Response of the Western European climate to a collapse of the thermohaline circulation |
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Authors: | A Laurian S S Drijfhout W Hazeleger B van den Hurk |
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Institution: | (1) KNMI, Royal Netherlands Meteorological Institute, De Bilt, The Netherlands |
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Abstract: | Two ensemble simulations with the ECHAM5/MPI-OM climate model have been investigated for the atmospheric response to a thermohaline
circulation (THC) collapse. The model forcing was specified from observations between 1950 and 2000 and it followed a rising
greenhouse gases emission scenario from 2001 to 2100. In one ensemble, a THC collapse was induced by adding freshwater in
the northern North Atlantic, from 2001 onwards. After about 20 years, an almost stationary response pattern develops, that
is, after the THC collapse, global mean temperature rises equally fast in both ensembles with the hosing ensemble displaying
a constant offset. The atmospheric response to the freshwater hosing features a strong zonal gradient in the anomalous 2-m
air temperature over Western Europe, associated with a strong land–sea contrast. Since Western Europe climate features a strong
marine impact due to the prevailing westerlies, the question arises how such a strong land–sea contrast can be maintained.
We show that a strong secondary cloud response is set up with increased cloud cover over sea and decreased cloud cover over
land. Also, the marine impact on Western European climate decreases, which results from a reduced transport of moist static
energy from sea to land. As a result, the change in lapse rate over the cold sea surface temperature (SST) anomalies west
of the continent is much larger than over land, dominated by changes in moisture content rather than temperature. |
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