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17 pags., 4 figs., 2 tabs., Sea ice plays a pivotal role in Earth's climate and its past reconstruction is crucial to investigate the connections and feedbacks with the other components of the climate system. Among the available archives that store information of past sea ice are marine and ice cores. Recent studies on the IP biomarker extracted from marine sediments has shown great skill to infer past changes of Arctic sea ice. In ice matrixes, sodium, bromine and iodine have shown potential to store the fingerprint of sea ice presence. The development of an unambiguous sea ice proxy from ice cores, however, has proven to be a challenging task especially in the Arctic realm. In this work we analyze the sodium, bromine and iodine records in the RECAP ice core, coastal eastern Greenland, to investigate the sea ice variability in the northern North Atlantic Ocean through the last 11,000 years of the current interglacial, i.e. the Holocene. We compare the RECAP records with marine sea ice proxy records available from the northern North Atlantic. We suggest that RECAP sodium concentrations can be associated with variability of sea ice extent, while the bromine-to-sodium ratios and iodine are associated respectively with seasonal sea ice and bioproductivity from open ocean and fresh sea ice surfaces. According to our interpretation, we find that sea ice was at its lowest extent and seasonal in nature during the early Holocene in all regions of the North Atlantic. Increasing sea ice signals are seen from ca. 8–9 ka b2k, in line with long-term Holocene cooling. The increasing sea ice trend appears uninterrupted in the Fram Strait and North Iceland while reaching a maximum ca. 5 ka b2k in the East Greenland region. Sea ice modifications during the last 5000 years display great variability in East Greenland with intermediate conditions between the early and mid Holocene, possibly associated with local fjord dynamics. The last sea ice maximum was reached across all regions 1000 years b2k., The RECAP ice coring effort was financed by the Danish Research Council through a Sapere Aude grant, the NSF through the Division of Polar Programs, the Alfred Wegener Institute, and the European Research Council under the European Community's Seventh Framework Programme (FP7/2007e2013)/ERC grant agreement The RECAP ice coring effort was financed by the Danish Research Council through a Sapere Aude grant, the NSF through the Division of Polar Programs, the Alfred Wegener Institute, and the European Research Council under the European Community's Seventh Framework Programme (FP7/2007e2013)/ERC grant agreement 610055 through the Ice2Ice project. Additional funding support for the paper has been provided by the Centre for Climate Dynamics at the Bjerknes Centre. through the Ice2Ice project. Additional funding support for the paper has been provided by the Centre for Climate Dynamics at the Bjerknes Centre.

Although there is a growing body of research on Arctic urbanisation and the development of cities in harsh polar climate, few studies focus on the long-term interaction of urban environments with t...

Century-long anomaly of Arctic sea-ice export leading the Little Ice Age may have arisen without external forcing., Arctic sea ice affects climate on seasonal to decadal time scales, and models suggest that sea ice is essential for longer anomalies such as the Little Ice Age. However, empirical evidence is fragmentary. Here, we reconstruct sea ice exported from the Arctic Ocean over the past 1400 years, using a spatial network of proxy records. We find robust evidence for extreme export of sea ice commencing abruptly around 1300 CE and terminating in the late 1300s. The exceptional magnitude and duration of this “Great Sea-Ice Anomaly” was previously unknown. The pulse of ice along East Greenland resulted in downstream increases in polar waters and ocean stratification, culminating ~1400 CE and sustained during subsequent centuries. While consistent with external forcing theories, the onset and development are notably similar to modeled spontaneous abrupt cooling enhanced by sea-ice feedbacks. These results provide evidence that marked climate changes may not require an external trigger.

Exploration and exploitation of oil and gas reserves of northern West Siberia has promoted rapid industrialization and urban development in the region. This development leaves significant footprints on the sensitive northern environment, which is already stressed by the global warming. This study reports the region-wide changes in the vegetation cover as well as the corresponding changes in and around 28 selected urbanized areas. The study utilizes the normalized difference vegetation index (NDVI) from high-resolution (250 m) MODIS data acquired for summer months (June through August) during 15 years (2000–2014). The results reveal the increase of NDVI (or “greening”) over the northern (tundra and tundra-forest) part of the region. Simultaneously, the southern, forested part shows the widespread decrease of NDVI (or “browning”). These region-wide patterns are, however, highly fragmented. The statistically significant NDVI trends occupy only a small fraction of the region. Urbanization destroys the vegetation cover within the developed areas and at about 5–10 km distance around them. The studied urbanized areas have the NDVI values by 15 % to 45 % lower than the corresponding areas at 20–40 km distance. The largest NDVI reduction is typical for the newly developed areas, whereas the older areas show recovery of the vegetation cover. The study reveals a robust indication of the accelerated greening near the older urban areas. Many Siberian cities become greener even against the wider browning trends at their background. Literature discussion suggests that the observed urban greening could be associated not only with special tending of the within-city green areas but also with the urban heat islands and succession of more productive shrub and tree species growing on warmer sandy soils.

Rapid changes in Helheim Glacier and other Greenland outlet glaciers since 2000 are well-known, but knowledge on earlier decades is fragmentary. Here we exploit the satellite image archives to produce and analyze a monthly-to-seasonal record of Helheim Glacier front position, 1980–2011. Statistical analysis identifies decadal periods with abrupt changes in variability and mean. The record also reveals evidence of volatile advance/retreat behavior in the 1980s. In one of several cases of large-amplitude subannual changes, the glacier front ‘surged’ forward in 1984/85, advancing ~6 km within a few months – surpassing its Little Ice Age maximum position – and afterward retreated ~5 km within a few weeks. These findings challenge the prevailing view of front position stability in the decades before the multi-year retreat in the early 2000s. Cold conditions including rigid ice mélange appear to be a factor in the high-amplitude seasonal advances in the 1980s. However the magnitude and abruptness of the changes in the record cannot be explained solely as a climatic response, such that glacio-dynamics must be invoked. Further, the volatile advance/retreat behavior in the cold 1980s resulted in increased dynamic ice loss, complicating the interpretation of increased calving activity as a response to warming.

This paper examines the multidecadal variability in the northern North Atlantic and the Arctic. Observations, modeling and paleo data provide evidence of a strong link between the atmospheric and physical oceanographic variability in these northern regions with Atlantic sea surface temperatures farther south as expressed by the Atlantic Multidecadal Oscillation (AMO). Air and sea temperatures over the past 100–150 years reveal cool periods in the late 1800s to early 1900s and in the 1970s to 1980s with warm periods during the 1920s to 1960s and from the 1990s through to the present, similar to the variability in the AMO index where a positive (negative) AMO index represents warm (cold) periods. Sea-ice extent in the north has also varied at multidecadal scales with the ice retreating during the warm periods and expanding during the cold periods. The presence of multidecadal variability is also suggested from marine sediment paleo data as well as ice-core oxygen isotope data. Observations of biological impacts of the multidecadal variability in the northern regions include a general increase in plankton and fish productivity, as well as expansion of the species distributions northward, in conjunction with the AMO warm periods and the opposite during AMO cold periods. In addition, a review of the mechanisms responsible for the AMO and a brief discussion of the linkages between the multidecadal variability in the northern and southern hemispheres, including between the Arctic and Antarctic, are presented.

Satellite studies using the normalized difference vegetation index (NDVI) have revealed changes in northern Eurasian vegetation productivity in recent decades, including greening in tundra and browning in the boreal forests. However, NDVI trends and relationships to climate depend on the temporal and spatial sampling, data and methods, underscoring the complexity of understanding the variability of vegetation productivity. Here utilizing Moderate Resolution Imaging Spectrometer (MODIS) data (2000–16), we perform a consistent analysis of NDVI and climate across four bioclimatic zones (tundra, forest–tundra, middle taiga and northern taiga) in northern West Siberia, further stratified into eight forest–landcover types. The study reveals that positive trends are most prevalent in the unforested and in the needleleaf deciduous forest (larch) within the tundra and the forest–tundra biomes, whereas negative trends are more prevalent in the coniferous forests in the taiga biome. Correlation analysis of NDVImax against SAT and precipitation averaged across each biome found two statistically significant results – between tundra and SAT, and middle taiga and precipitation. Spatial composites of NDVI and climate variables were effective for revealing insights and patterns of relationships. There was tundra greening in warmer summers, except when moisture is severely limited, e.g., 2016 record warmth coincident with negative anomalies in precipitation and soil moisture. The middle taiga has low NDVI in negative precipitation anomalies regardless of temperature, and high NDVI in moist and warm summers, i.e., higher temperature does not result in forest browning if moisture is adequate – no direct temperature stress. The transitional biomes share characteristics of their respective proximal biomes when greening, i.e., forest–tundra responds positively to higher temperatures, and northern taiga responds positively to precipitation. Spatial heterogeneity of NDVI patterns indicates divergent reactions of specific types of vegetation as well as local effects are clearly important on the background of regional climate response.

Satellite data suggest an Arctic sea ice-climate system in rapid transformation, yet its long-term natural modes of variability are poorly known. Here we integrate and synthesize a set of multicentury historical records of Atlantic Arctic sea ice, supplemented with high-resolution paleoproxy records, each reflecting primarily winter/spring sea ice conditions. We establish a signal of pervasive and persistent multidecadal (~60–90 year) fluctuations that is most pronounced in the Greenland Sea and weakens further away. Covariability between sea ice and Atlantic multidecadal variability as represented by the Atlantic Multidecadal Oscillation (AMO) index is evident during the instrumental record, including an abrupt change at the onset of the early twentieth century warming. Similar covariability through previous centuries is evident from comparison of the longest historical sea ice records and paleoproxy reconstructions of sea ice and the AMO. This observational evidence supports recent modeling studies that have suggested that Arctic sea ice is intrinsically linked to Atlantic multidecadal variability. This may have implications for understanding the recent negative trend in Arctic winter sea ice extent, although because the losses have been greater in summer, other processes and feedbacks are also important.

Arctic amplification of temperature change is theorised to be an important feature of the Earth’s climate system. For observational assessment and understanding of mechanisms of this amplification, which remain uncertain, thorough and detailed analyses of surface air temperature (SAT) variability and trends in the Arctic are needed. Here we present an analysis of Arctic SAT variability in comparison with mid-latitudes and the Northern Hemisphere (NH), based on an advanced SAT dataset – NansenSAT. We define an index for the Arctic amplification as the ratio between absolute values of the Arctic (65–90°N) and NH 30-yr running linear SAT trends. It is demonstrated that the temperature amplification in the Arctic is characteristic not only for the recent warming but also the early 20th century warming (ETCW) and subsequent cooling. The amplification appears to be weaker during the recent warming than in the ETCW, simply because the index values reflect the more pervasive nature of the recent warming that reflects the background of anthropogenic global warming. We also produced a new Arctic regionalisation created from hierarchical cluster analysis, which identifies six major natural regions in the Arctic that reflect SAT variability. Statistical comparison with several climate indices shows that the Atlantic Multidecadal Oscillation (AMO) is the mode of variability that is most significantly associated with the amplified warming–cooling in the Arctic, with a stronger correlation during the ETCW and recent warming than during the intermediate period. Regionally, differences are identified in terms of annual and seasonal rates of change and in their correlations with modes of variability. Keywords: Arctic amplification, surface air temperature, cluster analysis, modes of variability (Published: 17 May 2016) Citation: Tellus A 2016, 68, 28234, http://dx.doi.org/10.3402/tellusa.v68.28234

The Arctic has undergone substantial changes over the last few decades in various cryospheric and derivative systems and processes. Of these, the Arctic sea ice regime has seen some of the most rapid change and is one of the most visible markers of Arctic change outside the scientific community. This has drawn considerable attention not only from the natural sciences, but increasingly, from the political and commercial sectors as they begin to grapple with the problems and opportunities that are being presented. The possible impacts of past and projected changes in Arctic sea ice, especially as it relates to climatic response, are of particular interest and have been the subject of increasing research activity. A review of the current knowledge of the role of sea ice in the climate system is therefore timely. We present a review that examines both the current state of understanding, as regards the impacts of sea-ice loss observed to date, and climate model projections, to highlight hypothesised future changes and impacts on storm tracks and the North Atlantic Oscillation. Within the broad climate-system perspective, the topics of storminess and large-scale variability will be specifically considered. We then consider larger-scale impacts on the climatic system by reviewing studies that have focused on the interaction between sea-ice extent and the North Atlantic Oscillation. Finally, an overview of the representation of these topics in the literature in the context of IPCC climate projections is presented. While most agree on the direction of Arctic sea-ice change, the rates amongst the various projections vary greatly. Similarly, the response of storm tracks and climate variability are uncertain, exacerbated possibly by the influence of other factors. A variety of scientific papers on the relationship between sea-ice changes and atmospheric variability have brought to light important aspects of this complex topic. Examples are an overall reduction in the number of Arctic winter storms, a northward shift of mid-latitude winter storms in the Pacific and a delayed negative NAO-like response in autumn/winter to a reduced Arctic sea-ice cover (at least in some months). This review paper discusses this research and the disagreements, bringing about a fresh perspective on this issue. Highlights: ► Observed decrease in sea-ice extent is faster than predicted. ► September sea ice reaching record low in recent years. ► Reduction in the number of Arctic winter storms. ► Northward shift of midlatitude storms and uncertainty over changes in intensity. ► Most models simulate a negative NAO response when forced with less Arctic sea-ice.

Recent observations of ocean temperature in several Greenland fjords suggest that ocean warming can cause large changes in the outlet glaciers in these fjords. We have observed the Helheim outlet-glacier front in the Sermilik Fjord over the last three decades using satellite images, and the vertical fjord temperature and salinity during three summer expeditions, 2008-2010. We show that the subsurface water below 250 m depth is the warm saline Atlantic Water from the Irminger Sea penetrating into the fjord and exposing the lower part of the Helheim glacier to warm water up to 4C. Lagged correlation analysis spanning the 30-year time series, using the subsurface Atlantic Water temperature off the coast as a proxy for the variability of the subsurface warm Atlantic Water in the fjord, indicates that 24% of the Helheim ice-front movement can be accounted for by ocean temperature. A strong correlation (-0.75) between the ice-front position and the surface air temperature from a nearby meteorological station suggests that the higher air temperature causes melting and subsequent downward percolation of meltwater through crevasses leading to basal lubrication; the correlation accounts for 56% of the ice-front movement. The precise contribution of air temperature versus ocean temperature however, remains an open question, as more oceanographic and meteorological measurements are needed close to the glacier terminus.

Traditionally, turbulence energetics is characterized by turbulent kinetic energy (TKE) and modelled using solely the TKE budget equation. In stable stratification, TKE is generated by the velocity shear and expended through viscous dissipation and work against buoyancy forces. The effect of stratification is characterized by the ratio of the buoyancy gradient to squared shear, called Richardson number, Ri. It is widely believed that at Ri exceeding a critical value, Ric, local shear cannot maintain turbulence, and the flow becomes laminar. We revise this concept by extending the energy analysis to turbulent potential and total energies (TPE and TTE = TKE + TPE), consider their budget equations, and conclude that TTE is a conservative parameter maintained by shear in any stratification. Hence there is no "energetics Ric", in contrast to the hydrodynamic-instability threshold, Ric-instability, whose typical values vary from 0.25 to 1. We demonstrate that this interval, 0.25>1, clarify principal difference between turbulent boundary layers and free flows, and provide basis for improving operational turbulence closure models., Comment: 23 pages, 4 figures, Quarterly Journal of Royal Meteorological Society, in press

A continuous data set of Greenland Ice Sheet altimeter height from European Remote Sensing satellites (ERS-1 and ERS-2), 1992 to 2003, has been analyzed. An increase of 6.4 +/- 0.2 centimeters per year (cm/year) is found in the vast interior areas above 1500 meters, in contrast to previous reports of high-elevation balance. Below 1500 meters, the elevation-change rate is -2.0 +/- 0.9 cm/year, in qualitative agreement with reported thinning in the ice-sheet margins. Averaged over the study area, the increase is 5.4 +/- 0.2 cm/year, or approximately 60 cm over 11 years, or approximately 54 cm when corrected for isostatic uplift. Winter elevation changes are shown to be linked to the North Atlantic Oscillation.

This letter presents an analysis of a new homogenized time series of daily mean sea level pressure (SLP) from Reykjavik and Stykkisholmur, Iceland from 1823–1999. Time series statistical techniques including harmonic analysis are used to identify the seasonal march of pressure and its variability through the record. The results are assessed regarding the North Atlantic Oscillation (NAO) index. In addition to the annual cycle, the seasonal march of SLP has an appreciable semi-annual cycle, as well as anomalies (abrupt rises and falls) occurring at remarkably distinct times in spring and winter. Multidecadal variability is apparent in the seasonal cycle and the anomalies. A shift in intraseasonal variability is observed in recent decades, including an extension of the winter Icelandic Low enhancement period into March. Increasing failure or delay of the “usual” abrupt pressure rise in Iceland in late February has contributed to the unusually positive winter NAO index values since the 1960s.

The distribution of openings (leads and polynyas) in polar sea ice is not well known. This study estimates the large-scale distribution and variability of leads in the Arctic of the western hemisphere in winter, using a 5-year record of Defense Meteorological Satellite Program thermal- and visible-band imagery. The occurrence (density) and orientation of leads are derived from gridded maps made at 10-day intervals. Their mean value and interannual, seasonal, and monthly variabilities are estimated. Lead densities are observed to be highest in early winter, decreasing 20% from November through April. The highest densities are observed in the central Canada Basin, and the lowest are in the East Siberian Sea. There is limited interannual variability in the positions of maximum and minimum densities. Preferred lead orientations are identified as generally north-south in the Beaufort Sea sector and east-west in the East Siberian Sea sector, with transitional orientations in the intermediate area. The mean distributions of lead density and orientation are observed to be associated with large-scale mean fields of ice divergence and shear, respectively.

A 20-year time series from the Norwegian Coastal Current was explored to examine the effects of advected nutrient supply from the southern North Sea and of large-scale climate variability on hydrography, nutrients and particulate organic matter (seston), focusing on trends in the January to April period in the upper layers (0–30 m). The interannual variability in hydrography, nutrients and seston was correlated with the NAO index, mostly through the inflow of nutrient-rich waters from the southern North Sea. There was a long-term decrease in nutrient concentrations, which according to a water mass analysis followed a reduction in nutrients advected from the German Bight and southern North Sea. The concentrations of carbon and nitrogen in seston, dissolved organic nitrogen and the estimated fraction of non-autotrophic material increased significantly and non-linearly through a sharp transition between 1998 and 2000, and have remained at this level since. Humic coagulation was suggested as the mechanism behind the increase in the non-autotrophic fraction of seston, which could be connected with the reported “darkening” of the coastal Skagerrak and Baltic Sea. Concurrent with the thresholds in suspended material, a decimation of the sugar kelp forest and recruitment failure of key carnivorous fish was reported for the same region, suggesting that a regime shift took place in the early 2000s in the coastal waters of the Norwegian Skagerrak. Our data suggests that the effects of increased freshwater runoff, especially the increased inputs of terrestrial-derived, humic material, could play an important role in the observed, coastal responses.

Active and passive microwave sensor data from satellites are useful for studies of the sea ice in the polar regions, including the Eurasian Arctic. Their ability to acquire data regardless of darkness or cloudiness is an essential attribute in these regions. Here the scientific objective is to use the European Remote Sensing satellite (ERS-1) ERS-1 Synthetic Aperture Radar (SAR) data to investigate various sea ice phenomena and processes in the Kara Sea north of Russia. The technical objective is to assess the usefulness of ERS-1 SAR and Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) data for near-real-time monitoring of the ice conditions along the Northern Sea Route (the Northeast Passage) in support of navigation. The SAR data are seen to be very useful for small-scale ice studies, whereas the SSM/I data provide large-scale ice information, including the ice edge position. The SAR data are acquired in near real-time from the Tromso Satellite Station in order to transmit detailed sea ice maps to icebreakers operating in the Kara Sea. The use of SAR and SSM/I data in support of ice navigation there has been very successful.

The Arctic is part of the global climate system. To address the issue of climate, the fluxes of heat, salt, and fresh water must be considered. One of the most speculated reasons for rapid climate change in the subarctic North Atlantic, and the global conveyor belt, is a breakdown of the thermohaline circulation (THC) due to an increased fresh water supply. Whitehead’s (Estuaries 21:281–293, 1998) one-box dynamic model is used to show how multiple states and catastrophe can occur in the Arctic Mediterranean with variable freshening and cooling. The broader question is how this interacts with the global climate. In this chapter, we focus on the oceanic aspects of the arctic climate system, discuss processes, review the data, and speculate on the role this part of the globe has in the greater context of global climate. The interaction with the global system comprises the outflow of freshwater and ice, and deeper, freshened, and cooled seawater into the subarctic North Atlantic, via the Labrador Sea. An example of significant climate variability in the twentieth century is presented.

Greenland's marine-terminating glaciers drain large amounts of solid ice through calving of icebergs, as well as melting of floating glacial ice. Petermann Glacier, North Greenland, has the Northern Hemisphere's long floating ice shelf. A massive (~270 km2) calving event was observed from satellite sensors in August 2010. In order to understand this in perspective, here we perform a comprehensive retrospective data analysis of Petermann Glacier calving-front variability spanning half a century. Here we establish that there have been at least four massive (100+ km2) calving events over the past 50 years: (1) 1959–1961 (~153 km2), (2) 1991 (~168 km2), (3) 2001 (~71 km2) and (4) 2010 (~270 km2), as well as ~31 km2 calved in 2008. The terminus position in 2010 has retreated ~15 km beyond the envelope of previous observations. Whether the massive calving in 2010 represents natural episodic variability or a response to global and/or ocean warming in the fjord remains speculative, although this event supports the contention that the ice shelf recently has become vulnerable due to extensive fracturing and channelized basal melting.

The objectives of this paper are to summarise: (1) observed 20th-century and projected 21st-century changes in key components of the Arctic climate system and (2) probable impacts on the Arctic marine environment, with emphasis on the vulnerabilities of marine and sea ice–based ecosystems. Multi-decadal to century-scale observational data sets of surface air temperature (SAT) and sea ice indicate that the two pronounced 20th-century warming events, both amplified in the Arctic, were linked to sea-ice variability. Arctic sea-ice coverage has decreased *8% in the past quarter century, with record- and nearrecord low summer ice in observed recent years. A set of coupled atmosphere–ice–ocean global model simulations quantifies the expected changes in Arctic temperature and sea ice through the twenty-first century. Projected are polar-amplified increases in SAT and reductions in sea ice, with a predominantly ice-free Arctic Ocean in summer projected before the end of this century. A range of potential consequences of Arctic warming and a shrinking ice cover are foreseen. First, exposure of vast areas of the Arctic Ocean would greatly alter the coastal and shelf marine environment. Second, broad changes in the marine and sea ice–based ecosystem—e.g. changes in plankton due to less ice and greater inflow of melt water—could negatively impact Arctic and sub-Arctic marine biodiversity, not least the vulnerable ice-based mammals such as polar bears. Third, there would be a larger open area for potential Arctic fisheries, as well as increased offshore activities and marine transportation, including the Northern Sea Route north of Siberia. Changes in the physical environment of the Arctic Ocean are thus expected to be dramatic, and although projecting ecosystem changes several decades into twenty-first century is challenging, the impact of diminishing sea ice on Arctic marine and sea ice–based ecosystems will certainly be transformative. publishedVersion

[1] Recent observations suggest an arctic climate system in broad transformation, yet the regional marine-ecosystem response is poorly known. Here, we develop and analyze a comprehensive biogeophysical dataset of key water constituents – chlorophyll (chl), suspended sediments (sm) and dissolved organic matter (doc) – using satellite ocean-color data from the White Sea in the Russian Arctic, for the period 1998–2004. The revealed changes in chl, sm and doc are more pronounced in the bays (e.g., the southeastern bay trends are −20%, +18% and +11%, respectively) than in the central basin (−5%, +5% and +3%, respectively). The chlorophyll decreases reflect the impact of enhanced runoff on sm and doc, which make the water more turbid and less favourable for phytoplankton growth, in contrast to other arctic seas where increased phytoplankton is expected. This case study supports our hypothesis that the marine ecosystems of semi-enclosed arctic shelf seas respond rapidly to climate change and are thus particularly vulnerable to future global warming.

This chapter presents recent advances in remote sensing of the White Sea from satellite Earth observation (EO) sensors. Section 6.1 presents the methods for quantitative remote sensing of water quality in marine environments such as the White Sea. Section 6.2 describes the patterns of thermo-hydrodynamic processes and fields derived from the NOAA Advanced Very-High Resolution Radiometer (AVHRR) data. Section 6.3 discusses remote sensing of the White Sea using active (radar) and passive microwave sensors. Finally, Section 6.4 presents studies of the White Sea ice cover using passive microwave sensors, which are used to characterize the spatial variability of sea ice over the past two decades.

[1] The results of 12 coupled climate models participating in the Coupled Model Intercomparison Project (CMIP2) are compared together with observational data in order to investigate: 1) How the current generation of climate models reproduce the major features of the winter North Atlantic Oscillation (NAO), and 2) How the NAO intensity and variability may change in response to increasing atmospheric CO2 concentration. Long-term changes in the intensity and spatial position of the NAO nodes (Icelandic Low and Azores High) are investigated, and different definitions of the NAO index and the Arctic Oscillation (AO) are considered. The observed temporal trend in the NAO in recent decades lies beyond the natural variability found in the model control runs. For the majority of the models, there is a significant increase in the NAO trend in the forced runs relative to the control runs, suggesting that the NAO may intensify with further increases in greenhouse-gas concentrations.

A new set of multi-decadal and century-scale sea-ice data is compared with coupled atmosphere-ocean model simulations in order to understand Arctic sea ice and climate variability. It is evident that the two pronounced 20th-century warming events—both amplified in the Arctic—were linked to sea-ice variability. The area of sea ice is observed to have decreased by 8× 105km 2 (7.4%) since 1978, with record-low summer ice coverage in 2002. Model predictions are used to quantify changes in the ice cover through the 21st century. A predominantly ice-free Arctic in summer is predicted for the end of this century.

The global sea ice covers can be regularly and frequently monitored using satellite passive microwave sensors. Recent studies using passive microwave data have detected significant decreases in Arctic ice extent and ice area, with no significant changes in the Antarctic. The authors analyze microwave data through mid-1995, to identify changes in ice extent, ice area, and, for the first time, overall ice concentration. The authors find continued decreases in the Arctic ice extent and ice area, and establish a decrease in the ice concentration. They also include a seasonal analysis of the Arctic trends, which establishes the greatest decreases to be in summer and spring. The relatively large summer decreases imply a reduction in the multi-year ice area, suggesting reduced ice thickness, though observational data are lacking.

Sea ice is a sensitive component of the climate system, such that reductions in the Arctic and Antarctic sea ice covers could be indicative of greenhouse warming. The most consistent source of information on sea ice parameters is microwave remote sensing from polar-orbiting satellites. Previous analyses of Scanning Multichannel Microwave Radiometer (SMMR) data from 1978-87 revealed a significant decrease in Arctic sea ice extent with no changes in the Antarctic. The authors extend the record to 1994 by including data from the subsequent Special Sensor Microwave Imager (SSM/I). This involves intercalibrating the SMMR-SSM/I data during their six-week overlap period, achieved at the sea ice concentration level using the NORSEX algorithm. Time series analysis of the merged 1978-94 continuous sea ice time series reveals continued decreases in Arctic ice extent and area. Moreover, the authors find a slight decrease in Antarctic ice extent during the period. The brevity of the records reiterates the need for longer, continuous microwave time series to identify long-term trends.

There is growing evidence from observations that the Arctic sea ice cover is undergoing a significant change where both ice area and ice thickness are decreasing. Also climate simulation models predict that the sea ice will be reduced significantly in the next 50 years. The reduction has started and is documented by 20 years of satellite data observations. A reduced ice cover will facilitate sea transportation in the Northern Sea Route significantly. Several scenarios can be foreseen: less ice-covered areas will make it possible to use lower ice-class vessels and navigation can be carried out with less icebreaker support; longer melt season will extend the navigation period along the Siberian coast; finally, thinner ice-cover and more open leads in the central Arctic allows powerful icebreakers to sail efficiently across the North Pole, which is the shortest distance between the Barents Sea and the Bering Strait.

Recent research using microwave satellite remote sensing data has established that there has been a reduction of about 3 percent per decade in the areal extent of the Arctic sea ice cover since 1978, although it is unknown whether the nature of the perennial ice pack has changed. These data were used to quantify changes in the ice cover's composition, revealing a substantial reduction of about 14 percent in the area of multiyear ice in winter during the period from 1978 to 1998. There also appears to be a strong correlation between the area of multiyear ice and the spatially averaged thickness of the perennial ice pack, which suggests that the satellite-derived areal decreases represent substantial rather than only peripheral changes. If this apparent transformation continues, it may lead to a markedly different ice regime in the Arctic, altering heat and mass exchanges as well as ocean stratification.

Sea ice features and processes in the Barents Sea were studied during the Seasonal Ice Zone Experiment 1992 (SIZEX92), a dedicated ERS 1 satellite synthetic aperture radar (SAR) field campaign carried out in March 1992. SIZEX92 was based around the research vessels Polarsyssel and Håkon Mosby. In situ oceanographic, meteorological, and ice measurements were made, coordinated with low-altitude aerial observations and ERS 1 SAR data acquired in near real time. Fifty-eight low- and full-resolution SAR scenes were obtained during SIZEX92, which provided a validation data set for ERS 1 SAR backscatter under winter conditions in the Barents Sea. Analysis of SIZEX92 data has provided geophysical insights, including a better understanding of Barents Sea ice edge freezing processes and ice edge development in response to atmospheric forcings. In particular, areas of new ice formation were found to be related to bathymetric features through their influence on the circulation of Arctic and Atlantic water masses. ERS 1 SAR image sequences revealed rapid, mesoscale variations in new ice areas and the ice edge in response to wind conditions. In addition to geophysical insight, SIZEX92 demonstrated some of the technical capabilities and limitations of ERS 1 SAR to identify new ice areas, the ice edge, ice floes, and ice types. Mapping new ice areas using satellite SAR data may be considered the most promising new application. The limitations of ERS-1 SAR-derived ice classification and ice motion estimates in the marginal ice zone are identified.

The most consistent means of investigating the global sea ice cover is by satellite passive microwave sensors, as these are independent of illumination and cloud cover. The Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) and the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSMI) provide information on the global sea ice cover from 1978 to present. The two instruments flew simultaneously during a 6-week overlap period in July and August 1987, thus enabling intercomparison of the two sensors. Brightness temperatures are corrected for instrument drift and calibration differences in order to produce continuous time series of monthly averaged Arctic and Antarctic sea ice extent and sea ice area through the use of the NOrwegian Remote Sensing EXperiment (NORSEX) algorithm, which relates brightness temperatures to ice concentration. Statistical analysis on the time series estimates the decreases in Arctic ice extent and ice area to be 4.5% and 5.7%, respectively, during the 16.8-year observation period. The overall trends established here serve to better define and strengthen earlier assertions of a reduced ice cover, based on analysis of SMMR and SSMI data taken separately. These results are consistent with GCM simulations that suggest retreat of the sea ice cover under global warming scenarios.

The predominant feature of the Arctic Ocean is the presence of a perennial sea ice cover that shapes the climate of the region, by greatly altering the radiation budget, and restricting heat and mass exchanges between the ocean and atmosphere. The Arctic is believed to be particularly sensitive to global climate change that may result from increases in so-called greenhouse gases. Greenhouse warming scenarios using ocean-atmosphere general circulation models (GCMs) tend to predict enhanced warming in the polar regions, with the Arctic expected to warm about 3–4°C during the next half-century 1 . Some GCMs even show a complete or near-complete removal of the summer ice cover in the Arctic 2 . Thus, systematic, long-term observations of the Arctic ice cover may be useful for the early detection of global climate change 3 . The need is clearly indicated for operational climate monitoring of the Arctic ice cover, in the principles and framework of EuroGOOS. In particular, such a monitoring system fits well within the Climate Monitoring, Assessment and Prediction module. Also, because the Arctic ice cover greatly affects fisheries in the marginal seas, offshore oil and gas activities, and transport operations, such monitoring has relevance to the Marine Meteorological and Oceanographic Operational Services module. Here we describe 1) the existing climate monitoring program of the Arctic ice cover, developed by the Nansen Environmental and Remote Sensing Center (NERSC), Bergen, Norway, and 2) the potential for improvements, including the use of other observations and models of the Arctic ice cover.

Changes apparent in the arctic climate system in recent years require evaluation in a century-scale perspective in order to assess the Arctic's response to increasing anthropogenic greenhouse-gas forcing. Here, a new set of century- and multidecadal-scale observational data of surface air temperature (SAT) and sea ice is used in combination with ECHAM4 and HadCM3 coupled atmosphere-ice-ocean global model simulations in order to better determine and understand arctic climate variability. We show that two pronounced twentieth-century warming events, both amplified in the Arctic, were linked to sea-ice variability. SAT observations and model simulations indicate that the nature of the arctic warming in the last two decades is distinct from the early twentieth-century warm period. It is suggested strongly that the earlier warming was natural internal climate-system variability, whereas the recent SAT changes are a response to anthropogenic forcing. The area of arctic sea ice is furthermore observed to have decreased similar to8 x 10(5) km(2) (7.4%) in the past quarter century, with record-low summer ice coverage in September 2002. A set of model predictions is used to quantify changes in the ice cover through the twenty-first century, with greater reductions expected in summer than winter. In summer, a predominantly sea-ice-free Arctic is predicted for the end of this century.

New data on leads and surface-melt phenomena in the Arctic, based on mapping of DMSP visible images, are presented. Lead orientations in the Beaufort Sea are broadly correlated with geostrophic wind direction and show similar synoptic scale patterns. Preliminary results of airborne 1.06 μm lidar transects over Baffin Bay demonstrate its great potential for high-resolution mapping of open-water areas and, in winter/spring, their ice-crystal plumes. Some of these sub-visible plumes are observed to penetrate the Arctic inversion. Snow-melt maps for the entire Arctic prepared for four summer seasons have been used to derive surface-albedo data. From these data the variability of the surface-energy balance is estimated to be equivalent to 0.6 m of ice melt.

REMOTE sensing using an airborne infrared lidar has shown an unexpected capability to detect open leads in Arctic sea ice and their associated meteorology in winter. It is shown here that vertical profiles of backscattered radiation demonstrate strong returns from hydrometeor plumes originating from leads having a surface water temperature near -1.8 C. Recently refrozen leads are also distinguishable by the lidar backscatter from adjacent thicker, older sea ice. Wide leads release enough energy to create buoyant plumes which penetrate the Arctic boundary layer inversion, transporting heat and moisture into the troposphere. These results show that the role of the Arctic as a global heat sink may need to be reevaluated, and that lead plumes have a significant effect on the radiation budget.