Synoptic-scale perturbations in AGCM simulations of the present and Last Glacial Maximum climates |
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Authors: | M Kageyama P J Valdes |
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Institution: | (1) Laboratoire des Sciences du Climat et de l'Environnement, CE Saclay, L'Orme de Merisiers, Batiment 709, 91191 Gif-sur-Yvette Cedex, France E-mail: masa@lsce.saclay.cea.fr, FR;(2) Department of Meteorology, University of Reading, United Kingdom, GB |
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Abstract: | The conditions of development of mid-latitude depressions (synoptic eddies) in the winter Northern Hemisphere mid-latitudes
at the Last Glacial Maximum (LGM, 21 000 years ago) are very different from the present ones: this period is characterised
by a general cooling of the extra-tropics, with massive ice sheets over the Northern Hemisphere continents and sea-ice extending
very far south over the North Atlantic. The present work uses regression analysis to study the characteristics of the synoptic
eddies in present-day and LGM climate simulations by the Atmospheric General Circulation Model (AGCM) of the UK Universities'
Global Atmospheric Programme (UGAMP). In the LGM experiment, the structure of the Pacific eddies is similar to the present-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, determined
using a dry primitive equation model. A normal-mode stability analysis is carried both on zonally symmetric and asymmetric
flows for each of the 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 structure of the perturbations as retrieved from the AGCM, suggesting that
changes in the mean state (especially the temperature gradient) is the main driver of these changes. However in the case of
the present-day Atlantic storm-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 non-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 important in the case of the present-day Atlantic
storm-track.
Received: 29 September 1999 / Accepted: 17 November 1999 |
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