696
SESSION IIIB DISCUSSION
I would like to suggest that the eight-month lag is produced by
the asymmetry in the northern- and southern-hemisphere land masses.
If the transport of radioactivity from the stratosphere to troposphere
is predominately due to folding of the tropopause, which can be shown
to be related to cyclogenesis, then a difference between the hemispheres might be expected. Cyclogenesis over a rotating globe depends
on the vertical component of the earth’s vorticity, which is a function
of latitude, baroclinity, and hydrostatic stability. Fleagles’ results, I
think, are most applicable here. He showed that when the baroclinity
(north—south temperature gradient) was at a maximum at 45° latitude,
the most probable latitude for cyclogenesis was close to 60° and
cyclogenesis was prohibited from 30°
to the equator. Cyclogenesis
between 45° and 25° would require a larger baroclinity or a lower
hydrostatic stability. Cylconic development and tropopause folding
in this latitude range is probably responsible for the maximum deposition of radioactivity at the surface between the equator and 30°
latitude.
The temperature gradients are primarily produced by cooling
at high latitudes during fall and winter. Cyclones develop at high
latitudes and transport the cold air equatorward. This transport increases the baroclinity at lower latitudes. Thus an equatorward shift
of the zone of maximum baroclinity, and therefore the zone of maximum cyclogenesis, occurs during winter in both hemispheres.
However,
in the southern hemisphere when the cold air spreads
northward, it travels over water that is warmer than land (there is
very little land between 70 and 30°S). The resulting heating of the
air acts to reduce the equatorward shift of the baroclinity. In the ©
northern hemisphere the large land masses at high latitudes accelerate
the cooling trend. Even when they become snow covered, the land
masses at mid-latitudes can contribute to the cooling. Certainly, they
do not heat the air as does the warmer water. Therefore we might
expect more frequent and vigorous cyclogenesis between latitudes 25
and 45° to develop earlier in the northern-hemisphere winter than in
the southern-hemisphere winter.
The asymmetry in the land masses might also affect the concentration of radioactivity in the extruded stratospheric air. Since the
radioactivity in the stratosphere must descend to the lower cyclonic
stratosphere before it is transported into the troposphere, a diabatic
cooling process is required. I think radiative cooling plays an important part in the stratospheric descent of the air that brings down
the radioactivity to the level of the tropopause folding.
I suggest that both the asymmetric lag and the asymmetry in the
magnitude of the deviation from the bimonthly averages might be a
direct consequence of the asymmetry in the land masses of the two
hemispheres.