Erdfernerkundung - Numerische Physik: Modellierung

274 KAPITEL 5. DIE ERWEITERTE PERSPEKTIVE: CAWSES
Abbildung 5.41:
Changes total ozone
due to precipitating
solar energetic particles
during the solar cycle
(courtesy M. Sinnhuber,
U. Bremen)
§ 910 Figure 5.41 shows the variation in total ozone from 1989 to the middle of 2001, that
is one solar cycle. The figure shows a couple of remarkable features:
• the general temporal behavior of the ozone depletion reflects the variation of ion pair
production rates with the solar cycle as shown in Fig. 5.40: pronounced ozone depletion is
observed between 1989 and 1992 and after 1999; the minimum is around 1996.
• to the left, at the end of 1989, a strong decrease in ozone of some percent is visible in the
northern hemisphere. This is related to the large solar energetic particle of October 1989
as discussed in connection with Fig. 5.39. As mentioned there, the effect lasts for some
month.
• the ozone depletion on the right (middle of 2000) is related to the Bastille day event
discussed in connection with Fig. 5.30.
• in particular for the October 1989 event a strong hemispherical asymmetry is visible: the
ozone depletion is much more pronounced on the northern hemisphere than on the southern
one. In contrast, late in 1991 a solar energetic particle event causes an ozone depletion in
the southern hemisphere but has almost no influence on the northern hemisphere. This
hemispheric asymmetry basically is caused by two effects: (a) HOx and NOx life–times
are influenced by photochemical processes as are many other chemical processes in the
atmosphere. Thus even if all other conditions are equal, both hemispheres might exhibit
different patterns of ozone depletion because one is more strongly illuminated than the
other one. In the most extreme case, one hemisphere might be in polar night while the other
is in polar day. (b) Circulation patterns are very different in both hemispheres: while in the
northern hemisphere meridional transport happens all the year, in the southern hemisphere
a strong closed vortex persists in the stratosphere that inhibits meridional transport, in
particular in winter. In consequence, stratospheric temperatures can be extremely low
which influences ozone chemistry. This vortex also explains why an ozone hole is observed
at the southern pole while the northern hemisphere ozone hole is rather rudimentary.
• the spatial pattern of ozone depletion varies with the solar cycle: while during solar maximum
ozone depletion due to precipitating particles occurs almost down to the equator,
during solar minimum ozone depletion is limited to latitudes poleward of about 60 ◦ .
• ozone depletion occurs at latitudes where no particles precipitate: owing to the shape of the
geomagnetic field, particle precipitation is limited to the polar cap, that is to geographic
latitudes well polewards of 60 ◦ . Ozone depletion is not limited to these high latitudes but
occurs also close to the equator. This shift reflects the atmospheric circulation patterns and
the spatial variation in ozone production: the main ozone production is at low altitudes,
leading to high ozone concentrations at low altitudes, see also the right panel in Fig. 5.30.
The ozone-rich air then is transported polewards at high altitudes. At high latitudes ozone
is destroyed by photochemical reactions as well as energetic particles. The equation of continuity
requires also transport from the pole to low latitudes. This advects ozone depleted
air to equatorial latitudes and thus explains the reduction in equatorial ozone.
• in mid-latitudes (around 50 ◦ ) the solar-cycle variation in ozone due to precipitating energetic
particles is comparable to the observed variation – and it is opposite in sign to the
2. Juli 2008 c○ M.-B. Kallenrode