(Huyer et al. 1987). The second wave of that strong event reached far along the North
American coast with striking effects. However, strong events have been noted over
several centuries in colonial records. Those and other records along the tropical belt,
particularly records since 622 AD of the height of the Nile River, measured in
adjacent wells of that age (Quinn 1992), show that phenomena related to the Southern
Oscillation are long-term features of tropical zone climate. El Niño–Southern
Oscillation (ENSO) events correlate with hydrological cycles over much of the Earth.
Evaporation from the western Pacific warm pool supplies rainfall over Australia,
Indonesia, India, and distantly as far as the steppes of Russia. The El Niño condition
reduces this evaporation and can generate droughts of varying severity. The opposite
can occur around the eastern Pacific, where El Niño can bring unusually strong low-
to mid-latitude rains and flooding.
(^) An El Niño ends when the trade winds resume and slowly push warm surface layers
west, raising the eastern Pacific thermocline, allowing upwelling to come from
nutrient-rich layers again. Non-El Niño (in the colder instances called “La Niña”)
conditions are usually re-established 18–24 months after El Niño onset. This was
violated in the early 1990s when the Southern Oscillation Index remained negative
(pressure higher at Darwin than Tahiti; Fig. 16.25a), and eastern tropical SST was
above average for a half decade. This protracted run of warmth in the eastern Pacific,
coupled with a very strong El Niño in 1997–1998 suggests to some that global
warming may be increasing the frequency, duration, and intensity of El Niños. But,
again, data running back several hundred years do show similar series of very strong
and prolonged El Niños in the past.
(^) Impacts of El Niño on marine biota are strong. Those involving plankton, which
have short lifetimes and high population growth potential, come and go rapidly with
the events. Effects on larger, longer-lived animals, particularly seabirds, involve
substantial mortality and long population recovery times. Weakening of the trade
winds reduces equatorial upwelling, and deepening of the thermocline and nutricline
in the eastern tropical Pacific reduces nutrient supply to the surface from the
upwelling that does occur. General effects of El Niños on primary production and
higher trophic levels are discussed starting on page 265 of Chapter 11.
(^) El Niño transport along the coast displaces stocks of planktonic algae and animals;
they just move poleward with the flow. Latitudinal boundaries of tropical species shift
poleward, and corresponding boundaries of subpolar forms retreat upstream in the
California Current (Fig. 10.25) and Peru Current. Ochoa and Gómez (1987) found
from massive and recurring sampling campaigns before and during the 1982–1983 El
Niño that Protoperidinium obtusum, a subantarctic species normally dominant all
along the Peru coast, was completely replaced by the equatorial species Ceratium
breve, a shift of 16° of latitude for both. Such complementary shifts of 10° of latitude
or farther are commonly associated with strong El Niños. In strong El Niños, summer