Diapositiva 1

1
The role of Mediterranean mesoscale eddies on the
climate of the Euro-Mediterranean regionby A.
Bellucci1, S. Gualdi1,2, E. Scoccimarro2, A.
Sanna1, P. Oddo2, and A. Navarra1,2contact
alessio.bellucci_at_cmcc.it1. CMCC Centro
Euro-Mediterraneo per i Cambiamenti Climatici
(Euro-Mediterranean Centre for Climate Change),
Bologna, Italy2. INGV Istituto Nazionale di
Geofisica e Vulcanologia (National Institute of
Geophysics and Volcanology), Bologna, Italy
Introduction and Motivations Within the
CIRCE (Climate Change and Impact Research The
Mediterranean Environment) EU Project,
substantial efforts were devoted to enhance the
representation of the oceanic system in the
Mediterranean region. This was achieved by
developing coupled general circulation models
with ocean components which either explicitly
resolve, or simply permit, mesoscale circulation
features. The inclusion of the eddy variability
tail in the spectrum of the processes resolved
by the modelled system represents a particularly
relevant step forward with respect to the
previous CMIP3 generation of climate models , as
these were systematically based on coarse
resolution ocean components, leading in turn to
an extremely rough representation of the
Mediterranean Sea sub-system. In this study the
role of mesoscale oceanic features on the air-sea
interactions over the Mediterranean region was
analysed, in the context of one of the CIRCE
ensemble of climate models. To this aim, two
different simulations of the 20th Century
climate, performed with two distinct
con?gurations of the CMCC coupled general
circulation model featuring radically different
horizontal resolutions in the Mediterranean Sea
domain, were compared. This comparison highlights
the implications deriving from the inclusion of
energetic ocean mesoscale structures in the
variability spectrum of the coupled
ocean-atmosphere system and points to the need
for high-resolution ocean components in the
development of next generation climate model.
Impact of Med Sea horizontal resolution on
surface european climatebias reduction.
Models and experimental setup
H
AGCMECHAM5
AGCMECHAM5
Global SST
Global SST
Med Sea SST
L
OGCM OPA 8.2 (GLOBAL)
OGCM OPA 8.2 (GLOBAL)
Exp. H (turbulent)
NEMO (Med Sea)
Exp. L (laminar)
Exp. H (turbulent)

• Two 20C3M simulations forced with historical
  timeseries of GHG, aerosol volcanoes are
  performed, using two global CGCMs, only differing
  by the ocean space resolution over the Med Sea
  region (see Fig.1)
• Exp. H ECHAM5 T159L31 OPA8.2 (global ocean)
  LIM2 NEMO 1/16o (Med Sea)
• Exp. L ECHAM5 T159L31 OPA8.2 (global ocean)
  LIM2

Fig.1 Daily SST (1st Jan 1960) snapshots over
the Med Sea from experiments (top) H, and
(bottom) L.
In this study, two numerical simulations of the
20th Century climate performed with two global
GCMs are analysed. In the ?rst experiment (L), a
T159 atmosphere (equivalent to 80 Km horizontal
resolution) is coupled to a 2x2o global ocean
model, with a locally enhanced 1o resolution over
the Mediterranean Sea region. In the second
experiment (H), the same T159 atmosphere is
coupled to a global ocean model, except over the
Mediterranean Sea where a regional
high-resolution 1/16o ( 7 Km) ocean model is
used, which is in turn coupled to the
low-resolution global OGCM at Gibraltar Strait
(CMCC-Med Gualdi et al. 2011). Thus, in H, as
far as the Mediterranean area is concerned, the
atmosphere is locally coupled to an ocean model
which resolves mesoscale features (turbulent
ocean), whereas in L the atmosphere interacts
with a laminar oceanic system. Since these two
experiments are identical except for the
resolution of the ocean model over the
Mediterranean Sea, the systematic comparison of H
and L allows the assessment of the net effects on
the climate of the Euro-Mediterranean region from
explicitly resolving mesoscale oceanic features
in the coupled model.
Spectral analysis in the wavenumber domain
(Fig.2). Power spectra of surface temperature
in the wavenumber domain were computed for both H
and L experiments using daily zonal transects in
the Eastern Mediterranean basin, over a 4 years
long period (Fig.2). The spectra were diagnosed
from both the ocean model (sea-surface
temperature SST) and the atmospheric model
(surface air temperature over ocean grid-points
SAT) counterpart. The simple comparison between
SST and SAT power spectra for experiment H
highlights the existence of an upper cut-off
wavenumber set by the atmospheric resolution,
which inhibits the direct transfer of spatial SST
variance from the ocean to the atmosphere for
wavelenghts shorter than the smallest spatial
scale resolved by the atmosphere (80 Km).
Surface temperature fields display a typical
k-m power-law shape, i.e. with energy decaying
for larger wavenumbers. In H, m approximately
fits the theoretical -5/3 law of two-dimensional
turbulence within the 500-100 Km range, while a
steeper slope is revealed for the smaller-scale
dissipative range (SST power spectrum). On the
other hand, in L the ocean and the atmosphere
share a much similar horizontal resolution (80
and 111 Km, for the atmosphere and the ocean GCM,
respectively). Interestingly, SST (not shown)
and SAT power spectra display a steeper slope and
consistently lower energy in the 100-500 Km range
with respect to H. Thus, the absence of a
developed ocean eddy field in L seems to affect
the long-wave part of the common ocean-atmosphere
variability spectrum. The inclusion of a
vigorous oceanic eddy field in the coupled system
appears to indirectly affect the large scale part
of the variability spectrum. This may possibly
occur through the non-linear eddy-large scale
interactions taking place in the high-resolution
ocean component. In particular, the upscale
energy transfer, which typically takes place in
two-dimensional turbulent fluids (such as the
ocean) may play a role in this process.
Fig.3 Left H-L difference between long term
climatologies for (top) surface air temperature
(colour oC) (bottom) latent heat flux (W/m2).
Right SST climatology Model-OBS difference for
(top) H and (bottom) L. HadISST data were used as
SST OBS.
Patterns of H-L mean state differences (Fig.3
Left) reveal an overall 1 oK warming impact of
the enhanced ocean horizontal resolution over the
Med Sea,. Consistent H-L patterns of enhanced
evaporation (not shown) and latent heat losses
also emerge. The comparison of model SST
climatology with HadISST over the Mediterranean
region reveals a substantial SST bias reduction
in the high-resolution H experiment, with respect
to experiment L.
Med Sea interannual variability Fig.4 Power
spectra of Mediterranean basin-averaged SSTs
reveal enhanced variability around interannual
time-scales in the eddy resolving H experiment
(black), while the control L experiment (green)
shows a red-noise shaped structure, with enhanced
power at lower frequencies. AR1 95 confidence
levels are also shown.
Fig.2 Wavenumber spectra for SST and atmospheric
surface temperature from experiments L and H from
daily zonal transects in the Eastern
Mediterranean basin, from both the ocean and
atmospheric model. A constant slope theoretical
k-5/3 spectrum is also shown.
Concluding remarks The inclusion of a vigorous
eddy field in the ocanic component of a coupled
climate model substantially alters both the mean
state of the system and its space and time
variability. This comparison points to the need
for high-resolution ocean components in the
development of next generation climate models.
References Gualdi and Coauthors, 2011 The CIRCE
simulations a new set of regional climate change
projections performed with a realistic
representation of the Mediterranean Sea, to be
submitted to BAMS.
Acknowledgements This work was funded by the EU
FP7 CIRCE (Climate Change and Impact Research
the Mediterranean region and the global climate
system ) Integrated Project.