Winter Climate Variations Research Articles

Winter hydroclimate contrasts between southern and northern Europe at ∼ 5.2 ka

Winter climate variations in midlatitude regions have far-reaching effects on both natural and human systems worldwide. However, most climate reconstructions focus on either annual or summer climate conditions. Here we present a stalagmite stable isotope record from Urşilor Cave (Romania) where the δ18O values are controlled by winter temperature. Because of its East-Central Europe (ECE) location, the cave site is well suited to capture variations in precipitation and temperature in response to changes in the North Atlantic region. Present day precipitation and temperature in this region are controlled by two teleconnection patterns, the Arctic Oscillation (AO) and the East Atlantic (EA). By analyzing composite maps for four different scenarios of AO and EA phase, we suggest that between 5.6 and 5.2 ka the positive phases of AO and EA became increasingly dominant. This shift led to generally warmer winters in northern Europe, but drier conditions in southern Europe and the Levant. Considering the compound effect of AO and EA on the hydroclimate of ECE, our study highlights the importance of examining the combined impact of teleconnection patterns on climate proxies influenced by various forcing mechanisms.

Unraveling a century of global change impacts on winter bird distributions in the eastern United States.

One of the most pressing questions in ecology and conservation centers on disentangling the relative impacts of concurrent global change drivers, climate and land-use/land-cover (LULC), on biodiversity. Yet studies that evaluate the effects of both drivers on species' winter distributions remain scarce, hampering our ability to develop full-annual-cycle conservation strategies. Additionally, understanding how groups of species differentially respond to climate versus LULC change is vital for efforts to enhance bird community resilience to future environmental change. We analyzed long-term changes in winter occurrence of 89species across nine bird groups over a 90-year period within the eastern United States using Audubon Christmas Bird Count (CBC) data. We estimated variation in occurrence probability of each group as a function of spatial and temporal variation in winter climate (minimum temperature, cumulative precipitation) and LULC (proportion of group-specific and anthropogenic habitats within CBC circle). We reveal that spatial variation in bird occurrence probability was consistently explained by climate across all nine species groups. Conversely, LULC change explained more than twice the temporal variation (i.e., decadal changes) in bird occurrence probability than climate change on average across groups. This pattern was largely driven by habitat-constrained species (e.g., grassland birds, waterbirds), whereas decadal changes in occurrence probabilities of habitat-unconstrained species (e.g., forest passerines, mixed habitat birds) were equally explained by both climate and LULC changes over the last century. We conclude that climate has generally governed the winter occurrence of avifauna in space and time, while LULC change has played a pivotal role in driving distributional dynamics of species with limited and declining habitat availability. Effective land management will be critical for improving species' resilience to climate change, especially during a season of relative resource scarcity and critical energetic trade-offs.

Skillful prediction of winter Arctic Oscillation from previous summer in a linear empirical model

The winter Arctic Oscillation (WAO), as a primary atmospheric variability mode in the Northern Hemisphere, plays a key role in influencing mid-high-latitude climate variations. However, current dynamical seasonal forecasting systems have limited skills in predicting WAO with lead time longer than two months. In this study, we design a linear empirical model using two effective precursors from anomalies of the Arctic sea ice concentration (SIC) and the tropical sea surface temperature (SST) initiated in preceding late summer (August) which are both significantly correlated with WAO in recent four decades. This model can provide a skillful prediction of WAO at about half-year lead started from previous summer and perform much better than the dynamical models. Such a significantly prolonged lead time is owed to the stable precursor signals extracted from the SIC and SST anomalies over specific areas, which can persist from previous August and be further enhanced through autumn months. Validation results show that this model can produce a 20-year independent-validated prediction skill of 0.45 for 1999–2018 and a 39-year cross-validated skill of 0.67 for 1980–2018, providing a potentially effective tool for earlier predictions of winter climate variations at mid-high latitudes.

The Eurasia–North Pacific Oscillation in atmospheric mass variations independent of both IHO and AO and its possible impacts on winter climate

Using NCEP/NCAR reanalysis, we have investigated the features of migrations of atmospheric mass (AM) between land and ocean in Eurasia–North Pacific domain in boreal winter after having both signals of Inter-hemispheric Oscillation and Arctic Oscillation removed from the physical quantities. It is found that there is a Eurasia–North Pacific Oscillation (ENPO) in surface air pressure anomalies. This ENPO pattern characterizes with two oppositely signed anomalous surface pressure centers over Eurasia and North Pacific respectively, indicating strong connections between Siberian high and Aleutian low during period 1979–2012. The maintenance of this ENPO teleconnection is significantly associated with three factors including the anomalous AM flows and zonal circulation cell over Eurasia–North Pacific domain, the Rossby wave energy propagations, and the thermal forcing contrasts near the surface between Eurasia and North Pacific during boreal winter. The variations of both wintertime rainfall and temperature over Eurasia may be strongly affected by ENPO. When the ENPO index is positive (negative), there occurs the AM accumulation (depletion) over Eurasia with simultaneous depletion (accumulation) over mid-latitude North-Pacific. Correspondingly, this anomalous surface pressure pattern along with the related circulation anomalies at different isobaric levels possibly results in winter precipitation decreases (increases) over Siberian Plain and East China, whereas increases (decreases) over southeastern Europe, Xinjiang of China, and the west coast of Sea of Okhotsk. On the other hand, surface air temperature decreases (increases) over large areas of Eurasia. These results are helpful for our better understanding the mechanisms behind circulation and winter climate variations over Eurasia–North Pacific region.

Human-environment dynamics in European treeline ecosystems: a synthesis based on the DPSIR framework

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 73:17-29 (2017) - DOI: https://doi.org/10.3354/cr01454 Human-environment dynamics in European treeline ecosystems: a synthesis based on the DPSIR framework A. P. Kyriazopoulos1, O. Skre2,*, S. Sarkki3, F. E. Wielgolaski4, E. M. Abraham5, A. Ficko6 1Department of Forestry and Management of the Environment and Natural Resources, Democritus University of Thrace, 193 Pantazidou str., 68200 Orestiada, Greece 2Skre Nature and Environment, Fanaflaten 4, 5244 Fana, Norway 3Cultural Anthropology, Faculty of Humanities, PO Box 1000, University of Oulu, 90014 Oulu, Finland 4Department of Bioscience, University of Oslo, PO Box 1066 Blindern, 0316 Oslo, Norway 5Laboratory of Range Science, Department of Forestry and Natural Environment, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 6Biotechnical Faculty, Department of Forestry and Renewable Forest Resources, University of Ljubljana, Vecna pot 83, 1000 Ljubljana, Slovenia *Corresponding author: oddvar@nmvskre.no‑Advance View was available online May 18, 2017 ABSTRACT: The state of, and changes to, altitudinal and polar treeline ecosystems and their services in selected mountain regions in Europe were analyzed using the drivers-pressures-state-impacts-responses (DPSIR) framework. The analysis was based on 45 responses of experts from 19 countries to 2 semi-structured questionnaires on treeline ecosystem services (ESs), stakeholders and the DPSIR factors, and 11 case study descriptions of best management practices. The experts recognized climate and land-use changes as the main drivers, resulting in various pressures that contrasted among the regions. The impacts of the pressures were mainly considered as negative (e.g. loss of biodiversity, root rot diseases, moth and bark beetle outbreaks, wild fires, decrease of (sub)alpine grasslands, browsing), but also as positive (e.g. increase in forested area). The influence of climate warming, altered precipitation regimes, a longer growing season, annual variation in winter climate and increased ground-level ozone concentrations were considered less critical for recent treeline dynamics than land abandonment, increased tourism and livestock pressure. Current policy responses to emerging pressures and stakeholder demands were considered insufficient and incoherent. Mitigation, adaptation and restoration actions were rare and with no evident long-term impact. We conclude that (1) locally-specific human-environment interactions have greater influence on treeline dynamics than global warming; (2) ecological and social sustainability of the treeline areas can be enhanced by simultaneously promoting traditional land use and regulating tourism development; (3) ES users should look for new opportunities arising from environmental change rather than trying to sustain current levels of ESs indefinitely; and (4) to safeguard the unique ecological and social values of treeline areas, more coherent and proactive policies are needed. KEY WORDS: Mountain forests · DPSIR framework · Climate change · Land-use change · Ecosystem services · Disturbances · Biodiversity Full text in pdf format PreviousNextCite this article as: Kyriazopoulos AP, Skre O, Sarkki S, Wielgolaski FE, Abraham EM, Ficko A (2017) Human-environment dynamics in European treeline ecosystems: a synthesis based on the DPSIR framework. Clim Res 73:17-29. https://doi.org/10.3354/cr01454 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 73, No. 1-2. Online publication date: August 21, 2017 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2017 Inter-Research.

Timing of diapause termination in relation to variation in winter climate

AbstractIn temperate insects, winters are typically endured by entering diapause, which comprises a deep resting stage. Correct timing of diapause termination is vital for synchronization of emergence with conspecifics and for mobilizing resources when conditions for growth and reproduction become favourable. Although critical to survival, the intrinsic and extrinsic drivers of diapause termination timing are poorly understood. In the present study, we investigate diapause development under a range of durations (10–24 weeks) spent at different temperatures (−2 to 10 °C) in the pupal diapausing butterfly Pieris napi Linnaeus (Lepidoptera:Pieridae). We determine: (i) the maximum cold temperature for diapause development; (ii) if pupae in diapause count cold days or cold sums; and (iii) whether diapause termination is distinct or gradual. The results indicate large and idiosyncratic effects of high and low nonlethal temperatures on diapause development in P. napi. Although all temperatures tested lead to diapause termination, a thermal optimum between 2 and 4 °C is observed. Lower temperatures lead to decreased eclosion propensity, whereas higher temperatures slow down development and increase emergence desynchronization. These data suggest that, rather than a simple cold‐summing process with a distinct diapause termination point, there are trade‐offs between time and temperature at the low and high end of the thermal range, resulting in a nonlinear thermal landscape showing a ridge of increasing eclosion propensity at moderate temperatures. The present study suggests that the effects of temperature on diapause development should be included in projections on post‐winter phenology models of insects, including pest species.

Arctic and East Asia Winter Climate Variations Associated with the Eastern Atlantic Pattern

The wintertime Arctic temperature (T; surface–400 hPa) decreased from 1979 to 1997 and increased rapidly from 1998 to 2012, in contrast to the global mean surface air temperature. Here aspects of circulation variability that are associated with these temperature changes are examined using the NCEP–NCAR reanalysis and ERA-Interim products. It is found that the Nordic–Siberia seesaw of meridional winds near 70°N is associated with two-thirds of the variance of the Arctic winter mean T, possibly contributing to the cooling and warming trends. It is suggested here that the seesaw accounts for much of the difference in Arctic amplification between observations and climate models. Growth of sea ice in winter is hindered by southerly winds over the Nordic region (0°–60°E). Through modulation of the wind seesaw, the eastern Atlantic (EA) pattern is found to be significantly associated with Arctic and East Asia winter climate variations. In one phase of the EA pattern, a midlatitude North Atlantic ridge anomaly is associated with a poleward shift of the mean storm track, a weakened eddy-driven jet over Eurasia, and above-normal sea level pressure (SLP) over Siberia, most significantly in the region to the northwest of Lake Baikal. The EA pattern is associated with two-thirds of the variance of winter-average SLP over Siberia.

Nitrate and dissolved organic carbon mobilization in response to soil freezing variability

Reduced snowpack and associated increases in soil freezing severity resulting from winter climate change have the potential to disrupt carbon (C) and nitrogen (N) cycling in soils. We used a natural winter climate gradient based on elevation and aspect in a northern hardwood forest to examine the effects of variability in soil freezing depth, duration, and frequency on the mobilization of dissolved organic carbon (DOC) and nitrate (NO3 −) in soils over the course of 2 years. During a winter with a relatively thin snowpack, soils at lower elevation sites experienced greater freezing and especially variable freeze/thaw cycles, which in turn led to greater leaching of DOC from the organic horizon during the following growing season. In contrast to several previous field manipulation studies, we did not find changes in soil solution NO3 − concentrations related to soil freezing variables. Our results are consistent with a soil matrix disturbance from freezing and thawing which increases leachable C. These results build upon previous laboratory experiments and field manipulations that found differing responses of DOC and NO3 − following soil freezing, suggesting that mobilization of labile C may suppress NO3 − losses through microbial immobilization of N. This research highlights the importance of studying natural variation in winter climate and soil freezing and how they impact soil C and N retention, with implications for surface water runoff quality.

Hydrologic flowpaths during snowmelt in forested headwater catchments under differing winter climatic and soil frost regimes

Changes in hydrologic flowpaths have important impacts on the timing, magnitude and hydrochemistry of run-off during snowmelt in forested catchments, but how flowpaths are affected by variation in winter climate and the irregular presence of soil frost remains poorly understood. The depth and extent of soil frost may be expected to increase as snowpack decreases or develops later because of climate change. In this study, we used end-member mixing analysis to determine daily contributions of snow, forest floor soil water and groundwater to stream run-off during snowmelt under different soil frost regimes resulting from interannual and elevational variation at the Hubbard Brook Experimental Forest in New Hampshire, USA. We observed greater routing of run-off through forest floor flowpaths during early snowmelt in 2011, when the snowpack was deep and soil frost was minimal, compared with the early snowmelt in 2012 under conditions of deep and extensive soil frost. The results indicate that widespread soil frost that penetrated the depth of the forest floor decreased the flow signal through the shallowest subsurface flowpaths, but did not reduce overall infiltration of melt waters, as the contribution from the snow-precipitation end-member was similar under both conditions. These results are consistent with development of granular soil frost which permits vertical infiltration of melt waters, but either reduces lateral flow in the forest floor or prevents the solute exchange that would produce the typical chemical signature of shallow subsurface flowpaths in streamwater. Copyright © 2016 John Wiley & Sons, Ltd.

Holocene glacier variability and Neoglacial hydroclimate at Ålfotbreen, western Norway

Glaciers and small ice caps respond rapidly to climate perturbations (mainly winter precipitation, and summer temperature), and the mass-balance of glaciers located in western Norway is governed mainly by winter precipitation (Pw). Records of past Pw can offer important insight into long-term changes in atmospheric circulation, but few proxies are able to accurately capture winter climate variations in Scandinavia. Reconstructions of equilibrium-line-altitude (ELA) variations from glaciers that are sensitive to changes in Pw therefore provide a unique opportunity to quantify past winter climate in this region. Here we present a new, Holocene glacier activity reconstruction for the maritime ice cap Ålfotbreen in western Norway, based on investigations of distal glacier-fed lake sediments and modern mass balance measurements (1963–2010). Several lake sediment cores have been subject to a suite of laboratory analyses, including measurements of physical parameters such as dry bulk density (DBD) and loss-on-ignition (LOI), geochemistry (XRF), surface magnetic susceptibility (MS), and grain size distribution, to identify glacial sedimentation in the lake. Both radiocarbon (AMS 14C) and 210Pb dating were applied to establish age-depth relationships in the sediment cores. A novel approach was used to calibrate the sedimentary record against a simple ELA model, which allowed reconstruction of continuous ELA changes for Ålfotbreen during the Neoglacial (when Ålfotbreen was present, i.e. the last ∼1400 years). Furthermore, the resulting ELA variations were combined with an independent summer temperature record to calculate Neoglacial Pw using the ‘Liestøl equation’. The resulting Pw record is of higher resolution than previous reconstructions from glaciers in Norway and shows the potential of glacier records to provide high-resolution data reflecting past variations in hydroclimate. Complete deglaciation of the Ålfotbreen occurred ∼9700 cal yr BP, and the ice cap was subsequently absent or very small until a short-lived glacier event is seen in the lake sediments ∼8200 cal yr BP. The ice cap was most likely completely melted until a new glacier event occurred around ∼5300 cal yr BP, coeval with the onset of the Neoglacial at several other glaciers in southwestern Norway. Ålfotbreen was thereafter absent (or very small) until the onset of the Neoglacial period ∼1400 cal yr BP. The ‘Little Ice Age’ (LIA) ∼650–50 cal yr BP was the largest glacier advance of Ålfotbreen since deglaciation, with a maximum extent at ∼400–200 cal yr BP, when the ELA was lowered approximately 200 m relative to today. The late onset of the Neoglacial at Ålfotbreen is suggested to be a result of its low altitude relative to the regional ELA. A synthesis of Neoglacial ELA fluctuations along the coast of Norway indicates a time-transgressive trend in the maximum extent of the LIA, which apparently seems to have occurred progressively later as we move northwards. We suggest that this trend is likely due to regional winter precipitation differences along the coast of Norway.

Potential Mechanisms Driving Population Variation in Spatial Memory and the Hippocampus in Food-caching Chickadees.

Harsh environments and severe winters have been hypothesized to favor improvement of the cognitive abilities necessary for successful foraging. Geographic variation in winter climate, then, is likely associated with differences in selection pressures on cognitive ability, which could lead to evolutionary changes in cognition and its neural mechanisms, assuming that variation in these traits is heritable. Here, we focus on two species of food-caching chickadees (genus Poecile), which rely on stored food for survival over winter and require the use of spatial memory to recover their stores. These species also exhibit extensive climate-related population level variation in spatial memory and the hippocampus, including volume, the total number and size of neurons, and adults' rates of neurogenesis. Such variation could be driven by several mechanisms within the context of natural selection, including independent, population-specific selection (local adaptation), environment experience-based plasticity, developmental differences, and/or epigenetic differences. Extensive data on cognition, brain morphology, and behavior in multiple populations of these two species of chickadees along longitudinal, latitudinal, and elevational gradients in winter climate are most consistent with the hypothesis that natural selection drives the evolution of local adaptations associated with spatial memory differences among populations. Conversely, there is little support for the hypotheses that environment-induced plasticity or developmental differences are the main causes of population differences across climatic gradients. Available data on epigenetic modifications of memory ability are also inconsistent with the observed patterns of population variation, with birds living in more stressful and harsher environments having better spatial memory associated with a larger hippocampus and a larger number of hippocampal neurons. Overall, the existing data are most consistent with the hypothesis that highly predictable differences in winter climate drive the evolution and maintenance of differences among populations both in cognition and in the brain via local adaptations, at least in food-caching parids.

Regional Arctic sea ice variations as predictor for winter climate conditions

Seasonal prediction skill of winter mid and high northern latitudes climate from sea ice variations in eight different Arctic regions is analyzed using detrended ERA-interim data and satellite sea ice data for the period 1980–2013. We find significant correlations between ice areas in both September and November and winter sea level pressure, air temperature and precipitation. The prediction skill is improved when using November sea ice conditions as predictor compared to September. This is particularly true for predicting winter NAO-like patterns and blocking situations in the Euro-Atlantic area. We find that sea ice variations in Barents Sea seem to be most important for the sign of the following winter NAO—negative after low ice—but amplitude and extension of the patterns are modulated by Greenland and Labrador Seas ice areas. November ice variability in the Greenland Sea provides the best prediction skill for central and western European temperature and ice variations in the Laptev/East Siberian Seas have the largest impact on the blocking number in the Euro-Atlantic region. Over North America, prediction skill is largest using September ice areas from the Pacific Arctic sector as predictor. Composite analyses of high and low regional autumn ice conditions reveal that the atmospheric response is not entirely linear suggesting changing predictive skill dependent on sign and amplitude of the anomaly. The results confirm the importance of realistic sea ice initial conditions for seasonal forecasts. However, correlations do seldom exceed 0.6 indicating that Arctic sea ice variations can only explain a part of winter climate variations in northern mid and high latitudes.

Seasonal effects of Pacific‐based climate on recruitment in a predator‐limited large herbivore

1. Climate is an important factor influencing the population dynamics of large herbivores operating directly on individuals or through its effect on forage characteristics. However, the seasonal effect of climate may differ between forage- and predator-limited populations because of a climatic influence on predation rates. The influence of climate on predator-limited large herbivores is less well known than on forage-limited populations. Further, the effect of Pacific-based climate on large herbivore populations has been rarely assessed. 2. We investigated the effect of the Pacific Decadal Oscillation (PDO), across different seasons, on recruitment in 10 populations (herds) of mountain-dwelling caribou Rangifer tarandus caribou L. in the Yukon Territory, Canada. These low-density populations occur in highly seasonal environments and are considered predator-limited with high neonatal calf mortality. Hence, in most years females do not spend resources through lactational support during the summer and resource intake is devoted to self-maintenance. We predicted that climate affecting environmental conditions at calving would have a strong effect on recruitment via its influence on predation rates. We also predicted that climatic conditions prior to conception could have an effect on recruitment through its influence on female fecundity. We modelled recruitment (n = 165) by seasonal PDO values using generalized linear mixed-effects models with herd-varying coefficients. 3. We found that recruitment variability was best explained by variation in winter climate (beta = 0.110, SE = 0.007) prior to birth (in utero) and May climate (beta = 0.013, SE = 0.006) at calving. There was little support for a pre-conception climate effect influencing female body condition and hence fecundity. These results confirm that recruitment in these populations is limited by predation and that forage-limitation is not a significant factor in their population dynamics. There was considerable variability in herd-specific relationships between the PDO and recruitment. Incorporating herd-specific characteristics, such as variable predator densities or terrain characteristics within a herd range, may shed greater light on the complex relationship between climate and ungulate population dynamics.

Reconstruction of winter climate variations during the 19th century in Japan

AbstractAn attempt was made to reconstruct winter climate conditions in Japan for the period 1810/1811 to 1858/1859 on the basis of daily weather records documented in old diaries. Daily weather maps for each winter were drawn using 19th century weather records collected by our research group. Maps were divided into five types by classifying daily snowfall and rainfall distributions and the occurrence frequencies of each weather pattern for the period 1810/1811 to 1858/1859 were analysed. It was found that the occurrence frequencies of winter monsoon weather patterns were high from the late 1820s to the early 1840s. This period almost coincided with a summer cold period in the 19th century. The result implies that strengthening of a cold air mass around Japan occurred in the late 1820s, not only in summer but also in winter. The frequencies of the typical winter monsoon patterns correspond with the freezing dates of Lake Suwa, which have been used as an indicator of winter coldness in previous studies. On the basis of the frequencies of the winter monsoon weather patterns, mean January temperatures for western Japan were estimated. In the time series of estimated temperatures, a cooling period from the late 1820s to the early 1830s was revealed. Copyright © 2007 Royal Meteorological Society

Effects of climate on population fluctuations of ibex

AbstractPredicting the effects of the expected changes in climate on the dynamics of populations require that critical periods for climate‐induced changes in population size are identified. Based on time series analyses of 26 Swiss ibex (Capra ibex) populations, we show that variation in winter climate affected the annual changes in population size of most of the populations after accounting for the effects of density dependence and demographic stochasticity. In addition, precipitation during early summer also influenced the population fluctuations. This suggests that the major influences of climate on ibex population dynamics operated either through loss of individuals during winter or early summer, or through an effect on fecundity. However, spatial covariation in these climate variables was not able to synchronize the population fluctuations of ibex over larger distances, probably due to large spatial heterogeneity in the effects of single climate variables on different populations. Such spatial variation in the influence of the same climate variable on the local population dynamics suggests that predictions of influences of climate change need to account for local differences in population dynamical responses to climatic conditions.

Long‐term responses in arctic ungulate dynamics to changes in climatic and trophic processes

AbstractFollowing predictions from climatic general circulation models, the effects of perturbations in global climate are expected to be most pronounced in the Northern Hemisphere. Elaborating on a recently developed plant–herbivore–climate model, we explore statistically how different winter climate regimes and density‐dependent processes during the past century have affected population dynamics of two arctic ungulate species. Our analyses were performed on the dynamics of six muskox and six caribou populations. In muskoxen, variation in winter climate, mediated through the North Atlantic Oscillation (NAO), explained up to 24% of the variation in interannual abundance, whereas in caribou up to 16% was explained by the NAO. Muskoxen responded negatively following warm and snowy winters, whereas caribou responded negatively to dry winters. Direct and delayed density dependence was recorded in most populations and explained up to 32% and 90% of variations in abundance of muskoxen and caribou, respectively.

Climate patterns and the stochastic dynamics of migratory birds

We analyse time series data of 17 bird species trapped at Ottenby Bird Observatory, Sweden, during spring migration 1972–1999. The species have similar demography but respond differently to variation in the North Atlantic Oscillation (NAO) – a strong determinant of winter climate in the northern Hemisphere. Species wintering in northern Europe, compared to species having winter quarters in the Mediterranean area, tend to respond positively to variation in NAO. The variation within each group is high due to wide‐ranging winter‐distribution in many species, probably smoothing out the effect of spatial variation in NAO. Whereas mild winters (high NAO) is benign for many – but not all – birds wintering in northern Europe, the effect of drier‐than‐normal conditions in the Mediterranean area during high NAO index winters are uncertain. The work presented here goes beyond simple correlative studies and help identifying which species that are most affected by variation in winter climate. This is a first important step that calls for a more mechanistic approach when analysing possible changes to climate change.

Effects of density‐dependence and climate on the dynamics of a Svalbard reindeer population

To determine the main factors affecting the population dynamics of Svalbard reindeer, we analysed 21 yr of annual censuses, including data on population size, recruitment rate (calves per female) and mortality (number of deaths), from the Reindalen reindeer population. In accordance with previous studies on population dynamics of Svalbard reindeer, we found large inter‐annual variation in population size, mortality and recruitment rates within the study area. Population size decreased in years with low recruitment rate as well as high winter mortality and vice versa. Apparently. the fluctuations were due to both direct density‐dependent food limitation and variation in winter climate associated with high precipitation and icing of the feeding range. We found no delayed density‐dependence or effect of climatic conditions during summer on the population dynamics. The mortality during die‐off years was mainly of calves and very old individuals, indicating that the population was more vulnerable to high die oft in years following high recruitment rate. These results suggest an unstable interaction between the reindeer population and its food supply in these predator‐free environments.

Lag influences of winter circulation conditions in the tropical western Pacific on South Asian summer monsoon

By means of monthly mean NCEP/NCAR data analyses, this note investigates the lag influences of winter circulation conditions in the tropical western Pacific on South Asian summer monsoon through the methods of composite, correlation and statistical confident test. The results indicate clearly that winter climate variations in the equatorial western Pacific would produce significant influences on the following South Asian summer monsoon, and with the lapse of time the lag influences show clearly moving northward and extending westward features. When winter positive (negative) sea level pressure anomalies occupy the equatorial western Pacific, there is an anticyclonic (cyclonic) circulation anomaly appearing in the northwestern Pacific. With the lapse of time, the anticyclonic (cyclonic) circulation anomaly gradually moves to northeast, and its axis in the west-east directions also stretches, therefore, easterly (westerly) anomalies in the south part of the anticyclonic (cyclonic) circulation anomaly continuously expand westward to the peninsula of India. Undoubtedly, the South Asian summer monsoon is weak (strong).