Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 25

Chapter 2
Oscillations and teleconnections
2.1 History and definitions
The concept of atmospheric oscillation began with studies of the Asian mon-
soon. Following the great 1877 drought in India, the India Meteorological
Department was established under the leadership of H. F. Blanford. His task,
in part, was to examine whether any monsoon seasonal prediction could be
identified. Concentrating upon solar relations and climate, he could not report
success. It was, however, Sir Gilbert Walker who, as Director-General of
Observatories in the India Meteorological Department, initiated extensive stu-
dies of pressure patterns that eventually led to the identification of atmospheric
oscillations. After he retired in 1924, Walker observed a see-saw like oscillation
of sea level pressures in various parts of the Pacific Ocean (Walker 1923-4 ).
He labeled this the Southern Oscillation. Further studies in the 1920s and 1930s
saw identification of North Atlantic and North Pacific oscillations. Not a great
deal of attention was accorded this work, and it was not until many years later
that the contribution of Walker was recognized and the Walker circulation
named in his honor (Bjerknes 1966 ). It is interesting to note that the statistical
methods used by Walker were sophisticated enough to become the ''Yule-
Walker equations'' that refer to properties satisfied by the autocorrelations of
an autoregressive process (Katz 2002 ).
Throughout this text the discussion of regional climates and anomalies will, in
part, concern teleconnections. This chapter provides a background to the major
oscillations that relate to teleconnections. Of these, ENSO events are by far the
best known and, as a result, this topic and its extended influences are given
additional emphasis through an essay (Section 2.8 ).
2.1.1 Oscillations
Any phenomenon that tends to vary above or below a mean value in some sort of
periodic way is properly designated as an oscillation. Classical oscillation
research states that oscillation occurs when a system is disturbed from a position
of stable equilibrium. It may be recognized eventually as a predictable cycle,but
this term should not be used unless the period has a recognizable regularity. The
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