Synoptic-scale perturbations in AGCM simulations of the present and Last Glacial Maximum climates

The conditions of development of mid-latitude depressions (synoptic eddies) in the winter Northern Hemisphere mid-latitudes at the Last Glacial Maximu m (LGM, 21 000 years ago) are very different from the present ones: this pe riod is characterised by a general cooling of the extra-tropics, with massi ve ice sheets over the Northern Hemisphere continents and sea-ice extending very far south over the North Atlantic. The present work uses regression a nalysis to study the characteristics of the synoptic eddies in present-day and LGM climate simulations by the Atmospheric General Circulation Model (A GCM) of the UK Universities' Global Atmospheric Programme (UGAMP). In the L GM experiment, the structure of the Pacific eddies is similar to the presen t-day (PD) situation, but they are weaker. On the other hand, the Atlantic eddies show an increased zonal wavelength and a much shallower structure in the temperature and vertical wind perturbations. To understand the changes of these characteristics from present-day to LGM, we compare them to those computed for the most unstable modes of the corresponding mean flows, dete rmined using a dry primitive equation model. A normal-mode stability analys is is carried both on zonally symmetric and asymmetric flows for each of th e Northern Hemisphere storm-tracks. The changes in the most unstable normal modes found by both these analyses give a good account of changes in the s tructure of the perturbations as retrieved from the AGCM, suggesting that c hanges in the mean state (especially the temperature gradient) is the main driver of these changes. However in the case of the present-day Atlantic st orm-track, the growth rate of these modes is found to be very low compared to the other cases. A complementary analysis evaluates the importance of no n-modal growth, in the form of downstream development of perturbations, for each of the storm-tracks. This type of growth is found to be especially im portant in the case of the present-day Atlantic storm-track.