Northern Hemisphere Winter Storm Tracks of the Eemian Interglacial and the Last Glacial Inception

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Clim. Past, 3, 181–192, 2007 www.clim-past.net/3/181/2007/ Climate © Author(s) 2007. This work is licensed of the Past under a Creative Commons License.

Northern hemisphere winter storm tracks of the Eemian interglacial and the last glacial inception

F. Kaspar, T. Spangehl, and U. Cubasch Freie Universitat¨ Berlin, Institut fur¨ Meteorologie Received: 16 November 2006 – Published in Clim. Past Discuss.: 7 December 2006 Revised: 3 April 2007 – Accepted: 5 April 2007 – Published: 23 April 2007

Abstract. Climate simulations of the Eemian interglacial et al., 2002). The simulations approximately represent pe- and the last glacial inception have been performed by forc- riods with maximum and minimum summer insolation in the ing a coupled ocean-atmosphere general circulation model northern hemisphere. These periods are connected with dif- with insolation patterns of these periods. The parameters ferent states of the climate system that are visible in palaeo- of the Earth’s orbit have been set to conditions of 125 000 data, namely: The minimum of global ice volume occurred and 115 000 years before present (yr BP). Compared to to- at approx. 125 000 yr BP. We therefore consider this date day, these dates represent periods with enhanced and weak- as representative for the warm phase of the Eemian (here- ened seasonality of insolation in the northern hemisphere. after: EEM). After ca. 115 000 yr BP, open vegetation re- Here we analyse the simulated change in northern hemi- placed forests in northwestern Europe and at Vostok air tem- sphere winter storm tracks. The change in the orbital con- peratures dropped sharply (Kukla et al., 2002). We use this ﬁguration has a strong impact on the meridional tempera- date to represent the start of the last glaciation (hereafter: ture gradients and therefore on strength and location of the GI, “glacial inception”). Three parameters are responsible storm tracks. The North Atlantic storm track is strengthened, for the seasonal distribution of insolation: the eccentricity shifted northward and extends further to the east in the sim- of the Earth’s orbit, the angle of the axis of rotation (obliq- ulation for the Eemian at 125 kyr BP. As one consequence, uity) and the position of the equinoxes relative to the perihe- the northern parts of Europe experience an increase in winter lion. The combined effect of greater obliquity and eccentric- precipitation. The frequency of winter storm days increases ity, together with the fact that perihelion occurred in northern over large parts of the North Atlantic including the British hemisphere summer caused an ampliﬁcation of the northern Isles and the coastal zones of north-western Europe. Oppo- hemispheric seasonal cycle of insolation at 125 000 yr BP. site but weaker changes in storm track activity are simulated Figure 1 illustrates the latitudinal distribution of insolation for 115 kyr BP. during the seasons for 125 000 yr BP as anomaly from today. The changes are especially strong on the high northern latitudes during summer. At 65◦North the insolation in mid June was 11.7% higher than today (531 W/m2 instead of 1 Introduction 475 W/m2). The insolation in mid December was 1.9 W/m2 compared to 3.0 W/m2 today. At 115 000 yr BP the angle of Climate variations during the last 500 000 years are domi- perihelion was almost opposite compared to 125 000 yr BP nated by interglacial-glacial-cycles, which are generally be- and similar to today. The perihelion occurring in northern lieved to be responses to changes in insolation as a result hemisphere winter combined with greater eccentricity and of variations in Earth’s orbit around the sun (Berger, 1988). lower obliquity caused a weakening of the seasonal cycle of Palaeoclimatic records reveal that interglacials occurred ap- insolation in the northern hemisphere. This results in an al- proximately every 100 000 years in this period and lasted for most opposite distribution of the anomalies as for 125 000 yr 7 000 to 17 000 years. Here, we present simulations of the BP, but with smaller absolute differences. The insolation in Eemian interglacial, which was the last interglacial before the mid June at 65◦North was 7.6% lower (439 W/m2) than to- present one (approx. 130 000 yr BP–116 000 yr BP; Kukla day. In mid December it was 6.8 W/m2. For a detailed dis- Correspondence to: F. Kaspar cussion of insolation patterns during interglacials see Berger ([email protected]) et al. (2007).

Published by Copernicus GmbH on behalf of the European Geosciences Union. 182 F. Kaspar et al.: Storm tracks of the Eemian and the last glacial inception

(Hoskins and Hodges, 2002). Alternative measures for synoptic scale activity are based on the identification and tracking of individual systems (cyclones and anticyclones) from sea level pressure or near surface geopotential height fields (Lagrangian view). In contrast to the Eulerian view this La- grangian approach yields more detailed information on cyclone characteristics and enables to separate different types of systems (e.g. weak and strong cyclones). Here a number of methods have been developed and applied to general circulation models (GCMs) and reanalysis data (e.g. Knip- pertz et al., 2000; Hoskins and Hodges, 2002; Wernli and Schwierz, 2006). These methods require a smoothing of the input fields or the introduction of numerous parameters and thresholds to eliminate artificial and spurious lows. A di- rect comparison between different studies is often compli- Fig. 1. Insolation anomaly in W/m2 at 125 000 yr BP as a function cated as results are sensitive to both the resolution/spherical of latitude and the position on the Earth’s orbit. The values are truncation of the input fields and the setting of the individual shown as deviation from present-day conditions and are calculated parameters (Pinto et al., 2005). General circulation models according to Berger (1978). See Berger et al. (2007) for more details are able to simulate the present-day storm tracks fairly well on conditions during interglacials. (Hall et al., 1996). Whereas the main characteristics of the storm tracks are captured by models with lower spectral resolution (Stendel and Roeckner, 1998) the representation of Although absolute changes in insolation in the northern numerous features (e.g. the number of weak cyclones, the in- hemisphere are strongest in summer, northern high latitudi- tensity of strong cyclones, the representation of associated nal winter temperatures can also be strongly modified due to fronts and wind speeds) can be significantly improved by in- indirect effects, as for example via the influence of oceanic creased model resolution (Jung et al., 2006). or atmospheric dynamics or changes in sea-ice distribution. As GCMs indicate significant changes in storm tracks un- Therefore, the changed orbital configuration could lead to der climate change scenarios (Yin, 2005; Pinto et al., 2007), distinct changes in the seasonal cycle of temperature. As it is of interest to analyse the behaviour of the models un- the strength of this effect strongly depends on the latitude, der different conditions. Simulations of palaeoclimates offer it leads to significant changes in the meridional temperature the opportunity to compare variables that are linked to storm gradients. Due to the temperature gradients in midlatitudes tracks with reconstructed data, e.g. changes in precipitation. atmospheric circulation is characterised by the occurrence of Kageyama et al. (1999) analysed storm tracks in a number baroclinic waves. According to the theory of baroclinic insta- of GCMs for the last glacial maximum at 21 000 yr BP. At bility (Charney, 1947) these waves develop from small dis- that time orbital configuration was similar to today, but cli- turbances and depending on the background flow conditions mate was strongly influenced by the presence of large ice they amplify to mature synoptic scale systems. They interact sheets and differences in sea surface temperatures and sea with the background flow by reducing the meridional tem- ice cover. Nearly all models reacted with an eastward shift perature gradient and by dynamical feedbacks (Hoskins and of the northern hemispheric storm tracks. Hall and Valdes Valdes, 1990). Of relevance for the local weather and cli- (1997) analysed winter storm tracks in a GCM simulation mate are the synoptic scale cyclones and the associated fronts for 6000 years before present, also a period with enhanced as they are largely responsible for the precipitation and near summer insolation in the northern hemisphere. They found surface wind at these latitudes. an increase in baroclinicity in the storm track regions of the The regions of higher frequency atmospheric variability Atlantic and the Pacific. that are characterized by the frequent passage of cyclones Previous modelling experiments for the last glacial in- and anticyclones (in contrast to low frequency variability ception at approx. 115 000 yr BP focused on the question associated with blocking) have been intensively studied for if the change in insolation is sufficient to generate a peren- observed present-day climate (Blackmon, 1976; Blackmon nial snow coverage in polar latitudes. Early experiments with et al., 1977) as well as in model-based climate change projec- atmosphere-only models failed, but later experiments led to tions for the 21st century. “Storm tracks” defined after Black- better results by considering additional feedbacks, for exam- mon et al. (1977) as regions with large synoptic-scale activ- ple that of the ocean (Khodri et al., 2001) or vegetation (Gal- ity can be calculated as the standard deviation of the band- limore and Kutzbach, 1996). For a detailed review, see Vet- pass filtered geopotential height and are often used to quan- toretti and Peltier (2004). tify baroclinic wave activity (Eulerian view) as they are easy In this article, we analyse storm tracks in simulations of to apply and provide a general storm track activity measure the Eemian interglacial and the subsequent glacial inception.

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The climate model and the setup of the experiments are de- Table 1. Orbital parameters and greenhouse gas concentrations of scribed in Sect. 2. In Sect. 3 we give an overview over some the simulations. Orbital parameters are calculated following Berger general features of the simulations and present the results for (1978). The angle of perihelion refers to vernal equinox. Green- the northern hemispheric winter storm tracks and related cli- house gas concentrations are based on Vostok ice cores (CO2 and matic parameters. In Sect. 4 the results are discussed and CH4: Petit et al., 1999;N2O: Sowers, 2001) compared with simulations of future climate change. 125 kyr BP 115 kyr BP preindust. EEM GI PI 2 Model description and experimental setup Eccentricity 0.0400 0.0414 0.0167 ◦ 2.1 The ECHO-G model Obliquity [ ] 23.79 22.41 23.44 Perihelion [◦] 127.3 290.9 282.7 The ECHO-G model (Legutke and Maier-Reimer, 1999; CO [ppmv] 270 265 280 Legutke and Voss, 1999) consists of the ECHAM4 atmo- 2 CH4 [ppbv] 630 520 700 sphere model (Roeckner et al., 1992) coupled to the HOPE- N2O [ppbv] 260 270 265 G ocean model. The atmospheric component is a spectral CFCs [ppbv] 0 0 0 model with a horizontal resolution of T30 (≈3.75◦) and 19 vertical hybrid sigma-pressure levels with the highest level at 10 hPa. The ocean model HOPE-G is the global version of the Hamburg Ocean Primitive Equation Model (Wolff et al., cores (Petit et al., 1999; Sowers, 2001). Concentrations of 1997) and includes a dynamic-thermodynamic sea-ice model chlorofluorocarbons (CFCs) are set to zero. The models de- with a viscous-plastic rheology (Hibler, 1979). A gaussian fault values are retained for all the remaining boundary con- ◦ T42 Arakawa E-Grid (≈2.8 ) is used with a gradual merid- ditions, i.e. present-day conditions are used. A third simu- ◦ ional refinement reaching 0.5 in the tropical ocean between lation with preindustrial conditions is used for comparison ◦ ◦ 10 S and 10 N. The vertical resolution is given by 20 hor- (hereafter PI). We did not select a present-day simulation, izontal levels with eight levels within the top 2000 meters as greenhouse gas concentrations of the preindustrial simu- from the ocean surface. The atmospheric and oceanic com- lation are very similar to the values in the Eemian and there- ponents are coupled with a flux correction. The model time fore the simulated anomalies can be attributed more clearly steps are 30 min for ECHAM4 and 12 h for HOPE-G. The to insolation change. All parameters are shown in Table 1. resolution of the model is appropriate for palaeoclimatic ex- All simulations have been initialized with the same oceanic periments as it allows to simulate time-slices of several cen- state, i.e. potential temperature and salinity calculated from turies on modern computing facilities. The analysis of a the Levitus et al. (1994) climatology. 1000-year control simulation shows an overall good skill in simulating today’s climatology and interannual variability (Min et al., 2005a). The El Nino˜ Southern Oscillation 3 Results (ENSO) and the North Atlantic Oscillation (NAO) are also simulated reasonably well (Min et al., 2005b). 3.1 General behaviour of the simulations Fischer-Bruns et al. (2005) analysed midlatitude storm activity based on wind speed thresholds in ECHO-G simula- The simulations have been performed for at least 2000 years. ≈ tions with historical and climate change forcing. For the cli- During the first 100 years all simulations are dominated mate change experiments with a strong increase in green- by similar initial trends that are related to adaptations of house gas concentrations (IPCC SRES A2 and B2) they the ocean circulation. In this period global mean near sur- found a poleward shift in all three storm track regions and face temperature rises by approx. 0.4 K. Most features of increasing activity over the North Atlantic. the oceanic circulation reach quasi-stationary conditions after approx. 150 years in all three simulations. After this ini- 2.2 The experiments tial phase the simulations of the preindustrial and the Eemian climate are relatively stable. In the simulation for 115 kyr The simulations are performed as equilibrium experiments BP a significant long-term cooling trend occurs which is with constant boundary conditions. Therefore the simulated connected with an expansion of perennial snow-covered ar- temporal climate variations are only caused by the nonlin- eas over northern parts of North America and an increase in ear internal dynamics of the climate model. The orbital pa- northern hemispheric sea ice volume. rameters and greenhouse gas concentrations have been set to For the further analysis we selected an interval of 100 values of 125 kyr BP and 115 kyr BP (hereafter EEM and years during the early part of the simulations starting 300 GI). Orbital parameters have been calculated according to years after the beginning of the simulations. This period was Berger (1978). Greenhouse gas concentrations (CO2, CH4, selected, because the simulations have already reached their N2O) have been adapted to values obtained from Vostok ice equilibrium, but the perennial snow-covered areas over North www.clim-past.net/3/181/2007/ Clim. Past, 3, 181–192, 2007 184 F. Kaspar et al.: Storm tracks of the Eemian and the last glacial inception

Fig. 2. Winter (DJF) near-surface temperature difference [K] between the EEM simulation and the PI simulation.

Fig. 3. Winter (DJF) near-surface temperature difference [K] between the GI simulation and the PI simulation.

America are still very limited. Therefore the structure of the over North America and the lower northern latitudes (com- simulated patterns are mainly influenced by the orbital con- pared to the preindustrial simulation). The temperature re- figuration, whereas the land surface conditions are compara- duction over North America is stronger than over Siberia, as ble. Anomalies are shown relative to the same interval of the southward advection of polar air enhances the effect of the re- preindustrial simulation. duced insolation. However, temperatures are higher in large parts of the Arctic ocean, as a result of reduced sea ice cov- The changes in the latitude and season dependent distri- erage (Fig. 2), which is in turn a result of the increased sum- bution of insolation combined with the feedbacks of the cli- mer insolation. A strong temperature increase is simulated mate system, especially sea ice, result in a modification of for north-east Europe. In that region increased advection of the meridional temperature gradients. These have a strong oceanic heat due to stronger westerly winds leads to a strong impact on strength and location of the winter storm tracks. winter warming (Kaspar and Cubasch, 2007). The simulated Summer temperatures directly react to the strong changes of seasonal temperature patterns over Europe are in good agree- the high northern insolation, i.e. they are significantly higher ment with pollen-based temperature reconstructions (Kaspar for 125 000 yr BP and significantly lower for 115 000 yr BP. et al., 2005). This temperature change is especially strong on the continental areas. In contrast to that, winter temperatures are For 115 000 yr BP winter insolation was increased on al- mainly influenced by indirect effects, namely a combina- most all latitudes. However, the temperatures of the Arctic tion of changes in the large-scale atmospheric circulation, ocean are reduced, as they are dominated by the influence of the meridional ocean circulation and summer-season induced the increased sea ice coverage (again a result of the reduced changes in sea ice. Winter insolation is reduced at 125 000 yr summer insolation; Fig. 3). Increased temperatures occur in BP (cf. Fig. 1), leading to distinct temperature reductions a region in the north-west of the North Atlantic. This effect

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Fig. 4. Winter storm tracks of the preindustrial simulations [Pa], calculated as standard deviation of the ﬁltered sea level pressure.

Fig. 5. Winter storm tracks of the preindustrial simulations [gpm], calculated as standard deviation of the filtered geopotential height at 500 hPa. is caused by an enhanced North Atlantic current in this sim- culated the standard deviation for each grid point based on ulation, which is a consequence of the stronger annual mean the winter seasons (DJF). Storm tracks are then represented meridional temperature gradient. by the regions with high values of this standard deviation. A detailed description of general features of these simula- Figure 4 shows the results for the preindustrial simulation. tions can be found in Kaspar and Cubasch (2007). The maximum of the SLP-based Atlantic storm track is located at around 53◦ W and 52◦ N. The maximum of the Pa- 3.2 Winter storm tracks in the preindustrial simulation cific storm track is located at around 175◦ E and 48◦ N (SLP). The general structure and location of the storm tracks is quite Following Blackmon (1976), storm tracks are often quanti- well reproduced compared to ECMWF re-analyses data (see fied by the variability of the bandpass (typically 2.5–6 or 2.5– for example Kageyama et al., 1999). When storm tracks are 8 days) filtered geopotential height (GPH) fields at 500 hPa. calculated based on geopotential height (at 500 hPa) the max- Alternatively the calculation of storm tracks can be per- ima lie approx. 5◦ further south (Fig. 5). Moreover a third formed based on sea level pressure (SLP). As a change in maximum is pronounced over central Asia that is also known insolation directly influences surface conditions, we analysed from reanalysis data and is often called the Siberian storm the storm tracks based on SLP. In order to evaluate if the sim- track (e.g. Hoffmann, 1999). ulated changes persist at higher levels we also applied the analysis for 500 hPa. We used a Butterworth bandpass filter 3.3 Winter storm tracks during the Eemian (Hoffmann, 1999) with cut-off periods of 2.5 and 8 days and applied it to the time series of the 12-hourly SLP and GPH Figure 6 shows the SLP-based winter storm tracks in the sim- (at 500 hPa) fields. Based on this filtered time series we cal- ulation for 125 kyr BP as differences from the preindustrial www.clim-past.net/3/181/2007/ Clim. Past, 3, 181–192, 2007 186 F. Kaspar et al.: Storm tracks of the Eemian and the last glacial inception

Fig. 6. Eemian winter storm track anomaly [Pa] at 125 kyr BP (EEM-PI) calculated based on sea level pressure as in Fig. 4.

Fig. 7. Eemian winter storm track anomaly [gpm] at 125 kyr BP (EEM-PI) calculated based on geopotential height as in Fig. 5. simulation. The figure indicates a significant strengthening tures over the high latitudes of the western part of the North and northward shift of the Atlantic storm track. The region Atlantic and North America are responsible for a stronger with activity greater than 700 Pa also extends further east- temperature gradient in these regions at 125 kyr BP, which ward. In a large region close to the maximum of the North in turn enhances storm track activity in both regions. For Atlantic storm track at around 60◦ N changes exceed 50 Pa. Asia a temperature reduction is simulated for the southern In this region the relative change is around 10%. part, whereas slightly higher temperatures occur in the Arc- Storm track activity is also enhanced over the North Amer- tic ocean. In combination, the merdional temperature gra- ican continent, including Alaska. The Pacific storm track dient over Asia is reduced and a coinciding reduction in ◦ ◦ storm track activity (weaker Siberian storm track) is simu- is weakened mainly in its western part (120 E to 180 E). ◦ Its eastern part in the region of Alaska is shifted north- lated for the northern part of the continent (north of 50 N). ward. Over the Asian continent the activity is reduced, as Over the Mediterranean region the winter temperature gra- well as over the Mediterranean region (by about more than dient is reduced compared to the preindustrial case for two 20 Pa over large areas). When the analysis is done based reasons: over Northern Africa, reduced winter temperatures on geopotential height at 500 hPa, the spatial distribution of are simulated mainly because of reduced insolation. Over the anomalies is similar but more zonally oriented (Fig. 7). north-eastern Europe higher winter temperatures are caused Anomalies exceed 6 gpm over the Atlantic. The weakening by enhanced westerly winds and reduced sea-ice coverage. of the Pacific storm track is more distinct at this height. Consistently, reduced storm track activity is simulated for the Mediterranean region. There is a strong linkage between the simulated changes in storm tracks and the differences in the meridional tem- At 115 kyr BP the change in SLP-based storm track ac- perature gradients (cf. Fig. 2). Reduced winter tempera- tivity is approx. complementary compared to 125 kyr BP,

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Fig. 8. Winter storm track anomaly [Pa] at 115 kyr BP (GI-PI) calculated based on sea level pressure as in Fig. 4. but not as strong (Fig. 8). The western part of the Atlantic the Arctic north of East Asia and Alaska. These differences storm track is shifted slightly southward and it has a re- between the intervals are related to the long-term expansion duced strength especially at its eastern part. The reduction is of the sea-ice in the simulation of the glacial inception (GI). about >20 Pa east of 30◦ E. In this region the relative change During the selected periods strong changes in winter sea-ice is around 5%. In total, this results for the Atlantic storm are simulated for the eastern Beaufort Sea and the Sea of track in a more east-west-oriented structure of the changes Okhotsk. These changes influence the storm track activity in as compared to the EEM case. Storm track activity over the the mentioned adjacent regions. north-western parts of North America is reduced by a similar amount. The Pacific storm track activity is increased at its 3.4 Change in extreme wind speed and precipitation eastern part, whereas the change over Asia is relatively small. A distinct increase of activity also occurs in an area extend- Storm tracks are closely related to other climatic parameters ◦ ing from the eastern Mediterranean to approx. 75 E. As in that could be relevant for the interpretation of palaeoclimatic the EEM case, the anomalies have a stronger zonal structure data, as for example precipitation or the number of storm when calculated based on geopotential height at 500 hPa (not days (e.g. Raible et al., 2007). These two will be discussed shown). in this section. Again, the strong changes over North America are consis- The frequency of extreme wind events is an alternative tent to the temperature changes caused by the increased sea measure for storm activity. Fischer-Bruns et al. (2005) anal- ice coverage in the Arctic. The southward shift of the At- ysed storm activity of an ECHO-G simulation by counting lantic storm track is coherent to the increased temperatures the number of winter days reaching or exceeding a mini- over the western part of the North Atlantic. Relatively weak mum 10 m wind speed within a grid box. As threshold they changes occur over Asia. Here temperature changes are also used the lower limit of the WMO Beaufort wind speed scale comparatively small. The temperature decrease over the Arc- of 8 Bft (17.2 ms−1). Here we apply the same method and tic ocean close to Siberia does not result in a significant mod- threshold. The wind speed is calculated by the model ev- ification of storm track activity over Asia. ery 30 min. The 10 m wind is interpolated between the low- Although for both simulations a strong relation between est model level and the surface according to the logarithmic changes in meridional temperature gradients and storm wind profile based on a modified roughness length. At each tracks has been found, it should be noted that other mech- grid point the maximum value is determined for each day. anisms could have an impact on them, as e.g. latent heat re- As Fig. 9 indicates for the preindustrial case, the area lease (Hoffmann, 1999). where this wind speed is exceeded is almost solely restricted To test the robustness of the results, we performed the to the oceans. An annual average value of 50 storm days for same analysis for additional 100-year intervals (years: 200– the winter season (DJF) is exceeded in a small region over 299 and 400–499). This test showed that the spatial pattern of the Atlantic as well as over the Pacific. The maximum over ◦ ◦ the simulated changes is independent of the selected interval the Atlantic is located at 37 W/47 N. Over the Pacific it is ◦ ◦ for the EEM simulation. In the GI case, the spatial structure located at 175 W/40 N. of the results for the North Atlantic and the Mediterranean The change of the frequency of winter storm days is shown are also robust. However, significant differences between the in Fig. 10 for the EEM simulation. The North Atlantic north intervals occur for Alaska, the Western Pacific as well as for of 45◦ N is affected by an increase in the frequency, with the www.clim-past.net/3/181/2007/ Clim. Past, 3, 181–192, 2007 188 F. Kaspar et al.: Storm tracks of the Eemian and the last glacial inception

Fig. 9. Frequency of storm days (number of days with maximum daily 10m wind speed reaching or exceeding the lower threshold of 8 Bft) per winter season in the preindustrial simulation.

Fig. 10. Change in the average number of storm days (defined as in Fig. 9) per winter season in the simulation for 125 kyr BP (EEM-PI). strongest changes in the eastern part at around 55◦ N. The track activity. However, the change of activity in this region increase in this region in the neighbourhood of the British is relatively small (cf. Fig. 8). A northward shift of the storm Isles is of about 6 storm days, which is an increase of around frequency occurs over the Pacific. 20% compared to the preindustrial simulation (Fig. 9). The The Figs. 12 and 13 show the simulated change of win- European coastal regions from France to Norway are affected ter precipitation for EEM and GI as ratios to the simulated by an increase of at least 2 storm days. A belt with a reduced preindustrial precipitation. A strong change in precipitation number of storm days is visible at around 30◦ N. In total, the is visible for many regions of the North Atlantic and Europe changes on the North Atlantic indicate a north-eastward shift that are affected by the described changes in storm track ac- of the regions with frequent storm days. This is consistent tivity. In case of the EEM simulation, a large region is af- with the shift of the storm tracks described in the previous fected by a precipitation increase of around 10%. This region section. The results for the Pacific also reflect the pattern of includes Iceland, the British Isles and Scandinavia. It also in- changes in storm track activity. cludes the European continent north of 55◦ N and extends up In the case of the GI simulation (Fig. 11) the general to 80◦ N. In general, for most oceanic and coastal regions be- structure of the changes is also roughly consistent with the tween 30◦ N and 70◦ N the change in winter precipitation is changes of the storm tracks. On the eastern Atlantic the fre- roughly consistent with the change in storm track activity. quency is reduced between latitudes 35◦ N and 70◦ N. The In case of the GI simulation the simulated changes in decrease reaches 4 days in some areas. The western part of mid-latitudinal winter precipitation are less strong (Fig. 13). the North Atlantic is affected by an increase in storm fre- Compared to the preindustrial simulation, the relative change quency (north of 30◦ N). Over the north-western North At- is smaller than 5% for most continental areas and the Pacific lantic this increase occurs in a region with decreased storm north of 40◦ N. Although a small decrease in the order of

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Fig. 11. Change in the average number of storm days (deﬁned as in Fig. 9) per winter season in the simulation for 115 kyr BP (GI-PI).

Fig. 12. Winter precipitation anomaly in the EEM simulation as ratio relative to the preindustrial simulation (EEM/PI).

5% is simulated for the European Atlantic coast, the Euro- simulations with orbital forcing of two phases of the Eemian pean continent does not experience strong changes in winter interglacial: The Eemian warm phase at 125 kyr BP and the precipitation. Distinct changes in precipitation are simulated subsequent glacial inception at 115 kyr BP. The strongest for the Atlantic ocean, whereas the changes on the mid- and simulated change in storm track activity occurred in the sim- high latitudes of the Pacific are comparatively small. The ulation for 125 kyr BP namely a significant northward shift simulated changes in precipitation over the Atlantic show a of the axis of the North Atlantic storm track, combined with west-east-structure in rough consistency with the changes in a general increase in activity especially downstream of the storm track activity: Decreased precipitation in the eastern storm track maximum. For 115 kyr BP the changes are of op- part of the Atlantic and an increase in the western part at posite sign, but less strong. The frequency of storm days has around 35◦ N is consistent with the changes in storm tracks been used to identify regions with intense storm activity. As as in Fig. 8. However, in the north-western part of the At- with that measure similar regions of strongest change have lantic an increase of precipitation is simulated, which is in been identified, it can be concluded that this is a robust con- contrast to the slight decrease in storm track activity, but in clusion. It has also been shown that the results are indepen- consistency with the changes in the number of storm days dent of the selected height, i.e. performing the analysis based (Fig. 11). on sea level pressure or geopotential height at 500 hPa led to the same conclusions. In case of the warm phase, the results are also independent of the selected interval. As a long-term 4 Summary and conclusions cooling trend occurs in the simulation of the glacial inception, the simulated storm track activity is not stable in all re- Anomalies of northern hemispheric winter storm tracks as a gions for that simulation. However, for the North Atlantic the measure for synoptic-scale activity have been analysed for results are independent of the selected interval. One caveat is www.clim-past.net/3/181/2007/ Clim. Past, 3, 181–192, 2007 190 F. Kaspar et al.: Storm tracks of the Eemian and the last glacial inception

Fig. 13. Winter precipitation anomaly in the GI simulation as ratio relative to the preindustrial simulation (GI/PI). that the results are only based on one single coupled model structure and order of magnitude as simulated here for the with a relatively coarse resolution. The reproduction of ex- Eemian, with strongest changes in the neighbouring regions treme wind speeds by climate models significantly improves of the British Isles. In summary, the results suggest that with higher spatial and temporal resolution, esp. in coastal concerning the shift of winter storm tracks the Eemian in- areas (Leckebusch and Ulbrich, 2004). terglacial reveals a signal similar to future climate scenarios. The results show, that the changes in storm tracks are in When comparing Eemian and future changes in storm line with the changes in the meridional winter temperature tracks, it has to be noted that the forcings have different spa- gradients. For Europe these simulated changes in the tem- tial and seasonal structure. The similarity in the simulated perature gradient have been compared with proxy-data in an changes in storm tracks does not occur because the forcing earlier study (Kaspar et al., 2005). As in that study it has itself is similar, but because of the similar consequences on been shown that the simulated seasonal patterns of the tem- meridional gradients of the near-surface temperature of the perature anomalies are in good agreement with pollen-based winter months. reconstructions, we can assume at least for this region that Changes in North Atlantic storm tracks contribute to changes in temperature gradients are simulated realistically. changes in European precipitation. Information on this could The question whether the Eemian could be used as ana- be relevant for the interpretation of palaeoclimatic proxies. logue for a warmed future climate has been a matter of de- Our results suggest that an increase in winter precipitation bate (see for example Mearns et al., 2001). Analyses of storm does mainly occur in the northern parts of Europe at 125 kyr tracks with GCMs under greenhouse gas scenarios show sim- BP (north of 55◦ N). For southern Europe and for 115 kyr BP ilar spatial patterns in storm tracks as simulated here for the the changes in winter precipitation are comparatively small. Eemian at 125 kyr BP: Based on a tracking analysis Bengts- son et al. (2006) found a similar distribution of changes for Acknowledgements. This work has been performed within the a future climate change scenario simulation with the cou- German Climate Research Program DEKLIM of the German Min- pled GCM ECHAM5/MPI-OM: indications for a northward istry for Education and Research (BMBF). The simulations have shift, a weakening of the Mediterranean storm track and a been run on the NEC SX-6 supercomputer of the German Climate strengthening north of the British Isles (for IPCC-SRES- Computing Centre (DKRZ, Hamburg). We thank H. Wernli and an scenario A1B). These pattern agrees with the findings in anonymous reviewer for their comments that helped to improve the Sect. 3.3 (cmp. also Fig. 6). For the same model Pinto et al. manuscript. (2007) found mainly a large increase in storm track activity Edited by: J. Hargreaves at 500 hPa over Western Europe. Yin (2005) analysed an ensemble of 21st century simulations performed by 15 coupled climate models and also found a poleward shift and intensifi- cation of the northern hemispheric winter storm track simu- References lated by all but four models (for IPCC-SRES-scenario A1B). Fischer-Bruns et al. (2005) calculated the frequency of Bengtsson, L., Hodges, K. I., and Roeckner, E.: Storm tracks and storm days for an ECHO-G climate change simulation (for climate change, J. Climate, 19, 3518–3543, 2006. IPCC-SRES scenario A2 and B2). Again, the simulated Berger, A. L.: Long-term variations of daily insolation and Quar- change in frequency of winter storm days shows a similar ternary climate changes, J. Atmos. Sci., 35, 2362–2367, 1978.

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