Tree-ring-based reconstruction of August NAJ variability

Variability in the position of the summer NAJ is linked to jet stream variability over Central North America, Europe, and eastern Asia3 (Fig. 1). Jet anomalies in the eastern North Atlantic (10–30W)—the region that comprises the Icelandic Low and Azores High centers of action—are most influential for European climate (Fig. 1, Supplementary Figs. 1-4). Northern (southern) jet anomalies in this region correspond to northern (southern) jet anomalies over northwestern Europe and southeastern Europe and create a summer temperature seesaw between BRIT and NEMED (Fig. 1; Supplementary Figs. 1, 4b). August jet anomalies in the eastern North Atlantic are further representative for a broader temporal window (July and August) and a broader geographical range in the North Atlantic (58W to 8E; Supplementary Fig. 2 in ref.20).

Fig. 1 Latitudinal position of the August Northern Hemisphere Jet. Left wings of the violins represent the August Northern Hemisphere Jet latitudinal position distribution over the instrumental period (1920–2012) for 20° longitudinal slices. Right wings represent distribution during anomalous years when the North Atlantic Jet (NAJ; 10–30°W) latitudinal position exceeded 1 stdev northwards (a) or southwards (b). Gray shading indicates significant differences between the left and right distributions (one-sided Kolmogorov–Smirnov test; p < 0.05). Background map shows August surface temperature anomalies (°C; CRUTEM3.2160) composited over the anomalous years. Composite maps were created in R with color palette adapted from the KNMI Climate Explorer (https://climexp.knmi.nl) Full size image

The temperature seesaw created by August NAJ anomalies is reflected in regional tree-ring records10 (Fig. 2a, b, Supplementary Figs. 4c, 5b). We have compiled maximum latewood density (MXD) records for both regions (Supplementary Table 1) that represent interannual variability in regional August average surface temperature (Fig. 2a, b; Supplementary Fig. 5a). The two well-replicated (Fig. 2d) temperature proxies explain 52% (r = 0.72, p < 0.001; 1901–1978; Fig. 2a) and 44% (r = 0.66, p < 0.001; 1901–1980; Fig. 2b) of the variance in regional August average temperature variation in BRIT and NEMED, respectively, and thus illustrate the temperature dipole generated by anomalous NAJ positions. The BRIT and NEMED tree-ring records correlate significantly negatively with each other over their period of overlap (r = −0.29; p < 0.01; 1725–1978; Fig. 2c) and the negative correlation between the two tree-ring series weakens only in years when volcanic (e.g., Mount Tabora in 1816; r = −0.06) or other forcings (e.g., in 1740, r = −0.05) generate cold summers over the entire European continent (Fig. 2e).

Fig. 2 BRIT and NEMED tree-ring chronologies. a, b Pearson's correlation maps of BRIT (a) and NEMED (b) tree-ring chronologies with gridded 1° CRU TS4.047 August temperature anomaly fields (1901–1978) over Europe. Correlation coefficients higher than 0.3 are significant at the p < 0.01 significance level. c BRIT and NEMED (inversed) tree-ring chronologies (1725–1978) and d their sample replication over time. e The 31-year running Pearson's correlation coefficients between the BRIT and NEMED chronologies are consistently negative over the full period, except for years (1740, 1812, 1816) when external forcings created cold conditions throughout Europe. Correlation coefficients below −0.374 (dashed line) are significant at the p < 0.05 level. Correlation maps in a, b were created in the KNMI Climate Explorer (https://climexp.knmi.nl) Full size image

The BRIT and NEMED MXD chronologies are both significantly correlated with August NAJ position (r = 0.5 and r = −0.57, respectively; p < 0.001; 1920–1978) and their composite explains close to 40% of the variance in the August NAJ target (r = 0.63; R2 a = 0.39; p < 0.01; Fig. 3a). Calibration and verification trials show that August NAJ position can be skillfully reconstructed back to 1725 based on this combination (Fig. 3b), with 27 to 52% of the variance explained in the verification procedure and overall positive reduction of error (RE) and coefficient of efficiency (CE) statistics (Supplementary Table 2). Calibration/verification statistics, however, were weaker for the early calibration period (1920–1948) compared to the later period (1949–1978), which is possibly due to inhomogeneities in the earliest period of the twentieth century reanalysis target data set21.

Fig. 3 Summer NAJ reconstruction and variance. The reconstruction of the latitudinal position of the August NAJ was scaled and calibrated against NAJ position calculated based on twentieth century reanalysis data and explains 40% of its variance over the period of overlap (1920–1978; a). The NCEP/NCAR reanalysis data (1948–2016) are plotted for comparison in a. The full NAJ reconstruction (1725–1978) including combined error estimations is plotted in b. Running 31-year window number of NAJ anomalies (c), number of northern (N) and southern (S) NAJ anomalies (d), and persistence of anomalies (e) are plotted on the central year of the window for reconstructed (blue) and 20C Reanalysis (red) NAJ time series. Anomalies are defined as years when NAJ >1 stdev, with standard deviation calculated based on a merged time series of reconstructed (1725–1919) and 20C Reanalysis (1920–1978) NAJ values. Horizontal dashed lines in c–e represent the highest 31-year values over the reconstruction period (1725–1978) Full size image

300 years of high-summer NAJ variability

The reconstruction shows that late twentieth century NAJ positions fall within the latitudinal NAJ range of the preceding centuries, with the exception of the summer of 1976, which was the northernmost NAJ position in both the instrumental data and the reconstruction. The southernmost NAJ position of the target time series (44N in 1931) was exceeded only twice in the late eighteenth century (1782 and 1799; Fig. 3b). These two years respectively are the seventh and second wettest Augusts in England and Wales since record keeping began in 176622. It is worth noting that the NAJ can deviate further northward from its average position (51.6N) than southward (Fig. 1, Supplementary Fig. 1) and as a result European climate anomalies can be more extreme during northern compared to southern NAJ anomalies (Fig. 1, Supplementary Figs. 1-3).

NAJ variability is often characterized using circulation indices such as the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, which describe combined changes in NAJ position and speed14,20,23. When the NAJ is in an anomalously northerly position, it generates stronger than normal cyclonic conditions to the north (Icelandic Low) and anticyclonic conditions to the south (Azores High), corresponding to positive NAO and negative EA phases. Our August NAJ target correlates significantly with summer NAO (r = 0.37, p < 0.01) and EA (r = −0.49, p < 0.01) indices. The NAJ reconstruction also correlates significantly positively with a tree-ring-based summer NAO reconstruction24 over their full length of overlap (1725–1976; r = 0.59, p < 0.01; Supplementary Fig. 6). The two reconstructions are not fully independent, with 13% of the tree-ring series used in the NAJ reconstruction contributing to the summer NAO reconstruction, but still demonstrate the long-term linkages between North Atlantic summer atmospheric circulation and jet stream variability.

We used existing MXD data for our NAJ reconstruction, primarily collected by other researchers (Supplementary Table 1), which allowed us to optimize sample replication and climate sensitivity, but limited our ability to retain low-frequency variability (see Methods section). As a result, BRIT, NEMED, and the NAJ reconstruction are dominated by sub-decadal variability (~4 to 6 years; Supplementary Fig. 7) and no significant long-term poleward or equatorward trends were detected (Fig. 3b). We found no significant relationship between reconstructed NAJ position and past El Niño Southern Oscillation (ENSO) events, but found a southward NAJ shift 2 years after past volcanic eruptions (Fig. 4d). Furthermore, the NAJ time series shows a steep and unprecedented increase in the number of NAJ anomalies—and thus in variance—starting in the 1960s (Fig. 3c). An analysis of northward (positive) and southward (negative) anomalies separately (Fig. 3d) shows that anomalies of both signs are more frequent since the 1960s, but only the number of northward anomalies reaches unprecedented levels starting ca. 1980. The late twentieth century synchronicity by itself is unprecedented, however, with previous centuries showing a seesaw in the number of northward versus southward anomalies (Fig. 3d). Such a simultaneous increase of northward and southward anomalies and thus increased interannual meridional variability is likely indicative of a stronger NAJ “wobbling”13 and a more sinuous NAJ, rather than a southward (~1785–1810 CE) or northward (~1810–1870 CE) shift of the NAJ regime, as might have happened in the past (Fig. 3d). The increased number of anomalies further results in an enhanced persistence of anomalies, defined as the number of occasions when anomalies of the same sign occurred in consecutive years (Fig. 3e). Persistence of weather extremes related to such NAJ anomalies can be particularly challenging for agricultural and hydropower productivity and for natural ecosystems.

Fig. 4 NAJ positions during historical weather extremes and volcanic eruptions. Superposed epoch analyses (SEA) of NW Europe climate (a), SE Europe fire (b), North American temperature (c), and volcanic eruptions (d) event series with August NAJ reconstruction. Event series used are the England-Wales summer (June–August) precipitation (a; 1766–2014) and Central England summer temperature (a; 1725–2014) time series22, the Netherlands summer temperature time series58 (a; 1725–2000), a tree-ring-based fire record from Mt. Taygetos, Greece29 (b; 1823–1940), August temperature data from four meteorological stations in North America (Supplementary Fig. 1) with records dating back to the nineteenth century (c), and an ice-core-based volcanic event series59 (d; n = 25, 1725–1900). Filled symbols indicate statistical significance (p < 0.05). Event years in a and c are defined as >1.5 stdev (upward triangle) and <1.5 stdev (downward triangle) of the average of the time series. The analysis window includes up to 5 years before and after each event year Full size image

Our NAJ reconstruction provides a multi-century benchmark against which to test the late twentieth century increase in occurrence and persistence of anomalies that is visible in the reanalysis NAJ time series (Fig. 3c–e). The unprecedented character of the late twentieth century increase is independent of reconstruction scaling period (Supplementary Fig. 8), but we recognize that our NAJ reconstruction ends in 1978 and thus does not itself register the increase. Nevertheless, the NAJ reconstruction is scaled to the variance of the instrumental target and reconstruction and target are characterized by very similar skewness (0.9 and 0.94, respectively) and kurtosis (4.78 and 4.58, respectively) values over their period of overlap (1920–1978), suggesting that the NAJ variance is well represented by the reconstruction (Supplementary Fig. 9). More recent tree-ring collections are available for BRIT25 and NEMED26 that in future analyses can be used to measure MXD, then reconstruct regional summer temperature, extend the NAJ into the twenty-first century, and capture late twentieth century NAJ variance. High mean segment lengths (e.g., 400+ years for NEMED) and quasi-millennial-length time series (800+ years for BRIT, 1000+ years for NEMED) will allow to retain more low-frequency variability, to investigate decadal-scale drivers of summer NAJ variability such as the Atlantic Multidecadal Oscillation and sea ice fluctuations23, and to study centennial-scale NAJ variability during the Medieval Climate Anomaly and Little Ice Age27.

Extreme summer weather events in BRIT, NEMED, and the American Midwest are often associated with northward and southward NAJ deviations3,4,6. A comparison of our NAJ reconstruction with independent time series of past BRIT summer weather and NEMED fire occurrence (Fig. 4a, b) confirms that the position of the NAJ has impacted European extreme weather events since at least 1725 CE. Heatwaves and droughts in BRIT and the northwestern European Lowlands have consistently been associated with northern NAJ positions, whereas the NAJ showed southward anomalies during pluvials and cold summers (Fig. 4a). For instance, the southernmost NAJ position in the reconstruction occurred in 1782 (Fig. 3b), which was such a cold summer in Scotland that the grain harvest failed and famine arose28. A long rainy spell dominated the second most southern summer of 1799 in England, with only 8 days without rain between 22 June and 17 November28. August temperature data from four meteorological stations with records dating back to the nineteenth century (Supplementary Fig. 1) indicate that in the American Midwest, heatwaves have also consistently occurred during northern and summer cold spells during southern NAJ positions (Fig. 4c).

In NEMED, nineteenth and early twentieth century fire events occurred during southern NAJ summers (Fig. 4b) and hence during wet, cold BRIT summers (Supplementary Fig. 10a). Late summer (July–August) is the dominant fire season in NEMED forests29 and a southern NAJ position in this period creates hot and dry conditions that are favorable for wildfire occurrence and spread. Moreover, northern NAJ positions 3-year prior also induce fire conditions (Fig. 4d, Supplementary Fig. 10b), which could be due to the quasi-periodic character of NAJ with a 6-year peak (Supplementary Fig. 7b). However, such lagged moisture/fire relationships are also prominent in fire regimes in the American Southwest30, where preceding cool and wet summers increase fuel load and promote burning in subsequent dry summers, and could thus be prevailing in NEMED as well. Increased NAJ variance—with both northern and southern NAJ anomalies occurring more frequently—can thus create NEMED climate conditions that are conducive to widespread burning. Unlike in the American Southwest, however, the historical NEMED fire record is extremely sparse29 and additional regional paleofire records are needed to fully characterize NEMED fire–climate relationships.