Norbert Untersteiner

Snow Depth on Arctic Sea Ice

Snow depth and density were measured at Soviet drifting stations on multiyear Arctic sea ice. Measurements were made daily at fixed stakes at the weather station and once- or thrice-monthly at 10-m intervals on a line beginning about 500 m from the station buildings and extending outward an additional 500 or 1000 m. There were 31 stations, with lifetimes of 1‐7 yr. Analyses are performed here for the 37 years 1954‐91, during which time at least one station was always reporting. Snow depth at the stakes was sometimes higher than on the lines, and sometimes lower, but no systematic trend of snow depth was detected as a function of distance from the station along the 1000-m lines that would indicate an influence of the station. To determine the seasonal progression of snow depth for each year at each station, priority was given to snow lines if available; otherwise the fixed stakes were used, with an offset applied if necessary. The ice is mostly free of snow during August. Snow accumulates rapidly in September and October, moderately in November, very slowly in December and January, then moderately again from February to May. This pattern is exaggerated in the Greenland‐Ellesmere sector, which shows almost no net accumulation from November to March. The Chukchi region shows a steadier accumulation throughout the autumn, winter, and spring. The average snow depth of the multiyear ice region reaches a maximum of 34 cm (11 g cm22) in May. The deepest snow is just north of Greenland and Ellesmere Island, peaking in early June at more than 40 cm, when the snow is already melting north of Siberia and Alaska. The average snow density increases with time throughout the snow accumulation season, averaging 300 kg m23, with little geographical variation. Usually only two stations were in operation in any particular year, so there is insufficient information to obtain the geographical pattern of interannual variations. Therefore, to represent the geographical and seasonal variation of snow depth, a two-dimensional quadratic function is fitted to all data for a particular month, irrespective of year. Interannual anomalies for each month of each year are obtained relative to the long-term mean snow depth for the geographical location of the station operating in that particular year. The computed interannual variability (IAV) of snow depth in May is 6 cm, but this is larger than the true IAV because of inadequate geographical sampling. Weak negative trends of snow depth are found for all months. The largest trend is for May, the month of maximum snow depth, a decrease of 8 cm over 37 yr, apparently due to a reduction in accumulation-season snowfall.

Observations of melt ponds on Arctic sea ice

In an introductory section we review the physical processes influencing the formation and evolution of melt ponds on sea ice during the Arctic summer. As melt progresses, the changing properties of the surface interact strongly with the surface heat balance. The small interannual variability of the seasonal ice extent suggests an interannual variability of the surface heat balance of ±1 W m−2 or less. The interannual variance of atmospheric forcing represented by the transport of moist static energy into the Arctic is an order of magnitude greater. This appears to contradict the notion of a highly sensitive sea ice cover and emphasizes the need to generate albedo as an important internal variable in interactive models. Observations of melt ponds are needed in order to derive improved relationships between surface albedo and parameters such as the amount of snow, the onset and termination of melting, the ice thickness distribution, and ice deformation. Here classified (National Technical Means) imagery is used to measure melt pond coverage as it evolves over a summer on ice surrounding a drifting buoy. Local variability of pond cover is greatest at the beginning of the melt season, that is, pond coverage from 5% to 50% depending on ice type, as previously found by Russian investigators. An important distinction is found in the temporal change of pond cover: it decreases with time on thick ice, and it increases with time on thin ice (eventually leading to the disappearance of thin ice at the end of summer). An attempt to relate pond coverage to ice concentrations derived from passive microwave data proved unsuccessful.

The thinning of Arctic sea ice

The surplus heat needed to explain the loss of Arctic sea ice during the past few decades is on the order of 1 W/m 2 . Observing, attributing, and predicting such a small amount of energy remain daunting problems.

A study of the M2 tide in the Arctic Ocean

Numerical schemes for first and second order approximations on a 75-km grid are used to calculate amplitudes and phases of the M2 tide in the Arctic Basin. Assuming that all tidal energy enters from the Atlantic Ocean across a line approximately Tromso — Jan Mayen — Scoresby Sound, the computations yield a main amphidromic point off Prince Patrick Island at 81°30′ N, 133° W, confirming earlier assumptions. Several additional amphidromies appear to exist on the Eurasian Shelf. Frictional energy dissipation is assumed to be proportional to the square of horizontal velocity and the inverse square of water depth. Except for the region of Novaya Zemlya, observed and computed amplitudes and phases at points on the Eurasian and North American coast are in good agreement with observations. The results are shown on cotidal and corange maps, and a distribution of current ellipses. Also shown is a distribution of the maximum horizontal velocity divergence due to the M2 tide. Significant divergences, which are likely to affect the compactness of sea ice, are limited to the Greenland Sea, and the shallow waters of the Eurasian Shelf. From the computed velocity field, the amount of energy dissipated by bottom friction in the Arctic Basin averaged over one tidal period is estimated to be 5·1017 erg/s.

The Geophysics of Sea Ice: Overview

Terrestrial temperatures happen to lie in a range where, in the course of a year, the sea surface at high latitudes becomes cold enough to freeze. According to paleoclimatic studies of the sea floor sediments, this has been the case for at least several million years. During that time, however, the extent of sea ice underwent large fluctuations. For instance, at the last glacial maximum 18,000 years ago, Atlantic sea ice extended as far south as the shores of France and northern Spain in Europe, and the eastern coastal states in North America. The heat of fusion released by the sea surface to form an ice layer of, say, one meter thickness is an order of magnitude smaller than the mean annual total of either short-wave or long-wave radiation at the surface. In a single year, these annual totals may easily deviate ten percent from their mean values. Thus it is not surprising that small changes in the climatic forcing are accompanied by large changes in the extent of the ice-covered area.

Evaporation from Arctic sea ice in summer during the International Geophysical Year, 1957–1958

[1] Measurements of pan evaporation were made during the summers of 1957 and 1958 on an ice station drifting between 80° and 86°N. Using weather reports, measurements were either screened for absence of precipitation (to obtain evaporation, E) or not screened (to obtain P-E). Applying the screened data either to the entire month or only to the days without precipitation results in upper and lower limits to E. Monthly average values of E are positive in June and July, 3-5 and 5-8 mm/month, within the range of prior estimates, but are negative in August and September, indicating net deposition of frost or dew, at variance with prior estimates. The monthly averages of latent heat flux are small, 2-10 W m -2 by comparison to the individual components of net radiation, each on the order of 100-300 W m -2 .

The Thinning of Arctic Ice

The surplus heat needed to explain the loss of Arctic sea ice during the past few decades is on the order of 1W/m2. Observing, attributing, and predicting such a small amount of energy remain daunting problems.