Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 36

the Krakatoa volcano in the East Indies in August of 1883. Ash from this volcano
reached the lower stratosphere and drifted, in alternate years, either easterly
or westerly. The easterly winds were labeled the ''Krakatoa Easterlies'', named
for the volcano. The westerly flow was named the ''Berson Westerlies'' for the
German meteorologist A. Berson, who observed the west-east winds after
releasing observation balloons in East Africa (see Krakatoa Winds in
Fairbridge 1986 ). The actual discovery of the QBO is credited to R. J. Reed
and R. A. Ebdon based upon rawinsonde data obtained at Canton Island (Reed
et al. 1961 ). The term Quasi-biennial Oscillation was coined by Angell and
Korshover (1964). An extensive and thorough review of the QBO by Baldwin
et al.( 2001 ) provides an account of both the potential impacts and the theory of
the oscillation, and the following account draws, in part, upon that review.
The QBO is a quasi-periodic oscillation of the zonal wind, between easterlies
and westerlies that occur in the equatorial/tropical stratosphere. These alternat-
ing winds originate at the top of the lower stratosphere, above 30 km or approxi-
mately 10 hPa, and propagate downward at a rate of about 1 km each month until
they dissipate at the tropical tropopause. The easterlies are generally stronger
than the westerlies. The fastest observed oscillation has a period of 20 months,
and the slowest 36 months, with an average period of 28 months.
A theory to explain the QBO has been the subject of considerable debate.
Lindzen and Holton ( 1968 ) suggested that winds of the upper stratosphere, the
semi-annual oscillations, were a key; that is the QBO originated in the upper
stratosphere. In 1972 the same researchers outlined the role of gravity waves and
mixed Rossby waves, each of which provided the necessary easterly and wes-
terly momentum need for the QBO. The details of these are provided in a
historical review of QBO theory by Lindzen ( 1987 ). The current understanding
of the QBO certainly involves gravity waves as shown by computer and numer-
ical models.
The significance of the QBO is still being investigated. It is known to play a
role in stratospheric ozone mixing and in the potential for hurricane formation.
Promising relationships have been shown to exist between the QBO and
Northern Hemisphere winters. As noted, this is well illustrated in the work of
Baldwin et al.( 2001 ). The QBO is discussed in more detail in an essay given in
Section 3.3 .
2.7 The Arctic Oscillation (AO) and Antarctic
Oscillation (AAO)
In a lecture at the National Center for Atmospheric Research in 2000 (available
at tao.atmos.washington.edu/data/annularmodes), J. M. Wallace presented a
clear and highly informative summary of the interpretation of the AO and
AAO or, generically, the annular modes. One of the several points made in the
presentation was that both the identified oscillations are ''dynamical twins.''