Geoscience Reference
In-Depth Information
random and unpredictable swings in climate are largely brought about by
periodic changes in the Pacii c Ocean and the atmosphere above it, produc-
ing phenomena known as the El Niño-Southern Oscillation (ENSO) and
the Pacii c Decadal Oscillation (PDO). Together, the ENSO and the PDO
are responsible for climate patterns over most of the western United States,
including precipitation, wind patterns, and air temperature during various
seasons. h e ENSO arises from conditions in the tropical Pacii c Ocean;
the PDO is tied to conditions over portions of the northern Pacii c Ocean.
ENSO events are typically six to eighteen months in duration, whereas PDO
phases last two to three decades or more.
the el niño-southern oscillation (enso)
Conditions in the tropical Pacii c Ocean and in the air above it play a major
role in the year-to-year variability of climate in the North American West,
and they are ot en responsible for the region's climate extremes.
h e earliest scientii c observations leading to our present understanding
of oceanic and atmospheric variability in the tropical Pacii c were made in
the early twentieth century by Sir Gilbert Walker, director of the Indian
Meteorological Department. Walker's expertise was in mathematical physics
and statistics, not meteorology, but he had a passion for analyzing a wide
array of seemingly unrelated meteorological data to explain such phenomena
as droughts in India. He observed that there was a “swaying of pressure on a
big scale backward and forward between the Pacii c Ocean and the Indian
Ocean” (p. 22). Walker called this seesawing surface pressure the Southern
Oscillation, which is part of an east-west atmospheric cycle now known as
the “Walker Circulation.”
Later in the twentieth century, Norwegian meteorologist Jacob Bjerknes,
who later founded the Department of Meteorology at the University of
California, Los Angeles, observed oceanic and atmospheric patterns in
the tropical Pacii c Ocean during the strong 1957-58 El Niño (and during sub-
sequent events in 1963 and 1965). h ese conditions, characterized by anoma-
lously warm ocean surface waters in the eastern tropical Pacii c that occur
in late December, have been recognized for centuries by Peruvian i shermen,
who noticed signii cant reductions in their catch. h e episodes studied by
Bjerknes were also characterized by light easterly trade winds, heavy rainfall
in the eastern Pacii c, and the atmospheric pressure shit s described by Walker.
