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---
layout: default
title: Report Series
---
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="Author" content="Anna McCravy">
<meta name="GENERATOR" content="Mozilla/4.51 [en] (WinNT; U) [Netscape]">
<title>rpt32</title>
</head>
<body text="#000000" bgcolor="#FFFFFF" link="#0000EE" vlink="#551A8B" alink="#FF0000">
<h1>Report 32: Intercomparison of Low-Frequency Variability
of the Global 200 hPa Circulation for AMIP Simulations</h1>
<ul><b>Boyle, </b>James S.
<br>March 1996, 48 pp.
<hr>In the Atmospheric Model Intercomparison Project (AMIP) a number of
GCMs are integrated for a 10 year period, 1979-1988, all using the same
monthly mean sea surface temperature (SST). This permits a useful intercomparison
of the response of the models to the imposed SST. The variables used here
for the intercomparison are the 200 hPa divergence and streamfunction.
The data used are in the form of monthly averages and are filtered to a
spatial resolution of T10, although the actual spatial resolution of the
models varies from R15 to T42. The data are manipulated in this manner
to concentrate on the low frequency, large scale response. The tools of
the analysis are principal components analysis (PCA) and common principal
components (CPC). These analyses are carried out on the 120 months of data
with the seasonal cycle removed and in the case of the streamfunction with
the zonal average also removed. The 1979-1988 period encompasses two El
Niño / Southern Oscillation (ENSO) events (1982/83 and 1986/87),
and as could be expected the ENSO characteristic response has a prominent
impact in the model simulations.
<p>The results indicate that :
<ul>The PCA of the divergence has a dominant mode which is similar for
all the models and has the signature of an (ENSO) response. It has an east-west
dipole of divergence anomaly centered on the equator in the western Pacific.
This mode accounts for 29% to 53% of the explained variance for the models
considered.
<p>The streamfunction PC analysis also exhibits an ENSO type response as
the dominant mode, but this accounts for only 8% to 21% of the variance.
<p>The CPC analysis allows a direct comparison of the data from all the
models on a common set of vectors. The component identified with the ENSO
mode represents 27% to 52% of the variance explained for the divergence
in this formulation. These results indicate that the models share a basic
common pattern but there is a strong variation in the amplitude of the
corresponding modes.
<p>The variance explained by the leading mode for the CPC streamfunction
is between 5% and 19%, and there is less commonality in the higher components
than seen in the divergence. This appears to be related to the stronger
streamfunction response in the mid-latitudes, which is presumably more
affected by nonlinearity and intrinsic variability of the model integrations.
<p>Based on results using an ensemble of five decadal runs using the ECMWF
GCM an estimate is made of the variation of explained variance due to intrinsic
variability for a single model. It is found that in general the inter-model
variation is somewhat greater than the intra-model ensemble variation using
the ECMWF model.
<p>A probability density function (PDF) analysis in the space spanned by
the first two CPCs for the velocity potential ( which explain over 70%
of the variance for all but one model) yields distinctive dynamical signatures.
Some models populate a somewhat larger PDF space than others.</ul>
There is a strong implication that the models differ beyond the variation
due to intrinsic variability in the dynamical system. Some of the models
have distinctly different responses to a common SST forcing. The disparate
results indicate that consensus on the representation of the physics of
the atmosphere has not been reached, and the present uncertainty in the
parameterizations is greater than the intrinsic uncertainty of the model
system as shown by ensemble simulations. <a href="pdf/32.pdf">(pdf
file)</a>
</ul>
<p><font size=-1>UCRL-MI-123395</font></p>