Estimates of the North Atlantic Oscillation back to 1692 using a Paris London westerly index

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. : 8 48 () Published online 4 January in Wiley Online Library (wileyonlinelibrary.com) DOI:./joc.46 Estimates of the North Atlantic Oscillation back to 69 using a Paris London westerly index Richard C. Cornes, a * Philip D. Jones, a,b Keith R. Briffa a and Timothy J. Osborn a a Climatic Research Unit, School of Environmental Sciences, University of East Anglia, UK b Center of Excellence for Climate Change Research, Department of Meteorology, King Abdulaziz University, Jeddah, Saudi Arabia ABSTRACT: A westerly index for Europe is developed back to 69 using newly recovered and corrected Mean Sealevel Pressure (MSLP) data from London and Paris. The index is compared against various instrumental and proxy indices of the North Atlantic Oscillation (NAO). In the winter, the Paris-London index depicts a spatial pattern of atmospheric circulation that is bi-modal, with centres of action that are shifted eastwards compared to the NAO. Owing to asymmetry in the NAO the Paris-London index provides a good depiction of positive NAO conditions as well as extreme negative phases of the NAO that arise from reversals of the pressure centres, but less extreme negative NAO conditions are associated with westerly index values approaching zero. The merit in using the Paris-London index lies with its consistency over time as a measure of westerly wind flow, which may not be the case with other proxy-based indices. In the summer, the Paris-London index bears a close relationship to the reconstructed high-summer NAO series of Folland et al. (9) as well as the summer Luterbacher et al. (999) NAO reconstruction. An important finding is that the summer NAO was highly variable during the early nineteenth century but was predominately positive on the decadal time scale during that period. Since circa 97 the summer index has mostly been negative, indicating reduced westerlies and increased blocking conditions that are exceptional in the context of the last 5 years. Copyright Royal Meteorological Society KEY WORDS zonal flow; NAO reconstruction; Little Ice Age; sea-level pressure Received 5 May ; Revised 6 October ; Accepted 4 November. Introduction A tendency towards a more persistent positive state of the winter North Atlantic Oscillation (NAO) from the 97s, reaching a peak in the 99s, prompted questions in the scientific literature during the late 99s/early s about the role that greenhouse-gas forcing plays in influencing the variability of the NAO, and to what degree the observed high values were beyond the range of natural fluctuation (Gillett et al., ). During the first decade of the twenty-first century the NAO has weakened somewhat (Osborn, a, b) although much research is still concerned with examining the degree to which the variability of the NAO over the period of 96 was anomalous, particularly in respect to external forcing mechanisms (Deser and Phillips, 9). To answer that question, long time series of the NAO extending back to the early industrial or even pre-industrial era are required. Traditional indices of the NAO, which use the difference in Mean Sea-level Pressure (MSLP) between the Azores and Iceland during the winter months of the year (typically DJF or DJFM), extend back to the midnineteenth century. These indices show that the high positive values of the 99s were unprecedented over Correspondence to: R. C. Cornes, Climatic Research Unit, School of Environmental Sciences, University of East Anglia, NR4 7TJ, UK. r.cornes@uea.ac.uk that time period (Hurrell and van Loon, 997). The use of MSLP recorded at Gibraltar allows extension of the NAO index back to 8 with other Iberian stations allowing extension back to the 78s (Jones et al., ). The Gibraltar Reykjavik index has indicated that the high NAO index values of the 99s were not unprecedented but were of a magnitude similar to the values in the 9s (Jones et al., 997; Slonosky and Yiou, ). Questions about the variability of the NAO further back in time remain only partially answered (Hurrell et al., ). Proxy data are useful in this respect and have the potential to provide information on the state of the NAO over several hundreds of years (e.g. Appenzeller et al., 998; Cook et al., ; Luterbacher et al., a). However, these indices are often limited by non-stationary relationships between the proxy and the atmospheric circulation (Schmutz et al., ). Early pressure data from mainland Europe have been used for many years to provide information on the state of the atmospheric circulation in previous centuries (e.g. Mossman, 896; Lamb and Johnson, 96, 966; Kington, 98; Jones et al., 987, 999). The technique of using pressure gradients between two European sites was employed as early as the mid-eighteenth century by Short (767) who used the difference in pressure recorded at Plymouth and Yorkshire as a basis for explaining weather anomalies (Schove, 96). The earliest station-pair comparison that used data series of a useful length was Copyright Royal Meteorological Society

2 ESTIMATES OF THE NORTH ATLANTIC OSCILLATION 9 conducted by Mossman (896), and subsequently Mossman (9), who used the difference in MSLP recorded at London and Edinburgh. Brooks and Hunt (9) used the pressure difference between Edinburgh and Paris to assess the reliability of their long wind direction series for the British Isles. Later, during the 95s, the construction of station-pair pressure gradient indices became a popular research occupation (Hurrell et al., ). For example, according to Lamb and Johnson (959), Lysgaard (949) developed indices using Valentia Edinburgh, and Copenhagen Edinburgh: Schove (95, 96) used the difference in pressure recorded in the Scottish Lowlands and Thames valley (amongst other similar gradients) as a comparison to wind direction data from the British Isles. More recently, Slonosky et al. () have demonstrated that an index constructed from the difference in MSLP between London and Paris can provide a measure of westerly flow across western Europe, which has a close relationship to the traditional NAO indices, particularly during the winter months of the year. The Paris London index was extended back to 774 by Slonosky et al. () and allowed the authors to view the variability of the NAO in recent decades in the context of the last years. The condition of the NAO over the period of 97 observed in the traditional NAO indices did not appear as particularly significant when viewed in the context of the last years using the Paris London index. In addition, their results indicated increased variability in the annual index prior to circa 8. Slonosky et al. (b) demonstrated that a similar index could be constructed for the period of by using early instrumental observations recorded in London and Paris. Jones et al. () expanded these studies by examining the relationship of the Paris London index (774 ) to several NAO indices and surface temperature data series. They concluded that the Paris London index can provide a reliable extension to the traditional NAO indices and suggested that the series might be extended back to the late seventeenth century if long MSLP series for London and Paris could be constructed. This paper extends the work of Slonosky et al. (), Slonosky et al. (b) and Jones et al. () by using the newly recovered/homogenized London and Paris MSLP series (Cornes et al., a, b) to construct a westerly index that is nearly continuous back to 748, and is fragmentary back to 69. The homogeneity of the data has been assessed extensively, as detailed in Cornes et al. (a, b), and while concerns were raised in Cornes et al. (b) about the quality of the Paris data during the 67 68s, this does not affect the present study, which only uses the Paris data after 69. In addition to the increased length of the series, the results presented in this paper extend the previous studies by examining the Paris London index for all seasons of the year. The techniques employed in this paper follow those used by Slonosky et al. (; a) and Jones et al. (), with a combination of series-long correlations and running sub-period correlations being used to quantify the relationship of the Paris London index to alternative indices based on observed meteorological data and as inferred from various proxy climate data.. The construction of the Paris London index To construct a westerly index from the London and Paris MSLP data, the daily values were initially reduced to monthly means. Where the number of missing days in a month exceeded % the month was marked as missing. This is stricter than the 4% threshold used by Cornes () and eliminates certain outlying values in the early eighteenth century that result from a high number of missing values in certain seasons/years during that period. Anomalies were then calculated from these monthly data by subtracting the monthly mean for the period of to provide an index that represents deviations from modern-day conditions. These data were then normalized by dividing by the monthly standard deviation, which was also calculated from the period. Both these procedures were conducted on a month-by-month basis. Several different base periods were tested but the results from all tests were very similar. This normalisation technique was also used by Slonosky et al. (), Slonosky et al. (a,b) and Jones et al. (), although it should be noted that they used a normalisation period of This process of normalising the data is necessary to eliminate the expression of the annual cycle manifest in the un-standardized data and to ensure that a high temporal variance in the observations from either station does not bias the series. This approach has been used extensively in the calculation of station-pair indices for the Southern Oscillation Index (SOI) (Ropelewski and Jones, 987) and for the NAO indices (Jones et al., 997). However, as the standard deviation of MSLP at London and Paris is similar (Cornes, ) compared with, for example, the large difference between Ponta Delgada and Stykkisholmur in the NAO index, it may be considered less necessary here. Indices derived from both normalized and non-normalized data were tested, and without normalisation the variability of the index was very slightly reduced. The seasonal and annual values of the index were simply calculated as the mean of the monthly values in the respective seasons/years.. The relationship of the Paris London index to hemispheric MSLP, temperature and precipitation anomalies It has been suggested by Slonosky et al. () that the Paris London index is a zonal index of relevance for western Europe, and which is directly affected by the NAO despite being downstream of the NAO centres-ofaction. As the pressure gradient between London and Paris over climatological time scales declines from south to north, the Paris London index provides a measure of geostrophic flow on a line that approximates the position and direction of the English Channel. To provide Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

3 R. C. CORNES et al. Winter (DJF) Spring (MAM) 5W 8 5E 5W 8 5E W. E W.. E.... 9W E 9W 9E 6W E 6W.4.. 6E W E W E 5W Summer (JJA) 8 5E 5W Autumn (SON) 8 5E W E W... E 9W... 9E 9W. 9E 6W -. 6E 6W. 6E W. E W E Correlation (r)..6 Figure. Point correlation maps between gridded hemispheric MSLP and the Paris London index over the Period of The values of n during the winter and spring are and 4, respectively; for summer and autumn n =. The contour interval is.. The HadSLPr pressure data series was used and the data were converted to anomalies from the mean on a month-by-month basis. Although the HadSLPr data series begins in 85, the data prior to 88 were excluded due to their lower quality (Allan and Ansell, 6). This figure is available in colour online at wileyonlinelibrary.com/journal/joc further information on the exact pattern of atmospheric circulation that the Paris London index describes, correlation maps have been produced showing the association between gridded MSLP and the Paris London index (Figure ). In all seasons except summer, high values of the Paris London index are associated with low-pressure anomalies centred to the east of Iceland, and highpressure anomalies in an area encompassing the southern Mediterranean and northern Africa; the reverse is true for negative values of the index. The axis of this dipole follows the Greenwich Meridian, and hence, positive values are associated with increased westerly flow over Europe, while negative values are associated with decreased westerly or easterly flow. These results confirm the assertions of Slonosky et al. () that the Paris London index represents the influence of westerly flow over Europe, which is downstream of the NAO centres-of-action. It is important to note, however, that the pattern is most coherent in the winter (DJF) as shown by the higher correlation coefficients for that season. However, even during the winter the circulation pattern associated with the Paris London index differs somewhat from the typical NAO pattern and has elements of cluster pattern 4 (a European westerly pattern) and pattern 5 (an NAOtype pattern) following the classification by Philipp et al. (7). It also includes elements of the East Atlantic pattern as defined by Barnston and Livezey (987). During the summer, the pattern of the southern node is complicated and although the correlations are much weaker than during the other seasons the area of negative correlations is shifted south compared to winter and is centred on the North Sea. Two areas of positive correlation are evident: one area is located near to Tunisia, and a second weaker area is located in the Arctic. These results for summer Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

4 ESTIMATES OF THE NORTH ATLANTIC OSCILLATION indicate that the Paris London index produces a measure of the summer North Atlantic Oscillation (SNAO) as defined by Folland et al. (9), with the weak Arctic node and the North Sea node corresponding to the north and south nodes of the SNAO, respectively. However, the Paris London index primarily quantifies the southern node of the SNAO, and remains a westerly index. Therefore, an inverse relationship is to be expected between time series of the SNAO and the Paris London index. It must also be remembered that the SNAO is primarily a feature of high summer (July and August) and a different pattern is evident in June (Folland et al., 9). Comparison with the mean of JJA would therefore be likely to weaken the depiction of the SNAO in the results. A further important consideration when using the Paris London index as a proxy for the NAO is the relative proximity of the two locations. In contrast to stationpair indices of the NAO where the station series are highly negatively correlated, the Paris and London MSLP series are highly positively correlated (Cornes et al., b). Furthermore, the pressure difference between London and Paris is much smaller compared to the difference between stations representing the Azores and Icelandic pressure centres. This has an influence on the range of values in the Paris London index, which is of the order of five times lower than that of NAO station-pair indices. Furthermore, the relatively small pressure differences in the Paris London index render the index more sensitive to data errors in either or both MSLP series. For example, a data error of.4 hpa would be minor if the pressure-difference standard deviation was 4 hpa, but becomes more significant for a value of hpa. The proximity of London and Paris also has implications for the comparison of the Paris London index with NAO indices. Asymmetry in the NAO means that the pressure gradient between the two sites is greatly reduced during negative NAO conditions. In addition, since circa 97, the NAO centres-of-action have been situated farther to the east, with some suggestion that this may be related to asymmetry in the NAO along with the propensity of positive NAO conditions during the 97 period (Jung et al., ; Dong et al., ). The pattern of MSLP in the North Atlantic during the winter associated with the Paris London index (Figure ) is similar to the positive NAO conditions described in the winter regime analyses of Cassou et al. (4) and Hurrell and Deser (9). However, this pattern differs somewhat to negative NAO conditions when the northern node is shifted northwestward and the southern node is shifted slightly northeastwards. The Paris London index is only able to capture extreme negative NAO conditions that are associated with reversals of pressure around the centres-of-action (Moses et al., 987), which lead to strong easterly flows over western Europe and high gradients between London and Paris. More typical negative NAO conditions, where the pressure centres are simply more equitable and situated farther to the west would be associated with weak pressure gradients between London and Paris. This is demonstrated in Figure. Although the traditional NAO indices are similarly formed using (a) MSLP (b) MSLP High Paris-London Years n = 5 High PD-Styk Years n = (c) MSLP Low Paris-London Years n = MSLP Anomalies (hpa) (d) MSLP Low PD-Styk Years n = Figure. Average anomalies of winter (DJF) MSLP associated with high positive (>) and negative values (< ) in the scaled Paris London index and the scaled Pont-Delgada/Stykkisholmur NAO index (PD-Styk, see Table I for details). The HadSLPr pressure data were used, and the monthly means were converted to anomalies from the average. The number of years contributing to each plot are indicated, and only data for the period of 9 7 were used to allow comparison with Figure below. This figure is available in colour online at wileyonlinelibrary.com/journal/joc Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

5 R. C. CORNES et al. fixed-station data, the stations are suitably placed to remain close to the centres-of-action for both positive and negative NAO conditions. Despite these differences between the Paris London and NAO indices, the spatial pattern of surface temperature and precipitation anomalies across Europe during high and low phases is similar for both indices (Figure ). Indeed, the pattern of temperature anomalies across northwest Europe appears stronger for the Paris London index than for the NAO index. This clearly (a) Temperature High Paris-London Years n = 5 (b) Temperature High PD-Styk Years n = (c) Temperature Low Paris-London Years n = 4 (d) Temperature Low PD-Styk Years n = Temperature Anomalies ( C) (e) Precipitation High Paris-London Years n = 5 (f) Precipitation High PD-Styk Years n = (g) Precipitation Low Paris-London Years n = 4 (h) Precipitation Low PD-Styk Years n = Precipitation Anomalies (mm day ) Figure. Average anomalies of winter (DJF) surface temperature (HadCRUTv) and precipitation (GPCC version 5,.5.5 grid) associated with high positive (>) and negative values (< ) in the scaled Paris London index and the scaled Pont-Delgada/Stykkisholmur NAO index (PD-Styk, see Table I for details). The temperature data are the version of HadCRUTv available from the KNMI climate explorer database, which combine the sea-surface temperature and land-based temperature based on the proportion of land area per grid square (see Osborn, b for further details). Missing grid squares in the temperature series have been filled by the average of the neighbouring squares, as long as there are at least four such values. The anomalies for both temperature and precipitation are relative to the average. The number of years contributing to each plot are indicated, and only data for the period of 9 7 were used due to the time limit of the precipitation data. This figure is available in colour online at wileyonlinelibrary.com/journal/joc Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

6 ESTIMATES OF THE NORTH ATLANTIC OSCILLATION (a) NAO PC (b) NAO Zonal (c) PD-Styk (d) Iberia-Reyk (e) Luterbacher (f) Cook (g) Paris-London Figure 4. Time series plots of the NAO series listed in Table I (excluding Lisbon-Styk and Gib-Reyk) and the Paris London series during the extended winter season (DJFM). The Glueck series was not included in this plot as it has been reconstructed for the winter season expressed as DJF and is, therefore, not comparable with the other series. It should be noted that the data after 9 in the Luterbacher series are derived entirely from instrumental data. The thick black line shows the data smoothed with a -year Gaussian weighted filter. This figure is available in colour online at wileyonlinelibrary.com/journal/joc demonstrates the importance of using local-scale measures of the atmospheric circulation for assessing regional surface temperature and precipitation anomalies, which follows the reasoning of Jones and Lister (9). 4. Temporal variations in the Paris London and NAO indices In Figures 4, the annual and different seasonal Paris London westerly indices are compared against the various NAO indices detailed in Table I. The abbreviations for the indices listed in that table are used throughout the remainder of this paper. Both proxy and instrumental-based series were used in the comparison, following the example set by Cook (). 4.. Winter Two definitions of the winter season are used in this section: the conventional mean of data for the calendar months December to February, and the longer-period Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

7 4 R. C. CORNES et al. (a) NAO PC (b) NAO Zonal (c) PD-Styk (d) Iberia-Reyk (e) Luterbacher (f) Glueck (g) Paris-London Figure 5. As in Figure 4, but for winter (DJF). The Cook series has been excluded from this plot as it has been reconstructed for the extended winter season (DJFM) and is shown in Figure 4. This figure is available in colour online at wileyonlinelibrary.com/journal/joc mean of December to March. Common to all the time series (Figures 4 and 5) is the persistence of positive values during the 9 9 period. This period of predominately westerly flow has previously been recognized in many studies (e.g. Lamb, 977; Makrogiannis, 984). However, it must be noted that while the 9 9 period was dominated by strong westerly flows, it was punctuated by very strong easterly conditions during the winter of 96/97. The fact that this winter stands out in the series is reflective of the pattern of MSLP anomalies depicted by the Paris London index as described in Section above. The NAO indices in Figures 4 and 5 all show declining values from the 94s to the late 96s, and a recovery, thereafter, to a period of consistently high values during the late 98s/early 99s. However, the Paris London index shows a more suppressed variability, with generally low negative values during the 94 98s and a period of dominant high positive values during the 99s. This is reflective of the asymmetry in the NAO and the reduced Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

8 ESTIMATES OF THE NORTH ATLANTIC OSCILLATION 5 (a) NAO PC (b) NAO Zonal (c) PD-Styk (d) Iberia-Reyk (e) Luterbacher (f) Paris-London Figure 6. As in Figure 4, but for spring (MAM). This figure is available in colour online at wileyonlinelibrary.com/journal/joc ability of the Paris London index to capture typical negative NAO conditions. This low-frequency variability is also suppressed in the Iberia Styk index when compared to the other NAO index series, which indicates that this index series is also affected by asymmetry in the spatial distribution of MSLP during positive and negative phases of the NAO, although the locations of these two stations are close to the centres-of-action during typical negative NAO conditions, particularly when compared to London and Paris. Conversely, the NAO Zonal, and particularly the NAO PC indices, are better able to capture this asymmetry given the reduced reliance on fixed points in those indices. Portis et al. () developed a mobile NAO index to capture seasonal variations in the NAO. Clearly, a mobile NAO index is also warranted to capture interannual variations in the NAO following the asymmetric nature of the NAO. The results for the latter half of the eighteenth century and the first half of the nineteenth century show interesting features worthy of investigation. In the Paris London index, the period generally experienced weak westerly conditions, and the index was in a negative phase apart from the more westerly conditions of the mid-8s. The 8s and 8s were typified by strong westerly conditions, which were of a similar magnitude to the strong westerlies experienced during the 9 99 period, although the winter of 84/85 experienced strong easterly conditions. The DJF results (Figure 5) for the 8 8 period are quite different from those reconstructed by the Luterbacher and Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

9 6 R. C. CORNES et al. (a) NAO PC (b) NAO Zonal (c) PD-Styk (d) Iberia-Reyk (e) Luterbacher (f) Paris-London Figure 7. As in Figure 4, but for summer (JJA). This figure is available in colour online at wileyonlinelibrary.com/journal/joc Glueck proxy-based series, which show generally normal conditions during this period. In contrast, the synoptic situation at the turn of the nineteenth century has been described in several studies as being dominated by easterly conditions (Kington, 988). In the Paris London index, the 78s experienced anomalously weak westerly/easterly conditions, with the lowest negative value of the entire DJFM series occurring during the winter of 784/785. However, in the DJF Paris London series, the 784/785 winter does not show a very low index value, which would indicate that the seasonal mean during that year is strongly affected by predominant easterly conditions during March 785. Indeed, the weather maps constructed by Kington (988) indicate a dominant anticyclone over the British Isles. In addition, Manley s (964) London Weather Diary shows predominantly easterly winds in London during that month. Justification for this reconstruction also comes from temperature data, as March 785 was the coldest March in the CET series (Lamb, 995). The Luterbacher and Cook reconstructions also display negative values throughout the 78s, whereas the Glueck index indicates negative values in the second half of that decade. In the Paris London index, the 76s contain a wide range of extreme values, with the highest positive value of the entire DJF and DJFM series occurring in the winter of 76/76, and the most negative value in the DJF series during the winter of 764/765. These results contrast sharply with those of both the Luterbacher and Cook indices, which show generally negative values throughout the period. It is possible that errors in the London and Paris data may have led to these results. The Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

10 ESTIMATES OF THE NORTH ATLANTIC OSCILLATION 7 (a) SNAO (b) Paris-London Figure 8. A comparison between the Paris London index and the SNAO reconstruction for high summer (July and August). The SNAO index has been inverted from the original to provide a westerly index that is comparable to the Paris London index. The thick black lines show the values smoothed with a -year Gaussian low-pass filter. This figure is available in colour online at wileyonlinelibrary.com/journal/joc (a) NAO PC (b) NAO Zonal (c) PD-Styk (d) Iberia-Reyk (e) Luterbacher (f) Paris-London Figure 9. As in Figure 4, but for autumn (SON). This figure is available in colour online at wileyonlinelibrary.com/journal/joc Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

11 8 R. C. CORNES et al. (a) NAO PC (b) NAO Zonal (c) PD-Styk (d) Iberia-Reyk (e) Luterbacher (f) Paris-London Figure. As in Figure 4, but for annual means. This figure is available in colour online at wileyonlinelibrary.com/journal/joc transcription of the alternation of temperatures between Greenland and Germany published by van Loon and Rogers (978) indicates that the winter of 764/765 should be classed as NAO positive, with temperatures in Greenland very cold and in Germany moderate. It should be noted, however, that not all the Greenlandabove (below) winters described by van Loon and Rogers (978) are negative (positive) in the Paris London index (c.f. the results in Figure (a) (d)). Furthermore, the interannual variability in the Paris London index during the 76s bears a close relationship to that seen in the CET series (not shown). This independent information suggests that the Paris London index is reliable during the 76s. The results at the turn of the eighteenth century are limited by the high number of missing values in the Paris London index. There is an indication from those months that are represented that the index was generally negative during the winter months. This is consistent with the evidence from narrative sources (Lamb, 967, 977; Pfister, 994; Wanner et al., 995) and wind direction data from ship logbooks (Wheeler and Suarez- Dominguez, 6; Wheeler et al., 9). 4.. Spring Large differences between the various NAO time series and the Paris London series are apparent during the spring (Figure 6). Most notably, the NAO indices show that the 89 9 period was dominated by strong westerly flow, whereas the Paris London index displays near-neutral conditions during that period. In the twentieth century, the period was dominated by Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

12 ESTIMATES OF THE NORTH ATLANTIC OSCILLATION 9 Table I. The NAO indices used in this paper for comparison with the Paris London index. The two indices Gib/Reyk and Iberia/Reyk have been extended to 7 using Tim Osborn s update available at timo/datapages/naoi. htm. The dates in the Glueck series have been amended so that the year reflects the January of the winter season and, therefore, differs from the years quoted by the authors who took the winter dated from the December month. Period Data Abbreviation Source Instrument-based series 8 7 Normalized MSLP data from Gibraltar and Reykjavik Gib Reyk Jones et al. (997) 8 7 Normalized MSLP data from Iberia and Reykjavik Iberia Reyk Vinther et al. (a) Normalized MSLP data from Lisbon and Lisbon Styk Hurrell (995) Stykkisholmur/Reykjavik First eigenvector of North Atlantic MSLP NAO PC Hurrell (995) 87 7 Zonal pressure difference between 5 N and 65 N NAO Zonal Li and Wang () 865 Normalized MSLP data from Ponta-Delgada and Stykkisholmur PD Styk Rogers (984) Proxy-derived series Multiproxy reconstruction consisting of tree-ring Cook Cook et al. () chronologies from Europe and North America, and ice core records from Greenland Multiproxy reconstruction consisting of tree-ring Glueck Glueck and Stockton () chronologies from Morocco and Finland and ice core records from Greenland. 69 Multiproxy reconstruction of MSLP a (Azores/Iceland) Luterbacher Luterbacher et al. (999) based on pressure, temperature and rainfall series, and documentary data. MSLP from London were included in this reconstruction for the periods of , (excl. 77) and (December), and for Paris for the periods of 67 7, and (November) Reconstruction based on tree-ring data from Scandinavia SNAO Folland et al. (9) a The data after December 9 are an updated version of the gridded MSLP dataset developed by Trenberth and Paolino (98). strong westerly flows in the Paris London index, but this is only evident in the Iberia-Reyk index, and to a lesser extent in the Luterbacher series. Further back in time, the 8 85 period was dominated by negative values in the Paris London index, in partial agreement (i.e. the 8s, but not the 84s) with the Luterbacher reconstruction. Prior to this, the late eighteenth/early nineteenth century experienced generally strong westerly conditions, in contrast with the Luterbacher reconstruction, which shows strongly negative values in that period. The 78s are shown to have been a period of weak westerly flow in the Paris London index during the spring season; negative values of the index are also evident in the Luterbacher reconstruction during that time. This agrees well with the evidence for easterly conditions described by Le Roy Ladurie (97), which did great harm to the wine grape harvests in France at the time. The most negative value in the spring-season Paris London series occurred in 7, with the adjacent years also characterized by negative values of the index. This extreme 7 value is most probably a result of the low quality of the London pressure data during this period, which were obtained from the anonymous Holborn weather diary (Cornes et al., a). However, the Luterbacher et al. (a) reconstruction shows this short period to have been one with negative index values, although this magnitude is much less than in the Paris London index series. During the first decade of the 7s, the Paris London index shows generally strongly positive values, although prior to this there is an indication of predominately negative values. As with the winter series, the large number of missing values in the London pressure series during the 69s precludes any definite conclusions. However, according to Wanner et al. (995), the spring season in the 69s appears to have been characterized by strong easterly conditions; these results tentatively support this view. 4.. Summer During the summer months of the year (Figure 7), the Paris London index bears little relation to the instrumental NAO indices. Indeed, the instrumental NAO indices themselves show little temporal coherence and this is partly because the NAO is less well defined in the summer, and hence different indices represent different aspects of the atmospheric circulation. There is, however, close agreement between the Luterbacher reconstruction prior to 9 and the Paris London index. Of particular note is a phase during the period of when index values appear consistently high, with a peak at circa 8. Luterbacher et al. (a) indicate a jump in Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

13 4 R. C. CORNES et al. the reconstruction of summer (JJA) NAO conditions during the 77s, which coincides with the incorporation of many pressure series for locations in central and northern Europe. MSLP series from Paris and London contribute to the reconstruction beginning in 764 and 774, respectively, alongside 8 other series that are incorporated throughout the 76 77s. Thus there is a degree of circularity in the Figure-7 comparison as a result of the use of MSLP data from London and Paris in the Luterbacher series, although a clear delineation of circularity is hindered by variations in the number of predictors over time. This is addressed further in the comparison of correlation coefficients in Section 5 below. Briffa et al. (9) have indicated that this was a period of predominately high summer rainfall over northwest Europe, to an extent that was unprecedented in the -year rainfall record from Kew in London. Given the proximity of the Paris London index to the Kew station, a close association would be expected. These findings also agree with the interpretation of Lamb and Johnson (96), who attributed high values of their summer northwesterly index during the late eighteenth century to the frequent occurrence of a strong ridge of high-pressure extending from the Azores high into central Europe. Jacobeit et al. () also noted this increase in the frequency of westerly weather types during July and attributed the coincidental milder conditions across the north Atlantic/western European region at the time to negative vorticity anomalies within the broad westerly circulation. However, in contrast to the Luterbacher reconstruction, this 5- year period of predominately strong positive values was also a period of high interannual variability in the Paris London index. To some extent, this observation may be attributable to the temperature corrections applied to the barometer readings used in the London and Paris series. The quality of the temperature data in all seasons is likely to be low (Cornes et al., a, b), and there is the additional problem that in some cases the temperature data were not always recorded near to the barometer but were outside temperatures. In the winter months when the temperatures are lowest the corrections and, hence, the errors transferred to the pressure data are small. In the summer months, the temperature corrections applied to the barometer observations reach a maximum and, hence, the potential errors are also larger. Despite this limitation, the prolonged period of high values remains an important feature of the summer atmospheric circulation at the turn of the nineteenth century. A further notable feature of the Paris London index is that in the period since circa 97 the index has been in a predominately negative phase, and there have been only three summers when the index was positive. This feature agrees well with the results from the Central European zonal index developed by Jacobeit et al. () for the years Casty et al. (7) showed a similar result for their summer EOF pattern (a blocking-type pattern), which corresponds to the pattern depicted by the Paris London index in summer (Figure ) and the time series for which shows a similar variation to the Paris London index. These results also correspond closely to the results of Briffa et al. (9), which have indicated that this period is associated with a period of dryness in Europe that is anomalous in the context of the last years. Further supporting evidence is provided by weather-type catalogues for the UK. Both, the subjective Lamb Weather-type index, and the objective Jenkinson-Collinson classification show a pronounced shift in circa 97 to many more days in summer classified as anticyclonic (Wilby et al., ). In Figure 8, a comparison is shown between the Paris London index and the tree-ring-based SNAO reconstruction of Folland et al. (9) for high summer (July and August). Clearly, there is a very close positive agreement between these two independent series, with an interannual correlation of.4 over the period of (n = 6, p <. one-tailed t-test). Correlating the low-pass filtered series shown in Figure 8 produces a value of.7 (p <.5 one-tailed t-test, calculated as described in Section 5. below). However, while there is a close agreement between the series, the relationship is weaker further back in time, e.g. the correlation over the period of is. (n =, not significant), whereas over the period of it is.76 (n =, significant at p <. one-tailed t-test). Notably, the /76s are shown to be predominately negative while the Paris London index shows a period of relatively high positive values during most of the s. However, the low index values after circa 97 are evident in both series and are shown to be exceptional in the context of the last years. The Paris London index provides a longer time-frame context within which the values for the early twenty-first century may be compared. The unusually low index values of recent decades show no sign of abating, based on the series to 7 shown here. This is in line with modelled estimates of future conditions of the SNAO (Folland et al., 9) under assumed greenhouse-gas forcing. Importantly, this comparison is not limited by questions about common predictors, as is the case with the Luterbacher reconstruction, although it is important to note a similar exceptional period of westerly conditions in the late eighteenth/early nineteenth century that is common to all three series Autumn The time series for the autumn season (Figure 9) show some interesting results for the period of The Luterbacher reconstruction indicates that this period was one of weakly negative NAO conditions during the 69s and near-zero conditions during the 7s. In contrast, the Paris London index indicates that strong westerly conditions occurred in the 69 7 period, followed by strongly negative conditions in the 7s. It has been suggested by Lamb (967) that there was an increase in the frequency of wet autumns in the British Isles during the 69s, which would accord well with an increased frequency of westerly conditions across Europe. Copyright Royal Meteorological Society Int. J. 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14 ESTIMATES OF THE NORTH ATLANTIC OSCILLATION Annual To conclude this section, the annual mean time series are plotted in Figure. The results for the Paris London index agree with the findings of Slonosky et al. (), in that the variability in the Paris London index is much greater before 8 than after. This is attributable to the high variance apparent in the index during the summer months. As has been mentioned above, the quality of the pressure data is likely to be at a minimum during the summer on account of the lower quality of the thermometer data used in the temperature corrections. However, other studies (e.g. Kington, 98) have indicated that the atmospheric circulation in the period before the mid-nineteenth century was more variable than in the period after. This may, therefore, be a real feature of the atmospheric circulation and not merely an artefact of data inhomogeneity. 5. Correlations between the Paris London and NAO indices 5.. Interannual correlations The interannual correlation between various NAO indices for the winter season is well established (Osborn et al., 999; Wallace, ; Wanner et al., ; Li and Wang, ). As an aside, it should be noted that such correlation results for proxy-based NAO indices are often problematic because of common data input or proxy calibration forcing an apparent coincidence during the recent decade, which degrades prior to that. To gauge the relationship between the Paris London index and the NAO indices, a table of correlation coefficients is shown in Table II. The highest correlations are achieved with the NAO series that incorporate data from the Iberian Peninsula for the southern station (Gib-Reyk and Iberia- Reyk). The weakest correlations are achieved for the indices that are based on data located farthest west, in particular the PD Styk index. These results would be expected, given that the axis of the dipole quantified by the Paris London index roughly follows the Greenwich Meridian (Figure ). Interestingly the improvements made to the Gib Reyk index by Vinther et al. () appear to have raised the correlations with the Paris London index only during the winter months. In the summer months, lower correlation values are achieved with the Iberia-Styk index than for the original Jones et al. (997) Gib-Styk series, and this might be due to the temperature correction that Vinther et al. () applied to the Cadiz/San Fernando barometer readings. In the absence of barometer temperatures, contemporary outdoor temperatures were used, and it seems likely that these temperatures were too high in the summer months compared to the true barometer temperature, which led to an over-correction of the pressure data. However, this observation must be viewed in the context of a general Table II. Monthly, seasonal and annual Pearson correlation coefficients between the Paris London index and various NAO indices. Positive correlations in bold are significant at p<. and those in italics at p<.5 (one-tailed t-test). Correlation coefficients greater than.7 are underlined for emphasis. In the calculation of these p-values, the effective independent sample size has been estimated by scaling the sample size according to the degree of lag- autocorrelation (Slonosky et al., a, p68). The DJFM value for the PD-Styk index has been calculated from the average of the index for the component months and, therefore, differs from the calculation of the values during the other seasons which were calculated after averaging MSLP in the three months in the respective season. The values after December 9 in the Luterbacher series, which were gridded MSLP data, have been excluded in the calculation of these correlation coefficients. The full length of the other series have been used to compute these correlations. Instrument-based series Proxy-derived series Gib-Reyk Iberia-Reyk Lisbon-Styk NAO PC NAO Zonal PD-Styk Cook Glueck Luterbacher Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec DJFM DJF MAM JJA SON Annual Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()

15 4 R. C. CORNES et al. lower coherence in the atmospheric circulation in the summer, which is reflected by the generally lower correlations achieved in that season. The correlations in all series are at a maximum in the winter months, particularly in February, and show a minimum during the summer. This would be expected given that variations in the atmospheric circulation are most coherent during the winter. During the summer season (JJA), the correlation between the NAO PC index and the Paris London index is weakly negative, although significantly correlated (p <., one-tailed t-test). This is a reflection of the position of the southern node of the NAO, which is shifted northwards and is situated over the British Isles during these months. Thus, in a positive phase of this index, weak easterly flow is experienced across Europe. The correlations for the comparisons between the Paris London index and the three proxy-based indices show some interesting results. The Cook multiproxy reconstruction shows a much higher correlation than the Glueck reconstruction. However, the highest correlations are evident for the Luterbacher reconstruction. While the Luterbacher reconstruction is grouped under the proxy-derived category, instrumental data were included throughout the period considered here (69 9, see Table I), and hence, a higher correlation would be expected with the Paris London index compared to the two other proxy-derived indices. Furthermore, this result does not provide any indication of accuracy in the Luterbacher reconstruction during the earlier period when instrumental data were not used. Nonetheless, there are season variations in these correlations that are worthy of consideration. The correlations between the Paris London and Luterbacher indices are particularly high during the winter months when the correlation coefficients are comparable to those obtained for the PD- Styk series for the modern period. During the summer months, the correlations between the Luterbacher series are much higher than the PD-Styk index. Caution must be exercised when cross-comparing the results because the correlations were derived over different time periods, and because the sample sizes differ. However, this feature of the Luterbacher correlations would suggest that the reconstruction is biased towards mainland Europe. This is reflected in the results of Luterbacher et al. (b), which show that the highest skill in the reconstruction during the summer is achieved over Europe. Such a result might be expected given that for most of the reconstruction the predictors (both proxy and instrumental) are based in mainland Europe. In the winter when the atmospheric circulation is most coherent the reconstruction has a closer link to the pressure centres of the NAO, and this European bias becomes less important. 5.. Running correlations As an extension to the series-long correlation analysis above, running correlations based on a -year window have been calculated between the Paris London series and the various NAO series (Figure ). The running correlations during the winter (DJM and DJFM) are generally similar between the instrumental series. In the summer, larger differences are apparent. This would be expected given the series-long correlations listed in Table II. The correlation between the Paris London and Iberia- Reyk index is consistently the highest with values ranging between.7 and.9 during winter. Compared to the results of Jones et al. (), the running correlation between the Paris London index and Iberia Reyk here is more consistent over time, which is probably a reflection of the greater homogeneity of the Paris London index, but also the improvements made to the Gib Reyk index by Vinther et al. (). In the summer, the earliest correlations are below.. They rise to between. and.5 during the early twentieth century but gradually decline thereafter. A similar trend, although with generally higher correlation values, is evident during the autumn. As has been discussed above, the correlations during the earliest period may be reduced due to an over-correction of the Gibraltar station data by Vinther et al. (). A significant feature of the running correlations during the winter (DJF and DJFM) is the decline in the strength of correlations beginning in 9, which reaches a minimum for the -year period centred around 97; a recovery in the strength of the relationship can be observed thereafter. This result is most marked in the extended winter season (DJFM) and for the PC-based index. Jones et al. () also commented upon this feature of their running correlation analyses using their Paris London index. This result is probably due to the asymmetry in the NAO described earlier (Section ), with the centres-of-action situated farther east after the 97s, and the concomitant prevalence of positive NAO conditions. The period of extensive negative NAO conditions is poorly captured by the Paris London index leading to weaker correlations during those years. The two proxy-based NAO indices (Cook and Glueck) consistently show the lowest correlations compared to the other indices prior to 9 and 95, respectively, after which they are comparable with the instrumental NAO indices. The Cook index is considerably more highly correlated to the Paris London index than is the Glueck index series, which is weak (and generally negative) throughout much of the eighteenth and nineteenth centuries. In contrast to these two proxy series, the correlations with the Luterbacher series are generally high throughout the eighteenth and nineteenth centuries during the winter months. The correlations for the Luterbacher series after circa 9 are consistently higher compared to the PD-Styk series in the winter (DJF) season. The Luterbacher series during this time uses surface pressure data extracted from the five-degree gridded pressure set produced by the National Center for Atmospheric Research (NCAR) (Trenberth and Paolino, 98). The higher correlations may be attributed to the fact that the input data index are centred slightly to the east, and Copyright Royal Meteorological Society Int. J. Climatol. : 8 48 ()