Weather regimes in past climate atmospheric general circulation model simulations

We investigate the climates of the present-day, Inception of the Last Glaci ation (115000 y ago) and Last Glacial Maximum (21 000 y ago) in the extratr opical north Atlantic and Europe, as simulated by the Laboratoire de Meteor ologie Dynamique Atmospheric General Circulation Model. We use these simula tions to investigate the low-frequency variability of the model in differen t climates. The aim is to evaluate whether changes in the intraseasonal var iability, which we characterize using weather regimes, can help describe th e impact of different boundary conditions on climate and give a better unde rstanding of climate change processes. Weather regimes are defined as the m ost recurrent patterns in the 500 hPa geopotential height, using a clusteri ng algorithm method. The regimes found in the climate simulations of the pr esent-day and inception of the last glaciation are similar in their number and their structure. It is the regimes' populations which are found to be d ifferent for these climates, with an increase of the model's blocked regime and a decrease in the zonal regime at the inception of the last glaciation . This description reinforces the conclusions from a study of the differenc es between the climatological averages of the different runs and confirms t he northeastward shift to the tail of the Atlantic storm-track, which would favour more precipitation over the site of growth of the Fennoscandian ice -sheet. On the other hand, the Last Glacial Maximum results over this secto r are not found to be classifiable, showing that the change in boundary con ditions can be responsible for severe changes in the weather regime and low -frequency dynamics. The LGM Atlantic low-frequency variability appears to be dominated by a large-scale retrogressing wave with a period 40 to 50 day s. In addition weather regimes are found in a sector located further eastwa rd over the east Atlantic and European continent and are proved to be linke d to this low-frequency oscillation.