Summer Arctic Sea Ice Research Articles

Ensemble 1‐Year predictions of Arctic sea ice for the spring and summer of 2008

Ensemble predictions of arctic sea ice in spring and summer 2008 have been carried out using an ice‐ocean model. The ensemble is constructed by using atmospheric forcing from 2001 to 2007 and the September 2007 ice and ocean conditions estimated by the model. The prediction results show that the record low ice cover and the unusually warm ocean surface waters in summer 2007 lead to a substantial reduction in ice thickness in 2008. Up to 1.2 m ice thickness reduction is predicted in a large area of the Canada Basin in both spring and summer of 2008, leading to extraordinarily thin ice in summer 2008. There is a 50% chance that both the Northern Sea Route and the Northwest Passage will be nearly ice free in September 2008. It is not likely there will be another precipitous decline in arctic sea ice extent such as seen in 2007, unless a new atmospheric forcing regime, significantly different from the recent past, occurs.

Future abrupt reductions in the summer Arctic sea ice

We examine the trajectory of Arctic summer sea ice in seven projections from the Community Climate System Model and find that abrupt reductions are a common feature of these 21st century simulations. These events have decreasing September ice extent trends that are typically 4 times larger than comparable observed trends. One event exhibits a decrease from 6 million km2 to 2 million km2 in a decade, reaching near ice‐free September conditions by 2040. In the simulations, ice retreat accelerates as thinning increases the open water formation efficiency for a given melt rate and the ice‐albedo feedback increases shortwave absorption. The retreat is abrupt when ocean heat transport to the Arctic is rapidly increasing. Analysis from multiple climate models and three forcing scenarios indicates that abrupt reductions occur in simulations from over 50% of the models and suggests that reductions in future greenhouse gas emissions moderate the likelihood of these events.

The Thinning of Arctic Sea Ice, 1988–2003: Have We Passed a Tipping Point?

Abstract Recent observations of summer Arctic sea ice over the satellite era show that record or near-record lows for the ice extent occurred in the years 2002–05. To determine the physical processes contributing to these changes in the Arctic pack ice, model results from a regional coupled ice–ocean model have been analyzed. Since 1988 the thickness of the simulated basinwide ice thinned by 1.31 m or 43%. The thinning is greatest along the coast in the sector from the Chukchi Sea to the Beaufort Sea to Greenland. It is hypothesized that the thinning since 1988 is due to preconditioning, a trigger, and positive feedbacks: 1) the fall, winter, and spring air temperatures over the Arctic Ocean have gradually increased over the last 50 yr, leading to reduced thickness of first-year ice at the start of summer; 2) a temporary shift, starting in 1989, of two principal climate indexes (the Arctic Oscillation and Pacific Decadal Oscillation) caused a flushing of some of the older, thicker ice out of the basin and an increase in the summer open water extent; and 3) the increasing amounts of summer open water allow for increasing absorption of solar radiation, which melts the ice, warms the water, and promotes creation of thinner first-year ice, ice that often entirely melts by the end of the subsequent summer. Internal thermodynamic changes related to the positive ice–albedo feedback, not external forcing, dominate the thinning processes over the last 16 yr. This feedback continues to drive the thinning after the climate indexes return to near-normal conditions in the late 1990s. The late 1980s and early 1990s could be considered a tipping point during which the ice–ocean system began to enter a new era of thinning ice and increasing summer open water because of positive feedbacks. It remains to be seen if this era will persist or if a sustained cooling period can reverse the processes.

The Atmospheric Impact of Uncertainties in Recent Arctic Sea Ice Reconstructions

Abstract There are significant discrepancies between observational datasets of Arctic sea ice concentrations covering the last three decades, which result in differences of over 20% in Arctic summer sea ice extent/area and 5%–10% in winter. Previous modeling studies have shown that idealized sea ice anomalies have the potential for making a substantial impact on climate. In this paper, this theory is further developed by performing a set of simulations using the third Hadley Centre Coupled Atmospheric Model (HadAM3). The model was driven with monthly climatologies of sea ice fractions derived from three of these records to investigate potential implications of sea ice inaccuracies for climate simulations. The standard sea ice climatology from the Met Office provided a control. This study focuses on the effects of actual inaccuracies of concentration retrievals, which vary spatially and are larger in summer than winter. The smaller sea ice discrepancies in winter have a much larger influence on climate than the much greater summer sea ice differences. High sensitivity to sea ice prescription was observed, even though no SST feedbacks were included. Significant effects on surface fields were observed in the Arctic, North Atlantic, and North Pacific. Arctic average surface air temperature anomalies in winter vary by 2.5°C, and locally exceed 12°C. Arctic mean sea level pressure varies by up to 5 mb locally. Anomalies extend to 45°N over North America and Eurasia but not to lower latitudes, and with limited changes in circulation above the boundary layer. No statistically significant impact on climate variability was simulated, in terms of the North Atlantic Oscillation. Results suggest that the uncertainty in summer sea ice prescription is not critical but that winter values require greater accuracy, with the caveats that the influences of ocean–sea ice feedbacks were not included in this study.

Sea ice thickness measurement and its underside morphology analysis using radar penetration in the Arctic Ocean

Based on radar penetrating measurements and analysis of sea ice in the Arctic Ocean, the potential of radar wave to measure sea ice thickness and map the morphology of the underside of sea ice is investigated. The results indicate that the radar wave can penetrate Arctic summer sea ice of over 6 m in thickness; and the propagation velocity of the radar wave in sea ice is in the range of 0.142 m ∙ns −1 to 0.154 m ∙ns −1 . The radar images display the roughness and micro-relief variation of sea ice bottom surface. These features are closely related to sea ice types, which show that radar survey may be used to identify and classify ice types. Since radar images can simultaneously display the linear profile features of both the upper surface and the underside of sea ice, we use these images to quantify their actual linear length discrepancy. A new length factor is suggested in relation to the actual linear length discrepancy in linear profiles of sea ice, which may be useful in the further study of the area difference between the upper surface and bottom surface of sea ice.

Impact of underwater‐ice evolution on Arctic summer sea ice

A model is presented that describes the simultaneous growth and ablation of a layer of ice between an under‐ice melt pond and the underlying ocean. Such “false bottoms” are the only significant source of ice formation in the Arctic during summer. Analytical solutions for diffusional transport of heat and salt are calculated that illustrate the importance of salt transport in effecting phase change. The model is extended to account for turbulent transports and applied to make predictions of bottom ablation rates of sea ice given the far‐field properties of the ocean from the AIDJEX and SHEBA field experiments. The model predictions show that false bottoms may play a significant role in the summer heat budget of the ice‐ocean system, causing localized heat fluxes of more than 10 W m−2 into the mixed layer. The thickening of thin ice by false‐bottom formation leads to longer‐lasting sea ice and thus smaller ice‐free areas, which might be an important mechanism affecting the surface albedo.

Meltwater circulation and permeability of Arctic summer sea ice derived from hydrological field experiments

AbstractPermeability and meltwater flow have been studied in sea ice in the Siberian and central Arctic during the summers of 1995 and 1996. A bail-test technique has been adapted to allow for measurements of in situ permeability, found to range between 10−11 and 10−8 m2. Permeability varied by about a factor of 2 between 1995 (above-normal melt rates) and 1996 (below-normal melt rates). Release of fluorescent tracers (fluorescein, rhodamine) furthermore allowed the derivation of flow velocities and assessment of the relevant driving forces. Hydraulic gradients in rough ice and wind stress in ponded ice were found to be particularly important, driving meltwater over distances of several meters per day. The mid- to late-summer ice was found to be permeable enough to completely divert meltwater from the surface into the ice interior. It is shown, however, that lower permeabilities of the upper ice layers as well as refreezing of meltwater, particularly during the early melt season, foster the development of surface melt ponds.

Tracer studies of pathways and rates of meltwater transport through Arctic summer sea ice

At the Surface Heat Budget of the Arctic Ocean (SHEBA) program's field site in the northern Chukchi Sea, snow and ice meltwater flow was found to have a strong impact on the heat and mass balance of sea ice during the summer of 1998. Pathways and rates of meltwater transport were derived from tracer studies (H218O, 7Be, and release of fluorescent dyes), complemented by in situ sea‐ice permeability measurements. It was shown that the balance between meltwater supply at the surface (averaging between 3.5 and 10.5 mm d−1) and ice permeability (between <10−11 and >10−9 m2) determines the retention and pooling of meltwater, which in turn controls ice albedo. We found that the seasonal evolution of first‐year and multiyear ice permeability and surface morphology determine four distinct stages of melt. At the start of the ablation season (stage 1), ponding is widespread and lateral melt flow dominates. Several tens of cubic meters of meltwater per day were found to drain hundreds to thousands of square meters of ice through flaws and permeable zones. Significant formation of underwater ice, composed between <30 and >50% of meteoric water, formed at these drainage sites. Complete removal of snow cover, increase in ice permeability, and reductions in hydraulic gradients driving fluid flow mark stage 2, concurrent with a reduction in pond coverage and albedo. During stage 3, maximum permeabilities were measured, with surface meltwater penetrating to 1 m depth in the ice and convective overturning and desalination found to dominate the lower layers of first‐year and thin multiyear ice. Enhanced fluid flow into flaws and permeable zones was observed to promote ice floe breakup and disintegration, concurrent with increases in pond salinities and 7Be. Advective heat flows of several tens of watts per square meter were derived, promoting widening of ponds and increases in pond coverage. Stage 4 corresponds to freeze‐up. Roughly 40% of the total surface melt was retained by the ice cover within the ice matrix as well as in surface and under‐ice ponds (with a total net retention of 15%). Based on this work, areas of improvement for fully prognostic simulations of ice albedo are identified, calling for parameterizations of sea‐ice permeability and the integration of ice topography and refined ablation schemes into atmosphere‐ice‐ocean models.

Indirect measurements of the mass balance of summer Arctic sea ice with an electromagnetic induction technique

AbstractIn the framework of the Surface Heat Budget of the Arctic (SHEBA) study, indirect, non-invasive ice mass-balance measurements were carried out at a drifting station in the northern Chukchi Sea between May and August 1998. Ice thickness was derived from electromagnetic induction (EM) measurements of apparent conductivity along 13 profiles (60−900 m long). As shown through sensitivity studies with a one-dimensional model, the apparent conductivity data from individual points can be inverted to yield estimates of ice thickness and ablation with an accuracy of approximately 0.05 m (for 2 m thick level ice). Ablation rates were 8−18 mm d−1, with total ablation amounting to roughly 0.9−1.2 m. Measurements of thickness and melt rates along different profiles in undeformed multi-year ice corresponded closely, indicating that the sampling statistics are adequate. The roughness of undeformed ice has been found to increase during the summer due to deepening of melt ponds and enhanced bottom melt. Ice under melt ponds was disproportionately thinner, most likely a result of thicker snow cover reducing winter accretion.

Surface and radiative characteristics of the summer Arctic sea ice cover from multisensor satellite observations

Accurate quantification and characterization of the Arctic summer ice cover are needed for mass balance, heat flux, and modeling studies in the region. A general assessment of the state and basic characteristics of the ice cover can best be done in summer because it is when the perennial component is fully revealed. The main source of summer ice information has been passive microwave and to a lesser degree active microwave data. However, the emissivity and backscatter of sea ice are abnormal and difficult to resolve during this time period, causing large uncertainties in the interpretation of satellite data. In this study we examined the state of the sea ice cover by using special scanning microwave imager (SSM/I), synthetic aperture radar (SAR), and advanced very high resolution radiometer (AVHRR) satellite data synergistically. The surface and radiative characteristics of the summer ice cover were evaluated in the context of three special events: onset of melt, melt ponding, and freeze‐up. These events affect the emissivity and backscatter and may alter the albedo and ice structure. Onset of melt is readily detectable and is shown to migrate rapidly to the north in June. Melt ponding is not directly observable but is postulated to be the main cause of the decreases in brightness temperatures and large discrepancies between the SSM/I and SAR ice concentration results in many areas. In these areas, SAR and AVHRR results show concentrations near 100%, while the SSM/I data were as low as 70%. During freeze‐up the ice signatures are still quite different from those of midwinter ice, but the ice concentrations from SSM/I generally agree well with those from SAR data. Our results show that, generally, the average ice concentration within the pack is usually greater than 90% during the summer, which is substantially larger than that inferred previously from passive microwave data. The use of combined SAR and SSM/I data may also provide melt‐ponding fraction and first‐order estimate of albedo in the Arctic region.

Summer Arctic sea ice character from satellite microwave data

In situ microwave data for the microscale properties of sea ice are combined with satellite data on the macroscale properties to construct an interpretation method for satellite data on sea ice for the warm season. Scatterometer, or calibrated radar, data give information on ice surface conditions, and radiometer data give information on areal concentrations of ponds and leads. The pond data are determined by examining fractional areas of exposed ice and using parameterized lead areas. These results are used to develop diffuse‐illumination surface albedo estimates for the time of maximum ponding, near day 195, and that of minimum ice cover, near day 258. The estimated ponding albedos range from 0.5 to 0.6, and ice cover minimum albedos range from 0.6 to 0.8. During summer, the 37‐GHz radiometer data are nearly independent of sea ice properties and are dominated by atmospheric and snow cover conditions. Lower‐frequency data are increasingly useful for interpreting ice conditions. Substantial anisotropy in backscatter is recorded in scatterometer data at 14 GHz, with surface resolution of 50 km showing 90° anisotropy in the 0.5–1 dB range. In Nimbus 5 and Seasat data, some variation in satellite‐recorded microwave properties during the period between 195 and 258 cannot be directly interpreted but are thought to be due to weather.

The Horizontal Coherency of the Motion of Summer Arctic Sea Ice

Abstract Four ice floes in the permanent arctic ice pack were instrumented during the summer of 1975. Spectral and cross-spectral analysis of the complex time series of the ice motion and of the surface winds are given. The results show that the local low-frequency surface motion is linearly related to the local low-frequency winds. The higher frequency surface motion, including the significant inertial motion, cannot be entirely described by a linear model forced by the local winds. Cross-spectral analysis of the motion of floes separated by a hundred kilometers reveals a high coherency of the inertial motions. Two simple mathematical models are studied to explain observed cross spectra between stations. The spectral properties of these models are determined directly from the spectral properties of the winds. Qualitative agreement between the models and observations suggest that some coupling mechanism is at work in the ice pack even during summer.