180 Encyclopedia of the Solar System
convection—thunderstorms—predominate at low latitudes
(where abundant sunlight falls) but are rare near the poles;
in contrast, convection in the oceans is totally inhibited at
low latitudes and instead can occur only near the poles.
Third, much of the large-scale ocean circulation is driven
not by horizontal density contrasts, as in the atmosphere
(although these do play a role in the ocean), but by the
frictional force of wind blowing over the ocean surface. In
fact, the first simple models of ocean circulation developed
by Sverdrup, Stommel, and Munk in the 1940s and 1950s,
which were based solely on forcing caused by wind stress,
did a reasonably good job of capturing the large-scale hori-
zontal circulations in the ocean basins.
As in the atmosphere, the Earth’s rotation dominates
the large-scale dynamics of the ocean. Horizontal Coriolis
forces nearly balance pressure-gradient forces, leading to
geostrophy. As in the atmosphere, this means that ocean
currents flow perpendicular to horizontal pressure gradi-
ents. Rotation also means that wind stress induces currents
in a rather unintuitive fashion. Because of the existence of
the Coriolis force, currents do not simply form in the di-
rection of the wind stress; instead, the three-way balance
between Coriolis, pressure-gradient, and friction forces can
induce currents that flow in directions distinct from the
wind direction.
Averaged over time, the surface waters in most mid-
latitude ocean basins exhibit a circulation consisting of a
basin-filling gyre that rotates clockwise in the northern
hemisphere and counterclockwise in the southern hemi-
sphere. This circulation direction implies that the water in
the western portion of the basin flows from the equator
toward the pole, while the water in the eastern portion of
the basin flows from the pole toward the equator. How-
ever, the flow is extremely asymmetric: The equatorward
flow comprises a broad, slow motion that fills the eastern
90% of the ocean basin; in contrast, the poleward flow be-
comes concentrated into a narrow current (called a west-
ern boundary current) along the western edge of the ocean
basin. The northward-flowing Gulf Stream off the U.S. east-
ern seaboard and the Kuroshio Current off Japan are two
examples; these currents reach speeds up to∼1ms−^1 in
a narrow zone 50–100 km wide. This extraordinary asym-
metry in the ocean circulation results from the increasing
strength of the Coriolis force with latitude; theoretical mod-
els show that in a hypothetical ocean where Coriolis forces
are independent of latitude, the gyre circulations do not
exhibit western intensification. These gyres play an impor-
tant role in Earth’s climate by transporting heat from the
equator toward the poles. Their clockwise (counterclock-
wise) rotation in the northern (southern) hemisphere helps
explain why the water temperatures tend to be colder along
continental west coasts than continental east coasts.
In addition to the gyres, which transport water primar-
ily horizontally, the ocean also experiences vertical over-
turning. Only near the poles does the water temperature
become cold enough for the surface density to exceed the
deeper density. Formation of sea ice helps this process, be-
cause sea ice contains relatively little salt, so when it forms,
the remaining surface water is saltier (hence denser) than
average. Thus, vertical convection between the surface and
deep ocean occurs only in polar regions, in particular in
the Labrador Sea and near parts of Antarctica. On average,
very gradual ascending motion must occur elsewhere in the
ocean for mass balance to be achieved. This overturning
circulation, which transports water from the surface to the
deep ocean and back over∼1000 year timescales, is called
the thermohaline circulation.
The thermohaline circulation helps explain why deep-
ocean waters have near-freezing temperatures worldwide:
All deep-ocean water, even that in the equatorial oceans,
originated at the poles and thus retains the signature of po-
lar temperatures. Given the solar warming of low-latitude
surface waters, the existence of a thermocline is thus natu-
rally explained. However, the detailed dynamics that control
the horizontal structure and depth of the thermocline are
subtle and have led to major research efforts in physical
oceanography over the past 4 decades.
Despite the importance of the basin-filling gyre and ther-
mohaline circulations, much of the ocean’s kinetic energy
resides in small eddy structures only 10–100 km across.
The predominance of this kinetic energy at small scales re-
sults largely from the natural interaction of buoyancy forces
and rotation. Fluid flows away from pressure highs toward
pressure lows, but Coriolis forces short-circuit this process
by deflecting the motion so that fluid flows perpendicular
to the horizontal pressure gradient. The stronger the influ-
ence of rotation relative to buoyancy, the better this process
is short-circuited, and hence the smaller are the resulting
eddy structures. In the atmosphere, this natural length scale
(called the deformation radius) is 1000–2000 km, but in the
oceans it is only 10–100 km. The rings and meddies de-
scribed earlier provide striking examples of oceanic eddies
in this size range.4.3 Salinity
When one swims in the ocean, the leading impression is of
saltiness. The ocean’s global-mean salinity is 3.5% by mass
but varies between 3.3 and 3.8% in the open oceans and can
reach 4% in the Red Sea and Persian Gulf; values lower than
3.3% can occur on continental shelves near river deltas. The
ocean’s salt would form a global layer 150 m thick if precipi-
tated into solid form. Sea salt is composed of 55% chlorine,
30% sodium, 8% sulfate, 4% magnesium, and 1% calcium
by mass. The∼15% variability in the salinity of open-ocean
waters occurs because evaporation and precipitation add or
remove freshwater, which dilutes or concentrates the local
salt abundance. However, this process cannot influence the
relative proportions of elements in sea salt, which therefore
remain almost constant everywhere in the oceans.