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J. Oerlemans

Abstract

An objective approach to breaks in the weather is presented. A simple mathematical procedure is described that makes it possible to detect changes in a time series. The method is based on a comparison of a typical change, prescribed a priori, with an interval of a local record of daily temperature, for instance. A break quality is defined as the ratio of the amplitude of the change (computed by a least-squares method) to the corresponding rms difference between the typical and observed change. In this way for each day a break quality may he computed. The real breaks are found by requiring that the break quality have a maximum value with respect to time. Since the break quality is a dimensionless quantity, different weather elements may be compared immediately.

Results are given of a break analysis of daily values of temperature, sunshine and precipitation as observed at De Bilt during the period 1949-74. Daily mean temperature shows the highest break qualities while precipitation hardly shows clear breaks. The correlation between break qualities of different weather elements appears to be small.

The method was also applied to the geopotential height field over the North Atlantic Ccean. The existence of the preferred region of blocking over the northeastern part of the Atlantic is seen-very clearly by means of the break analysis.

Finally, some. remarks are made concerning the. construction of a statistical significance test and some further possibilities of the break analysis are indicated.

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J. Oerlemans and H. F. Vugts

Preliminary results are described from a glaciometeorological experiment carried out in the margin (melting zone) of the Greenland ice sheet in the summers of 1990 and 1991. This work was initiated within the framework of a Dutch research program on land ice and sea level change. Seven meteostations were operated along a transect running from the tundra well onto the ice sheet. At the ice edge, humidity, temperature, and wind profiles were obtained with a tethered balloon. On the ice sheet, 90 km from the edge, a boundary-layer research unit, including a sound detecting and ranging system (SODAR) and a radio acoustic sounding system (RASS), was established.

Although focusing on the relation between surface energy balance, glacier mass balance, and ice flow, the experiment has also delivered a unique dataset on the dynamics of the atmospheric boundary layer around the warm tundra–cold ice sheet transition.

Unexpected behavior was found for the surface albedo during the melt season. Lowest values are not found close to the ice edge, which is usual for glaciers, but higher on the ice sheet. Meltwater accumulation due to inefficient surface drainage was found to be the cause for this.

The wind regime is dominated by katabatic flow from the ice sheet. The katabatic layer is typically 100–200 m thick. Close to the ice edge, the flow exhibits a very regular daily rhythm, with maximum wind speed in the afternoon. Farther on the ice sheet, the regime changes, and wind speed reaches maximum values in late night/early morning.

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B. K. Reichert, L. Bengtsson, and J. Oerlemans

Abstract

Glacier fluctuations exclusively due to internal variations in the climate system are simulated using downscaled integrations of the ECHAM4/OPYC coupled general circulation model (GCM). A process-based modeling approach using a mass balance model of intermediate complexity and a dynamic ice flow model considering simple shearing flow and sliding are applied. Multimillennia records of glacier length fluctuations for Nigardsbreen (Norway) and Rhonegletscher (Switzerland) are simulated using autoregressive processes determined by statistically downscaled GCM experiments. Return periods and probabilities of specific glacier length changes using GCM integrations excluding external forcings such as solar irradiation changes, volcanic, or anthropogenic effects are analyzed and compared to historical glacier length records. Preindustrial fluctuations of the glaciers as far as observed or reconstructed, including their advance during the “Little Ice Age,” can be explained by internal variability in the climate system as represented by a GCM. However, fluctuations comparable to the present-day glacier retreat exceed any variation simulated by the GCM control experiments and must be caused by external forcing, with anthropogenic forcing being a likely candidate.

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B. K. Reichert, L. Bengtsson, and J. Oerlemans

Abstract

A process-oriented modeling approach is applied in order to simulate glacier mass balance for individual glaciers using statistically downscaled general circulation models (GCMs). Glacier-specific seasonal sensitivity characteristics based on a mass balance model of intermediate complexity are used to simulate mass balances of Nigardsbreen (Norway) and Rhonegletscher (Switzerland). Simulations using reanalyses (ECMWF) for the period 1979–93 are in good agreement with in situ mass balance measurements for Nigardsbreen. The method is applied to multicentury integrations of coupled (ECHAM4/OPYC) and mixed-layer (ECHAM4/MLO) GCMs excluding external forcing. A high correlation between decadal variations in the North Atlantic oscillation (NAO) and mass balance of the glaciers is found. The dominant factor for this relationship is the strong impact of winter precipitation associated with the NAO. A high NAO phase means enhanced (reduced) winter precipitation for Nigardsbreen (Rhonegletscher), typically leading to a higher (lower) than normal annual mass balance. This mechanism, entirely due to internal variations in the climate system, can explain observed strong positive mass balances for Nigardsbreen and other maritime Norwegian glaciers within the period 1980–95. It can also partly be responsible for recent strong negative mass balances of Alpine glaciers.

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J. Oerlemans and H. M. Van Den Dool

Abstract

A zonally averaged Climate model of the energy-balance type is examined. Recently published satellite measurements were used to improve existing parameterizations of planetary albedo and outgoing radiation in term of surface and sea level temperature. A realistic constant for the diffusion of energy was found by tuning the model to the present climate. For the actual solar constant both the present climate and an ice-covered earth are solutions of the model. They are extremely stable for temperature perturbations.

The effect of variation of the solar constant was investigated in detail. If the solar constant is decreased by 9–10% the warm solution (partial ice cover) jumps to the cold one (complete ice cover). Transition from the cold to the warm solution requires an increase of the solar constant to 109–110% of its present value. Therefore, we conclude that the model climate is much more stable with regard to variations in the solar input than has been assumed so far. This is caused mainly by our updated formulation of the outgoing radiation. Further experiments showed that our model is much more sensitive to changes in the outgoing radiation than to changes in the diffusivity for energy.

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J. L. Nap, H. M. Van Den Dool, and J. Oerlemans

Abstract

Monthly forecasts of temperature, rainfall and sunshine have been verified during the period 1970–79. The predictions were based on seven different schemes. Of the seven methods, five refer to De Bilt (The Netherlands), one to southeast England and one to the Federal Republic of Germany. The results are not very encouraging for any of the methods. The skill is negligible except for a few schemes that predicted the monthly mean temperature ∼ 10% better than climatology.

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J. R. de Wolde, R. Bintanja, and J. Oerlemans

Abstract

Estimates of sea level rise during the period 1856–1991 due to thermal expansion are presented. The estimates are based on an ocean model that consists of three zonally averaged ocean basins representing the Atlantic, Pacific, and Indian Oceans. These basins are connected by a circumpolar basin that represents the Southern Ocean. The ocean circulation in the model was prescribed. Surface ocean forcing was calculated from observed sea surface temperatures. Global mean forcing and regionally varying forcing were distinguished. Different parameterizations of ocean heat mixing were incorporated. According to the model presented, global mean sea level rise caused by thermal expansion over the last hundred years ranged from 2.2 to 5.1 cm, a best estimate being 3.5 cm.

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J. M. Gregory, N. J. White, J. A. Church, M. F. P. Bierkens, J. E. Box, M. R. van den Broeke, J. G. Cogley, X. Fettweis, E. Hanna, P. Huybrechts, L. F. Konikow, P. W. Leclercq, B. Marzeion, J. Oerlemans, M. E. Tamisiea, Y. Wada, L. M. Wake, and R. S. W. van de Wal

Abstract

Confidence in projections of global-mean sea level rise (GMSLR) depends on an ability to account for GMSLR during the twentieth century. There are contributions from ocean thermal expansion, mass loss from glaciers and ice sheets, groundwater extraction, and reservoir impoundment. Progress has been made toward solving the “enigma” of twentieth-century GMSLR, which is that the observed GMSLR has previously been found to exceed the sum of estimated contributions, especially for the earlier decades. The authors propose the following: thermal expansion simulated by climate models may previously have been underestimated because of their not including volcanic forcing in their control state; the rate of glacier mass loss was larger than previously estimated and was not smaller in the first half than in the second half of the century; the Greenland ice sheet could have made a positive contribution throughout the century; and groundwater depletion and reservoir impoundment, which are of opposite sign, may have been approximately equal in magnitude. It is possible to reconstruct the time series of GMSLR from the quantified contributions, apart from a constant residual term, which is small enough to be explained as a long-term contribution from the Antarctic ice sheet. The reconstructions account for the observation that the rate of GMSLR was not much larger during the last 50 years than during the twentieth century as a whole, despite the increasing anthropogenic forcing. Semiempirical methods for projecting GMSLR depend on the existence of a relationship between global climate change and the rate of GMSLR, but the implication of the authors' closure of the budget is that such a relationship is weak or absent during the twentieth century.

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