Earth as a Planet: Atmosphere and Oceans 181
In contrast to seawater, most river and lake water is rel-
atively fresh; for example, the salinity of Lake Michigan is
∼200 times less than that of seawater. However, freshwater
lakes always have both inlets and outlets. In contrast, lakes
that lack outlets—the Great Salt Lake, the Dead Sea, the
Caspian Sea—are always salty. This provides a clue about
processes determining saltiness.
Why is the ocean salty? When rain falls on continents,
enters rivers, and flows into the oceans, many elements
leach into the water from the continental rock. These el-
ements have an extremely low abundance in the conti-
nental water, but because the ocean has no outlet (unlike
a freshwater lake), these dissolved trace components can
build up over time in the ocean. Ocean-seafloor chemical
interactions (especially after volcanic eruptions) can also in-
troduce dissolved ions into the oceans. However, the com-
position of typical river water differs drastically from that
of sea salt—typical river salt contains∼9% chlorine, 7%
sodium, 12% sulfate, 5% magnesium, and 17% calcium by
mass. Although sodium and chlorine comprise∼85% of sea
salt, they make up only∼16% of typical river salt. The ratio
of chlorine to calcium is 0.5 in river salt but 46 in sea salt.
Furthermore, the abundance of sulfate and silica is much
greater in river salt than in sea salt. These differences re-
sult largely from the fact that processes act to remove salt
ions from ocean water, but the efficiency of these processes
depends on the ion. For example, many forms of sea life
construct shells of calcium carbonate or silica, so these bio-
logical processes remove calcium and silica from ocean wa-
ter. Much magnesium and sulfate seems to be removed in
ocean water–seafloor interactions. The relative inefficiency
of such removal processes for sodium and chlorine appar-
ently leads to the dominance of these ions in sea salt despite
their lower proportion in river salt.
Circumstantial evidence suggests that ocean salinity has
not changed substantially over the past billion years. This
implies that the ocean is near a quasi-steady state where
salt removal balances salt addition via rivers and seafloor–
ocean interactions. These removal processes include bio-
logical sequestration in shell material, abiological seafloor–
ocean water chemical interactions, and physical processes
such as formation of evaporate deposits when shallow seas
dry up, which has the net effect of returning the water to
the world ocean while leaving salt behind on land.
4.4 Atmosphere–Ocean Interactions
Many weather and climate phenomena result from a cou-
pled interaction between the atmosphere and ocean and
would not occur if either component were removed. Two
major examples are hurricanes and El Ni ̃no.
Hurricanes are strong vortices, 100–1000 km across, with
warm cores and winds often up to∼70 m s−^1 ; the tempera-
ture difference between the vortex and the surrounding air
produces the pressure differences that allow strong vortex
winds to form. In turn, the strong winds lead to increased
evaporation off the ocean surface, which provides an en-
hanced supply of water vapor to fuel the thunderstorms that
maintain the warm core. This enhanced evaporation from
the ocean must continue throughout the hurricane’s life-
time because the thermal effects of condensation in thun-
derstorms inside the hurricane provides the energy that
maintains the vortex against frictional losses. Thus, both the
ocean and atmosphere play crucial roles. When the ocean
component is removed—say, when the hurricane moves
over land—the hurricane rapidly decays.
El Ni ̃no corresponds to an enhancement of ocean tem-
peratures in the eastern equatorial Pacific at the expense
of those in the western equatorial Pacific; increased rainfall
in western North and South America result, and drought
conditions often overtake Southeast Asia. El Ni ̃no events
occur every few years and have global effects. At the crud-
est level, “normal” (non-El Ni ̃no) conditions correspond
to westward-blowing equatorial winds that cause a thick-
ening of the thermocline (hence producing warmer sea-
surface temperatures) in the western equatorial Pacific;
these warm temperatures promote evaporation, thunder-
storms, and upwelling there, drawing near-surface air in
from the east and thus helping to maintain the circulation.
On the other hand, during El Ni ̃no, the westward-blowing
trade winds break down, allowing the thicker thermocline
to relax eastward toward South America, hence helping to
move the warmer water eastward. Thunderstorm activity
thus becomes enhanced in the eastern Pacific and reduced
in the western Pacific compared to non-El Ni ̃no conditions,
again helping to maintain the winds that allow those sea-
surface temperatures. Although El Ni ̃no differs from a hur-
ricane in being a hemispheric-scale long-period fluctuation
rather than a local vortex, El Ni ̃no shares with hurricanes
the fact that it could not exist were either the atmosphere or
the ocean component prevented from interacting with the
other. To successfully capture these phenomena, climate
models need accurate representations of the ocean and the
atmosphere and their interaction, which continues to be a
challenge.4.5 Oceans on other Worlds
TheGalileospacecraft provided evidence that subsurface
liquid-water oceans exist inside the icy moons Callisto,
Europa, and possibly Ganymede. The recent detection of
a jet of water molecules and ice grains from the south pole
of Enceladus raises the question of whether that moon has
a subsurface reservoir of liquid water. Theoretical models
suggest that internal oceans could exist on a wide range of
other bodies, including Titan, the smaller moons of Saturn
and Uranus, Pluto, and possibly even some larger Kuiper
Belt objects. These oceans of course differ from Earth’s
ocean in that they are ice-covered; another difference is that
they must transport the geothermal heat flux of those bodies