Storminess, palaeoclimate and coastal dunes in Portugal: an aeolian record of negative North Atlantic Oscillation

   

Michèle Clarke1 & Helen Rendell 2 
1School of Geography, University of Nottingham, University Park, Nottingham NG7 2RD.  
2Geography Department, Loughborough University, Loughborough, Leicestershire LE11 3TU   
 
Between the Duoro and Tejo river estuaries, the littoral fringe of western Portugal is extensively blanketed by dunes and sand sheets that extend inland more than 10km (Figure 1). A variety of dune forms, including hairpin parabolic, compound parabolic and crescentic ridge dunes, are currently stabilised by maritime pine forests in this coastal region. Climatic deterioration during Little Ice Age (LIA) is believed to have driven increased sediment supply and coastal dune accretion along the Iberian Peninsula in Portugal (Dias et al., 2000) and southern Spain (Borja et al., 1999). Archival records testify to pre-Little Ice Age sand inundation of medieval cemeteries at Fão and Chafé, (Granja, 1999; Granja and Soares de Carvalho, 1992) and medieval salt pans at Belinho, Sublago and Marinhas Esposende (Granja, 1999). Stabilisation measures, to curtail sand drift in Portugal, occurred as early as the 13th century when a royal decree forced planting of maritime pine (Pinus pinaster) to establish the forest of Pinhal do Rei (Clarke et al., 2002) in order to protect the town of Marinha Grande from sand invasion.  
 
 
 

Absolute dating of dune sand, using luminescence techniques, has advantages over radiocarbon dating of enclosing peat layers or soils in that the event being dated is the dune building episode rather than a period of soil or mire development. This study focuses on absolute dating of dune forms between Furaduoro and Nazaré along the western coast of Portugal to establish the timing of sand invasion and its palaeoclimatic significance. 

Stratigraphical, geomorphological and chronological investigation of historical sand drift in northwest Europe has revealed episodic sand invasion on exposed Atlantic coasts in Spain (Borja et al., 1999) France (Clarke et al., 1999; 2002; Meur et al., 1992), Britain (Lewis, 1993; Gilbertson et al., 1999; Wilson, 2002) and Ireland (Wilson and Braley,1997; Wintle et al., 1998) and on the North Sea coasts of Denmark (Christiansen et al., 1990; Lamb & Freyandahl, 1991; Clemmensen et al., 1996; Clemmensen et al., 2001a; 2001b) and the Netherlands (Koster et al., 1993; Jelgersma et al., 1995). Along these coastlines, historical sand drift, causing problems for human settlement and agriculture (Lamb & Freyendahl, 1991; Lamb, 1995), has been linked to periods of increased North Atlantic storminess during the Little Ice Age (Björck and Clemmensen, 2004; Clarke et al., 2002; Clemmensen et al., 2001; Gilbertson et al., 1999; Lewis, 1992; Wilson, 2002; Wintle et al., 1998). Periods of increased storminess have been linked to the North Atlantic Oscillation and the extent of sea ice (Dawson et al., 2002; Lamb 1995). 

Mean wind speed, intensity, direction and number of storms over the Atlantic is determined by the strength and sign of the North Atlantic Oscillation (NAO) (Hurrell & Dickson, 2004), controlled by the surface pressure gradients between the sub-tropical Azores high-pressure system and the high latitude Aleutian and Iceland low pressure centres (Hurrell and Van Loon, 1997; Walter and Graf, 2004). During boreal winters in a positive NAO phase (NAO+), higher than normal surface pressures south of 55 N combine with a broad region of low pressure throughout the arctic and sub-arctic (figure 2a). This creates a strengthening of sub-polar westerlies (Thompson et al., 1999),  bringing mild, wet and stormy winters to the British Isles and Scandinavia (Parker & Folland, 1988) and dry weather to the Iberian Peninsula (Dickson et al., 2000) while south of the Azores high-pressure centre, enhanced easterly trade winds carry Saharan dust to the Caribbean (Moulin et al., 1997). During the negative phase (NAO-), both the Icelandic-low and the Azores high-pressure centres are weaker than normal with the result that both the mid-latitude westerlies and the sub-tropical tradewinds are also weak (Hurrell & Dickson, 2004).  European wintertime temperatures are frequently lower than normal, dominated by cold air from the north and east (van Loon & Williams, 1976; Moses et al., 1987). The NAO- phase is also associated with an increase in the extent of sea ice in the Nordic Seas (Vinje, 2001), while a weakening of the Azores anticyclone allows the westerlies to bring rain across the Iberian Peninsula and into the Mediterranean (Rogers, 1997; Qian and Saunders, 2003; Hurrell and Van Loon, 1997; Hurrell et al., 2003).  

In Aquitaine, southwest France, sand invasion has been shown to correspond to an increased frequency of North Atlantic storms across northern Europe (Clarke et al., 2002) driven by northward displacement of the westerlies during a postive North Atlatnic Oscillation (NAO). Contemporary example of this type of behaviour was shown by the severe storms of December 1999 (Pearce et al., 2001) which occurred during a winter (DJFM) period with NAOi +2.8 (Hurrell, 2005). To the south and west of Aquitaine, Portugal is well placed to examine the synchroneity of coastal responses to NAO-related storminess. If periods of sand invasion and dune building in the coastal areas of Western Portugal are related to periods of persistent storminess, we would expect the timing of sand accretion to reflect periods of dominant negative NAO values, and asynchronous both with the Aquitaine record and the existing record in the rest of Europe. 
 

Journal Papers associated with this research: 

Clarke, M.L. & Rendell, H.M. (2006) The effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal. The Holocene, 16 (3), 341-355. [ e-mail me for a pdf!]

Clarke, M.L., Rendell, H.M., Tastet, J-P., Clave, B. & Masse, L. (2002) Late Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France The Holocene, 12, 231-238. [Download]  

Clarke, M.L., Rendell, H.M., Pye, K., Tastet, J-P., Pontee, N.I. & Masse, L. (1999) Evidence for the timing of dune development on the Aquitaine Coast, southwest France. Zeitschrift fur Geomophologie Supplement Band, 116, 147-163.  

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