The future of the western Baltic Sea: two possible scenarios

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  • Ocean Dynamics (2013) 63:901921DOI 10.1007/s10236-013-0634-0

    The future of the western Baltic Sea: two possible scenariosUlf Grawe Rene Friedland Hans Burchard

    Received: 2 August 2012 / Accepted: 5 June 2013 / Published online: 3 July 2013 Springer-Verlag Berlin Heidelberg 2013

    Abstract Globally coupled climate models are generallycapable of reproducing the observed trends in the glob-ally averaged atmospheric temperature. However, the globalmodels do not perform as well on regional scales. Here, wepresent results from four 100-year, high-resolution oceanmodel experiments (resolution less than 1 km) for the west-ern Baltic Sea. The forcing is taken from a regional atmo-spheric model and a regional ocean model, imbedded intotwo global greenhouse gas emission scenarios, A1B and B1,for the period of 2000 to 2100 with each two realisations.Two control runs from 1960 to 2000 are used for valida-tion. For both scenarios, the results show a warming with anincrease of 0.52.5 K at the sea surface and 0.72.8 K below40 m. The simulations further indicate a decrease in salinityby 1.52 practical salinity units. The increase in water tem-perature leads to a prolongation of heat waves based onpresent-day thresholds. This amounts to a doubling or eventripling of the heat wave duration. The simulations showa decrease in inflow events (barotropic/baroclinic), whichwill affect the deepwater generation and ventilation of thecentral Baltic Sea. The high spatial resolution allows usto diagnose the inflow events and the mechanism that willcause future changes. The reduction in barotropic inflowevents correlates well with the increase in westerly winds.The changes in the baroclinic inflows can be consistentlyexplained by the reduction of calm wind periods and thusa weakening of the necessary stratification in the westernBaltic Sea and the Danish Straits.

    Responsible Editor: Aida Alvera-Azcarate

    U. Grawe () R. Friedland H. BurchardDepartment of Physical Oceanography and Instrumentation,Leibniz Institute for Baltic Sea Research,Warnemunde, Germanye-mail: ulf.graewe@io-warnemuende.de

    Keywords Regional ocean models Climate change Baltic Sea Baltic inflow

    1 Introduction

    Climate change and variability affect the coastal zone, themarine ecosystem and fisheries in several ways. First, tem-perature has a direct influence on metabolism and growth;see, for example, Jobling (1996). Climate may also havesecondary effects, affecting a species by changes in foodavailability, competitors, or predators. For the North Sea andBaltic Sea, there are several recent studies on the effects ofclimate change on fish stock and plankton (Clark et al. 2003;Isla et al. 2008; Margonski et al. 2010). Temperature andsalinity changes may also act as proxies for other climatemechanisms such as circulation alterations and changes invertical mixing and stratification.

    The Baltic Sea is a marginal, semi-enclosed water body,with a highly stratified water column. The salinity con-tent and stratification in the Baltic Sea is controlled bythe excess of riverine freshwater, the vertical mixing andepisodic inflows of North Sea water (Reimann et al. 2009).These events transport saline, oxygen-rich North Sea waterinto the Baltic Sea and are an important climate variable(Omstedt et al. 2004; Meier 2007). In addition to the influ-ence on the salinity, these inflows are also a source ofnutrients and zooplankton (Feistel et al. 2008). Due tothis highly variable environment, life in the Baltic Sea isstrongly adapted and often reaches its physiological limits(e.g., Flinkman et al. 1998; Koster et al. 2003). Moreover,large-scale variability in the atmospheric forcing, and hencechanges in local climate, might lead to a significant increasein coastal erosion due to changes in storm surges or waveaction (Meyer et al. 2008; Zhang et al. 2010).

  • 902 Ocean Dynamics (2013) 63:901921

    The climate changes projected to occur within the next100 years will have a considerable impact on the physi-cal conditions of the Baltic Sea (BACC 2008). The pro-jected warming varies between 3 and 5 K, with a tendencytowards a reduced salinity. To study the implications ofthe projected climate change (Meier et al. 2004, 2006;Wang et al. 2008; van Roosmalen et al. 2010), a vari-ety of scenarios of future climate are needed. Such sce-narios are produced by globally coupled atmosphereocean circulation models. However, for shallow seaslike the western Baltic Sea, the present generation (butalso the next generation) of such global models do nothave the necessary resolution to properly resolve thecomplex topography. Typically, they also lack impor-tant shelf sea physical processes like turbulent mix-ing, overflows and fronts. Hence, there is an increasingneed to use regional ocean models to provide valuable,high-resolution information to governments, stakeholdersand coastal engineers (Adlandsvik and Bentsen 2007;Melsom et al. 2009; Holt et al. 2010; Grawe and Burchard2012; Olbert et al. 2012).

    This paper uses dynamical downscaling to regionalisefuture global climate scenarios for the western Baltic Sea.Model studies on the western Baltic Sea allow to seecumulative effects of the climate change in the Baltic Sea,because all water masses that enter or leave the Baltic haveto pass the Danish Straits (Great Belt and resund; seeFig. 1). The modelling is done by forcing a well-calibratedhigh-resolution local ocean model (Burchard 2009) with

    atmospheric forcing and open ocean lateral boundarydescription from a regional atmospheric and a regionalocean model. The final spatial resolution of the local modelis less than 1 km, which allows for a realistic descriptionof topographic features like sills, sounds and coastlines. Itwill be shown that such a high spatial resolution is neces-sary for properly reproducing inflow events into the westernBaltic Sea. The forcing used in the present study are twogreenhouse gas emission scenarios proposed by the Inter-governmental Panel on Climate Change (IPCC 2007), A1Band B1, with each two realisations. The latter scenario isthe more optimistic one, with less greenhouse gas emis-sions. The presented transient simulations range from 1960to 2100 and are a novel feature in regionalised ocean cli-mate modelling, because they do not rely on the assumptionthat the underlying system has reached a dynamical steadystate (see also Neumann 2010). Meier and Kauker (2003b)discussed that for the Baltic Sea with an average exchangetime of 35 years, the usual 30-year time slice experimentsmust fail, especially for salinity scenarios. The memory ofthe Baltic Sea is longer than the simulations itself would be.Thus, only the sudden transition phase could be observedand not the response of the Baltic Sea to a slowly changeclimate.

    The outline of the paper is as follows: In the next sec-tion, we briefly recall the hydrodynamics of the westernBaltic Sea and the driving mechanisms of salt inflows intothe Baltic Sea. In Section 3, we explain our modelling strat-egy and the necessity of using a high-resolution model for

    Fig. 1 Model domain of thelocal model, open boundariesand location of the westernBaltic Sea. The colouringindicates the depth below meansea level in metre. Upper panel:a map of the whole Baltic Seashowing the location of themodel domain. The location ofobservation stations are asfollows: KB, Kiel buoy; DSB,Darss Sill buoy; OBB, OderBank buoy; and ROS Rostock.Model output is used at GRT,Gedser Rev transect;ABB-Arkona Basin buoy; andBBS, Bornholm Basin station.Further, GB denotes the GreatBelt and OS, the resund. Theblue circles denote the positionof river mouths. The thick bluelines indicate the location ofopen boundaries

    Kattegat

    KB

    OBB

    ABB

    BBS

    DSB

    ROS

    GRT

    OS

    GB

    Bornholm Basin

    Arkona Basin

    Odra lagoonGermanyGermany

    Sweden

    Denmark

    Denmark

    Poland

    0 50 100 km50

    10 E 12 E 14 E 16

    E

    54 N

    55 N

    56 N

    57 N

    100

    80

    60

    40

    20

    0

    0 10 E 20 E

    55 N

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  • Ocean Dynamics (2013) 63:901921 903

    downscaling. We give details on the forcing scenarios andthe performed numerical experiments. Moreover, we intro-duce diagnostic measures like heat wave duration, potentialenergy anomaly, salt and volume fluxes to quantify changesin the climate projections. Section 4 deals with the vali-dation of present-day conditions and sets the stage for thediscussion of changes in the climate runs which are anal-ysed in Section 5. Here, a special focus is put onto shifts inthe inflow dynamics. Finally, we end the paper with a shortconclusion in Section 6.

    2 Dynamics of the western Baltic Sea

    The transition region between the Kattegat and the BalticSea, the western Baltic Sea (Fig. 1), is characterised byshallow and narrow straits (the resund and Great Belt,also called Danish Straits; OS and GB in Fig. 1), flow-limiting sills (Darss Sill; GRT in Fig. 1 and Drogden Sillat the southern end of the resund), but also several basinswith depths of 50100 m (Arkona Basin and BornholmBasin) (Fennel and Sturm 1992; Siegel et al. 2005). Thewater exchange through the western Baltic Sea can be sep-arated into three components. At first, there exist a nearlypermanent outflow of approximately 15,000 m3/s of brack-ish Baltic Sea water with a salinity of 7 practical salinityunits (psu). This flow directing into the Kattegat is causedby the excess of freshwater input from rivers discharginginto the Baltic and the net precipitation (Hordoir and Meier2010). Secondly, the baroclinic pressure gradient across theDanish Straits, fed by the salinity difference of 2025 psubetween the Kattegat (34 psu) and the Arkona Basin (7 psu),causes episodic inflows of saline water into the Baltic Sea(Sellschopp et al. 2006; Reimann et al. 2009). Finally, thebarotropic sea level differences between the Kattegat andthe Baltic Sea can additionally trigger the inflow of Katte-gat water into the western Baltic Sea (Matthaus and Franck1992). The abovementioned Baltic inflows manifest as bot-tom gravity currents. These inflow events occur irregularly,from repeated events within a single year to stagnation peri-ods lasting for a decade (Matthaus 2006). What they havein common is that they only take the route via the GreatBelt and Darss Sill. The significant impact of such eventson the physical, chemical and biological status of the BalticSea has intensively been investigated in the past 50 years.A recent review on these studies was given by Matthaus(2006). These inflow events have the potential for deepwaterventilation of the Gotland Basin (Feistel et al. 2008).

    To characterise the barotropic inflows, also called majorBaltic inflows (MBIs), Matthaus and Franck (1992) usedan indicator time series of the bottom salinity at GedserRev (GRT, see Fig. 1). Matthaus and Schinke (1994)showed that as a preconditioner, the water level within

    the Baltic has to be lowered by easterly winds, lasting for24 weeks. These easterly winds have to be followed bystrong westerly winds to create a sea level difference inthe order of 1 m between the Kattegat and the ArkonaBasin.

    The baroclinic inflow events are driven by baroclinicpressure gradients, especially horizontal salinity differencesbetween the Kattegat and Bornholm Basin (Feistel et al.2004, 2008). They appear during persistently calm windconditions lasting for more than 14 days (usually in the latesummer) and are characterised by a significant stratifica-tion at Darss Sill. Whereas the driving mechanism of thebaroclinic inflows is well understood, long-term statistics ofthese events are missing. Based on recent observations andmodelling studies (Feistel et al. 2004; Meier et al. 2004),it is estimated that the salt transport associated with baro-clinic inflows is at least a factor of 510 smaller comparedto MBIs. Moreover, Burchard et al. (2005) showed that thebalance in gravity-driven plumes in the Arkona Sea is notonly between pressure gradients and the Coriolis acceler-ation, but also the bottom friction plays a substantial role.They concluded that the local topography plays an importrole in balancing the gravity currents.

    Besides the barotropic dynamics and the saline flow, thewestern Baltic Sea shows a summer temperature stratifi-cation leading to a three-layer system with a well-mixedseasonal thermocline in the upper 20 m and a permanenthalocline through inflows and the dense bottom water poolin the deeper basins.

    Concluding, due to the narrow channels and sills incombination with the thermohaline dynamics, the westernBaltic Sea is a challenging region to model. Thus, high spa-tial and vertical resolution is required to capture verticalstratification and fronts due to inflows (Umlauf et al. 2007).

    3 Methods and data

    3.1 Modelling strategy

    In this study, we use a high-resolution local ocean modelthat is the last part of a model chain. The downscaling startswith a global climate model and subsequent nestings of aregional ocean and atmospheric model. Finally, the Gen-eral Estuarine Transport Model (GETM) is used as the innermodel (local model) forced by the regional scale models.The usage of the high-resolution local model is motivatedby the following reasons: Fischer and Matthaus (1996) andLintrup and Jakobsen (1999) showed the importance of theresund for the water exchange of the Baltic Sea. Thisnarrow strait (a width of 3 km at the narrowest position)is difficult to resolve, even in regional-scale ocean mod-els (Meier 2006; Neumann 2010). For instance, the 5-km

  • 904 Ocean Dynamics (2013) 63:901921

    (3-nm) Modular Ocean Model (MOM) setup of Neumann(2010) with its B-grid, needs at least two grid cells to obtaina velocity cell, so that a channel has a minimal broadnessof 10 km. This also implies that the cross-sectional areaof the Danish Straits is changed (keeping the depth con-stant), or the depth of the straits has to be changed (keepingthe cross-sectional area constant). Both changes will alterthe stratification characteristics, the flow structure and thevolume transport in the Danish Straits.

    An advantage of GETM is its usage of bottom-following coordinates. Although -style coordinate systemsare known to cause problems due to discretisation errorsof the internal pressure gradient (Haney 1991), they areadvantageous for modelling gravity currents, and they donot need an additional overflow/gravity currents parameter-isation like in geopotential ocean models (Beckmann andDoscher 1997). Further, Griffies et al. (2000) and Ezer andMellor (2004) (and references therein) discussed the prob-lems of artificial numerical mixing using z-level coordinatesin modelling overflows/gravity currents. Due to this addi-tional mixing in z-level models, the salt flux through theDanish Straits needs to be increased for geopotential modelsto match the long-term salinity content in the deeper basinsof the Baltic. GETM is used as a nested model; there is noneed to have this artificially higher salt transport throughthe Danish Straits. A detailed validation of the salt transportis discussed in Section 4.3. To further stress the importanceof...

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