The North Atlantic Oscillation (NAO) describes the interaction o f sub-polar low pressure, centring over Iceland and anticyclonic high pressure in the Azores (Hurrell et al., 2003, W anner et al., 2001). The relationship between these pressure systems creates a sea level pressure (SLP) dipole between the North and the South Atlantic (Hurrell et al., 2003, Barlow et al., 1993, Hurrell and Deser, 2009). This SLP dipole drives air mass m ovement, controlling prevailing w ind directions and rainfall over the European continent (Hurrell et al., 2001, Pena et al., 2010, Trouet et al., 2009, Hurrell and VanLoon, 1997, Jones et al., 1997). The NAO's effect is dom inant during the boreal w inte r (Dec-Feb), representing th e m ajor cause of seasonal to inter-decadal variability in Northern hemisphere atmospheric circulation and clim ate (W anner et al., 2001, Hurrell, 1995, Hurrell et al., 2003, Hurrell et al., 2001).
2 .1 1 .1 Phases and Effects
The NAO Index is created by normalising the seasonal pressure difference between w eather stations in The Azores and Iceland (Figure 2.14), and can be described as being in a positive or negative phase (Jones et al., 1997, Wanner et al., 2001, Hurrell and Deser, 2009)
NAO
1
A
I860
1880
1900
1920
1940
1960
1980
2005
Figure 2.14: Index showing positive and negative fluctuations in the NAO (Hurrell et al., 2003).
Positive phases occur during periods o f high pressure over the Azores and low pressure over Iceland (Hurrell et al., 2003, Hurrell et al., 2001, W anner et al., 2001). Counter clockwise rota tion o f low pressure systems to the north and clockwise rotation o f high pressure to the
1997, Barlow et al., 1993, Hurrell, 1995, Jones et al., 1997, Hurrell and Deser, 2009). W esterly winds lead to cool summers and warm w et winters in the north, and cool dry w inters and dry summers in the south (Jones et al., 1997, Hurrell and Deser, 2009).
Negative NAO phases are associated w ith weak low pressure systems over Iceland and weak high pressure systems over the Azores (Hurrell et al., 2001), bringing colder w inters and hot summers to the north o f Europe and w e tte r conditions in central and Southern Iberia (Pauling et al., 2006). NAO phase changes manifest as variations in rainfall am ount (Roig et al., 2009, Vicente-Serrano and Cuadrat, 2007) and therefore oxygen isotope ratio in areas o f N orthern Iberia which have exhibit a strong rainfall am ount effect; making palaeorecords from this region ideally suited to recording long term NAO change, and quantifying its influence on regional climate (Moreno et al., 2010, Hurrell and Deser, 2009).
2 .1 1 .2 In s tru m e n ta l records
W eather station records enable the NAO Index to be calculated until the 1860's (Figure 2.15) (Jones et al., 1997).
NAO Index (Dec-Mar) 1864-1995
JUu„ 4 - 3 - -2\ 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 Year
Figure 2.15: NAO index using normalised SLP fo r Lisbon and Stykkisholmur, a predom inately positive trend is clearly evident from 1970 - 2000 AD (Hurrell and VanLoon 1997).
Positive and negative NAO phases during the 20th century correlate w ell w ith climatic variations, highlighting the NAO's role as a m ajor controlling fa cto r in European climate (Hurrell and VanLoon, 1997, Wanner et al., 2001, Semenov et al., 2008). Fluctuation between positive and negative NAO index occur w ith a 6-10 year periodicity during the la tte r half of the 20th century (Hurrell and VanLoon, 1997, Proctor et al., 2002, Appenzeller et al., 1998, Black et al., 1999, Proctor et al., 2000). These fluctuations however, occur during a tim e of
rapid anthropogenic change, w ith some studies suggesting modal fluctuations are controlled by factors external to the natural atmospheric system (Semenov et al., 2008, Hurrell and Deser, 2009).
However, using modern GNIP site data Baldini et al., (2008) show th a t central Europe and the northern Iberian cost line have a significant (p> 0.1) correlation (0.4; spearman's rank) between the w inte r NAO index (December, January, February, March) and rainfall S180 (Figure 2.16). This significant correlation indicates that northern Iberia is w ell suited fo r the production of NAO records which are reflected as changes in rainfall 5180 .
\G N IP Stations
O > 10 yrs DFJM oieOp data
x NAO-tS^Op^ not significant ip > 0.1)
2ov)6 E m o t)J-T » ~ cln u>u'ij'
Figure 2.16: Contour map of Spearman's rank correlation coefficients (rs) between w in te r (December, January, February, March; DJFM) NAO index and DJFM (8lsO) from 43 GNIP stations w ith >10 years of complete DJFM 5180 data.
2 .1 1 .3 P alaeorecords o f th e NAO
Palaeoclimate and environm ental records which record an NAO signature are prim arily derived from lake (Trouet et al., 2009), ocean (Olsen et al., 2012), ice (O'Brien et al., 1995)
in s tru m e n ta l d ata on NAO phase. These clim ate records can be analysed fo r a large range o f proxies, o fte n unique to th e record being used. H o w ever, th e accurate analysis o f proxies can o ffe r indirect evidence o f th e NAO e.g. evidence o f en hanced sea sto rm conditions re co rd ed as an increase in sea salt sodium flux ice cores, re la ted to intense positive NAO co n dition s (O 'B rein e t al., 1995). A spatial range o f p alae o c lim a te records (fo r e x am p le th ro u g h o u t Europe) related to th e NAO can th e re fo re be used to u nd erstan d changes in NAO phase during m a jo r clim atic events, and possibly aid o u r und erstan d in g o f h o w th e NAO m ay have c o n trib u te d to clim atic conditions (rainfall intensity o r te m p e ra tu re ) durin g th o se even ts (A p p en zelle r et al., 1998, Sem enov et al., 2 0 08 , T ro u e t e t al., 2 0 0 9 , Olsen e t al., 2 0 1 2 ).
M o s t high resolution records suggest th e NAO played a role during m id d le to late H o locene c lim a te change, th ro u g h o u t Europe (T ro uet e t al., 20 09 , Barlow e t al., 1 9 9 3 ). M o jta h id e t al., (2 0 1 3 ) have identified m illennial scale fluctuatio n s in th e NAO fro m ocean cores ex tra c te d fro m th e n o rth e rn coast o f Spain. These cycles in NAO style atm osph eric positioning ind icate th e NAO has exacted a strong control o ver clim ate variab ility th ro u g h o u t Europe durin g th e H o lo cen e. H o w ever, th e m ajo rity o f Holocene scale NAO records b reakd o w n during th e early H o lo c en e possibly due to th e existence o f m a jo r ice masses in th e n o rth e rn h em is p h ere (G irau deau e t al., 2 0 1 0 ). This b reakd o w n in NAO forcing during th e YD and early H o locene e ith e r indicates th a t th e NAO did not d evelop as a m a jo r clim atic fo rce until th e mid H o lo c en e o r th a t cu rren t records o f th e NAO are poorly resolved / positioned to record a long d u ratio n NAO signal. The fo rm a tio n o f longer d uration , NAO sensitive palaeorecords m ay help to id e n tify w hen th e NAO began to co n tro l European clim ate.
2.11.4 Speleothems
D ue to th e ir sequential g ro w th and long g ro w th d u ratio n , sp eleoth em s o ffe r an ideal archive ty p e to record a long d uration NAO signal. Production o f high resolution sp ele o th em records o f th e NAO relies h ow ever, upon having a suitable cave site. Baldini e t al. (2 0 0 6 b ) suggest th e fo llo w in g criteria: 1) high am plitud es in th e m e teo ric w a te r 6 180 , 2) strong com parison b e tw e e n m e te o ric 6 180 and NAO index (Baldini e t al., 2 0 0 8 ), 3) m in im al re te n tio n o f w a te r in soil and karst zones, and finally 4) high s p ele o th em g ro w th rates. Several sp e le o th e m studies have reprod u ced a long te rm (< 3000yrs) records o f NAO using s p e le o th e m g ro w th layer thickness, w hich indicate changes in th e hydrological system re la ted to th e intensity o f p re c ip ita tio n driven by th e NAO (P ro cto r e t al., 2 0 0 0 , P roctor e t al., 2 0 0 2 , Jackson e t al., 2 0 0 8 . A ccu rately assessing annual g ro w th layer w id th is h o w ev er, co m p lex and o fte n in accurate. T h e re fo re , in cave sites such as those id en tified by Baldini e t al. (2 0 0 6 b ), high
re so lu tio n analysis o f chem ical proxies preserved w ith in sp eleoth em s should be u n d e rta k e n , m a n y o f w hich m ay be able to o ffe r insight into changing hydrological co n dition s and th e re fo r e variations in th e NAO.