Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 149

most likely transferred through the Rossby wave train, associated with topical
forcing of the equatorial to polar circulation processes.
Turner ( 2004 ) found that the strongest relationship was between height anoma-
lies over the Amundsen-Bellingshousen Sea and the SOI. During El Ni˜o, height
anomalies are positive; during La Ni˜a, they are negative. Pressure anomalies
change from positive to negative across the SOI minimum. During El Ni˜othere
tend to be fewer cyclones in the Bellingshousen Sea, winds exhibit a greater
southerly direction, and the polar front jet tends to be weaker. However, there are
no linear relationships, and considerable variability from event to event, with
some El Ni˜o years or periods having no effect. Considerable further research is
needed to define and resolve ENSO/Antarctic climate relationships.
5.7 Polar night jet and stratospheric ozone depletion
Toward the end of winter and into early spring (August to mid-October), the
combination of a strong polar-equatorial temperature gradient, strong land-
ocean temperature contrasts, and the coreless winter, create the strongest cir-
cumpolar vortex during the year. The PNJ, the leading edge of the circumpolar
vortex, shifts poleward, and downward in altitude to the lower stratosphere and
upper troposphere (Kuroda and Kodera 2001 ), and intensifies to over 75 m s 1 .
Transport of warmer, more humid air from the mid-latitudes toward the poles,
usually through planetary waves, is blocked, and the atmosphere over the poles
becomes isolated. The clear skies and high altitude of the Antarctic ice plateau
encourage longwave radiation loss to space from the surface, enhancing the
development of very cold temperatures. Anomalously cold stratospheric air
descends into the upper troposphere (see section 5.3 ). Air temperatures fall to
190 K in the upper troposphere and lower stratosphere over much of the con-
tinent (Simmonds 1998 ).
The lack of interaction with lower-latitude air, and the extremely cold tem-
peratures have a major influence on stratospheric ozone levels over Antarctica,
and are the major controllers of the chemistry creating the Antarctic ''ozone
hole.'' Stratospheric ozone absorbs ultraviolet radiation in the 280 to 320 nm
wavelength band (UV-B), which would cause skin cancers on human beings, and
be harmful to other life at the Earth's surface (GEO 2003 ). Damage to the ozone
layer is of global concern because of this threat.
Background Box 5.1 summarizes the chemistry process and the elements
involved. Since the 1920s, a wide range of chlorine (Cl, chlorofluorocarbons) and
bromine (Br) compounds, have been released into the troposphere through human
activities (Turco 2002 ). After several decades (depending on the compound), these
molecules reach the stratosphere, where they are dissociated by ultraviolet radia-
tion. The now-free Cl and Br atoms react with ozone, creating the more stable
molecule oxygen, and ClO or BrO. The latter molecules are dissociated by ultra-
violet radiation, or through reaction with single oxygen atoms, and the cycle begins