Downscaling of current and future rainfall climatologies for southern Morocco. Part II: Climate change signals

摘要

Climate simulations from ECHAM4/OPYC3 (IS92a scenario) and ECHAM5/MP1-OM1 (SRES A1B scenario) are analysed regarding their precipitation change signal in southern Morocco at the border of the Saharan desert. Results are obtained from statistical-dynamical downscaling, using circulation weather types (CWTs) as driving fields. The trends obtained from the two climate models between their respective control (1969-1989) and scenario (2060-2089) periods are added to climatology obtained form NCEP reanalysis data (1958-1997). The resulting changes are compared to general circulation models (GCM) simulated rainfall changes. Application of the climate trends between the respective control and scenario periods of the two data sets to the climatology obtained from NCEP reanalyses leads to an increase of days with CWTs that favour rainfall in the region. This would lead to an enhanced rainfall amount by 20% for the ECHAM4/OPYC3 IS92a run and rainfall increase of 8% for the ECHAM5/MPI-OM1 model A1B run. These findings are rather surprising, since the simulated rainfall developments from ECHAM5/MP1-OM1 model AM run gives a negative precipitation tendency for the area (ca-25 mm/a) and the ECHAM4/OPYC3 IS92a run depicts a slight rise in rainfall rates (ca +25 mm/a), due to enhanced moisture transport in a warmer atmosphere. Thermodynamic aspects are not considered here since the used approach is purely dynamical. The contradicting rainfall tendencies are attributed to the coarse resolution of the GCMs and the important role the High Atlas Mountain range plays in the mechanisms generating precipitation in the region. (C) 2007 Royal Meteorological Society