Winds transport moisture from low to high latitudes on average, where some of the moisture condenses to form clouds. In the Arctic, changes in cloudiness are associated with changes in cyclone frequency (Serreze et al., 2000). However, it is still unclear to what extent changes in cloud cover (or inversely sunshine) are due to local processes (e.g. evaporation) or to large-scale circulation patterns like the North Atlantic Oscillation (NAO) (Hurrell, 1995; Wang and Key, 2003). The NAO is not stationary but deviates from one phase to another and produces large changes in the behaviour of Atlantic cyclones including the intensity and number of storms and their paths, among others (Hurrell et al., 2003). There is thus great interest in the possibility of predicting the NAO for society and the environment. However, to understand the modern NAO deviation, its current change, and its future requires the long-term perspective of the NAO deviation.

Here we use palynological data from a lacustrine sediment core collected in southern Greenland (Qipisarqo Lake) to derive climate records spanning about 9000 years. In particular, the present study aims at reconstructing past cloud cover variability, with special attention paid to large-scale circulation patterns like the NAO and their relationship with climate parameters including air temperature and precipitation. The transfer functions used to reconstruct Holocene climate conditions in southern Greenland are based on the modern analogue technique. The reference database used includes 39 taxa and 831 reference sites from the Boreal, Subarctic and Arctic biomes of North America and Greenland (Fréchette et al., submitted). The location of Qipisarqo Lake (61°00’N, 47°45’W, 7 m a.s.l.) is suitable to document changes in large-scale circulation patterns since it lies in the trajectory of one of the principal North Atlantic cyclone track (cf. Tsukernik et al., 2007 and references therein).

The sediment core of Qipisarqo Lake comprises lower organic lacustrine sediments (gyttja), which are capped by faintly laminated minerogenic, organic-poor sediments, and then by organic lacustrine sediments. The lacustrine organic sedimentation began at ca. 9100 cal. yr BP, was interrupted around 500 cal. yr BP, at the onset of the Little Age Ice (LIA) cooling, and restarted recently in the 20th century (Kaplan et al., 2002). After a pioneer phase dominated by herbs, dwarf shrub heaths (Ericaceae) shortly followed by willows (Salix), colonized the landscape (Fig. 1A). At that time, the summer temperature was cold and the sky as sunny as today (Fig. 1B). Around 7000 cal. yr BP, low and high shrub taxa (Alnus and Betula) immigrated, the summer temperature became warmer than today and the sky sunnier. Around 5000 cal. yr BP, the increase in Betula frequencies suggests a warming of winter temperature and higher depth of snow cover (cf. Fredskild, 1991). Indeed, from the mid-late Holocene transition to today, January air temperature and annual precipitation increased at Qipisarqo Lake, whereas July air temperature and July sunshine decreased (or cloud cover increased) (Fig. 1B). We explain the increase in cloudiness by increasing cyclonic activity from ca. 7000 cal. yr BP onwards in the North Atlantic west of Greenland.

The palynological signature of the LIA period is interesting. Low frequencies of Alnus and coniferous trees (Picea and Pinus) pollen are observed during the LIA (Fig. 1A). The summer and winter temperatures were also colder than today and the sky clearer (Fig. 1B). The Picea and Pinus pollen grains registered in Qipisarqo Lake sediments are brought to the site by long-distance transport. They mainly come from the boreal forest of Labrador and Newfoundland, and were brought to southern Greenland by northeastward winds. During the positive phases of the NAO, the Atlantic cyclone tracks are pushed southward allowing the polar air masses from the Canadian Arctic Archipelago to follow a southward track and disturb the climate of southern Greenland (Cappelen et al., 2001). In contrast, during the negative phases of the NAO, Atlantic cyclones follow a northward track towards Greenland. More storms, stonger winds and indirectely cloudier sky, are observed in southern Greenland during the negative phases of the NAO. Following this, the low frequencies of Picea and Pinus observed during the LIA at Qipisarqo Lake might suggest that southward polar air masses, associated with the positive phases of the NAO, prevailed during the LIA. In this presentation, we will verify if there is indeed a relationship between the Picea and Pinus frequency and the sunshine (or cloud cover) condition in southern Greenland. If so, the reconstruction of cloud cover variability could be helpful to understand the mechanisms behind shifts in cyclone tracks (cf. Previdi and Veron, 2007; Tsukernik et al., 2007) and then document long-term changes in synoptic scale atmospheric circulation patterns, like the NAO.