Download General Circulation and Climate Zones

Ocean Stratification
and Circulation
Martin Visbeck
DEES, Lamont-Doherty Earth Observatory
[email protected]
Outline
•
Review of Climate Variability
•
Ocean Stratification
Ocean Circulation
Review of Oceans Role in Climate
•
•
General Atmosphere Ocean Circulation
The surface energy balance
Top of
atmosphere
seafloor
Imbalance of energy flux at the top can
be balanced by:
Atmospheric Heat Transport
Oceanic Heat Transport
Air-sea
interface
Radiative Energy Balance
Ocean
Atmosphere
Top of
atmosphere
The imbalance
of the top of the
atmosphere
radiation
implies that
there must be
an internal heat
transport by the
combined
action of ocean
and atmosphere
of ~6 1015 W
30°N/S.
The Oceans Role in Climate
•
The annual heat flux between ocean and
atmosphere is formed by the sum of all of the
heat transfer process:
•
Solar radiation
+168
•
Terrestial radiation
-66
•
Evaporation
-78
•
Heat conduction
-24
Effect of Atmospheric Forcing
on Ocean
Ocean Basins
•
The ocean composes 70.8% of the earth's surface.
•
Sea water fills the basins separating the continents with an
average depth of 3795 meters.
Ocean Basins
•
The transition from the continental to the deep ocean or
continental margins extend from the sea shore to around
2500 meters depth, it covers 40.7% of the ocean (29% of
Earth surface).
•
The deep basins composes of extensive flat plains of 4000
to 5000 meters depth, and the mid-ocean ridge, marking
the axis of sea floor spreading where the crust tectonic
plates of the earth form.
•
The deep ocean covers about 59.3% of the ocean's surface
(42% of Earth¹s surface).
Ocean Basins
•
The deepest ocean is found in the trenches where the
plates are subducted, the Mariana Trench is 11035 meters
deep (compared to the 8848 meter height of Mount
Everest). If the solid earth were made into a flat plain, the
sea water would cover the entire earth to a depth of 2440
meters.
•
If all of the water vapor in the atmosphere were converted
to liquid it would cover the smoothed earth surface by about
an 1 inch.
Ocean Basins
•
•
The ocean basins are divided into three main Oceans:
•
The Pacific Ocean is the largest and deepest (52% of the ocean
area, mean depth of 4028 meters);
•
The Indian Ocean (20% area, mean depth of 3897 m) and
•
The Atlantic Ocean, the shallowest because of the rather narrow
deep basins (25% area, mean depth of 3332 m).
•
The Arctic is considered part of the Atlantic Ocean;
•
The southern parts of the three Oceans are referred to as the
Southern Ocean.
The northern hemisphere has less ocean than the southern
hemisphere, only about 61% ocean versus 81% for the
southern hemisphere.
Water in the Ocean
• The ocean holds
98% of the 1.4 billion
cubic kilometers of
water on the planet.
• Exchange of this
water with the water
vapor within the
atmosphere is a key
part of the global
hydrological cycle.
Water Properties
• In addition, the high
heat capacity and
density of water
relative to the
atmosphere, and the
great amount of energy
required to change its
phases (solid to liquid
to gas) makes the
ocean a powerful factor
of the Earth's climate
system.
General Circulation
The seasonal cycle revisited
• Strong Temperature
contrast in high
latitudes
January
July
• Strong over
Land...why?
Properties of Water and Climate
• The ocean attenuates
winter/summer and
day/night changes in
air temperature, and
moves significant
amounts of heat from
low to higher
latitudes, it provides
moisture to the
atmosphere.
Sea Surface Temperature
•
The link between ocean and climate depends on exchange of energy (mainly
heat and radiation) and materials (water, gases) across the sea surface. T
atmosphere 'sees', influences and responds to the sea surface temperature
(SST), by way of sea-air heat flux. SST generally cools with increasing latitude,
but important deviations from a pure latitudinal dependence occurs. These are
generally due to the movement of sea water in both the horizontal and vertical
directions. Temperature and density of ocean water are related inversely: warm
water means low density, cold water means denser sea water.
Ocean Salinity
• Sea water is about a
3.49% salt solution,
the rest is freshwater.
• The major salt
constituent is Chloride
•
The more saline, the
denser the sea water
Ocean Salinity
• As the range of salt concentration in the ocean
varies from about 3.2 to 3.8%, oceanographers,
who refer to salt content as 'salinity', express salt
concentration as parts per thousand; 34.9 ppt is
Evaporation and Precipitation
and Ocean Surface Salinity
•
As sea water evaporates the
salt remains behind, only the
freshwater is transferred from
the ocean to the atmosphere.
A region of excess
evaporation, such as the
subtropics tend to become
salty, while the areas of
excess rainfall become
fresher. Salinity reflects the
workings of the hydrological
cycle: the movement of
freshwater through the
earth/ocean/atmosphere
Sea Ice and Salinity
•
Sea ice formation also removes freshwater from the ocean,
leaving behind a more saline solution. The resulting ice
contains only part of the sea water salt, about 0.5% (5 ppt),
hence ice formation like evaporation, concentrates salt in
the remaining body of sea water.
Sea Ice and Salinity
•
Along the shores of Antarctica this causes very dense water
(cold and salty), which in some regions in the Southern
Ocean leads to deep reaching convection, called Antarctic
Bottom water. Southern ocean ice exhibits lots of seasonal
variability, and is generally only 0.5 meters thick.
Sea Ice and Salinity
•
The very low salinity of the Arctic prohibits the development
of deep reaching convection. In sharp contrast is the Arctic
sea where the sea ice is usually about 2 to 3 meters thick
with a lesser amount of seasonality, and a water column
which is very stratified. There is some evidence that global
warming is reducing the area of year round sea ice in Arctic,
but not (yet?) within the Southern Ocean.
Stratification
• What sets the stratification in the ocean?
• Where is the warm water?
• Where is the cold water?
Ocean simulation in tank
Equator
Pole
Basin
Stratification
• The surface and
deep Atlantic is
relatively saline.
This water is
derived from the
sinking of chilled
saline surface
water in the
northern North
Atlantic, and is
called North
Atlantic Deep
Water (NADW).
Basin
Stratification
• In contrast the
deep Pacific is
relatively fresh, as
it experiences no
deep convection of
cooled salty
surface water, its
surface layer is too
fresh and buoyant
to sink.
Basin
Stratification
• Towards the sea
floor, temperatures
are near 0°C
marking the
presence of
Antarctic Bottom
Water derived from
the dense water
along the shores of
Antarctica.
Intermediate
Water
• At the base of the
thermocline is the
low salinity
Antarctic
Intermediate water
derived from the
Antarctic
Circumpolar
Current.
How does the Ocean move
heat?
Ocean
Atmosphere
Top of
atmosphere