Page 63 - (DK) Ocean - The Definitive Visual Guide

UNDERWATER CIRCULATION 61

Deep-water Circulation

Seawater circulates slowly through the deeper parts of the oceans, driven by water
sinking in major downwelling zones, such as in the north Atlantic. Any specific
downwelling of
cold, salty water mass of deep water has, at some time, sunk in one of these zones. Once it sinks, its
in north Atlantic properties, such as its salinity, remain stable for long periods—thus, every mass of
diffuse upwelling
in Indian Ocean deep water contains a “memory” of where it originally sank. By analyzing seawater
warm surface
cold, dense flow in South samples from various parts of the deep oceans, it is possible to piece together the
water moves Equatorial general pattern of deep-water flow. The indications are that there is a large-scale
at depth Current circulation involving all the oceans, called the global conveyor. A specific mass of
through
Atlantic seawater takes about 1,000 years to complete a lap of this circuit.
diffuse upwelling in north
Pacific Ocean

warm surface
flow of North
Equatorial
Atlantic water is Current in
joined here by more central Pacific
cold water formed
near Antarctica
DISCOVERY warm flow of
Equatorial surface
SEAL AID current through
Indonesian
THE GLOBAL CONVEYOR archipelago
This deep-diving elephant seal The conveyor starts with cold,
is helping to gather information salty water sinking in the north
about underwater circulation in Atlantic. Moving south at depth, it
the south Atlantic. A measuring flows around Antarctica, branching combined mass of
device—attached to its head into the Indian and Pacific oceans, and cold water moves slowly cold, dense water flows
with glue that sloughs off when returns to the surface by mixing with around Antarctica, at depth north at depth into the
the animal molts—collects data warmer waters above. Finally, warm Pacific Ocean
about temperature and salinity at surface currents return it to the Atlantic.
varying depths. The information
gained may also help to conserve
elephant seal populations. Circulation Cells
One type of circulation that affects only the upper 70 ft (20 m) of the
ocean, but is more complex than either a simple horizontal or vertical
flow of water, is known as Langmuir circulation. This is wind-driven and
consists of rows of long, cylinder-shaped cells of water, aligned in the
direction in which the wind is blowing and each rotating in the
opposite direction from its neighbor—alternate cells rotate clockwise
and counterclockwise. Each cell is about 30–160 ft (10–50 m) wide and
can be hundreds of yards long. On the sea surface, the areas between
adjacent cells where seawater converges are visible as long white streaks
of foam, or congregations of seaweed, called INTRODUCTION
windrows. The whole pattern of circulation LANGMUIR WINDROWS
These long streaks of foam on the
was first explained in 1938 by an American
sea surface are the windrows of
chemist named Irving Langmuir, after he Langmuir circulation cells. The
crossed the Atlantic in an ocean liner. distance between windrow lines
It was subsequently named in his honor. increases with the wind speed.