Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Appendix – Glacial vs. interglacial

climate – the mean states

Global and NH extra-tropical means

(Table A1)

The global and NH extra-tropical mean SAT and

precipitation values (see Table A1) are largely controlled

by orbital forcing as there are much larger

differences between different time periods (PI,

LGM, MIS4) than for different ice sheet configurations

(compare MIS4 LIN , MIS4 LGM and MIS4 125 ). That

means that on global scale, the ice sheet sensitivity

is clearly of second order importance.

For DJF, the global mean SAT of the LGM is about

5.8 °C colder than PI; the MIS4 temperature is

about 5.1 °C colder. For the NH extra-tropics the

difference is even more distinct with a cooling of

up to 10.5 °C for the LGM (8.5 °C for MIS4) during

the glacial state. Regarding winter precipitation

both the global and the NH extra-tropics show a

reduction in precipitation during glacial conditions.

In the most extreme case, i.e., for the LGM NH

extra-tropics, this reduction is about 25% of the

PI precipitation.

Similar to the winter means, the glacial summer

(JJA) conditions are colder and drier compared

with the recent past. The global mean changes

are of similar magnitude as in winter whereas the

reduction in the glacial NH extra-tropical JJA temperatures

is not as distinct as for DJF. The reduction

in summer precipitation is about 20% for all glacial

simulations for both the global and the NH extratropical

domain.

Winter (DJF) mean climate in the NH

during PI and glacial maximum conditions

(Figure A1)

The PI winters generally show moderate temperatures

over the North Atlantic and European continent

due to warm surface waters in the North

Atlantic. The main precipitation band results from

the storm track which goes along the eddy-driven

jet stream pointing from the US east coast towards

the British Isles and Scandinavia. The present ice

sheet configuration also lead to a zonal flow over

the North Atlantic resulting from a high pressure

band over the subtropics and a low pressure band

over the high latitudes. During a glacial maximum

(shown for MIS4 125 ) the North Atlantic surface

waters are much colder which leads to very cold

winters over much of the North Atlantic and European

region. The presence of extensive ice sheets

results in a redirection of the atmospheric flow,

e.g., the Laurentide ice sheet leads to a southward

shift of the North Atlantic storm track so precipitation

increases over the Iberian peninsula and to

some extent also in the Alpine region. The glacial

topography also fosters meridional circulations

patterns rather than a zonal flow as over the ice

sheets a cyclonic circulation of cold air (indicated

through high SLP values) is simulated.

Summer (JJA) mean climate in the NH

during PI and glacial maximum conditions

(Figure A2)

The changes from PI to MIS4 125 during the summer

season are generally less distinct than the winter

equivalents. Simulations with glacial boundary

conditions show a colder and drier summer climate,

in particular over the ice sheets itself. Due to

the same mechanical shift as in winter, the North

Simulation DJF global DJF 20N-90N JJA global JJA 20N-90N

SAT [°C]

PI 10.61 – 2.70 14.33 15.98

LGM 4.89 –13.27 9.07 9.69

MIS4 LIN 5.54 –11.00 10.21 11.27

MIS4 LGM 5.45 –11.24 10.23 11.37

MIS4 125 5.39 –11.30 10.23 11.37

Precipitation [mm/day]

PI 2.88 2.05 2.90 2.02

LGM 2.52 1.54 2.55 1.70

MIS4 LIN 2.58 1.63 2.62 1.76

MIS4 LGM 2.57 1.64 2.62 1.78

MIS4 125 2.57 1.65 2.62 1.78

Table A1: Global and

Northern Hemisphere

extra-tropics (20N-

90N) averages of surface

air temperature

(SAT [°C]) and total

precipitation [mm/day]

for the pre-industrial

and four glacial simulations.

ENSI Erfahrungs- und Forschungsbericht 2012 277

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