Page 346 - Earth's Climate Past and Future

322 PART V • Historical and Future Climate Change

FIGURE 17-19 El Niño and the
Southern Oscillation (ENSO)
Temperatures are warmer off western
South America and across the
eastern tropical Pacific during El
Niño episodes which occur every
2 to 7 years. (Adapted from E. M.
Rasmussen, “El Niño and Variations in
Climate,” American Scientist 73 [1985]:
108–77.)

A

Warmer

~
El Nino years
Temperature

Colder 1940 1950 1960 1970 1980
B Year

The North Atlantic Oscillation (NAO) is a fluctu- ous negative years in the 1960s and 1970s, and a large
ation in atmospheric pressure between a subpolar low- and persistent strengthening during the 1980s and early
pressure center near Iceland and a high-pressure center 1990s (Figure 17–21B). Some scientists have claimed
in the Azores-Gibraltar region. The NAO is best devel- that the rapid retreat of Arctic sea ice during the 1990s
oped in winter. The “positive” NAO mode features was affected by the North Atlantic oscillation. How-
lower pressure over Iceland, higher pressure over the ever, the North Atlantic oscillation weakened during
Azores, and a strengthening of westerly winds across the late 1990s, but the retreat of sea ice has not only
intervening latitudes of the Atlantic Ocean between continued but even intensified.
these two pressure centers (Figure 17–21A). The sub- Still another possibility is that recent climate may
tropical Atlantic Ocean is warmer in a large region have been influenced by solar forcing at the relatively
extending from the mid-Atlantic and southeast coast of weak and short-term cycles of 440 years or less that
the United States eastward to the Azores Islands. The were identified by differences between tree ages derived
14
warm, moisture-bearing winds arriving from this part by counting rings and by C dating (Chapter 14). Some
of the Atlantic make Europe warmer and wetter than high-resolution records show small tendencies toward
during years of negative NAO. Cooler temperatures cyclic behavior at these periods, but many of them also
occur off the west coast of Africa, where strong trade show similar tendencies at other periods that are not
winds also send extra amounts of dust out across the connected to known solar changes. In addition, many
ocean toward the Caribbean Sea. other records show no evidence of short-term solar
The NAO has varied in strength over decadal scales, forcing. At this point, the possibility of short-term
with persistent positive years in the early 1900s, numer- cycles in response to solar forcing is not settled.