North Atlantic Oscillation Reconstructions Research Articles

Oxygen Isotopes in Tree Rings from Greenland: A New Proxy of NAO

We present the first Greenlandic tree ring oxygen isotope record (δ18OGTR), derived from four birch trees collected from the Qinguadalen Valley in southwestern Greenland in 1999. Our δ18O record spans from 1950–1999 and is significantly and positively correlated with winter ice core δ18O from southern Greenland. δ18OGTR records are positively correlated with southwestern Greenland January–August mean temperatures. North Atlantic Oscillation (NAO) reconstructions have been developed from a variety of proxies, but never with Greenlandic tree rings, and our δ18OGTR record is significantly correlated with NAO (r = −0.64), and spatial correlations with sea-level pressure indicate a classic NAO pressure seesaw pattern. These results may facilitate a longer NAO reconstruction based on long time series of tree ring δ18O records from Greenland, provided that subfossil wood can be found in areas vacated by melting glaciers.

A 2,000-year Bayesian NAO reconstruction from the Iberian Peninsula

The North Atlantic Oscillation (NAO) is the major atmospheric mode that controls winter European climate variability because its strength and phase determine regional temperature, precipitation and storm tracks. The NAO spatial structure and associated climatic impacts over Europe are not stationary making it crucial to understanding its past evolution in order to improve the predictability of future scenarios. In this regard, there has been a dramatic increase in the number of studies aimed at reconstructing past NAO variability, but the information related to decadal-scale NAO evolution beyond the last millennium is scarce and inconclusive. We present a new 2,000-year multi-annual, proxy-based reconstruction of local NAO impact, with associated uncertainties, obtained by a Bayesian approach. This new local NAO reconstruction is obtained from a mountain lacustrine sedimentary archive of the Iberian Peninsula. This geographical area is not included in previous NAO reconstructions despite being a widely used region for instrumental-based NAO measurements. We assess the main external forcings (i.e., volcanic eruptions and solar activity) on NAO variability which, on a decadal scale, show that a low number of sunspots correlate to low NAO values. By comparison with other previously published NAO reconstructions in our analyses we can test the stationarity of the solar influence on the NAO signal across a latitudinal gradient based on the position of the employed archives for each NAO reconstruction. Inconclusive results on the volcanic forcing on NAO variability over decadal time-scales indicates the need for further studies. Moreover, we highlight the potential role of other North Atlantic modes of variability (i.e., East Atlantic pattern) on the non-stationary behaviour of the NAO throughout the Common Era, likely via solar forcing.

A Euro-Mediterranean tree-ring reconstruction of the winter NAO index since 910 C.E.

We develop a new reconstruction of the winter North Atlantic Oscillation (NAO) index using a network of 97 Euro-Mediterranean tree-ring series. The reconstruction covers the period 910–2018 C.E., making it the longest annually resolved estimate of winter NAO variability available. We use nested correlation-weighted principal components regression and the Maximum Entropy Bootstrap method to generate a 2400-member ensemble of reconstructions for estimating the final reconstruction and its quantile uncertainties. Extensive validation testing of the new reconstruction against data withheld from the calibration exercise demonstrates its skill. The skill level of the new reconstruction is also an improvement over two NAO reconstructions published earlier. Spectral analyses indicate that the new reconstruction behaves like a ‘white noise’ process with intermittent band-limited power, suggesting that the winter NAO is stochastically forced. The ‘white noise’ properties of our reconstruction are also shown to be consistent with the spectral properties of long instrumental NAO indices extending back to 1781 and NAO indices extracted from a large number of forced climate model runs covering the last millennium. In contrast, an annually resolved multi-proxy NAO reconstruction of comparable length, based in part on speleothem data, behaves more like externally forced ‘red noise’ process, which is inconsistent with our reconstruction, long observations, and forced model runs.

Reconstructing Late Holocene North Atlantic atmospheric circulation changes using functional paleoclimate networks

Abstract. Obtaining reliable reconstructions of long-term atmospheric circulation changes in the North Atlantic region presents a persistent challenge to contemporary paleoclimate research, which has been addressed by a multitude of recent studies. In order to contribute a novel methodological aspect to this active field, we apply here evolving functional network analysis, a recently developed tool for studying temporal changes of the spatial co-variability structure of the Earth's climate system, to a set of Late Holocene paleoclimate proxy records covering the last two millennia. The emerging patterns obtained by our analysis are related to long-term changes in the dominant mode of atmospheric circulation in the region, the North Atlantic Oscillation (NAO). By comparing the time-dependent inter-regional linkage structures of the obtained functional paleoclimate network representations to a recent multi-centennial NAO reconstruction, we identify co-variability between southern Greenland, Svalbard, and Fennoscandia as being indicative of a positive NAO phase, while connections from Greenland and Fennoscandia to central Europe are more pronounced during negative NAO phases. By drawing upon this correspondence, we use some key parameters of the evolving network structure to obtain a qualitative reconstruction of the NAO long-term variability over the entire Common Era (last 2000 years) using a linear regression model trained upon the existing shorter reconstruction.

The effect of the East Atlantic pattern on the precipitation δ18O-NAO relationship in Europe

The North Atlantic Oscillation (NAO) is known to influence precipitation δ18O (δ18Op) through its control on air temperature and on the trajectory of the westerly winds that carry moisture onto Europe during boreal winters. Hence, paleoclimate studies seeking to reconstruct the NAO can exploit the δ18O signal that is commonly preserved in natural archives such as stalagmites, ice cores, tree rings and lake sediments. However, such reconstructions should consider the uncertainties that arise from non-stationarities in the δ18Op-NAO relationship. Here, new insights into the causes of these temporal non-stationarities are presented for the European region using both observations (GNIP database) and the output of an isotope-enabled general circulation model (ECHAM5-wiso). The results show that, although the East Atlantic (EA) pattern is generally uncorrelated to δ18Op during the instrumental period, its polarity affects the δ18Op-NAO relationship. Non-stationarities in this relationship result from spatial shifts of the δ18Op-NAO correlated areas as a consequence of different NAO/EA combinations. These shifts are consistent with those reported previously for NAO-winter climate variables and the resulting non-stationarities mean that δ18O-based NAO reconstructions could be compromised if the balance of positive and negative NAO/EA states differs substantially in a calibration period compared with the period of interest in the past. The same approach has been followed to assess the relationships between δ18Op and both winter total precipitation and winter mean surface air temperature, with similar results. Crucially, this study also identifies regions within Europe where temporal changes in the NAO, air temperature and precipitation can be more robustly reconstructed using δ18O time series from natural archives, irrespective of concomitant changes in the EA.

Aeolian sediment reconstructions from the Scottish Outer Hebrides: Late Holocene storminess and the role of the North Atlantic Oscillation

Northern Europe can be strongly influenced by winter storms driven by the North Atlantic Oscillation (NAO), with a positive NAO index associated with greater storminess in northern Europe. However, palaeoclimate reconstructions have suggested that the NAO-storminess relationship observed during the instrumental period is not consistent with the relationship over the last millennium, especially during the Little Ice Age (LIA), when it has been suggested that enhanced storminess occurred during a phase of persistent negative NAO. To assess this relationship over a longer time period, a storminess reconstruction from an NAO-sensitive area (the Outer Hebrides) is compared with Late Holocene NAO reconstructions. The patterns of storminess are inferred from aeolian sand deposits within two ombrotrophic peat bogs, with multiple cores and two locations used to distinguish the storminess signal from intra-site variability and local factors. The results suggest storminess increased after 1000 cal yrs BP, with higher storminess during the Medieval Climate Anomaly (MCA) than the LIA, supporting the hypothesis that the NAO-storminess relationship was consistent with the instrumental period. However the shift from a predominantly negative to positive NAO at c.2000 cal yrs BP preceded the increased storminess by 1000 years. We suggest that the long-term trends in storminess were caused by insolation changes, while oceanic forcing may have influenced millennial variability.

Daily precipitation in Northern Iberia: Understanding the recent changes after the circulation variability in the North Atlantic sector

AbstractTrends in the characteristics of daily precipitation over Northern Iberia (NIB) are analyzed after 14 daily records covering the last 40 years plus seven century‐length time series. Results reveal an evolution to drier conditions with subregional variations: in western and central subregions, the decrease in both the occurrence and the intensity of wet days results in a statistically significant decline of total precipitation. Contrary to other regions in the Iberian Peninsula, the lack of correlation between the North Atlantic Oscillation (NAO) index and the observed rainfall anomalies within NIB suggests that the dominance of a positive NAO mode from the late 1970s to 2002 cannot explain the trends. The land‐sea and mountain distribution creates sharp rainfall transitions depending on the pathways of the moisture‐laden winds. Consequently, NIB and other mountainous regions in the margins of the European continental water divide are sensitive to small changes in the NAO pressure centers and can exhibit both positive and negative rainfall anomalies for each NAO mode. A novel methodology for identifying changes in the NAO modes, consistent with the observed pressure/precipitation anomalies at both continental and regional scales, is presented. After the disaggregation of each dominant NAO wet season into two categories, the main rainfall changes in the NIB region can be explained in the context of the NAO variability during the last decades. The reported (sub)regional rainfall differences stress the need of caution when using NAO reconstructions, based in site specific rainfall anomalies, for the interpretation of the past climate precipitation variability in areas of complex terrain.

A model-tested North Atlantic Oscillation reconstruction for the past millennium.

The North Atlantic Oscillation (NAO) is the major source of variability in winter atmospheric circulation in the Northern Hemisphere, with large impacts on temperature, precipitation and storm tracks, and therefore also on strategic sectors such as insurance, renewable energy production, crop yields and water management. Recent developments in dynamical methods offer promise to improve seasonal NAO predictions, but assessing potential predictability on multi-annual timescales requires documentation of past low-frequency variability in the NAO. A recent bi-proxy NAO reconstruction spanning the past millennium suggested that long-lasting positive NAO conditions were established during medieval times, explaining the particularly warm conditions in Europe during this period; however, these conclusions are debated. Here, we present a yearly NAO reconstruction for the past millennium, based on an initial selection of 48 annually resolved proxy records distributed around the Atlantic Ocean and built through an ensemble of multivariate regressions. We validate the approach in six past-millennium climate simulations, and show that our reconstruction outperforms the bi-proxy index. The final reconstruction shows no persistent positive NAO during the medieval period, but suggests that positive phases were dominant during the thirteenth and fourteenth centuries. The reconstruction also reveals that a positive NAO emerges two years after strong volcanic eruptions, consistent with results obtained from models and satellite observations for the Mt Pinatubo eruption in the Philippines.

δ18O values of cave drip water: a promising proxy for the reconstruction of the North Atlantic Oscillation?

We present cave monitoring data and two drip water δ18O models for the Herbstlabyrinth-Adventhohle (HL) cave system, Germany. Winter climate of the cave region is influenced by the North Atlantic Oscillation (NAO), as documented by the positive correlations between winter temperature, winter rainfall δ18O values and the winter NAO index. Cave monitoring data show that recharge of the aquifer at the HL cave system mainly occurs during winter months, confirming our drip water model, which calculates recharge as the difference between precipitation and potential evapo-transpiration. Cave drip water from the HL cave system should, thus, be a sensitive proxy for reconstruction of the NAO. The infiltration models accounting for mixing effects occurring in the karst aquifer show a very good agreement with the monitoring data. Comparison of the modelled and the monitored drip water δ18O values suggests a mean residence time of the water in the aquifer of 12 months and a transmission time of 10 months. Using these parameters, a long-term dataset from the nearby meteorological station Frankfurt am Main and the rainfall δ18O data from our monitoring program, we model the δ18O values of the cave drip water for the last 144 years. Despite of the favourable conditions of the HL cave system for NAO reconstruction, the modelled drip water δ18O time series only shows a moderate correlation with the winter NAO index (r = 0.33). The major reason for the relatively low correlation is the substantial contribution of precipitation from the remaining seasons to the drip water, for instance, due to heavy precipitation events occurring during summer. These rainfall events bias the seasonality of infiltration and weaken the winter NAO signal recorded in the drip water δ18O values. Reconstruction of the NAO from speleothems is, thus, challenging, in particular considering further complications arising from chronological uncertainties and calcite precipitation under conditions deviating from isotopic equilibrium. These problems may, at least in part, be avoided by multi-proxy studies of annually laminated stalagmites.

Witnessing North Atlantic westerlies variability from ships’ logbooks (1685–2008)

A monthly index based on the persistence of the westerly winds over the English Chanel is constructed for 1685–2008 using daily data from ships’ logbooks and comprehensive marine meteorological datasets. The so-called Westerly Index (WI) provides the longest instrumental record of atmospheric circulation currently available. Anomalous WI values are associated with spatially coherent climatic signals in temperature and precipitation over large areas of Europe, which are stronger for precipitation than for temperature and in winter and summer than in transitional seasons. Overall, the WI series accord with the known European climatic history, and reveal that the frequency of the westerlies in the eastern Atlantic during the twentieth century and the Late Maunder Minimum was not exceptional in the context of the last three centuries. It is shown that the WI provides additional and complementary information to the North Atlantic Oscillation (NAO) indices. The analysis of WI series during the industrial era indicates an overall good agreement with the winter and high-summer NAO, with the exception of several multidecadal periods of weakened correlation. These decoupled periods between the frequency and the intensity of the zonal flow are interpreted on the basis of several sources of non-stationarity affecting the centres of the variability of the North Atlantic and their teleconnections. Comparisons with NAO reconstructions and long instrumental indices extending back to the seventeenth century suggest that similar situations have occurred in the past, which call for caution when reconstructing the past atmospheric circulation from climatic proxies. The robustness and extension of its climatic signal, the length of the series and its instrumental nature make the WI an excellent benchmark for proxy calibration in Europe and Greenland.

Impact of precipitation intermittency on NAO-temperature signals in proxy records

Abstract. In mid and high latitudes, the stable isotope ratio in precipitation is driven by changes in temperature, which control atmospheric distillation. This relationship forms the basis for many continental paleoclimatic reconstructions using direct (e.g. ice cores) or indirect (e.g. tree ring cellulose, speleothem calcite) archives of past precipitation. However, the archiving process is inherently biased by intermittency of precipitation. Here, we use two sets of atmospheric reanalyses (NCEP (National Centers for Environmental Prediction) and ERA-interim) to quantify this precipitation intermittency bias, by comparing seasonal (winter and summer) temperatures estimated with and without precipitation weighting. We show that this bias reaches up to 10 °C and has large interannual variability. We then assess the impact of precipitation intermittency on the strength and stability of temporal correlations between seasonal temperatures and the North Atlantic Oscillation (NAO). Precipitation weighting reduces the correlation between winter NAO and temperature in some areas (e.g. Québec, South-East USA, East Greenland, East Siberia, Mediterranean sector) but does not alter the main patterns of correlation. The correlations between NAO, δ18O in precipitation, temperature and precipitation weighted temperature are investigated using outputs of an atmospheric general circulation model enabled with stable isotopes and nudged using reanalyses (LMDZiso (Laboratoire de Météorologie Dynamique Zoom)). In winter, LMDZiso shows similar correlation values between the NAO and both the precipitation weighted temperature and δ18O in precipitation, thus suggesting limited impacts of moisture origin. Correlations of comparable magnitude are obtained for the available observational evidence (GNIP (Global Network of Isotopes in Precipitation) and Greenland ice core data). Our findings support the use of archives of past δ18O for NAO reconstructions.

Testing the robustness of a precipitation proxy-based North Atlantic Oscillation reconstruction

The reconstruction of past atmospheric circulation is crucial for the understanding of natural climate change and its driving factors. A recent reconstruction suggests that, during Medieval times, the European region was dominated by a persistent positive phase of the North Atlantic Oscillation (NAO), followed by a shift to a more oscillatory behavior. We test this hypothesis and the concept underlying the reconstruction in a pseudo-proxy approach using instrumental records, reanalysis data sets and millennial simulations with four different climate models. While a shift from a more positive to a more negative phase of the NAO seems to be likely, the amplitude and persistence of the reconstructed positive phase cannot be reproduced by models. The analysis further reveals that proxy locations that were used in the reconstruction are not always sufficient to describe the NAO. This is reflected in a failure of the reconstruction to verify against instrumental records of the NAO in the 19th century. By adding complementary proxies, the robustness of an NAO reconstruction can be improved to the degree that it would withstand the tests presented here.

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

AbstractA westerly index for Europe is developed back to 1692 using newly recovered and corrected Mean Sea‐level 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. (2009) as well as the summer Luterbacher et al. (1999) 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 1970 the summer index has mostly been negative, indicating reduced westerlies and increased blocking conditions that are exceptional in the context of the last 250 years. Copyright © 2012 Royal Meteorological Society

Past and recent changes in the North Atlantic oscillation

AbstractThe North Atlantic oscillation (NAO) is under current climate conditions the leading mode of atmospheric circulation variability over the North Atlantic region. While the pattern is present during the entire year, it is most important during winter, explaining a large part of the variability of the large‐scale pressure field, being thus largely determinant for the weather conditions over the North Atlantic basin and over Western Europe. In this study, a review of recent literature on the basic understanding of the NAO, its variability on different time scales and driving physical mechanisms is presented. In particular, the observed NAO variations and long‐term trends are put into a long term perspective by considering paleo‐proxy evidence. A representative number of recently released NAO reconstructions are discussed. While the reconstructions agree reasonably well with observations during the instrumental overlapping period, there is a rather high uncertainty between the different reconstructions for the pre‐instrumental period, which leads to partially incoherent results, that is, periods where the NAO reconstructions do not agree even in sign. Finally, we highlight the future need of a broader definition of the NAO, the assessment of the stability of the teleconnection centers over time, the analysis of the relations to other relevant variables like temperature and precipitation, as well as on the relevant processes involved. WIREs Clim Change 2012, 3:79–90. doi: 10.1002/wcc.150This article is categorized under: Paleoclimates and Current Trends > Earth System Behavior

Mediterranean Sea level and barotropic flow through the Strait of Gibraltar for the period 1958–2001 and reconstructed since 1659

Sea level values from a two‐dimensional model of the Mediterranean Sea forced by atmospheric pressure and wind are used to estimate the barotropic flow through the Strait of Gibraltar for the period 1958–2001. The Mediterranean mean sea level derived from the model ranges between ±20 cm with a standard deviation of 5.5 cm and is correlated to the North Atlantic Oscillation (NAO) index. Thus NAO historical data and reconstructions are used to derive the Mediterranean Sea level variability from 1659 until 2001. The accuracy of the reconstruction is estimated in 2.7 cm for monthly mean values, 0.41 cm for annual mean values, and 0.22 cm for decadal mean values (0.48 cm for decadal winter mean sea level). The barotropic flow through the strait is computed from the model output as the time derivative of the total volume of the basin. During the period 1958–2001 the estimated daily flow ranges between ±2.7 Sv, with a standard deviation of 0.56 Sv. The dominant periodicities are in between 1 and 2 weeks. At these scales the model successfully reproduces previously published flow estimates based on current meter observations, which confirms that atmospheric pressure and wind dominate the intraseasonal variability of the flow. For the annual cycle the variability of the atmospherically induced flow is similar to the variability of the flow induced by the evaporation‐precipitation (E‐P) budget (±0.025 Sv), though absolute values of the first are about a third of the latter. At longer timescales the atmospheric contribution is much smaller than the E‐P induced flow.

Preliminary Reconstructions of the North Atlantic Oscillation and the Southern Oscillation Index from Measures of Wind Strength and Direction Taken During the Cliwoc Period

Using measures of wind strength and direction taken onboard ships during the 1750–1850 (CLIWOC project) period, preliminary reconstructions are attempted for the North Atlantic Oscillation (NAO) and the Southern Oscillation Index (SOI). The reconstructions are based on regression equations developed using similar data from the ICOADS dataset. Although the regression relationships developed over a calibration period (1881–1940) work almost as well over an independent verification period (1941–1997), application to the earlier 1750–1850 period results in barely statistically significant correlation coefficients when compared with a number of other NAO and SOI reconstructions from other proxy and long instrumental sources. A number of possibilities are investigated to attempt to determine the cause, the most likely of which is that the number of observations available for the CLIWOC period is just too low in some regions. As large numbers of ships' logbooks remain to be digitised, the regression relationships will prove useful to focus effort in future digitisation endeavours.

Scale-Dependent Reconstruction of the NAO Index

A method is presented to reconstruct decadal variations of the North Atlantic Oscillation (NAO). The spectral characteristics of the NAO on time scales of decades and longer are of particular interest for the understanding of North Atlantic ocean‐atmosphere interactions. The reconstruction is based on a transfer model calibration that uses bandpass-filtered time series. The maximum overlap discrete wavelet transform (MODWT) is applied for decomposing the time series variance into different time scales. A total of 43 proxies, including Greenland ice cores and European tree-ring chronologies, are selected and regionally grouped providing four independent reconstructions for the period 1700‐1978. The mean reconstruction agrees well with two recently published reconstructions during most of the time period. However, there are considerable differences in the earliest part before 1750. Running correlations between the reconstructions indicate that time-dependent relations exist among the different NAO reconstructions. The results suggest that the geographical distribution of proxies strongly affects the reconstruction and could explain some of the apparent discrepancies among the reconstructions recently published in literature. In the early eighteenth century, external forcing (solar, volcanic) seems to mask the NAO signature within the proxies.

NAO signal recorded in the stable isotopes of Greenland ice cores

The winter δ18O signal is extracted from 7 Greenland ice cores covering the past ∼700 years. To filter out noise and local variations in the 7 isotope records a principal component analysis is carried out on the ice core data. A comparison between the time series of the first principal component (PC1) with 67 years of winter (December to March) temperature measurements from 3 southern Greenland synoptic stations shows highly significant correlations. Southern Greenland winter temperatures are known to be greatly influenced by the North Atlantic Oscillation (NAO). A good proxy for southern Greenland temperatures is therefore expected to reveal at least parts of the NAO signal. It is shown that the PC1 time series indeed is significantly correlated to the NAO during the winter months. The inclusion of ice core winter season δ18O time series in future multiproxy NAO reconstructions is therefore recommended.

Reconstruction of the North Atlantic Oscillation, 1429–1983

AbstractThe North Atlantic Oscillation (NAO) is considered to be the dominant mode of winter atmospheric variability in the Northern Hemisphere (Barnston AG, Livezey RE. 1987. Classification, seasonality and persistence of low frequency atmospheric circulation patterns. Monthly Weather Review 115: 1083–1126), especially in the North Atlantic region. A better understanding of its recent variability in the context of pre‐instrumental period variations is critical for prediction purposes. A 555‐year (1429–1983) multi‐proxy reconstruction of the cool season NAO, calibrated against the Lisbon–Iceland (LISJHI) NAO, is presented. Predictor variables include tree‐ring chronologies from Morocco and Finland, GISP2 δ18O annual series, and a GISP2 snow accumulation record. Although the reconstructed values are generally lower than the instrumental values during the calibration period (1863–1983), the final reconstruction does capture the low frequency of the instrumental NAO. The reconstruction compares favourably with existing shorter NAO reconstructions and with the instrumental NAO. The variability in the reconstructed NAO is also discussed within the context of lengthy regional climate records. Results suggest that the occurrence and length of the recent persistently high phase of the NAO are not unusual over the 555‐year period of time, but that the magnitude of some of the instrumental values may, in fact, be unique. Copyright © 2001 Royal Meteorological Society

Grape harvest dates for checking NAO paleoreconstructions

Dates of the grape harvests in northeastern France and Switzerland exhibit significant correlations with the North Atlantic Oscillation (NAO), a large‐scale mode of climate variability over North Atlantic. These correlations, established for the monthly values of the NAO index over the last 175 years, reveal characteristic signatures as a function of frequency. Conversely, these signatures may be used to test the robustness of existing NAO paleoreconstructions, thanks to the availability of harvest dates for northwestern Europe since the end of the fifteenth century. Climatic signal appears prevailing over possible human factors in these series. The results demonstrate the importance of including old instrumental data and of providing monthly values rather than annual means in the NAO reconstructions. They suggest that the available long harvest dates series could constitute an interesting proxy to constrain such reconstructions.