oceans. At times, the Peru Current weakens causing rains in the Atacama Desert
and catastrophic die-offs of ocean life; this phenomenon is known as El Nin ̃o.
A thermohaline circulation connecting the ocean currents of the topwaters with
the currents’ deep waters has become better known in the last couple of decades.
Whereas the surface ocean currents are driven by winds, deep currents are driven
by differences in density. There are two ways in which the density differences
are maintained: the first is by salinity differences and the second is by temperature
differences. Mainly in the North Atlantic and near the Antarctic periphery cold
surface water sinks into the deep waters. The ocean bottom geography is key as
the dense flow seeks the lowest ocean bottoms. The deep currents are prevented
from flowing to certain locations because of the relative high ocean bottom, like
mid-oceanic ridges blocking the way. The rate of travel of these waters is a mere
fraction of surface currents. At these tortoise-like rates, water sinking in the North
Atlantic may take on the order of 1,000 years to become surface water again in the
North Pacific and the Indian Ocean. The thermohaline is sometimes likened to a
conveyor belt. Long-term variations in the thermohaline circulation are speculated
to play a role in climate change.Oceans
Approximately 71 percent ofEarth’ssurfaceiscoveredbyaworldocean.The
water is salty, deep, and some of it is in transit. In that human life is largely246 Oceans
El Nin ̃o
An occasional cessation of the flow of the Humboldt Current and upwelling off of Peru. The
phenomenon was first documented by the Spanish in the 1600s, though South American
fisherman knew about it long before that. The name means “the child” for the Christ child
in that the onshore effects of El Nin ̃o are visible starting in December. El Nin ̃o leads to abnor-
mal flooding rains and the local collapse of the oceanic food chain. Appearing irregularly every
3 to 6 years, El Nin ̃o usually lasts 9 to 12 months. It has some Northern Hemisphere effects
such as an association with wet winters in the southern United States and low numbers of
hurricanes in the North Atlantic. El Nin ̃o is now known to be part of the gigantic Southern
Oscillation, which is a pressure reversal over the Southern Pacific Ocean affecting weather
conditions in Australia and impacting the South Asianmonsoon.ElNin ̃o sometimes alter-
nates with La Nin ̃a which is associated with very dry conditions in western South America.
El Nin ̃o is now monitored by satellite so that onset can be predicted months in advance.
The ultimate cause(s) of El Nin ̃o is unknown and under investigation.