Jörg Bareiss

Surface properties and processes of perennial Antarctic sea ice in summer

Ice core and snow data from the Amundsen, Bellingshausen, and Weddell Seas, Antarctica show that the formation of superimposed ice and the development of seawater-filled gap layers with high algal standing stocks is typical of the perennial sea ice in summer. The coarse-grained and dense snow had salinities mostly below 0.1 per mil. A layer of fresh superimposed ice had a mean thickness ranging from 0.04-0.12 m. 0.04 to 0.08 m thick gap layers extended downwards from 0.02 to 0.14 m below the water level. These gaps were populated by diatom standing stocks up to 439 µg/l chlorophyll a. We propose a comprehensive heuristic model of summer processes, where warming and the reversal of temperature gradients cause major transformations in snow and ice properties. The warming also causes the re-opening of incompletely frozen slush layers caused by flood-freeze cycles during winter. Alternatively, superimposed ice forms at the cold interface between snow and slush in the case of flooding with negative freeboard. Combined, these explain the initial formation of gap layers by abiotic means alone. The upward growth of superimposed ice above the water level competes with a steady submergence of floes due to bottom and internal melting and accumulation of snow.

A model study of differences of snow thinning on Arctic and Antarctic first-year sea ice during spring and summer

Abstract The one-dimensional snow model SNTHERM is validated using field measurements of snow and superimposed ice thickness and surface energy fluxes. These were performed during the spring-to-summer transition in Svalbard and in the Weddell Sea, Antarctica. Both the seasonal snow-thickness decrease and the formation of superimposed ice are well reproduced by the model. During the three observation periods, observed and modeled snow thickness differ only by 13.1–27.1mm on average. In regional studies, the model is forced with atmospheric re-analysis data (European Centre for Medium-Range Weather Forecasts) and applied to several meridional transects across the Arctic and Southern Ocean. These show fundamental regional differences in the onset, duration and magnitude of snow thinning in summer. In the central Arctic, snowmelt onset occurs within a narrow time range of ±11 days and without significant regional differences. In contrast, the snow cover on Antarctic sea ice begins to melt about 25 days earlier and the length of the Antarctic snow-thinning season increases with increasing latitude. The importance of melting and evaporation for the modeled snow-thickness decrease is very different in the two hemispheres. The ratio of evaporated snow mass to melted snow mass per unit area is derived from the model, and amounts to approximately 4.2 in the Antarctic and only 0.75 in the Arctic. This agrees with observations and model results of the surface energy balance, and illustrates the dominance of surface cooling by upward turbulent fluxes in the Antarctic.

Impact of River Discharge and Regional Climatology on the Decay of Sea Ice in the Laptev Sea during Spring and Early Summer

Summer sea-ice conditions in the Laptev Sea are characterized by high interannual variability. The impact of Lena River discharge, one of the Arctics major rivers discharging roughly 525 km3 annually onto the Laptev shelf, and the regional meteorological regime affect the spring and summer ice regime of the Laptev Sea. Using ground and remote-sensing data and statistical analyses, it is shown that river discharge plays an insignificant role in the large-scale decay of the Laptev Sea ice cover. Hydrological and remote sensing data for the period 1979-1990 show that discharge/sea-ice interactions are confined to the coastal regions, with Lena River water flooding a fast-ice belt, roughly 25 km wide, in early to mid-June. Sea-ice decay and summer ice extent are shown to be affected most strongly by dynamic atmospheric forcing and by opening and enlargement of coastal polynyas in early spring.

The importance of diurnal processes for the seasonal cycle of sea-ice microwave brightness temperatures during early summer in the Weddell Sea, Antarctica

Abstract Over the perennial Sea ice in the western and central Weddell Sea, Antarctica, the onset of Summer is accompanied by a Significant decrease of Sea-ice brightness temperatures (Tb) as observed by passive-microwave radiometers Such as the Special Sensor Microwave/Imager (SSM/I). The Summer-specific Tb drop is the dominant feature in the seasonal cycle of Tb data and represents a conspicuous difference to most Arctic Sea-ice regions, where the onset of Summer is mostly marked by a rise in Tb. Data from a 5 week drift Station through the western Weddell Sea in the 2004/05 austral Summer, Ice Station POLarstern (IsPOL), helped with identifying the characteristic processes for Antarctic Sea ice. In Situ glaciological and meteorological data, in combination with SSM/I Swath Satellite data, indicate that the cycle of repeated diurnal thawing and refreezing of Snow (‘freeze–thaw cycles’) is the dominant process in the Summer Season, with the absence of complete Snow wetting. The resulting metamorphous Snow with increased grain Size, as well as the formation of ice layers, leads to decreasing emissivity, enhanced volume Scattering and increased backscatter. This causes the Summer Tb drop.

Satellite microwave observations of the interannual variability of snowmelt on sea ice in the Southern Ocean

[1] Snowmelt processes on Antarctic sea ice are examined. We present a simple snowmelt indicator based on diurnal brightness temperature variations from microwave satellite data. The method is validated through extensive field data from the western Weddell Sea and lends itself to the investigation of interannual and spatial variations of the typical snowmelt on Antarctic sea ice. We use in-situ measurements of physical snow properties to show that despite the absence of strong melting, the summer period is distinct from all other seasons with enhanced diurnal variations of snow wetness. A microwave emission model reveals that repeated thawing and refreezing cause the typical microwave emissivity signatures that are found on perennial Antarctic sea ice during summer. The proposed melt indicator accounts for the characteristic phenomenological stages of snowmelt in the Southern Ocean and detects the onset of diurnal snow wetting. An algorithm is presented to map large-scale snowmelt onset based on satellite data from the period between 1988 and 2006. The results indicate strong meridional gradients of snowmelt onset with the Weddell, Amundsen, and Ross Seas showing earliest (beginning of October) and most frequent snowmelt. Moreover, a distinct interannual variability of melt onset dates and large areas of first-year ice where no diurnal freeze thawing occurs at the surface are determined.

Observing snowmelt dynamics on fast ice in Kongsfjorden, Svalbard, with NOAA/AVHRR data and field measurements

Temporal snowmelt dynamics on fast ice in Kongsfjorden/Svalbard are studied for the period 1990-2003, using visible and near-infrared channels of the Advanced Very High Resolution Radiometer (AVHRR). Long-term radiation data from an adjacent Baseline Surface Radiation Network station, as well as extensive glaciological and meteorological field measurements on the melting ice in 2002 and 2003, are used to validate a snowmelt index derived from the satellite data. This study shows that the remote sensing data are in good agreement with the field observations. However, the temporal variability of atmospheric water vapour has an impact on the snowmelt index, and must be accounted for through atmospheric correction. The analysis of long-term satellite data provides valuable insight into the strength and rate of the snowvolume decay, and reveals a strong interannual variability of the snowmelt intensity. However, a precise date for determining melt onset requires clear-sky AVHRR data throughout the onset period.

New data set of onset of annual snowmelt on Antarctic sea ice

The annual onset of snowmelt on sea ice is essential for climate monitoring since it triggers a decrease in surface albedo that feeds back into a stronger absorption of shortwave radiation—a process known as the snowmelt-albedo feedback—and thus strongly modifies the surface energy balance during summer [Curry et al., 1995]. Algorithms designed for the detection of snowmelt on Arctic sea ice and based on longterm passive-microwave data [Anderson, 1997; Drobot and Anderson, 2001] revealed the melt season in the Arctic from 1979 to 1998 to be significantly elongated and the onset of melt to be shifted toward earlier dates [Drobot and Anderson, 2001; Belchansky et al., 2004]. In the Antarctic, however, little effort has been made so far in detecting the length of the summer melt season on sea ice by means of satellite microwave data. This results from the fact that surface melting in the Antarctic differs significantly from corresponding processes in the Arctic [Nicolaus et al., 2006]. The hemispheric differences are supported by extensive field measurements [Massom et al., 2001; Haas et al., 2001 ] and find expression in a reversal of the general surface radar backscatter and brightness temperature (TB) tendencies during summer [Haas, 2001; Kern and Heygster, 2001] : In the Antarctic, sea ice backscatter increases and TB decreases when summer approaches, contrary to the Arctic. Hence, algorithms developed for Arctic sea ice are not applicable on its southern counterpart. As summer air temperatures in the Antarctic rarely rise above 0°C, classical surface melt ponds have never been observed to the extent they appear in the Arctic and the sea ice surface typically remains snow-covered year-round. Drinkwater and Liu [2000] investigate snowmelt on Antarctic sea ice based on a method that identifies a decrease in surface radar backscatter. However, they detect melt to be lasting for only some days and exclusively on first-year ice. Presumably, the backscatter decrease they observe is due to flooding of the snow before the ice underneath finally deteriorates.

An observational and modelling analysis of Laptev Sea (Arctic Ocean) ice variations during summer

Abstract In this study we investigate the relationship of the atmospheric circulation and the sea-ice distribution in the Laptev Sea, Arctic Ocean, in the summers 1979−96. Sea-ice data from passive-microwave radiometers, global atmospheric data analyses, cyclone statistics and simulations of the regional climate model HIRHAM4 were analyzed to find out if periods of reduced or increased sea-ice concentrations are linked to synoptic patterns (circulation anomalies, cyclone activity). A canonical correlation analysis between Arctic sea-level pressure and sea-ice concentration anomalies confirms large-scale relationships among these variables. We did not find a simple relationship between sea-ice area anomalies and cyclone activity in the Laptev Sea area