Page 36 - (DK) Ocean - The Definitive Visual Guide

34 OCEAN WATER

Temperature and Salinity

OCEAN WATER IS NOT UNIFORM BUT VARIES in several physical attributes,
including temperature, salinity, pressure, and density. These vary vertically
(dividing the oceans into layers), horizontally (between tropical and temperate
regions, for example), and seasonally. The basic variables, temperature and salinity,
in turn produce variations in density that help drive deep-water ocean circulation.

Temperature
Temperature varies considerably over the upper areas of the oceans. In the tropics and LA NIÑA TEMPERATURE ANOMALY IN PACIFIC
subtropics, solar heating keeps the ocean surface warm throughout the year. Below the The Pacific experiences long-term fluctuations in
surface, the temperature declines steeply to about 8–10˚C (46-50˚F) at a depth of 1,000m the temperature patterns of its surface waters,
(3,300ft). This region of steep decline is called a thermocline. Deeper still, temperature which are linked to climatic disturbances known
decreases more gradually to a uniform, near-freezing value of about 2˚C (36˚F) on the sea as El Niño and La Niña. This visualization, based on
satellite data, shows a strong La Niña-type surface
floor – this temperature subsists throughout the deep oceans. In mid-latitudes there is a temperature pattern that developed in late 2010,
much more marked seasonal variation in surface with a blue band indicating lower than normal
temperature. In high latitudes and polar temperatures in the eastern Pacific.
oceans, the water is constantly
cold, sometimes below
0˚C (32˚F).

cool surface
waters caused
by cold current warm tropical
moving up coast
water, with
temperatures
constantly above
OCEAN SURFACE
25˚C (77˚F)
TEMPERATURE
This map shows average region of variable surface constantly cold
temperature, fluctuating
surface temperatures in seasonally from 7 to 20˚C water with warm surface
March. Proximity to the (45–68˚F) temperatures in waters caused
equator is the main factor the range 0–3˚C by warm current
determining surface North America (32–37˚F) moving down
southeast coast
temperature, but ocean
currents also play a role. constantly cold
water off
Greenland
constantly warm pool of
water in Caribbean Sea
thermocline, KEY
where temperature 90°F 32°C
declines rapidly
INTRODUCTION cold bottom water at TEMPERATURE AND DEPTH 70°F 20°C
South America
30°C
with depth
Shown in early summer, the
vast bulk of ocean water in
this part of the north Atlantic
50°F
10°C
is uniformly cold (below
5˚C/41˚F). Only a thin surface
layer from the tropics into
0°C
a uniform temperature
mid- latitudes is warmed
30°F
of 2˚C (36˚F)
above this base level.