Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Figure 8:

The frequencies of the

DJF daily wind direction

(on the 1000 hPa

level) over Switzerland.

The frequencies are

split into 4 categories

indicating the strength

of the wind.

wind speed is slightly increased and – more importantly

– a large Laurentide ice sheet as in MIS4 LGM ,

MIS4 125 and LGM leads to a predominantly southsouthwesterly

circulation. This is consistent with

a reconstruction that suggests a more southerly

atmospheric circulation for the Alps during LGM,

but not so during less pronounced glacial states

([6]). Generally, southern wind leads to more precipitation

at the southern side of the Alps. Consequently

the change from predominantly westerly

to southerly circulation will increase precipitation

especially in the southern parts of Switzerland.

Summary and Conclusions

This study has identified the Laurentide ice sheet

as the most important driver of changes in atmospheric

dynamics and glacial precipitation for the

European region ([3] and [5]). On a more regional

scale, the large-scale circulation changes in glacial

periods lead to increased precipitation and a shift

to more southern winds over Switzerland. In consequence,

the results suggest that under deep glacial

conditions, where the Laurentide ice sheet is large

(as in LGM or higher), increased accumulation has

to be expected for the glaciers in the Southern

Alps. However, the uncertainties remain large. For

a detailed evaluation of local glacier advances and

their potential for deep erosion further modelling

studies with included alpine ice sheet and sediment

models are needed.

Assessment 2012

The aims of the project extension were to further

analyse the simulations performed during the first

year of the project, to summarize these results in

a peer-reviewed publication and to perform an

additional simulation with a «super-glacial» ice

sheet in order to investigate its potential impact

on Europe and Switzerland. All aims have been

fulfilled in the last months. The project was scientifically

very successful as illustrated by the two

peer-reviewed publications and the international

interest experienced at several conferences and

workshops. The additional analysis of the existing

simulations exhibits that the ice sheet height is

crucial for the atmospheric circulation not only for

long-term mean changes but also on the weather

scale. Applying an atmospheric circulation classification

on daily data clearly shows that during

glacial times with an increased Laurentide ice sheet

the weather patterns are predominantly meridional

representing an west-east dipole structure. This

is different to the more zonal north-south dipole

patterns, which dominate the present day climate

and the recent past. In our study, we were further

able to show that these changes in the circulation

types are responsible for a substantial part of the

precipitation differences when comparing glacial

times with today. The additional «super-glacial» ice

sheet simulation shows that a further increase of

the Laurentide ice sheet also leads to an additional

increase in precipitation over southern and central

Europe during winter. This means that under the

constraint of an extreme high Laurentide ice sheet

the European-Alpine ice sheet has the potential to

grow which in turn will imply enhanced deep erosion

due to glaciers. Focusing on Switzerland we

see that the increased precipitation during glacial

times goes along with a shift to more southerly

flow. Thus, the simulations suggest that in particular

the Southern Alps receive more precipitation

during glacial times with high ice sheets. The

«super-glacial» ice sheet simulation additionally

increases the precipitation on the northern side of

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