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layout: AMIP
title: Abstract
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<B>Harzallah, A. </B>and R. Sadourny.&nbsp; 1995:&nbsp; Internal versus
SST-forced atmospheric variability as simulated by an atmospheric general
circulation model.&nbsp; <I>Journal of Climate,</I> <B>8</B>, 474-475.&nbsp;
<HR WIDTH="100%">The variability of atmospheric flow is analyzed by separating
it into an internal part due to atmospheric dynamics only and an external
(or forced) part due to the variability of sea surface temperature forcing.
The two modes of variability are identified by performing an ensemble of
seven independent long-term simulations of the atmospheric response to
observed SST (1970-1988) with the LMD atmospheric general circulation model
The forced variability is defined from the analysis of the ensemble mean
and the internal variability from the analysis of deviations from the ensemble
mean. Emphasis is put on interannual variability of sea level pressure
and 500-hPa geopotential height for the Northern Hemisphere winter. In
view of the large systematic errors related to the relatively small number
of realizations, unbiased variance estimators have been developed Although
statistical significance is not reached in some extratropical regions,
large significant extratropical responses are found at the North Pacific-Alaska
sector for SLP and over western Canada and the Aleutians for 500-hPa geopotential
height The influence of SST variations on internal variability is also
examined by using a 7-year simulation using the climatological SST seasonal
cycle. It is found that interannual SST changes strongly influence the
geographical distribution of internal variability; in particular, it tends
to increase it over oceans. Patterns of internal and external variability
of the 500-hPa geopotential height are further examined by using EOF decompositions.
showing that the model realistically simulates the leading observed variability
modes. The geographical structure of internal variability patterns is found
to be similar to that of total variability, although similar modes tend
to evolve rather differently in time. The zonally symmetric seesaw dominates
the internal variability for both observed and climatologically prescribed
SST. The Pacific-North American (PNA ) and Western Pacific (WP) patterns,
on the other hand, are the dominant modes associated with patterns of SST
variability; the latter is related to Atlantic anomalies. while the former
responds to both El Ni&ntilde;o events and extratropical forcing.