Yutaka Ageta

Effect of summer accumulation on glacier mass balance on the Tibetan Plateau revealed by mass-balance model

The characteristics and sensitivities of a cold-based glacier on the Tibetan Plateau, where the summer monsoon provides most of the mass input to glaciers, are discussed using an energy-balance model incorporating the process of water refreezing. The model accurately represents the observational results related to the mass balance of Xiao Dongkemadi glacier on the central plateau during 1992/93. Our data revealed that the mass balance of cold glaciers cannot simply be described by the surface mass/heat balances, because about 20% of infiltrated water is refrozen and thus does not run off from the glacier. Model calculations demonstrate that glaciers in an arid environment can maintain their mass since the monsoon provides precipitation during the melting season. Snowfall in summer keeps surface albedo high and largely restrains ablation. Nevertheless, the calculations also make clear that glaciers on the plateau are more vulnerable than those of other regions because of summer accumulation. In the monsoon climate, warming would cause not only a decrease in accumulation, but also a drastic increase in ablation in combination with surface-albedo lowering. Therefore, although glaciers on and around the plateau can be sustained by summer accumulation, they are more vulnerable to warming than winter-accumulation-type glaciers.

Predictions of changes of glacier mass balance in the Nepal Himalaya and Tibetan Plateau : a case study of air temperature increase for three glaciers

Annual mass exchange differs between maritime and continental glaciers. A common characteristic of these glaciers in Asian high-mountain areas is that most of the annual accumulation occurs in summer. Since variations in mass balance of a summer-accumulation type of glacier are quite sensitive to variations in summer air temperature, shrinkages of such glaciers due to climate warming are predicted by the use of simplified experimental relations between air temperature and mass balance, disregarding variation of other climatic variables such as cloudiness and precipitation. The results predict that both small and large maritime glaciers are more sensitive to warming than a continental ice cap. A small glacier would disappear in a few decades if the air temperature persisted a few degrees above that of an equilibrium state of mass balance.

Superimposed ice in glacier mass balance on the Tibetan Plateau

The relations betw een mass bal ance and meltwater refre ezing were examined on the basis of glac iolo gical obse rvations carried out in summer 1993 on Xiao Dongkemadi Glacier, Tanggula Mountai ns, central Tibetan Plateau. On this glacier, a part of melt water refreezes at the snovv/ice int erface as superimp osed ice. The amount of supe rimp osed ice formation was determ ined by both meltw ater supply and temperat ure condit ion of the glaci er. Snow-lay er thic kness on the glacier ice body is less than 2 m, even in the higher accu mulat ion zone. About 60% of meltw ater generate d in the accumula tion zone for the period ?v1ay-September was trapped at the snow/ice interface by refre ezing , and was not dis charged out of the glacie r. About 26% of accumulated snow to the glacier surface was repl aced on the snow/ice interface by refreez ing in the accumulation zone. These facts indicate that superim posed ice formation is quit e signifi cant for water retention in glaciers unde r low-pre cipitation conditions.

Mass balance of Xiao Dongkemadi glacier on the central Tibetan Plateau from 1989 to 1995

Abstract Data on the mass balance of Xiao Dongkemadi glacier in the Tanggula mountains, central Tibetan Plateau, were obtained over 5 5 years from 1989 to 1995. These are the first continuous mass-balance data for a continental-type glacier on the Tibetan Plateau, where the glacier accumulates during the summer monsoon (summer-accumulation-type glacier). Mass-balance vs altitude profiles were steeper in the negative than in the positive mass-balance years. This is considered to have resulted from the effect of summer accumulation. The annual mass balance is compared with air temperature, precipitation, and black-body temperature in the area including the glacier, which is calculated from infrared radiation observations by theJapanese Geostationary Meteorological Satellite. It was found that the interannual variation in the glacier mass balance was not closely related to maximum monthly mean air temperature, while it did have a relatively good correlation with maximum monthly mean black-body temperature.

Application of a mass-balance model to a Himalayan glacier

A mass-balance model based on the energy balance at the snow or ice surface is formulated, with particular attention paid to processes affecting absorption of radiation. The model is applied to a small glacier, Glacier AX010 in the Nepalese Himalaya, and tests of its mass-balance sensitivity to input and climatic parameters are carried out. Calculated and observed area-averaged mass balances of the glacier during summer 1978 (June-September) show good agreement, namely-0.44 and -0.46 m w.e., respectively. Results show the mass balance is strongly sensitive to snow or ice albedo, to the effects of screening by surrounding mountain walls, to areal variations in multiple reflection between clouds and the glacier surface, and to thin snow covers which alter the surface albedo. In tests of the sensitivity of the mass balance to seasonal values of climatic parameters, the mass balance is found to be strongly sensitive to summer air temperature and precipitation but only weakly sensitive to relative humidity.

Temporal Changes in Elevation of the Debris-Covered Ablation Area of Khumbu Glacier in the Nepal Himalaya since 1978

Abstract We evaluated elevation changes at four sites on debris-covered ablation area of Khumbu Glacier, Nepal Himalaya, since 1978. In 2004, we carried out a ground survey by differential GPS in the upper- and lowermost areas of the ablation area. The amount of surface lowering was calculated by comparing digital elevation models (DEMs) with 30-m grid size, as generated from survey data corrected in 1978, 1995, and in the present study. Because we could not access the middle parts of the debris-covered area due to surface roughness, for this area we used an ASTER-DEM calibrated by the ground survey data. The amount of surface lowering during the period 1978–2004 was insignificant near the terminus. A remarkable acceleration of surface lowering was found in the middle part of the debris-covered ablation area, where the glacier surface is highly undulating. In the uppermost area, surface lowering has continued at a steady rate. Surface flow speeds have decreased since 1956, revealing that the recent decrease in ice flux from the upper accumulation area would have accelerated the rate of surface lowering of the debris-covered area of Khumbu Glacier during the period 1995–2004.

State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling

Global cooling in intermediate glacial climate with northern ice sheets preconditions climatic instability with bipolar seesaw. Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets.

Climatic Information from the Chongce Ice Cap, West Kunlun, China

Shallow corings and pit studies were carried out in 1987 on the Chongce Ice Cap in the West Kunlun Mountains which lie along the southern edge of the Taklamakan Desert. Stratigraphic observations, as well as measurements of insoluble particles, chemical constituents and liquid conductivity , on the core and pit samples allowed identification of annual layers, although visible dirt layers are not alwa ys annual signals. A vertical profile of stable isotope content showed a periodic variation near the surface, strongly attenuated with depth.

Estimation of atmospheric transmissivity of solar radiation from precipitation in the Himalaya and the Tibetan Plateau

Abstract Though solar radiation is important for glacier mass-balance simulation, solar radiation data are not always available. As a result of analyzing meteorological data measured in the Himalaya and the Tibetan Plateau, a favorable correlation between precipitation and atmospheric transmissivity of solar radiation is found in terms of monthly values. Monthly mean solar radiation is derived from the relationship between atmospheric transmissivity of solar radiation and precipitation with input of monthly precipitation, latitude, skyline and time. The differences between estimated and observed monthly mean solar radiation are <40Wm−2 in most cases. However, the differences at some sites are significantly large. The error in the estimated solar radiation during the monsoon season can be large when the monthly mean precipitation rate is about 5 mm d−1. Though the error in the estimated solar radiation during the non-monsoon season is generally small due to low precipitation in the Himalaya and the Tibetan Plateau during this season, it can exceed 100 W m−2.