplitudeof these radiative imbalances are related mostly to the presence of H2O in theclimate system. Firstly, the heterogeneous distribution of water in the climatesystem, through the greenhouse effect (Le Treut et al. 2007), acts to trap adisproportionate amount of Earths terrestrial heat at the equator with respectto the poles. Secondly, snow and sea-ice surfaces in the high latitudes reflectincoming solar radiation back space, thus contributing to the TOA radiativedeficit there.

Figure 2.1: (a) Earths global mean energy budget (Kiehl and Trenberth 1997)and (b) The climatological North-South distribution of the net radiative fluxes atthe top of the atmosphere. Adapted from Pidwirny (2006). Dashed black lineindicates the global mean value of the net radiative fluxes; F ≈ 235 W m−2

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At first glance, these energy imbalances seem to imply a continual cool-ing of the poles and warming of the equator, when in reality the temperaturein these regions remains relatively bounded over time. The atmosphere andoceans play a critical role in this respect, acting to balance the radiative deficitsin higher latitudes by a flow of energy from the equator towards the poles. Itis this meridional transport of energy that gives rise to the global circulationof the atmosphere and oceans. Without the circulation, the equator would beabout 15 K warmer and the poles about 24 K cooler (Pidwirny 2006). The bulkof this energy transport occurs within the atmosphere (Figure 2.2a), peaking atabout 35◦ latitude in both hemispheres – energy transport in the ocean domi-nates only in the narrow equatorial band between 20◦N and S (Trenberth andCaron 2001; Wunsch 2005). Beyond this band, the ocean begins to lose a greatdeal of its heat to the atmosphere – so that virtually all the energy transportedin the high-latitudes occurs within the atmosphere. Energy transport in the at-mosphere can be further split into its dry and moist components (Figure 2.2b),constituting the transport of temperature, potential and kinetic energy, and thetransport of water vapour, respectively. This thesis has focused primarily ontransport of the moist component and it’s influence on Arctic climate vari-ability and climate change. This line of inquiry has garnered a great deal ofattention within the community over the past few years, and it is along this linethat this thesis has mades it’s major contributions to scientific understanding.

The meridional transport of energy is in turn complicated by the Earth rota-tion via the Coriolis force – whereby poleward moving parcels of air, converg-ing towards Earths axis of rotation, feel an eastward force as a consequence ofconservation of angular momentum. More generally, the Coriolis force accel-erates the fluid to the right and left in the northern and southern hemispheres,respectively. It is this force that gives rise to the characteristic trade windsand westerlies of the tropics and mid-latitudes, respectively. As a consequenceof Earths rotation, the atmospheric circulation becomes increasingly zonal to-ward the poles, meaning that direct thermodynamic cells spanning each hemi-sphere are impossible. Poleward of about 30◦, the thermodynamically directHadley Cell breaks down (Frierson et al. 2007) and the meridional transportof heat occurs primarily within quasi-stationary planetary waves, about 10,000km across, and smaller transient eddies (synoptic systems), both cyclonic (lowpressure) and anticyclonic (high pressure), existing on timescales of 2-7 daysand with a spatial scale on the order of 1000 km. As much as 94% of the totalenergy transport into the Arctic is carried within the transient eddy systems(Dufour et al. 2016). As such, the link between the cyclonic systems of themid-latitudes and climate variability and change in the northern high-latitudesrepresents an interesting avenue of research, and has been one of the mainthemes of this thesis.

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Figure 2.2: Climatologies of the zonal mean meridional energy transport in (a)the atmopshere and ocean (Trenberth and Caron 2001) and (b) the atmospherealone – split into the dry and moist (latent) components (Graversen and Burtu2016).

The transient systems themselves cause the mid-latitudes to have highlyvaried and unpredictable weather. The intensity and regularity of these systemsis in turn closely related to the bulk meridional temperature gradient across theregion, which induces strong positive wind shear through the thermal windrelation, and hence acts as a source of baroclinic instability (Charney 1947;Eady 1949). Reduction of the temperature gradient between the equator andpoles could therefore have implications for mid-latitude weather (Barnes andScreen 2015; Schneider et al. 2015; Screen 2014; Shaw et al. 2016). In thisrespect, the Arctic plays a critical role in the climate system as a heat sink

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for Earths global circulation. As we will see, global warming has not beenhomogenous – the Arctic is warming at a disproportionally faster rate thanat lower latitudes. Arctic amplification therefore acts to lower the meridionaltemperature gradient, and as such may have implications for the global climatesystem. Although the effects of a reduced mid-latitude temperature gradient onArctic climate variability and change has not been assessed in this thesis, theprocesses that drive polar amplification are clearly a key priority in the broadercontext of the climate community – both in understanding the global climatesystem today, and how it is likely to change in the future.

2.2 The Arctic winter climate

The Arctic winter climate is characterised by a surfaced based temperature in-version, predominantly occurring over land and sea ice (Serreze et al. 1992;Tjernström and Graversen 2009). Surface based inversions are understood asconsequence of radiative cooling at the surface and horizontal advection ofheat from lower latitudes (Overland and Guest 1991; Zhang et al. 2011). Assuch, the Arctic winter boundary layer is typically characterised by periodsof stably stratified conditions, which can persist for weeks at a time (Bradleyand Keimig 1992). The presence of the temperature inversion has importantimplications for the magnitude and sign of the turbulent exchanges of heat andmoisture (Bintanja et al. 2011) and momentum (and hence sea ice drift, Over-land and Guest 1991) between the surface and atmosphere. As the majority of

Figure 2.3: The surface energy budget averaged over the area north of 70◦N.Fsfc denotes the mean total energy flux at the surface. SWsfc and LWsfc are theshortwave and longwave radiative flux components, respectively. QH and QEare the sensible and latent heat flux components, respectively. Positive valuesindicate an energy flux into the surface. Taken from Serreze et al. (2007).

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the region north of 70◦N is covered by ice during the winter, local sources ofheat and moisture are cut off from the atmosphere, making the Arctic winterclimate extremely dry and cold. As a result, net longwave radiation (NetLW)constitutes virtually the entirety of the surface energy budget (SEB) of the Arc-tic during winter (Figure 2.3). Thus, the Arctic winter climate is particularlysensitive to processes which influence radiative transfer in the atmosphere.Transport of water vapour from lower latitudes is an obvious mechanism bywhich large thermodynamic perturbations may be induced, particularly at thesurface (Doyle et al. 2011; Park et al. 2015c; Raddatz et al. 2013; Woods et al.2013). Warming of the surface in turn acts to erode the temperature inversion,which further impacts the SEB though reduced stability and increased turbu-lent fluxes at the surface (Woods and Caballero 2016). The processes by whichmoist air masses erode the temperature inversion, and the role of clouds, hasbeen addressed in a recent modelling study by Pithan et al. (2014), building onthe work of Wexler (1936) and Curry (1983).

In this sense, the Arctic winter climate can be considered as falling into twodistinct states – conditions oscillate between radiatively clear and cloudlessconditions, in which there is vigorous surface cooling (NetLW ∼ -40 W m−2)and a strong temperature inversion, and moist and cloudy conditions in whichthe surface NetLW is close to zero (NetLW ∼ 0 W m−2) and the inversion isweak (Morrison et al. 2011; Stramler et al. 2011). This behaviour is apparent

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in the strikingly bimodal distribution of NetLW observed during the SurfaceHeat Budget of the Arctic Ocean (SHEBA) experiment (Uttal et al. 2002) –an observational campaign which collected high frequency temporal data at anice-locked drifting site in the Beaufort Sea during the winter of 97/98. Even asmall change in the frequency of occupancy of these states could significantlyeffect the SEB and thus winter sea ice thickness (Morrison et al. 2011). Evi-dence has also linked these states to the general circulation of the atmosphere,with the clear and opaque states tending to occur during periods of high andlow surface pressure respectively, suggesting a link with dynamics of lowerlatitudes (Figure 2.4). Paper I of this thesis focused mainly on this connection.Our results showed that intense filamentary flows of moisture across the Arcticbasin, termed “moisture intrusion events” (akin to atmospheric rivers (Gimenoet al. 2014)), drive a significant amount of the temperature variability north of70◦N during winter. The phenomenology of these events are described in detailin Paper I and II. Moisture intrusion events typically originate over the oceanicbasins of the Arctic boundary and within the northward advecting branches ofmid-latitude cyclones held in place by blocking highs to the east. This resultwas confirmed in a later study by Liu and Barnes (2015). Changes in Arcticclimate may thus be related to dynamical and thermodynamic changes occur-ring in lower latitudes (Screen et al. 2012; Screen and Francis 2016; Screenand Simmonds 2013).

2.3 Arctic amplification

One of the most striking features of global climate change is the disproportion-ally large surface warming occurring in the Arctic with respect to the globalmean (Cohen et al. 2014; Screen and Simmonds 2010b; Serreze et al. 2009;Serreze and Francis 2006). This warming is in turn related to another promi-nent feature of global climate change: the dramatic sea ice loss occurring in theArctic ocean, which has been largest during summer (Hartmann et al. 2013).Declining sea ice extent has previously been thought of as the main driverof Arctic amplification though the surface albedo feedback mechanism, firstproposed by Arrhenius (1896) over a century ago – whereby increasing tem-perature leads to snow and ice melt in the high-latitudes and enhanced absorp-tion of solar radiation at the surface (Kwok et al. 2009; Serreze et al. 2009;Serreze and Francis 2006). However,the strength of this feedback mechanismhas proven difficult to quantify. Firstly, the increase in solar energy absorbedby the surface is partly compensated by the enhanced radiative cooling andevaporation from ice free surfaces. These cooling processes induced by sea iceloss may also continue throughout the whole year, not just the summer months.How these energy fluxes balance out in the long term remains difficult to quan-

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tify precisely (Flanner et al. 2011). Furthermore, climate model experiments inwhich the surface albedo feedback is suppressed still show polar amplification(Alexeev et al. 2005; Caballero and Langen 2005; Graversen and Wang 2009),suggesting that other mechanisms are also a factor. All else being equal, Arc-tic amplification is expected in response to global warming on account of thePlanck feedback. Given an homogeneous global mean forcing ∆F , induced bya doubling of CO2 concentrations in the atmosphere for example (Griggs andNoguer 2002), the temperature change expected from the Stefan-Boltzmannlaw, ∆T = ∆F/(4σT 3), depends on the background temperature T . Polar re-gions, being colder than lower latitudes, will therefore tend to warm more fora given forcing. Arctic amplification is also expected from the outset due tothe lapse-rate feedback – whereby the stably stratified boundary layer trapsheat disproportionately close the the surface and leads to a bottom amplifiedwarming signal (Manabe and Wetherald 1975).

A body of literature addressing other mechanisms of Arctic amplificationhas grown rapidly over the last 10 years. Ocean heat transport (Sato et al.2014; Spielhagen et al. 2011), propagating planetary waves from the tropics(Ding et al. 2014; Lee 2012; Lee et al. 2011; Yoo et al. 2011) and increaseddownwelling longwave radiation associated with moisture transport from mid-latitudes (Bintanja et al. 2011; Francis and Hunter 2006; Graversen and Wang2009; Kapsch et al. 2013; Park et al. 2015a,b,c; Woods and Caballero 2016)have all contributed to observed Arctic amplification. Paper II focused on thelatter of these mechanisms. A positive trend in the frequency of moisture in-trusion events during Dec-Jan was shown to have driven approximately 45%of the surface temperature trends and 35% of the sea ice trends observed inthe Barents Sea over the 1990–2012 period. Our results lead us to suggest amore nuanced view of the observed bottom amplified warming structure. Pre-viously, bottom amplified warming has been thought of as a consequence ofincreased upward turbulent heat flux due to sea ice decline. Moisture intru-sions tracking over marginal sea ice zones increase the radiative and turbulentenergy fluxes into the surface over a relatively short time period (about 2 dayson average) and erodes the temperature inversion, yielding an overall bottom-amplified warming (Cronin and Tziperman 2015; Pithan et al. 2014). The re-sponse of the sea ice has a much longer memory however – negative anomaliespersist up to two weeks after the initial thermodynamic perturbation providedby the intrusion. In the time mean therefore, intrusions act to increase the tur-bulent heating of the atmosphere via removal of sea ice. A positive trend inintrusions can therefore be thought of as driving both sea ice decline, bottomamplified warming and increased turbulent heating of the lower atmosphere(which itself produces bottom amplified warming) in the Barents Sea.

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3. Data

This chapter discusses the various datasets used during the course of this the-sis. Firstly there is an overview of the reanalysis data which comprised themajority of data used in Paper I, II and IV. A brief discussion of the in-situ me-teorological data used in Paper II then follows. In the second section I discussthe climate model data used in Paper III. Finally, the chapter concludes witha discussion of some of the modelling difficulties and frame Paper III and IV,which focused on global climate model (GCM) uncertainties, in this context.

3.1 Reanalysis and in-situ data

Reanalysis data is a blend of in-situ and remotely-sensed observations, assim-ilated within a computer model that forecasts the weather. The advantage ofthis approach is that many sources of data from the Global Observing System(GOS, Figure 3.1) can be combined into a smooth, homogenous and consis-tent dataset with a much larger temporal and spatial coverage than any of thedatasets could provide alone. Forecasting of weather involves the integrationof a computer model forward in time from an initial state based on the mostcurrent observations. Reanalyses of the past are constructed in much the sameway, but with one crucial difference – observations from the past are availableat each time step of the integration, allowing for the model derived fields to becontinuously augmented towards reality. This leads to a dataset with a muchhigher accuracy than a dataset made purely of forecasts alone. Of course, re-analyses can only be constructed over time periods for which observations areavailable. This thesis is primarily based on reanalysis data obtained from theEuropean Centre for Medium Range Weather Forecasting (ECMWF). This in-stitution is considered the best global forecasting centre in the world, and theirreanalysis products are used widely throughout the climate science commu-nity. Paper I, II, III and IV all made used of the ERA-Interim reanalysis set(Dee et al. 2011), which has been shown to outperform other reanalyses inthe Arctic (Jakobson et al. 2012). Reanalysis data facilitates the analysis oflarge scale phenomenon in the atmosphere, for example moisture intrusionsand their associated circulations.

In Paper II we made use of in-situ measurements from radiosonde sound-

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Figure 3.1: A schematic of the Global Observing System. Numbers indicate theapproximate number of observations made every hour. Adapted from a presenta-tion by Dr. Tony McNally: The Global Observing System (DA Training Course).Made available from the ECMWF website.

ings along the Arctic coastline. The stations locations can be found in Fig-ure 1 of Paper II. This data was used to validate results using reanalysis data(see Figure 4a-d and 6 in Paper II). Radiosondes provide measurements of thetemperature, pressure and humidity with height. They are typically deployedbetween 2–4 times per day at thousands of locations around the globe.

3.2 Climate model data

Climate model data is constructed in much the same manner as reanalysisdata, but without the assimilation of observations. The value of these climatedatasets is in the time mean of their output therefore. GCMs also differ withrespect to weather prediction models, like that used at the ECMWF, in that theyare typically coupled to a global ocean circulation model (OGCM). Processesacting to exchange energy between the atmosphere and ocean therefore play acritical role in accurate climate simulations over long timescales. Furthermore,while a weather forecast model may routinely be integrate over 10 days, GCMsare often integrated over century and even millennial timescales. As such sev-eral trade offs are apparent between the numerical systems operated; includingreduced resolution in the horizontal and vertical and and an increased role forparametrisation of sub grid processes. One of the aims of climate model analy-

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sis is to minimise the discrepancies between the model output and the observedmodern climate as represented by reanalysis data. This issue was addressed inPaper III in relation to known and systematic Arctic biases in the climate mod-els. Identifying sources of these biases constitutes a route to improving theaccuracy of future climate projections.

Paper III made use of the phase 5 Couple Model Intercomparison Project(CMIP5) simulation archives. We used the historical present day simulationsfrom 31 models and evaluated the representation of moisture intrusions. De-tails of the models used and their operational grids can be found in Table 1 ofPaper III.

3.3 Modelling errors and uncertainties

A number of issues in the ERA-Interim reanalysis, in relation to the winterclimate, have been addressed before. Most notably, mixed-phase clouds havebeen shown to contain an excessive amount ice (Engström et al. 2014), whichacts to reduces cloud opacity and likely leads to an underestimation of thethermodynamic perturbations induced by moisture intrusions. Furthermore,the surface albedo of ERA-Interim is prescribed from climatological valuesbased on Ebert and Curry (1993), and as such, interactions between ice, oceanand atmosphere are likely misrepresented (Karlsson and Svensson 2013).

A weak Atlantic Meridional Overturn Circulation (AMOC) in CMIP5 mod-els is known to bias the Arctic region cold (Wang et al. 2014). Furthermore,a large spread in the radiative impacts of clouds are a source of uncertainty(Karlsson and Svensson 2013). Paper III assessed the representation of mois-ture intrusion events in 31 of the CMIP5 models. Zappa et al. (2013) re-ported biases in the storm tracks of the models, showing that the Atlanticstorm tracks are typically shifted southward and are overly zonal, while thePacific storm tracks are typically shifted northward. We confirmed this re-sult in Paper III, and further demonstrated that similar complimentary biasesin the northward moisture transport at 70◦N existed. The overall structure ofbiases in the CMIP5 models is largely characterised by a dipole patter cen-tred over the north pole, with cold/dry and warm/moist biases in the Atlanticand Pacific sectors respectively. The dynamical biases are the likely sourceof biases in the northward moisture transport (Figure 2 Paper III), which arepredominantly contributed by a misrepresentation of the intense fluxes. Mis-representation of intense moisture transport therefore impacts the distributionof moisture injection events around 70◦N. As a result, the CMIP5 models overestimate the number of intrusion originating the Atlantic sector, and under-estimate the number of intrusion events originating in the Pacific sector eachseason. Using a similar methodology to that developed in Paper II, we showed

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that biases in the frequency of moisture intrusion events cause about 17% ofthe surface temperature and 24% of the surface downward longwave radiationbiases over the Atlantic sector during winter. Furthermore, our methodologyexplained about 14% and 16% of the gradients of biases between the Atlanticand Pacific sectors in these fields, respectively. The predicted gradients, whilesmall in amplitude, were highly correlated with the true bias gradient in themodels. The high skill of our methodology, coupled with the small amplitudein the predicted signal, lead us to conclude that strong feedbacks likely existbetween intense moisture transport at 70◦N and sea ice in the CMIP5 models.

Paper IV dealt with the method-related uncertainties in the parameterisa-tion of turbulent air-sea fluxes (TASFs). TASFs constitute the primary mech-anism by which heat, moisture and momentum are exchanged between theocean and atmosphere. As such, they represent a critical component of theocean-atmosphere coupling in climate models (Brodeau et al. 2010). TASFsalso impact the atmosphere, most notably in mid-latitude western boundarycurrent regions where they can modulate the atmospheric responses at meso-and synoptic-scales (Minobe et al. 2008; Small et al. 2008; Zolina and Gulev2003). Direct measurements of TASFs, however, tend to be idealised, infre-quent and highly localised in space. As such, the datasets cannot be used tobuild global climatologies of TASFs. Rather, TASF climatologies are routinelyestimated using what are called “bulk formulae” – expressions which relatethe TASFs to more widely available surface meteorological variables, such asthose available in reanalysis datasets. The traditional bulk formulae used toestimate TASFs are shown in equation 1 of Paper IV. Accurate estimates ofTASFs depend primarily on the bulk transfer coefficients (BTCs), which arewind speed and stability dependent parameters that determine the strength ofthe coupling between the atmosphere and ocean. BTCs are determined in aniterative manner by what is referred to as the “bulk algorithm”. Discrepanciesbetween the most common bulk algorithms in use today have been shown tolead to large uncertainties in the estimated TASFs (Brunke et al. 2002). InPaper IV we reported a disagreement of about 10% between three of the mostcommon bulk algorithms. The main focus of this study was, however, relatedto the uncertainties arising due to assumptions regarding the density of air,sea surface saturation humidity, sea level pressure, cool skin and warm layereffects (a consequence of radiative fluxes during day and night), and sea sur-face currents. Previously, it has been common practice to ignore uncertaintiesassociated with these approximations, as they typically lead to much smallerdisagreements, compared to those arising due to the choice of bulk algorithm,when tested individually. We demonstrated in Paper IV however, that multipleparameter approximations, when implemented in tandem, can lead to disagree-ments between TASF climatologies on the the order of 20%. This highlights

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the importance of making appropriate choices in the configuration of parame-ter approximations and bulk algorithms when computing TASF climatologies.

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4. Conclusions

This thesis has predominantly focused on high latitude circulation and extrememoisture transport into the Arctic during winter in a number of published arti-cles and manuscripts. An atmospheric phenomenon termed a "moisture intru-sion" is defined, and it’s phenomenology studied. The long tail in the distribu-tion of northward moisture fluxes at 70◦N highlights the outsized role playedby extreme events and motivates the episodic based approach used in Papers I,II and III.

4.1 Summary of papers

Paper I investigates the impacts of moisture intrusion events on the inter-annualvariability of the Arctic winter climate, as well as the large-scale circulationassociated with their emergence. We demonstrate that extreme episodes ofnorthward moisture transport are typically characterised by a low pressure sys-tem held in place by a blocking high to the east. This result was confirmed ina later study by Liu and Barnes (2015). Paper I also shows that moisture intru-sions events exert a strong control on Arctic climate variability, despite onlycontributing a minority of the total moisture transport. The residual majority ofthe moisture transport has been shown to have no significant climate impacts,again highlighting the outsized role of these events.

Paper II is a logical continuation from Paper I, aiming to quantify the con-tribution of moisture intrusion events to observed Arctic amplification duringwinter. While Paper I is concerned with inter annual variability, Paper II fo-cuses on the long term changes in temperature and sea ice induced by intrusionevents. Moisture intrusions were shown to have driven approximately 45% ofthe surface temperature and 35% of the sea ice concentration trends in theBarents Seas during December and January over the past two decades. Wealso show that moisture intrusions induce a bottom amplified warming signal,even within the ice-locked region north of 80◦N. An increase in the frequencyof moisture intrusions events can therefore drive a bottom amplified warm-ing trend even in the absence of sea ice loss. This vertical warming structurehad previously been attributed to increased upward turbulent heat fluxes dueto sea ice loss (Screen and Simmonds 2010a). Lagged composites focusing

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on impacts in the Barents Sea show that upward turbulent heat flux anomaliesemerge about 10 days after the passage of a moisture intrusion through the re-gion. Paper II concludes by offering a more nuanced view of the link betweentrends in surface turbulent fluxes and bottom amplified warming. Moisture in-trusions modify the vertical temperature structure, remove ice, and lead, in thetime mean, to an increase of upward turbulent heat fluxes from the newly openocean and thus further bottom amplified warming.

Paper III focuses on the representation of moisture intrusion events in 31of the models from phase 5 of the Coupled Model Intercomparison Project(CMIP5). We aim at quantifying the contribution of the misrepresentation ofmoisture intrusions to systematic biases evident in the models during winter.We show that the Arctic winter biases in the CMIP5 model are characterisedby dipole pattern centred over the north pole, with cold/dry and warm/moistbiases in the Atlantic and Pacific sectors respectively. The cold temperature bi-ases in the Arctic sector are in turn associated with very large positive biases inthe sea ice concentration there. We also confirmed reports of dynamical biasesrelated to under- and overestimation of synoptic activity in the Atlantic andPacific sectors, respectively (Zappa et al. 2013). Biases in the moisture fluxesaround 70◦N are shown to exhibit the same qualitative dipole pattern alreadynoted, with negative and positive biases in the Atlantic and Pacific sectors, re-spectively. Interestingly, these biases tend to compensate in the zonal mean,leading to small net-baises overall in the models. We note that the biases ineach sector are predominantly contributed by the high-intensity instantaneousfluxes – the same fluxes which were shown to have significant climate impactsin Paper I and II. These biases lead, in turn, to negative and positive biases inthe frequency of moisture intrusion events tracking over the Atlantic and Pa-cific sectors. We quantify the contribution of these moisture intrusion biases tothe true biases in the Atlantic and Pacific sectors using the methodology devel-oped in Paper II. Our predicted biases, while perhaps small in amplitude, werevery highly correlated with the true biases in the models. This fact, coupledwith the excessive sea ice concentration in the Atlantic sector, lead us to con-clude that strong feedbacks may exist in the CMIP5 models between intensemoisture transport at 70◦N and sea ice. Correction of this potential feedback,as well as improvement in the representation of mid-latitude storm tracks rep-resent a clear avenue for mitigating Arctic climate biases in some key fields.

Paper IV focuses on quantifying the uncertainties associated with variousapproximations in the parameterisation of turbulent air-sea fluxes (TASFs).This study is written mostly for the benefit of the general climate-model prac-titioner – someone without expertise in turbulent heat, moisture and momen-tum exchanges between the air and sea, but who nonetheless needs to make adecision regarding which configuration of parameter approximations and bulk

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algorithm are appropriate for the given needs. We compute TASFs over a seriesof sensitivity experiments using the surface state variables from ERA-Interim– each sensitivity experiment differs from a pre defined control experiment inonly one aspect of the parameterisation (see table 1 in Paper IV for a summaryof the aspects tested). In this way we can directly quantify the uncertainties inTASF climatologies associated with each approximation, as well as the choiceof the bulk algorithm itself. Not surprisingly, the largest sources of uncertaintyarise due of the choice of bulk algorithm itself. Disagreements between thebulk algorithms highlight the need for validation of these methods with respectto observations. The majority of the approximations and assumptions testedhave limited impacts by comparison. However, in cases where multiple ap-proximations are implemented in tandem, computed TASF climatologies maydiffer by as much as 20%. Paper IV demonstrates that uncertainties related toparameter approximations in the bulk formulae used to compute TASFs cannot always be neglected.

4.2 Outlook

Paper I and Paper II highlighted the important role played by mid-latitude dy-namics in driving Arctic climate variability and change during winter. Theinterplay between transient eddies and the larger-scale quasi-stationary wavesconstitutes a large part of this connection. As such, this avenue of research hasbeen highlighted as an area in need of research recently by Dufour et al. (2016).Dufour et al. (2016), in an analysis of seven reanalysis products, also demon-strated that the proportion of total poleward moisture flux contained withinthe transient eddies has been increasing over the past few decades. Determin-ing the precise connection between these different wave scales is therefore aninteresting research avenue with respect to Arctic amplification – especiallyin light of some recent studies reporting positive trends in poleward moisturetransport at the Arctic boundary (Mattingly et al. 2016; Maturilli and Kayser2016; Zhang et al. 2013).

Paper III touched briefly on a potential link between moisture intrusionevents and stratospheric dynamics (see Figure 5). Moisture intrusions originat-ing in Atlantic and Pacific sectors were associated with a cooling and warmingof the stratosphere, respectively. One may conclude from this point that thestratospheric response to intrusions is not primarily determined by thermody-namic impacts in the troposphere. Rather, the upper-level response suggestsa link with the heat flux in the stratosphere, which is in turn associated withvertical coupling of waves between the troposphere and stratosphere (Shawand Perlwitz 2013). We have confirmed that the stratospheric temperature per-turbations associated with intrusion events are indeed correlated with strato-

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spheric heat flux, with negative and positive stratospheric heat flux anomaliesassociated with intrusions originating in the Atlantic and Pacific sectors re-spectively.

Extending our moisture intrusion methodology to other seasons and hemi-spheres is an obvious avenue to pursue. Kapsch et al. (2013) showed thatpositive downward longwave radiation anomalies in spring exerted a strongcontrol on sea ice extent the following summer . Intrusions could potentiallybe a proximate cause of these early radiative anomalies. If so, intrusions mayrepresent a source of inter seasonal variability, as well as the inter annual vari-ability demonstrated in Paper II. Unlike the Arctic, the Antarctic region hasnot experienced significant polar amplification over recent decades. Moistureintrusions into the Antarctic may not play as central a role as in the Arctic dueto the lack of zonal land-ocean contrasts and orography in the Antarctic Cir-cumpolar Current (ACC), leading to unfavourable conditions for the formationof large-scale quasi-stationary planetary waves. As the interaction between thetransient and stationary waves represents a critical feature of the moisture in-trusions in the northern high-latitudes, one might expect moisture intrusionsin the southern hemisphere to be less frequent. Applying the methodology inPaper I and Paper II in the southern hemisphere may be illuminating.

Finally, a logical follow up to Paper III would be an assessment of thechanges in statistics of moisture intrusion events under global warming in sim-ulations of the 21st century made for CMIP5. With many models predictingan ice free summer before century’s end (Wang and Overland 2012), quantify-ing the potential contribution of moisture intrusion events to this decline is aninteresting open question.

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Acknowledgements

First and foremost, I would like to express a deep sense of gratitude towards mymain supervisor Rodrigo Caballero. Thank you for giving me the opportunityto study here at MISU and for the extremely thoughtful and involved guidanceyou have shown to me over the past few years. Your passion for science hasleft a very meaningful impression on me, and it has been a real privilege towork with you. I also wish to thank my second supervisor Gunilla Svenssonfor all the illuminating and stimulating discussions over the years, as well asthe expert scientific guidance she has shown to me. I thank you both for thetime we have shared together. Beyond my two supervisors, I would also likethank those with whom I have collaborated with over the years. These includeProf. Eyal Heifetz of Tel Aviv University, Prof. Raymond Pierrehumbert ofOxford University (formerly University of Chicago), Prof. Nili Harnik of TelAviv University, and Dr. Laurent Brodeau of the Barcelona SupercomputingCentre (formerly MISU). Thank you all for your excellent suggestions, sci-entific discussions and contributions to my growth as a scientist. I thank theBolin Centre for Climate Research, the Swedish Research Council, and theSwedish e-Science Research Centre (SeRC) for supporting my research andtravel during the course of my Ph.D studies.

To all of you who make MISU such a wonderful place to work, I extendthe warmest of thanks. Firstly to my office mates Marie, Wing and Eva for pro-viding such wonderful company. I thank Marie in particular for her depth ofknowledge on Arctic climate which she shared with me frequently. Thanks toFabien, Quentin, Abubakr and Marin for entertaining my political/economicinterests over the years and for the great discussions we’ve had over beer.Anna, thank you for always taking care of my sourdough when I was on ex-tended travel! Thanks to Erik, Lina, and Kerstin for the amazing job they didtranslating my abstract into Swedish! Sara, thank you for all the invitations toparties and the many delicious home-brews you’ve shared with me. To Malin,one of the honorary great friends! It’s been a lot of fun working and hangingout with you. Henrik, it’s been a great to see oursleves grow over the pastyears, and I wish you luck with you thesis. There are so many people to name,so many who have already left MISU. You have all left me with such fondmemories. I wish you all the best life can offer.

To my great friends Laurent, Saeed and John. I’m actually quite emotional

as I type this. Your friendships have meant so much to me over the past years,and continue to do so. All the time we have spent together, be it holidays,nights out, great friend lunches or sharing the ups and downs of our lives, hasbeen the most memorable part of my Swedish journey. I feel honoured toconsider you among my best of friends, and although we may move apart inlife from now, I will always cherish our friendships.

Finally, I wish to thank my family for all their support and frequent visitsover the years. To my mother and father, thank you for all the love you’veshown and for consoling me in times of doubt. To my two brother, your visitswere a real boon over the years and something I always looked forward to totake my mind of the stress. My love to you all.

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