plankton distribution patterns (Chapter 10). El Niño, Spanish for “the boy”, gets its
name from Christmas, the usual season of its appearance off Peru. The eastern tropical
Pacific Ocean warms during these events. Upwelling may or may not cease, but when
it continues, fewer nutrients are brought to the surface. There are strong effects on all
parts of the ecosystem: phytoplankton, zooplankton, the anchoveta stock, guano birds,
and global fish-meal markets.
(^) “Southern Oscillation” refers to the variable atmospheric pressure difference (Fig.
16.25a) along the equator in the Pacific, usually measured between Easter Island or
Tahiti and Darwin, Australia. This difference, normally high pressure in the east, low
in the west, is a measure of the driving force for the east-to-west trade winds. These
persistent winds (20–30 knots night and day, year around) push ocean water west in
the equatorial zone, generating an upward slope in sea level from east to west. It is
sun-warmed surface waters that move, so the layer of warm water in the west
becomes deep, and isopleths of temperature, density, and most other variables slope
upward (relative to the surface) to the east. “Permanent” thermoclines are at 350–400
m in the west, only 100–120 m in the east. Equatorial upwelling in the east can thus
reach down to rich subsurface layers, making the eastern tropical Pacific a nutrient-
rich (apart from iron limitation), relatively high production zone. The western “warm
pool” is as oligotrophic as a central gyre. This is the “normal” condition.
Fig. 16.25 Comparison of (a) a Southern Oscillation Index (SOI, anomaly of a
standardized atmospheric pressure difference between Tahiti and Darwin, Australia,
that is filtered to remove oscillations shorter than 8 months) and (b) a multivariate
ENSO index (MEI). The SOI is from www.cgd.ucar.edu/cas/catalog/climind/soi.html.
The smoothing line is a decadal running average. The MEI is based on six variables
across the tropical Pacific: sea-level pressure; zonal and meridional components of the
surface wind; sea-surface temperature; surface air temperature; and total cloudiness
fraction. The MEI has an arbitrary scale, high when SOI is low. It is a NOAA product
generated by Klaus Wolter (www.esrl.noaa.gov/psd/people/klaus.wolter/MEI/).