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With permafrost warming, the observed discharge of the Kolyma River in northeastern Siberia decreased between 1930s and 2000; however, the underlying mechanism is not well understood. To understand the hydrological changes in the Kolyma River, it is important to analyze the long-term hydrometeorological features, along with the changes in the active layer thickness. A coupled hydrological and biogeochemical model was used to analyze the hydrological changes due to permafrost warming during 1979–2012, and the simulated results were validated with satellite-based products and in situ observational records. The increase in the active layer thickness by permafrost warming suppressed the summer discharge contrary to the increased summer precipitation. This suggests that the increased terrestrial water storage anomaly (TWSA) contributed to increased evapotranspiration, which likely reduced soil water stress to plants. As soil freeze–thaw processes in permafrost areas serve as factors of climate memory, we identified a two-year lag between precipitation and evapotranspiration via TWSA. The present results will expand our understanding of future Arctic changes and can be applied to Arctic adaptation measures.

Arctic precipitation (PG) that occurs as rainfall (Pr) or snowfall (Ps) depending on the prevailing climatic conditions results in seasonally specific hydrological events. Climate change can affect...

Surface air temperature is an important factor for the permafrost thermal state in the Northern Hemisphere. It is therefore necessary to understand the variations and regional differences in air temperature to determine the interactions between permafrost degradation and climate change. In this study, we used observational data from the National Centers for Environmental Information, the China Meteorological Administration, and the World Data Centre for Meteorology to quantitatively analyze the variations and regional differences in air temperature from 1980 to 2018. The results demonstrated that the annual mean air temperatures were low in continuous permafrost regions and high in sporadic and isolated permafrost regions, with a significant warming rate of 0.371 ± 0.086 °C/decade. Air temperatures warmed the slowest during the winter and fastest during the spring, and no “warming hiatus” was observed in the permafrost regions of the Northern Hemisphere. The spatial patterns of freezing degree-days (FDDs) and thawing degree-days (TDDs) had different spatial characteristics. The decreasing rate of FDDs was −6.97 °C·days/year, while the increasing rate of TDDs was 6.4 °C·days/year. The air temperatures and warming trends had largely regional differences with respect to high latitude, transitional, and high altitude permafrost regions. Air temperature and its warming trend was the highest in high altitude regions. In addition, air temperature warming trends gradually decreased from the continuous permafrost zone to the island permafrost zone. The FDDs had a significant decreasing trend from the continuous permafrost zone to the island permafrost zone, whereas TDDs exhibited the opposite trend. The results indicate that the air temperature warming rate in the permafrost regions was approximately 2.0 times that of the global warming rate, and 1.3 times the global land warming rate from 1980 to 2018. These findings offer a perspective on the differences in permafrost and its thermal state across different regions under climate change.

The 11th Symposium on Polar Science/Special session: [S] Accelerating Arctic research: Recent progress and future prospect of Arctic research, Poster presentations, Mon. 16 Nov. - Fri. 18 Dec.

Positive river heat–sea ice feedback doubles the river heat effect on declining Arctic sea ice., Arctic river discharge increased over the last several decades, conveying heat and freshwater into the Arctic Ocean and likely affecting regional sea ice and the ocean heat budget. However, until now, there have been only limited assessments of riverine heat impacts. Here, we adopted a synthesis of a pan-Arctic sea ice–ocean model and a land surface model to quantify impacts of river heat on the Arctic sea ice and ocean heat budget. We show that river heat contributed up to 10% of the regional sea ice reduction over the Arctic shelves from 1980 to 2015. Particularly notable, this effect occurs as earlier sea ice breakup in late spring and early summer. The increasing ice-free area in the shelf seas results in a warmer ocean in summer, enhancing ocean–atmosphere energy exchange and atmospheric warming. Our findings suggest that a positive river heat–sea ice feedback nearly doubles the river heat effect.

Evapotranspiration (ET) is a key component in global water and energy cycles. This chapter presents and discusses recent research advances about ET over northern regions and watersheds. ET in northern regions tends to increase with the decrease of latitude. The largest ET typically appears in the forest ecosystem, while the grasslands and shrublands have small ET. While the seasonal variations in ET are usually high, the interannual variability in annual ET is usually low over the Arctic regions. Sublimation from snow cover accounts for about 15–25% of winter precipitation. Many factors, such as soil moisture, vegetation type and productivity, and ecosystem features affect ET over northern regions. In addition, precipitation plays a key role in impacting ET. ET is more sensitive to precipitation in the early growing season than in the late growing season. Furthermore, changes in freeze–thaw processes due to warming also affect land surface conditions and the ET processes. During 1983–2005, ET increased significantly in the Arctic region with a rate of 3.8 mm decade-1 because of regional warming and vegetation greening. Such an increase in ET may exert significant impacts on the regional hydrology and water resources. Advanced models can simulate past ET change over the large northern watersheds. Remote sensing has provided new ET data and information that support climate and hydrology research and applications. There is a key question: Will Arctic landscapes become wetter or drier as climate changes? According to global models and data analyses, annual ET has increased over the northern regions. In the future, summer PE is projected to decrease much of Canada, increase over Alaska, decrease over the western and northern Eurasian subarctic, and increase over parts of northeastern Russia. Over most of these areas, the sign of the projected change is not robust across the models at the 95% confidence level. Many factors contribute to the uncertainty in the projected changes in Arctic surface wetness. There is certainly a need to better quantify and narrow the uncertainties in global models in the northern regions.

Long-term observations and data analyses of water temperatures and discharge over the northern regions determine water temperature regimes and quantify river heat flux into the ocean system. This chapter presents an overview of thermal regime and heat flux for the large rivers in northern regions. This chapter compares the results for the Siberian and North American arctic watersheds/regions, and highlights the differences and changes in water temperature and heat flux due to climate variation and human effect. Given the limited water temperature observations, this chapter is unable to discuss the variability and change in river thermal conditions over the northern regions of the North America. There is a knowledge gap in river temperature and heat contribution along the arctic coast of North America. Advanced hydrologic models and remote sensing data have provided opportunities to improve our understanding of river thermal characteristics across the northern regions. There is a need to continue to explore remote sensing data/products in the investigations of river water temperature and heat transport processes and to develop coupled land–ocean models, in order to better quantify land–ocean linkage and connections across the northern coastal regions.

River ice is critical for northern hydrology and ecosystems, such as the magnitude and timing of hydrologic extremes, i.e., low flows and floods. Historical data analyses and model studies clearly show widespread decreases in river ice thinness and duration due to climate warming across the northern regions. Reductions in river ice jam flooding may have major positive benefits for communities and infrastructure along the river margins, but could also alter the ecology of deltaic riparian and coastal marine ecosystems. The reduction in river and lake ice will influence transportation opportunities in remote regions. In situ observations are important to monitor the ongoing changes in river and lake ice features across the northern regions. Modeling and remote sensing tools are very useful to the understanding of ice processes, attributions of past changes, and projection of future ice conditions in a warming Arctic. More effort is necessary to combine observations, models, and remote sensing technology to investigate ice hydrology over the broader northern regions.

Over the recent decades, the warming in Arctic has affected changes in the terrestrial hydrologic processes. Unfortunately, the number of hydrometeorological observing stations in the region has decreased. To reduce the limitation in observation, a number of process-based and distributed models have been developed for simulating the hydrological processes in a changing climate. The current generations of models are able to reasonably reproduce the prominent cold region hydrologic processes, such as degrading permafrost, decreasing snow extent, increasing river discharge and evapotranspiration, and increasing streamflow temperature. These models enhance our understanding of the response of Arctic terrestrial processes to climate change and variation. However, the model representations for some of the Arctic hydrological processes are still not yet sufficient and need further improvements. This chapter provides an overview of changes in key processes and conditions of the Arctic terrestrial hydrology based on a synthesis of observations and model simulations, and presents recommendations for further development and improvement of cold region hydrologic models.

The winter of northern Arctic regions is characterized by strong winds that lead to frequent blowing snow and thus heterogeneous snow cover, which critically affects permafrost hydrothermal processes and the associated feedbacks across the northern regions. However until now, observations and models have not documented the blowing snow impacts. The blowing snow process has coupled into a land surface model CHANGE, and the improved model was applied to observational sites in the northeastern Siberia for 1979–2016. The simulated snow depth and soil temperature showed general agreements with the observations. To quantify the impacts of blowing snow on permafrost temperatures and the associated greenhouse gases, two decadal experiments that included or excluded blowing snow, were conducted for the observational sites and over the pan-Arctic scale. The differences between the two experiments represent impacts of the blowing snow on the analytical components. The blowing snow-induced thinner snow depth resulted in cooler permafrost temperature and lower active layer thickness; this lower temperature limited the vegetation photosynthetic activity due to the increased soil moisture stress in terms of larger soil ice portion and hence lower ecosystem productivity. The cooler permafrost temperature is also linked to less decomposition of soil organic matter and lower releases of CO2 and CH4 to the atmosphere. These results suggest that the most land models without a blowing snow component likely overestimate the release of greenhouse gases from the tundra regions. There is a strong need to improve land surface models for better simulations and future projections of the northern environmental changes.

Recent years have seen an obvious warming trend in the Arctic. Streamflow and water temperature Tw are important parameters representing the changes of Arctic rivers under climate change. However, few quantitative assessments of changes in river Tw have been conducted at the pan-Arctic scale. To carry out such an assessment, this study used a modeling framework combining a land process model [the coupled hydrological and biogeochemical model (CHANGE)] with models of river discharge Q, ice cover, and Tw dynamics. The Tw model was improved by incorporating heat exchange at the air–water interface and heat advection from upstream through the channel network. The model was applied to pan-Arctic terrestrial rivers flowing into the Arctic Ocean over the period 1979–2013 and quantitatively assessed trends of Tw at regional and pan-Arctic scales. The simulated Tw values were consistent with observations at the mouths of major pan-Arctic rivers. The model simulations indicated a warming trend of Tw by 0.16°C decade−1 at the outlets of the pan-Arctic rivers, including widespread spatial warming consistent with increased air temperature Ta. The strong impact of Ta on Tw was verified by model sensitivity analysis based on various scenarios involving changes in the Ta and Q forcings. Finally, this study demonstrated the warming of Tw in Arctic rivers induced by Ta warming, suggesting the potential for warming Tw of Arctic rivers under future climate change scenarios.

Arctic Oscillation–induced temperature increase drives earlier snow melting and fire activity over southeastern Siberia., Carbon release through boreal fires could considerably accelerate Arctic warming; however, boreal fire occurrence mechanisms and dynamics remain largely unknown. Here, we analyze fire activity and relevant large-scale atmospheric conditions over southeastern Siberia, which has the largest burned area fraction in the circumboreal and high-level carbon emissions due to high-density peatlands. It is found that the annual burned area increased when a positive Arctic Oscillation (AO) takes place in early months of the year, despite peak fire season occurring 1 to 2 months later. A local high-pressure system linked to the AO drives a high-temperature anomaly in late winter, causing premature snowmelt. This causes earlier ground surface exposure and drier ground in spring due to enhanced evaporation, promoting fire spreading. Recently, southeastern Siberia has experienced warming and snow retreat; therefore, southeastern Siberia requires appropriate fire management strategies to prevent massive carbon release and accelerated global warming.

Active-layer thickness (ALT) is one of the most robust measures used to assess the impact of climate change on terrestrial permafrost. Testing of a handheld dynamic cone penetrometer showed that it was capable of measuring ALT with the same level of accuracy as conventional methods in boreal and tundra sites in eastern Siberia. The penetrometer also characterised the vertical structure of ground hardness within the active layer. The vertical profile of penetrometer measurements corresponded closely with soil plasticity and the liquid limit in high-centred polygons produced by thermokarst subsidence in dry grassland areas at a boreal site at Churapcha. The ALT was markedly deeper (>70 cm) at gravelly slope points adjacent to a wet tundra plain (

Permafrost extent (PE) and active layer thickness (ALT) are important for assessing high northern latitude (HNL) ecological and hydrological processes, and potential land–atmosphere carbon and climate feedbacks. We developed a new approach to infer PE from satellite microwave remote sensing of daily landscape freeze–thaw (FT) status. Our results document, for the first time, the use of satellite microwave FT observations for monitoring permafrost extent and condition. The FT observations define near-surface thermal status used to determine permafrost extent and stability over a 30-year (1980–2009) satellite record. The PE results showed similar performance against independent inventory and process model (CHANGE) estimates, but with larger differences over heterogeneous permafrost subzones. A consistent decline in the ensemble mean of permafrost areas (− 0.33 million km 2 decade − 1 ; p − 1 ; p 2 ) of permafrost areas are vulnerable to degradation based on the 30-year PE record. ALT estimates determined from satellite (MODIS) and ERA-Interim temperatures, and CHANGE simulations, compared favorably with independent field observations and indicate deepening ALT trends consistent with widespread permafrost degradation under recent climate change.

The Arctic warming observed in recent decades has amplified changes in terrestrial ecohydrological processes. A number of process-based models have been developed to simulate the ecohydrological processes in cold regions under changing climate conditions. These models have simulated prominent changes in ecohydrological processes in regions of eastern Siberia at point and watershed scales, such as degrading permafrost, decreasing snow extent, and increasing river discharge and evapotranspiration induced by warming; model results have been consistent with observations. These modeling results improve our understanding of the responses of ecohydrological processes to the warming climate, in turn contributing to further model improvement and better projection of future changes in hydrologic processes. However, model representations of some cold region hydrological processes remain insufficient and need further improvement. This chapter summarizes changes in key processes and ecohydrology conditions of the Lena watershed based on a synthesis of observations and model simulations.

A land process model [the coupled hydrological and biogeochemical model (CHANGE)] is used to quantitatively assess changes in the ice phenology, thickness, and volume of terrestrial Arctic rivers from 1979 to 2009. The CHANGE model was coupled with a river routing and discharge model enabling explicit representation of river ice and water temperature dynamics. Model-simulated river ice phenological dates and thickness were generally consistent with in situ river ice data and landscape freeze–thaw (FT) satellite observations. Climate data indicated an increasing trend in winter surface air temperature (SAT) over the pan-Arctic during the study period. Nevertheless, the river ice thickness simulations exhibited a thickening regional trend independent of SAT warming, and associated with less insulation and cooling of underlying river ice by thinning snow cover. Deeper snow depth (SND) combined with SAT warming decreased simulated ice thickness, especially for Siberian rivers, where ice thickness is more strongly correlated with SND than SAT. Overall, the Arctic river ice simulations indicated regional trends toward later fall freezeup, earlier spring breakup, and consequently a longer annual ice-free period. The simulated ice phenological dates were significantly correlated with seasonal SAT warming. It is found that SND is an important factor for winter river ice growth, while ice phenological timing is dominated by seasonal SAT. The mean total Arctic river ice volume simulated from CHANGE was 54.1 km3 based on the annual maximum ice thickness in individual grid cells, while river ice volume for the pan-Arctic rivers decreased by 2.82 km3 (0.5%) over the 1979–2009 record. Arctic river ice is shrinking as a consequence of regional climate warming and coincident with other cryospheric components, including permafrost, glaciers, and sea ice.

In the last decade, increases in both soil temperature and active layer thickness have been observed in the central Lena River basin, an area located in the centre of continuous permafrost in the Eurasian continent. Increased soil temperatures have been accompanied by corresponding increases in soil moisture within the active layer at many sites throughout the region. These underground hydro-thermal changes are believed to be primarily due to perennially wet climate conditions, rather than the atmospheric warming alone that has resulted in abnormally large amounts of winter snow accumulation and late summer rainfall in the Lena River basin. This study aimed to clarify the linkage between atmospheric and land surface variations in regions of continuous permafrost in eastern Siberia based on in situ observation and atmospheric climatic data analyses. These wet conditions in 2004–2008 were likely caused by the enhancement of cyclones over the Arctic Ocean and eastward propagation of storm activity during late summer and early winter. Water vapour flux was enhanced in conjunction with the manifestation of precipitation in eastern Siberia. As a result, consecutive positive anomalies of winter snow accumulation and precipitation during the subsequent summers effectively humidified land surfaces in the permafrost region from 2005 onwards, which resulted in abrupt soil warming and wetting of both the active layer and the upper part of the permafrost. The positive effects on increasing soil temperatures of both snowfall in winter and rainfall in summer have been observed in eastern Siberia since the 1990s, and have also been demonstrated through land surface simulation. Results of these hydro-climatic changes indicated that the near-surface permafrost in eastern Siberia is vulnerable to the climatic conditions that have arisen due to the acceleration of the hydrological cycle that has resulted from the aforementioned period of warming.

As part of the terrestrial branch of the Japan-funded Arctic Climate Change Research Project (GRENE-TEA), which aims to clarify the role and function of the Arctic terrestrial system in the climate system, and assess the influence of its changes on a global scale, this model intercomparison project (GTMIP) is planned and being conducted to (1) enhance communication and understanding between the "minds and hands" (i.e., between the modelling and field scientists) and (2) assess the uncertainty and variations stemming from variability in model implementation/design and in model outputs due to climatic and historical conditions in the Arctic terrestrial regions. This paper provides an overview and the experiment protocol of Stage 1 of the project, site simulations driven by statistically fitted data created using the GRENE-TEA site observations for the last three decades. The target metrics for the model evaluation cover key processes in both physics and biogeochemistry, including energy budgets, snow, permafrost, phenology, and carbon budgets. The preliminary results on four metrics (annual mean latent heat flux, annual maximum snow depth, gross primary production, and net ecosystem production) already demonstrate the range of variations in reproducibility among existing models and sites. Full analysis on annual as well as seasonal time scales, to be conducted upon completion of model outputs submission, will delineate inter-dependence among the key processes, and provide the clue for improving the model performance.

River discharges from Siberia are a large source of freshwater into the Arctic Ocean, although the cause of the long-term variation in discharge is still unclear. The observed river discharges of the Lena in the east and the Ob in the west indicated different relationships in each of the epochs during the past seven decades. The correlations between the two river discharges were negative during the 1980s to mid-1990s, positive during the mid-1950s to 1960s, and became weak after the mid-1990s. Long-term records of tree-ring-reconstructed discharges during the past two centuries have also shown differences in the correlations in each epoch. However, it is noteworthy that the correlations obtained from the reconstructions tend to be negative. Such negative correlations have also been obtained from precipitations over the Lena and Ob in observation, and in simulations with an atmospheric general circulation model (AGCM) and multi-coupled models conducted for the Fourth Assessment Report of the IPCC. The AGCM control simulation further demonstrated that an east–west seesaw pattern of summertime atmospheric large-scale circulation frequently emerges over Siberia as an atmospheric internal variability, resulting in the negative correlation between the Lena and Ob. Consequently, the summertime atmospheric internal variability of east–west seesaw pattern over Siberia is a key factor influencing the long-term variation in precipitation and river discharge, i.e., the water cycle in this region.

This study quantitatively evaluated how insulation by snow depth (SND) affected the soil thermal regime and permafrost degradation in the pan-Arctic area, and more generally defined the characteristics of soil temperature (TSOIL) and SND from 1901 to 2009. This was achieved through experiments performed with the land surface model CHANGE to assess sensitivity to winter precipitation as well as air temperature. Simulated TSOIL, active layer thickness (ALT), SND, and snow density were generally comparable with in situ or satellite observations at large scales and over long periods. Northernmost regions had snow that remained relatively stable and in a thicker state during the past four decades, generating greater increases in TSOIL. Changes in snow cover have led to changes in the thermal state of the underlying soil, which is strongly dependent on both the magnitude and the timing of changes in snowfall. Simulations of the period 2001–2009 revealed significant differences in the extent of near-surface permafrost, reflecting differences in the model’s treatment of meteorology and the soil bottom boundary. Permafrost loss was greater when SND increased in autumn rather than in winter, due to insulation of the soil resulting from early cooling. Simulations revealed that TSOIL tended to increase over most of the pan-Arctic from 1901 to 2009, and that this increase was significant in northern regions, especially in northeastern Siberia where SND is responsible for 50 % or more of the changes in TSOIL at a depth of 3.6 m. In the same region, ALT also increased at a rate of approximately 2.3 cm per decade. The most sensitive response of ALT to changes in SND appeared in the southern boundary regions of permafrost, in contrast to permafrost temperatures within the 60°N–80°N region, which were more sensitive to changes in snow cover. Finally, our model suggests that snow cover contributes to the warming of permafrost in northern regions and could play a more important role under conditions of future Arctic warming.

A parameter set expressing the potential response used to simulate evapotranspiration was inferred from pooled data of leaf-level stomatal conductance in the Far East. We used Jarvis-type parameterization, which consists of restriction terms of light, air dryness, temperature, and soil water. Two parameter sets, pooled common (PC) and within-site (WS)

This study is a broad-scale synthesis of information on climate changes in two Arctic terrestrial regions, eastern Siberia and the Alaska–Yukon area of North America. Over the past 60 years (1951–2010), the trends of temperature and precipitation in the two regions are broadly similar in their seasonality. However, atmospheric advection influences the two regions differently during winter. The differential advective effects are much weaker in the other seasons. The Pacific Decadal Oscillation is the strongest correlator with interannual variability in the two regions, followed by the Arctic Oscillation and the El Nino/Southern Oscillation. Projected changes by the late 21st Century are qualitatively similar to the changes that have been ongoing over the past 60 years, although the rate of change increases modestly under mid-range forcing scenarios (e.g., the A1B scenario). The greatest warming is projected to occur farther north over the Arctic Ocean in response to sea ice loss. Precipitation is projected to increase by all models, although increases in evapotranspiration preclude conclusions about trends toward wetter or drier land surface conditions. A notable feature of the future climate simulations is a strong maximum of pressure decreases in the Bering Sea region, implying further advective changes.

This study not only examined the spatiotemporal variations of active-layer thickness (ALT) in permafrost regions during 1948–2006 over the terrestrial Arctic regions experiencing climate changes, but also identified the associated drivers based on observational data and a simulation conducted by a land surface model (CHANGE). The focus on the ALT extends previous studies that have emphasized ground temperatures in permafrost regions. The Ob, Yenisey, Lena, Yukon, and Mackenzie watersheds are foci of the study. Time series of ALT in Eurasian watersheds showed generally increasing trends, while the increase in ALT in North American watersheds was not significant. However, ALT in the North American watersheds has been negatively anomalous since 1990 when the Arctic air temperature entered into a warming phase. The warming temperatures were not simply expressed to increases in ALT. Since 1990 when the warming increased, the forcing of the ALT by the higher annual thawing index (ATI) in the Mackenzie and Yukon basins has been offset by the combined effects of less insulation caused by thinner snow depth and drier soil during summer. In contrast, the increasing ATI together with thicker snow depth and higher summer soil moisture in the Lena contributed to the increase in ALT. The results imply that the soil thermal and moisture regimes formed in the pre-thaw season(s) provide memory that manifests itself during the summer. The different ALT anomalies between Eurasian and North American watersheds highlight increased importance of the variability of hydrological variables.

The Central Yakutian permafrost landscape is rapidly being modified by land use and global warming, but small-scale thermokarst process variability and hydrological conditions are poorly understood. We analyze lake-area changes and thaw subsidence of young thermokarst lakes on ice-complex deposits (yedoma lakes) in comparison to residual lakes in alas basins during the last 70 years for a local study site and we record regional lake size and distribution on different ice-rich permafrost terraces using satellite and historical airborne imagery. Statistical analysis of climatic and ground-temperature data identified driving factors of yedoma- and alas-lake changes. Overall, lake area is larger today than in 1944 but alas-lake levels have oscillated greatly over 70 years, with a mean alas-lake-radius change rate of 1.663.0 m/yr. Anthropogenic disturbance and forest degradation initiated, and climate forced rapid, continuous yedoma-lake growth. The mean yedoma lake-radius change rate equals 1.261.0 m/yr over the whole observation period. Mean thaw subsidence below yedoma lakes is 6.261.4 cm/yr. Multiple regression analysis suggests that winter precipitation, winter temperature, and active-layer properties are primary controllers of area changes in both lake types; summer weather and permafrost conditions additionally influence yedoma-lake growth rates. The main controlling factors of alas-lake changes are unclear due to larger catchment areas and subsurface hydrological conditions. Increasing thermokarst activity is currently linked to older terraces with higher ground-ice contents, but thermokarst activity will likely stay high and wet conditions will persist within the near future in Central Yakutian alas basins.

A supersite has been established and is operated under the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the International Arctic Research Center (IARC) Collaboration Study (JICS), with special emphasis on activities in Research Area 2 (Terrestrial Processes and Variation), which aims understanding multi-scale interactions in the Arctic system, quantifying the impact of observing system components, and utilizing observations in tests and validation experiments for modeling and remote sensing. We have shown that the formation of a supersite under JICS can promote the effective integration of observational studies, modeling, and satellite remote sensing research targeting various components of the processes occurring in the Arctic system. Through the study of these processes, we can enhance our understanding of the nature of the Arctic and the regional aspects of global environmental change, and improve the predictability of climatic fluctuations. We plan to maintain the supersite for the five years of JICS between 2009 JFY and 2013 JFY, as well as to improve the operation of the supersite.

Urbanisation is increasing tremendously in some parts of the world. Consequently, many rural forests may become depleted, although many opportunities exist for urban forests to increase. However, few studies have quantified the carbon (C) sequestration capacities of urban and rural forests in specific climatic zones. The present study compared carbon sequestration in two temperate deciduous forests located in Nagoya and Toyota, central Japan. The Nagoya University forest represented an urban forest, and a site in Toyota represented a rural forest. The urban forest at Nagoya University had comparatively smaller areas of green space and larger areas of buildings and roads. Land uses for building and road, which are typical of urban areas, result in smaller diurnal temperature ranges but higher air temperature, vapour pressure deficit, and atmospheric carbon dioxide (CO 2 ) concentration. The urban forest in this study exhibited higher gross ecosystem exchange (GEE), especially in the active growing season from May to September, suggesting the possible effect of CO 2 fertilisation. However, higher air temperatures caused comparatively smaller net ecosystem exchange (NEE) because of higher ecosystem respiration (RE). Although both forests functioned as CO 2 sinks at annual time scales, the rural Toyota forest site (5.43 t C ha –1 yr –1 ) had 36% higher net ecosystem production (NEP=–NEE; the negative sign indicates uptake by the forest ecosystem from the atmosphere) than that at the urban forest. The higher normalised respiration (i.e., RE/GPP ratio; GPP=–GEE where GPP represents gross primary production) at the Nagoya University forest might be attributable to factors associated with the degree of urbanisation. Thus, in temperate forests, factors associated with urbanisation may reduce the atmospheric carbon sink function by accelerating respiration. This is an issue of global interest, as many countries are experiencing rapid urbanisation.

Marked increases in active-layer and upper permafrost temperatures occurred in the central Lena River basin in association with abrupt increases in active-layer soil moisture following the summer of 2005. The positive trend in soil temperature-moisture relations was observed at monitoring sites in the Yakutsk area, regardless of vegetation and soil type. The increase in soil temperature appears to have started in response to the large amounts of snow that accumulated in the winter of 2004. Abnormally high pre-winter rainfall and snowfall in the following three years accelerated soil warming through the effects of greater latent heat of freezing and insulation from atmospheric cooling in winter. The consecutive positive anomalies of snow depth and rainfall, which occurred widely in the central and southern Lena River basin during this three-year period, increased soil moisture and appear to have altered the active-layer thermal properties, which likely induced widespread warming of the surface layer of permafrost in this region. Copyright © 2009 John Wiley & Sons, Ltd.

Spatio-temporal variations in the energy budget and evapotranspiration (ET) in eastern Siberia were examined for a period of 19 years, from 1986 to 2004, for the Lena and Kolyma watersheds, which are underlain by permafrost. The energy budget and ET were calculated with a land surface model [Yamazaki, T., Yabuki, H., Ishii, Y., Ohta, T., Ohata, T., 2004. Water and energy exchanges at forests and a grassland in eastern Siberia evaluated using a one-dimensional land surface model. J. Hydrometeol. 5, 504–515] that considered vegetation, snow, and soil processes. Station data were used to compile gridded time series of forcing data. The spatial distribution of the energy budget showed that the latent heat flux (QE) was a significant term in the surface energy budget in forested regions, while the sensible heat (QH) was significant in the tundra region. A similar result was also obtained for the seasonal variation; in the Kolyma watershed, for instance, QH dominated the seasonal energy budget, even during the growing season. In the Lena watershed, 53% of precipitation (PG) was lost through ET, which occurred during a very sharply pronounced growing season. Transpiration (ET) was a major component of ET in the Lena watershed, while in the Kolyma watershed, evaporation from the forest floor (ES) was dominant. This suggested clear relations between the energy budget and the difference in land surface and climate. In the Lena and Kolyma watersheds, a time series of PG exhibited strong annual variations with an increasing trend during 1986–2004. ET also exhibited an increasing trend in the two watersheds. However, the interannual variation was weaker than that of PG, reflecting the influence of extensive permafrost in the area. The ET components also showed an increasing trend. In contrast, ES decreased during 1986–2004, the trend of ES thus being inversely correlated to that of PG. Interestingly, annual ET also exhibited an inverse correlation with PG in the Lena and Kolyma watersheds, although this relationship was weak. In contrast, ET displayed a strong correlation with air temperature (Ta) and the timing of the disappearance of snow.

We used an aerodynamic method to objectively determine a representative canopy height, using standard meteorological measurements. The canopy height may change if the tree height is used to represent the actual canopy, but little work to date has focused on creating a standard for determining the representative canopy height. Here we propose the ‘aerodynamic canopy height’ h a as the most effective means of resolving the representative canopy height for all forests. We determined h a by simple linear regression between zero-plane displacement d and roughness length z 0, without the need for stand inventory data. The applicability of h a was confirmed in five different forests, including a forest with a complex canopy structure. Comparison with stand inventory data showed that h a was almost equivalent to the representative height of trees composing the crown surface if the forest had a simple structure, or to the representative height of taller trees composing the upper canopy in forests with a complex canopy structure. The linear relationship between d and z 0 was explained by assuming that the logarithmic wind profile above the canopy and the exponential wind profile within the canopy were continuous and smooth at canopy height. This was supported by observations, which showed that h a was essentially the same as the height defined by the inflection point of the vertical profile of wind speed. The applicability of h a was also verified using data from several previous studies.

We explored the relationship between directional variation (changes in direction from a reference point) in vegetation and wind profiles, and propose an empirical wind profile model that may reproduce the wind profile within the canopy (such as secondary wind maximum) and reduce calculation loads. Based on the results of our observations in secondary broad-leaved forest, we clarified the variation in secondary wind maximum and aerodynamic parameters in wind direction, and assessed the influence of forest structural heterogeneity on the wind profile. Wind blew from specific directions depending on the time of day and season, and secondary wind maxima were observed in particular wind directions. Outlier estimations of aerodynamic parameters were determined for wind directions of 45°, 120–210°, and 300°, and these did not show a logarithmic distribution. Our proposed model reproduced the wind profile within and above the canopy, closely resembling K-theory and closure models, and reduced the required number of calculations and observations. The influence of forest structural heterogeneity was greatest in the upper part of the canopy (9.14 m), and reproducibility of the wind profile was improved by 0.05 m/s (3.4% on average) by changing the forest structural parameters of the model according to the wind direction. It was clear that forest structural heterogeneity did not cause critical errors in estimation of the wind profile, even at our study site, which had complex forest structure.

第6回極域科学シンポジウム[OM] 極域気水圏11月16日（月） 国立極地研究所１階交流アトリウム