Caple AUF900 Uživatelský manuál stáhnout pdf (Strana 283)

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ENSI Erfahrungs- und Forschungsbericht 2011

duced precipitation over the tropical Atlantic and

a northward shift of the significant mid- and high-

latitude changes in precipitation.

Overall, the different ocean surface forcings for

LGM1 and LGM2 do not fundamentally alter the

large-scale precipitation anomaly patterns com-

pared to PI, even though the impact on winter

SATs is strong. The main difference between the

simulations is a modulation of the amplitude in

several regions, e.g. in the Nordic Seas. The effect

of the changed radiative forcing in MWLGM glob-

ally affects SATs and also the precipitation in the

tropics, but it is not possible to directly address its

impact on the precipitation in the North Atlantic

region as the major differences seen in this region

can be at least partly related to changes in the

ocean surface.

b) Sensitivity to ice sheet height

The impacts of the topography, i.e., the height of

the ice sheets, on the atmospheric dynamics and

the precipitation pattern are investigated using the

four MW sensitivity simulations (MWLIN, MWEU,

MWUS and MWNS). The comparison of these re-

sults to the sensitivity to ocean surface or radiative

forcing changes enables us to categorize the pro-

cesses with respect to their relevance in being re-

sponsible for precipitation changes over Europe.

As the mean altitude in the MW simulations is re-

duced compared to the LGM topography (due to

the lowering of the ice sheets), the global mean

SATs in the four simulations are slightly higher than

in MWLGM. Generally, the strongest SAT differ-

ences between the simulations are found in re-

gions where the altitudes are changed. Over the

ocean, the only significant difference between

the MW simulations is a warming (compared to

MWLGM) in winter between 40° and 50°N across

the North Atlantic that is most pronounced in

MWUS and MWNS reaching up to 8 °C.

The global mean precipitation is not affected by

the topography changes and does not significant-

ly differ in the MW simulations. Regarding win-

ter precipitation, however, a strong impact of the

height of the ice sheets is evident (Fig. 3g and i).

The band of positive precipitation anomalies in the

mid-latitudes that occurs in all three LGM topog-

raphy simulations is reduced and the reduction

is stronger the more the Laurentide ice sheet is

lowered. While the significant anomalies are only

slightly diluted in MWEU (Laurentide ice sheet

height at 76 % of the LGM value), they are strong-

ly reduced in MWLIN (height at 67 %) especially in

the eastern part and do no longer form a continu-

ous band. In MWUS and MWNS (height at 46 %)

the positive precipitation anomalies are limited to

the western part between 40°W to 70°W except

for a few small patches around Spain in MWUS.

The discrepancy between MWUS and MWNS

around Spain is attributed to internal variability,

as the difference between the two simulations is

not significant in this region. Additionally, in other

areas precipitation anomalies consistently change

with the height of the Laurentide ice sheet. A low-

er altitude of the Laurentide ice sheet corresponds

to a precipitation increase over the eastern part

of its slope, in the Labrador Sea and in the North

Atlantic at 20°N. The impact of the Fennoscandi-

an ice sheet is less pronounced and mainly affects

the precipitation at its southeastern slope of the

ice sheet. There, the precipitation is significantly

increased in the simulations with a lowered alti-

tude of the Fennoscandian ice sheet compared to

MWLGM while no significant changes are found

for MWUS and MWNS.

For summer precipitation the differences between

the four simulations are much smaller and not sig-

nificant for most regions (Fig. 3h and j). The few

significant changes point to a similar but much

weaker impact of the Laurentide ice sheet as in the

winter with increased precipitation in the high-lat-

itudes and a band of reduced precipitation across

the Atlantic compared to MWLGM, but overall

the anomaly patterns are similar for all MW sim-

ulations.

Analyzing the MW simulations indicates a strong

impact of the topography on the winter precipi-

tation pattern. For most of the anomalies in the

North Atlantic region the height of the Lauren-

tide ice sheet is identified as the dominant factor.

Together with the changed winter SATs over the

North Atlantic – even though the lower boundary

forcing is the same – the results suggest a change

of the atmospheric dynamics.

Importance of the atmospheric dynamics

To investigate the impact of the boundary condi-

tions on the synoptic scale variability in our simu-

lations two different methods are considered: an

Eulerian measure, which is defined as the band-

pass filtered (2.5–6 days) standard deviation of the

500 hPa geopotential height [6] and a Lagrang-

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Caple AUF900 Uživatelský manuál Strana 283

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