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 to- day. The changes are especially strong on the high north- ern 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 syn- optic scale activity are based on the identification and track- ing 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 cy- clone characteristics and enables to separate different types of systems (e.g. weak and strong cyclones). Here a num- ber 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 res- olution (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.