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of ITCZ are 12ºN during boreal summer (August, 2006) and 7ºS during boreal winter
(February, 2007).
Figure 10.
Seasonal variation of the mean CTH retrieved from CALIOP measurements performed
between June, 2006 and May, 2007 against the latitude (dotted line). The gray area gives the peak
width of tropical CTH distribution (Figure 9) taken arbitrary at 8 km height. The black full line
gives the annual cycle of the ITCZ computed from 17 years of HRC (Hightly Reflective Cloud) data
(January, 1971 to December, 1987) by Waliser et al. (1993).
The ITCZ tends to reside longer in the Northern hemisphere than in the Southern
hemisphere (7 months and 5 months, respectively). The width of the tropical CTH
distribution at an arbitrary height of 8 km is shown as the gray area in the Figure 10.
This belt seem to be largest between November and March (close to 7ºS), and minimal
between July and October (close to 12ºN).
This asymmetry, which could seem in contradiction with the symmetry of the solar
radiation to the equator, has been well documented, and modeled by numerous authors
such as Philander et al. (1996), Li (1997), and Hu et al. (2007). The fundamental cause
of the asymmetry in the eastern Pacifi c is the tilt of the western coast of the Americas,
which perturbs the sea surface temperature in the vicinity of the coastal region through
a coastal wind-upwelling mechanism. The asymmetry in the Atlantic results from the
land--ocean thermal contrast between the bulge of northwestern Africa and the ocean
to the south. The ocean--atmosphere interactions act as an amplifi er to enhance the
asymmetry that is set up by the continental or coastal morphology (Li, 1997).
Influence of Polar Stratospheric Clouds
We highlighted the potential impact of the PSC on the CTH statistics for the southern
polar latitudes. PSC are both high altitude and optically thin clouds but they can be
detected from lidar measurements (see Figure 7 under 60ºS). They play a major role
in stratospheric chemistry and in particular on ozone depletion (i.e., Solomon, 1999).
