Summer North Atlantic Oscillation Research Articles

Drought variability in eastern Mongolian Plateau and its linkages to the large-scale climate forcing

A robust regional tree-ring chronology with a span of 1819–2009 was developed for the Hulun Buir steppe, China, a region in the eastern edge of the Mongolian Plateau. This chronology exhibited significantly positive correlation with precipitation in June, and negative correlations with temperature from April to September except for May. Highest correlation was found between tree rings and the average April–August standardized precipitation evapotranspiration index (SPEI), suggesting that pines growth strongly respond to the seasonal drought conditions. Accordingly, the average April–August SPEI reconstruction was performed for the period 1854–2009, explaining 45.5 % variance of the calibration period 1953–2009. New reconstruction shows some synchrony with regional-scale events found in other reconstructions to the west Mongolian Plateau. The recent droughts in late 1990 to present are not unusual in the context of the past several centuries. Spectrum analyses suggested that the average April–August SPEI variations, especially severe droughts in the late 1870s-early 1880s, 1920s and since the late 1990s could be associated with large-scale climate forcing, such as the El Nino-Southern Oscillation, the Pacific Decadal Oscillation and the summer North Atlantic Oscillation. Significant teleconnections indicated drought variability during the past several centuries in eastern Mongolian Plateau existed close connections with large-scale synoptic features.

A tree-ring field reconstruction of Fennoscandian summer hydroclimate variability for the last millennium

Hydroclimatological extremes, such as droughts and floods, are expected to increase in frequency and intensity with global climate change. An improved knowledge of its natural variability and the underlying physical mechanisms for changes in the hydrological cycle will help understand the response of extreme hydroclimatic events to climate warming. This study presents the first gridded hydroclimatic reconstruction (0.5° × 0.5° grid resolution), as expressed by the warm season Standardized Precipitation Evapotranspiration Index (SPEI), for most of Fennoscandia. A point-by-point regression approach is used to develop the reconstruction from a network of moisture sensitive tree-ring chronologies spanning over the past millennium. The reconstruction gives a unique opportunity to examine the frequency, severity, persistence, and spatial characteristics of Fennoscandian hydroclimatic variability in the context of the last 1,000 years. The full SPEI reconstruction highlights the seventeenth century as a period of frequent severe and widespread hydroclimatic anomalies. Although some severe extremes have occurred locally throughout the domain over the fifteenth and sixteenth centuries, the period is surprisingly free from any spatially extensive anomalies. The twentieth century is not anomalous in terms of the number of severe and spatially extensive hydro climatic extremes in the context of the last millennium. Principle component analysis reveals that there are two dominant modes of spatial moisture variability across Fennoscandia. The same patterns are evident in the observational record and in the reconstructed dataset over the instrumental era and two paleoperiods. The 500 mb pressure patterns associated with the two modes suggests the importance of the summer North Atlantic Oscillation.

Annual thawing and freezing indices changes in the China Tianshan Mountains

Annual thawing/freezing indices (TIs and FIs) for ten meteorological stations in the China Tianshan Mountains (CTM) are analyzed based on the daily maximum and minimum temperature records during 1961–2010. These indices are submitted for trend analysis and change-point analysis. The correlations between the TIs and FIs and the annual mass balance (AMB) for the No. 1 glacier, a rapid regressing small-scale glacier with the solely longest observed time series of the AMB at the headwater of the Urumqi River in the CTM, were analyzed in order to demonstrate the implication behind variations in the TIs and FIs. Furthermore, to explain atmospheric controls of the TI and FI variations, correlations between large-scale climatic forcing, e.g., the mean NAO indices and the mean Arctic Oscillation (AO) and the TIs and the FIs were analyzed. The results show that there were obvious upward trends in the TIs and downward trends in the FIs occurred, which demonstrates an increasing thawing and decreasing freezing potential in the CTM during the period of 1961–2010. Change-point analysis revealed evident change point around the year 1997 for the TIs at most of the stations, while various change points for the FIs. Obvious negative relationships between the TIs and the AMB for the No. 1 glacier have been found. Also, significant change point around the year 1996/1997 has been detected for the AMB time series. The revealed tight links between the TIs and the AMB, and also temporal consistency of change points indicates that changes in the TIs could be one of the main factors resulted in changes in the AMB of No. 1 glacier. Furthermore, significant negative correlation between the TIs and FIs and the mean summer NAO index for most stations are also found, demonstrating that the variations in TIs and FIs in the CTM were possibly related with changes in NAO.

Geomorphological records of extreme floods and their relationship to decadal-scale climate change

Extreme rainfall and flood events in steep upland catchments leave geomorphological traces of their occurrence in the form of boulder berms, debris cones, and alluvial fans. Constraining the age of these features is critical to understanding (i) landscape evolution in response to past, present, and future climate changes; and (ii) the magnitude–frequency of extreme, ungauged floods in small upland catchments. This research focuses on the Cambrian Mountains of Wales, UK, where lichenometric dating of geomorphological features and palaeohydrological reconstructions is combined with climatological data and documentary flood records. Our new data from Wales highlight a distinct flood-rich period between 1900 and 1960, similar to many other UK lichen-dated records. However, this study sheds new light on the underlying climatic controls on upland flooding in small catchments. Although floods can occur in any season, their timing is best explained by the Summer North Atlantic Oscillation (SNAO) and shifts between negative (wetter than average conditions with regular cyclonic flow and flooding) and positive phases (drier than average conditions with less frequent cyclonic flow and flooding), which vary from individual summers to decadal and multidecadal periods. Recent wet summer weather, flooding, and boulder-berm deposition in the UK (2007–2012) are related to a pronounced negative phase shift of the SNAO. There is also increasing evidence that recent summer weather extremes in the mid-latitudes may be related to Arctic amplification and rapid sea ice loss. If this is the case, continuing and future climate change is likely to mean that (i) unusual weather patterns become more frequent; and (ii) upland UK catchments will experience heightened flood risk and significant geomorphological changes.

A Tree-Ring Based Late Summer Temperature Reconstruction (AD 1675–1980) for the Northeastern Mediterranean

This article presents a late summer temperature reconstruction (AD 1675–1980) for the northeastern Mediterranean (NEMED) that is based on a compilation of maximum latewood density tree-ring data from 21 high-elevation sites. This study applied a novel approach by combining individual series from all sites into one NEMED master chronology. This approach retains only the series with a strong and temporally robust common signal and it improves reconstruction length. It further improved the regional character of the reconstruction by using as a target averaged gridded instrumental temperature data from a broad NEMED region (38–45°N, 15–25°E). Cold (e.g. 1740) and warm (e.g. 1945) extreme years and decades in the reconstruction correspond to regional instrumental and reconstructed temperature records. Some extreme periods (e.g. cold 1810s) reflect European-wide or global-scale climate conditions and can be explained by volcanic and solar forcing. Other extremes are strictly regional in scope. For example, 1976 was the coldest NEMED summer over the last 350 years, but was anomalously dry and hot in northwestern Europe and is a strong manifestation of the summer North Atlantic Oscillation (sNAO). The regional NEMED summer reconstruction thus contributes to an improved understanding of regional (e.g. sNAO) vs. global-scale (i.e. external) drivers of past climate variability.

A Tree-Ring Based Late Summer Temperature Reconstruction (AD 1675–1980) for the Northeastern Mediterranean

This article presents a late summer temperature reconstruction (AD 1675–1980) for the northeastern Mediterranean (NEMED) that is based on a compilation of maximum latewood density tree-ring data from 21 high-elevation sites. This study applied a novel approach by combining individual series from all sites into one NEMED master chronology. This approach retains only the series with a strong and temporally robust common signal and it improves reconstruction length. It further improved the regional character of the reconstruction by using as a target averaged gridded instrumental temperature data from a broad NEMED region (38–45°N, 15–25°E). Cold (e.g. 1740) and warm (e.g. 1945) extreme years and decades in the reconstruction correspond to regional instrumental and reconstructed temperature records. Some extreme periods (e.g. cold 1810s) reflect European-wide or global-scale climate conditions and can be explained by volcanic and solar forcing. Other extremes are strictly regional in scope. For example, 1976 was the coldest NEMED summer over the last 350 years, but was anomalously dry and hot in northwestern Europe and is a strong manifestation of the summer North Atlantic Oscillation (sNAO). The regional NEMED summer reconstruction thus contributes to an improved understanding of regional (e.g. sNAO) vs. global-scale (i.e. external) drivers of past climate variability.

Teleconnection between rainfall over South China and the East European Plain in July and August

In the present reported work, we identified that there is a significant negative relationship between rainfall over South China (SC) and the East European Plain (EEP) in the months of July and August, and investigated the possible reason for this negative relationship. The correlation coefficients between SC and the EEP rainfall were calculated to be −0.42 for July and −0.35 for August, both significant at the 95 % confidence level. We report that a wave-like train of circulation anomalies and a pathway of wave-activity flux stretching from Europe to East China connect the anticyclonic anomaly over Europe and the cyclonic anomaly over central and southern China, which are responsible for less EEP rainfall and more SC rainfall. We suggest that the teleconnection between SC and EEP rainfall results from the extension of stationary Rossby waves in the mid-latitudes in the upper troposphere for both July and August. This stationary Rossby wave is contributed to by summer North Atlantic Oscillation (NAO) and its extension features are determined by the location and intensity of the climatological upper-tropospheric westerly jet. Furthermore, we found that there was an interdecadal change around the mid-1970s in the negative SC–EEP rainfall relationship for both July and August. The negative correlation was significant and strong in the period 1976–2005, but much weaker in the period 1955–1975. The extension of stationary Rossby waves from Europe to East China was responsible for the significant negative relationship during the period 1976–2005.

Variability of the North Atlantic summer storm track: mechanisms and impacts on European climate

The summertime variability of the extratropical storm track over the Atlantic sector and its links to European climate have been analysed for the period 1948–2011 using observations and reanalyses. The main results are as follows. (1) The dominant mode of the summer storm track density variability is characterized by a meridional shift of the storm track between two distinct paths and is related to a bimodal distribution in the climatology for this region. It is also closely related to the Summer North Atlantic Oscillation (SNAO). (2) A southward shift is associated with a downstream extension of the storm track and a decrease in blocking frequency over the UK and northwestern Europe. (3) The southward shift is associated with enhanced precipitation over the UK and northwestern Europe and decreased precipitation over southern Europe (contrary to the behaviour in winter). (4) There are strong ocean–atmosphere interactions related to the dominant mode of storm track variability. The atmosphere forces the ocean through anomalous surface fluxes and Ekman currents, but there is also some evidence consistent with an ocean influence on the atmosphere, and that coupled ocean–atmosphere feedbacks might play a role. The ocean influence on the atmosphere may be particularly important on decadal timescales, related to the Atlantic Multidecadal Oscillation (AMO).

Influence of the circumglobal wave-train on European summer precipitation

We investigate European summer (July–August) precipitation variability and its global teleconnections using the NCEP/NCAR reanalysis data (1950–2010) and a historical Coupled Model Intercomparison Project climate simulation (1901–2005) carried out using the ECHAM6/MPIOM climate model. A wavelike pattern is found in the upper tropospheric levels (200 hPa) similar to the summer circumglobal wave train (CGT) extending from the North Pacific to the Eurasian region. The positive phase of the CGT is associated with upper level anomalous low (high) pressure over western (eastern) Europe. It is further associated with a dipole-like precipitation pattern over Europe entailing significantly enhanced (reduced) precipitation over the western (eastern) region. The anomalous circulation features and associated summer precipitation pattern over Europe inverts for the negative CGT phase. Accordingly, the global teleconnection pattern of a precipitation index summarizing summer precipitation over Western Europe entails an upper level signature which consists of a CGT-like wave pattern extending from the North Pacific to Eurasia. The imprint of the CGT on European summer precipitation is distinct from that of the summer North Atlantic Oscillation, despite the two modes of variability bear strong similarities in their upper level atmospheric pattern over Western Europe. The analysis of simulated CGT features and of its climatic implications for the European region substantiates the existence of the CGT-European summer precipitation connection. The summer CGT in the mid-latitude therefore adds to the list of the modes of large-scale atmospheric variability significantly influencing European summer precipitation variability.

Climate variability and outbreaks of infectious diseases in Europe.

Several studies provide evidence of a link between vector-borne disease outbreaks and El Niño driven climate anomalies. Less investigated are the effects of the North Atlantic Oscillation (NAO). Here, we test its impact on outbreak occurrences of 13 infectious diseases over Europe during the last fifty years, controlling for potential bias due to increased surveillance and detection. NAO variation statistically influenced the outbreak occurrence of eleven of the infectious diseases. Seven diseases were associated with winter NAO positive phases in northern Europe, and therefore with above-average temperatures and precipitation. Two diseases were associated with the summer or spring NAO negative phases in northern Europe, and therefore with below-average temperatures and precipitation. Two diseases were associated with summer positive or negative NAO phases in southern Mediterranean countries. These findings suggest that there is potential for developing early warning systems, based on climatic variation information, for improved outbreak control and management.

Enhanced Central European summer precipitation in the late 19th century: a link to the Tropics

AbstractIn Central Europe, an unusually large number of floods were recorded in the summers and autumns of the late 19th century, causing considerable damage. Different factors contributing to the recurrence of these floods have been discussed in the literature, including the unusually high precipitation anomalies recorded at the time. Based on the frequency and spatial pattern of these floods, previous studies suggest that changes in the large‐scale circulation may have played a relevant role. Here, we use an atmospheric global climate model forced with observed sea surface temperatures (SSTs) and atmospheric forcings (greenhouse gas concentrations and aerosol emissions) to test this hypothesis and identify the causes for the associated atmospheric circulation pattern.We find that our model is able to reproduce the high summer precipitation anomalies observed in Central Europe in the late 19th century, and we show that between 1875 and 1890, transient SSTs, as opposed to climatological ones, are the primary driver for this precipitation increase. Using a series of numerical experiments, we find that the SST variability in the central and eastern tropical Pacific Ocean (associated with the El Niño–Southern Oscillation region) was the primary driver for these anomalies. In addition, SST variability in the Indian Ocean as well as increasing anthropogenic aerosol emissions substantially enhanced European precipitation.The atmospheric circulation anomalies associated with this precipitation increase show a Pacific North American (PNA)‐like pattern over North America and a substantially weakened mid‐troposphere westerly flow over Europe and the North Atlantic, as well as increased pressure over Greenland and reduced pressure over Europe, resembling the negative phase of the summer North Atlantic Oscillation (NAO).

Predictable signals of seasonal precipitation in the Yangtze–Huaihe River Valley

ABSTRACTAn effort is made to identify the ‘more predictable’ signals in seasonal precipitation in the Eastern Yangtze–Huaihe River Valley of China based on the observations recorded for the period of 1951–2004 at a network of 23 stations in this region. A recently developed methodology for decomposing the interannual variance of seasonal mean climate fields is applied to precipitation time series at the stations. This allows the total interannual variance to be separated into the variance of a slow component, or predictable signal, and the variance of a more noisier component of rainfall associated with intraseasonal variability. The potential predictability (signal‐to‐total ratio) is generally moderate in this region and is lower during winter from January to March (JFM) and higher in summer from May to July (MJJ) and autumn from November to January (NDJ). Empirical orthogonal function (EOF) analysis is then applied to the predictable‐, intraseasonal‐ and total‐covariance matrices, respectively. Leading EOF modes of the total component more often resemble the EOF modes of the intraseasonal component when the potential predictability is lower and vice versa. Temporal variation of the leading modes in the predictable components is more closely linked to the interannual variability of Eastern Pacific sea surface temperature (SST) in early summer (MJJ), North Atlantic SST in late summer (JAS) and El Niño/Southern oscillation in late autumn (NDJ). The rainfall SST connections found in these seasons persist throughout the entire 54‐year period. The predictable rainfall modes also have apparent linkages to the intensity and position of the Western Pacific subtropical high during the early summer and late autumn with more rainfall occurring in the region when the height pattern is more intensive and extends further west. During the late summer, the predictable rainfall variability is accompanied by a large variation of tropospheric advection from north of the region, indicating the impact of an anomalous atmospheric circulation associated with the variation of summer North Atlantic Oscillation.

Low flows in France and their relationship to large-scale climate indices

Summary This study explores the relationship between low flows and large-scale climate variability in France. To this aim, a national low flow reference network of near-natural catchments, consisting of 236 gauging stations, was set up. A subset of 220 daily streamflow records for the period 1968–2008 was used to detect trends in a number of severity and timing drought indices. In addition to testing temporal trends, correlations with four climate indices were also evaluated: the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the frequency of two Weather Patterns corresponding to circulation types associated to wet (WP2) and dry (WP8) conditions over France. Due to their specific dynamics, NAO and WPs were also analyzed seasonally. Results show a consistent increase of drought severity in southern France. Correlations with NAO and AMO show a similar spatial pattern. Additionally, significant relationships with WPs were found throughout France, with the exception of the Mediterranean coast. Timing indices appear to be less related to large-scale climate indices, whereas some evidence of negative temporal trends was found (e.g. earlier drought start). To assess the robustness of the above relationships, a subset of 28 stations with longer records was studied over a 60 year period (1948–2008). The results show that, when shifting the time window of the analysis, the correlations between low flow indices and climate indices remain stable, whereas those with respect to time do not. Seasonal climate indices appear to have stronger links with low flow indices than their annual counterparts. For instance, the summer NAO shows a strong link with severity indices in the northern half of the country. This link is found again for the winter WP2. The above results indicate that temporal trends should only be used for descriptive purposes, whereas seasonally lagged climate indices are potential candidates as predictors of summer low flows.

Impact of Preceding Summer North Atlantic Oscillation on Early Autumn Precipitation over Central China

This study examined the impact of the preceding boreal summer(June–August) North Atlantic Oscillation(NAO) on early autumn(September) rainfall over Central China(RCC). The results show that a significant positive correlation exists between the preceding summer NAO and the early autumn RCC on the interannual timescale. In order to understand the physical mechanism between them, the role of ocean was investigated. It was found that the strong summer NAO can induce a tripole sea surface temperature anomaly(SSTA) in the North Atlantic; this SSTA pattern can persist until early autumn. The diagnostic analysis showed that the tripole SSTA pattern excites a downstream Atlantic-Eurasian(AEA) teleconnection, which contributes to an increase in RCC. The circulation anomalies related to SSTA caused by the weak NAO are opposite, so the RCC is less than normal. The results imply that the preceding summer NAO may be regarded as a forecast factor for the early autumn RCC.

Exploring teleconnections between the summer NAO (SNAO) and climate in East Asia over the last four centuries – A tree-ring perspective

The summer North Atlantic Oscillation (SNAO), derived from the first EOF of mean sea level pressure over the extratropical North Atlantic in July and August, has a close association with climate variability over the North Atlantic region, and beyond, on both short and long time scales. Recent findings suggested a teleconnection, through the SNAO, linking climate variability over Northern Europe with that of East Asia in the latter part of the twentieth century. Here we investigate the temporal stability of that teleconnection for the last four centuries using 4261 tree-ring width series from 106 sites and, additionally, ten climate reconstructions from East Asia. Our results showed a great potential in using tree-ring width (TRW) data to extend analyses of the SNAO influence on East Asian climate beyond the instrumental period, but preferably with a denser network. The strongest SNAO-TRW associations were found in central East Asia (in and around Mongolia) and on the eastern edge of the Tibetan Plateau. In addition, the analysis showed that the association between the SNAO and East Asian climate over the last 400 years has been variable, both among regions and at specific sites. Moreover, a clear difference in the SNAO-TRW associations was found on two examined time scales, being stronger on longer timescales. Our results indicate that TRW data can be a useful tool to explore the remote influence of the SNAO on East Asian climate in the past.

Physical Mechanism of the Impacts of the Tropical Atlantic Sea Surface Temperature on the Decadal Change of the Summer North Atlantic Oscillation

In this study,physical mechanism of the impacts of the tropical Atlantic sea surface temperature(SST) on decadal change of the summer North Atlantic Oscillation(SNAO) was explored using an atmospheric general circulation model(AGCM) developed at the International Centre for Theoretical Physics(ICTP).The simulation results indicate that the decadal warming of the SST over the tropical Atlantic after the late 1970s could have significantly enhanced the convection over the region.This enhanced convection would have strengthened the local meridional circulation over the Eastern Atlantic-North Africa-Western Europe region,exciting a meridional teleconnection.This teleconnection might have brought the signal of the tropical Atlantic SST to the Extratropics,consequently activating the variability of the eastern part of the SNAO southern center,which led to an eastward shift of the SNAO southern center around the late 1970s.Such physical processes are highly consistent with the previous observations.

The summer North Atlantic Oscillation in CMIP3 models and related uncertainties in projected summer drying in Europe

This paper discusses uncertainties in model projections of summer drying in the Euro‐Mediterranean region related to errors and uncertainties in the simulation of the summer NAO (SNAO). The SNAO is the leading mode of summer SLP variability in the North Atlantic/European sector and modulates precipitation not only in the vicinity of the SLP dipole (northwest Europe) but also in the Mediterranean region. An analysis of CMIP3 models is conducted to determine the extent to which models reproduce the signature of the SNAO and its impact on precipitation and to assess the role of the SNAO in the projected precipitation reductions. Most models correctly simulate the spatial pattern of the SNAO and the dry anomalies in northwest Europe that accompany the positive phase. The models also capture the concurrent wet conditions in the Mediterranean, but the amplitude of this signal is too weak, especially in the east. This error is related to the poor simulation of the upper‐level circulation response to a positive SNAO, namely the observed trough over the Balkans that creates potential instability and favors precipitation. The SNAO is generally projected to trend upwards in CMIP3 models, leading to a consistent signal of precipitation reduction in NW Europe, but the intensity of the trend varies greatly across models, resulting in large uncertainties in the magnitude of the projected drying. In the Mediterranean, because the simulated influence of the SNAO is too weak, no precipitation increase occurs even in the presence of a strong SNAO trend, reducing confidence in these projections.

Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers

Abstract. Greenland ice sheet mass loss has accelerated in the past decade responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area. Using satellite-derived surface albedo with calibrated regional climate modeled surface air temperature and surface downward solar irradiance, we determine the spatial dependence and quantitative impact of the ice sheet albedo feedback over 12 summer periods beginning in 2000. We find that, while albedo feedback defined by the change in net solar shortwave flux and temperature over time is positive over 97% of the ice sheet, when defined using paired annual anomalies, a second-order negative feedback is evident over 63% of the accumulation area. This negative feedback damps the accumulation area response to warming due to a positive correlation between snowfall and surface air temperature anomalies. Positive anomaly-gauged feedback concentrated in the ablation area accounts for more than half of the overall increase in melting when satellite-derived melt duration is used to define the timing when net shortwave flux is sunk into melting. Abnormally strong anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007, enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward shortwave flux, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net infrared and solar radiation for the high elevation accumulation area approached positive values during this period. Thus, it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.

Changes of the connection between the summer North Atlantic Oscillation and the East Asian summer rainfall

In this study, the relationship between the summer North Atlantic Oscillation (SNAO) and the East Asian summer rainfall was statistically diagnosed based on the European Centre for Medium‐Range Weather Forecasts (ECMWF) 40‐year and interim reanalysis data (ERA‐40 and ERA‐Interim) as well as precipitation data from the Global Precipitation Climatology Centre (GPCC). The results show that the decadal change of the SNAO pattern around the late 1970s significantly enhanced its connection with summer rainfall over central and northern East Asia. Over the period before the late 1970s, the SNAO‐related circulations were dominant over the North Atlantic region. Consequently, there was a weak connection between the SNAO and the East Asian summer rainfall. However, over the period after the late 1970s, the SNAO pattern experienced a decadal change, with the southern center shifting eastward. Such changes in the SNAO pattern can alter the stationary wave activity over the Eurasian Continent, producing an anomalous meridional dipole pattern over East Asia. This dipole pattern can then change the divergence circulation, vertical motion, water vapor, and total cloud cover, which would consequently provide beneficial conditions for more (less) summer rainfall over central (northern) East Asia in a positive (negative)‐phase SNAO year.

A pan-European summer teleconnection mode recorded by a new temperature reconstruction from the northeastern Mediterranean (ad 1768–2008)

The dominant atmospheric circulation pattern that governs European summer climate is a blocking-like pattern over the British Isles that co-occurs with a low over southeastern Europe. The meridionally oriented configuration of this circulation pattern favours the intrusion of warm air over the northeastern Mediterranean during one mode and over northwestern Europe during its opposite mode. We present a summer temperature reconstruction (1768–2008) for the southeastern node of this teleconnection pattern. This reconstruction is based on maximum latewood density (MXD) measurements of 23 Pinus heldreichii trees from a high-elevation stand in the Pirin Mountains in Bulgaria. The temperature signal in trees from high-elevation, summer-dry regions is stronger in MXD measurements compared with tree-ring width data and our reconstruction reflects well interannual- to decadal-scale summer temperature fluctuations in the Balkans as instrumentally recorded over the twentieth century. Fluctuations in our Bulgarian reconstruction correspond to summer temperature variability in the Balkans, Italy, and the southern Carpathians, but opposing temperature variability patterns are manifested over the British Isles and southern Scandinavia. The strong and consistent anti-phase relationship between our reconstruction and a reconstruction of the summer North Atlantic Oscillation (sNAO) suggests that the sNAO pattern is a main driver of the teleconnection between summer temperatures in southeastern versus northwestern Europe. This teleconnection is most pronounced on interannual timescales and has been stable over the last two centuries.