Visualizing Environmental Science

276 CHAPTER 11 The Ocean and Fisheries

PROCESS DIAGRAM

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some cooler, and some warmer than usual. Normally,

westward-blowing trade winds restrict the warmest waters

to the western Pacific near Australia (ˆ}ÕÀiÊ££°Ó>).

Every three to seven years, however, the trade winds

weaken, and the warm mass of water expands eastward

to South America, increasing surface temperatures in

the usually cooler east Pacific (ˆ}ÕÀiÊ££°ÓL). Ocean

currents, which normally flow westward in this area, slow

down, stop altogether, or even reverse and go eastward.

The name for this phenomenon, El Niño (in

Spanish, “the boy child”), refers to the Christ

child: The warming usually reaches the fish-

ing grounds off Peru just before Christmas.

Most ENSOs last between one and two years.

ENSO can devastate the fisheries off

South America. Normally, the colder, nutri-

ent-rich deep water is about 40 m (130 ft)

below the surface and upwells (comes to the

surface) along the coast, partly in response

sinks. The deep current flowing southward in the North

Atlantic is, on average, 8°C (14.4°F) cooler than the

shallow current flowing northward.

Scientific evidence indicates that the ocean con-

veyor belt shifts from one equilibrium state to another.

Historically, these shifts are linked to major changes in

global climate.

Ocean–Atmosphere Interaction

The ocean and the atmosphere are strongly

linked, with wind from the atmosphere af-

fecting the ocean currents and heat from the

ocean affecting atmospheric circulation. One

of the best examples of the interaction between

ocean and atmosphere is the El Niño–Southern

Oscillation (ENSO) event, which is responsible

for much of Earth’s interannual (from one

year to the next) climate variability. As a result

of ENSO, some areas are drier, some wetter,

El Niño–Southern

Oscillation (ENSO)

A periodic, large-scale

warming of surface

waters of the tropical

eastern Pacific Ocean

that temporarily

alters both ocean

and atmospheric

circulation patterns.

What are the differences in air

circulation patterns across the Arctic Ocean and

over North America and Asia during normal climate

conditions and ENSO conditions?

Think Critically

Warm, moist air rises,

causes low pressure

and heavy rainfall.

2

Accumulation

of warm water.

1

Upwelling of nutrient-rich

water (Humboldt Current).

4

Cool, dry air descends

with high pressure and

little rainfall.

3

Equator

Southeast trade winds

South Equatorial Current

L H

Warm water flows

eastward to

South America.

1

Upwelling blocked

by warm water.

4

Descending air causes

high pressure and

warm, dry weather.

(^2) Warm air rises, causing
low pressure and
heavy rainfall.
3
Equator
Weak trade winds
H L
Adapted from Figure 6.31 on p. 148 in A. F. Arbogast.
Discovering
Physical Geography

. Hoboken, NJ: John Wiley and Sons, Inc. (2007).

a. Normal climate conditions

ENSO events depend on the relationship of atmospheric circulation

to surface water flow in the Pacific. Normal conditions occur when

strong easterly flow pushes warm water into the western Pacific.

b. ENSO conditions

An ENSO event occurs when easterly flow weakens, allowing warm

water to collect along the South American coast. Note the

relationship between precipitation and the location of pressure

systems. During an ENSO event, northern areas of the contiguous

United States are typically warmer during winter, whereas southern

areas are cooler and wetter.