Analysis of the mid-latitude weather regimes in the 200-year control integration of the SINTEX model

Abstract

Recent results indicate that climate predictions require models which can simulate accurately natural circulation regimes and their associated variability. The main purpose of this study is to investigate whether (and how) a coupled model can simulate the real world weather regimes. A 200-year control integration of a coupled GCM (the «SINTEX model») is considered. The output analysed consists of monthly mean values of Northern Hemisphere extended winter (November to April) 500-hPa geopotential heights. An Empirical Orthogonal Function (EOF) analysis is first applied in order to define a reduced phase space based on the leading modes of variability. Therefore the principal component PDF in the reduced phase space spanned by two leading EOFs is computed. Based on a PDF analysis in the phase space spanned by the leading EOF1 and REOF2, substantial evidence of the nongaussian regime structure of the SINTEX northern winter circulation is found. The model Probability Density Function (PDF) exhibits three maxima. The 500-hPa height geographical patterns of these density maxima are strongly reminiscent of well-documented Northern Hemisphere weather regimes. This result indicates that the SINTEX model can not only simulate the non-gaussian structure of the climatic attractor, but is also able to reproduce the natural modes of variability of the system.

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ANNALS OF GEOPHYSICS, VOL. 46, N. 1, February 2003
27
Analysis of the mid-latitude weather
regimes in the 200-year control
integration of the SINTEX model
Susanna Corti (1), Silvio Gualdi (2) and Antonio Navarra (2)
(1) Istituto di Scienze dell’Atmosfera e del Clima, CNR, Bologna, Italy
(2) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy
Abstract
Recent results indicate that climate predictions require models which can simulate accurately natural circulation
regimes and their associated variability. The main purpose of this study is to investigate whether (and how) a
coupled model can simulate the real world weather regimes. A 200-year control integration of a coupled GCM
(the«SINTEX model») is considered. The output analysed consists of monthly mean values of Northern Hemisphere
extended winter (November to April) 500-hPa geopotential heights. An Empirical Orthogonal Function (EOF)
analysis is first applied in order to define a reduced phase space based on the leading modes of variability. Therefore
the principal component PDF in the reduced phase space spanned by two leading EOFs is computed. Based on a
PDF analysis in the phase space spanned by the leading EOF1 and REOF2, substantial evidence of the non-
gaussian regime structure of the SINTEX northern winter circulation is found. The model Probability Density
Function (PDF) exhibits three maxima. The 500-hPa height geographical patterns of these density maxima are
strongly reminiscent of well-documented Northern Hemisphere weather regimes. This result indicates that the
SINTEX model can not only simulate the non-gaussian structure of the climatic attractor, but is also able to
reproduce the natural modes of variability of the system.
1. Introduction
Previous studies have assumed that anthro-
pogenic (and natural) climate change can be un-
derstood in terms of a linear superposition of a
response to external forcing on unchanging
background variability. While this may be an
adequate description of large-scale temperature
changes, there is evidence that changes in atmos-
pheric circulation may be better characterized as
certain naturally-occurring weather regimes
becoming more or less prevalent in response to
a given external forcing. In particular, based on
analyses of mid-tropospheric geopotential data,
a recent study (Corti et al., 1999) suggested that
trends in northern-hemispheric climate over the
latter decades can be interpreted in terms of a
change in the relative probability of naturally-
occurring atmospheric circulation regimes, like
the «Cold Ocean Warm Land» (Wallace et al.,
1996) and «Arctic Oscillation» (Thomson and
Wallace, 1998) patterns, rather than a simple
linear shift in the mean climate with super-
imposed noise. This result supports a non-linear
dynamical paradigm that the climate response to
anthropogenic forcing may project principally
on the dominant patterns of natural climatic
variability (Palmer, 1999).
Mailing address: Dr. Susanna Corti, Istituto di Scienze
dell’Atmosfera e del Clima, CNR, Via Gobetti 101, 40129,
Bologna, Italy; email: s.corti@isac.cnr.it
Key words Coupled General Circulation Model −
systematic error − non-linear dynamics − flow regimes

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Susanna Corti, Silvio Gualdi and Antonio Navarra
If valid, this new conceptual model has im-
portant implications both for the interpretation
of the observed signal and for future climate
change. In particular, considering this nonlinear
perspective, the prediction of anthropogenic
climate change require models which can sim-
ulate accurately natural circulation regimes and
their associated variability, even though the
dominant timescale of such variability may be
much shorter than the climate change signal
itself. More generally, these models should be
able to simulate the non-gaussian characteristics
of the climate attractor. To date, Atmospheric-
Ocean General Circulation Models (A-OGCMs)
have not been tested in this way to any great
extent. The main purpose of the present study is
to explore how well the SINTEX model (Gualdi
et al., 2003) is capable of simulating the kind of
regime behaviour outlined above.
The paper is structured as follows. The 500
hPa geopotential height model wintertime cli-
matology is presented in Section 2, while Section
3 is devoted to the EOF analysis of the monthly-
mean of the Northern Hemisphere extended
winter (November to April) 500-hPa geopotential
height as simulated by the model. In Section 4
geographical patterns of the four (simulated)
atmospheric regimes, together with the corre-
sponding probability density estimation, are
shown. Preliminary conclusions are to be found
in Section 5.
2. Simulation of the wintertime climatology
This study analyses a 200-year integration of
a coupled GCM (the «SINTEX model»). The
atmospheric component of the coupled model is
ECHAM4 (at T30 resolution), developed at the
Max-Planck Institute (Hamburg). The oceanic
component is the OPA model developed by
LODYC (Paris). More detailed information on
these models and the coupling strategies are to
be found in a companion paper in this special
issue (Gualdi et al., 2003).
The output analysed consists of monthly
mean values of Northern Hemisphere extended
winter (November to April) 500-hPa geopotential
height. Figure 1a-f compares the cold-season
model climatology (of 500-hPa geopotential
height) with the corresponding NCEP/NCAR
climatology over the period 1958-1998 (Kalnay
et al., 1996). The full field mean error and the
model and observed standard deviations are
shown in the left column, while panels on the
right column show the eddy component of the
fields (i.e. their deviations from zonal means).
Just looking at the full height fields error in
panel (a), it is evident that the northern extra-
tropical mean flow has a level of realism com-
parable to that of higher resolution GCMs, with
systematic errors reaching a maximum amplitude
of about 80 m in the North Pacific and Polar
regions. The error pattern exhibits a quite zonal
symmetry: there is a general overestimation of
the mean height north of 60°N, partially counter-
balanced, a part for South Europe, by a negative
error south of this latitude. The model distribution
of low-frequency variability (compare standard
deviation patterns in panel (b) and panel (c)) is
very realistic: the three maxima in the North
Pacific (shifted south-eastward though), North
Atlantic and North-Western Siberia are fairly
well reproduced, albeit about 10 to 20 % lower
than observed.
Comparing the eddy fields in panel (d) and
panel (e), the simulated amplitude of the stationary
waves is fairly realistic in the Pacific sector, over-
estimated by 20 % in the Euro-Atlantic region,
and about one third of the observed amplitude
over the eastern part of North America. However,
the systematic model error seems dominated by
its eddy component (cf. panels (a) and (f))
everywhere except for North-Eastern Asia.
3. Empirical Orthogonal Function (EOF)
analysis
The leading variability patterns of the model
atmosphere were searched for by calculating
Empirical Orthogonal Functions (EOFs) of
monthly 500 hPa height anomalies (the season-
al cycle has been removed from the data by
computing anomalies with respect to the long-
term monthly mean). Figure 2 shows the first six
EOFs patterns from the 200-years model in-
tegration, whilst the corresponding patterns
computed from the observed data set (NCEP
1958-1998 reanalyses) are displayed in fig. 3.

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Analysis of the mid-latitude weather regimes in the 200-year control integration of the SINTEX model
Fig. 1a-f. Statistics of 500 hPa geopotential height from NDJFMA 200-year integration of the SINTEX model
and from the 41-year (1958-1998) of NCEP reanalysis. a) Mean model error (SINTEX-NCEP), contour interval
20 m; b,c) model and observed standard deviation, contour interval 10 m; d) eddy component of the 500 hPa
model mean field, contour interval 30 m; e) as in (d) but for NCEP reanalysis; f) eddy model error (SINTEX-
NCEP), contour interval 20 m.
a
e
b
c
d
f
f

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Susanna Corti, Silvio Gualdi and Antonio Navarra
Fig. 2. The SINTEX model simulation: geographical distribution of 500-hPa-geopotential height associated with
the six dominant Empirical Orthogonal Functions. EOFs, contour interval 10 m.

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Analysis of the mid-latitude weather regimes in the 200-year control integration of the SINTEX model
Fig. 3. As fig. 2, but for the observed fields (1958-1998 NCEP reanalysis). Contour interval 10 m.

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Susanna Corti, Silvio Gualdi and Antonio Navarra
Fig. 4. Observed and simulated leading EOFs at 500-hPa-geopotential height. Left column: first leading EOFs
from the NCEP reanalysis data set; right column: first EOF (EOF1) and second rotated EOF (REOF2) from the
SINTEX model integration, contour interval 10 m.
The first observed mode (EOF1) is charac-
terised by two dominant centres of opposite sign
(signs of anomalies are immaterial in EOF
analysis) over the North Pacific around 150°W
in the Gulf of Alaska and over the North Atlantic
around 45°W on the southern part of Greenland.
To the southwest and to the southeast of the
Atlantic centre, there exist weaker anomaly
centres contributing together to the wave train-
like appearance in the Atlantic-to-Eurasian sector.

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Analysis of the mid-latitude weather regimes in the 200-year control integration of the SINTEX model
The first model EOF bears a strong resemblance
to its observed counterpart in the Pacific sector,
while the Atlantic wave train-like is reduced to a
north-south dipole. The observed EOF2 is also
characterised by two dominant centres, of equal
sign in this case, over slightly shifted positions
in the North Pacific and in the North Atlantic.
This time, the Atlantic feature is consistent with
the structure of the North Atlantic Oscillation
(van Loon and Rogers, 1978). This observed
mode does not have a counterpart in the second
model EOF, however the two patterns do not
differ too much. The wavetrain-like which char-
acterises the global structure of EOF3 is cap-
tured by the model and almost all the centres
of action characterising the remaining EOFs
are reproduced, even though partially, in the
SINTEX model simulation.
4. Weather regimes in the SINTEX model
Following the line of Corti et al. (1999), we
defined a reduced two-dimensional phase space
based on the leading patterns (EOFs) of the
model low-frequency variability. But, in order
to compare model and observed results, model
EOFs 2 and 4 was rotated to obtain a pattern as
similar as possible to the observed EOF2. We
shall refer to such a pattern, shown in fig. 4, as to
rotated-EOF2 (REOF2).
Figure 5 compares the PDF of the projection
coefficients of the monthly-mean high field, in
the reduced phase space spanned by EOF1 and
REOF2 and the «observed PDF» found by Corti
et al. (1999). The same interactive gaussian-
kernel estimator has generated both distributions.
The model PDF exhibits three maxima labelled
A′, B′ and C′, which appear to be consistent with
the corresponding, observed ones. The fourth
observed maximum, labelled D, does not have a
counterpart in the SINTEX PDF, however, close
to its expected location, there is a large plateau
well separated from the other maxima (this
plateau has been labelled D′ in the figure).
The geographical patterns of the three density
maxima and that corresponding to the mean value
of plateau D′, are illustrated in fig. 6, where they
are shown as departures from the 200-year mean
500 hPa height.
?????????????????????????????
Fig. 5. Evidence of the non-gaussian characteristic of
the climate attractor. Top panel: atmospheric state
vector PDF based on monthly-mean 500-hPa geo-
potential height in the space spanned by the two
dominant EOFs (see fig. 4, left column). There are four
maxima labelled A, B, C, D.; bottom panel: atmos-
pheric state vector PDF based on monthly-mean 500-
hPa geopotential height from the SINTEX 200 year
integration in the space spanned by EOF1 and REOF2
(see fig. 4, right column). There are three maxima
labelled A′, B′, C′ and a large plateau labelled D′.

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Susanna Corti, Silvio Gualdi and Antonio Navarra
All the simulated regimes have an observed
counterpart (see fig. 7 for comparison) well
documented in the literature (e.g., Mo and Ghil,
1988; Molteni et al., 1990; Cheng and Wallace,
′
′
Fig. 6. Geographical patterns of the four atmospheric regimes in the SINTEX model. Shown is the geographical
distribution of 500-hPa geopotential height anomaly associated with clusters A′ (the «Cold Ocean Warm Land»
regime), B′, C′ and D′ (the «Arctic Oscillation» regime). Contour interval 10 m.
′
′
1993; among others). The circulation associated
with cluster A′ is characterised by a strong jet
stream, displaced southward of its climatological
mean latitude over the Central Pacific, with a

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Analysis of the mid-latitude weather regimes in the 200-year control integration of the SINTEX model
strongly diffluent flow downstream leading into
a long-wave ridges over the Rockies and, over
the Euro-Atlantic region, by the positive polarity
of the North Atlantic Oscillation, i.e. a strength
of the westerlies along the node in the North-
Atlantic dipole pattern. Furthermore, this kind
of atmospheric flow is associated with above
normal temperatures over Alaska, Western
Canada, Europe, most of Russia and North-
Eastern United States. Both clusters B′ and C′
contain elements of the PNA pattern in its
negative polarity, and are associated with high
Fig. 7. As fig. 6, but for the observed data set (after Corti et al., 1999). Same contour interval.

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Susanna Corti, Silvio Gualdi and Antonio Navarra
amplitude ridges (Alaska blocking) over North-
Eastern Pacific and with cold air outbreaks over
the U.S. Pacific Northwest. Cluster B′ projects
onto the positive North Atlantic Oscillation as
well. Finally cluster D′ resembles the NAO in
its negative polarity and it is well correlated with
the 500 hPa height component of the Arctic
Oscillation (Thompson and Wallace, 1998). It
is associated with blocking episodes over the
Atlantic and is characterised by cold weather
over the North-Eastern U.S. and South-Eastern
Canada and Scandinavia.
5. Concluding remarks
The previous sections presented the SINTEX
model’s skill in simulating the low-frequency
variability features of the winter midlatitude
atmospheric circulation. The model perfor-
mances have been assessed comparing first the
simulated mean state and variability with the
corresponding statistics from observational data
sets and then evaluating the model capability of
simulating the non-gaussian structure of the
climatic attractor.
Taking into account the limitations imposed
by the «coarse» atmospheric horizontal reso-
lution, the overall performance of the SINTEX
model in simulating the leading patterns of the
Northern Hemisphere low-frequency variability
is quite accurate. It was found that the SINTEX
model not only can simulate the non-gaussian
structure of the climatic attractor, but is also able
to reproduce the natural modes of variability of
the system.
The main results of our analyses can be
summarised as follows.
– The cumulative variance explained by the
first six EOFs, 56% of total, is comparable to
the observed one (59%).
– All the leading EOFs, apart EOF2 and
EOF4, have an observed counterpart which ex-
plains almost the same amount of total variance.
– Furthermore, SINTEX EOF2 and EOF4
are not so far from observed EOF2 and EOF4,
rather they can be thought of as a linear com-
bination of these patterns.
– Based on a PDF analysis in the phase space
spanned by the leading EOF1 and REOF2,
substantial evidence of the non-gaussian regime
structure of the SINTEX northern winter circu-
lation has been found.
– The model PDF exhibits three maxima
labelled A′, B′ and C′, which have an observed
counterpart.
– The spatial structure of regime A′ (the most
populated) is strongly reminiscent of the pattern
found by linear regression analysis between
monthly-mean Northern Hemisphere mean
surface air temperature and 500 hPa height
(Wallace et al., 1996). Essentially, this pattern
denotes the 500-hPa geopotential height
manifestation of the so-called «Cold Ocean
Warm Land» pattern, which describes recent
climate change in surface air temperature on
decadal timescale. The fact that this pattern has
been detected, as a regime, in a control run (i.e.
no anomalous forcing) of a coupled model
represents further evidence that COWL does have
meaning not only in relation to hemispheric sur-
face air temperature, but it is a natural mode of
atmospheric variability.
– The height anomalies associated with
clusters B′ and C′ have both a projection onto
the negative Pacific North American (PNA)
pattern, but cluster B′ projects onto the positive
North Atlantic Oscillation (NAO) as well.
– Cluster D′, which is not a PDF maximum,
but corresponds to the mean value of plateau D′,
is the least populated in the model (and also in
the real world). It is extremely well correlated
with the 500 hPa height component of the Arctic
Oscillation (Thompson and Wallace, 1998) in its
negative phase; furthermore it is strongly remi-
niscent of regime G′ of Cheng and Wallace
(1993) and of Kimoto and Ghil’s (1993) regime
BNAO.
The results outlined above indicate that the
SINTEX model is able to reproduce the natural
modes of variability of the system. However, we
have not so far discussed the model bias in the
regime frequencies. In the 200-year model
integration, cluster A′ is overwhelmingly the
most populated cluster, whereas, for example,
cluster D′ is very much a secondary regime. In
other words, on a monthly time-scale, the model
circulation is somehow biased toward the A′
regime. A possible explanation of this behaviour
can be found in the fact that there is spatial

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Analysis of the mid-latitude weather regimes in the 200-year control integration of the SINTEX model
correspondence between the systematic error
patterns (fig. 1a,f) and the cluster A′ anomaly.
In fact, the spatial correlation between the eddy
systematic error (projected on the subspace
spanned by EOF1 and REOF2) and the cluster
A′ pattern is 96%. This result supports the notion
(Palmer, 2001) that the systematic error could
be interpreted in terms of model bias towards
the most intensely populated regime. Following
this perspective, (most of) the systematic error
is determined by the model overestimation of
the dominant regime (and consequent under-
estimation of the secondary regimes) which could
be due to an underestimation, by the model, of
transient variability. Let us assume that the cluster
A is the most stable regime (in the Pacific/North
American sector this circulation regime projects
onto the positive phase of the Pacific North
American pattern (Wallace and Gutzler, 1981)
which is known to be a relatively stable pattern
(Palmer, 1988)) and that the SINTEX model
can simulate the circulation regimes but with
inadequate small-scale variability to trigger re-
gime transitions. In this case, as suggested by
Molteni and Tibaldi (1990), the most stable
circulation regime will become overpopulated.
In order to investigate this hypothesis a detailed
analysis of the model simulation of the high-
frequency variability patterns related to regime
transitions is currently under way.
Acknowledgements
This work was supported by the EC SINTEX
project under contract ENV4-CT98-0714.
REFERENCES
CHENG, X. and J.M. WALLACE (1993): Cluster analysis of
the Northern Hemisphere wintertime 500-hPa height
field: spatial patterns, J. Atmos. Sci., 50, 2674-2696.
CORTI, S., F. MOLTENI and T.N. PALMER (1999): Signature
of recent climate change in frequencies of natural
atmospheric circulation regimes, Nature, 398, 799-802.
GUALDI, S., A. NAVARRA, E. GUILYARDI and P. DELECLUSE
(2003): Assessment of the tropical Indo-Pacific climate
in the SINTEX CGCM, Ann. Geophysics, 46 (1), 1-26
(this volume).
KALNAY, E., M. KANAMITSU, R. KISTLER, W. COLLINS, D.
DEAVEN, L. GANDIN, M. IREDELL, S. SAHA, G. WHITE,
J. WOOLLEN, Y. ZHU, M. CHELLIAH, W. EBISUZAKI, W.
HIGGINS, J. JANOWIAK, K.C. MO, C. ROPELEWSKI, J.
WANG, A. LEETMAA, R. REYNOLDS, R. JENNE and D.
JOSEPH (1996): The NCEP/NCAR 40-Year Reanalysis
Project, Bull. Am. Meteorol. Soc., 77, 437-471.
KIMOTO, M. and M. GHIL (1993): Multiple flow regimes in
the Northern Hemisphere winter, part I: methodology
and hemispheric regimes, J. Atmos. Sci.,50, 2674-2696.
MO, K.C. and M. GHIL (1988): Cluster analysis of multiple
planetary flow regimes, J. Geophys. Res.,93D, 10,927-
10,952.
MOLTENI, F. and S. TIBALDI (1990): Regimes in the win-
tertime circulation over northern extratropics, part II:
consequences for dynamical predictability, Q. J. R.
Meteorol. Soc., 116, 1263-1288.
MOLTENI, F., S. TIBALDI and T.N. PALMER (1990): Regimes
in the wintertime circulation over northern extratropics,
I: observational evidence, Q. J. R. Meteorol. Soc., 116,
31-67.
PALMER, T.N. (1988): Medium and extended range pre-
dictability and stability of the Pacific North American
mode, Q. J. R. Meteorol. Soc., 114, 691-713.
PALMER, T.N. (1999): A nonlinear dynamical perspective on
climate prediction, J. Climate, 12, 575-591
PALMER, T.N. (2001): A nonlinear dynamical perspective on
model error: a proposal for non-local stochastic-dynamic
parametrization in weather and climate prediction
models, Q. J. R. Meteorol. Soc., 127, 279-304.
THOMPSON, D.W. and J.M. WALLACE (1998): The Arctic
Oscillation signature in the wintertime geopotential
height and temperature fields, Geophys. Res. Lett., 25,
1297-1300.
VAN LOON, H. and J.C. ROGERS (1978): The seesaw in winter
temperatures between Greenland and Northern Europe,
part I: general description, Mon. Weather Rev.,118, 2056-
2081.
WALLACE, J.M. and D.S. GUTZLER (1981): Teleconnections
in the geopotential height field during the northern
hemisphere winter, Mon. Weather Rev., 109, 784-812
WALLACE, J.M., Y. ZHANG and L. BAJUK (1996): Inter-
pretation of interdecadal trends in Northern Hemi-
sphere surface air temperature, J. Climate, 9, 249-259