Geoscience Reference
In-Depth Information
Pueblo and other cultures across the Southwest and California, may well have
been caused by extended La Niña conditions in the Pacii c Ocean.
Another intriguing result of the Palmyra coral record reveals that El
Niño events became more frequent and intense during the Little Ice Age—
particularly in the seventeenth century—than we have experienced for most
of the twentieth century. Today, we know that El Niño events are associated
with increased precipitation in the Southwest. As discussed in chapter 10, the
cooler climate during the Little Ice Age coincided with wetter conditions and
increased l ooding in California and the American West.
the enso over the ages
Studies of the past behavior of the ENSO have led to new understandings
elsewhere as well. For example, southern Ecuador is a region adjacent to
the tropical Pacii c with a climate closely tied to El Niño. Two geologists,
Donald Rodbell and the late Geof rey Seltzer, selected a study site there, Lake
Pallcacocha, to assess the ef ects of the ENSO over the entire Holocene.
El Niño winters typically involve periods of heavy rainfall and l ood-
ing in this region, leading to rapid sediment erosion from the surrounding
mountains. h is erosion results in thick layers of light gray sediment being
deposited on the lake bottom. Rodbell, Seltzer, and their team analyzed these
layers in long sediment cores taken from Lake Pallcacocha to calculate the
frequency and intensity of past El Niño events. h ey found that, prior to
5,000 years ago, during the early to mid-Holocene, l ood layers occurred on
average every i t een years, whereas during the late Holocene, l ood layers
occurred every two to ten years. h is result suggests that El Niño events have
been occurring more frequently in recent times.
variability in the pacific
decadal oscillation
Researchers have also looked for clues to past climate changes in the
strength of the Pacii c Decadal Oscillation (PDO)—another pattern of
ocean-atmosphere interaction that inl uences precipitation in the American
West (as described in chapter 4). When the PDO is in a “negative,” or cool,
phase, sea surface temperatures are typically cooler than average; conversely,
