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phytoplankton to decrease, leaving less food for salmon and thus reducing
their populations.
Mantua and his colleagues analyzed the oceanographic and i sheries data
and found that these cycles of sea-surface temperature and salmon popula-
tions lasted decades. h ey discovered that sea-surface temperatures in the
northeast Pacii c were warmer than average from 1925 to 1947 and from 1977
to 1999, and that they were cooler than average from 1947 to 1977 and at er
about 1999. h e team called these broad oceanographic cycles the Pacii c
Decadal Oscillation, or PDO. h
e warm and cold phases each lasted two to
three decades.
h e PDO is an elegant demonstration of the linkages between climate,
oceans, and marine life, and it also helps explain the enigmatic shit s that
occur every few decades in the West's climate as measured by rainfall and
streaml ow across the region. During a positive (warm) phase of the PDO,
surface waters of the central North Pacii c are anomalously cold while surface
waters along the northwestern coast of North America are unusually warm.
h ere is also warming of tropical surface waters in the eastern Pacii c. h is
PDO phase brings higher than average winter precipitation to the American
Southwest, with increased risk of l ooding, while lower than average winter
precipitation occurs in the Pacii c Northwest. h e patterns reverse during a
negative (cool) PDO phase, with cooling of surface waters along the north-
western coast of North America and in the tropical eastern Pacii c. During
this phase, average winter precipitation is lower in the American Southwest
and higher in the Pacii c Northwest (see i gure 12B).
Two full PDO cycles have occurred in the past century: cool, or negative,
regimes occurred from 1890 to 1924 and from 1947 to 1976; warm, or positive,
regimes occurred from 1925 to 1946 and from 1977 to 1998. Regime shit s—
transitions between positive and negative phases of the PDO—occur every
few decades and can also have dramatic ef ects on the climate. For instance,
the winter of 1976-77 marked the transition from a cool to a warm phase
of the PDO, and, during that year, extreme drought had societal impacts
beyond temporary water restrictions. It was the driest year on record in
California, the Pacii c Northwest, and other regions of the North American
West. In the Pacii c Northwest, the human population and agriculture had
expanded during the wetter, cool-phase PDO between 1945 and 1976. h e
1977 drought introduced the i rst of several dry decades in the region—and
the i rst of many conl icts between farmers, i shermen, and other water users
accustomed to wetter times.
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