Erdfernerkundung - Numerische Physik: Modellierung

5.5. CAWSES 275
variation expected from the solar-cycle variation of the UV radiation.
Vanishing Geomagnetic Field
Abbildung 5.42:
Variation in total
ozone during the solar
cycle without geomagnetic
field (courtesy M.
Sinnhuber, U Bremen)
§ 911 A worst case estimate for the ozone balance during a magnetic field reversal is obtained
in case of a vanishing geomagnetic field. Then particles precipitate into the atmosphere at all
latitudes and not only at high latitudes. Figure 5.42 shows model calculations or this case.
The results are quite similar to the ones obtained for the present day geomagnetic field in
Fig. 5.41:
• total ozone depletion is higher during solar maximum than during solar minimum,
• the effects of individual events are visible,
• ozone depletion in individual events shows a hemispheric asymmetry,
• the spatial pattern of ozone depletion varies with the solar cycle.
Despite all these similarities (in fact, there is one similarity too much) there is also a fundamental
and surprising difference between the two figures: ozone depletion in high latitudes
is much more pronounced in case of a vanishing geomagnetic field although the number of
particles precipitating at high latitudes does not change! The difference in particle precipitation
between the present-day field and the vanishing field, on the other hand, occurs at
low- and mid-latitudes: here the difference in ozone depletion between the two runs is less
pronounced than at polar latitudes where no difference would be expected. This again reflects
the complex relation between photochemical processes and atmospheric circulation.
§ 912 A closer inspection of Fig. 5.42 suggests ozone depletion by 10–20% at high latitudes
lasting for a few month. Ozone has radiative properties, in particular, it absorbs UV
radiation and is also a greenhouse gas, that is, it affects the terrestrial long-wave emission.
In consequence, a local change in ozone concentration can cause a local change in temperature.
Since pressure gradients resulting from temperature gradients are the driving forces for
atmospheric motion, a change in ozone concentration might cause a change in atmospheric
velocity fields and thus modify climate.
Consequences for Climate?
§ 913 A first indication for such a process is given in Fig. 5.43: in the top panel ozone
depletion at 75N is shown for a large solar energetic particle event at the time of polarity
reversal, that is for a vanishing geomagnetic field. Ozone depletion can amount to more than
50% in the stratosphere and stay at such high levels for almost a year. The lower panel in
Fig. 5.43 shows the resulting changes in temperature (sorry for the color-coding). Although
temperature changes are rather small (rarely larger than 5 K), temperature gradients can
become quite large. In consequence, wind fields change. And once the wind fields are modified,
the entire temperature distribution in the atmosphere is modified, which certainly will affect
climate.
c○ M.-B. Kallenrode 2. Juli 2008