Transcript Part-1

Ocean and Atmosphere
Coupling
El-nino -- Southern Oscillation
Martin Visbeck
DEES, Lamont-Doherty Earth Observatory
[email protected]
Outline
• Review
• Meridional heat and freshwater transfer
by the atmosphere and ocean
• Fluxes across the sea surface interface:
Outline
•
Changes in equatorial Pacific ocean and
atmosphere circulation associated with El Niño and
La Niña events.
•
Two dynamic feedback processes which act to
intensify El Niño and La Niña events.
•
Why this system oscillates and the time-scale of
this oscillation.
•
Effects of El Niño/La Niña on regional and global
climate.
Atmospheric Processes
Horizontal Forces: Sea Breeze
Sea Breeze /Monsoon
Seasonal cycle of heating produces the monsoon
winds akin to the daily occurring sea-breeze.
Dynamic Meteorology
Pressure is routinely plotted on maps and isobars (contours of
constant pressure) are drawn.
Dynamic Meteorology
The Coriolis Force / geostrophic flow
Geostrophic Balance: f u = - (D p / D y) / r
Pressure Gradient Force (PGF) = Coriolis Force (CF)
Dynamic Meteorology
Geostrophic flow and Friction
The Ekman balance is important for forming weather patterns
in the atmosphere:
General Circulation
The surface energy balance
Atmosphere/Ocean Heat Transport
I ~ sT4
General Circulation
Large scale energy transport
A non
rotating
planet might
have a
circulation
like this.
What will the
Coriolis
force do?
General Circulation
The surface energy balance
What will the
Coriolis
force do?
A three cell
structure
emerges:
Hadley Cell
Mid-L. Cell
Polar Cell
General Circulation
The surface wind and pressure
• Polar
Easterlies
• Westerlies
• Trade Winds
• ITCZ (Low)
• Subtropical
High
General Circulation
The surface pressure
• Northern
Winter
(January)
• High over
Land,
Low over
Ocean
General Circulation
The surface pressure
• Northern
Summer
(July)
• High over
Ocean,
Low over
Land
General Circulation
Climate Zones Evaporation - Precipitation
Evaporation:
where air is subsiding
Precipitation:
where air is rising
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
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.
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.
Ocean Atmosphere Coupling
• In low latitudes the
ocean moves more
heat poleward than
does the
atmosphere, but at
higher latitudes the
atmosphere
becomes the big
carrier.
• Note the figure is
old atmosphere to
The Oceans Role in Climate
The ocean role in climate would be zero if there
were a impervious lid over the ocean, but there is
not, across the sea surface pass heat, water,
momentum, gases and other materials.
The Oceans Role in Climate
Much of the direct and diffuse solar short wave
radiation that reaches the sea surface penetrates
the ocean (the ocean has a low albedo), heating
the sea water down to about 100 meters,
depending on the water clarity. It is in this sunlit
surface layer of the ocean that the process of
photosynthesis can occur.
Solar heating of the ocean on a global average is
168 watts per square meter.
The Oceans Role in Climate
The ocean transmits electromagnetic radiation into the atmosphere
in proportion to the fourth power of the sea surface temperature
(°K). This radiation is at much longer wave lengths (greater
than 10 microns, in the infrared range) than that of the sun,
because the ocean surface is far cooler that the sun's surface.
The net long wave radiation from the ocean surface is surprisingly
uniform over the global. Why? The infrared radiation emitted
from the ocean is quickly absorbed and re-emitted by water
vapor and carbon dioxide, and other greenhouse gases residing
in the lower atmosphere.
The Oceans Role in Climate
Much of the radiation from the atmospheric gases, also in the infra
red range, is transmitted back to the ocean, reducing the net
long wave radiation heat loss of the ocean.
The warmer the ocean the warmer and more humid is the air,
increasing its greenhouse abilities. Thus it is very difficult for
the ocean to transmit heat by long wave radiation into the
atmosphere, it just gets kicked back by the greenhouse gases,
notably water vapor whose maximum concentration is
proportional to the air temperature.
Net back radiation cools the ocean, on a global average by
66 watts per square meter.
The Oceans Role in Climate
When air is contact with the ocean is at a different temperature
than that the sea surface, heat transfer by conduction takes
place.
The ocean is on global average about 1 or 2 degrees warmer than
the atmosphere so on average ocean heat is transferred from
ocean to atmosphere by conduction.
The heated air is more buoyant than the air above it, so it convects
the ocean heat into the lower atmosphere. If the ocean were
colder than the atmosphere (which of course happens, just not
quite as common as a warmer ocean) the air in contact with the
ocean cools, becoming denser and hence more stable, more
stratified.
The Oceans Role in Climate
As such it does a poor job of carrying the ocean 'cool' into
the lower atmosphere. This occurs over the subtropical
upwelling regions of the ocean (Cape Verde climate)
The transfer of heat between ocean and atmosphere by
conduction is more efficient when the ocean is warmer
than the air it is in contact with.
On global average the oceanic heat loss by conduction
is only 24 watts per square meter.
The Oceans Role in Climate
The largest heat loss for the ocean is evaporation, (which links
heat exchange with hydrological cycle). On global average
the heat loss by evaporation is 78 watts per square meter.
Why so large? Its because of the large heat of vaporization (or
latent heat) of water, a product of the polar bonding of the H2O
molecule.
The water vapor leaving the ocean is transferred by the
atmosphere eventually condensing into water droplets
forming clouds, releasing its latent heat of vaporization
in the atmosphere.
Evaporation and Latent Heat Flux
Scientist use what is called a "bulk formula" to calculate the
evaporative heat flux. This is an empirically-derived formula
(meaning it was derived after countless hours in the lab using real
data, rather than derived from theoretical principals).
•
•
•
Qe = Lt W Ae rair (qs - qa)
W is the wind speed in m/s
Ae = 1.3 x 10-3 (w/o dimension) is an exchange coefficient
•
qs is the saturated mixing ratio for water vapor in kg/kg
•
qa is the mixing ratio for water vapor in kg/kg
•
Lt = 2494 - 2.2T
•
rair = 1.2 kg/m3 is the density of air
kJ/kg where T is the temperature of the water in ºC
Evaporation and Latent Heat Flux
Qe = Lt W Ae rair (qs qa)
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
The Oceans Role in Climate
While the ocean gains heat in
low latitudes and losses
heat in high latitudes, the
largest heat loss is drawn
from the warm Gulf Stream
waters off the east coast of
the US during the winter.
An equivalent pattern is
found near Japan, where
the Kuroshio current is
influenced by the winter
winds off Asia. It is in these
regions that the atmosphere
takes over as the major
meridional heat transfer
agent.
Ocean Atmosphere Coupling
• To maintain an approximate steady state
climate the ocean and atmosphere must
move excess heat from the tropics to the
heat deficit polar regions.
• Additionally the ocean and atmosphere must
move freshwater to balance regions with
excess dryness with those of excess rainfall.
• The movement of freshwater in its vapor,
liquid and solid state is referred to as the
hydrological cycle.
The Oceans Role in Climate
The annual freshwater flux
between ocean and
atmosphere reflects the
water vapor content
(relative humidity) of the
atmosphere, resulting
from the general
circulation of the
atmosphere. The dry
regions of the subtropics
where the air subsides
along the poleward edges
of the Hadley Cell; the
rainy Intra Tropical
Convergence Zone
(ITCZ) where the trades
winds of northern and
southern hemisphere
meet, forcing updrafts of
air.
Climate Variability
•
Changes in equatorial Pacific ocean and
atmosphere circulation associated with El Niño and
La Niña events.