Geoscience Reference
In-Depth Information
Hydrosphere
The liquid water in oceans, interior seas, lakes, rivers, and subterranean waters
constitute the Earth's hydrosphere. Oceans cover about 70% of the Earth's surface
and therefore intercept more total solar energy than land surfaces. Most of the
energy leaves oceanic surfaces in the form of latent heat in water vapor, but this is
not necessarily the case for land surfaces. Consequently, maritime air masses are
very different to continental air masses. The atmosphere and oceans are strongly
coupled by the exchange of energy, matter (water vapor), and momentum at their
interface, and precipitation strongly influences ocean salinity. The mass and
specific heat of the water in oceans is much greater than for air and understanding
this difference is very important in the context of seasonal changes in the atmos-
phere. The oceans represent an enormous reservoir for stored energy. As a result,
changes in the sea surface temperature happen fairly slowly and this moderates the
rate of change of associated features in the atmosphere, thereby greatly aiding
seasonal climate prediction.
The oceans are also denser than the atmosphere and have a larger mechanical
inertia, so ocean currents are much slower than atmospheric flows, and oceanic
movement at depth is particularly slow. The atmosphere is heated from below by
the Sun's energy intercepted by the underlying surface, but oceanic heating is from
above. Consequently, there is a profound difference in the way buoyancy acts in
these two fluid media. The higher temperature at the surface of the sea means
oceanic mixing by surface winds tends to be suppressed, and such mixing is lim-
ited to the active surface layer that has a thickness on the order of 100 m. A strong
gradient of temperature below this surface layer separates it from the deep ocean.
The response time for oceanic movement in the upper mixed layer is weeks to
months to seasons. In the deep ocean, however, movement due to density variations
associated with changes in temperature and salinity occur over time scales from
centuries to millennia. There are eddies in the upper ocean but turbulence is in
general much less pronounced than in the atmosphere. Ocean currents are
important because they move heat from the tropical regions, where incidental
solar radiation is greatest, toward colder mid-latitude and polar regions where
radiation is least. Currents in the upper layer of the ocean are driven by the
prevailing wind patterns in the atmosphere. Ocean flow is from east to west in the
tropics (in response to the trade winds), poleward on the eastern side of continents,
then back toward the equator on the western side of continents.
Lakes, rivers, and subterranean waters make up the remainder of the hydro-
sphere. They can have significant hydrometeorological and hydroclimatological
significance in continental regions, particularly at regional and local scale. The
contrast between the influence on the atmosphere of open water on the one hand
and land surfaces on the other is significant. This is responsible for 'lake effect'
snowfall in the US Great Lakes and 'river breeze' effects near the Amazon River,
for example. River flow into oceans also has an important influence on ocean
salinity near coasts.
