Showing papers on "Ice-albedo feedback published in 1997"

Modeling Sea Ice as a Granular Material, Including the Dilatancy Effect

Abstract: A dynamic sea ice model based on granular material rheology is presented. The sea ice model is coupled to both a mixed layer ocean model and a one-layer thermodynamic atmospheric model, which allows for an ice albedo feedback. Land is represented by a 6-m thick layer with a constant base temperature. A 10-year integration including both thermodynamic and dynamic effects and incorporating prescribed climatological wind stress and ocean current data was performed in order for the model to reach a stable periodic seasonal cycle. The commonly observed lead complexes, along which sliding and opening of adjacent ice floes occur in the Arctic sea ice cover, are well reproduced in this simulation. In particular, shear lines extending from the western Canadian Archipelago toward the central Arctic, often observed in winter satellite images, are present. The ice edge is well positioned both in winter and summer using this thermodynamically coupled ocean–ice–atmosphere model. The results also yield a sea ic...

The changing albedo of the Greenland ice sheet: implications for climate modeling

Abstract: Although the snow albedo feedback mechanism has been shown to amplify global warming effects in nearly all models of global climate, it continues to be represented as a simplistic parameterization. Here, we demonstrate how changes in snow-pack energy-balance drive the seasonal fluctuations in snow albedo for the Greenland ice sheet. For a detailed, point-based investigation of the relationship between snowpack energy balance and albedo, two models are coupled together; one that calculates snow grain-size and the other that uses those grain-size data as input to a radiative-transfer code to obtain spectral albedo. These data indicate that in the near-infrared wavelengths, albedo values drop nearly 20%, during a 10 day period during which grain-sizes increased dramatically. Satellite data were used to map monthly changes in albedo over the entire Greenland ice sheet during the spring and summer months. These monthly albedo images indicate albedo reductions of as much as 80% in coastal regions. Even in areas that experience little or no melt, albedo decreases of 10–20% were common. From these results, it is clear that snow albedo parameterizations for climate models must incorporate the dynamics of snowpack energy balance.

Predicted reduction in basal melt rates of an Antarctic ice shelf in a warmer climate

Abstract: Floating ice shelves are vulnerable to climate change at both their upper and lower surfaces. The extent to which the apparently air-temperature-related retreat of some northerly Antarctic Peninsula ice shelves1 presages the demise of their much larger, more southerly, counterparts is not known, but air-temperature effects are unlikely to be important in the near future. Oceanographic measurements from beneath the most massive of these southerly ice shelves—the Filchner–Ronne Ice Shelf2,3,4—have confirmed that dense sea water resulting from sea-ice formation north of the ice shelf flows into the sub-ice-shelf cavity. This relatively warm so-called High Salinity Shelf Water (HSSW) is responsible for the net melting at the ice shelf's base. Here I present temperature measurements, from the same sub-ice-shelf cavity, which show a strong seasonality in the inflow of HSSW. This seasonality results from intense wintertime production of sea ice, and I argue that the seasonal springtime warming can be used as an analogue for climate warming. For the present mode of oceanographic circulation, the implication is that warmer winters (a climate warming, leading to lower rates of sea-ice formation, would cause a reduction in the flux of HSSW beneath the ice shelf. The resultant cooling in the sub-ice cavity would lead, in turn, to a reduction in the total melting at the ice shelf's base. A moderate warming of the climate could thus lead to a basal thickening of the Filchner–Ronne Ice Shelf, perhaps increasing its longevity.

Spatial variations in the rate of sea level rise caused by the present-day melting of glaciers and ice sheets

Abstract: The redistribution of surface water mass associated with the melting of glacial ice causes uplift near areas of mass depletion, depression of the seafloors, and changes in the earth's gravitational field which perturb the ocean surface. As a result, local spatial variations exist in the rate of sea level rise. Tide gauges on continental coastlines measure a sea level rise 5% smaller than the global average. Tide gauges in the hemisphere opposite a source of continental mass depletion measure sea level rise 10 to 20% greater than the global average produced by that source while satellites make measurements 10% too low. Because most long duration tide gauges are in the northern hemisphere, if the sources of sea level rise are unbalanced between the two hemispheres, estimates of global sea level rise could be in error by 10 to 20%. Individual tide gauges could be more seriously unrepresentative if they are near regions of significant present-day mass depletion.

Energy and water transport in climates simulated by a general circulation model that includes dynamic sea ice

Abstract: We analyze energy and water transport in present, doubled CO 2 , and tripled CO 2 climates simulated by the Mark 2 CSIRO nine-level general circulation model with a mixed layer ocean. The model differs from the Mark 1 version by the inclusion of dynamic sea ice, a semi-Lagrangian water vapor transport, and an enhanced land-surface scheme, and it includes prescribed ocean heat transport. We describe a 30-year climatology of the 1xCO 2 simulation, emphasizing the sea ice and the mean meridional energy and water transport. The ice depths, concentrations, and velocities are moderately realistic in both hemispheres. Poleward energy transport is inferred (calculated indirectly from vertical energy fluxes) for both the atmosphere and ocean, although the oceanic flux is much weaker than observational estimates for the southern hemisphere. Atmospheric water transport is also poleward outside the tropics and compares well with observations. Energy transport within the ice layer has been evaluated by both direct and indirect methods. As it is largely due to the latent heat of ice formation, it is closely proportional to the water transport by ice. The meridional transports by ice of both energy and water are relatively important at high latitudes. The divergence of the ice energy transport corresponds to a significant component of the surface energy budget, reaching ±10 W m -2 or more at some polar locations. The equilibrated doubled CO 2 global mean surface warming of the Mark 2 mixed layer model is 4.3°C. The reduction from the Mark 1 result (4.8°C) follows largely from a 40% reduction of the warming over high-latitude oceans. This is attributed to the presence of dynamically induced leads in the ice cover. The equilibrated warming for 3 x CO 2 is 6.8°C. The model atmosphere transports less heat poleward in the doubled CO 2 climate, largely as a response to increased solar radiation absorbed at high latitudes. This behavior contrasts with the change at CO 2 doubling in a transient simulation by the Mark 2 model coupled to a full ocean model, in which heat is taken up in the midlatitudes, particularly by the Southern Ocean, and supplied by a net top-of-atmosphere radiative imbalance distributed over all latitudes (global mean, 1.8 W m -2 ). The atmospheric water transport is enhanced by 10-20% in the warmer climates at most latitudes.

The influence of superimposed-ice formation on the sensitivity of glacier mass balance to climate change

Abstract: . The formation of superimposed ice at the surface of high-Arctic glaciers is an important control on glacier mass balance, but one which is usually modelled in only a schematic fashion. A method is developed to predict the relationship between the thick­ness of superimposed ice formed and the mean annual air temperature (which approxi­mates the ice temperature at 14 m depth). This relationship is used to investigate the dependence of the proportion of snowpack water equivalent which forms superimposed ice on changes in mean annual temperature and patterns of snow accumulation. Increased temperatures are likely to reduce the extent of the zone of superimposed-ice accumulation and the thickness of superimposed ice formed. This will have a negative effect on glacier mass balance. This is true even if warming occurs only in the winter months, since near-surface ice temperatures will respond to such warming. ForJohn Evans Glacier, Ellesmere Island, Nunavut, Canada (79°40'N, 74°00'W), a 1°C rise in mean annual air temperature due solely to winter warming is predicted to reduce the specific mass balance of the glacier by 0.008 m a-I as a result of decreased superimposed-ice formation. Although such a response is small in comparison to the changes which might result from summer warming, it is nonetheless significant given the very low specific mass balance of many high-Arctic glaciers. INTRODUCTION Global circulation models (GCMs) are consistent in pre­dicting that "greenhouse gas" -induced climate warming will be particularly marked at northern high latitudes, espe­cially in winter (Hansen and others, 1981; Cao and others, 1992; Manabe and others, 1992; McGinnis and Crane, 1994; Lynch and others, 1995). For a doubling of atmospheric CO2 from the present level of 340 ppmv, GCMs predict a warm­ing of 8-12°C for areas above the Arctic Circle in Decem­ber, January and February (Houghton and others, 1990, 1992). Summer warming is, however, predicted to be less than 4°C in the same areas. Summer precipitation is pre­dicted to increase slightly, while little change is expected in winter precipitation. Attempts to simulate the response of the mass balance of Arctic glaciers to climate changes such as these have utilised both energy-balance and degree-day mass-balance models (Huybrechts and others, 1991; Oerle­mans and Fortuin, 1992; B0ggild and others, 1994). Results generally suggest that the increase in melting due to summer warming is likely to exceed any increase in accumulation, such that Arctic glacier mass balance is likely to become more negative as climate warms. Winter warming and the associated reduction in seasonality of the thermal climate are generally considered to have no significant impact on mass balance (Braithwaite and Olesen, 1993). The mass-balance response of Arctic glaciers to climate warming is complicated by the process of superimposed-ice formation, which must therefore be incorporated in mass-Present address: School of Geography, University of Leeds, Leeds LS2 9JT, England. 186 balance models (Reeh, 1991; Oerlemans and others, 1993). Superimposed ice forms at the base of the snow pack on Arctic glaciers, where percolating meltwater refreezes on contact with underlying cold ice (Baird, 1952; Arnold, 1965; Koerner, 1970; Palosuo, 1987; Jonsson and Hansson, 1990). Ice formed in this way has to be melted again before it can run off from the glacier surface, effectively reducing the net ablation produced by a given input of energy to the glacier surface. Under current climatic conditions, superimposed­ice formation is an important process of accumulation on high-Arctic glaciers, and it is the dominant form of accum­ulation on many ice masses in the Canadian high Arctic, including the Meighen, Barnes, Bylot and Devon Ice Caps (Baird, 1952; Koerner, 1970). Thus, changes in the rate of superimposed-ice formation might be expected to play an important role in mediating the relationship between climate change and the mass balance of these glaciers. Typically, mass-balance simulation models treat the for­mation of superimposed ice in a highly schematic fashion. A common approach is to set some limit to the thickness of superimposed ice which can develop (Reeh, 1991; Huy­brechts and others, 1991; Letreguilly and others, 1991). This is done using a parameter, P-max, which is the proportion of snowpack water equivalent which forms superimposed ice at a given site. In general, P-max has been assigned a value of 0.6, largely on intuitive grounds, although some field studies do support a value of this order. For instance, Wolfe (1995) obtained a value of 0.67 from studies on Quvia­givaa Glacier, Ellesmere Island, Northwest Territories, Canada. In all studies of which we are aware, the value of P-max is held constant, even when climatic boundary conditions are altered substantially from those of the present day.

Atmospheric meridional circulation impacts on contrasting winter sea ice extent in two years in the Pacific sector of the Southern Ocean

Abstract: An explanation is sought for the marked variation in maximum sea ice extent observed between 2 years in the 2 areas of greatest interannual variability in winter ice extent in the South Pacific sector of the Southern Ocean. The role of ice recession in controlling ice extent is highlighted, and the adjustments in the near-surface atmospheric meridional circulation and air temperature that attend winter periods of ice retreat and advance are noted. Distinct meridional flow and air temperature adjustments attend periods of sea ice retreat that limit ice to higher than normal latitudes as well as ice advance to lower than normal latitudes. Ice advance leading to above-normal ice extent, for instance, takes place only when a lowering of air temperatures is accompanied by equatorward flow. A lack of warm air advection is, however, needed to adequately account for the development and maintenance of above-normal ice extent. Systematic meridional circulation changes also take place during the development and over the duration of ice extent anomalies. These are shown to emanate from adjustments of the semi-annual cycle in the extra-tropical South Pacific atmospheric circulation.

The Role of sea ice in 2×CO2 climate model sensitivity: Part II: Hemispheric dependencies

Abstract: How sensitive are doubled CO2 simulations to GCM control-run sea ice thickness and extent? This issue is examined in a series of 10 control-run simulations with different sea ice and corresponding doubled CO2 simulations. Results show that with increased control-run sea ice coverage in the Southern Hemisphere, temperature sensitivity with climate change is enhanced, while there is little effect on temperature sensitivity of (reasonable) variations in control-run sea ice thickness. In the Northern Hemisphere the situation is reversed: sea ice thickness is the key parameter, while (reasonable) variations in control-run sea ice coverage are of less importance. In both cases, the quantity of sea ice that can be removed in the warmer climate is the determining factor. Overall, the Southern Hemisphere sea ice coverage change had a larger impact on global temperature, because Northern Hemisphere sea ice was sufficiently thick to limit its response to doubled CO2, and sea ice changes generally occurred at higher latitudes, reducing the sea ice-albedo feedback. In both these experiments and earlier ones in which sea ice was not allowed to change, the model displayed a sensitivity of ∼0.02°C global warming per percent change in Southern Hemisphere sea ice coverage.

The relation between ice deformation, oceanic heat flux and the ice thickness distribution in the Arctic Ocean

Abstract: A one-dimensional ocean model, coupled with an ice model where the ice cover is partitioned into a number of thickness categories, is used to investigate how the ice thickness distribution in the Arctic Ocean depends on the two deformation processes, export (net divergence) and ridging. The model standard case generates vertical profiles of temperature, salinity and an ice thickness distribution similar to the observations. The standard case oceanic heat flux agrees well with recently deduced heat fluxes from Arctic Ice Dynamics Joint Experiment (AIDJEX) data. The oceanic heat flux is about 18 W m−2 during summer and about 1 W m−2, during winter. It is found that the mean ice thickness decreases with increasing ice export in close agreement with other investigations. However, the dependence of the mean ice thickness on ridging activity is found to be very weak. The change of the mean ice thickness is only 12 cm when increasing the ridging activity in the model from 0 to 0.4 yr−1; where the larger value represents an estimate of the actual ridging activity in the Arctic Ocean. In a sensitivity study, different types of ridging and albedo parameterizations are tested, and this weak dependence is found to be very robust. Other properties of the model ice cover are sensitive to ridging though: The equilibrium thickness of undeformed ice decreases with both export and ridging activity. The annual maximum open water fraction and the amplitude of the seasonal thickness variation increases with export and ridging.

On the surface energy budget of sea ice

Abstract: The surface energy budget was investigated during a cruise through the pack ice in the Southern Ocean. The time of observation was close to mid-summer. Some of the more important findings were: The mean albedo varied from 11% for open water to 59% for 10/10 ice cover. Hourly values span the range from 6% (open water) to 76% (total ice cover). The net heat flux into the ocean (B) was on average 109 W m -2 . If this energy were used solely for melting of sea ice, 30 mm could be melted each day. For low surface albedos (ice concentration below 7/10), the net radiation increased with decreasing cloudiness. However, the opposite was the case for a high surface albedo. The last point shows the importance of clouds on the surface energy budget. Not only should their presence or absence be known but also the reflectivity of the underlying surface, as it might change the net radiation in opposite ways.

Effects of clouds, ice sheet, and sea ice on the Earth radiation budget in the Antarctic

Abstract: The effects of clouds, the continental ice sheet, and sea ice on the radiation budget in the Antarctic are examined by using Earth Radiation Budget Experiment, International Satellite Cloud Climatology Project, and special sensor microwave/imager data in 1987/1988. The continental ice sheet affects not only the albedo but also the surface temperature because of elevation and hence the outgoing longwave radiation (OLR). The high elevation of the Antarctic continent makes the radiation budget in both polar regions asymmetric. At elevations below 2 km the OLR is reduced at the rate of 5–10 W/m2/km; above 2 km the rate is about 20 W/m2/km. Sea ice, which is a critical climate feedback factor, appears to have less impact on radiation than do clouds. Between 60° and 65°S in October, sea ice increases the top of the atmosphere albedo by about 0.2 and reduces the OLR by 7–10 W/m2; this seems smaller than the formal cloud forcing, which increases the albedo by 0.3 and reduces the OLR by 30–40 W/m2. However, these numbers do not fully differentiate the independent effects of sea ice and cloudiness. A more detailed analysis shows that the independent effect of sea ice is as large as clouds, with clouds masking the radiative effect of sea ice by more than one half.

Different melt regimes indicated by surface albedo measurements at the Greenland ice sheet margin : Application of TM image

Abstract: In eastern North Greenland it is observed that the surface originating from Wisconsin (the last Ice Age) is less reflectant than the adjacent Holocene ice of the present interglacial period. Due to deformation and flow, the oldest ice of Wisconsin origin it presently only constitutes the outermost 710 m of the ca. 10 km wide surface of the ablation zone, which only has a limited implication on the present surface ablation (melt) rates. However, the steady ice flow from the centre towards the margins, suggests that at the termination of the Ice Age the entire surface of the ablation zone constituted of this low reflectant ice, giving higher ablation rates than at present and may explain the fast retreat/disintegration of the former ice sheets. Ice core results document high dust content in Wisconsin ice, originating from intense fallout from the atmosphere due to a period with higher storminess than present. Therefore, the lower surface albedo of Wisconsin ice may very well relate to the dust content incorporated in the ice. Inside the Wisconsin/Holocene transition, i.e. between 710 m and 10 km from the ice margin, the age of the ice becomes gradually younger and is of present age at the equilibrium line. Over this distance resembling the Holocene epoch, deep ice cores document constant atmospheric dust fallout over the Greenland ice sheet. However, application of Landsat TM in this study documents a sharp albedo gradient within Holocene ice, which increases the annual ablation rate by a factor of 2-4. With the constant dust loading within the Holocene ice, it is argued that the albedo variations relate to the physical properties of the ice, i.e. the way in which dust and other impurities will become distributed on the surface, when it is melted free. The impurities can either concentrate in cryoconite holes (a high surface albedo) or remain homogeneously distributed on the surface (a low surface albedo). A critical crystal size of the ice may control which of the surface types become dominant.

Experimental and Theoretical Studies of Ice-Albedo Feedback Processes in the Arctic Basin

Abstract: : Our overall goal is to develop a quantitative understanding of processes that collectively make up the ice-albedo feedback mechanism. This mechanism is generally believed to be a key factor in amplifying natural variations within the earth's climate system. Central to achieving this understanding is learning more about how shortwave radiation is absorbed and distributed in the ice pack and upper ocean, and how this distribution affects the regional heat and mass balance of the ice cover. Complicating the problem are a variety of issues related to the extreme sub-grid scale variability of the Arctic ice cover and to how such variability can be accounted for in large-scale models. Our long-term goal is to develop accurate formulations of major ice-albedo feedback processes in a form suitable for inclusion in climate and general circulation models.