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Scientific Report 2007-2009
Geophysics
G1. Theory and observations of climate and its changes
The main objective in Climate Dynamics is the understanding
of the origin of the atmospheric general circulation,
i.e. climatic zones. Aristotele devoted a volume to
the description of atmospheric phenomena and climatic
zones, while Galileo was the first who studied the origin
of trade winds. Today, there are rational bases for an
explanation of climatic zones, but the nonlinear character
of the involved physical processes makes difficult the
development of a comprehensive and established theory.
Furthermore, nowadays the problem of future changes of
the Earth’s climate is attracting an increasing interest.
However, the complex nature of the physical problem
necessitates a major scientific effort to make possible assessments
of likely future climate changes, regardless of
whether these may be natural or man-made.
In this framework, the Climate Dynamics group of
Roma carried out theoretical studies and numerical simulations
to investigate the origin of the observed atmospheric
general circulation features in relation to the
role of the stratosphere, of the baroclinic eddies through
their heat and momentum transports, and of changes
in the imposed meridional temperature gradient in the
troposphere. The formation and variability of tropospheric
double-jet patterns observed in the Southern
Hemispheres during the transition seasons (Fig. 1) has
been investigated using a quasigeostrophic and a simplified
general circulation model (GCM) [1].
NCEP reanalysis data for the last 50 years [3]. The analysis
suggested the importance of baroclinic adjustment
processes for midlatitude tropopause dynamics.
Using both NCEP reanalysis and observations space
and time variability of drought and wetness at large-scale
has been investigated also in relation to a changing climate.
An updated analysis for the European area has
been carried out computing the Standardized Precipitation
Index (SPI) and applying the Principal Component
Analysis (PCA) to the SPI field [4]. Linear and nonlinear
trends of drought and wetness were compared for
two time sections: 1949–1997 and 1949–2009 (Fig. 2).
a)
April 2000
b)
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pressure (mb)
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Figure 1: a) Latitude-pressure cross-section of the monthly
mean zonal wind for April 2000, and b) latitude-time diagram
of the zonal mean zonal wind at 200 mb [1].
The role of eddy heat fluxes in generating the observed
double-jet pattern was ascertained using an analytical
Eady model with stratospheric easterlies. Sensitivity of
the results to the meridional temperature gradient in
the troposphere showed a regime change from a prevailing
subtropical jet to a midlatitude one. The intermittent
nature of the tropospheric double jets has been also
studied for the Northern Hemisphere winter and Southern
Hemisphere summer [2]. The impact of baroclinic
eddies on the mean tropopause height (a key parameter
in climate change detection) has been assessed using
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Figure 2: a) First loading of SPI field and b) first principal
component score time series with the fitting linear and
nonlinear trends for the whole period and the shorter period
[4].
The study showed that the SPI time series are not
stationary and have multi-year fluctuations. Linear
trends highly depend on the time section considered and
classical statistics, commonly used in hydrology, should
be revised under the hypothesis of a varying climate.
Finally, in the last 3 years the group is participating
to the development of the ground segment of ASI
ROSA satellite mission (launch occurred in September
2009 on board of OCEANSAT-2) that uses the radio
occultation technique for sounding the atmosphere
(http://www.asi.it/Rosa/RosaIT/ROSA.htm).
References
1. I. Bordi et al., J. Atmos. Sci. 66, 1366 (2009).
2. I. Bordi et al., Mon. Wea. Rev. 135, 3118 (2007).
3. A. Dell’Aquila et al., Clim. Dyn. 28, 325 (2007).
4. I. Bordi et al., Hydrol. Earth Syst. Sci. 13, 1519 (2009).
Authors
I. Bordi, A. Sutera
http://romatm13.phys.uniroma1.it/
Sapienza Università di Roma 166 Dipartimento di Fisica