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Oxygen stable isotopes measured in tree rings have been useful for reconstructing climate variability and explaining changes in physiological processes occurring in forests, complementing other more widely studied tree-ring parameters such as ring width. Here, we analyzed the relationships between different climate parameters and annually resolved tree-ring δ18O records (d18OTR) from white spruce (Picea glauca [Moench]Voss) trees located near Tungsten, Northwest Territories, Canada, and used the NASA GISS ModelE2 isotopically equipped general circulation model (GCM) to interpret the relationships in an idealized sense. The d18OTR series were primarily related to temperature variations in spring and summer, likely through temperature effects on the precipitation δ18O with a combination of evaporative enrichment at leaf level in summer. The GCM simulations showed significant positive relationships between modelled precipitation δ18O over the study region and surface temperature and geopotential height over northwestern North America, with stronger patterns during fall winter than during spring-summer. The modelled precipitation δ18O was only significantly associated with moisture transport during the fall-winter season. The d18OTR showed similar correlation patterns to modelled precipitation δ18O during spring-summer, with significant positive correlations with surface temperature and geopotential height, but no correlations with moisture transport. Overall, the d18OTR records for northwestern Canada reflect the same significant large-scale climate patterns as precipitation δ18O for spring-summer, and therefore have potential for reconstructions past atmospheric dynamics in addition to temperature variability.

Traditionally, high-latitude dendroclimatic studies have focused on measurements of total ring width (RW), with the maximum density of the latewood (MXD) serving as a complementary variable. Whilst MXD has typically improved the strength of the growing season climate connection over that of RW, its measurements are costly and time-consuming. Recently, a less costly and more time-efficient technique to extract density measurements has emerged, based on lignin’s propensity to absorb blue light. This Blue Intensity (BI) methodology is based on image analyses of finely-sanded core samples, and the relative ease with which density measurements can be extracted allows for significant increases in spatio-temporal sample depth. While some studies have attempted to combine RW and MXD as predictors for summer temperature reconstructions, here we evaluate a systematic comparison of the climate signal for RW and latewood BI (LWBI) separately, using a recently updated and expanded tree ring database for Labrador, Canada. We demonstrate that while RW responds primarily to climatic drivers earlier in the growing season (January–April), LWBI is more responsive to climate conditions during late spring and summer (May–August). Furthermore, RW appears to be driven primarily by large-scale atmospheric dynamics associated with the Pacific North American pattern, whilst LWBI is more closely associated with local climate conditions, themselves linked to the behaviour of the Atlantic Multidecadal Oscillation. Lastly, we demonstrate that anomalously wide or narrow growth rings consistently respond to the same climate drivers as average growth years, whereas the sensitivity of LWBI to extreme climate conditions appears to be enhanced.

This research was funded by grants from the US National Science Foundation P2C2 program (AGS-1501856, and AGS-1502224). BIC was supported by the NASA Modeling, Analysis, and Prediction program (NASA #80NSSC17K0265). Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atmospheric circulation anomalies in the Atlantic, but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial timescales. Here we analyze the coupled patterns of drought variability on either side of the North Atlantic Ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin-wide moisture anomalies to ocean and atmosphere processes. A leading mode of broad-scale moisture variability is characterized by distinct patterns of North Atlantic atmosphere circulation and sea surface temperatures. We infer a negative phase of the North Atlantic Oscillation and colder Atlantic sea surface temperatures in the middle of the 15th century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age. Publisher PDF

Although extended or ‘protracted’ El Niño and La Niña episodes were first suggested nearly 20 years ago, they have not received the attention of other ‘flavours’ of the El Niño–Southern Oscillation (ENSO) or low-frequency ‘ENSO-like’ phenomena. In this study, instrumental variables and palaeoclimatic reconstructions are used to investigate the most recent ‘protracted’ El Niño episode in 2014–2016, and place it into a longer historical context. Although just reaching the threshold for such an episode, the 2014–2016 ‘protracted’ El Niño had very severe societal, agricultural, environmental and ecological impacts, particularly in western Pacific regions like eastern Australia. We show that although ‘protracted’ ENSO episodes of either phase cause similar, near-global modulations of weather and climate as during more ‘classical’ events, impacts associated with ‘protracted’ episodes last longer, with strong influences in eastern Australia. The latter is a response to the dominance of Niño 4 sea surface temperature (SST) and associated atmospheric teleconnection anomalies during ‘protracted’ ENSO episodes. Importantly, while Niño 4 SST anomalies recorded during the austral summer of 2016 were the highest values on record, an analysis of long-term palaeoclimate records indicates that there may have been episodes of greater magnitude and duration than seen in instrumental observations. This suggests that shorter instrumental observations may underestimate the risks of possible future ENSO extremes compared with those observed from multi-century palaeoclimate records. Improved knowledge of ENSO and the potential to forecast ‘protracted’ episodes would be of immense practical benefit to communities affected by the severe impacts of ENSO extremes.

The association between climate variability and episodic events, such as the antecedent moisture conditions prior to wildfire or the cooling following volcanic eruptions, is commonly assessed using Superposed Epoch Analysis (SEA). In SEA the epochal response is typically calculated as the average climate conditions prior to and following all event years or their deviation from climatology. However, the magnitude and significance of the inferred climate association may be sensitive to the selection or omission of individual key years, potentially resulting in a biased assessment of the relationship between these events and climate. Here we describe and test a modified double-bootstrap SEA that generates multiple unique draws of the key years and evaluates the sign, magnitude, and significance of event-climate relationships within a probabilistic framework. This multiple resampling helps quantify multiple uncertainties inherent in conventional applications of SEA within dendrochronology and paleoclimatology. We demonstrate our modified SEA by evaluating the volcanic cooling signal in a Northern Hemisphere tree-ring temperature reconstruction and the link between drought and wildfire events in the western United States. Finally, we make our Matlab and R code available to be adapted for future SEA applications.

The interdecadal Pacific Oscillation (IPO) represents the decadal mode of the El Nino-Southern Oscillation phenomenon. As such the IPO is one of the dominant modes of decadal climate variability on both sides of the Pacific Ocean basin. For this paper we utilized a newly developed tree-ring data network comprised of five multi-centennial Vietnamese cypress ring-width chronologies that range from around 11°N–23°N latitude. We combined these data with an average of tree-ring derived drought indices from the North American Drought Atlas over the south central USA, from a box that spans from 27.5°N–35.0°N, 85.0°–110.0°W and contains 55 gridpoints and more than 100 tree ring site chronologies, these 2 locations exhibit rainfall variations that are strongly, negatively correlated with each other and are representative of the influence of the IPO on terrestrial rainfall. The final reconstruction model, weighted most heavily on the three most southerly of the five Vietnamese cypress records, spans from 1350 to 2004, and explains nearly 57% of the variance in the original IPO data for the 5-month season of October–February. The reconstruction model passes all standard statistical tests using a split calibration-verification scheme. We reveal 15 positive and 15 negative phase shifts of the IPO prior to the period of instrumentation, suggesting that the IPO has been active for at least the past seven centuries with varying degrees of intensity. We compare our reconstruction with two related millennial records: the MacDonald and Case (Geophys Res Lett 32(8):L08703, 2005) tree ring-derived reconstruction of the Pacific Decadal Oscillation, and an ice core-derived reconstruction of the IPO from the Law Dome Ice core in Antarctica by Vance et al. (Geophys Res Lett 33(6):L06712, 2015). While there is good general agreement with the latter record, there are three key periods of the past where the two records are out of phase, and we explore the reasons for this disparity. Direct comparison with the related Tripole Index (TPI) shows weaker correlation, likely owing to the stronger relationship between our tree ring data and the equatorial Pacific region relative to the north and south regions of the Pacific that combine to comprise the TPI calculation.

We present a 350-year hydroclimatic year (HY) index for northern Vietnam derived from three discrete seasonal reconstructions from tree rings: an index of autumn rainfall from the earlywood widths of Chinese Douglas fir (Pseudotsuga sinensis), the first such record from this species, and two nearby published Palmer Drought Severity Index (PDSI) reconstructions from cypress (Fokienia hodginsii) tree rings for spring and summer, respectively. Autumn rainfall over the study region constitutes only around 9% of the annual total, but its variability is strongly linked to the strength of the atmospheric gradient over Asia during the transition from the boreal summer to winter monsoons. Deficit or surplus of autumn rainfall enhances or mitigates, respectively, the impact of the annual winter dry season on trees growing on porous karst hillsides. The most protracted HY drought (dry across all seasons) occurred at the turn of the twentieth century at a time of relative quiet, but a mid-to-late eighteenth century multi-year HY drought coincided with a period of great societal turmoil across mainland Southeast Asia and the Tay Son Rebellion in northern Vietnam. A mid-nineteenth century uprising accompanied by a smallpox epidemic, crop failure and famine, occurred during the worst autumn drought of the past two and a half centuries but only moderate drought in spring and summer. The ���Great Vietnamese Famine��� of the mid-twentieth century was dry only in autumn, with a wet spring and an average summer.

Supported by the German Science Foundation, grants # Inst 247/665-1 FUGG and ES 161/9-1. SSG acknowledges support by the Alexander von Humboldt Foundation, KJA by US National Science Foundation grants AGS-1501856 and NSF AGS-1501834, and JL and LS by the Belmont Forum and JPI-Climate, Collaborative Research Action INTEGRATE. Over the past two decades, the dendroclimate community has produced various annually resolved, warm season temperature reconstructions for the extratropical Northern Hemisphere. Here we compare these tree-ring based reconstructions back to 831 CE and present a set of basic metrics to provide guidance for non-specialists on their interpretation and use. We specifically draw attention to (i) the imbalance between (numerous) short and (few) long site chronologies incorporated into the hemispheric means, (ii) the beneficial effects of including maximum latewood density chronologies in the recently published reconstructions, (iii) a decrease in reconstruction covariance prior to 1400 CE, and (iv) the varying amplitudes and trends of reconstructed temperatures over the past 1100 years. Whereas the reconstructions agree on several important features, such as warmth during medieval times and cooler temperatures in the 17th and 19th centuries, they still exhibit substantial differences during 13th and 14th centuries. We caution users who might consider combining the reconstructions through simple averaging that all reconstructions share some of the same underlying tree-ring data, and provide four recommendations to guide future efforts to better understand past millennium temperature variability. Postprint

This paper describes two dominant patterns of Asian winter climate variability: the Siberian high (SH) pattern and the Asia–Arctic (AA) pattern. The former depicts atmospheric variability closely associated with the intensity of the Siberian high, and the latter characterizes the teleconnection pattern of atmospheric variability between Asia and the Arctic, which is distinct from the Arctic Oscillation (AO). The AA pattern plays more important roles in regulating winter precipitation and the 850-hPa meridional wind component over East Asia than the SH pattern, which controls surface air temperature variability over East Asia. In the Arctic Ocean and its marginal seas, sea ice loss in both autumn and winter could bring the positive phase of the SH pattern or cause the negative phase of the AA pattern. The latter corresponds to a weakened East Asian winter monsoon (EAWM) and enhanced winter precipitation in the midlatitudes of the Asian continent and East Asia. For the SH pattern, sea ice loss in the prior autumn emerges in the Siberian marginal seas, and winter loss mainly occurs in the Barents Sea, Labrador Sea, and Davis Strait. For the AA pattern, sea ice loss in the prior autumn is observed in the Barents–Kara Seas, the western Laptev Sea, and the Beaufort Sea, and winter loss only occurs in some areas of the Barents Sea, the Labrador Sea, and Davis Strait. Simulation experiments with observed sea ice forcing also support that Arctic sea ice loss may favor frequent occurrence of the negative phase of the AA pattern. The results also imply that the relationship between Arctic sea ice loss and winter atmospheric variability over East Asia is unstable, which is a challenge for predicting the EAWM based on Arctic sea ice loss.

Warming over Mongolia and Central Asia has been unusually rapid over the past few decades, particularly in the summer, with surface temperature anomalies higher than for much of the globe. With few temperature station records available in this remote region prior to the 1950s, paleoclimatic data must be used to understand annual-to-centennial scale climate variability, local response to large-scale forcing mechanisms, and the significance of major features of the past millennium such as the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) both of which can vary globally. Here we use an extensive collection of living and subfossil wood samples from temperature-sensitive trees to produce a millennial-length, validated reconstruction of summer temperatures for Mongolia and Central Asia from 931 to 2005 CE. This tree-ring reconstruction shows general agreement with the MCA (warming) and LIA (cooling) trends, a significant volcanic signature, and warming in the 20th and 21st Century. Recent warming (2000–2005) exceeds that from any other time and is concurrent with, and likely exacerbated, the impact of extreme drought (1999–2002) that resulted in massive livestock loss across Mongolia.

Volcanic eruptions have global climate impacts, but their effect on the hydrologic cycle is poorly understood. We use a modified version of superposed epoch analysis, an eruption year list collated from multiple datasets, and seasonal paleoclimate reconstructions (soil moisture, precipitation, geopotential heights, and temperature) to investigate volcanic forcing of spring and summer hydroclimate over Europe and the Mediterranean over the last millennium. In the western Mediterranean, wet conditions occur in the eruption year and the following 3 years. Conversely, northwestern Europe and the British Isles experience dry conditions in response to volcanic eruptions, with the largest moisture deficits in post-eruption years 2 and 3. The precipitation response occurs primarily in late spring and early summer (April-July), a pattern that strongly resembles the negative phase of the East Atlantic Pattern. Modulated by this mode of climate variability, eruptions force significant, widespread, and heterogeneous hydroclimate responses across Europe and the Mediterranean.

Climate in the Gulf of Alaska (GOA) reflects large-scale ocean-atmosphere variability of the North Pacific climate system. Ring-width (RW) records from the GOA have yielded a valuable long-term perspective for North Pacific changes on decadal to longer time scales in prior studies, but express a broad winter to late summer seasonal response. Similar to the highly climate-sensitive maximum latewood density (MXD) proxy, the Blue Intensity (BI) parameter has recently been shown to correlate well with year-to-year warm-season temperatures for a number of sites at northern latitudes. Since BI records are much less expensive and labor intensive to generate than MXD, such data hold great potential value for future tree-ring studies in the GOA and other regions at mid-to-high latitudes. Here we highlight the potential for improving tree-ring based reconstructions using combinations of RW and BI-related parameters (latewood BI (LWB) and delta BI (DB)) from an experimental sub-set of samples from eight mountain hemlock (Tsuga mertensiana) sites along the GOA. This is the first such study for the hemlock genus using BI data. We find that using either LWB or DB can improve the amount of explained temperature variance by > 10 % compared to RW alone although the optimal target season changes to June–September, which may have implications for studying ocean-atmosphere variability in the region. However, one challenge in building these BI records is that resin extraction did not remove colour differences between the heartwood and sapwood, so long term trend biases, expressed as relatively warm temperatures in the 18th century, were noted when using the LWB data. Using DB appeared to overcome these trend biases resulting in a reconstruction expressing 18th–19th century temperatures ca. 0.5 °C cooler than the 20th/21st centuries. This cool period agrees well with previous dendroclimatic studies and the glacial advance record in the region. Continuing BI measurement in the GOA region must focus on sampling more trees per site (> 20) and more sites to overcome site specific factors effecting climate response while sub-fossil material will extend the reflectance records back over 1000 years. DB appears to capture long term secular trends that agree with other proxy archives in the region but great care is needed when implementing different detrending options. Finally, more experimentation is needed to assess the utility of DB for different conifer species around the Northern Hemisphere.

A new ring-width record from the eastern flanks of the Eastern Sakhalin Range, Sakhalin Island, Russian Federation is significantly correlated with summer temperatures and allows for the reconstruction of May–July average temperatures for the past 400 years. The reconstruction explains 37 % of the variance in May–July temperatures and shows a strong cooling between 1680 and 1710 CE coincident with the Maunder solar minimum and in agreement with other independent tree-ring reconstructions and glacier histories from sites along the margin of the Sea of Okhotsk. While recent decades are among the warmest in the record they are rivaled by periods centered on 1650 and 1850 CE. Warming in the observational record and the reconstruction is consistent with the influence of the declining strength of the Siberian High and loss of sea ice over the same interval. Decadal (17–25 year) variability persists throughout the reconstruction. At interannual timescales the Sakhalin reconstruction is most strongly correlated with local and central North Pacific sea surface temperatures over the past 120 years, whereas at decadal timescales there is an additional association with Asian land surface temperatures.

We show evidence for the influence of the Pacific Decadal Oscillation (PDO) on Myanmar's monsoonal hydroclimate using both instrumental and 20th century reanalysis data, and a tree-ring width chronology from Myanmar's central Dry Zone. The ‘regime shifts’ identified in the instrumental PDO for the past century are clearly evident in the Myanmar teak. The teak record and PDO index correlate most significantly and positively during December–May, at r = 0.41 (0.002, n = 109). We generated composite climate anomalies for southern Asia and adjacent ocean areas during negative and positive PDO phases and above/below average teak growth for the May–September wet monsoon season. They show that negative (positive) PDO phases correspond to dry (wet) conditions, due to reduced (enhanced) moisture flux into central Myanmar. Multitaper Method (MTM) and Singular Spectrum Analysis (SSA) spectral analyses reveal considerable multidecadal variability over the past several centuries of the teak chronology, consistent with the PDO.

In the Original publication of the article, a grant number was not included in the in acknowlegement section. The correct information is as follows.

A 1200-year-long tree-ring width record from living and subfossil mountain hemlock wood is used to reconstruct February through August temperatures for the Gulf of Alaska, providing a record of past climate variability for the Northeast Pacific sector that captures interannual to centennial timescales. The moderate elevation at the tree-ring sites has allowed these trees to retain their temperature signal without evidence of the so-called divergence effect, or underestimation of tree-ring inferred temperature trends, which is observed at many northern latitude forest locations. This ‘divergence-free’ reconstruction reveals centennial trends that include a warm interval centered on ad 950 for coastal Alaska that occurs around the time of the ‘Medieval Warm Period’, a warming that is only rivaled by recent decades. Spectral analysis of this reconstruction supports the centennial pacing identified as a 170–220-year cadence consistent with solar variability. On the decadal to bidecadal scale, the reconstruction reveals ~10- and 18-year cycles, which have been observed elsewhere in climate records for western North America and are linked to solar and lunar tidal forcing, respectively. Temperature minima that occur at ad 969–970 and 1698–1700 correspond with the timing of major volcanic events. This tree-ring reconstruction supports centennial modes of solar forcing as a driver of surface air temperatures in the Gulf of Alaska, with lunar tidal, solar variability, internal variability, and volcanism, impacting climate on annual to decadal timescales.

There is a need to understand the long-term variability Asian-Pacific Oscillation (APO) due to its close linkages with large-scale ocean–atmosphere anomalies. The first principal component (PC) of a network of 130 temperature proxies covering East Asia and the northeastern Pacific Ocean was employed to reconstruct the instrumental APO index from 500 to 2006. It explains 46.7% of the instrumental APO variance and correlates significantly with middle to upper tropospheric temperatures. The reconstructed APO reaches its peak in the Little Ice Age (LIA) and is low during the Medieval Climate Anomalies (MCA) period and since the 1850. The APO variations are caused by the temperature difference between East Asia and north Central Pacific Ocean. The Pacific Decadal Oscillations (PDOs) and tropical ocean-atmospheric features [the El Nino–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD)] are reversely connected to APO variations particularly on long timescale (e.g. centennial timescale) and short timescale (e.g. interannual), respectively, possibly via modulating the strength of the Asian summer monsoon. However, the relationship between the APO and both external forcings (e.g. solar irradiation) and the PDO are quite variable, which is significant only when the influences of the coupled ocean-atmosphere patterns are weakened. The APO is positively correlated with indices of the hydroclimate of Monsoonal Asia, and is negatively associated with climatic indices for western North America.

The authors acknowledge the National Science Foundation for funding much of the research presented herein. RW's Scottish work is currently funded through the UK Leverhulme Trust and Natural Environment Research Council (NERC) projects, “RELiC: Reconstructing 8000 years of Environmental and Landscape change in the Cairngorms (F/00 268/BG)” and “SCOT2K: Reconstructing 2000 years of Scottish climate from tree rings (NE/K003097/1).”

Using NCEP–NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) data, this paper investigates the association between winter sea ice concentration (SIC) in Baffin Bay southward to the eastern coast of Newfoundland, and the ensuing summer atmospheric circulation over the mid- to high latitudes of Eurasia. It is found that winter SIC anomalies are significantly correlated with the ensuing summer 500-hPa height anomalies that dynamically correspond to the Eurasian pattern of 850-hPa wind variability and significantly influence summer rainfall variability over northern Eurasia. Spring atmospheric circulation anomalies south of Newfoundland, associated with persistent winter–spring SIC and a horseshoe-like pattern of sea surface temperature (SST) anomalies in the North Atlantic, act as a bridge linking winter SIC and the ensuing summer atmospheric circulation anomalies over northern Eurasia. Indeed, this study only reveals the association based on observations and simple simulation experiments with SIC forcing. The more precise mechanism for this linkage needs to be addressed in future work using numerical simulations with SIC and SST as the external forcings. The results herein have the following implication: Winter SIC west of Greenland is a possible precursor for summer atmospheric circulation and rainfall anomalies over northern Eurasia.

Water availability is fundamental to societies and ecosystems, but our understanding of variations 55 in hydroclimate (includingextreme events, flooding, and decadal periods of drought) is limited because of a paucity of modern instrumental observationsthat are distributed unevenly across the globe and only span parts of the 20th and 21st centuries. Such data coverage isinsufficient for characterizing hydroclimate and its associated dynamics because of its multidecadal-to-centennial variability andhighly regionalized spatial signature. High-resolution (seasonal to decadal) hydroclimatic proxies that span all or parts of the60 Common Era (CE) and paleoclimate model simulations are therefore important tools for augmenting our understanding ofhydroclimate variability. In particular, the comparison of the two sources of information is critical for addressing theuncertainties and limitations of both, while enriching each of their interpretations. We review the principal proxy data availablefor hydroclimatic reconstructions over the CE and highlight contemporary understanding of how these proxies are interpreted ashydroclimate indicators. We also review the available last-millennium simulations from fully-coupled climate models and65 discuss several outstanding challenges associated with simulating hydroclimate variability and change over the CE. A specificreview of simulated hydroclimatic changes forced by volcanic events is provided, as well as a discussion of expectedimprovements in estimated forcings, models and their implementation in the future. Our review of hydroclimatic proxies andlast-millennium model simulations is used as the basis for articulating a variety of considerations and best practices for how toperform proxy-model comparisons of CE hydroclimate. This discussion provides a framework for how best to evaluate70 hydroclimate variability and its associated dynamics using these comparisons, as well as how they can better informinterpretations of both proxy data and model simulations. We subsequently explore means of using proxy-model comparisonsto better constrain and characterize future hydroclimate risks. This is explored specifically in the context of several examplesthat demonstrate how proxy-model comparisons can be used to quantitatively constrain future hydroclimatic risks as estimatedfrom climate model projections. Fil: Smerdon, Jason E.. Columbia University; Estados Unidos Fil: Luterbacher, Jürg. University Of Giessen; Alemania Fil: Phipps, Steven J.. University of Tasmania; Australia Fil: Anchukaitis, Kevin J.. University of Arizona; Estados Unidos Fil: Ault, Toby. Cornell University; Estados Unidos Fil: Coats, Sloan. State University of Colorado at Boulder; Estados Unidos Fil: Cobb, Kim M.. Instituto Tecnológico de Georgia; Estados Unidos Fil: Cook, Benjamin I.. Nasa Goddard Institute For Space Studies; Estados Unidos Fil: Colose, Chris. Nasa Goddard Institute For Space Studies; Estados Unidos Fil: Felis, Thomas. Universitat Bremen; Alemania Fil: Gallant, Ailie. Monash University; Australia Fil: Jungclaus, Johann H.. Max-Planck-Institute for Meteorology; Alemania Fil: Konecky, Bronwen. State University of Colorado at Boulder; Estados Unidos Fil: LeGrande, Allegra. NASA Goddard Institute for Space Studie; Estados Unidos Fil: Lewis, Sophie. The Australian National University; Australia Fil: Lopatka, Alex S.. University of Maryland; Estados Unidos Fil: Man, Wenmin. Chinese Academy of Sciences; República de China Fil: Mankin, Justin S.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos. NASA Goddard Institute for Space Studies; Estados Unidos Fil: Maxwell, Justin T.. Indiana University; Estados Unidos Fil: Otto-Bliesner, Bette L.. National Center for Atmospheric Research; Estados Unidos Fil: Partin, Judson W.. University of Texas at Austin; Estados Unidos Fil: Singh, Deepti. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Stevenson, Samantha. National Center for Atmospheric Research; Estados Unidos Fil: Tierney, Jessica E.. Arizona State University; Estados Unidos Fil: Zanchettin, Davide. Ca’ Foscari University of Venice; Italia Fil: Zhang, Huan. Justus Liebig University; Alemania Fil: Atwood, Alyssa R.. Georgia Institute of Technology; Estados Unidos. Brown University; Estados Unidos Fil: Andreu Hayles, Laia. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Baek, Seung H.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Buckley, Brendan. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Cook, Edward. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: D'Arrigo, Rosanne. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Dee, Sylvia G.. Brown University; Estados Unidos Fil: Griffiths, Michael L.. William Paterson University; Estados Unidos Fil: Kulkarni, Charuta. City University of New York; Estados Unidos Fil: Kushnir, Yochanan. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Lehner, Flavio. National Center for Atmospheric Research; Estados Unidos Fil: Leland, Caroline. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Linderholm, Hans W.. University of Gothenburg; Suecia Fil: Okazaki, Atsushi. University of Tokyo; Japón Fil: Palmer, Jonathan. University of New South Wales; Australia Fil: Piovano, Eduardo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Raible, Christoph C.. University of Bern; Suiza Fil: Rao, Mukund P.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Scheff, Jacob. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Schmidt, Gavin A.. NASA Goddard Institute for Space Studie; Estados Unidos Fil: Seager, Richard. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Widmann, Martin. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Park Williams, A.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Xoplaki, Elena. Justus Liebig University; Alemania

In arid and semi-arid regions of the world, such as Mongolia, the future of water resources under a warming climate is of particular concern. The influence of increasing temperatures on precipitation is difficult to predict because precipitation trends in coming decades could have a high degree of spatial variability. In this study, we applied a rotated principal component analysis (RPCA) to a network of 20 tree-ring chronologies across central Mongolia from 1790 to 1994 to evaluate spatial hydroclimatic variability and to place recent variability in the context of the past several centuries. The RPCA results indicate that the network consists of four tree-growth anomaly regions, which were found to be relatively stable through time and space. Correlation analyses reveal spatial linkages between the tree-growth anomalies and instrumental data, where annual streamflow variability was strongly associated with tree-growth anomalies from their respective regions from 1959 to 1994 ( r = 0.52–0.64, p

We compared tree-ring width (TRW) and maximum latewood density (MXD) chronologies to remotely sensed indices of productivity (NDVI) and snowmelt since 1981 and to the instrumental temperature record at four arctic treeline sites in North America. Our results show that at these sites, TRW chronologies reflect temperatures less consistently than the MXD chronologies do and that the NDVI does not correlate significantly with TRW at high-frequency, i.e. when comparing yearly values. In contrast, the MXD chronologies correlate positively and significantly with NDVI and temperature during the growing season at all sites. Neither TRW or MXD chronologies appeared consistently influenced by the annual timing of snowmelt. A comparison of tree-ring chronologies and temperatures since 1900 confirms that MXD has tracked growing season temperature at these treeline sites throughout the past century. A spatial evaluation of the correlations further reveals that each of the MXD chronologies investigated here reflects interannual variation in NDVI and growing season temperatures across a large geographic region. As a result, they collectively provide a spatially comprehensive record of historic early-season canopy status as well as growing season temperatures for the high latitudes of North America.

Drought patterns across monsoon and temperate Asia over the period 1877���2005 are linked to Indo-Pacific climate variability associated with the El Ni��o-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Using the Monsoon Asia Drought Atlas (MADA) composed of a high-resolution network of hydroclimatically sensitive tree-ring records with a focus on the June���August months, spatial drought patterns during El Ni��o and IOD events are assessed as to their agreement with an instrumental drought index and consistency in the drought response amongst ENSO/IOD events. Spatial characteristics in drought patterns are related to regional climate anomalies over the Indo-Pacific basin, using reanalysis products, including changes in the Asian monsoon systems, zonal Walker circulation, moisture fluxes, and precipitation. A weakening of the monsoon circulation over the Indian subcontinent and Southeast Asia during El Ni��o events, along with anomalous subsidence over monsoon Asia and reduced moisture flux, is reflected in anomalous drought conditions over India, Southeast Asia and Indonesia. When an IOD event co-occurs with an El Ni��o, severe drought conditions identified in the MADA for Southeast Asia, Indonesia, eastern China and central Asia are associated with a weakened South Asian monsoon, reduced moisture flux over China, and anomalous divergent flow and subsidence over Indonesia. Insights into the relative influences of Pacific and Indian Ocean variability for Asian monsoon climate on interannual to decadal and longer timescales, as recorded in the MADA, provide a useful tool for assessing long-term changes in the characteristics of Asian monsoon droughts in the context of Indo-Pacific climate variability.

We used monthly mean surface wind data from the National Centers for Environmental Prediction/National Centers for Atmospheric Research (NCEP/NCAR) reanalysis dataset during the period 1979–2010 to describe the first two patterns of Arctic surface wind variability by means of the complex vector empirical orthogonal function (CVEOF) analysis. The first two patterns respectively account for 31 and 16% of its total anomalous kinetic energy. The leading pattern consists of the two subpatterns: the northern Laptev Sea (NLS) pattern and the Arctic dipole (AD) pattern. The second pattern contains the northern Kara Sea (NKS) pattern and the central Arctic (CA) pattern. Over the past two decades, the combined dynamical forcing of the first two patterns has contributed to Arctic September sea ice extent (SIE) minima and its declining trend. September SIE minima are mainly associated with the negative phase of the AD pattern and the positive phase of the CA pattern during the summer (July to September) season, and both phases coherently show an anomalous anticyclone over the Arctic Ocean. Wind patterns affect September SIE through their frequency and intensity. The negative trend in September SIE over the past two decades is associated with increased frequency and enhanced intensity of the CA pattern during the melting season from April to September. Thus, it cannot be simply attributed to the AD anomaly characterised by the second empirical orthogonal function mode of sea level pressure north of 70°N. The CA pattern exhibited interdecadal variability in the late 1990s, and an anomalous cyclone prevailed before 1997 and was then replaced by an anomalous anticyclone over the Arctic Ocean that is consistent with the rapid decline trend in September SIE. This paper provides an alternative way to identify the dominant patterns of climate variability and investigate their associated Arctic sea ice variability from a dynamical perspective. Indeed, this study investigates only the role of surface wind dynamical forcing in resulting September SIE minima and trend in terms of CVEOF, without even considering contributions from other factors. Keywords: Arctic surface wind patterns; Arctic dipole pattern; central Arctic pattern; September sea ice extent minima; trend, interdecadal variability (Published: 18 May 2012) Citation: Tellus A 2012, 64 , 18590, http://dx.doi.org/10.3402/tellusa.v64i0.18590

article i nfo We describe a millennial length (~1500-yr) tree-ring chronology developed from West Virginia (WVA), USA red cedar (Juniperus virginiana) ring widths that is significantly correlated with local to regional temperature and precipitation for the region. Using ensemble methods of tree-ring standardization, above average ring widths are indicated for the period between ~1000 and 1300 CE, the approximate time of the Medieval Climate Anomaly (MCA), the most recent major warm episode prior to the modern era. The chronology then transitions to more negative overall growth persisting through much of the subsequent period known as the Little Ice Age (LIA). While WVA cedar growth levels during the MCA are broadly similar to the 20th century mean, the most positive values during the MCA are associated with RCS-standardized chronologies, which pseudoproxy tests reveal are likely biased artificially positive, warranting further investigation. This cedar record is significantly correlated with the NAO, due to the tendency for warmer, wetter conditions to occur in the eastern-central USA during the NAO's positive phase. These types of conditions are inferred for this cedar chronology during the MCA period, during which NAO reconstructions suggest a persistently-positive NAO state.

This study presents the first multi-proxy reconstruction of rainfall variability from the mid-latitude region of south-eastern Australia (SEA). A skilful rainfall reconstruction for the 1783–1988 period was possible using twelve annually-resolved palaeoclimate records from the Australasian region. An innovative Monte Carlo calibration and verification technique is introduced to provide the robust uncertainty estimates needed for reliable climate reconstructions. Our ensemble median reconstruction captures 33% of inter-annual and 72% of decadal variations in instrumental SEA rainfall observations. We investigate the stability of regional SEA rainfall with large-scale circulation associated with El Nino–Southern Oscillation (ENSO) and the Inter-decadal Pacific Oscillation (IPO) over the past 206 years. We find evidence for a robust relationship with high SEA rainfall, ENSO and the IPO over the 1840–1988 period. These relationships break down in the late 18th–early 19th century, coinciding with a known period of equatorial Pacific Sea Surface Temperature (SST) cooling during one of the most severe periods of the Little Ice Age. In comparison to a markedly wetter late 18th/early 19th century containing 75% of sustained wet years, 70% of all reconstructed sustained dry years in SEA occur during the 20th century. In the context of the rainfall estimates introduced here, there is a 97.1% probability that the decadal rainfall anomaly recorded during the 1998–2008 ‘Big Dry’ is the worst experienced since the first European settlement of Australia.

A preliminary study of a point-by-point spatial precipitation reconstruction for northwestern (NW) China is explored, based on a tree-ring network of 132 chronologies. Precipitation variations during the past ~200–400 yr (the common reconstruction period is from 1802 to 1990) are reconstructed for 26 stations in NW China from a nationwide 160-station dataset. The authors introduce a “search spatial correlation contour” method to locate candidate tree-ring predictors for the reconstruction data of a given climate station. Calibration and verification results indicate that most precipitation reconstruction models are acceptable, except for a few reconstructions (stations Hetian, Hami, Jiuquan, and Wuwei) with degraded quality. Additionally, the authors compare four spatial precipitation factors in the instrumental records and reconstructions derived from a rotated principal component analysis (RPCA). The northern and southern Xinjiang factors from the instrumental and reconstructed data agree well with each other. However, differences in spatial patterns between the instrumentation and reconstruction data are also found for the other two factors, which probably result from the relatively poor quality of a few stations. Major drought events documented in previous studies—for example, from the 1920s through the 1930s for the eastern part of NW China—are reconstructed in this study.

Our ability to predict El Nino/Southern Oscillation (ENSO) activity is hampered by the relatively short length of the instrumental record. An annually resolved record of ENSO variability over the past millennium based on tree rings indicates that ENSO amplitude varies on a 50–90 year cycle, providing an important constraint for improving predictions. The El Nino/Southern Oscillation (ENSO) is the dominant mode of interannual climate variability on Earth, alternating between anomalously warm (El Nino) and cold (La Nina) conditions in the tropical Pacific at intervals of 2–8 years1,2. The amplitude of ENSO variability affects the occurrence and predictability of climate extremes around the world3,4, but our ability to detect and predict changes in ENSO amplitude is limited by the fact that the instrumental record is too short to characterize its natural variability5,6,7,8. Here we use the North American Drought Atlas9,10—a database of drought reconstructions based on tree-ring records—to produce a continuous, annually resolved record of ENSO variability over the past 1,100 years. Our record is in broad agreement with independent, ENSO-sensitive proxy records in the Pacific and surrounding regions. Together, these records indicate that ENSO amplitude exhibits a quasi-regular cycle of 50–90 years that is closely coupled to the tropical Pacific mean state. Anomalously warm conditions in the eastern Pacific are associated with enhanced ENSO variability, consistent with model simulations11. The quasi-periodic ENSO amplitude modulation reported here offers a key observational constraint for improving models and their prediction of ENSO behaviour linked to global warming.

Tree-ring width and glacier mass balance are two highly sensitive climatic proxies which are often used as indicators of biological and geophysical changes in high-altitude ecosystems. Tree-ring data have been widely used to reconstruct past temperatures and also to reconstruct past glacier mass balance. Here we show that tree-ring chronologies from a high-altitude Pinus cembra L. dendroclimatic network and glaciers from the same region in the European Alps have non-stationary responses to air temperature, and have also been responding non-proportionally to temperature extremes in recent decades. Both ring-width chronologies and the mass-balance series of some glaciers from the same region have shown an increasing sensitivity to summer (JJA) temperatures. Our results demonstrate that the sensitivity to climate of tree-ring chronologies and glacier mass balance may change over time and has been increasing in recent decades, posing some limitations to tree-ring-based glacier mass-balance reconstruc...

Moisture sensitive trees from the central Tien Shan area, northwest China show a pattern of extremely low growth in 1945. Evidence from a number of sources indicates that a remarkable drought event, in both spatial and temporal terms, occurred over the central Tien Shan and surrounding areas in that year. Climatological analysis reveals that this significant growth reduction is likely a combined effect of unusually dry and warm conditions. We also show that this extreme drought is associated with a La Nina or cold phase El Nino-Southern Oscillation (ENSO) event. Meanwhile, cooler sea surface temperatures (SSTs) in the northern Indian Ocean, particularly in the Arabian Sea, also appear to have contributed to this severe drought episode. Overall, our case study of 1945 extreme drought event in the central Tien Shan area suggests there might be a close link between extreme climate conditions in remote north central Asia and coupled ocean-atmosphere dynamics in the tropics.

Of Monsoons and Megadroughts The Asian monsoon is the weather system that has the greatest effect on the greatest number of people in the world. Naturally then, knowing better how climate change might affect the monsoon is tremendously important. One obstacle that prevents a better understanding of future behavior is a poor knowledge of its past. Cook et al. (p. 486 ; see the Perspective by Wahl and Morrill ) help to fill this gap with a 700-year reconstruction of the monsoon from tree-ring data obtained throughout Asia. The reconstruction chronicles monsoon failures and megadroughts, as well as patterns of precipitation, and can thus be compared with other relevant climate records to allow links with sea-surface temperatures to be better understood.

We statistically reconstruct austral summer (winter) surface air temperature fields back to ad 900 (1706) using 22 (20) annually resolved predictors from natural and human archives from southern South America (SSA). This represents the first regional-scale climate field reconstruction for parts of the Southern Hemisphere at this high temporal resolution. We apply three different reconstruction techniques: multivariate principal component regression, composite plus scaling, and regularized expectation maximization. There is generally good agreement between the results of the three methods on interannual and decadal timescales. The field reconstructions allow us to describe differences and similarities in the temperature evolution of different sub-regions of SSA. The reconstructed SSA mean summer temperatures between 900 and 1350 are mostly above the 1901-1995 climatology. After 1350, we reconstruct a sharp transition to colder conditions, which last until approximately 1700. The summers in the eighteenth century are relatively warm with a subsequent cold relapse peaking around 1850. In the twentieth century, summer temperatures reach conditions similar to earlier warm periods. The winter temperatures in the eighteenth and nineteenth centuries were mostly below the twentieth century average. The uncertainties of our reconstructions are generally largest in the eastern lowlands of SSA, where the coverage with proxy data is poorest. Verifications with independent summer temperature proxies and instrumental measurements suggest that the interannual and multi-decadal variations of SSA temperatures are well captured by our reconstructions. This new dataset can be used for data/model comparison and data assimilation as well as for detection and attribution studies at sub-continental scales

Spatial reconstructions of drought for central High Asia based on a tree-ring network are presented. Drought patterns for central High Asia are classified into western and eastern modes of variability. Tree-ring based reconstructions of the Palmer drought severity index (PDSI) are presented for both the western central High Asia drought mode (1587–2005), and for the eastern central High Asia mode (1660–2005). Both reconstructions, generated using a principal component regression method, show an increased variability in recent decades. The wettest epoch for both reconstructions occurred from the 1940s to the 1950s. The most extreme reconstructed drought for western central High Asia was from the 1640s to the 1650s, coinciding with the collapse of the Chinese Ming Dynasty. The eastern central High Asia reconstruction has shown a distinct tendency towards drier conditions since the 1980s. Our spatial reconstructions agree well with previous reconstructions that fall within each mode, while there is no significant correlation between the two spatial reconstructions.

The Citarum river basin of western Java, Indonesia, which supplies water to 10 million residents in Jakarta, has become increasingly vulnerable to anthropogenic change. Citarum’s streamflow record, only ~45 years in length (1963-present), is too short for understanding the full range of hydrometeorological variability in this important region. Here we present a tree-ring based reconstruction of September–November Citarum streamflow (AD 1759–2006), one of the first such records available for monsoon Asia. Close coupling is observed between decreased tree growth and low streamflow levels, which in turn are associated with drought caused by ENSO warm events in the tropical Pacific and Indian Ocean positive dipole-type variability. Over the full length of record, reconstructed variance was at its weakest during the interval from ~1905–1960, overlapping with a period of unusually-low variability (1920–1960) in the ENSO-Indian Ocean dipole systems. In subsequent decades, increased variance in both the streamflow anomalies and a coral-based SST reconstruction of the Indian Ocean Dipole Mode signal the potential for intensified drought activity and related consequences for water supply and crop productivity in western Java, where much of the country’s rice is grown.

In this study we compare three newly developed independent NINO3.4 sea surface temperature (SST) reconstructions using data from (1) the central Pacific (corals), (2) the TexMex region of the USA (tree rings) and (3) other regions in the Tropics (corals and an ice core) which are teleconnected with central Pacific SSTs in the 20th century. Although these three reconstructions are strongly calibrated and well verified, inter-proxy comparison shows a significant weakening in inter- proxycoherenceinthe 19thcentury.Thisbreakdownin commonsignalcouldberelatedtoinsufficient data, dating errors in some of the proxy records or a breakdown in El Nino-Southern Oscillation's (ENSO's) influence on other regions. However, spectral analysis indicates that each reconstruction portraysENSO-likespectralproperties.Superposedepochanalysisalsoshowsthateachreconstruction shows a generally consistent 'El Nino-like' response to major volcanic events in the following year, while duringyearsT þ4toT þ7'La Nina-like'conditions prevail.Theseresults suggest thateachofthe series expresses ENSO-like 'behaviour', but this 'behaviour' does not appear to be spatially or temporally consistent. This result may reflect published observations that there appear to be distinct 'types' of ENSO variability depending on location within the tropical Pacific. Future work must address potential dating issues within some proxies (i.e. sampling of multiple coral heads for one location) as well as assessing the time stability of local climate relationships with central Pacific SSTs. More emphasis is needed on sampling new and extending old coral proxy records from the crucial central and eastern tropical Pacific region. Copyright # 2009 John Wiley & Sons, Ltd. and # Crown Copyright 2009.

We present a Palmer Drought Severity Index reconstruction (r = 0.61, P < 0.01) from 1440 to 2007 for the southeastern Tibetan Plateau, based on tree rings of the forest fir (Abies forrestii). Persistent decadal dry intervals were found in the 1440s-1460s, 1560s-1580s, 1700s, 1770s, 1810s, 1860s and 1980s, and the extreme wet epochs were the 1480s-1490s, 1510s-1520s, 1590s, 1610s-1630s, 1720s-1730s, 1800s, 1830s, 1870s, 1930s, 1950s and after the 1990s. Comparisons of our record with those identified in other moisture related reconstructions for nearby regions showed that our reconstructed droughts were relatively consistent with those found in other regions of Indochina, suggesting similar drought regimes. Spectral peaks of 2.3-5.5 years may be indicative of ENSO activity, as also suggested by negative correlations with SSTs in the eastern equatorial and southeastern Pacific Ocean. Significant multidecadal spectral peaks of 29.2-40.9 and 56.8-60.2 years were identified. As indicated by the spatial correlation patterns, the decadal-scale variability may be linked to SST variations in the northern Pacific and Atlantic Oceans.

Tree-ring records can provide longer, high-resolution records of climate variability in remote regions such as western Mongolia, where recorded data are extremely limited. Here, we use three absolutely dated tree-ring-width chronologies to reconstruct the Palmer Drought Severity Index (PDSI) for a grid point in western Mongolia (48.75°N, 88.75°W). A reconstruction of the June–September PDSI for this region extends from 1565 to 2003 and explains 41% of the total variance in the instrumental PDSI. The 439-year reconstruction shows that starting in the 20th Century and continuing into the 21st Century there is a large-scale regional increase in growing-season moisture conditions compared to the prior centuries, a trend not seen in central or eastern Mongolian tree-ring reconstructions. This increasing trend in western Mongolia is consistent with station observations and other nearby proxy records. The wettest 5-year period of the reconstruction is 1741–1745 followed by 1993–1997, and the driest period is 1755–1759, followed by 1882–1886. Spectral analysis shows significant periodicities at approximately 22, 11, 7 and 5 years. The reconstruction shows similar trends to a PDSI reconstruction from NW China during the 20th Century and also correlates with lake-level data from nearby Khar-Us Nuur, Mongolia. Copyright © 2008 Royal Meteorological Society

We have reconstructed the Palmer Drought Severity Index (PDSI) from 1618 to 2005 for the Guiqing Mountain area, north central China, using tree rings of Pinus tabulaeformis. The reconstruction explains 49% of the actual PDSI variance during the common period 1951–2005. The PDSI reconstruction is representative of moisture conditions over a large area to the south and east of the sampling site, the core area for the 1997 severe drought. It shows slightly different variations when compared to other available PDSI reconstructions north and west of the sampling sites. The drought epoch since 1994 is most severe for our study region, with least intensity for the Helan Mountain area. The drought in 1997 was centered in the Guiqing Mountain area, and appears to be the most extreme drought event over the length of reconstruction. Significant spectral peaks are identified at 2.1, 2.3, 2.5, 3.2, 3.6, 42.7 and 53.8 years. Our reconstructed droughts are significantly correlated with sea surface temperature in the northern Indian Ocean, western equatorial Pacific Ocean, eastern equatorial Pacific Ocean, Eastern China Sea and Sea of Japan. The linkages to the Indian and Pacific Oceans suggest the connection of regional moisture variations to the Asian monsoon circulations. Copyright © 2009 Royal Meteorological Society

We present white spruce ( Picea glauca ) tree-ring width and maximum latewood density chronologies for two latitudinal treeline sites in northern interior Canada: along the Coppermine River in the Northwest Territories (NWT); and in the Thelon River Sanctuary, Nunavut. These chronologies provide climate and tree growth information for these two remote locations, filling a sizeable gap in spatial coverage of proxy records used to reconstruct temperature variability for the Northern Hemisphere. They represent some of the longest high-resolution proxies available for northern North America, dating as far back as AD 1046 for Coppermine ring widths. These chronologies correlate significantly with hemispheric-scale annual temperature reconstructions for the past millennium. Density records from both sites show a positive relationship with warm-season temperature data since ∼ the mid-20th century, although this link is somewhat weaker in recent decades (since ∼ 1980). Both ring-width chronologies demonstrate even greater loss of temperature sensitivity, and in the Thelon ring-width series a sustained reduction in growth appears linked to increased drought stress in this recent period. Diminishing correlations with temperature are also found when the Thelon ring-width and climate data are prewhitened, indicating that any low frequency uncertainties in the instrumental or tree-ring data (e.g., artifacts from the standardization process) cannot entirely account for this result. Our findings therefore suggest a recent loss of temperature sensitivity at these northern treeline locations that varies with the parameter and site studied. These and other uncertainties in the tree-ring as well as instrumental data will need to be resolved in future efforts to relate northern tree-ring records to temperature variability on a range of spatial scales.

The effect of volcanism on low-latitude climate has been difficult to quantify. A compilation of tropical and subtropical annually resolved climate reconstructions shows a correlation between low sea surface temperatures and low-latitude volcanic activity over the past four centuries. Palaeoclimate records have demonstrated links between high-latitude climate changes and tropical as well as high-latitude volcanic activity1,2,3,4,5. However, little is known about the impact of high-or low-latitude volcanic eruptions on tropical climate, particularly for the period preceding the instrumental record6,7,8,9. Here we use annually resolved temperature-related records from corals, tree rings and ice cores to investigate the relationship between volcanism and low-latitude climate. Over the past 450 years, we find an association between low-latitude volcanic events and lower sea surface temperatures in the tropical oceans. The longest sustained cold period in recent centuries occurred in the early nineteenth century, following the eruption of Tambora and a second, unidentified but presumably tropical1, volcano. We therefore conclude that the tropical ocean–atmosphere system has been sensitive to changes in radiative forcing caused by tropical volcanism over the past several centuries.

The authors examine the effects of latitude and life history on the timing of relationships between satellite measures of normalized difference vegetation index (NDVI) and ground-based measures of tree-ring width in forests at mid- and high latitudes in the Northern Hemisphere. Results indicate a correlation between NDVI and tree rings over the entire growing season for all areas analyzed. For sites south of 40°N, a correlation appears in early spring and late fall while a correlation appears during summer months north of 40°N. For conifers, the correlation appears in summer while deciduous trees show the relationship during early spring and late fall. Of these two correlations, the effect of life history seems to dominate the effect of latitude. The timing of these correlations may help clarify the relationship between climate and tree rings and the effect of canopy duration on carbon uptake by trees.

Moisture variability across China and Mongolia (hereafter, CM) during 1951–2005 was investigated using the recently developed monthly Palmer Drought Severity Index (PDSI) dataset. In total there are 206 PDSI grid points across CM, based on a 2.5° × 2.5° gridding system. For CM as a whole a significant decreasing trend in mean moisture availability was observed during 1951–2005, but with strong decadal (17.1-year) and interannual (5.0-year, 3.2-year, 2.4–2.8 year) variations. The areas affected by moderate and severe moisture deficit over CM have increased significantly since the mid-1950s. In contrast, there is a significant decreasing trend for areas affected by moderate wetness since the mid-1950s, and no significant trend was found for the areas affected by severe wetness. Ten moisture-related spatial patterns were objectively defined for CM using rotated Empirical Orthogonal Function (REOF) analysis. These patterns are related to distinct geographical areas and are associated with distinct temporal variations. Four of these patterns, in Northeast China (NE), North China (NC), Central China (CC), and East China (EC), generally demonstrate a significant decreasing trend in moisture availability. Two patterns located in western areas of Northwest China (NW) and the Tibetan Plateau (TP) show a significant moisture increase, while four patterns in Mongolia (MN), far western China (FW), South China (SC), and Southwest China (SW) do not have significant moisture trends during 1951–2005. Based on REOF results we propose that CM should be divided into ten coherent moisture divisions. Moisture variations within each division are generally coherent, but may show either similar or contrasting covariability with adjacent divisions.

The world's hydrological cycle is believed to intensify with global warming, yet current climate models have only a limited ability to assess moisture responses at regional scales. Tree-ring records are a valuable source of information for understanding long-term, regional-scale moisture changes, particularly for large regions such as the Tibetan Plateau (TP), where the observational data are short and sparse. Here, we present a new ring-width chronology developed from Qilian Juniper (Sabina przewalskii) wood at two sites on the northeastern TP. This chronology, combined with others from the same region, demonstrates that tree growth anomalies are linked to regional late spring to early summer moisture availability. Although late monsoon season precipitation in the study area decreased during recent decades, tree growth continued to increase due to persistent moisture availability in the early monsoon season. Comparison with global sea surface temperatures (SSTs) indicates that early (late) monsoon season precipitation is closely related to tropical Pacific (Indian Ocean) SSTs, suggesting a possible seasonal shift in the dominant moisture source area for monsoonal precipitation over the northeastern TP. It is further shown that there is a very high degree of coherency regarding low-frequency tree growth anomalies over the northeastern TP during the last six centuries. The most prominent drought epoch occurred during ca. 1450–1500, which may have been caused by a significant decrease in the thermal gradient between the Eurasian continent and the tropical oceans. A persistent tree growth increase since the 1880s is coincident with global warming, suggesting an intensified early monsoon season moisture regime in the study area.

An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings. The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle. Herein we review the current literature published on the divergence problem to date, and assess its possible causes and implications. The causes, however, are not well understood and are difficult to test due to the existence of a number of covarying environmental factors that may potentially impact recent tree growth. These possible causes include temperature-induced drought stress, nonlinear thresholds or time-dependent responses to recent warming, delayed snowmelt and related changes in seasonality, and differential growth/climate relationships inferred for maximum, minimum and mean temperatures. Another possible cause of the divergence described briefly herein is ‘global dimming’, a phenomenon that has appeared, in recent decades, to decrease the amount of solar radiation available for photosynthesis and plant growth on a large scale. It is theorized that the dimming phenomenon should have a relatively greater impact on tree growth at higher northern latitudes, consistent with what has been observed from the tree-ring record. Additional potential causes include “end effects” and other methodological issues that can emerge in standardization and chronology development, and biases in instrumental target data and its modeling. Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations. © 2007 Elsevier B.V. All rights reserved.

Indonesia is impacted by severe droughts that cause major food shortages over much of the country, and have long been linked with El Ni˜ no events in the tropical Pacific Ocean. Despite seasonal forecasts based on relatively complex climate models, the 1997-1998 'El Ni˜ no of the century' was still followed by huge shortfalls in crop production (e.g. a loss of 3 million tons of rice in Java alone), reflecting the large gap that can exist between these forecasts and their actual utilization in agricultural planning. Alternatively, simple predictive models of rainfall and crop yields for Indonesia and other Indian Ocean rim countries have utilized an index of equatorial Pacific sea surface temperatures (Ni˜ SST), and related this index directly to indices of rainfall and crop productivity. However, these latter models have not included climatic information from the Indian Ocean, also implicated as a likely cause of drought in western Indonesia. Here we show that an index of Indian Ocean SSTs, when combined with the previously utilized Ni˜ no-3.4 SSTs, provides a significantly more accurate model of Sept-Dec drought (PDSI) over Java, Indonesia, than Ni˜ no-3.4 SSTs alone (ar 2 64.5% vs 37.9%). Based solely on data for the month of August, this model provides the best tradeoff between model skill and adequate lead time for the Sept-Dec rice planting season. Such simple models can be used to generate readily usable early warning forecasts of drought and crop failure risk in Indonesia, particularly in the western part of the country that is most influenced by Indian Ocean climate variability. Copyright  2008 Royal Meteorological Society

We describe tree-ring reconstructions of spring (May-June) precipitation and spring-summer (May-August) streamflow for north-western Turkey, both beginning in AD 1650. These are among the first such reconstructions for the region, and the streamflow reconstruction is among the first of its kind for Turkey and the entire Middle East. The reconstructions, which both emphasize high-frequency variations, account for 34 and 53% of their respective instrumental variance. Comparison to precipitation and runoff data provides some means of verification for the instrumental streamflow record, which is very short (30 years). Drought and flood events in the reconstructions are compared to historical archives and other tree-ring reconstructions for Turkey. The results reveal common climatic extremes over much of the country. Many of these events have had profound impacts on the peoples of Turkey over the past several centuries. Copyright © 2007 Royal Meteorological Society

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала. Large-scale millennial length Northern Hemisphere (NH) temperature reconstructions have been progressively improved over the last 20 years as new datasets have been developed. This paper, and its companion (Part II, Anchukaitis et al. in prep), details the latest tree-ring (TR) based NH land air temperature reconstruction from a temporal and spatial perspective. This work is the first product of a consortium called N-TREND (Northern Hemisphere Tree-Ring Network Development) which brings together dendroclimatologists to identify a collective strategy for improving large-scale summer temperature reconstructions. The new reconstruction, N-TREND2015, utilises 54 records, a significant expansion compared with previous TR studies, and yields an improved reconstruction with stronger statistical calibration metrics. N-TREND2015 is relatively insensitive to the compositing method and spatial weighting used and validation metrics indicate that the new record portrays reasonable coherence with large scale summer temperatures and is robust at all time-scales from 918 to 2004 where at least 3 TR records exist from each major continental mass. N-TREND2015 indicates a longer and warmer medieval period (~900 e1170) than portrayed by previous TR NH reconstructions and by the CMIP5 model ensemble, but with better overall agreement between records for the last 600 years. Future dendroclimatic projects should focus on developing new long records from data-sparse regions such as North America and eastern Eurasia as well as ensuring the measurement of parameters related to latewood density to complement ring-width records which can improve local based calibration substantially.