Summer Melt Season Research Articles

A decade of environmental change in the Pacific Arctic region

Over the decade 2004–2013 environmental changes in the Pacific sector of the Arctic have been dramatic enough to suggest that a ‘new normal’ climate is emerging. The clearest indicator of this change is the dramatic loss of sea ice during the summer, which in some years has already resulted in essentially ice-free conditions in this region. For example, between 7 August and 11 October 2012, sea ice concentration in the Beaufort and Chukchi Seas between 70 and 80°N fell below 20% (with a record minimum concentration of only 5% on 2 September). Thick multiyear sea ice (older than 2years) has almost entirely disappeared, replaced by thin and more mobile first-year ice. Year-to-year variability in ice concentration is associated with anomalous wind forcing linked to larger-scale atmospheric circulation patterns, which also affect ocean currents, including transport through Bering Strait. With reduced sea ice extent the area of ice-free ocean susceptible to rapid solar heating has increased. More heat is stored in the upper ocean early in the summer melt season and persists later into the autumn freeze-up. Monthly surface air temperature anomalies greater than 6°C have occurred frequently in the autumn in the Beaufort and Chukchi seas. Environmental variations in the Bering Sea over the last decade have been subtle by comparison, and include an increase in winter sea-ice extent between 2007 and 2013.

Variations in Firn Line Altitude and Firn Zone Area on Qiyi Glacier, Qilian Mountains, Over the Period of 1990 to 2011

Firn line altitude is an indicator of equilibrium line altitude (ELA) and plays an important role in predicting climate behavior and global change. Landsat 30-m resolution data from clear sky at the end of 21 summer melt seasons (July and August, 1990–2011) were used to obtain the firn lines and extract the glacier boundary at Qiyi Glacier in the Qilian Mountains of China. High-resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer 30-m resolution Global Digital Elevation Model (ASTER GDEM) was employed to obtain the firn line altitudes. The firn line altitudes on the Qiyi Glacier increased from 4540 m a.s.l. to 5000 m a.s.l. during the study period, a rise of 460 m. Changes in firn line altitudes had good positive correlations with air temperature data and were in good agreement with measured and simulated ELA changes. The calculated firn zone area decreased (increased) with the increase (decrease) of firn line altitude. Linear regression was used to fit the variational trend, indicating that the firn zone area decreased by ∼1.19 km2, with a change rate 0.057 km2 a-1, during the study period. The firn zone area ratio decreased from 76.2% to 18.1% during the same time.

Air-Deployed Microbuoy Measurement of Temperatures in the Marginal Ice Zone Upper Ocean during the MIZOPEX Campaign

AbstractAir-deployed microbuoys (ADMBs) were developed as a means of measuring subsurface temperatures in the marginal ice zone (MIZ) over campaign-duration time scales to better understand how MIZ surface layer heat content accelerates melt rates at the edge of the ice pack. ADMBs are small, low-cost buoys deployable from unmanned aircraft and are capable of measuring temperatures to 0.1°C absolute accuracy at the surface, 1-m, and 2-m depth, along with GPS position. Each ADMB contains a microcontroller, GPS, 900-MHz radio, flash electrically erasable programmable read-only memory (EEPROM), battery, and a set of temperature sensors to monitor conditions for up to 10 days. A communications board on an overflying aircraft autonomously deploys each ADMB and collects data from previously deployed ADMBs for analysis. The 2013 Marginal Ice Zone Observations and Processes Experiment (MIZOPEX) campaign deployed ADMBs into the summer melt season MIZ north of Oliktok Point, Alaska, collecting over 400 h of data from two clusters of buoys during the short field campaign. Initial results indicate that SST is a good measure of upper-ocean temperature in the MIZ when conditions are well mixed, but that is often not the case. In areas with higher ice concentration, surface temperatures tend to underestimate the temperature of the subsurface, while in areas of low ice concentration, SSTs overestimate the subsurface temperature.

Heat exchange processes and thermal dynamics of a glacier‐fed alpine stream

AbstractGlacier‐fed river thermal regimes vary markedly in space and time; however, knowledge is limited on the fundamental processes controlling alpine stream temperature dynamics. To address the research gap, this study quantified heat exchanges at the water surface and bed of the Taillon glacier‐fed stream, French Pyrénées. Hydro‐meteorological observations were recorded at 15‐min intervals across two summer melt seasons (2010 and 2011), and energy balance components were measured or estimated based on site‐specific data. Averaged over both seasons, net radiation was the largest heat source (~80% of total flux); sensible heat (~13%) and friction (~3%) were also sources, while heat exchange across the channel–streambed interface was negligible (<1%). Latent heat displayed distinct interannual variability and contributed 5% in 2010 compared with 0.03% in 2011. At the sub‐seasonal scale, latent heat shifted from source to sink, possibly linked to the retreating valley snowline that changed temperature and humidity gradients. These findings represent the first, multiyear study of the heat exchange processes operating in a glacier‐fed stream, providing fundamental process understanding; the research highlights the direct control antecedent (winter) conditions that have on energy exchange and stream temperature during summer months. In particular, the timing and volume of snowfall/snowmelt can drive thermal dynamics by the following: (1) altering the length of the stream network exposed to the atmosphere and (2) controlling the volume and timing of cold water advection downstream. Finally, this study highlights the need to develop long‐term hydro‐meteorological monitoring stations to improve the understanding of these highly dynamic, climatically sensitive systems. Copyright © 2015 John Wiley & Sons, Ltd.

Multiyear ice replenishment in the Canadian Arctic Archipelago: 1997–2013

AbstractIn the Canadian Arctic Archipelago (CAA), multiyear ice (MYI) replenishment from first‐year ice aging (CAAMYI‐Oct‐1) and Arctic Ocean MYI exchange (CAAMYI‐exchange) contribute to the CAA's relatively heavy sea ice conditions at the end of the summer melt season. We estimate these components using RADARSAT and the Canadian Ice Service Digital Archive and explore processes responsible for interannual variability from 1997 to 2013. CAAMYI‐Oct‐1 (52 ± 36 × 103 km2) provides a larger contribution than CAAMYI‐exchange (13 ± 11 × 103 km2). CAAMYI‐Oct‐1 represents ∼10% of the amount that occurs in the Arctic Ocean. CAAMYI‐exchange represents ∼50% of Nares Strait MYI export to Baffin Bay and ∼12% of Fram Strait MYI export to the Greenland Sea. CAAMYI‐Oct‐1 exhibits dependence on warmer (cooler) summers that increase (decrease) melt evident from strong relationships to surface air temperature (SAT), albedo and total absorbed solar radiation (Qtotal). CAAMYI‐exchange is influenced by summer sea level pressure (SLP) anomalies over the Beaufort Sea and Canadian Basin which shifts the primary source of CAAMYI‐exchange between less obstructed M'Clure Strait (low SLP anomalies) and the more obstructed Queen Elizabeth Islands (high SLP anomalies). Over the 17‐record, appreciable replenishment occurred for most years from 1997 to 2004, reduced replenishment from 2005 to 2012, and large replenishment in 2013. The reduced replenishment period was associated with positive SAT, negative albedo, and positive Qtotal anomalies that facilitated more melt and less CAAMYI‐Oct‐1, together with high SLP anomalies that facilitated less CAAMYI‐exchange. Large replenishment in 2013 was primarily from CAAMYI‐Oct‐1 attributed to strongly negative SAT and Qtotal anomalies and strongly positive albedo that impeded melt.

Seasonal release of anoxic geothermal meltwater from the Katla volcanic system at Sólheimajökull, Iceland

Understanding patterns of geothermal and volcanic activity at many of Iceland's most active volcanic systems is hampered by thick overlying ice, which prevents direct observation and complicates interpretation of geophysical signals. Katla is a prime example, being a large and restless volcanic system covered by the 740m thick Mýrdalsjökull ice cap, whose eruptions have triggered some of the most powerful known meltwater floods in historical times. To shed new light on geothermal and subglacial hydrological processes at Katla, we have determined the sulphate isotopic composition of a series of glacial meltwater samples discharged from Sólheimajökull, a valley glacier of Mýrdalsjökull, between 2009 and 2012. Dual isotopic analysis of δ34S and δ18O in dissolved sulphate allows identification of source mixing processes and chemical evolution during subglacial meltwater transport. Strikingly, meltwater δ18OSO4 signatures indicate redox conditions at the glacier bed, which are inverse to those normally encountered at Arctic and Alpine glaciers. Discharge of reduced, anoxic meltwater in summer, rather than winter, points towards seasonal release of geothermally derived volatile gases. We attribute this to headward expansion of the channelized subglacial drainage system during the summer melt season, accessing key areas of geothermal activity within the Katla caldera. Volatile release may be further enhanced by unloading of overburden pressure due to snowpack melting in the summer season. In winter, restriction of subglacial channels to lower elevations effectively seals geothermal fluids and dissolved gases beneath the ice cap, with only sporadic release permitted by periodic increases in subglacial water pressure. When the subglacial drainage configuration permits access to key geothermal areas, sulphate isotopic signatures thereby form sensitive indicators of geothermal activity occurring deep beneath the Mýrdalsjökull ice cap.

Constraints on snow accumulation and firn density in Greenland using GPS receivers

AbstractData from three continuously operating GPS sites located in the interior of the Greenland ice sheet are analyzed. Traditionally these kinds of GPS installations (where the GPS antenna is placed on a pole deployed into the firn) are used to estimate the local horizontal speed and direction of the ice sheet. However, these data are also sensitive to the vertical displacement of the pole as it moves through the firn layer. A new method developed to measure snow depth variations with reflected GPS signals is applied to these GPS data from Greenland. This method provides a constraint on the vertical distance between the GPS antenna and the surface snow layer. The vertical positions and snow surface heights are then used to assess output from surface accumulation and firn densification models, showing agreement better than 10% at the sites with the longest records. Comparisons between the GPS reflection method and in situ snow sensors at the Dye-2 site show good agreement, capturing the dramatic changes observed in Greenland during the 2012 summer melt season. The geocentric elevation of the snow surface can be inferred by subtracting the snow surface height estimates from the vertical position measurements. It should be possible to use those surface elevation estimates to help validate elevation results obtained from satellite altimetry.

Variability in the annual cycle of the Río Atuel streamflows and its relationship with tropospheric circulation

ABSTRACTClimate‐induced changes in the annual regime of snow‐fed rivers have serious implications for water resource management. In the Central Andes (CA, 28°–36°S) of Argentina–Chile, the snow accumulated in high‐elevation mountains in winter is the dominant component of streamflows during the spring–summer melting season. Although topography introduces complexity in snowpack responses to the annual temperature cycle, streamflow series over a century in length make the CA particularly suitable for identification of long‐term hydrological changes. Principal component (PC) analysis of Río Atuel annual hydrographs from 1906 to 2012 discriminates between precipitation‐ and temperature‐related components associated with variations in snow accumulation (49% of variance) and advances/delays of the streamflow annual peak (21% of variance), respectively. The temporal evolution of PC1 loadings reveals a predominant negative period from 1917 to 1976 and from 1988 to present, suggesting the propensity to undergo long periods with reduced flows. In turn, the PC2 pattern is predominantly positive from 1948 to the present, revealing a tendency to more frequent peak flows in late spring since the mid‐20th century. Above‐average streamflows related to abundant snowfalls in the CA are associated with northward shifts in stormtracks that are remotely induced by above‐average sea surface temperatures in the equatorial Pacific. On the other hand, earlier streamflow peaks in November‐December are concurrent with above‐average temperatures across the Atuel basin induced by enhanced meridional circulation from the Tropics due to the strengthening of the South Atlantic anticyclone. These circulation anomalies are linked to the persistent positive phase of the Southern Annular Mode during the last decades. Additionally, years with reduced streamflows in January, and proportionally larger flow contributions in November‐December, are associated with anomalous air cooling at high levels induced by low pressure centres over the region as part of a quasi‐zonal stationary Rossby wave train that extends from Australia to the South American–South Atlantic sector.

An Arctic sea ice thickness variability revealed from satellite altimetric measurements

A modified algorithm taking into account the first year (FY) and multiyear (MY) ice densities is used to derive a sea ice thickness from freeboard measurements acquired by satellite altimetry ICESat (2003-2008). Estimates agree with various independent in situ measurements within 0.21 m. Both the fall and winter campaigns see a dramatic extent retreat of thicker MY ice that survives at least one summer melting season. There were strong seasonal and interannual variabilities with regard to the mean thickness. Seasonal increases of 0.53 m for FY the ice and 0.29 m for the MY ice between the autumn and the winter ICESat campaigns, roughly 4-5 month separation, were found. Interarmually, the significant MY ice thickness declines over the consecutive four ICESat winter campaigns (2005-2008) leads to a pronounced thickness drop of 0.8 m in MY sea ice zones. No clear trend was identified from the averaged thickness of thinner, FY ice that emerges in autumn and winter and melts in summer. Uncertainty estimates for our calculated thickness, caused by the standard deviations of multiple input parameters including freeboard, ice density, snow density, snow depth, show large errors more than 0.5 m in thicker MY ice zones and relatively small standard deviations under 0.5 m elsewhere. Moreover, a sensitivity analysis is implemented to determine the separate impact on the thickness estimate in the dependence of an individual input variable as mentioned above. The results show systematic bias of the estimated ice thickness appears to be mainly caused by the variations of freeboard as well as the ice density whereas the snow density and depth brings about relatively insignificant errors.

Surface albedo as a proxy for the mass balance of Greenland’s terrestrial ice

Satellite observations are critical to understanding the mass balance of Greenland’s terrestrial ice (Fig. 1). The Gravity Recovery and Climate Experiment (GRACE) satellite constellation provides monthly gravimetry observations that can directly assess mass balance. Temporal data gaps have begun to appear in the GRACE record due to declining satellite function. In anticipation of further deterioration in the coverage of GRACE, we have explored an empirical relation between ice-surface albedo (or reflectance) and ice-mass balance to fill the gaps in the gravimetry record of Greenland’s ice-mass balance. As surface albedo observed by the moderate-resolution imaging spectroradiometer (MODIS) aboard the Terra satellite is available in near real-time, employing a MODIS-derived proxy permits near real-time estimates of Greenland ice-mass balance. The Geological Survey of Denmark and Greenland has begun employing the albedo – mass-balance relation described here to issue near real-time estimates of Greenland ice-mass balance during the summer melt season at www.polarportal.org.

Temporal and spatial changes in Western Himalayan firn line altitudes from 1998 to 2009

Understanding changes in glacier mass balance is important because it is indicative of changes in climate and the hydrologic cycle. The latter also has particular influence on people living near glaciers and/or glacier-fed rivers. The Western Himalayas remain one of the regions where recent changes in glacier mass balance are not well-known. The temporal and spatial changes in firn line altitudes are an indicator of equilibrium line altitudes and thus reflect changes in glacier mass balance. Here, we use Himalayan Landsat TM/ETM+ data in July and August (the late summer melt season) to quantify changes in firn line altitudes from 1998 to 2009. We produced reflectance maps through radiometric calibration and atmospheric correction and use topographic correction to remove or reduce terrain or shadow effects. The real ‘surface albedo’ is obtained by narrowband-to-broadband (NTB) albedo conversion from the combined solar radiation. The firn line altitude was then extracted by combining the ‘surface albedo’ with pre-registered digital elevation model. The individual firn line altitude varies by region. The Western Himalayas display the largest range of firn line variability, where the firn line altitudes vary from 4840m a.s.l. to 5770m a.s.l. The individual glacier mean firn line altitude from 1998 to 2009 rose from 5072±77m a.s.l. to 5640±74m a.s.l. in the Western Himalayas. The mean firn line altitude increased from 1998 to 2009. The lowest mean recorded firn line altitude recorded was 5237±166m a.s.l. in 1998, whereas the highest was 5397±135m a.s.l. in 2000. We also observed a difference between the changes in fine line altitudes of northern and southern slopes of the western Himalayans, as the northern slope glaciers display a greater increase in firn line altitudes than the southern slope glaciers. In the southern slope, changes in firn line altitudes correlate with NCDC-NOAA temperature and precipitation data. This sustained increase of firn line altitudes and associated loss of glacier mass imply a persistent loss of stored freshwater in the Western Himalaya.

Enhanced Arctic Ice Concentration Estimation Merging MODIS Ice Surface Temperature and SSM/I Sea-Ice Concentration

Ice concentration data from passive microwave sensors are not reliable during the summer melt season. In this study passive microwave ice concentration estimates are improved upon through the assimilation of ice surface temperature data from the Moderate-Resolution Imaging Spectroradiometer (MODIS). Ice concentration from the analysis is converted to ice extent and compared with data from the Interactive Multisensor Snow and Ice Mapping System (IMS). Ice concentration analysis from data assimilation is in better agreement with the IMS data than the original passive microwave data, with the largest improvements occurring during sea-ice melt.

Glacier slip and seismicity induced by surface melt

Many of the key processes governing fast glacier flow involve interaction between a glacier and its basal hydrological system, which is hidden from direct observation. Passive seismic monitoring has shown promise as a tool for remotely monitoring basal processes, but lack of glacier-bed access prevents clear understanding of the relationships between subglacial processes and corresponding seismic emissions. Here we describe direct measurements of basal hydrology, sliding, and broadband seismicity made in a unique subglacial facility in Norway during the onset of two summer melt seasons. In the most pronounced of these episodes, rapid delivery of surface meltwater to the bed briefly enhanced basal slip following a period of elevated high-frequency seismic activity related to surface crevassing. Subsequent ground tilt derived from ultralong-period seismic signals was associated with subglacial bedrock deformation during transient pressurization of the basal hydraulic system. These signals are interpreted to represent hydraulic jacking as the supply of water to the bed exceeded the capacity of the hydraulic system. Enhanced slip terminated 2.5 h after it started, when ice-bed decoupling or increased connectivity in the basal cavity network relieved cavity overpressure. The results support theoretical models for hydraulic jacking and illustrate how melt-induced increases in speed can be short lived if cavity growth or ice-bed decoupling allows basal water more efficient drainage.

Isotopic investigation of runoff generation in a glacierized catchment in northern Sweden

In this study, summer rainfall contributions to streamflow were quantified in the sub-arctic, 30% glacierized Tarfala (21.7 km2) catchment in northern Sweden for two non-consecutive summer sampling seasons (2004 and 2011). We used two-component hydrograph separation along with isotope ratios (δ18O and δD) of rainwater and daily streamwater samplings to estimate relative fraction and uncertainties (because of laboratory instrumentation, temporal variability and spatial gradients) of source water contributions. We hypothesized that the glacier influence on how rainfall becomes runoff is temporally variable and largely dependent on a combination of the timing of decreasing snow cover on glaciers and the relative moisture storage condition within the catchment. The results indicate that the majority of storm runoff was dominated by pre-event water. However, the average event water contribution during storm events differed slightly between both years with 11% reached in 2004 and 22% in 2011. Event water contributions to runoff generally increased over 2011 the sampling season in both the main stream of Tarfala catchment and in the two pro-glacial streams that drain Storglaciaren (the largest glacier in Tarfala catchment covering 2.9 km2). We credit both the inter-annual and intra-annual differences in event water contributions to large rainfall events late in the summer melt season, low glacier snow cover and elevated soil moisture due to large antecedent precipitation. Together amplification of these two mechanisms under a warming climate might influence the timing and magnitude of floods, the sediment budget and nutrient cycling in glacierized catchments. Copyright © 2012 John Wiley & Sons, Ltd.

Changes in melt season characteristics on Devon Ice Cap, Canada, and their association with the Arctic atmospheric circulation

AbstractUsing June–August surface meteorological measurements for 2004–10 we found that the duration of the summer melt season at elevations of 1800, 1300 and 1000 m a.s.l. on Devon Ice Cap, Canada, increased at a rate of 3.4 (p < 0.1; weak significance), 6.1 (p< 0.01) and 8.8 (p < 0.05) days a−1, respectively. The calculated surface melt rate at 1300m a.s.l. increased from 74 to 133 cm w.e. a−1 between 2007 and 2010. These changes are linked to two types of change in the Arctic atmospheric circulation. Strengthening of the 500 hPa ridge over the Arctic in June–July resulted in increases in both the advection of warm air into the region and the occurrence of cloud-free conditions over the ice cap, causing the available melt energy to increase by 4–24% relative to June–July 2007–10 mean conditions. More frequent southwesterly low-pressure systems in August after 2004 accounted for a 12–38% increase in available melt energy relative to the August 2007–10 daily mean due to advection of warm air into the Arctic, reduced incoming shortwave radiation and increased net longwave radiation. This delayed the timing of freeze-up by an average of 5.5 days a–1 (p < 0.05) at the three sites over 2004–10.

Controls on short-term variations in Greenland glacier dynamics

AbstractShort-term ice-dynamical processes at Greenland’s Jakobshavn and Kangerdlugssuaq glaciers were studied using a 3 day time series of synthetic aperture radar data acquired during the 2011 European Remote-sensing Satellite-2 (ERS-2) 3 day repeat campaign together with modelled meteorological parameters. The time series spans the period March–July 2011 and captures the first ∼30% of the summer melting season. In both study areas, we observe velocity fluctuations at the lower ∼10 km of the glacier. At Jakobshavn Isbræ, where our dataset covers the first part of the seasonal calving-front retreat, we identify ten calving episodes, with a mean calving-front area loss of 1.29 ± 0.4 km2. Significant glacier speed-up was observed in the near-terminus area following all calving episodes. We identify changes in calving-front geometry as the dominant control on velocity fluctuations on both glaciers, apart from a <15% early-summer speed-up at Kangerdlugssuaq Glacier during a period of calving-front advance, which we attribute to enhanced surface melt-induced basal lubrication. Our 3 day velocity maps show new spatial characteristics of the ice melange flow variability in the Jakobshavn and Kangerdlugssuaq fjord systems, which are primarily controlled by calving-front dynamics and fjord geometry.

Clear‐sky thermodynamic and radiative anomalies over a sea ice sensitive region of the Arctic

Monthly clear‐sky anomalies of atmospheric temperature and water vapor over the East Siberian and Laptev Sea regions of the Arctic for 2003–2010 are examined here. This region experiences significant interannual variations in sea ice concentration and is also where ice loss was most apparent in the record year 2007. Clear‐sky thermodynamic profiles come from the Atmospheric Infrared Sounder (AIRS) sensor onboard the Aqua satellite. Associated longwave (LW) and shortwave (SW) radiation‐flux anomalies are estimated through radiative transfer modeling. Anomalies of temperature (±10 K) and water vapor (±1 g kg−1) often positively covary, resulting in distinct signatures in the clear‐sky downwelling LW (LWD) anomalies, occasionally larger than ±10 W m−2around the 2003–2010 climatology. Estimates of mean greenhouse anomalies indicate a shift from negative to positive anomalies midway through the 8‐year record. Sensitivity tests suggest that temperature anomalies are the strongest contributor to both LWD and greenhouse anomalies, relative to water‐vapor anomalies; monthly averaging of column precipitable water yields relatively small anomalies (order 1 mm) that produce a linear response in greenhouse anomalies. Finally the clear‐sky contribution to 2007 monthly ice thickness is estimated. Anomalous clear‐sky radiation retards the total 2007 ice thickness by 0.3 m (15–30% of ice‐thickness climatology), and anomalous LW radiation is most important for preconditioning the ice during the months prior to, and after, the summer melt season. A highly sensitive interaction between cloud fraction, surface albedo and LWD anomalies is found, and we develop a metric for determining clear‐sky anomalous ice melt potential.

Seasonal sediment fluxes forcing supraglacial melting on the Wright Lower Glacier, McMurdo Dry Valleys, Antarctica

AbstractCold‐based glaciers exist in low temperature and low humidity environments in which shortwave radiation is the largest source of energy to the glacier surface and the energy budget is very sensitive to the surface albedo. Consequently, the presence of relatively low volumes of debris on glacier surfaces has a significant impact on the timing, magnitude and rate of ablation at the surface. The aim of this study is to understand how the presence of sediment on the glacier surface at the start of the melt season can affect meltwater generation and delivery on a cold‐based glacier. A combination of field measurements, energy balance modelling and chemical mixing modelling were used on the Wright Lower Glacier, McMurdo Dry Valleys, Antarctica, between October 2005 and January 2006 to address this aim. In this system, sediment was transported onto the glacier surface during the winter months (March–October) by foehn winds, which reduced surface albedo at the start of the summer melt season. The areas of the glacier on which sediment accumulated began to melt earlier than other parts of the glacier and experienced a longer melt season. Over the study period, the total ablation on the dirty surfaces was nine times greater than for clean ice. Ablation on the dirty surfaces is dominated by melting, whereas sublimation dominates the clean ice. As the sediment was unevenly distributed over the glacier surface, the variation in melt amount and timing drove the development of a cryoconite hole system. Copyright © 2012 John Wiley & Sons, Ltd.

Annual Arctic sea ice less reflective than old ice

In the Arctic Ocean the blanket of permanent sea ice is being progressively replaced by a transient winter cover. In recent years the extent of the northern ocean's ice cover has declined. The summer melt season is starting earlier, the winter freeze is happening later, the areal extent of the ice has decreased, and more ice is failing to last through the summer. A key uncertainty in this ongoing climate transformation is how seasonal sea ice affects and responds to climate dynamics as compared to the traditional multiyear sea ice. Tackling an important branch of this issue, Perovich and Polashenski analyze how the albedo of seasonal sea ice changes throughout the summer melt season. The ice's albedo affects how much sunlight enters the system and hence influences biological productivity, ice extent, and future rates of warming.

Monitoring the melt season length of the Barnes Ice Cap over the 1979–2010 period using active and passive microwave remote sensing data

AbstractThe Barnes Ice Cap (BIC) located on Baffin Island (Nunavut, Canada) is one of the most southern ice caps of the Canadian Arctic Archipelago. Observational data provide evidence of increased melting, thinning and contour recession due to recent climate warming in the Arctic. The duration of the summer melt season for the BIC, over the period 1979–2010, was derived using a threshold algorithm for 19 GHz horizontal polarization brightness temperature data; the passive microwave satellite measurements included data from the quasi‐daily Scanning Multichannel Microwave Radiometer and the Special Sensor Microwave Imager. Our results show the melt season lengthened by 33% from 65.6 ± 6 days at the beginning of the period (1979–1987) to 87.1 ± 7.8 days towards the end (2002–2010). The interannual variations of the number of melt days were in agreement with those derived from active microwave backscatter data from the QuikSCAT scatterometer for the overlapping 2000–2009 period. In addition, elevation change data from the ICESat altimeter confirmed the thinning of the BIC at a mean rate of −0.75 m/year for the 2003–2009 period. For the 32‐year period that we analysed, correlations with summer and annual air temperature and annual sum of positive days were examined for both the North American Regional Reanalysis and the Clyde River Automatic Weather Station data. Correlations with land surface temperature data from MODIS were also examined over the last decade. The results of these investigations showed that these climate indicators did not adequately explain the observed melt variations for the BIC. Ground‐based snow and ice measurements collected near the BIC summit during a 10‐day field campaign in March 2011 provided insights onto the surface properties and confirm the relevance of the remote sensing invariant threshold algorithm used for melt detection. Copyright © 2012 John Wiley & Sons, Ltd.