15115249
Divergence in seasonal hydrology across northern Eurasia: Emerging trends and water cycle linkages
[1] Discharge from large Eurasia rivers increased during the 20th century, yet much remains unknown regarding details of this increasing freshwater flux. Here, for the three largest Eurasian basins (the Ob, Yenisei, and Lena) we examine the nature of annual and seasonal discharge trends by investigating the flow changes along with those for precipitation, snow depth, and snow water equivalent. On the basis of a multiperiod trend analysis and examination of station data, we propose two characteristic regimes to explain the long-term discharge increase from these large Eurasian rivers. Over the early decades from approximately 1936 to 1965, annual precipitation correlates well with annual discharge, and positive discharge trends are concurrent with summer/fall discharge increases. The latter decades were marked by a divergence between winter/ spring flows, which increased, amid summer/fall discharge declines. A comparison of cold season precipitation (CSP) and spring discharge trends across subbasins of the Ob, Yenisei, and Lena shows limited agreement with one precipitation data set but good agreement (R 2 > 0.90) when a second is used. While natural variability in the Arctic system tends to mask these emerging trends, spatial and temporal changes can generally be characterized by increased solid precipitation, primarily to the north, along with a drier hydrography during the warm season.
128425266
A projection of severe near‐surface permafrost degradation during the 21st century
[1] The current distribution and future projections of permafrost are examined in a fully coupled global climate model, the Community Climate System Model, version 3 (CCSM3) with explicit treatment of frozen soil processes. The spatial extent of simulated present-day permafrost in CCSM3 agrees well with observational estimates – an area, excluding ice sheets, of 10.5 million km2. By 2100, as little as 1.0 million km2 of near-surface permafrost remains. Freshwater discharge to the Arctic Ocean rises by 28% over the same period, largely due to increases in precipitation that outpace increases in evaporation, with about 15% of the rise directly attributable to melting ground ice. Such large changes in permafrost may provoke feedbacks such as activation of the soil carbon pool and a northward expansion of shrubs and forests.
46970385
Variability in Springtime Thaw in the Terrestrial High Latitudes: Monitoring a Major Control on the Biospheric Assimilation of Atmospheric CO2 with Spaceborne Microwave Remote Sensing
Abstract Evidence is presented from the satellite microwave remote sensing record that the timing of seasonal thawing and subsequent initiation of the growing season in early spring has advanced by approximately 8 days from 1988 to 2001 for the pan-Arctic basin and Alaska. These trends are highly variable across the region, with North America experiencing a larger advance relative to Eurasia and the entire region. Interannual variability in the timing of spring thaw as detected from the remote sensing record corresponded directly to seasonal anomalies in mean atmospheric CO2 concentrations for the region, including the timing of the seasonal draw down of atmospheric CO2 from terrestrial net primary productivity (NPP) in spring, and seasonal maximum and minimum CO2 concentrations. The timing of the seasonal thaw for a given year was also found to be a significant (P < 0.01) predictor of the seasonal amplitude of atmospheric CO2 for the following year. These results imply that the timing of seasonal thawing...
19705156
Role of Land-Surface Changes in Arctic Summer Warming
A major challenge in predicting Earth's future climate state is to understand feedbacks that alter greenhouse-gas forcing. Here we synthesize field data from arctic Alaska, showing that terrestrial changes in summer albedo contribute substantially to recent high-latitude warming trends. Pronounced terrestrial summer warming in arctic Alaska correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by about 3 watts per square meter per decade (similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric CO2). The continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating by two to seven times.
17046207
The Arctic freshwater system: Changes and impacts
[1] Dramatic changes have been observed in the Arctic over the last century. Many of these involve the storage and cycling of fresh water. On land, precipitation and river discharge, lake abundance and size, glacier area and volume, soil moisture, and a variety of permafrost characteristics have changed. In the ocean, sea ice thickness and areal coverage have decreased and water mass circulation patterns have shifted, changing freshwater pathways and sea ice cover dynamics. Precipitation onto the ocean surface has also changed. Such changes are expected to continue, and perhaps accelerate, in the coming century, enhanced by complex feedbacks between the oceanic, atmospheric, and terrestrial freshwater systems. Change to the arctic freshwater system heralds changes for our global physical and ecological environment as well as human activities in the Arctic. In this paper we review observed changes in the arctic freshwater system over the last century in terrestrial, atmospheric, and oceanic systems.
32069335
Disappearing Arctic Lakes
Historical archived satellite images were compared with contemporary satellite data to track ongoing changes in more than 10,000 large lakes in rapidly warming Siberia. A widespread decline in lake abundance and area has occurred since 1973, despite slight precipitation increases to the region. The spatial pattern of lake disappearance suggests (i) that thaw and "breaching" of permafrost is driving the observed losses, by enabling rapid lake draining into the subsurface; and (ii) a conceptual model in which high-latitude warming of permafrost triggers an initial but transitory phase of lake and wetland expansion, followed by their widespread disappearance.
129858352
An improved precipitation climatology for the Arctic Ocean
Bias correction methods from the World Meteorological Organization Solid Precipitation Measurement Intercomparison Project are applied to 31 years of the Arctic Ocean drifting station records. Observation biases of wind-induced gauge undercatch and trace amount of precipitation are corrected on a daily basis. Gauge-measured mean annual precipitation is increased significantly from 150mm to 260mm due to the bias corrections. The seasonal cycle of bias corrected precipitation over the Arctic Ocean is characterized by a minimum in April and a maximum in September; this regime is consistent with other precipitation climatologies for the Arctic Ocean and is also in general agreement with several climate model simulations. The results of this work will have a significant impact on the Arctic fresh water budget, and they will also be useful for validation of global and regional climate models.
36435454
Increasing River Discharge to the Arctic Ocean
Synthesis of river-monitoring data reveals that the average annual discharge of fresh water from the six largest Eurasian rivers to the Arctic Ocean increased by 7% from 1936 to 1999. The average annual rate of increase was 2.0 ± 0.7 cubic kilometers per year. Consequently, average annual discharge from the six rivers is now about 128 cubic kilometers per year greater than it was when routine measurements of discharge began. Discharge was correlated with changes in both the North Atlantic Oscillation and global mean surface air temperature. The observed large-scale change in freshwater flux has potentially important implications for ocean circulation and climate.
4736425
Recent temperature and precipitation increases in West Siberia and their association with the Arctic Oscillation
Surface air temperature and precipitation records for the years 1958-1999 from ten meteorological stations located throughout West Siberia are used to identify climatic trends and determine to what extent these trends are potentially attributable to the Arctic Oscillation (AO). Although recent changes in atmospheric variability are associated with broad Arctic climate change, West Siberia appears particularly susceptible to warming. Furthermore, unlike most of the Arctic, moisture transport in the region is highly variable. The records show that West Siberia is experiencing significant warming and notable increases in precipitation, likely driven, in part, by large-scale Arctic atmospheric variability. Because this region contains a large percentage of the world's peatlands and contributes a significant portion of the total terrestrial freshwater flux to the Arctic Ocean, these recent climatic trends may have globally significant repercussions. The most robust patterns found are strong and prevalent springtime warming, winter precipitation increases, and strong association of non-summer air temperatures with the AO. Warming rates for both spring (0.5-0.8 °C/decade) and annual (0.3-0.5°C/decade) records are statistically significant for nine often stations. On average, the AO is linearly congruent with 96% (winter), 19% (spring), 0% (summer), 67% (autumn) and 53% (annual) of the warming found in this study. Significant trends in precipitation occur most commonly during winter, when four of ten stations exhibit significant increases (4-13 %/decade). The AO may play a lesser role in precipitation variability and is linearly congruent with only 17% (winter), 13% (spring), 12% (summer), 1% (autumn) and 26% (annual) of precipitation trends.
28575516
Thermohaline circulation, the achilles heel of our climate system: will man-made CO2 upset the current balance?
During the last glacial period, Earth's climate underwent frequent large and abrupt global changes. This behavior appears to reflect the ability of the ocean's thermohaline circulation to assume more than one mode of operation. The record in ancient sedimentary rocks suggests that similar abrupt changes plagued the Earth at other times. The trigger mechanism for these reorganizations may have been the antiphasing of polar insolation associated with orbital cycles. Were the ongoing increase in atmospheric CO2 levels to trigger another such reorganization, it would be bad news for a world striving to feed 11 to 16 billion people.
128910306
Sensitivity of the thermohaline circulation to Arctic Ocean runoff
[1] Arctic Ocean river runoff increases over the 20th century raise concerns of the potential impact it may have on the thermohaline circulation (THC) and thus global climate. This study investigates how changes in Arctic river discharge may control THC by a series of experiments with an intermediate complexity global climate model. The experiments show an inverse relationship between THC strength and changes to riverine freshwater discharge, similar to the response of THC to surface freshening of the North Atlantic. Arctic Ocean freshwater export and volume were more sensitive to river runoff than sea ice export. A strong linear relationship between the THC strength and the steric height gradient (depth integrated density anomaly and an important driver for the western boundary current) suggests that the Arctic freshwater pools and fluxes are very effective in translating changes in runoff to THC strength by regulating the ocean water density in the North Atlantic.
129936504
Simulation and Projection of Arctic Freshwater Budget Components by the IPCC AR4 Global Climate Models
Abstract The state-of-the-art AOGCM simulations have recently (late 2004–early 2005) been completed for the Intergovernmental Panel on Climate Change (IPCC) in order to provide input to the IPCC’s Fourth Assessment Report (AR4). The present paper synthesizes the new simulations of both the twentieth- and twenty-first-century arctic freshwater budget components for use in the IPCC AR4, and attempts to determine whether demonstrable progress has been achieved since the late 1990s. Precipitation and its difference with evapotranspiration are addressed over the Arctic Ocean and its terrestrial watersheds, including the basins of the four major rivers draining into the Arctic Ocean: the Ob, the Yenisey, the Lena, and the Mackenzie. Compared to the previous [IPCC Third Assessment Report (TAR)] generation of AOGCMs, there are some indications that the models as a class have improved in simulations of the Arctic precipitation. In spite of observational uncertainties, the models still appear to oversimulate area-a...
39195357
Projected changes in Arctic Ocean freshwater budgets
[1] Arctic Ocean freshwater budgets are examined from 10 models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. This includes an analysis of sea ice transport and storage, ocean transport and storage, and net surface flux exchange. Simulated budgets for the late 20th century are compared to available observations, followed by an analysis of simulated changes from 1950 to 2050. The consistent theme over this period is an acceleration of the Arctic hydrological cycle, which is expressed as an increase in the flux of water passing through the hydrologic elements. Increased freshwater inputs to the ocean from net precipitation, river runoff, and net ice melt result. While generally attended by a larger export of liquid freshwater to lower latitudes, primarily through Fram Strait, liquid freshwater storage in the Arctic Ocean increases. In contrast, the export and storage of freshwater in the form of sea ice decreases. The qualitative agreement between models for which the only common forcing is rising greenhouse gas concentrations implicates this greenhouse gas loading as the cause of the change. Although the models perform quite well in their simulations of net precipitation over the Arctic Ocean and terrestrial drainage, they differ significantly regarding the magnitude of the trends and their representation of contemporary mean ocean and sea ice budget terms. To reduce uncertainty in future projections of the Arctic freshwater cycle, the climate models as a group require considerable improvement in these aspects of their simulations.
129117452
Human influence on increasing Arctic river discharges
[1] Climate models predict an intensification of hydrological cycle as anthropogenic greenhouse gas emissions to the atmosphere increase. As part of the process, high latitude precipitation and consequently river runoffs are expected to increase. Some observations have indicated that such a process may have started already during the late half of the 20th century. Arctic river flow changes simulated in HadCM3 with all historical external factors agree with river monitoring data reported by Peterson et al. [2002]. Model simulated total river discharges into the Arctic Ocean have increased by an annual rate of 8.73 km3 since the 1960s. Increasing high latitude precipitation is contributing a substantial part to the upward trend, which is likely to be the early stage of intensifying global hydrological cycle caused by anthropogenic factors, as we do not see the trend in the same model forced with natural factors alone.
53405204
Increasing river discharge in the Eurasian Arctic: Consideration of dams, permafrost thaw, and fires as potential agents of change
[1] Discharge from Eurasian rivers to the Arctic Ocean has increased significantly in recent decades, but the reason for this trend remains unclear. Increased net atmospheric moisture transport from lower to higher latitudes in a warming climate has been identified as one potential mechanism. However, uncertainty associated with estimates of precipitation in the Arctic makes it difficult to confirm whether or not this mechanism is responsible for the change in discharge. Three alternative mechanisms are dam construction and operation, permafrost thaw, and increasing forest fires. Here we evaluate the potential influence of these three mechanisms on changes in discharge from the six largest Eurasian Arctic rivers (Yenisey, Ob’, Lena, Kolyma, Pechora, and Severnaya Dvina) between 1936 and 1999. Comprehensive discharge records made it possible to evaluate the influence of dams directly. Data on permafrost thaw and fires in the watersheds of the Eurasian Arctic rivers are more limited. We therefore use a combination of data and modeling scenarios to explore the potential of these two mechanisms as drivers of increasing discharge. Dams have dramatically altered the seasonality of discharge but are not responsible for increases in annual values. Both thawing of permafrost and increased fires may have contributed to changes in discharge, but neither can be considered a major driver. Cumulative thaw depths required to produce the observed increases in discharge are unreasonable: Even if all of the water from thawing permafrost were converted to discharge, a minimum of 4 m thawed evenly across the combined permafrost area of the six major Eurasian Arctic watersheds would have been required. Similarly, sensitivity analysis shows that the increases in fires that would have been necessary to drive the changes in discharge are unrealistic. Of the potential drivers considered here, increasing northward transport of moisture as a result of global warming remains the most viable explanation for the observed increases in Eurasian Arctic river discharge. INDEX TERMS: 1655 Global Change: Water cycles (1836); 1803 Hydrology: Anthropogenic effects; 1833 Hydrology: Hydroclimatology; 1860 Hydrology: Runoff and streamflow; KEYWORDS: Arctic river discharge, global change Citation: McClelland, J. W., R. M. Holmes, B. J. Peterson, and M. Stieglitz (2004), Increasing river discharge in the Eurasian Arctic: Consideration of dams, permafrost thaw, and fires as potential agents of change, J. Geophys. Res., 109, D18102,
18342072
Siberian Lena River hydrologic regime and recent change
[1] The long-term (1935–1999) monthly records of temperature, precipitation, stream flow, river ice thickness, and active layer depth have been analyzed in this study to examine Lena River hydrologic regime and recent change. Remarkable hydrologic changes have been identified in this study. During the cold season (October–April), significant increases (25–90%) in stream flow and decrease in river ice thickness have been found due to warming in Siberia. In the snowmelt period (May–June), strong warming in spring leads to an advance of snowmelt season into late May and results in a lower daily maximum discharge in June. During summer months (July–September) the changes in stream flow hydrology are less significant in comparison to those for winter and spring seasons. A slight stream flow increase is discovered for both July and August, mainly owing to precipitation increase in May and June. Discharge in September has a slight downward trend due to precipitation decrease and temperature increase in August. The magnitudes of changes in stream flow and river ice thickness identified in this study are large enough to alter the hydrologic regime. Investigation into the hydrologic response of the Lena River to climate change and variation reveals strong linkages of stream flow with temperature and precipitation. We therefore believe that Lena River hydrologic regime changes are mainly the consequence of recent climate warming over Siberia and also closely related to changes in permafrost condition.
129343164
Simulation of reservoir influences on annual and seasonal streamflow changes for the Lena, Yenisei, and Ob' rivers
[1] Since the 1930s, combined streamflow from the largest Eurasian rivers discharging to the Arctic Ocean has been increasing. For many of these rivers, an increase in annual streamflow volume has been accompanied by a shift in seasonality (e.g., earlier snowmelt runoff in the spring). These changes in annual and seasonal streamflow may be due to direct effects of climate change (e.g., increased precipitation, or changes in snow accumulation and ablation patterns), indirect effects of climate change (e.g., changes in permafrost), or human effects (e.g., storage and release of river runoff in reservoirs). We develop and describe a method to estimate the potential contributions of artificial reservoirs to long-term changes in annual and seasonal streamflow between 1937 and 1998. Reservoir effects on downstream flow are simulated using a reservoir routing model coupled off-line to the Variable Infiltration Capacity (VIC) land surface hydrology model for the Lena, Yenisei and Ob' river basins. The effects of reservoirs on basin average evaporation are also represented. We perform trend analysis on long-term (≥30 years) time series of seasonal and annual streamflow and isolate the effects of reservoirs. Although reservoirs have had little effect on trends in annual discharge from the Lena, Yenisei, and Ob' river basins, we conclude that they are responsible for many of the seasonal changes that have been observed. For the Lena, reservoirs account for up to 80% and 30% of the observed winter and spring trends, respectively. For the Yenisei, reservoirs account for up to 100%, 40%, and 60% to 100% of the observed winter, spring, and late summer to early fall trends, respectively. For the Ob', reservoirs may account for more than 70% of the observed trends during the months of January to March. A result of this study is a set of reconstructed streamflow at the outlets of the Lena, Yenisei, and Ob' river basins which can be used in subsequent studies to improve the understanding of climate change effects on runoff generation in the Eurasian Arctic.
128824128
Discharge Characteristics and Changes over the Ob River Watershed in Siberia
Abstract This study analyzes long-term (1936–90) monthly streamflow records for the major subbasins within the Ob River watershed in order to examine discharge changes induced by human activities (particularly reservoirs and agricultural activities) and natural variations. Changes in streamflow pattern were found to be different between the upper and lower parts of the Ob watershed. Over the upper Ob basin, streamflow decreases in summer months and increases in the winter season. The decreases in summer are mainly due to water uses along the river valley for agricultural and industrial purposes and to reservoir regulation to reduce the summer peak floods. The increases in winter streamflow are caused by reservoir impacts to release water for power generation over winter months. In the lower Ob regions, however, streamflow increased during midsummer and winter months and weakly decreased in autumn. These increases in summer flow are associated with increases in summer precipitation and winter snow cover ov...
14188964
Temporal and spatial variations in maximum river discharge from a new Russian data set
[1] Floods cause more damage in Russia than any other natural disaster, and future climate model projections suggest that the frequency and magnitude of extreme hydrological events will increase in Russia with climate change. Here we analyze daily discharge records from a new data set of 139 Russian gauges in the Eurasian Arctic drainage basin with watershed areas from 16.1 to 50,000 km2 for signs of change in maximum river discharge. Several hypotheses about changes in maximum daily discharge and their linking with trends in precipitation over the cold season were tested. For the magnitude of maximum daily discharge we found relatively equal numbers of significant positive and negative trends across the Russian Arctic drainage basin, which draws into question the hypothesis of an increasing risk of extreme floods. We observed a significant shift to earlier spring discharge, which is consistent with documented changes in snowmelt and freeze-thaw dates. Spatial analysis of changes in maximum discharge and cold season precipitation revealed consistency across most of the domain, the exception being the Lena basin. Trends in maximum discharge of the small- to medium-sized rivers were generally consistent with aggregated signals found for the downstream gauges of the six largest Russian rivers. Although we observe regional changes in maximum discharge across the Russian Arctic drainage basin, no evidence of widespread trends in extreme discharge can be assumed from our analysis.
128561119
Application of New Precipitation and Reconstructed Streamflow Products to Streamflow Trend Attribution in Northern Eurasia
River runoff to the Arctic Ocean has increased over the last century, primarily during the winter and spring and primarily from the major Eurasian rivers. Some recent studies have suggested that the additional runoff is due to increased northward transport of atmospheric moisture (and associated increased precipitation), but other studies show inconsistencies in long-term runoff and precipitation trends, perhaps partly due to biases in the observational datasets. Through trend analysis of precipitation, temperature, and streamflow data, the authors investigate the extent to which Eurasian Arctic river discharge changes are attributable to precipitation and temperature changes as well as to reservoir construction and operation between the years of 1936 and 2000. Two new datasets are applied: a gridded precipitation product, in which the low-frequency variability is constrained to match that of long-term bias-corrected precipitation station data, and a reconstructed streamflow product, in which the effects of reservoirs have been minimized using a physically based reservoir model. It is found that reservoir operations have primarily affected streamflow seasonality, increasing winter discharge and decreasing summer discharge. To understand the influences of climate on streamflow changes, the authors hypothesize three cases that would cause precipitation trends to be inconsistent with streamflow trends: first, for the coldest basins in northeastern Siberia, streamflow should be sensitive to warming primarily as a result of the melting of excess ground ice, and for these basins positive streamflow trends may exceed precipitation trends in magnitude; second, evapotranspiration (ET) in the warmer regions of western Siberia and European Russia is sensitive to warming and increased precipitation, therefore observed precipitation trends may exceed streamflow trends; and third, streamflow from the central Siberian basins should respond to both effects. It is found that, in general, these hypotheses hold true. In the coldest basins, streamflow trends diverged from precipitation trends starting in the 1950s to 1960s, and this divergence accelerated thereafter. In the warmest basins, precipitation trends consistently exceeded streamflow trends, suggesting that increased precipitation contributed to increases in both ET and streamflow. In the central basins, permafrost degradation and ET effects appear to be contributing to long-term streamflow trends in varying degrees for each basin. The results herein suggest that the extent and state of the permafrost underlying a basin is a complicating factor in understanding long-term changes in Eurasian Arctic river discharge.
129933497
Streamflow Characteristics and Changes in Kolyma Basin in Siberia
Abstract This study documents major changes in streamflow hydrology over the Kolyma watershed due to climatic variations and human impacts. Streamflow seasonal cycles over the basin are characteristic of the northern region, with the lowest runoff in April and peak flow in June. Analyses of monthly flows and trends show that reservoir construction and operation have considerably affected streamflow regimes. Comparisons of mean monthly discharge records between pre- and post-1986 dam periods indicate that the mid–lower basin (downstream of the dam) experienced significant increase in low flows and decrease in peak flows after dam construction. For example, mean monthly flows during the post-dam period at the Ust’-Srednekan station (located 1423 km downstream of the dam) has strongly increased by about 205 m3 s−1 (or 522%–3157%) during December–April, and decreased by 133 m3 s−1 (41%) in June. Long-term monthly discharge data reveal an overall increase in streamflow during low flow seasons; the increase is ...
53468298
Compatibility analysis of precipitation and runoff trends over the large Siberian watersheds
[1] The consistency of long-term yearly precipitation and runoff trends over the largest Arctic watersheds (Ob, Yenisei and Lena Rivers) is examined. Three gridded precipitation datasets (Climatic Research Unit, University of Delaware, NCEP) are used for comparative analyses with runoff data collected at basin outlets. The results generally demonstrate inconsistency in long-term changes of basin precipitation and runoff. The Yenisei River runoff increases significantly, while precipitation data show mostly negative trends. The Ob River does not show any significant trend either in precipitation or runoff. Positive trend in the Lena River runoff is accompanied by a weak precipitation increase; however, the precipitation increase is not strong enough to support the observed runoff change. The inconsistency identified in basin precipitation and runoff trends suggests uncertainty in both the quality of basin precipitation and runoff datasets, as well as the perceived hydrologic factors impacting runoff change. INDEX TERMS: 9315 Information Related to Geographic Region: Arctic region; 1860 Hydrology: Runoff and streamflow; 1854 Hydrology: Precipitation (3354); 1833 Hydrology: Hydroclimatology; 1620 Global Change: Climate dynamics (3309). Citation: Berezovskaya, S., D. Yang, and D. L. Kane (2004), Compatibility analysis of precipitation and runoff trends over the large Siberian watersheds, Geophys. Res. Lett., 31, L21502, doi:10.1029/2004GL021277.
129460871
Intercomparison of four global precipitation data sets and their correlation with increased Eurasian river discharge to the Arctic Ocean
[1] Recent increases in Eurasian river discharge to the Arctic Ocean have attracted considerable scientific attention but remain poorly understood. Previous studies have examined fire frequency, permafrost thaw, and dam construction as potential mechanisms. Here we focus on precipitation as a driver, using 198 dam-free Eurasian river basins ranging from 151 to 897,000 km2. Using R-ArcticNet monthly discharge data and four observational and reanalysis precipitation products from the University of Delaware (UDel), University of Washington (UW), NCEP/NCAR (NCEP), and ECMWF (ERA-40), we (1) assess which precipitation data sets best capture spatially realistic patterns as inferred from agreement with river discharge (198 basins; 1958–1989); and (2) determine to what extent observed discharge trends follow Udel precipitation changes (66 basins; 1936–1999). Results from the precipitation intercomparison show for the 74 (of 198) basins displaying statistically significant discharge trends (24 positive, 50 negative; −74% to +89%, mean = −1%), interpolated precipitation products significantly outperform reanalysis data sets, perhaps owing to the fine-scale resolutions examined here. Agreement between discharge and precipitation is 42–86% and 42–97% for UDel and UW, respectively, but approaches zero for NCEP and ERA-40. Comparison of precipitation and discharge trends suggests that precipitation increases play a significant role in observed long-term discharge increases. For the 40 (of 66) basins displaying statistically significant trends in discharge (32 positive, 8 negative; −23% to +50%, mean = +11%), 29 display corresponding trends in precipitation with 35–62% agreement between discharge and precipitation trend. Comparison of discharge trends with basin permafrost properties indicates a possible, but not strong role for permafrost thaw in the observed increases.
128686697
The Changes in Russian Winter Snow Accumulation during 1936–83 and Its Spatial Patterns
Abstract Winter snow depth observations from 119 Russian stations during the years 1936–83 are selected. These irregularly spaced station data are then interpolated into 220 regular grids of 2° lat × 5.24° long that cover a region of 50°–70°N, 30°–140°E. The spatial variation patterns of the annual Russian winter snow accumulation during the period of 1936–83 are identified by using principal components analyses. Statistically significant trends in major snow depth variation patterns are detected. A method is constructed to estimate the spatial distributions of the total amount of snow depth change based on the significant trends of component scores during the period of 1936–83. The study found that snow depth has increased over most of northern Russia and decreased over most of southern Russia during the study period. Exceptions are found in northern European Russia, where a slight decrease in snow depth has occurred and in southern west Siberia where the snow depth has increased. The total amount of sno...
130171183
Changes in transitional snowfall season length in northern Eurasia
[1] An earlier study suggested that snow season length over large parts of northern Eurasia has increased due to earlier dates of first snowfall and later dates of last snowfall during 1937–94. In this study, much smaller increases in the length of continuous snow cover have been found over very limited areas of northern Eurasia, while decreases in continuous snow cover have been found over southern Siberia. The transitional snowfall season, defined by the difference between the first (last) snowfall date and the first (last) date of continuous ground snow cover, showed significant increases over much of Siberia. The increase in spring transitional snowfall season length is partly attributable to a later last snowfall date and partially due to earlier disappearance of continuous snow cover. The increase in fall transitional snowfall season length is mostly attributable to an earlier starting date of first snowfall.
16054880
Evaluation of trends in derived snowfall and rainfall across Eurasia and linkages with discharge to the Arctic Ocean
[1] To more fully understand the role of precipitation in observed increases in freshwater discharge to the Arctic Ocean, data from a new archive of bias-adjusted precipitation records for the former USSR (TD9813), along with the CRU and Willmott-Matsuura data sets, were examined for the period 1936–1999. Across the six largest Eurasian river basins, snowfall derived from TD9813 exhibits a strongly significant increase until the late 1950s and a moderately significant decrease thereafter. A strongly significant decline in derived rainfall is also noted. Spatially, snowfall increases are found primarily across north-central Eurasia, an area where the rainfall decreases are most prominent. Although no significant change is determined in Eurasian-basin snowfall over the entire 64 year period, we note that interpolation from early, uneven station networks causes an overestimation of spatial precipitation, and that the local snowfall trends determined from gridded TD9813 data are likely underestimated. Yet, numerous uncertainties in historical Arctic climate data and the sparse, irregular nature of Arctic station networks preclude a confident assessment of precipitation-discharge linkages during the period of reported discharge trends.
134209988
Bias corrections of long‐term (1973–2004) daily precipitation data over the northern regions
[1] A consistent daily bias correction procedure was applied at 4802 stations over high latitude regions (North of 45°N) to quantify the precipitation gauge measurement biases of wind-induced undercatch, wetting losses, and trace amount of precipitation for the last 30 years. These corrections have increased the gauge-measured monthly precipitation significantly by up to 22 mm for winter months, and slightly by about 5 mm during summer season. Relatively, the correction factors (CF) are small in summer (10%), and very large in winter (80–120%) because of the increased effect of wind on gauge undercatch of snowfall. The CFs also vary over space particularly in snowfall season. Significant CF differences were found across the USA/Canada borders mainly due to differences in catch efficiency between the national gauges. Bias corrections generally enhance monthly precipitation trends by 5–20%. These results point to a need to review our current understanding of the Arctic fresh water budget and its change.
59475246
Estimating continental and terrestrial precipitation averages from rain-gauge networks
© 1994 by the Royal Meteorological Society. Available from the publisher's website at http://dx.doi.org/10.1002/joc.3370140405
128845383
Assessment of contemporary Arctic river runoff based on observational discharge records
We describe the contemporary hydrography of the pan-Arctic land area draining into the Arctic Ocean, northern Bering Sea, and Hudson Bay on the basis of observational records of river discharge and computed runoff. The Regional Arctic Hydrographic Network data set, R-ArcticNET, is presented, which is based on 3754 recording stations drawn from Russian, Canadian, European, and U.S. archives. R-ArcticNET represents the single largest data compendium of observed discharge in the Arctic. Approximately 73% of the nonglaciated area of the pan-Arctic is monitored by at least one river discharge gage giving a mean gage density of 168 gages per 106 km2. Average annual runoff is 212 mm yr−1 with approximately 60% of the river discharge occurring from April to July. Gridded runoff surfaces are generated for the gaged portion of the pan-Arctic region to investigate global change signals. Siberia and Alaska showed increases in winter runoff during the 1980s relative to the 1960s and 1970s during annual and seasonal periods. These changes are consistent with observations of change in the climatology of the region. Western Canada experienced decreased spring and summer runoff.
129797696
Widespread decline in hydrological monitoring threatens Pan-Arctic Research
Operational river discharge monitoring is declining in both North America and Eurasia. This problem is especially severe in the Far East of Siberia and the province of Ontario, where 73% and 67% of river gauges were closed between 1986 and 1999, respectively. These reductions will greatly affect our ability to study variations in and alterations to the pan-Arctic hydrological cycle.
131577592
Streamflow changes over Siberian Yenisei River Basin
Abstract This study analyzes long-term (1935–99) monthly discharge data for the major sub-basins within the Yenisei River watershed in order to document significant streamflow changes induced by reservoir regulations and by natural variations/changes. The results show that both the unregulated upper basin and major lower streams of the watershed experienced streamflow decreases in the early melt period and discharge increases in the late melt season. These changes in snowmelt runoff pattern suggest a delay in snowcover melt in the Yenisei basin perhaps associated with cooling trends during the snowmelt months over central Siberia. This study also demonstrates that the reservoir regulation has significantly altered the monthly discharge regimes in northeast and the upper portions of the Yenisei basin. Constructions of four large dams in the northeast Yensiei regions reduced the summer peak flows in the Angara valley by 15–30% and increased the winter low flows by 5–30%. Operations of two large reservoirs in the upper Yenisei regions enhanced the winter flows by 45–85% and reduced the summer flows by 10–50%. These alterations lead to a streamflow regime change toward less seasonal variation over the eastern and lower Yenisei basin. Because of reservoir regulations, discharge records collected at the Yenisei basin outlet do not always represent natural changes and variations, they tend to underestimate the natural streamflow trends in summer and overestimate the trends in winter and fall seasons. Cold season discharge increase over the Yenisei river is not natural-caused, but mainly the effect of reservoir regulations in the Yenisei basin.
56012220
A pan‐arctic evaluation of changes in river discharge during the latter half of the 20th century
[1] Several recent publications have documented changes in river discharge from arctic and subarctic watersheds. Comparison of these findings, however, has been hampered by differences in time periods and methods of analysis. Here we compare changes in discharge from different regions of the pan-arctic watershed using identical time periods and analytical methods. Discharge to the Arctic Ocean increased by 5.6 km3/y/y during 1964–2000, the net result of a large increase from Eurasia moderated by a small decrease from North America. In contrast, discharge to Hudson/James/Ungava Bays decreased by 2.5 km3/y/y during 1964–2000. While this evaluation identifies an overall increase in discharge (∼120 km3/y greater discharge at the end of the time period as compared to the beginning for Hudson/James/Unvaga Bays and the Arctic Ocean combined), the contrasting regional trends also highlight the need to understand the consequences of adding/removing freshwater from particular regions of the arctic and subarctic oceans.
140681903
Large‐scale hydro‐climatology of the terrestrial Arctic drainage system
[1] The large-scale hydro-climatology of the terrestrial Arctic drainage system is examined, focusing on the period 1960 onward. Special attention is paid to the Ob, Yenisey, Lena, and Mackenzie watersheds, which provide the bulk of freshwater discharge to the Arctic Ocean. Station data are used to compile monthly gridded time series of gauge-corrected precipitation (P). Gridded time series of precipitation minus evapotranspiration (P−ET) are calculated from the moisture flux convergence using NCEP reanalysis data. Estimates of ET are obtained as a residual. Runoff (R) is obtained from available discharge records. For long-term water-year means, P−ET for the Yenisey, Lena, and Mackenzie watersheds is 16–20% lower than the observed runoff. In the Ob watershed, the two values agree within 9%. Given the uncertainties in P−ET, we consider the atmospheric and surface water budgets to be reasonably closed. Compared to the other three basins, the mean runoff ratio (R/P) is lower in the Ob watershed, consistent with the high fraction of annual precipitation lost through ET. All basins exhibit summer maxima in P and minima in P−ET. Summer P−ET in the Ob watershed is negative due to high ET rates. For large domains in northern Eurasia, about 25% of July precipitation is associated with the recycling of water vapor evapotranspirated within each domain. This points to a significant effect of the land surface on the hydrologic regime. Variability in P and P−ET has generally clear associations with the regional atmospheric circulation. A strong link with the Urals trough is documented for the Ob. Relationships with indices of the Arctic Oscillation and other teleconnections are generally weak. Water-year time series of runoff and P−ET are strongly correlated in the Lena watershed only, reflecting extensive permafrost. Cold-season runoff has increased in the Yenisey and Lena watersheds. This is most pronounced in the Yenisey watershed, where runoff has also increased sharply in spring, decreased in summer, but has increased for the year as a whole. The mechanisms for these changes are not entirely clear. While they fundamentally relate to higher air temperatures, increased winter precipitation, and strong summer drying, we speculate links with changes in active layer thickness and thawing permafrost.
129756645
State of the ground : Climatology and changes during the past 69 years over northern eurasia for a rarely used measure of snow cover and frozen land
Abstract Significant climatic changes over northern Eurasia during the twentieth century are revealed in numerous variables including those affecting and characterizing the state of the cryosphere. In addition to commonly used in situ observations of snow cover such as snow depth and snow courses, synoptic archives in the former Soviet Union contain regular daily and semidaily reports about the state of the ground in the area surrounding the station. Information about frozen, dry, wet, ponded, and snow-covered land, and in the case of snow-covered land, about the characteristics of snow cover, is available in these reports. A new Global Synoptic Data Network (GSDN) consisting of 2100 stations within the boundaries of the former Soviet Union created jointly by the National Climatic Data Center (NCDC) and Russian Institute for Hydrometeorological Information (RIHMI) was used to assess the climatology of snow cover, frozen and unfrozen ground reports, and their temporal variability for the period from 1936 t...
4797539
Rising minimum daily flows in northern Eurasian rivers: A growing influence of groundwater in the high‐latitude hydrologic cycle
[1] A first analysis of new daily discharge data for 111 northern rivers from 1936–1999 and 1958–1989 finds an overall pattern of increasing minimum daily flows (or “low flows”) throughout Russia. These increases are generally more abundant than are increases in mean flow and appear to drive much of the overall rise in mean flow observed here and in previous studies. Minimum flow decreases have also occurred but are less abundant. The minimum flow increases are found in summer as well as winter and in nonpermafrost as well as permafrost terrain. No robust spatial contrasts are found between the European Russia, Ob', Yenisey, and Lena/eastern Siberia sectors. A subset of 12 unusually long discharge records from 1935–2002, concentrated in south central Russia, suggests that recent minimum flow increases since ∼1985 are largely unprecedented in the instrumental record, at least for this small group of stations. If minimum flows are presumed sensitive to groundwater and unsaturated zone inputs to river discharge, then the data suggest a broad-scale mobilization of such water sources in the late 20th century. We speculate that reduced intensity of seasonal ground freezing, together with precipitation increases, might drive much of the well documented but poorly understood increases in river discharge to the Arctic Ocean.
140544299
Increasing winter baseflow and mean annual streamflow from possible permafrost thawing in the Northwest Territories, Canada
[1] Increasing surface air temperatures from anthropogenic forcing are melting permafrost at high latitudes and intensifying the hydrological cycle. Long-term streamflow records (≥30 yrs) from 23 stream gauges in the Canadian Northwest Territories (NWT) indicate a general significant upward trend in winter baseflow of 0.5–271.6 %/yr and the beginning of significant increasing mean annual flow (seen at 39% of studied gauge records), as assessed by the Kendall-τ test. The NWT exports an average discharge of ≥308.6 km3/yr to the Beaufort Sea, of which ≥120.9 km3/yr is baseflow. We propose that the increases in winter baseflow and mean annual streamflow in the NWT were caused predominately by climate warming via permafrost thawing that enhances infiltration and deeper flowpaths and hydrological cycle intensification. To provide hydroclimatic context, we present evidence of a statistically significant positive link between the Northern annular mode and annual NWT streamflow at the interannual-to-decadal timescales.
129728666
Response of Northern Hemisphere extratropical cyclone activity and associated precipitation to climate change, as represented by the Community Climate System Model
[1] The projected effects of rising CO2 levels on Northern Hemisphere extratropical cyclone activity and cyclone-associated precipitation are examined for September–May, using output from version 3 of the Community Climate System Model (CCSM3). A cyclone identification algorithm was applied to a five member ensemble of CCSM3 20th and 21st century output, along with a method of isolating precipitation produced by each cyclone. Mean seasonal statistics describing cyclone activity and the character of associated precipitation were calculated over several study regions for 20 a periods. The dominant change in cyclone activity is a marked midlatitude decrease in frequency during autumn, winter, and spring. Few significant shifts in storm tracks or cyclone intensity were identified. Total daily precipitation from these events is found to increase into the 21st century, largely because of increases in available atmospheric moisture with rising temperatures. This thermodynamic increase in precipitation leads to large rises in total seasonal cyclone-associated precipitation over high latitudes, while over midlatitudes the thermodynamic increase is offset by the dynamic effect associated with decreased cyclone frequency.
59377392
Historical trends in the jet streams
[1] Jet streams, the meandering bands of fast winds located near the tropopause, are driving factors for weather in the midlatitudes. This is the first study to analyze historical trends of jet stream properties based on the ERA-40 and the NCEP/NCAR reanalysis datasets for the period 1979 to 2001. We defined jet stream properties based on mass and mass-flux weighted averages. We found that, in general, the jet streams have risen in altitude and moved poleward in both hemispheres. In the northern hemisphere, the jet stream weakened. In the southern hemisphere, the sub-tropical jet weakened, whereas the polar jet strengthened. Exceptions to this general behavior were found locally and seasonally. Further observations and analysis are needed to confidently attribute the causes of these changes to anthropogenic climate change, natural variability, or some combination of the two. Citation: Archer, C. L., and K. Caldeira (2008), Historical trends in the jet streams, Geophys. Res. Lett., 35, L08803, doi:10.1029/2008GL033614.
54953175
The large‐scale freshwater cycle of the Arctic
This paper synthesizes our understanding of the Arctic's large-scale freshwater cycle. It combines terrestrial and oceanic observations with insights gained from the ERA-40 reanalysis and land surface and ice-ocean models. Annual mean freshwater input to the Arctic Ocean is dominated by river discharge (38%), inflow through Bering Strait (30%), and net precipitation (24%). Total freshwater export from the Arctic Ocean to the North Atlantic is dominated by transports through the Canadian Arctic Archipelago (35%) and via Fram Strait as liquid (26%) and sea ice (25%). All terms are computed relative to a reference salinity of 34.8. Compared to earlier estimates, our budget features larger import of freshwater through Bering Strait and larger liquid phase export through Fram Strait. While there is no reason to expect a steady state, error analysis indicates that the difference between annual mean oceanic inflows and outflows (∼8% of the total inflow) is indistinguishable from zero. Freshwater in the Arctic Ocean has a mean residence time of about a decade. This is understood in that annual freshwater input, while large (∼8500 km3), is an order of magnitude smaller than oceanic freshwater storage of ∼84,000 km3. Freshwater in the atmosphere, as water vapor, has a residence time of about a week. Seasonality in Arctic Ocean freshwater storage is nevertheless highly uncertain, reflecting both sparse hydrographic data and insufficient information on sea ice volume. Uncertainties mask seasonal storage changes forced by freshwater fluxes. Of flux terms with sufficient data for analysis, Fram Strait ice outflow shows the largest interannual variability.
130384526
Regional sensitivities of mean and peak river discharge to climate variability in Europe
[1] Variability of atmospheric circulation is thought to be the most important factor causing annual and decadal variability of fresh water fluxes from the continents. Previous studies, however, have rarely established the sensitivity of the basins to atmospheric circulation variability. Here we present an analysis of long-term (>30 years) links between atmospheric forcing and winter (December–February) precipitation, and sensitivities of annual mean and maximum winter discharges observed at up to 608 stations across Europe. Links to four atmospheric indices are examined: the North Atlantic Oscillation (NAO) index, the Arctic Oscillation (AO) index, the frequency of west circulation (FWC) as described by the subjective Groswetterlagen classification, and the north to south sea level pressure difference across the European continent (SLPD). The results show that annual maximum discharges are more sensitive to variability of atmospheric circulation than mean discharges. Mean discharges vary on average between 8 and 44%, while peak discharges vary between 10 and 54% per unit index change. Discharges in Iberia and Scandinavia are more sensitive than those in central and northwest Europe. Discharge closely follows variability of atmospheric circulation. Compared with FWC and SLPD, the NAO and AO indices have only limited use for analyzing climate impacts in river basins in northwest Europe.
62879350
Recent radical shifts of atmospheric circulations and rapid changes in Arctic climate system
[1] Arctic climate system change has accelerated tremendously since the beginning of this century, and a strikingly extreme sea-ice loss occurred in summer 2007. However, the greenhouse-gas-emissions forcing has only increased gradually and the driving role in Arctic climate change of the positively-polarized Arctic/North Atlantic Oscillation (AO/NAO) trend has substantially weakened. Although various contributing factors have been examined, the fundamental physical process, which orchestrates these contributors to drive the acceleration and the latest extreme event, remains unknown. We report on drastic, systematic spatial changes in atmospheric circulations, showing a sudden jump from the conventional tri-polar AO/NAO to an unprecedented dipolar leading pattern, following accelerated northeastward shifts of the AO/NAO centers of action. These shifts provide an accelerating impetus for the recent rapid Arctic climate system changes, perhaps shedding light on recent arguments about a tipping point of global-warming-forced climate change in the Arctic. The radical spatial shift is a precursor to the observed extreme change event, demonstrating skilful information for future prediction.