Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Project goals

Deep geological repositories for radioactive waste

need to be save for up to one million years. For

such a long perspective glacial periods and the

potential influence of deep erosion due to glaciers

have to be considered for the repository sites. The

aim of this project is to investigate the impact of

different glacial boundary conditions on the atmospheric

dynamics and the precipitation pattern,

and to identify the influence of the changes on

the glaciers in Switzerland.

In doing so, a set of glacial simulations has been

conducted and thoroughly analysed. Additionally,

the simulations are also considered as input

to future modelling studies, e.g., using regional

climate models or ice sheet models. However, such

regional simulations are not in the scope of the

current project.

Work carried out

and results obtained

This section is divided in two parts. First, a brief

summary of the experimental setup and of the

results of 2011 is given to set the framework of

the study. Then, the new results are presented in

three subsections focusing on the additional simulation,

on circulation type classification, and on the

precipitation change in Switzerland.

Experimental setup and summary 2011

The project is based on simulations with a global

atmosphere general circulation model (Community

Climate System Model version 4, [1]) with prescribed

sea surface temperatures (SST) and sea ice

extent. The model is run in a 0.9°x1.25° horizontal

resolution and the prescribed lower boundary conditions

are taken from simulations with a fully-coupled

but lower resolved atmosphere-ocean general

circulation model.

A pre-industrial simulation (1850 AD; hereafter PI)

that serves as reference simulation and a set of

glacial experiments based on two states of the last

glacial period have been conducted under different

perpetual forcing conditions (see Table 1). The two

states corresponds to the Last Glacial Maximum

(LGM, 21 ka ago) and to Marine Isotope Stage 4

(MIS4, 65 ka ago). In the glacial simulations the

boundary conditions differ from PI as follows: lower

concentrations of greenhouse gases, changed

Earth's orbital parameters, added major continental

ice sheets (different scenarios based on [2]; see

Table 1), and a sea level that is lower than today

(120 m difference for LGM and 80 m for MIS4).

The main results of the analysis of the mean differences

are summarized in Fig. 1 (see [3] for

more details). In the glacial simulations the winter

(December to February, DJF) storm track in

the North Atlantic is shifted to the south. As a

consequence of these changes, the precipitation is

increased over southern and eastern Europe. Contrastingly,

precipitation over the North Atlantic and

the extensive ice sheets is strongly reduced. Furthermore,

the set of simulations allows to clearly

identify the Laurentide ice sheet as the main driver

of the changed circulation, as the southward shift

of the storm track – and consequently the changes

in precipitation – are more pronounced in the

simulations with an enhanced elevation of this ice

sheet. Other boundary conditions, e.g., the radiative

forcing or the ocean surface, are of second

order importance.

The impact for the glaciers in Switzerland is, however,

difficult to address, as the mean precipitation

signal is only partially significant in this region. To

clarify the impact further analyses are performed

Description Simulation Ice sheet heights

Laurentide Fennoscandian Others

1850 AD simulation PI 0% 0% 0%

21 ka simulation LGM 100% 100% 100%

65 ka simulations MIS4 LT 46% 100% 100%

MIS4 LIN 67% 67% 67%

MIS4 FS 76% 33% 76%

MIS4 LGM 100% 100% 100%

MIS4 125 125% 125% 125%

Table 1: Overview of the simulations and the ice sheet topographies. The values for the ice sheet heights indicate how much

of the LGM–present-day topography changes are applied for the Laurentide, the Fennoscandian and all other (mainly Greenland

and Antarctica) ice sheets. MIS4 125 is the new simulation that has not been available in the first part of the project.

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ENSI Erfahrungs- und Forschungsbericht 2012